阅读说明：本技术 基于核共振吸收的血管检查 (Vascular examination based on nuclear resonance absorption ) 是由 H·德尔 于 2019-08-06 设计创作，主要内容包括：本发明涉及一种用于确定血管部分的特征的系统和方法,所述血管部分包括血液,所述血液含有在特定能量下呈现X射线光子的共振吸收的造影剂。所述系统包括发射X射线辐射的可调谐单色X射线源(21)、用于在X射线辐射已经穿过所述血管部分之后探测X射线辐射的X射线探测器设备(22)。控制单元(26)改变X射线源(21)的调谐,以改变由X射线源(21)发射的X射线辐射的能量,并且评估单元(27)确定出现入射到所述血管部分上的X射线辐射的核共振吸收处的X射线源(21)的调谐,并基于所确定的调谐来估计所述特征。所述特征具体地可以是所述血管部分中的血液速度。(The present invention relates to a system and method for determining characteristics of a vascular segment comprising blood containing a contrast agent exhibiting resonant absorption of X-ray photons at a specific energy. The system comprises a tunable monochromatic X-ray source (21) emitting X-ray radiation, an X-ray detector device (22) for detecting X-ray radiation after it has passed through the blood vessel portion. A control unit (26) changes the tuning of the X-ray source (21) to change the energy of the X-ray radiation emitted by the X-ray source (21), and an evaluation unit (27) determines the tuning of the X-ray source (21) at which nuclear resonance absorption of the X-ray radiation incident on the vessel portion occurs and estimates the characteristic based on the determined tuning. The characteristic may specifically be the blood velocity in the vessel portion.)

1. A system for determining at least one characteristic of a portion of a blood vessel of a patient (24), the portion of the blood vessel comprising blood containing a contrast agent exhibiting nuclear resonance absorption of X-ray photons at a particular energy, the system comprising:

a tunable monochromatic X-ray source (21, 21') configured to emit X-ray radiation,

an X-ray detector device (22, 22') arranged to detect X-ray radiation after the X-ray radiation has passed through the portion of the blood vessel and to provide a detection signal indicative of an intensity of the detected X-ray radiation,

-a control unit (26, 26 ') adapted to change the tuning of the X-ray source (21, 21 '), thereby changing the energy of the X-ray radiation emitted by the X-ray source (21, 21 '), and

an evaluation unit (27, 27 ') configured to determine, based on the detection signals, a tuning of the X-ray source (21, 21') at which nuclear resonance absorption of the X-ray radiation incident on the portion of the blood vessel occurs, and to estimate the at least one characteristic based on the determined tuning.

2. The system of claim 1, wherein the contrast agent comprises iodine-127.

3. The system according to claim 1, wherein the evaluation unit (27, 27 ') is configured to determine the tuning of the X-ray source (21, 21 ') at which nuclear resonance absorption of the X-ray radiation incident on the portion of the blood vessel occurs by determining, based on the detection signals, the tuning of the X-ray source (21, 21 ') at which a maximum attenuation of the X-ray radiation through the portion of the blood vessel occurs.

4. The system of claim 1, wherein the at least one characteristic of the portion of the blood vessel comprises a velocity of the blood flowing in the portion of the blood vessel.

5. The system according to claim 4, wherein the evaluation unit (27, 27') is further configured to determine the velocity of the blood in the portion of the blood vessel based on an orientation of the portion of the blood vessel.

6. The system according to claim 5, wherein the orientation of the blood vessel is determined based on a three-dimensional image, in particular a computed tomography image, of the portion of the blood vessel comprising the blood of the contrast agent.

7. The system as defined in claim 4, wherein the evaluation unit (27, 27') is further configured to determine the velocity of the blood based on an angle between a longitudinal direction of the portion of the blood vessel and a direction of travel of X-ray photons comprised in the X-ray radiation.

8. The system according to claim 4, wherein the patient (24) is positioned relative to the X-ray source (21, 21') such that X-ray photons comprised in the X-ray radiation pass through the portion of the blood vessel at an angle of not 90 ° between a direction of travel of the X-ray photons and a longitudinal direction of the portion of the blood vessel.

9. The system according to claim 6, wherein the X-ray source (21 ') and the X-ray detector (22') are movable relative to the patient such that the X-ray radiation emitted by the X-ray source (21 ') passes through the portion of the blood vessel at different angles and such that the X-ray detector (22') registers projection values of the blood vessel corresponding to the different angles, and wherein the three-dimensional image is generated from the projection values according to a computed tomography reconstruction.

10. The system according to claim 3, further comprising a gating unit (28, 28 ') adapted to provide a gating signal for controlling the X-ray source (21, 21 ') to emit X-ray radiation only during a time corresponding to a predetermined portion of the patient's cardiac cycle.

11. The system according to claim 1, wherein the characteristic of the portion of the blood vessel comprises its anatomy and/or a spatial distribution of calcium comprised in the portion of the blood vessel.

12. The system of claim 11, further configured to generate an X-ray image based on the detector signal acquired at the tuning of the X-ray source (21, 21') at which a maximum attenuation of the X-ray radiation passing through the blood vessel occurs.

13. The system as defined in claim 12, wherein the evaluation unit (27, 27') is configured to determine a location of calcium in the portion of the blood vessel and/or a degree of calcification of the portion of the blood vessel based on the generated image.

14. A method for determining at least one characteristic of a portion of a blood vessel of a patient (24), the portion of the blood vessel comprising blood containing a contrast agent exhibiting nuclear resonance absorption of X-ray photons at a particular energy, the method comprising:

-controlling a tunable monochromatic X-ray source (21, 21') to emit X-ray radiation,

obtaining detection signals of an X-ray detector (22, 22') detecting the X-ray radiation after the X-rays have passed through the portion of the blood vessel,

-changing the tuning of the X-ray source (21, 21') so as to change the energy of the X-ray radiation emitted by the X-ray source,

-determining, based on the detection signals, a tuning of the X-ray source (21, 21') at which nuclear resonance absorption of the X-ray radiation incident on the portion of the blood vessel occurs; and is

-estimating the at least one characteristic based on the determined tuning.

15. Computer program comprising a program code for instructing a computer device to execute the method according to claim 14 when the program code is executed in the computer device.

Technical Field

The present invention relates to a system and a method for determining at least one characteristic of a portion of a blood vessel of a patient. The characteristic may correspond to a velocity of blood flowing in a portion of a blood vessel. Similarly, the features may correspond to the anatomy of a portion of a blood vessel and/or the spatial distribution of calcium included therein.

Background

A stenosis in a blood vessel can be detected and examined using angiographic imaging. In angiography, a contrast agent, typically comprising iodine, is inserted into a blood vessel and the blood vessel is imaged using a suitable imaging modality, such as X-ray imaging, Computed Tomography (CT) imaging or Magnetic Resonance (MR) imaging. In the final image of the blood flowing through the vessel, the stenosis can be detected and its extent can be determined in order to decide on the treatment, e.g. the insertion of a stent.

However, anatomical obstructions detected in the angiographic images may not significantly obstruct blood flow. Therefore, if a decision about the intervention is made based on the angiographic image only, unnecessary intervention for implanting the stent into the patient may be performed. Thus, a catheter-based blood pressure measurement may be made to determine if blood flow is significantly reduced and if a stent implantation must be performed. This measurement has proven to be a reliable method for examining stenosis. However, they are invasive techniques that require intervention. Furthermore, it may be difficult or impossible to place a measurement catheter in certain vessels, and in such cases measurements may not be possible.

Alternatively, a computerized blood flow model generated based on angiographic CT images can be used to determine blood flow through the vessel. However, for example, in the case of patients with pacemakers, internal defibrillators, or prosthetic heart valves, the CT images may be degraded by metal artifacts. For such patients, modeling of blood flow may not be possible.

Furthermore, stenosis is often caused by calcified plaque in the blood vessel. However, calcium attenuates X-ray radiation similarly to iodine, and therefore calcium and iodine cannot be distinguished in angiographic images. Thus, angiographic images do not allow the determination of the spatial distribution of calcified plaque in a vessel comprising a stenosis.

Disclosure of Invention

In view of this, it is an object of the invention to allow an improved non-invasive examination of a portion of a blood vessel, in particular to characterize a stenosis in a portion of a blood vessel.

In one aspect, the invention proposes a system for determining at least one characteristic of a portion of a blood vessel of a patient, the portion of the blood vessel comprising blood containing a contrast agent exhibiting nuclear resonance absorption of X-ray photons at a specific energy. The system comprises: a tunable monochromatic X-ray source configured to emit X-ray radiation; an X-ray detector device arranged to detect X-ray radiation after it has traversed a portion of a blood vessel and to provide a detection signal indicative of an intensity of the detected X-ray radiation; and a control unit adapted to change the tuning of the X-ray source, thereby changing the energy of the X-ray radiation emitted by the X-ray source. Furthermore, the system comprises an evaluation unit configured to determine, based on the detection signals, a tuning of the X-ray source at which nuclear resonance absorption of X-ray radiation incident on the portion of the blood vessel occurs, and to estimate the at least one characteristic based on the determined tuning.

The characteristic may correspond to a velocity of blood flowing in a portion of a blood vessel. In this case, the system allows the blood velocity to be determined in a non-invasive procedure. This has advantages over conventional invasive techniques for measuring blood velocity. Additionally or alternatively, the features may correspond to the anatomy of a portion of a blood vessel and/or to the spatial distribution of calcium included therein. In this regard, the system may specifically generate an angiographic image of a portion of a blood vessel, where calcium (and thus calcified plaque) can be distinguished from other materials (including contrast agents) so that the spatial distribution of calcium is visible.

The contrast agent may comprise iodine-127. The species exhibits nuclear transitions at 57.6keV, which induce nuclear resonance absorption at the same energy. This energy is in the energy range of X-ray radiation commonly used in medicine for diagnostic purposes. Thus, the system also allows for the acquisition of diagnostic X-ray images. In addition, iodine-127 has been widely used as a contrast agent in radiology, making the use of approved contrast agents in the system.

In an embodiment, the evaluation unit is configured to determine the tuning of the X-ray source at which nuclear resonance absorption of X-ray radiation incident on the portion of the blood vessel occurs by determining the tuning of the X-ray source at which a maximum attenuation of X-ray radiation passing through the portion of the blood vessel occurs based on the detection signals. Since the attenuation of X-ray radiation is higher in the case of nuclear resonance absorption compared to the "normal" interaction between X-ray radiation and material, the tuning at which the maximum attenuation of X-ray photons occurs corresponds to the tuning at which nuclear resonance absorption occurs.

Each tuning of the X-ray source corresponds to a particular energy of the X-ray radiation emitted by the X-ray source. Based on the information about the energy at which the nuclear resonance absorption occurs or the corresponding tuning of the X-ray source, the velocity of the blood flowing through the portion of the blood vessel can be determined, and based on the detection signals or tuning of the X-ray detector acquired at this energy, an angiographic image can be constructed, which allows to distinguish between contrast agent and calcium.

The determination of blood velocity in the proposed system relies on the relative doppler shift of the photon energy required to induce nuclear resonance absorption, caused by the relative motion of the X-ray source and the contrast agent nuclei. Since this motion corresponds to the relative motion of the X-ray source and the blood flowing in the portion of the vessel under examination, the blood velocity can be estimated based on the doppler shift of the photon energy required to induce nuclear resonance absorption relative to the known transition energy of the nuclear transition corresponding to nuclear resonance absorption in a stationary frame of nuclei of the contrast agent. Here, the photon energy of the doppler shift corresponds to the shift tuning of the X-ray source compared to the tuning at resonance absorption that would occur when the nuclei of the contrast agent are stationary. Thus, the blood velocity can be determined based on the tuning of the X-ray source where resonance absorption occurs (i.e., the tuning where maximum photon attenuation occurs).

In an embodiment, the evaluation unit is further configured to determine the velocity of the blood in the portion of the blood vessel based on the orientation of the portion of the blood vessel. In a related embodiment, the orientation of the blood vessel is determined based on a three-dimensional image (in particular, a computed tomography image) of a portion of the blood vessel including blood containing the contrast agent. Furthermore, in order to take into account the orientation of the blood vessel, the evaluation unit may specifically be configured to determine the velocity of the blood based on an angle between a longitudinal direction of the portion of the blood vessel and a direction of travel of the X-ray photons comprised in the X-ray radiation.

These embodiments allow for the observation that the doppler shift of the photon energy required to induce nuclear resonance absorption also depends on the angle between the atomic nucleus and the direction of travel of the X-ray photon. In the system, the direction of travel of the X-ray photons is known from the arrangement of the X-ray source and the X-ray detector. The direction of travel of the nuclei of the contrast agent corresponds to the direction of movement of the blood and this direction corresponds to the orientation of the vessel portion to be examined, which can be determined on the basis of a three-dimensional image of the vessel portion. In particular, the direction is substantially parallel to the longitudinal axis of the portion of the blood vessel under examination. Thus, by determining the blood velocity based on the orientation of the portion of the blood vessel being examined, and in particular based on the longitudinal axis of the portion of the blood vessel being examined, the dependence of the doppler shift on the angle between the nuclei and the direction of travel of the X-ray photons can be taken into account.

Furthermore, at least in the so-called non-relativistic limit, i.e. when the relative velocity of the X-ray source and the nuclei of the contrast agent is small compared to the speed of light, no doppler shift of the photon energy occurs if the angle between the nuclei and the direction of travel of the X-ray photons is 90 °. Thus, the patient is preferably positioned relative to the X-ray source such that X-ray photons comprised in the X-ray radiation traverse the portion of the blood vessel at an angle other than 90 degrees between the direction of travel of the X-ray photons and the longitudinal direction of the portion of the blood vessel. This allows the blood velocity to be determined using the relativistic doppler effect even in cases where the blood velocity is small compared to the speed of light (as is often the case).

Other X-ray systems configured as CT systems may be used to acquire the above-mentioned three-dimensional image for determining the orientation of the portion of the blood vessel to be examined. However, it is equally possible to use a tunable monochromatic X-ray source and an X-ray detector for acquiring three-dimensional images.

In a related embodiment, the X-ray source and the X-ray detector are movable relative to the patient such that X-ray radiation emitted by the X-ray source traverses portions of the blood vessel at different angles, and the X-ray detector registers projection values of the blood vessel corresponding to the different angles, and generates a three-dimensional image from the projection values according to a computed tomography reconstruction. In this embodiment, the system itself can be used to acquire a CT image comprising a three-dimensional image to determine the orientation of the blood vessel.

Furthermore, blood velocity varies during the patient's cardiac cycle, and it is desirable to determine the blood velocity in a particular portion of the cardiac cycle. An embodiment therefore comprises that the system further comprises a gating unit adapted to provide a gating signal for controlling the X-ray source to emit X-ray radiation only during a time corresponding to a predetermined portion of a cardiac cycle of the patient. In this embodiment, the measurement of the blood velocity can be performed with respect to a predetermined portion of the cardiac cycle. The gating signal may be derived from electrocardiogram data.

In an alternative embodiment, the evaluation unit is configured to determine the portion of the cardiac cycle in which the maximum attenuation of the X-ray radiation through the blood vessel occurs. In this embodiment, the measurement can also be performed continuously, i.e. the X-ray source continuously emits X-ray radiation with varying photon energies, which is detected in the radiation detector, in order to determine the photon energy at which nuclear resonance absorption occurs, and the evaluation unit can retrospectively determine in which part of the cardiac cycle the maximum attenuation of the X-ray radiation occurs. Such a determination may be made based on electrocardiogram data acquired during irradiation of the portion of the blood vessel with X-ray photons.

As mentioned above, the system also allows determining an image of a portion of a blood vessel showing its anatomy, including the spatial distribution of calcium contained in the portion of the blood vessel. In a related embodiment, the system is configured to generate an X-ray image based on detector signals acquired at a tuning of the X-ray source at which a maximum attenuation of X-ray radiation passing through the blood vessel occurs. In this image, the contrast agent can be distinguished from calcium. Although both substances "generally" exhibit similar attenuation properties with respect to X-ray photons and are therefore not distinguishable in conventional X-ray images, the attenuation properties of the contrast agent change when nuclear resonance absorption occurs (i.e. when the maximum attenuation of X-ray radiation through the blood vessel occurs). Therefore, the image acquired in this embodiment shows the contrast agent and calcium contained in the portion of the blood vessel with different contrasts.

This allows determining the spatial distribution of calcified plaques in the portion of the blood vessel. Hence, an embodiment comprises an evaluation unit configured to determine a location of calcium in the portion of the blood vessel and/or a degree of calcification of the portion of the blood vessel based on the generated image.

In another aspect, the invention proposes a method for determining at least one characteristic of a portion of a blood vessel of a patient, the portion of the blood vessel comprising blood containing a contrast agent exhibiting resonant absorption of X-ray photons at a specific energy. The method comprises the following steps: (i) controlling a tunable monochromatic X-ray source to emit X-ray radiation, (ii) obtaining a detection signal of an X-ray detector detecting the X-ray radiation after the X-ray radiation has traversed a portion of the blood vessel, (iii) changing a tuning of the X-ray source, thereby changing an energy of the X-ray radiation emitted by the X-ray source, (iv) determining, based on the detection signal, a tuning of the X-ray source at which nuclear resonance absorption of the X-ray radiation incident on the portion of the blood vessel occurs; and, (v) estimating at least one characteristic based on the determined tuning.

Furthermore, the invention proposes a computer program comprising program code for instructing a computer device to execute the method when the program code is executed in the computer device.

It shall be understood that the system of claim 1, the method of claim 14 and the computer program of claim 15 have similar and/or identical preferred embodiments, in particular preferred embodiments as defined in the dependent claims.

It shall be understood that preferred embodiments of the invention can also be any combination of the dependent claims or the above embodiments with the respective independent claims.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

Drawings

In the following drawings:

figure 1 shows schematically and exemplarily an angle between a direction of motion of nuclei of a contrast agent and a direction of travel of X-ray photons,

figure 2 shows schematically and exemplarily components of a system for determining a characteristic of a portion of a blood vessel of a patient in one embodiment,

fig. 3 shows schematically and exemplarily components of a system for determining a characteristic of a portion of a blood vessel of a patient in another embodiment, an

Fig. 4 schematically and exemplarily shows steps of a method for determining a characteristic of a portion of a blood vessel of a patient.

Detailed Description

The invention proposes to determine the velocity of blood flowing in a part of a blood vessel of a patient's body based on excitation of the nuclei of a contrast agent introduced into the blood vessel. Furthermore, in measuring the blood velocity, an X-ray image showing plaque (including calcified plaque) in the portion of the blood vessel under examination can be acquired. Thus, if desired, two characteristics of a portion of a blood vessel can be determined in one measurement: blood velocity and distribution of calcified plaque. Similarly, blood velocity can be determined or angiographic images showing the distribution of calcified plaque can be generated.

The blood vessel may be a coronary artery in a region of the heart of the patient. Similarly, however, it is possible to determine the blood velocity in blood vessels in other parts of the patient's body. The part of the blood vessel to be examined may have been identified in advance (e.g. in an angiographic image) and may comprise a stenosis to be further examined.

The contrast agent is selected such that the nuclei contained therein exhibit nuclear resonance absorption of X-ray photons at a defined energy. Absorption results in the transition of the nucleus from a non-excited state to an excited state, where the non-excited state and the excited state have an energy difference, also referred to herein as the transition energy. Sometimes, this energy is also referred to as moesbauer energy or moesbauer line. The present invention proposes to use the presence of nuclear resonance absorption to measure blood velocity and/or to acquire angiographic images showing calcified plaque in blood vessels. The latter is particularly possible if calcium cannot be distinguished from the contrast agent in "normal" angiographic images (which is often the case), since the attenuation of X-ray radiation by the contrast agent changes when the X-ray radiation induces nuclear resonance absorption, while the attenuation characteristics of calcium (which does not exhibit nuclear transitions) do not change. Thus, in images acquired at photon energies where nuclear resonance absorption occurs in the contrast agent, the contrast agent can be distinguished from calcified plaque.

In order for nuclear resonance absorption to occur, the energy of the X-ray photon in the stationary frame of the nucleus must correspond to the transition energy in order to induce a state transition. If the source emitting the X-ray photons and the nuclei of the contrast agent move relative to each other, the required photon energy in a stationary frame of the source will be displaced according to the relative velocity between the source of the X-ray photons and the nuclei due to the relativistic doppler effect.

On this basis and based on the observation that the velocity of the nuclei of the contrast agent substantially corresponds to the velocity of the blood, the velocity of the blood can be estimated by determining the photon energy at which resonance absorption occurs. To determine this photon energy, the portion of the blood vessel to be examined can be irradiated with variable energy X-ray radiation by changing the tuning of the tunable monochromatic X-ray source, and the tuning associated with the highest photon absorption rate in the contrast agent can be determined. This tuning corresponds to the energy at which resonance absorption occurs in the contrast agent and, based on this tuning, the blood velocity in the vessel portion can be determined.

If the nucleus moves at a velocity v at an angle θ between the direction of motion of the nucleus and the direction of travel of the X-ray photon, the photon energy E seen by the nucleus is given by:

wherein E is₀Is the energy of the photons emitted by the X-ray source in its reference frame, and c is the speed of light. The angle θ is shown in fig. 1, wherein the nuclei are provided with reference numeral 1, the X-ray source with reference numeral 2, the direction of movement of the nuclei with reference numeral 3, and the direction of travel of the X-ray photons with reference numeral 4. If the velocity v is significantly less than the speed of light, as is the case for the velocity of blood, for example, the above formula reduces to:

E＝E₀·(1-v/c·cosθ)

in order to induce a nuclear transition, the energy E must be equal to the transition energy E of the nuclear transition at rest_Trans. In this case, for a given angle θ, the speed of the nuclei is given in non-relativistic limits by:

wherein E is_ResRepresenting the photon energy at which resonance absorption occurs in the reference frame of the X-ray source.

The velocity of the nuclei of the contrast agent corresponding to the velocity of the blood can thus be determined in particular on the basis of the energy of the X-ray radiation measured in the stationary frame of the X-ray source at which the resonance absorption occurs and on the basis of the transition energy of the nuclear transition.

When nuclear resonance absorption occurs, the X-ray radiation incident on the blood is attenuated to a particularly high degree, in particular, above the attenuation due to the "normal" interaction of the X-ray radiation with the material (i.e. the interaction between the X-ray radiation and the electron shells of the atoms of the material). Thus, the X-ray attenuation as a function of the energy of the X-ray photon has a maximum at the energy at which the nuclear resonance absorption occurs.

To take advantage of this effect to determine blood velocity, it is proposed to irradiate the blood with a monochromatic X-ray source and to change the tuning of the X-ray source to change the energy of the X-ray photons emitted by the X-ray source. Then, using an X-ray detector that detects X-ray radiation that has passed through the blood, the tuning of the X-ray source at which the maximum attenuation of the X-ray radiation occurs can be determined. This tuning corresponds to the tuning where the intensity of the X-ray radiation is reduced to a maximum extent once the X-ray radiation has passed through the blood. Based on this tuning, and based on the known transition energy of the nuclei of the contrast agent, the blood velocity can be determined, in particular according to the above formula. To this end, the tuning of the X-ray source may be related to the respective energy of the X-ray photons, so that the blood velocity can be determined according to the above formula.

Furthermore, the angle between the directions of motion of the nuclei of the contrast agent is preferably taken into account when determining the blood velocity. Furthermore, the angle is preferably adjusted to a value not equal to 90 ° because if the angle θ happens to be 90 °, no doppler shift occurs in the non-relativistic limits.

The direction of motion of the nuclei of the contrast agent substantially corresponds to the direction of blood flow, which is due to the orientation of the part of the blood vessel to be examined in space and the direction of blood flow within this blood vessel part. For determining the blood velocity it may be assumed that the direction of motion of the nuclei of the contrast agent corresponds to the longitudinal direction of the vessel portion, wherein this direction is parallel to the longitudinal extension of the vessel portion and the point in the main direction of blood flow. This longitudinal direction of the vessel portion may be determined based on a three-dimensional angiographic image. Thus, angiographic imaging may be performed similarly in connection with determining blood velocity to obtain information about the direction of blood flow, and this information may be used in determining blood velocity.

The contrast agent may specifically comprise iodine-127. The material exhibits nuclear transitions with transition energies of 57.6 keV. The transition has a natural linewidth of 1.5x10-9 keV. Thus, the motion of the nuclei with a velocity of 1cm/s in the direction of travel of the exciting X-ray photon results in a doppler shift of the photon energy in the natural line width range. In the case of healthy blood vessels, the blood velocity in a human blood vessel is typically in the range of tens of cm/s. Within the stenosis, blood velocity can reach 100-. Thus, a doppler shift of the photon energy is indeed required in order for the X-ray photon to be absorbed by the iodine nucleus, which moves at typical blood velocities. Thus, it is possible to determine the blood velocity based on the excitation of the 57.6keV transition of iodine-127. In addition, iodine-127 (which is the only stable isotope of natural iodine) has been widely used as a contrast agent in conventional X-ray based medical imaging. Thus, there is no need to approve new contrast agents.

Also, another contrast agent may be used, which comprises a material having a nuclear transition in a suitable energy range (i.e. in an energy range that is capable of irradiating the human body without causing unacceptable detrimental effects) and having a suitably narrow linewidth.

Fig. 2 shows schematically and exemplarily components of an embodiment of a system for determining a blood velocity in a selected portion of a blood vessel of a patient. In this embodiment, the system comprises an X-ray source 21 and an X-ray detector 22, which are arranged relative to each other such that the registration is performed in the X-ray detector 22 if an X-ray photon emitted by the X-ray source 21 is not absorbed or deflected en route from the X-ray source 21 to the X-ray detector 22. Between the X-ray source 21 and the X-ray detector 22 there is a measurement region 23 in which a body 24 of a patient is positioned to examine a portion of a blood vessel of the patient on the basis of nuclear resonance absorption. At least during the measurement, in the embodiment shown in fig. 2 the X-ray source 21 and the X-ray detector 22 are kept in a fixed position relative to the body 24 of the patient.

The X-ray source 21 is configured as a tunable monochromatic X-ray source. Thus, the X-ray source 21 emits X-ray photons having substantially a defined energy and the energy can be varied by varying the tuning of the X-ray source 21, wherein the tuning is characterized by the relevant operating parameters of the X-ray source 21 which determine the energy of the emitted X-ray photons. Preferably, the energy bandwidth of a photon in an X-ray beam of a certain nominal energy is no more than about 10 to 100 times the natural linewidth of a nuclear resonance transition of a nucleus of the contrast agent. Therefore, the energy bandwidth of the X-ray source 21 is preferably in the sub- μ eV range.

In order to generate X-ray radiation with a small energy bandwidth in this range and with a sufficiently high intensity, the X-ray source 21 may comprise a primary X-ray generator for generating X-ray radiation with a high intensity in a frequency range around the required output photon energy of the X-ray source 21, and the X-ray source 21 may additionally comprise an X-ray monochromator for selecting X-ray photons from a sufficiently small portion of the energy spectrum of the X-ray photons generated by the primary X-ray generator.

Exemplary primary X-ray generators include synchrotrons or free electron lasers that generate X-ray radiation within a desired energy range in a manner known to those of ordinary skill in the art. Another example is an X-ray generator based on Thomson scattering (also called inverse compton scattering) of laser pulses relative to an electron beam generated by a linear electron accelerator. This X-ray source is described in "Thomson scattering X-ray source" of z. a novel tool for monoclonal formulated tomogry "(Proc. SPIE 10391, Developments in X-Ray Tomogrpahy XI, 103910Z (9.2017, 19 th), doi: 10.1117/12.2273136). The X-ray generator is more compact and less costly than an X-ray generator comprising a synchrotron or free electron laser.

For example, the monochromator may be configured as a Si crystal monochromator. Such a monochromator allows to generate monochromatic X-ray radiation with an energy resolution deltae/E (where deltae is the energy bandwidth and E is the photon energy) in the range of 10-9keV, which is sufficient to obtain the small energy bandwidth required for determining the blood velocity. Examples of such monochromators are described in the publication "X-ray monochromator with energy resolution of 8X 10-9at 14.41 keV" (Review of Scientific Instruments72, 4080 (2001); https:// doi. org/10.1063/1.1406925) by M.Yabashi et al.

In one embodiment, the X-ray source 21 emits X-ray radiation in a diverging X-ray beam. In another embodiment, the X-ray source 21 emits an X-ray radiation beam having a parallel beam geometry. For detecting X-ray radiation, the X-ray detector 22 may comprise a one-or two-dimensional detector array comprising detector elements (so-called pixels) for detecting X-ray radiation incident into the detector elements, and for each detector element the X-ray detector 22 may provide a measurement signal indicative of the intensity of the incident radiation. The X-ray detector 22 may be configured as an indirect conversion detector known to those of ordinary skill in the art. However, the X-ray detector 22 may similarly be configured in another manner, for example as a direct conversion detector. The detector array is configured and arranged such that it covers at least a part of the cross-section of the X-ray beam emitted by the X-ray source 21 at the position of the X-ray detector 22. Furthermore, the X-ray detector 22 has a defined position relative to the X-ray source 21, so that for each detector element the connection to the X-ray source 21 is known. These connecting lines correspond to the trajectories of the X-ray photons detected in the detector elements after having traveled along a straight line through the measurement region 23.

In the measurement region 23, the patient's body 24 can be positioned on a support 25, and the support 25 can be adjustable in order to position the patient in such a way that: the angle between the longitudinal direction of the blood vessel portion to be examined and the direction of travel of the X-ray photons emitted by the X-ray source differs from 90 deg.. In a related embodiment, the X-ray source 21 and the X-ray detector 22 are arranged such that the X-ray beam emitted by the X-ray source 21 travels in a substantially vertical direction, for example in a downward direction, and the patient body 24 is positioned with a suitably tilted longitudinal axis in the measurement region 23 between the X-ray source 21 and the X-ray detector 22. This generally allows to achieve an angle between the direction of travel of the X-ray beam and the longitudinal direction of the blood vessel which is different from 90 °. Alternatively, the X-ray source 21 and the X-ray detector 22 are arranged such that the X-ray beam emitted by the X-ray source travels in a substantially horizontal direction and the patient body 24 can be positioned with its longitudinal axis inclined with respect to a horizontal axis, so as to achieve an angle between the direction of travel of the X-ray beam and the longitudinal direction of the blood vessel to be examined which is different from 90 °.

The X-ray source 21 is coupled to a control unit 26, the control unit 26 controlling the tuning of the X-ray source 21 by controlling relevant operating parameters of the X-ray source 21, wherein each tuning corresponds to a specific energy of an X-ray photon emitted by the X-ray source 21. The control unit 26 may also receive a gating signal provided by a gating unit 28, as will be explained below, and may control the X-ray source 21 in accordance with the gating signal. Furthermore, the X-ray detector 22 and the control unit 26 are coupled to an evaluation unit 27. The evaluation unit 27 receives detection signals from the X-ray detector 22, preferably individually for each detector element, which detection signals comprise information about the intensity of the X-ray radiation detected in the X-ray detector 22, and evaluates the detection. The evaluation unit 27 receives information from the control unit 26 indicating the tuning of the X-ray source 21, so that the evaluation unit 27 can associate the radiation intensity measured at a certain time with the tuning of the X-ray source 21 at the same time. In addition, the evaluation unit 27 may receive information about the intensity of the X-ray radiation emitted by the X-ray source 21. Furthermore, the evaluation unit 27 may control the overall operation of the system, and in doing so may also control the operation of the control unit 26 associated with the X-ray source 21.

The control unit 26 and the evaluation unit 27 may be configured as computer devices, each comprising a microprocessor programmed for performing the functions provided by the control unit 26 or the evaluation unit 27. For this purpose, a corresponding computer program may be stored and executed in the computer device. Furthermore, at least the evaluation unit 27 may comprise a suitable user interface for interacting with a user, which may comprise a display device and suitable input means. Via the user interface, the user may input control commands, for example to start a measurement procedure provided by the system, and may receive an output of the evaluation unit 27, such as the result of the determination of the blood velocity. Furthermore, it is also possible to integrate the control unit 26 and the evaluation unit 27 in one computer device.

Using the above system, it is possible to determine the velocity of blood flowing through a portion of a patient's blood vessel and/or to acquire angiographic images in which contrast agents are distinguished from calcium, including calcified plaque, as will now be explained in detail:

for determining the blood velocity, the angles between the longitudinal direction of the blood vessel portion to be examined and the direction of travel of the X-ray photons in three dimensions are taken into account. To determine and adjust the angle, an angiographic image of a region of the patient's body 24 including the vessel portion may be acquired. The image may be acquired using a CT device 29 in a manner known to those of ordinary skill in the art. In the CT device 29, the patient body 24 can be supported by a movable support which can also be moved into the measurement region 23 of the system for blood velocity determination. Thus, the same support 25 may be used for supporting the patient during acquisition of the angiographic image and performing measurements for determining the blood velocity, so that the patient does not have to be repositioned between acquiring the angiographic image and measuring the blood velocity. This ensures that the position and orientation of the part of the blood vessel to be examined does not change between the acquisition of the angiographic image and the determination of the blood velocity.

For acquiring an angiographic image, a suitable contrast agent may be injected into the vessel comprising the part to be examined, wherein the contrast agent preferably corresponds to the contrast agent used for determining the blood velocity. After the contrast agent has been administered, a conventional CT image of the relevant region of the patient's body 24 may be acquired by the CT device 29 as the contrast agent flows through the blood vessels.

In the obtained CT image, a blood vessel including a portion to be inspected is clearly visible, so that the position and the longitudinal direction of the portion can be determined. For this purpose, vessel segmentation may be performed as known to those of ordinary skill in the art. Then, based on the segmented blood vessel, the position and orientation of its relevant part can be determined in a suitable manner. For example, a centerline of the vessel portion may be estimated in order to determine an orientation of the vessel portion. In addition, a direction of blood flow in the vessel portion is determined, and a longitudinal direction of the vessel portion is determined based on the orientation of the vessel portion and the direction of blood flow therein. If the blood vessel is curved, its portion to be examined may be selected such that it is substantially straight. In this case, the longitudinal direction of the vessel portion can approximate the direction of the tangent to the vessel in the area of the selected portion thereof.

After the position and the longitudinal direction of the part of the blood vessel to be examined have been determined, the patient's body 24 is positioned in the examination area 23 of the system in order to determine the blood velocity in this part. Using information derived from angiographic CT images, the patient's body 24 is specifically positioned in the following manner: the X-ray beam emitted by the X-ray source 21 passes through a portion of the blood vessel and the angle between the longitudinal direction of the portion of the blood vessel and the direction of travel of the X-ray photons is not equal to 90 °. Preferably, the angle is chosen to be as small as possible by properly positioning the patient's body 24, since a smaller angle results in a larger doppler shift of the photon energy used to induce nuclear resonance absorption, thereby enabling a more accurate determination of blood velocity.

Under the control of the control unit 26, the X-ray source 21 is subsequently operated to irradiate the vessel portion with an X-ray beam of varying energy. The energy may vary within a predetermined range around a transition energy of the nuclei of the contrast agent at rest, wherein the range may comprise the necessary possible displacement energy to induce nuclear resonance transitions when the nuclei of the contrast agent move at a speed corresponding to a typical blood velocity. To vary the energy within this range, the tuning of the X-ray source 21 is varied within the corresponding range by varying the relevant operating parameters of the X-ray source 21. Within this range, the tuning may be changed according to a predetermined step, e.g. by starting with the tuning corresponding to the lowest energy value within the energy range, and by adjusting the tuning such that the photon energy is increased stepwise.

The X-ray beam emitted by the X-ray source 21 travels through a vessel portion and X-ray photons that have not been absorbed or attenuated are registered in the X-ray detector 22. Typically, several detector elements of the X-ray detector 22 detect X-ray radiation that has traveled through the blood vessel portion. These detector elements may be determined based on the known arrangement of the X-ray source 21 and the X-ray detector 22, the known geometry of the X-ray beam and the position and orientation of the vessel portion as determined based on the angiographic CT images.

For each tuning of the X-ray source 21 and for each relevant detector element (i.e. each detector element registers X-ray photons that have traveled through the part of the blood vessel to be examined), the intensity of the radiation incident on the detector element is measured in the X-ray detector 22 and forwarded to the evaluation unit 27. For each detector element, evaluation unit 27 then determines the amount of photon attenuation for the various tunings of X-ray source 21 and determines the tuning at which the maximum photon attenuation occurs. This tuning corresponds to the energy at the maximum number of photons that have been absorbed by the nuclei of the contrast agent and thus corresponds to the photon energy at which nuclear resonance absorption has occurred.

Based on the determined tuning for the different detector elements, the blood velocity is then determined in the evaluation unit 27. For this purpose, the angle between the longitudinal direction of the relevant portion of the blood vessel as determined on the basis of the angiographic image and the direction of travel of the X-ray photons impinging on the X-ray detector 22 is determined for the detector elements. In case the X-ray source 21 emits a diverging X-ray beam, the angle varies for different detector elements. Therefore, the angle is preferably determined individually for each detector element. In the case of a parallel beam geometry, the angles for all detector elements are substantially the same. In this case, one angle can be determined for all detector elements.

For each detector element, the evaluation unit 27 may then determine a value of the blood velocity based on the angle between the longitudinal direction of the vessel portion and the direction of travel of the X-ray photons and on the photon energy at which the minimum intensity has been detected in the detector element. The determination can be made based on the formula given above, taking into account the known transition energy of the nuclear transition of the nuclei of the contrast agent. Furthermore, the determination may be made based on a known relationship between the tuning of the X-ray source 21 and the energy of the emitted X-ray photons. One possible procedure for determining this relationship is described herein below.

From the values of the blood velocity determined for the detector elements receiving the X-ray photons having traveled through the portion of the blood vessel to be examined, the evaluation unit 26 may determine an average value. This may be done at least if the value differs from the threshold by no more than an amount, so that a substantially uniform velocity of the blood in the vessel portion may be assumed. This average value can then be taken into account as the blood velocity in the vessel portion in its further evaluation, for example in the diagnosis of a stenosis in the vessel portion and in deciding whether a treatment of a stenosis has to be carried out.

To determine the tuning of the X-ray source 21 at which the maximum photon attenuation occurs, the X-ray source 21 may be operated to emit X-ray radiation with a constant intensity for all photon energies to which the X-ray source 21 is tuned. In this embodiment, the tuning where the maximum photon attenuation occurs is the tuning where the minimum photon intensity is measured in each detector element.

In an alternative embodiment, the emitted photon energy may be determined by comparing the intensity of the radiation emitted by the X-ray source 21 with the intensity of the radiation measured in the X-ray detector 22 for each tuning of the X-ray source 21. Based on the difference between these intensities, the evaluation unit 26 determines the photon attenuation for each tuning and for each detector element of the X-ray detector 22. The evaluation 27 may then compare the determined photon attenuation to determine the energy at which the maximum photon attenuation occurs and estimate the blood velocity based on this energy, as described above. In this embodiment, the X-ray source 21 is not operated to emit radiation with a constant intensity for all photon energies.

The emitted radiation intensity may be estimated based on various operating parameters of the X-ray source 21 using a model of the X-ray source 21. Alternatively, the emitted radiation intensity may be measured using an additional X-ray detector arranged to detect the X-ray radiation before it reaches the patient body 24. The X-ray detector may in particular be integrated into the X-ray source 21. Furthermore, if the X-ray source 21 generates a fan-shaped radiation beam, the additional X-ray detector may be arranged such that it detects radiation only in a small part of the cross-section of the X-ray beam. In this portion, the additional X-ray detector may block the X-ray beam, but the remainder of the X-ray beam may sufficiently illuminate the patient's body 24 to measure blood velocity. Alternatively, additional X-ray detectors may be moved in and out of the X-ray radiation beam. In this embodiment, an additional radiation detector may be moved into the radiation beam in order to measure the emitted radiation intensity in at least a part of the cross section of the X-ray beam for each tuning of the X-ray source 21, and then moved out of the radiation beam again. The patient's body 24 may be irradiated before or after the measurement.

Furthermore, calibration of the X-ray source 21 may be required in order to tune the X-ray source 21 precisely to a particular energy and to correlate the tuning of the X-ray source 21 to the corresponding photon energy. Accordingly, a corresponding calibration measurement can be made before the actual measurement of the blood velocity is performed, and preferably also before the patient's body 24 is positioned in the measurement region 23. In particular, calibration measurements may be made at once in order to calibrate the system for measurement of blood velocity for a plurality of patients.

For performing calibration measurements, a reference probe comprising a contrast agent (or an included isotope exhibiting nuclear resonance absorption) may first be positioned in a fixed position (i.e. stationary) in the measurement region 23 and irradiated with X-ray radiation. During irradiation, X-ray radiation passing through the probe can be detected by means of the X-ray detector 22 and the tuning of the X-ray photons can be changed. The detection signals acquired in the process can be evaluated by an evaluation unit 27 in order to determine the tuning at which the reference probe attenuates the X-ray photons to the maximum extent, wherein such a determination can be made identically during the above-mentioned actual measurement. This tuning corresponds to a photon energy equal to the transition energy of the nuclei of the contrast agent.

Furthermore, the reference probe may be moved in the measurement region in a predetermined direction at one or more known speeds, such that there is a known angle between the direction of movement and the direction of travel of the X-ray photons (wherein the angle may vary for different detector elements in case of a divergence of the X-ray beam). For each velocity, the tuning of the X-ray source 21 at which the maximum photon attenuation occurs can be determined as described above. For this tuning, the corresponding photon energy may then be determined based on the known speed of the probe, the known angle between the direction of the probe and the direction of travel of the X-ray photon, and the known transition energy.

Based on these measurements, the relation between the tuning of the X-ray source 1 and the photon energy can be determined, and the blood velocity can be determined using this relation as described above. Alternatively, the relation between the tuning of the X-ray source 1 and the quantity v · cos (θ) may be determined for the relevant contrast agent, and the blood velocity may be determined based on this relation.

In the presence of a substantially linear dependence of photon energy on tuning, the above measurements may be made for a stationary reference probe and for one velocity, and the relationship between the tuning of the X-ray source 21 and the photon energy or quantity v · cos (θ) for all tunings may be determined based on the determination for that velocity and based on the determination for the stationary probe. As an alternative, which may be applied in particular in case the dependency of the photon energy on the tuning is non-linear, the determination may be made for a plurality of velocities, which preferably correspond to the possible velocities of the blood, and the overall relation between the tuning of the X-ray source 21 and the photon energy or quantity v · cos (θ) may be determined based on these determinations in a manner known to a person skilled in the art, e.g. based on a non-linear fitting procedure.

As an alternative to the foregoing method, the reference probe may be moved in the measurement zone 23 along a known path (i.e. along a known direction) at a varying speed corresponding to a known speed profile. At the same time, the tuning of the X-ray source 21 may be changed such that the attenuation of the X-ray photons is constantly kept at its maximum, i.e. such that a constant resonance absorption occurs during the movement. The required change of tuning can be controlled by means of a suitable closed loop modulation. Furthermore, the tuning of the X-ray source 21 may be tracked and the relation between the tuning of the X-ray source 21 and the photon energy or amount v · cos (θ) may be determined based on the tracked tuning and the known velocity profile and the known movement path of the reference probe.

Furthermore, the velocity of the blood flowing through the part of the blood vessel to be examined is not constant, but changes during the cardiac cycle. In this regard, the velocity of blood in a selected portion of the cardiac cycle needs to be determined. For this purpose, the measurement of the blood velocity may be made at a time corresponding to a selected portion of the cardiac cycle. To achieve this, the operation of the X-ray source 21 may be gated accordingly. This means that the X-ray source 21 emits X-ray radiation only during selected parts of the cardiac cycle. To enable such a gating operation, the system may comprise a gating unit 28 adapted to determine the occurrence of a predetermined portion of the cardiac cycle. Such a determination may be made, for example, based on Electrocardiogram (EKG) data known to those of ordinary skill in the art. Furthermore, the gating unit 28 outputs a gating signal indicating a time corresponding to a selected part of the cardiac cycle for determining the blood velocity, and the X-ray source 21 may be operated based on the gating signal.

If the blood velocity needs to be determined in several cardiac cycles, a separate measurement can be made in each cardiac cycle based on the corresponding gating signal. Here, each gating signal may indicate a time corresponding to an associated portion of the cardiac cycle, and in each associated portion of the cardiac cycle, the blood velocity may be measured separately as described above.

Also, the blood velocity can be measured independently of the cardiac cycle. In this embodiment, nuclear resonance absorption will be observed in any portion of the cardiac cycle, and thus the blood velocity in any portion of the cardiac cycle is measured. To determine the relevant part of the cardiac cycle, EKG data can be acquired during the measurement and, based on these data, it can be determined which part of the cardiac cycle the nuclear resonance absorption occurs during the measurement. This method allows, in contrast to gated measurements, to perform measurements to determine blood velocity without interruption.

Fig. 3 shows schematically and exemplarily components of a system for determining blood velocity in another embodiment. This embodiment differs from the above-described embodiment in that the X-ray source 21 ' and the X-ray detector 22' are movable relative to the body 24 of the patient such that the X-ray radiation emitted by the X-ray source 21 ' passes through the body 24 at different angles. This provides improved flexibility in positioning the patient body 24 such that the angle between the longitudinal direction of the vessel portion and the direction of travel of the X-ray photons is small. Furthermore, in this embodiment, the same X-ray source 21 'and the same X-ray detector 22' as used for determining the blood velocity can be used for acquiring the CT image.

In one embodiment, the X-ray source 21 ' and the X-ray detector 22' may be mounted on a gantry (not shown in the figures) which is rotatable around a measurement region 23 ' in which a body 24 of a patient is arranged. In this embodiment, the arrangement of the X-ray source 21 'and the X-ray detector 22' is similar to that in the conventional CT apparatus. The patient's body 24 can be positioned in the measuring region 23' in a substantially horizontal position. However, it is also possible to position the patient's body 24 such that its longitudinal axis is inclined with respect to the horizontal. In other embodiments, the positions of the X-ray source 21 ' and the X-ray detector 22' may be fixed and the body 24 of the patient may be rotated in the measurement region 23 ' in order to acquire CT images, as schematically shown in fig. 3. This embodiment also allows the acquisition of CT images also with the use of an X-ray source 21' that is not compact enough to be mounted on a gantry. In this embodiment, the body 24 of the patient can be positioned in the measuring region in a substantially upright position and fixed in this position by means of a suitable support 25'. Also, the support 25 'may be configured to enable the patient's body 24 to be positioned with its longitudinal axis tilted with respect to the vertical.

In both configurations, the X-ray source 21 'and the X-ray detector 22' may be configured generally as described in connection with the embodiment shown in fig. 2. Furthermore, the X-ray source 21 'may again be controlled by the control unit 26', which control unit 26 'is specifically configured to change the tuning of the X-ray source 21', as already described above. In addition, the system may further comprise a gating unit 28 'for allowing gated X-ray measurements and/or for registering measurements to a portion of the patient's cardiac cycle. Furthermore, the system also comprises an evaluation unit 27 'for evaluating detection signals acquired using the X-ray detector 22'.

The aforementioned configuration allows for a flexible positioning of the patient body 24 such that the angle between the longitudinal direction of the blood vessel portion to be examined and the direction of travel of the X-ray photons is as small as is required for determining the blood velocity. The position and orientation of the vessel portion may again be determined on the basis of a three-dimensional angiographic image of a part of the patient body 24 comprising the vessel portion as described above, on the basis of which the patient body 24 is positioned in the measurement region 23'. The image may be acquired before the measurement of the blood velocity is made.

In this respect, three-dimensional angiographic CT images can be acquired using the X-ray source 21 'and the X-ray detector 22' of the system for determining blood velocity. For acquiring the images, the X-ray source 21 ' and the X-ray detector 22' are rotated around the body 24 of the patient, or the body 24 of the patient is rotated in the measurement region 23 ', so that the X-ray radiation emitted by the X-ray source irradiates the body 24 at different angles. For each angle, the X-ray detector 22 'registers projection values of the irradiated portion of the patient's body 24 and, based on these projection values, generates a three-dimensional image of the body portion by means of a CT reconstruction process, as known to a person skilled in the art.

The determination of the blood velocity can then be performed in the same manner as described above in connection with the system shown in fig. 2. In particular, a suitable relative positioning of the vessel portion with the X-ray source 21 'and the X-ray detector 22' may be used for making the determination. For the determination, the tuning of the X-ray source 21 ' may be changed by the control unit 26 and on the basis of the detection signals acquired by means of the X-ray detector 22', the evaluation unit 27 ' may determine the tuning at which the maximum photon attenuation occurs and may estimate the blood velocity on the basis of this tuning, as described above. Using the gating signal provided by the gating unit 28 ', the determination may again be performed with respect to one or more specific portions of the patient's cardiac cycle.

In other embodiments, an angiographic image is acquired showing contrast agent and calcified plaque with different contrast, thereby enabling determination of the spatial distribution of calcified plaque in the vessel portion under examination. Images can be acquired at the tuning of the X-ray source 21, 21' where nuclear resonance absorption occurs using a system as described above. In this case, the contrast agent attenuates the X-ray photons to a higher degree than if no nuclear resonance absorption occurs. Thus, the contrast agent exhibits a higher photon attenuation than the calcium contained in the calcified plaque, which has similar attenuation characteristics as the contrast agent "normally" (i.e., when no nuclear resonance absorption occurs).

In one embodiment, angiographic images are acquired during measurement of blood velocity in a vessel portion under examination. In this embodiment, the detection signals of the X-ray detectors 22, 22' acquired for the tuning of the X-ray source 21 at which nuclear resonance absorption occurs are used to generate an image. Using such detection signals, angiographic images can be generated in a conventional manner known to those of ordinary skill in the art.

If the X-ray source 21, 21 'emits a diverging X-ray beam, the photon energy at which nuclear resonance absorption occurs may be different for the individual detector elements of the X-ray detector 22, 22' due to the varying angle between the direction of travel of the X-ray photons and the direction of travel of the blood or contrast agent registered in the detector elements. In this case, a selection of detection signals for constructing an angiographic image may be made separately for each detector element, and an angiographic image may then be constructed from such selected detection signals. Thus, images may be generated from detection signals of individual detector elements acquired at different points in time. In order to still generate an angiographic image with uniform contrast, the X-ray source 21, 21' may be operated to emit X-ray radiation at a constant intensity for all photon energies, or the intensity difference may be corrected during reconstruction of the image.

In the case of a parallel beam geometry, the angle between the direction of travel of the registered X-ray photons in the detector elements and the direction of travel of the blood or contrast agent, which substantially corresponds to the longitudinal direction of the examined vessel portion, is substantially the same for all detector elements. Thus, in the case of a parallel beam geometry, an image can be generated from the detection signals of the detector elements which are acquired simultaneously at the same point in time.

This provides an advantage over conventional digital subtraction angiography, which is typically used to visualize calcification of blood vessels. In this method, a first image of the blood vessel is acquired before injecting a contrast agent into the blood vessel, wherein calcified plaques are visible in the image. The first image is then subtracted from a second image of the blood vessel acquired after the contrast agent has been injected into the blood vessel. This enables generation of a differential image showing only the contrast agent. However, since the two images are acquired at different points in time and the blood vessels may move between these points in time, the difference image may comprise motion artifacts. This is particularly true for blood vessels near the heart. These motion artifacts can be avoided by acquiring images showing calcified plaque based on detection signals measured at a single point in time, as described above.

In other variations, angiographic images are acquired without simultaneously determining blood velocity. In this embodiment, the detection signals used for generating the angiographic image also correspond to the detection signals acquired at the tuning of the X-ray source 21, 21' where nuclear resonance absorption occurs and are acquired as described above. Thus, the part of the blood vessel to be examined is irradiated with X-ray radiation and the tuning of the X-ray source 21, 21' may be changed to change the energy of the X-ray photons. Then, as described above, the tuning at which the resonance absorption occurs is determined for each detector element of the X-ray detector 22, 22', and the detection signals of the detector elements acquired at these tuning are selected for constructing the angiographic image.

However, when the patient body 24 is positioned such that the angle between the longitudinal axis of the examined vessel portion and the direction of travel of the X-ray photons differs from 90 ° in order to determine the blood velocity, such positioning is not necessary for acquiring an angiographic image, since the acquisition of this image is independent of the relativistic doppler effect. In fact, it is not necessary to know the angle between the longitudinal axis of the examined vessel portion and the direction of travel of the X-ray photons in order to generate the angiographic image. Instead, it suffices to tune the X-ray source 21, 21' to the X-ray photon energy at which nuclear resonance absorption occurs, and to construct an angiographic image based on the detection signals acquired at this tuning.

In the above manner, a two-dimensional angiographic image of the blood vessel portion under examination can be generated. In a variant of the aforementioned embodiment, it is also possible to acquire three-dimensional angiographic images of the vessel portion. For this purpose, the detection signals for generating the image may be acquired at several angles between the X-ray beam and the patient body 24 in the above-described manner in order to reconstruct a CT image of the vessel portion. This can be done using the X-ray system shown in fig. 3.

In the above embodiments, the characteristics of the portion of the blood vessel of the patient can be determined based on nuclear resonance absorption. As illustrated, the characteristic may be the velocity of blood flowing through the portion of the blood vessel. Also, the characteristic may be a part of the anatomy of the blood vessel and/or the spatial distribution of calcium contained therein.

Fig. 4 shows systematically and exemplarily the steps of determining relevant characteristics of a vessel portion performed in a system according to the preceding embodiment: having correctly positioned the patient's body 24 in the measurement region 23, 23' of the system used in step 401, an X-ray measurement for determining the desired characteristics of the vessel portion can be started. During the measurement, in step 402, the control unit 26, 26 'may control the X-ray source 21, 21' to emit X-ray radiation irradiating the portion of the blood vessel to be examined. During irradiation of the vessel portion, the control unit 26, 26 ' changes the tuning of the X-ray source 21, 21 ', thereby changing the energy of the X-ray radiation emitted by the X-ray source 21, 21 ' (step 403). After the X-ray radiation has passed through the vessel portion, it is detected by means of the X-ray detectors 22, 22' of the system (step 404). In step 405, the detection signals of the X-ray detectors 22, 22 'are provided to an evaluation unit 27, 27' of the system. Based on the detection signals obtained in this step, the evaluation unit 27, 27 'determines the tuning of the X-ray source 21, 21' at which the maximum attenuation of the X-ray radiation travelling through the portion of the blood vessel occurs (step 406). Based on the determined tuning, the evaluation unit 27, 27' then determines the desired characteristics of the vessel portion as described above (step 407).

Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

A computer program may be provided with or stored/distributed as part of other hardware on a suitable medium, such as an optical storage medium or a solid-state medium, but may also be distributed in other forms, such as via the internet or other wired or wireless telecommunication systems.

Any reference signs in the claims shall not be construed as limiting the scope.