阅读说明：本技术 X射线管电流分布的自动确定 (Automatic determination of current distribution in X-ray tubes ) 是由 A·克劳斯 于 2021-04-02 设计创作，主要内容包括：本发明涉及X射线管电流分布的自动确定。描述了用于利用X射线成像装置控制管电流以采集检查对象的检查范围的至少一个X射线图像的方法。在方法中,执行对检查对象的检查范围的预采集。基于预采集确定检查范围的三维调制的X射线衰减。基于确定的X射线衰减,确定初始管电流分布。针对管电流的后续实时修改规定容差范围。在X射线管不发生过热的情况下确定最大容许管电流分布。基于初始管电流分布确定潜在患者剂量的期望值并基于规定的容差范围确定潜在患者剂量的最大值。在实际X射线图像采集中测量实际X射线衰减。基于实际X射线衰减和初始规划的管电流分布确定适合的管电流分布。最后根据确定的调整的管电流分布调整管电流。(The invention relates to an automatic determination of an X-ray tube current profile. A method for controlling a tube current with an X-ray imaging device for acquiring at least one X-ray image of an examination region of an examination object is described. In the method, a preliminary acquisition of an examination region of an examination object is performed. The three-dimensionally modulated X-ray attenuation of the examination area is determined on the basis of the pre-acquisition. Based on the determined X-ray attenuation, an initial tube current distribution is determined. The specified tolerance range is modified in real time for subsequent tube current. The maximum allowable tube current distribution is determined without overheating the X-ray tube. An expected value of the potential patient dose is determined based on the initial tube current distribution and a maximum value of the potential patient dose is determined based on a specified tolerance range. The actual X-ray attenuation is measured in the actual X-ray image acquisition. An appropriate tube current distribution is determined based on the actual X-ray attenuation and the initially planned tube current distribution. And finally, adjusting the tube current according to the determined adjusted tube current distribution.)

1. A method for controlling tube current with an X-ray imaging apparatus (1, 1a) for acquiring at least one X-ray image of an examination volume of an examination object (3), the method comprising the steps of:

performing a pre-acquisition of the examination volume of the examination object (3);

determining a three-dimensionally modulated X-ray attenuation (W) of the examination area on the basis of the pre-acquisition_ap(z)、W_lat(z))；

Determining an initial tube current distribution (I) based on the determined X-ray attenuation_ap(z)、I_lat(z))；

Specifying a tolerance range for subsequent real-time modification of the tube current, based on the initial tube current profile (I)_ap(z)、I_lat(z)) determining a maximum allowable tube current distribution without overheating one X-ray tube (8) of said X-ray imaging apparatus (1, 1 a);

based on the initial tube current profile (I)_ap(z)、I_lat(z)) and the specified tolerance range, determining an expected value and a maximum value for a potential patient dose;

during the acquisition of the X-ray image, an actual X-ray attenuation (V) is measured_ap、V_lat)；

Based on the actual patient attenuation (V)_ap、V_lat) And initially planned tube current distribution (I)_ap(z)、I_lat(z)), determining an adjusted tube current profile (J)_ap(z)、J_lat(z))；

According to the determined adjusted tube current distribution (J)_ap(z)、J_lat(z)) to adjust the tube current.

2. The method according to claim 1, wherein the pre-acquisition comprises a tomography of the examination volume of the examination object (3).

3. The method according to claim 1, wherein the pre-acquisition comprises an optical image acquisition of the examination range of the examination object (3).

4. The method of any preceding claim, wherein the three-dimensional isThe determination of the modulated X-ray attenuation comprises determining an X-ray attenuation vector (W)_ap(z)、W_lat(z))。

5. Method according to claim 4, wherein the X-ray attenuation vector (Wt) is determined in a anteroposterior direction (ap) and a transverse direction (lat) by means of the pre-acquisition_ap(z)、W_lat(z))。

6. Method according to any of the preceding claims, wherein the initial tube current distribution (I) is corrected on the basis of the determined X-ray attenuation_ap(z)、I_lat(z)) in the front-back direction (ap) and in the lateral direction (lat).

7. The method according to any of the preceding claims, wherein the actual X-ray attenuation (V)_ap、V_lat) Is carried out in a front-back direction (ap) and a lateral direction (lat).

8. The method according to any of the preceding claims, wherein a maximum allowed boost of patient dose is displayed to the operator during the X-ray image acquisition by a configurable parameter.

9. Method according to any of the preceding claims, wherein the X-ray attenuation (W) is performed by simply dividing the logarithmized attenuation data by the linear absorption coefficient (mu) of water_ap(z)、W_lat(z)) is determined.

10. The method according to any of the preceding claims, wherein said adjusting is performed based on a function (F) to adjust said initially planned tube current, said function comprising said actual patient attenuation (V)_ap、V_lat) With the X-ray attenuation vector (W)_ap(z)、W_lat(z)) and the product of the absorption coefficient of water (mu).

11. Tube current control device for acquiring at least one X-ray image of an examination area of an examination object (3), the tube current control device comprising:

a control unit (27) for controlling the operation of the motor,

for controlling an X-ray radiation source (8) for an X-ray image acquisition

And is

For acquiring X-ray raw data from an X-ray radiation detector (9);

a pre-acquisition control unit (23) for controlling a pre-acquisition of the examination volume of the examination object (3);

an X-ray attenuation estimation unit (24) for estimating a three-dimensionally modulated X-ray attenuation (W) based on the pre-acquisition_ap(z)、W_lat(z))；

A distribution defining unit (25) for defining a distribution based on the estimated X-ray attenuation (W)_ap(z)、W_lat(z)) to determine an initial tube current profile (I)_ap(z)、I_lat(z))；

A range specification unit (26) for specifying a tolerance range for a subsequent real-time modification of the tube current, wherein the tolerance range is based on the initial tube current distribution (I)_ap(z)、I_lat(z)) determining a maximum allowable tube current profile without overheating the X-ray tube;

a dose determination unit (27) for determining a dose based on the initial tube current distribution (I)_ap(z)、I_lat(z)) determining an expected value of a potential patient dose and determining a maximum value of a potential patient dose based on the specified tolerance range;

an X-ray attenuation determination unit (28) for determining an actual X-ray attenuation (V) during an X-ray image acquisition on the basis of X-ray raw data acquired therein_ap、V_lat)，

An adjustment unit (29) for determining an adjusted tube current distribution (J) based on the actual X-ray attenuation and an initially planned tube current distribution_ap(z)、J_lat(z))；

Wherein the control unit (27) is configured to adjust the tube current profile (J) in dependence on the determined adjusted tube current profile_ap(z)、J_lat(z)) to adjust the tube current of the X-ray source (8).

12. An X-ray imaging apparatus (1a) having:

an X-ray radiation source (8) having an X-ray tube;

an X-ray radiation detector (9);

a tube current control device (12a) according to claim 11.

13. A computed tomography system (1) with an X-ray imaging apparatus (1a) according to claim 12.

14. A computer program product with a computer program which can be loaded directly into a memory means of an X-ray imaging apparatus, the computer program having program segments for performing all the steps of the method as claimed in any one of claims 1 to 10 when the computer program is executed on the X-ray imaging apparatus.

15. A computer readable medium having stored thereon program segments readable and executable by an operator to perform all the steps of the method according to any one of claims 1 to 10 when the program segments are executed by the operator.

Technical Field

The invention relates to a method for controlling a tube current with an X-ray imaging device for acquiring at least one X-ray image of an examination area of an examination object. The invention also relates to a tube current control device. The invention also relates to an X-ray imaging apparatus. Furthermore, the invention relates to a computer tomography system.

Background

Imaging X-ray machines, such as C-arm X-ray apparatus instruments or computed tomography devices, are increasingly used to interpret medical problems. With X-ray radiation, patients are increasingly exposed to radiation, so that the dose is required to be used and optimized appropriately on the basis of ALARA ("as reasonably low as possible") principle at each examination. Accordingly, the aim of medical imaging is to apply as small a dose of X-ray radiation as possible to the patient to generate one or more X-ray images.

In view of this, in modern CT scanners, the tube current and thus the dose is automatically adjusted according to the attenuation characteristics of the patient under examination. For example, CARE Dose4D is such an automatic Dose control system. In order to determine the attenuation characteristics of the patient, the attenuation profile of the patient in the anteroposterior and lateral directions must be accurately known before the actual image acquisition is started.

Known automatic dose control systems are based on tomography. Tomography corresponds to a conventional two-dimensional X-ray overlay acquisition. Tomography measures the respective X-ray attenuation distribution through the patient in a particular projection direction of the X-ray radiation through the patient and represents this distribution by means of different gray values. The dose automation system takes this X-ray attenuation into account to determine a suitable tube current profile or modulation tube current. It is known to acquire tomograms of a patient in the lateral direction and in the anteroposterior direction, respectively, before the acquisition of an X-ray image and to determine the X-ray attenuation distribution of the patient in the respective direction on the basis of the gray value distribution.

The attenuation of the patient can also be estimated by means of an optical 3D camera. Such processes are described, for example, in DE 102015204449 and US 2019/0214135.

If only one tomogram is created, inaccurate results can be obtained in estimating patient attenuation if the patient is not lying in the optimal manner at the center of rotation of the scanner. Even when the patient is centered in an optimal manner, inaccuracies can result due to movement of the patient between the slice and the tomography. This may be due to, for example, tomography and different breathing states between tomography.

When using optical 3D cameras, inaccuracies are caused by the presence of clothing or coverings and by the unusual relationship between the patient's body surface and the patient's attenuation.

The inaccuracies mentioned in the estimation of patient attenuation before the actual X-ray image acquisition lead to a sub-optimal X-ray dose for the patient.

EP 1172069 a1 describes a so-called CT exposure automatic control system in which the tube current is adjusted in real time to achieve a previously predetermined image noise. However, in this method, technical limitations for the X-ray tube, such as tube overheating in case of overload or inertia of the tube current modulation, cannot be taken into account. In addition, the described method does not allow to estimate the patient's dose before performing the scan.

Disclosure of Invention

It is therefore an object of the present invention to achieve an improved accuracy in the adjustment of the X-ray tube current distribution in accordance with the individual X-ray attenuation characteristics of the patient, while at the same time running the total dose applied to the patient is further reduced.

According to the invention, this object is achieved by a method for controlling a tube current with an X-ray imaging apparatus for acquiring at least one X-ray image of an examination range of an examination object according to claim 1, and by a tube current control device according to claim 11, by an X-ray imaging apparatus according to claim 12 and by a computed tomography system according to claim 13.

In a method according to the invention for controlling a tube current with an X-ray imaging device for acquiring at least one X-ray image of an examination area of an examination object, a pre-acquisition of the examination area of the examination object is performed. The pre-acquisition is used to derive information about the X-ray attenuation that occurs at the time of the subsequent actual X-ray image acquisition. In other words, based on the pre-acquisition, the three-dimensionally modulated X-ray attenuation of the examination volume is determined. In this context, the examination area shall comprise the area of the examination object to which the X-ray radiation is subsequently applied at the time of the actual X-ray image acquisition. For example, the range may include a part of the body of the patient or may include the entire body of the patient in a general examination.

An initial tube current distribution is then calculated based on the determined X-ray attenuation. The initial tube current distribution is determined such that the previously specified signal-to-noise ratio and the image quality associated therewith are achieved. In addition, a tolerance range is specified for subsequent real-time modification of the tube current, wherein a maximum allowable tube current distribution without overheating of the X-ray tube is determined. The tube current distribution adjusted during the subsequent X-ray imaging is also not allowed to exceed the specified maximum value. In this context, a tube current profile is understood to be a current-time curve which represents a tube current or a corresponding time-dependent current intensity generated by the X-ray tube during the examination time or at least one predetermined time period of the examination.

An expected value of the potential patient dose is specified based on the initial tube current profiles iap (z), ilat (z), and a maximum value of the potential patient dose is specified based on the specified tolerance range. In the case of actual X-ray imaging, the maximum value can be taken into account in the subsequent adjustment of the tube current, for example by previously limiting the tolerance range accordingly, so that the maximum dose is not exceeded. However, it is also possible to determine the already given X-ray dose during X-ray imaging and to set a smooth and variable limitation of the tolerance range in real time based on the desired X-ray dose predicted for the remaining X-ray imaging procedure.

Then, during the actual X-ray image acquisition, the actual X-ray attenuation is regularly determined based on the measured raw data or projections. Preferably, the X-ray attenuation is calculated by simply dividing the logarithmized attenuation data by the linear absorption coefficient of water.

Then, an adjusted tube current distribution is calculated based on the actual X-ray attenuation values and the initially planned tube current distribution. Since the actual X-ray attenuation values provide information about the extent to which the previously performed estimates of the X-ray attenuation deviate from the actual values. Finally, a tube current adjustment is made based on the determined adjusted tube current profile. During X-ray imaging, the adjustments and steps for determining the actual X-ray attenuation of the examination object may be repeated or updated a number of times, thereby achieving a high quality X-ray acquisition with a minimum X-ray dose.

The tube current control device according to the invention for acquiring at least one X-ray image of an examination area of an examination object comprises a control unit for controlling an X-ray radiation source for the X-ray image acquisition and for acquiring X-ray raw data from an X-ray radiation detector. A pre-acquisition control unit for controlling the pre-acquisition of the examination range of the examination object is also part of the tube current control device according to the invention. The tube current control device according to the present invention further comprises a distribution specifying unit for determining an initial tube current distribution based on the estimated X-ray attenuation.

In order to take into account technical limitations, the tube current control device according to the invention further comprises a range specification unit for modifying the specified tolerance range for subsequent real-time modification of the tube current, wherein the maximum allowable tube current distribution is determined without overheating of the X-ray tube.

In order to protect the patient against overdosing, the tube current control device according to the invention further comprises a dose determination unit for determining a desired value of the potential patient dose based on the initial tube current distribution and for determining a maximum value of the potential patient dose based on the specified tolerance range.

An X-ray attenuation specification unit for determining the actual X-ray attenuation during X-ray image acquisition on the basis of the acquired X-ray raw data is also part of the tube current control device according to the invention.

The tube current control device according to the invention further comprises an adjustment unit for determining an adjusted tube current distribution based on the actual X-ray attenuation and the initially planned tube current distribution. The control unit is configured to adjust a tube current of the X-ray source in accordance with the determined adjusted tube current distribution. The tube current control device according to the invention also has the advantage of the method according to the invention for controlling the tube current.

An X-ray imaging apparatus according to the present invention comprises an X-ray radiation source having an X-ray tube, an X-ray radiation detector, and a tube current control device according to the present invention. The X-ray imaging apparatus according to the present invention also has the advantages of the tube current control device according to the present invention.

The X-ray imaging apparatus may be part of an X-ray machine designed for acquiring a plurality of X-ray projections from different projection angles, for example a computed tomography apparatus with an annular rotating frame or a C-arm X-ray machine. The acquisition can take place during a particularly continuous rotational movement of the acquisition unit with an X-ray radiation source and an X-ray radiation detector cooperating with the X-ray radiation source. The X-ray radiation source may in particular be an X-ray tube with a rotating anode. X-ray radiation detectors for computed tomography apparatuses are, for example, multi-line detectors. The X-ray detector used for the C-arm X-ray apparatus is, for example, a flat panel detector. For X-ray detectors, the design can be made both in terms of energy resolution and also in terms of counting.

Most of the basic components of the X-ray tube control device according to the invention may be constructed in the form of software components. This relates in particular to a control unit, a pre-acquisition control unit for controlling a pre-acquisition of an examination range of an examination object, an X-ray attenuation estimation unit for estimating a three-dimensionally modulated X-ray attenuation on the basis of the pre-acquisition, a distribution specification unit, a range specification unit, a dose specification unit, an X-ray attenuation specification unit and an adjustment unit.

In principle, however, these components can also be implemented partially in software-assisted hardware, for example FPGAs or the like, in particular when particularly fast calculations are involved. Likewise, the required interfaces can be designed as software interfaces, for example, when only data reception from other software components is involved. The interfaces can also be designed as hardware-based interfaces, which are controlled by suitable software.

The main advantage of the software implementation is that even the control devices of the X-ray imaging apparatus (for example a computed tomography system) that have been used up to now can be updated in a simple manner by software upgrades in order to operate in the manner according to the invention. In this connection, the object is achieved by a corresponding computer program product with a computer program which can be loaded directly into a memory device of an X-ray imaging apparatus or a memory device of a control apparatus and which comprises program segments for carrying out all the steps of the method according to the invention when the computer program is executed in the control apparatus of the X-ray imaging apparatus or in the control apparatus of a computed tomography system.

Such a computer program product may, if applicable, comprise, in addition to the computer program, additional components (e.g. documents) and/or additional components, as well as hardware components, such as hardware locks (dongles, etc.) for the use of software.

By means of a software implementation, the method can be reproducibly implemented on different computers and is less sensitive to errors.

For the transfer of the memory device of the image data generating device or of the control device of the computer tomography system and/or for the storage on the image data generating device or on the control device of the computer tomography system, a computer-readable medium can be used, for example a memory stick, a hard disk or a generally transferable or fixedly mounted data carrier on which program segments of a computer program are stored that are readable and executable by an arithmetic unit of the X-ray imaging device. The arithmetic unit can have, for example, one or more microprocessors or the like which work together.

The dependent claims and the following description each contain particularly advantageous embodiments and refinements of the invention. In this case, the claims of one category of claims can also be modified in a manner analogous to the dependent claims of another category of claims. Furthermore, various features of the different exemplary embodiments and the claims may also be combined into new embodiments within the scope of the invention.

According to one embodiment of the method according to the invention, the pre-acquisition includes a tomography of the examination region of the examination object. By the method according to the invention, the accuracy of an automatic dose control device based on tomography in the case of a patient lying not centrally can be improved.

According to one aspect of the method according to the invention, the pre-acquisition comprises an optical image acquisition of an examination area of the examination object. If the pre-acquisition is performed without tomography based on a relatively inaccurate estimation of the X-ray attenuation by means of data of an optical sensor, e.g. a camera, the accuracy of the dose automation device will be significantly improved by the method according to the invention. For acquiring images of the examination object, the X-ray imaging device preferably comprises an optical sensor at least in the area of the body to be imaged.

In a particularly practical variant of the method according to the invention, the determination of the X-ray attenuation of the three-dimensional modulation comprises the determination of an X-ray attenuation vector.

Preferably, the X-ray attenuation vectors are determined in the anteroposterior and lateral directions by means of a pre-acquisition. If the examination object is a patient, the patient can be considered to have the smallest measure in the front-to-back direction in the transverse plane and the largest measure in the lateral direction. Therefore, the smallest X-ray attenuation can also be expected for X-ray projections in the anteroposterior direction, and the strongest X-ray attenuation is presumed in the lateral direction. Thus, the two estimates of the X-ray attenuation in the anteroposterior direction and in the lateral direction can be considered as a minimum and a maximum, between which the X-ray attenuation varies in a continuous transition. Advantageously, the evaluation of the pre-acquisition can be limited to a few directions, thereby reducing the computational cost.

Thus, based on the X-ray attenuation determined in the front-to-back direction and the lateral direction, then an initial tube current distribution determination may be made.

It is particularly preferred that the measurement of the actual X-ray attenuation is also carried out in the front-rear direction and in the lateral direction, since, as mentioned above, boundary values of the value intervals of the possible X-ray attenuation values can be expected in both directions. Advantageously, the calculation of the temporal X-ray tube current distribution can be simplified due to the selection of only two directions of the particularly effective force, whereby the real-time function of the adaptation of the X-ray current distribution can also be realized more simply.

In order to maintain a maximum patient dose, the maximum allowed boost of patient dose is preferably displayed to the operator by configurable parameters during X-ray image acquisition. In this variant, the variation parameters and their variation ranges adapted to the tube current are specified in advance.

Alternatively, the value range of the variation range may also be adjusted in real time during the X-ray acquisition depending on the already applied X-ray dose and the X-ray dose expected in the further X-ray imaging procedure. In this variant, maximum image quality can be achieved while maintaining the permissible or predetermined X-ray dose

Preferably, the adjustment of the tube current is performed based on a function for adjusting the initially planned tube current, which function comprises an exponential function of the actual patient attenuation with the product of the difference of the X-ray attenuation vectors determined based on the pre-acquisition and the absorption coefficient of water.

In the case of a CT system, the adjustment of the tube current is preferably performed with a delay of 180 °. That is, the correction of the tube current at a particular position z of the CT helical scan is based on the measurement of the patient attenuation in the previous gantry half cycle. Thus, maximum timeliness of the data basis for tube current regulation is achieved.

Drawings

The invention will be described and explained in more detail hereinafter with the aid of embodiments shown in the drawings.

In which is shown:

FIG. 1 is a schematic view of an X-ray imaging apparatus having a tube control apparatus according to one embodiment of the present invention;

FIG. 2 is a computed tomography system according to one embodiment of the present invention;

fig. 3 is a flow chart of a method for controlling a tube current for acquiring an X-ray image of an examination region of an examination object according to an embodiment of the invention;

FIG. 4 is a tomographic scan of a thoracic model at different distances from an image acquisition unit;

FIG. 5 is three different initial tube current profiles for the three different tomography scans shown in FIG. 4;

fig. 6 is a tube current distribution map adjusted for the initial tube current distribution shown in fig. 5.

Detailed Description

In fig. 1, an X-ray imaging apparatus 1a having a tube control device 12a is schematically shown according to one embodiment of the present invention. The X-ray imaging apparatus 1a includes an X-ray source 8 and an X-ray detector 9 in addition to the mentioned tube control device 12 a.

The X-ray tube control device 12a comprises a pre-acquisition control unit 23 for controlling pre-acquisition of an examination range of an examination object. That is, the control unit 27 is actuated by means of the pre-acquisition control unit 23, with which the actuation of the X-ray radiation source 8 for the X-ray image acquisition and for the pre-acquisition is carried out. The control unit 27 is further configured for acquiring X-ray raw data of the X-ray radiation detector 9. Also as part of the X-ray tube control device 12a is an X-ray attenuation estimation unit 24 configured to determine an X-ray attenuation vector W in the front-rear direction and the lateral direction based on the tomogram_ap(z)、W_lat(z). Determined X-ray attenuation vector W_ap(z)、W_lat(z) is then used by the distribution specification unit 25 to calculate the initial tube current distribution I_ap(z)、I_lat(z)。

The pipe control device 20a also has a range specifying unit 26. The range specification unit is used for modifying the specified tolerance range for subsequent real-time modification of the tube current. The tolerance range specifies a maximum allowable tube current or a maximum allowable tube current distribution in a situation in which the X-ray tube has not yet been overheated.

Based on the initial tube current distribution I_ap(z)、I_lat(z), it can now be specified in advance that the degree of the tube current is allowed to be adjusted (if applicable) during the X-ray image acquisition.

Also as part of the X-ray tube control device 12a is a dose determination unit 27 configured for, based on the initial tube current profile I_ap(z)、I_lat(z) and the specified tolerance ranges, determining expected and maximum values for potential patient doses. In other words, it is possible to base the initial tube current profile I_ap(z)、I_lat(z) An expected value of the X-ray dose for the patient is calculated. The maximum value is then obtained with the aid of the specified tolerance range. If the maximum value is higher than the predetermined X-ray dose, the tolerance range may be limited accordingly so as not to exceed the predetermined X-ray dose value.

The X-ray tube control apparatus 12a further comprises an X-ray attenuation determination unit 28 configured for determining, during X-ray image acquisition, an actual X-ray attenuation V in a back-and-forth direction and in a lateral direction based on the determined X-ray raw data_ap、V_lat. In order to determine the actual X-ray attenuation mentioned in the anteroposterior and lateral directions, the projections at 3, 9 and 6, 12 tube positions were evaluated. In fig. 2, the mentioned positions are denoted by "3 h", "9 h", "6 h", "12 h".

The actual X-ray attenuation is determined by simply dividing the projected logarithmized attenuation values by the linear absorption coefficient of water. The adjustment unit 29 is likewise part of the X-ray tube control device 12a, which determines an adjusted tube current distribution in the front-back direction and the lateral direction on the basis of the actual X-ray attenuation and the initial tube current distribution in the front-back direction and the lateral direction.

The adjusted tube current distribution is used by the control unit 27 to adjust the tube current of the X-ray source 8 in accordance with the determined adjusted tube current distribution.

Fig. 2 shows an X-ray imaging apparatus 1 using an example of an X-ray computed tomography scanner. The computer tomography scanner shown here has an acquisition unit 17 which comprises a radiation source 8 in the form of an X-ray source and a radiation detector 9 in the form of an X-ray detector. The acquisition unit 17 rotates about the system axis 5 during acquisition of the X-ray projections, and the X-ray source 8 emits a beam 2 in the form of an X-ray beam during acquisition. The X-ray source 8 is an X-ray tube. The X-ray detector is a multi-line detector.

The patient 3 lies on a patient bed 6 while the projections are acquired. The patient bed 6 is connected with the bed base 4 such that the bed base supports the patient bed 6 with the patient 3. The patient bed 6 is designed to move the patient 3 through the opening 10 of the acquisition unit 17 in the acquisition direction. The acquisition direction is usually given by the system axis 5 pointing in the z-direction. The acquisition unit 17 rotates around the z-axis when acquiring the X-ray projections. In this example, the body axis of the patient is the same as the system axis 5. Both axes lie on the z-axis of a three-dimensional cartesian coordinate system. In the case of a helical acquisition, the patient bed 6 is continuously moved through the opening 10 while the acquisition unit 17 rotates around the patient 3 and acquires X-ray projections. Thus, the X-ray beam describes a spiral on the surface of the patient 3.

The X-ray imaging apparatus 1 has a computer 12 which is connected to a display unit 11, for example for graphically displaying the X-ray acquisition, and to the input unit 7. The display unit 11 may be, for example, an LCD screen, a plasma screen, or an OLED screen. It may also relate to a touch screen which is also designed as an input unit 7. Such a touch screen may be integrated in the imaging instrument or designed as part of a portable instrument. The input unit 7 is for example a keyboard, a mouse, a so-called "touch screen" or even a microphone for speech input. The input unit 7 may also be configured to recognize the user's motion and convert it into a corresponding command. The user can modify, for example, the reference data set selected for the preparation of the imaging by means of the input unit 7.

The computer 12 is connected to a rotatable acquisition unit 17 for data exchange. On the one hand, control signals for the acquisition of the X-ray image are transmitted from the computer via the connection 14 to the acquisition unit 17, and on the other hand projection data acquired for the image reconstruction are transmitted via the connection 14 to the computer 12. The connection 14 is realized in a known manner, either wired or wireless.

The computer 12 has an arithmetic unit 16. The operator 16 is designed as an image or image data processing unit. The operator is configured to perform all data processing steps related to the method according to the invention. The arithmetic unit 16 can cooperate with the computer-readable data carrier 13, in particular in order to carry out the method according to the invention by means of a computer program having a program code. In addition, the computer program may be stored in a retrievable manner on a machine-readable carrier. The machine-readable carrier may be, inter alia, a CD, DVD, blu-ray disc, memory stick or hard disk. The arithmetic unit 16 may be designed in hardware or in software. For example, the operator 16 is designed as an FPGA ("field programmable gate array" acronym), or includes an arithmetic logic unit.

In the embodiment shown here, at least one computer program is stored in a memory of the computer 12, which computer program, when being executed on the computer 12, carries out all the steps of a method for controlling a tube current with an X-ray imaging device for acquiring at least one X-ray image of an examination area of an examination object. The computer program for performing the steps of the method according to the invention comprises program code. In addition, the computer program is designed as an executable file, and/or is stored on a computing system different from the computer 12. For example, the X-ray imaging apparatus 1 may be designed such that the computer 12 loads a computer program for executing the method according to the invention into its internal working memory via an intranet or the internet.

In fig. 3, a flow chart of a method for controlling a tube current for acquiring at least one X-ray image of an examination area of an examination object with an X-ray imaging apparatus according to an embodiment of the invention is shown. In step 3. i, a preliminary acquisition of the examination area of the examination subject (in this example a patient) is first carried out. The pre-acquisition can be performed, for example, in the form of a tomography scan. In tomography, X-ray image acquisitions are established from a plurality of directions by means of an acquisition unit of an X-ray imaging device. Alternatively, it is also possible to acquire images of the patient from a plurality of directions by means of an optical sensor (e.g. a camera) in order to determine the size of the patient in three dimensions.

In step 3 II, an X-ray attenuation vector W is determined in the anteroposterior and transverse directions on the basis of the generated pre-acquisitions_ap(z)、W_lat(z). It can be considered that, for example, the patient has the smallest measure in the anteroposterior direction and the largest measure in the lateral direction. In a subsequent X-ray image acquisition, maximum X-ray attenuation values are expected in the lateral direction and minimum X-ray attenuation values are expected in the anteroposterior direction. In the collectionThe X-ray attenuation values fluctuate between these two values as expected when the unit is rotated a quarter turn.

In step 3. III, based on the determined X-ray attenuation vector W_ap(z)、W_lat(z) calculating an initial tube current distribution I in the anteroposterior direction and the lateral direction_ap(z)、I_lat(z)。

Additionally, in steps 3. IV, the tolerance ranges for subsequent real-time modifications of the tube current are determined. The tolerance range must be selected such that the maximum allowable tube current is not exceeded, since otherwise overheating of the X-ray tube would occur. The tolerance range is based on the initial tube current distribution I_ap(z)、I_lat(z) determined.

Furthermore, in step 3. V, based on the initial tube current profile I_ap(z)、I_lat(z) and the specified tolerance ranges to calculate expected and maximum values for potential patient doses. In other words, the desired value of the X-ray dose is derived from an initial tube current profile, which of course represents a time-dependent quantity. In contrast, the maximum value of the potential patient dose is derived from the allowable tube current. In order to prevent the X-ray dose from now exceeding the healthily allowable dose, in step 3. vi the tolerance range is limited such that the healthily allowable X-ray dose is not expected to be exceeded. As will be explained further below, the tolerance range can additionally be reduced in real time during the X-ray imaging acquisition on the basis of the actual tube current distribution in order to avoid exceeding the healthily permissible X-ray dose anyway when the actual tube current distribution deviates from the initial tube current distribution.

In step 3 vii, the actual X-ray imaging of the examination area of the patient is started. In step 3. VIII, the actual X-ray attenuation V is determined during the X-ray image acquisition based on the X-ray attenuation data acquired in real time in the anteroposterior and lateral directions_ap、V_lat。

In step 3. IX, an adjusted tube current distribution is calculated based on the actual patient attenuation and the initially planned tube current distribution. Using the absorption coefficient mu of water and an appropriately selected parameter b (0)<b<1) Calculating adjusted tube currents J in the front-rear direction and the lateral direction_ap(z)、J_lat(z), giving:

and

in step 3. X, the tube current is adjusted according to the determined adjusted tube current profile.

In fig. 4, a comparison 40 of three different tomograms 40a, 40b, 40c in the anterior-posterior direction of one thorax model is shown. The individual tomograms 40a, 40b, 40c differ from one another in the positioning of the thorax model. In the left picture 40a, the thorax model is located in a centered position with respect to the circular gantry or acquisition unit (see fig. 2). In the intermediate picture 40b, the thorax model is, in contrast, located deeper, i.e. at a greater distance from the X-ray source. Thus, the thorax model has a smaller size. The opposite effect can be seen in the right picture 40 c. In picture 40c, the thorax model is positioned closer to the X-ray source, thereby obtaining an enlarged thorax model map.

For the different tomograms 40a, 40b, 40c, different initial tube currents or tube current distributions, which are shown in fig. 5, are obtained as a basis for planning for CT-X-ray imaging. In fig. 5, a graph 50 is shown, the graph 50 showing the distribution of three different tube currents. The current or its current intensity I over time t is plotted in units mA. Tube current distribution I₁From the left tomogram 40 a. The middle fault map 40b corresponds to the tube current distribution I₂. It can be seen that the tube current I2 associated with the middle tomogram 40b is significantly lower in magnitude than the tube current I associated with the left tomogram₁The amplitude of (c). The right tomogram 40c and the tube current I with the greatest amplitude₃And (4) associating. Now if the tube current I determined by acquiring the middle tomogram 40b is used₂To perform X-rayLine image acquisition, only a small X-ray dose will be applied to the patient, but a reduced image quality may result. At the tube current I determined by acquiring the right tomogram 40c₃Then an excessively high X-ray dose may be applied to the patient.

In fig. 6, a modified tube current profile J is shown₁、J₂、J₃Fig. 60. In this case, the adjusted tube current profile J₁With initial tube current distribution I₁Correlated, adjusted tube current profile J₂With initial tube current distribution I₂Correlated and adjusted tube current profile J₃With initial tube current distribution I₃And (4) associating. It can be seen that although for the calculation of the initial tube current distribution I₁、I₂、I₃The middle and right tomograms 40b, 40c shown in fig. 5 that are used are erroneous, but the amplitudes of the individual tube currents are very similar to one another. By using the method according to the invention, dose differences due to erroneous pre-acquisitions can be corrected automatically.

Finally, it should be reiterated that the above-described method and apparatus are merely preferred embodiments of the present invention and that those skilled in the art may make modifications to the invention without departing from the scope of the invention as defined by the appended claims. The method and the X-ray imaging apparatus are therefore described primarily by means of a system for acquiring medical image data. However, the invention is not limited to use in the medical field, but may in principle also be applied to the acquisition of images for other purposes. For the sake of completeness, it is pointed out that the use of the indefinite article "a" or "an" does not exclude the possibility that more than one of the relevant features is present. Likewise, the term "unit" does not exclude the possibility that the unit is made up of a plurality of components, which may also be spatially distributed, if possible.