阅读说明：本技术 用于检测受试者的运动的方法和设备 (Method and apparatus for detecting motion of a subject ) 是由 S.比伯 于 2021-04-16 设计创作，主要内容包括：本发明涉及用于检测在磁共振成像设备内部的受试者(30)的运动的方法和系统,该方法包括以下步骤：(a)提供至少一个输入信号(10、12)；(b)使用至少一个输入信号(10、12)来产生较高导频音信号(16),其具有的发射频率是磁共振成像设备的拉莫尔频率的至少两倍高；(c)使用发射天线(26)朝向受试者(30)发射较高导频音信号(16)；(d)使用接收天线(28)接收发射的较高导频音信号(18)；(e)使用至少一个输入信号(12)将发射的较高导频音信号(18)的频率转换为等于或低于输入信号(12)的频率的中频,从而产生发射的导频音信号；以及(f)将发射的导频音信号(20)转发到分析系统(32),以检测由受试者(30)的运动引起的发射的较高导频音信号(20)的变化。(The invention relates to a method and a system for detecting motion of a subject (30) inside a magnetic resonance imaging device, the method comprising the steps of: (a) providing at least one input signal (10, 12); (b) using at least one input signal (10, 12) to generate an upper pilot tone signal (16) having a transmit frequency at least twice as high as the larmor frequency of the magnetic resonance imaging apparatus; (c) transmitting an upper pilot tone signal (16) towards a subject (30) using a transmit antenna (26); (d) receiving the transmitted higher pilot tone signal (18) using a receive antenna (28); (e) converting the frequency of the transmitted higher pilot tone signal (18) to an intermediate frequency equal to or lower than the frequency of the input signal (12) using at least one input signal (12), thereby generating a transmitted pilot tone signal; and (f) forwarding the transmitted pilot tone signal (20) to an analysis system (32) to detect changes in the transmitted higher pilot tone signal (20) caused by motion of the subject (30).)

1. A method for detecting motion of a subject (30), the subject (30) being located inside a magnetic resonance imaging apparatus, the method comprising the steps of:

a) providing at least one input signal (10, 12);

b) using the at least one input signal (10, 12) to generate an upper pilot tone signal (16) having a transmit frequency at least twice as high as the larmor frequency of the magnetic resonance imaging apparatus;

c) transmitting an upper pilot tone signal (16) towards a subject (30) using a transmit antenna (26), the transmitted upper pilot tone signal (18) interacting with the subject (30);

d) receiving the transmitted higher pilot tone signal (18) using a receive antenna (28);

e) converting the frequency of the transmitted higher pilot tone signal (18) to an intermediate frequency equal to or lower than the frequency of the or one of the at least one input signal (12) using the at least one input signal (12), thereby generating a transmitted pilot tone signal; and

f) the transmitted pilot tone signal (20) is forwarded to an analysis system (32) to detect changes in the transmitted higher pilot tone signal (20) caused by motion of the subject (30).

2. The method of claim 1, wherein the first and second light sources are selected from the group consisting of,

wherein the at least one input signal comprises at least one local oscillator signal (12) and a pilot tone signal (10), and the at least one local oscillator signal (12) is used in step (b) to convert the frequency of the pilot tone signal (10) to a transmission frequency, thereby generating an upper pilot tone signal (16).

3. The method according to any of the preceding claims, wherein the frequency of the higher pilot tone signal (16) is a combination of the frequencies of the at least one input signal (10, 12), in particular of the at least one local oscillator signal (12) and the pilot tone signal (10).

4. A method according to claim 2 or 3, wherein the step of converting the frequency of the pilot tone signal (10) and/or the step of converting the transmitted higher pilot tone signal (18) comprises mixing the respective signal with the at least one local oscillator signal (12) or a frequency multiplied local oscillator signal having a carrier frequency.

5. The method of any one of claims 2 to 4,

wherein step b comprises multiplying or dividing the frequency of the at least one local oscillator signal (12) to obtain a multiplied or divided local oscillator signal (12), and

the multiplied/divided local oscillator signal is mixed with a pilot tone signal (10) to obtain an upper pilot tone signal (16).

6. The method of any one of claims 2 to 5,

wherein the step of converting the transmitted higher pilot tone signal (18) to a lower intermediate frequency comprises the steps of:

down-mixing the transmitted higher pilot tone signal (18) with the at least one local oscillator signal (12) or a multiplied local oscillator signal to the frequency of the pilot tone signal to obtain a transmitted pilot tone signal (20); and

optionally, the transmitted pilot tone signal (20) is mixed down to an intermediate frequency that is lower than the frequency of the pilot tone signal (10).

7. The method of any one of the preceding claims, wherein the pilot tone signal (10) is a modulation signal having a frequency close to or equal to the larmor frequency of the magnetic resonance imaging device.

8. The method according to any one of the preceding claims, wherein the transmit antenna (26) and the receive antenna (28) are provided at a local RF coil (40), the local RF coil (40) being placed in proximity of a body part of the subject (30).

9. The method according to any of the preceding claims, wherein the transmitted higher pilot tone signal (18) has a transmission frequency higher than 300 MHz.

10. The method of any preceding claim, wherein the method is performed simultaneously using a first transmit antenna (26) and a corresponding first receive antenna (28) transmitting at a first transmit frequency and a second transmit antenna (26') and a corresponding second receive antenna (28') transmitting at a second transmit frequency.

11. The method of claim 10, wherein the first and second transmit frequencies differ by more than 100 MHz.

12. The method according to any of the preceding claims, wherein the higher pilot tone signal (16) is transmitted in the form of a Frequency Modulated Continuous Wave (FMCW).

13. A system for detecting motion of a subject (30), the subject (30) being located inside a magnetic resonance imaging apparatus, the system comprising:

-an interface adapted to receive at least one input signal (10, 12),

-a multiplier and/or divider (34) and/or first mixer (22) configured to use the at least one input signal (10, 12) to generate an upper pilot tone signal (16) having a transmit frequency at least twice as high as the Larmor frequency of the magnetic resonance imaging device,

-a transmit antenna (26), the transmit antenna (26) being adapted to transmit a higher pilot tone signal (16),

-a receiving antenna (28), the receiving antenna (28) being adapted to receive the transmitted higher pilot tone signal (18),

-a second mixer (24) configured to convert the frequency of the transmitted higher pilot tone signal (18) into an intermediate frequency equal to or lower than the frequency of the or one of the at least one input signal (10, 12) using the at least one input signal (12), and

-an interface adapted to forward the transmitted pilot tone signal (20).

14. The system of claim 13, wherein the system is part of a local RF coil (40), or wherein the system is adapted as part of a local RF coil (40) or as an additional part attached to the outside of a local RF coil (40).

15. A local RF coil (40) for a magnetic resonance imaging apparatus, the local RF coil (40) being adapted for detecting a motion of a portion of a body of a subject (30) located in its vicinity, the local RF coil (40) comprising:

-one or more coil elements adapted to transmit magnetic resonance signals;

-a coil connector adapted to receive at least one input signal, in particular a local oscillator signal (12) and a pilot tone signal (10); and

-at least one system for detecting motion of a subject (30) according to any one of claims 13 to 14.

Technical Field

The invention relates to a method for detecting motion of a subject, a system for detecting motion of a subject and a local RF coil for a magnetic resonance imaging device adapted for detecting motion.

Background

In Magnetic Resonance Imaging (MRI), it is necessary to avoid or at least detect motion of the imaged subject or patient throughout the image acquisition time. Motion of the subject may lead to artifacts, which may increase the difficulty of image interpretation, lead to incorrect medical findings and/or may require repetition of the image acquisition process. Limbs are sometimes fixed within local RF coils to inhibit motion, but for the head such fixation is less acceptable for most patients. For certain imaging procedures, it is important to detect motion caused by heartbeat or respiration, e.g. triggering image acquisition accordingly.

Prior art methods have attempted to obtain information about motion by sensing Radio Frequency (RF) signals that are close to but outside the band of RF signals used for MR imaging (i.e., the Larmor frequency). These RF signals are referred to as Pilot Tone (PT) signals and the Method is first disclosed in Peter Speier et al, "PT-Nav: A Novel Respiratory Navigation Method for Continuous Acquisition of Pilot Tone Modulation in the MR receiver", ESMRMB,129:97-98,2015.doi:10.1007/s 10334-015-0487-2. The frequency of the pilot tone signal is outside the frequency band of the MRI system to avoid mutual interference, however it is close enough that it can be transmitted and processed by the RF coil and other devices used to transmit and process the magnetic resonance signals. The method using the pilot tone has been focused on applications in respiration and heartbeat. Yet another problem is the movement of other body parts of the subject, such as head movement or limb movement, including the knees, hands or feet.

In the pilot tone method, the frequency of the pilot tone signal must be close to the frequency of the MR signal because the signal must be received using the existing narrowband local RF coil. Furthermore, existing signal processing systems may not be suitable for operating also at significantly higher frequencies. This results in a frequency of less than 130MHz at typical magnetic fields of 3 tesla or less. In this frequency range, the interaction with tissue (especially human tissue) is very weak. Most of the effects of the pilot tone approach are caused by variations in eddy currents in the tissue. This effect is weak because tissue has low conductivity at frequencies of about 100 MHz. Furthermore, due to the long wavelength, there is only a near field effect, with little standing wave or reflection effect.

Another method of detecting motion during MR acquisition is to attach optical markers to the subject, which are tracked by a camera. For example, so-called "motion sensors" include intra-bore real-time patient viewing systems that allow for close patient monitoring and prospective motion correction for nervous system MRI examinations. However, this method requires the installation of additional hardware (camera), as well as the additional step of attaching the marker to the subject. Moreover, it is not always possible to allow the camera to view the subject unimpeded. Although this solution may lead to very accurate results, a method that requires less effort and expense is desired.

Disclosure of Invention

It is therefore an object of the present invention to provide a method and related system that enables efficient and accurate detection of motion of a subject or patient being imaged in a magnetic resonance imaging apparatus.

According to a first aspect, the invention provides a method for detecting motion of a subject, the subject being located inside a Magnetic Resonance Imaging (MRI) device. The method comprises the following steps:

(a) at least one of the input signals is provided,

(b) generating an upper pilot tone signal using at least one input signal, having a transmit frequency at least twice as high as a Larmor frequency of the magnetic resonance imaging device,

(c) transmitting an upper pilot tone signal toward the subject using a transmit antenna, the transmitted upper pilot tone signal interacting with the subject,

(d) the transmitted higher pilot tone signal is received using a receive antenna,

(e) using the at least one input signal to convert the frequency of the transmitted higher pilot tone signal to an intermediate frequency equal to or lower than the frequency of the at least one input signal or at least one of them, thereby generating a transmitted pilot tone signal, an

(f) The transmitted pilot tone signal is forwarded to an analysis system to detect changes in the transmitted higher pilot tone signal caused by the subject's motion.

The subject may be a human being, in particular a patient to be imaged in a magnetic resonance apparatus, or an animal. A subject is located inside an MRI device, meaning that at least a part of its body is located within the sensitive region of such an MRI device, e.g. within the bore of the main magnet. In some embodiments, the body part to be imaged is also located within the sensitive area of the local RF coil, in particular the RF coil to be placed close to the body part from which the image is to be acquired. The body part may be the head, a part of a limb (such as a knee, a leg or a part thereof, a foot, an arm, a hand) etc., or it may be the chest and/or abdomen, or it may be an organ such as the heart, the lung system, the liver, the kidney etc. The local RF coils may be of any type, which may be head coils, knee coils or flexible array coils for abdominal or thoracic imaging. However, the method can also be used without a local coil, i.e. the MR signals are acquired by a body coil fixedly integrated in the MRI device.

The MRI apparatus is for example a commercially available medical MRI apparatus and the method of the present invention may advantageously be used with any existing apparatus. For example, an MRI device may use a main magnetic field strength of 0.5-3 Tesla, which corresponds to a Larmor frequency between about 20MHz and 130 MHz. Typically, the larmor frequency will be between about 40 to 90MHz, corresponding to about 1 to 2 tesla. In some embodiments, the MRI device may operate at different larmor frequencies because it may acquire signals from various chemical elements, or because the main magnetic field may change. In most embodiments, larmor frequency refers to the frequency of the MRI signal used for MRI acquisition (imaging or spectroscopy) of a subject inside the MRI apparatus, which is typically the larmor frequency of hydrogen.

The method allows detecting motion of a subject, in particular a body part located inside the sensitive region of an MRI device, in particular a local RF coil. The detected motion may be natural and unavoidable motion, e.g. caused by breathing or heartbeat, but may also be voluntary and involuntary motion of the subject or of a body part thereof, such as the head or limbs, caused by anxiety or intolerance. In most embodiments, the method of the present invention allows for detecting whether motion has occurred or is sufficient to interfere with MRI measurements. In most embodiments, this method does not allow tracking of motion, i.e. detecting how much of such motion has occurred and in which direction. The main object of the invention is to detect whether a movement has occurred and when such a movement is detected further measures can be taken, for example the MRI acquisition can be repeated or interrupted and MRI navigator images can be acquired to correct other measurements.

In a preferred embodiment, the method of the invention is performed while performing a Magnetic Resonance (MR) measurement of the subject using an MRI device, such as spectroscopic MR imaging.

The at least one input signal may be any periodically oscillating electronic signal, such as a sine wave or a square wave, or it may be based on such a signal, such as a modulation signal. It may be pulsed or continuous. The input signal may be generated by a frequency generator or local oscillator, which may be, for example, a crystal oscillator (optionally stabilized by a Phase Locked Loop (PLL)), an oven controlled crystal oscillator (OXCO), or a Direct Digital Synthesizer (DDS). Such an electronic oscillator is preferably placed outside the sensitive region of the MRI system, in particular outside the bore of the main MR magnet, for example in the space surrounding the main MR magnet and the RF coil outside the faraday cage.

Preferably, the invention makes use of at least one input signal, e.g. a Local Oscillator (LO) signal, provided to a local RF coil in currently available MR architectures, e.g. an interface providing the LO signal to an MR system to which the local RF coil is connected (e.g. using a connector) during MR acquisition. The frequency conversion of steps b) and e) is preferably performed on the local RF coil, or within a separate transmitting and receiving device that can be attached to any local RF coil.

Method steps b) and e) are preferably carried out using analog signals, since high transmission frequencies are easier to produce in analog than in digital. However, in most embodiments, the output signal (the transmitted pilot tone signal) will eventually be digitized, in particular by sampling at a predetermined sampling frequency, which may be between 1 and 30MHz, preferably between 5 and 20MHz, for example 8 to 12 MHz.

The frequency of the at least one input signal is preferably in the range of 5MHz to 1000MHz, more preferably in the range of 10MHz to 500MHz, most preferably in the range of 50MHz to 150 MHz. According to an embodiment, the at least one input signal comprises at least one local oscillator signal. In a preferred embodiment, there are at least two different input signals, in particular at least two input signals with different frequencies. By mixing different signals having different frequencies, signals having various frequencies can be generated. In an embodiment, one input signal is an information-bearing (e.g., modulated) signal and at least one other input signal is a local oscillator signal providing a frequency. For example, as described below, the input signal may include a local oscillator signal and a pilot tone signal, but the invention may also be performed with a single input signal.

The input signal is used to generate a higher pilot tone signal having a transmission frequency preferably higher than the frequency of the input signal, e.g. it may be a multiple of the frequency of the input signal. In the case of multiple input signals, the frequency of the higher pilot tone signal may be a combination of the frequencies of the input signals, and it may be generated by frequency multiplication and/or frequency division and/or mixing. Preferably, at least some or all of the frequency doubling, division and/or mixing steps are followed by a band-pass filtering step in order to pass signal components in the desired frequency range, i.e. the frequency range to be obtained by the frequency doubling and/or division or mixing step. In a preferred embodiment, frequency multiplication requires the generation of harmonics having a frequency n times the input frequency, where n is an integer (n ═ 1, 2, 3 …). The frequency division preferably divides the frequency by an integer, preferably by an exponent of 2, i.e. 2 to the power m, where m is an integer. Thus, for example, the frequency of the input signal may be multiplied by a factor n, where n is 1, 1/2, 1/4, 1/8, 1/16 …. The frequency of the higher pilot tone signal is referred to as the transmit frequency because it is the frequency at which the signal radiates towards the subject or patient, and therefore the higher frequency radio waves interact with the subject.

The transmit frequency is at least twice as high as the larmor frequency at which the MR measurement is made. Thus, the method of the present invention has the advantage over the pilot tone method of using higher frequencies, thereby significantly improving the interaction with body tissue. At such higher frequencies, the signal variations caused by patient motion will be an order of magnitude higher than can be observed in current pilot tone approaches. The motion of the subject may, for example, cause a change in eddy currents, which in turn may alter the transmitted higher pilot tone signal, e.g., may cause a change in signal amplitude or phase. Advantageously, the higher frequency of the transmitted higher pilot tone signal results in stronger interaction with tissue and therefore more significant changes in the signal up to about an order of magnitude, relative to prior art pilot tone signals. This is due to the increased conductivity of the tissue at higher frequencies, resulting in stronger interactions. Therefore, the motion can be detected more easily and more accurately.

Preferably, the emission frequency is significantly higher than the larmor frequency, for example about 2-128 times higher, preferably 4-64 times higher, more preferably 8-32 times higher. For example, the transmission frequency may be higher than 300MHz, for example between 300MHz and 20GHz, more preferably between 400MHz and 10GHz, still more preferably between 600MHz and 6 GHz. In a preferred embodiment, the frequency of the transmitted higher pilot tone signal is within the ISM radio band. This may simplify or eliminate the need for licensing of the respective system applying the method. It may also help to avoid interference with other signals or other medical equipment.

The higher pilot tone signal is then transmitted towards the subject, thereby preferably converting the electronic signal into a radio signal, which is radiated towards the subject via the transmitting antenna. Thus, the transmitted higher pilot tone signal may be considered a radio-based signal. Interacting with the subject refers to the emitted higher pilot tone signal interacting, for example, with the tissue of the subject, for example, by eddy currents in the tissue of the subject. The radiated (transmitted) higher pilot tone signal is then received by the receive antenna, and in particular the transmitted signal.

The transmit antenna and the receive antenna are adapted for use in the desired frequency range, i.e. in the frequency range of the transmitted pilot tone signal. Advantageously, these antennas are capable of transmitting and receiving higher frequencies than previously available in MRI systems. In other words, in most embodiments, the transmit and receive antennas are not RF coils for transmitting MR signals, but are separate antennas. For example, such transmit and receive antennas may be attached to local RF coils, in particular head coils or local RF coils for knees, vertebrae, wrists or breasts. It is also contemplated to connect the antenna to a flexible anterior coil for imaging the abdominal or cardiovascular system. The transmit and receive antennas may also be disposed at other locations within or near the sensitive area of the MRI device, for example they may be attached to the inside of the magnet bore. In order to be sensitive to movements of the body part currently being examined, the antenna should be less than 500mm, preferably less than 300mm, from such body part. Can be in the range of about 5X 5cm²In the area of the antenna, additional required electronics and/or antennas for transmission and/or reception. Such an antenna and required electronics may be provided separately, e.g. may be attached to an existing local RF coil, or may be placed in other ways near the subject's body, e.g. attached to the bore of the primary MR magnet. Alternatively, such additional antennas and electronics may be integrated into the MRI device, or into the local RF coil.

After being received by the receiving antenna, the frequency of the transmitted higher pilot tone signal is converted to an intermediate frequency which is equal to or lower than the frequency of the input signal or one of them, whereby a so-called transmitted pilot tone signal (also referred to as output signal) is generated at such an intermediate frequency. Thus, the signal is more easily processed with existing electronic components. Furthermore, the relatively low frequency of the input signal (e.g., local oscillator) and the output signal facilitates processing of the connection cables, and interference between cables at lower frequencies will be less.

Preferably, the conversion of the transmitted higher pilot tone signal is performed using mixing with the at least one input signal or with a signal generated from the at least one input signal, for example by frequency multiplication or frequency division, wherein preferably band-pass filtering is performed after the mixing step.

Converting the frequency to a frequency even lower than that of the input signal makes it even easier to process the signal (e.g. by converting it from analog to digital, amplifying it, etc.) and transmit it to the analysis system. The analysis system may be any digital processing system, for example it may be part of the control computer of the MRI system. The analysis system may be implemented by any processing unit, such as a CPU, and may be on any computer, laptop or tablet, but it is also contemplated that the signals may be transmitted to the remote analysis system over a local area network or wireless LAN.

The analysis system may then, for example, detect changes in the transmitted pilot tone signal, thereby inferring motion of the subject. According to an embodiment, the motion of the subject is detected if the frequency and/or phase and/or amplitude variation of the transmitted pilot tone signal exceeds a predetermined threshold. In this embodiment the higher pilot tone signal may be a continuous signal of constant frequency and the transmitted pilot tone signal itself is observed for changes, i.e. over time, for example by constantly monitoring the frequency, phase and/or amplitude, preferably only the frequency and/or phase, and observing whether they have changed by at least a predetermined threshold, for example within a predetermined time window. Alternatively, the transmitted pilot tone signal (output signal) may be compared with the input signal from which the higher pilot tone signal was generated, thereby detecting any changes above a predetermined threshold.

According to an embodiment, each time a movement of the subject is thus detected, the analysis system may record that such movement has occurred and use this information, for example to ignore any data acquired after such movement, or to restart the current image acquisition process. Alternatively, a new position may be captured, for example by measuring the new position of the subject using the MR navigator, and the scan axis of the current image acquisition protocol is adjusted accordingly by translation and/or rotation.

According to a preferred embodiment, the at least one input signal comprises at least one local oscillator signal and a pilot tone signal. Preferably, the at least one local oscillator signal is used in step (b) to convert the frequency of the pilot tone signal to the transmit frequency, thereby generating the higher pilot tone signal.

The at least one local oscillator signal may be any periodically oscillating electronic signal, such as a sine wave or a square wave, which may be provided by an electronic oscillator. Preferably, the pilot tone signal is also a periodically oscillating electronic signal. In a preferred embodiment, the frequency of the pilot tone signal is close to the larmor frequency of the MRI system, for example at a magnetic field of 1.5 tesla, the frequency of the pilot tone may be about 63.5 MHz. This allows the advantageous use of already existing electronic components. In particular, it is preferred to use pilot tone signals that are already incorporated in existing MRI systems. The costs are minimized due to the small number of additional electronic components and connections. More generally, the pilot tone signal may have a frequency between 10MHz and 500MHz, more preferably in the range of 50MHz to 150 MHz. It may be a modulated signal (as opposed to a constant sine wave).

According to an embodiment, the step of converting the frequency of the pilot tone signal to a transmission frequency and/or the step of converting the transmitted higher pilot tone signal comprises mixing the respective signal with at least one input signal, in particular with a Local Oscillator (LO) signal or a multiplied or divided local oscillator signal having a carrier frequency. The carrier frequency may be generated by multiplying the LO signal by an integer or by dividing by an integer, preferably by the nth power of 2, where n is an integer, i.e. the frequency may be multiplied by 1, 1/2, 1/4, 1/8, 1/16, etc. When mixing the two signals, preferably a multiplicative mixing, in particular a frequency mixing, is applied, wherein the frequency of the local oscillator signal or a frequency multiplied local oscillator signal is added to the frequency of the pilot tone signal to generate the higher pilot tone signal. On the other hand, processing the transmitted higher pilot tone signal may include subtracting the frequency of the local oscillator signal or the frequency of the multiplied local oscillator signal from the frequency of the transmitted higher pilot tone signal. After the step of mixing the pilot tone signal or the transmitted higher pilot tone signal with the at least one local oscillator signal or the multiplied local oscillator signal, a band pass filter may be applied to filter the desired harmonics or intermodulation products. By adding multiplied local oscillator signals even higher frequencies can be achieved, thereby further increasing the interference to the subject tissue.

Advantageously, using at least one local oscillator signal to convert the pilot tone signal to a higher frequency and down-converting the transmitted higher pilot tone signal to a frequency equal to the pilot tone signal makes it easier to, for example, sample and analyze the pilot tone signal. In a preferred embodiment, the same local oscillator signal may be used in steps b) and e), so that only one signal processing chain is required for the local oscillator signal.

In a useful embodiment, the pilot tone signal is multiplied or divided in frequency before being mixed with the local oscillator signal or the multiplied local oscillator signal. Multiplying the frequencies of the pilot tone signal constitutes an alternative and/or additional way of achieving higher frequencies. In addition, the low frequency input signal may be used as a pilot tone signal. For example, the frequency of the pilot tone signal may be significantly lower than the larmor frequency of the MRI system, for example in the range of 8 to 12 MHz. Alternatively, the frequency of the pilot tone signal may be similar to or equal to the larmor frequency, while the multiplied pilot tone signal is twice or three times or many times the larmor frequency.

According to another embodiment, the frequency of the higher pilot tone signal is a combination of the frequencies of the at least one local oscillator signal and the pilot tone signal. Typically, the frequency of the higher pilot tone signal is thus created by adding the frequency of the pilot tone signal multiplied (or divided) by a predetermined integer and the frequency of the at least one local oscillator signal multiplied (or divided) by another predetermined integer. Optionally, one or more local oscillator signals having a frequency multiplied or divided by another predetermined integer may also be added to produce the higher pilot tone signal. This may result in the following form of frequency f relative to the pilot tone signal, for example_PTFrequency f of the first local oscillator signal_LO1And the frequency f of the second local oscillator signal_LO2Of the higher pilot tone signal f_HPT：

f_HPT＝N₀f_PT+N₁f_LO1+N₂f_LO2，

Wherein N is₀、N₁、N₂Is an integer, preferably an index with a base 2. In embodiments of the present invention, these integers may be between 0 and 256, more preferably between 0 and 64, and typically between 8 and 32. In a useful embodiment, N₂Is zero, i.e. only one local oscillator signal is required. In some embodiments, N₀＝1。

According to an embodiment, step b comprises multiplying (and/or dividing) the frequency of the at least one local oscillator signal to obtain a multiplied local oscillator signal, and mixing the multiplied local oscillator signal with the pilot tone signal to obtain the higher pilot tone signal. The multiplication of the signal may be achieved, for example, by a frequency multiplier, allowing the higher pilot tone signal to be transmitted at a frequency significantly higher than the frequency of the input signal, i.e., the frequency of the pilot tone signal and the frequency of the local oscillator. This allows for greater flexibility in measuring the movement of the subject. Furthermore, the frequency may be adjusted to the respective requirements of the measurement, for example to measure different parts of the subject, which may be composed of different materials, or to focus on objects at different depths within the subject.

According to one embodiment, the step of converting the transmitted higher pilot tone signal to a lower intermediate frequency comprises: down-mixing the transmitted higher pilot tone signal with at least one local oscillator signal or a frequency multiplied local oscillator signal to the frequency of the pilot tone signal to obtain a transmitted pilot tone signal; and optionally, down-mixing the transmitted pilot tone signal to an intermediate frequency that is lower than the frequency of the pilot tone signal. As mentioned above, it may also comprise a step of frequency division. The second down-mix signal produces an even lower frequency, which further simplifies the signal processing. Preferably, the frequency below the frequency of the pilot tone signal is in the range of 5 to 30MHz, more preferably in the range of 5 to 15MHz, most preferably in the range of 8 to 12 MHz.

According to an embodiment, the pilot tone signal is a modulation signal having a frequency close to or equal to the larmor frequency of the magnetic resonance imaging device. Using pilot tone signals or signalsA frequency close to or equal to the Larmor frequency makes it possible to easily combine this method with existing systems, such as Siemens Sola @, in which a local oscillator signal and a pilot tone signal are already availableProvided is a system.

According to another embodiment, the transmit antenna and the receive antenna are arranged at a local RF coil, which is placed near the body part of the subject. The method can be implemented more easily in existing systems by simply updating an existing local RF coil or using a new local RF coil already equipped with a corresponding antenna, placing the antenna on the local RF coil.

According to another embodiment, the transmitted higher pilot tone signal has a transmission frequency above 300MHz, preferably above 600 MHz. Higher frequencies may increase the interference of the signal to the subject, particularly to body tissue. Generally, higher frequencies will increase interference and therefore observable changes in the transmitted pilot tone signal, making it easier to detect motion of the subject, and therefore it is desirable to utilize high frequencies. On the other hand, higher frequencies will make signal processing more difficult. This problem is at least partially avoided by up-and-down mixing of the pilot tone signal.

According to another embodiment, the method is performed simultaneously using a first transmit antenna and a corresponding first receive antenna transmitting at a first transmit frequency and a second transmit antenna and a corresponding second receive antenna transmitting at a second transmit frequency. The use of two or more transmit and receive antennas makes it possible to detect motion of multiple portions of the subject at a time and/or to view one portion of the subject from different angles at a time. The transmit antenna and/or the receive antenna may be placed in close proximity to each other, e.g., multiple antennas may be attached to one local RF coil, or they may be located at different locations, e.g., on different local RF coils. For example, it is conceivable to detect motion of different parts of the subject at a time by placing the antennas at different positions, and/or to detect motion of the same part of the subject from different angles, thus facilitating detection of motion in different directions, for example.

In a useful embodiment, the first and second transmission frequencies differ by more than 100MHz to 20GHz, preferably more than 500MHz to 10GHz or even more than 1 to 5 MHz. Using frequencies that are far from each other makes it possible to focus on different objects of a subject located at different depths of the subject. Different frequencies and different spacings between frequencies may be suitable depending on the respective depths of the different objects and the characteristics of the subject at that location related to radiation penetration. In an alternative embodiment, the first and second transmission frequencies may be close to each other, for example they may differ by less than 100MHz, for example between 10 and 80 MHz. The different frequencies but close to each other make it possible to observe a part of the subject from different directions using different antennas sensitive to different frequencies or by applying band pass filters without different signals interfering with each other, which only pass the corresponding frequencies of the signals that should be received by the antennas. Thus, it can be ensured that different signals can be distinguished.

According to an alternative embodiment, the higher pilot tone signal is transmitted in the form of a Frequency Modulated Continuous Wave (FMCW). For example, by changing the frequency up and down over a period of time, the frequency difference between the transmit signal and the reflected receive signal makes it possible to determine the distance via the frequency difference between the transmit signal and the receive signal. By continuously observing this distance, motion will be detected. In an alternative embodiment, the higher pilot tone signal is transmitted in the form of a pulse radar. Thereby, a pulse is transmitted and time is measured until the pulse is reflected back to the receiving antenna, so that distance can be measured and by comparing successive pulses also motion can be measured.

According to another aspect, the invention provides a system for detecting motion of a subject, the subject being located inside a magnetic resonance imaging apparatus, the system comprising:

an interface adapted to receive at least an input signal, in particular at least a local oscillator signal and a pilot tone signal,

a multiplier and/or divider and/or first mixer configured to generate an upper pilot tone signal using at least one input signal, having a transmit frequency at least twice as high as the Larmor frequency of the magnetic resonance imaging device, in particular by converting the frequency of the pilot tone signal to the transmit frequency using at least one local oscillator signal,

a transmit antenna adapted to transmit a higher pilot tone signal,

a receive antenna adapted to receive the transmitted higher pilot tone signal,

a second mixer configured to convert the frequency of the transmitted higher pilot tone signal using at least one input signal, in particular a local oscillator signal, to an intermediate frequency equal to or lower than the frequency of at least one of the input signals or thereof, in particular lower than the frequency of the pilot tone signal or at least one local oscillator signal, thereby generating a transmitted pilot tone signal (also referred to as output signal), an

An interface adapted to forward the transmitted pilot tone signal.

All features of the method may be adapted to the system and vice versa. The interface adapted to receive signals and the interface adapted to forward signals may optionally both be part of one input/output interface.

According to a preferred embodiment, the system is part of the local RF coil, or the system is adapted as part of the local RF coil or as an additional part attached to the outside of the local RF coil. In particular, the transmitting antenna and/or the receiving antenna may be part of the local RF coil or adapted as part of the local RF coil or as an additional part attached to the outside of the local RF coil.

According to an embodiment, the interface adapted to receive the input signal, in particular the at least one local oscillator signal and the pilot tone signal, the first multiplier, the first mixer and the transmit antenna may be part of the local RF coil, or adapted as an additional part, which may be attached to the local RF coil or elsewhere. Likewise, the receiving antenna, the second multiplier, the second mixer and the interface adapted to forward the transmitted pilot tone signal may be part of the local RF coil, or may be adapted as an additional part, which may be attached to the local RF coil or elsewhere. Components that are not part of the local RF coil may be attached or attachable to the bore of the MRI system. On the one hand, this may save space on the respective local RF coil. On the other hand, it may cause the system to observe the subject, detecting its motion from other different angles by sending a transmitted higher pilot tone signal towards the subject from another/alternative location.

According to another aspect, the invention provides a local RF coil for a magnetic resonance imaging apparatus, the local RF coil being adapted to detect motion of a portion of a subject's body located in its vicinity, the local RF coil comprising:

one or more coil elements adapted to transmit magnetic resonance signals,

coil connector adapted to receive at least one input signal, in particular a local oscillator signal and a pilot tone signal, an

At least one system for detecting motion of a subject according to any of the embodiments of the system described herein.

By providing local RF coils with a system for detecting motion, updating an existing MRI system with the motion detection system may be facilitated by simply adding and/or replacing and/or updating the respective local RF coils. Updating an existing local RF coil is particularly advantageous as it may require only minor changes with only minor costs, especially if the existing local RF coil and/or the existing MRI system already includes providing a pilot tone signal and/or one or more local oscillator signals.

Drawings

Embodiments of the present invention will now be described with reference to the accompanying drawings.

Fig. 1 shows a schematic flow chart representing an embodiment of a method for detecting a motion of a subject according to the present invention.

Fig. 2 shows a schematic cross section of a local RF coil for an MRI apparatus with a system for detecting motion of a subject according to an embodiment of the present invention.

Detailed Description

Fig. 1 shows an embodiment of a method for detecting motion of a subject 30 according to the present invention.In this embodiment, a Pilot Tone (PT) signal 10 is provided. Furthermore, a first local oscillator signal LO 112 and a second local oscillator signal LO 214 are provided. The frequency of the pilot tone signal 10 may be multiplied by a factor N₀. The frequency of the local oscillator signal 12 may be multiplied by a factor N₁The frequency of the second local oscillator signal 14 may be multiplied by a factor N₂，N₀、N₁And N₂Is an integer. Instead of or in addition to frequency multiplication, frequency division may also be applied. In a next step, the frequencies of the pilot tone signal 10, the local oscillator signal 12 and optionally the second local oscillator signal 14 are added by the first mixer 22 to generate the higher pilot tone signal 16, which frequency is the sum of the frequencies of the (multiplied) pilot tone signal 10, the (multiplied) local oscillator signal 12 and optionally the (multiplied) second local oscillator signal 14.

The higher pilot tone signal 16 is transmitted via an antenna 26 towards a subject 30, in this case a patient. After interaction with the patient 30, the transmitted higher pilot tone signal 18 is received by the receive antenna 28. By using the second mixer 24, the frequency of the transmitted higher pilot tone signal 18 is subtracted by the same integer number N multiplied₁And optionally multiplied by an integer N, of the local oscillator signal 12₂Thereby resulting in a transmitted pilot tone signal 20 having the same frequency as the original pilot tone signal 10. In a final step, the transmitted pilot tone signal 20 is then forwarded to an analysis system 32 (in this case a computer). A computer can then be used to detect changes in the transmitted pilot tone signal 20 and trace these changes back to the motion of the patient 30.

Fig. 2 shows an embodiment of a local RF coil 40 for an MRI apparatus with a system for detecting subject motion, in this case the body part being the head 30 of a patient. The head 30 is shown in axial cross-section inside the head coil 40. The system is adapted to receive a local oscillator LO1 signal 12 and a pilot tone PT signal 10. For example, the LO1 signal is generated by an oscillator (e.g., PLL stabilized quartz or OXCO) that is phase locked to a local oscillator signal provided to the local RF coil. L isThe frequency of O1 may be 75MHz, and the LO1 signal is multiplied by multiplier/divider 34, e.g., to 600MHz (N)₁8) frequency. The multiplier/divider 34 preferably comprises diodes and/or transistors or digital gates, as is known in the art. The multiplication and/or division 34 is followed by a band pass filter 36 to filter the desired harmonics to obtain multiplied and/or divided local oscillator signals.

Further, the system comprises a first mixer 22, preferably a multiplying mixer, which mixes the multiplied local oscillator signal with the pilot tone signal 10. In some embodiments, the mixer includes or is followed by a band pass filter that suppresses unwanted sidebands and passes signals in the desired transmit frequency range, e.g., some linear combination, e.g., sum, of the multiplied frequency (600MHz) of the local oscillator signal 12 and the frequency (63.5MHz) of the pilot tone signal 10, e.g., 663.65MHz, to produce the higher pilot tone signal 16. The system further includes a transmit antenna 26 attached to the interior of the local RF coil 40, the transmit antenna 26 adapted to transmit the higher pilot tone signal 16 to the patient's head 30. In addition, the system includes a receive antenna 28 adapted to receive the transmitted higher pilot tone signal THPT 18. THPT18 is first filtered by another band pass filter 36a to remove any noise and optionally pre-amplified by a pre-amplifier (not shown). The received THPT signal 18 is then mixed with a signal having a frequency N in a second mixer 24₁*f_LOMultiplied and filtered local oscillator signal mixing at 600 MHz. The second mixer 24 is followed by a third band-pass filter 36b adapted to filter the desired frequency of the transmitted pilot tone signal 20, in this case f_THPT-N₁*f_LO663.5 MHz-600 MHz 63.5 MHz. In other words, the multiplied frequency of the local oscillator signal 12 is subtracted from the frequency of the transmitted higher pilot tone signal 18 to produce a transmitted pilot tone signal 20 having a frequency approximately equal to the frequency of the pilot tone signal 10 (in this case 63.5 MHz). This signal 20 is further mixed by a third mixer 38, again using the 75MHz first local oscillator LO1 signal 12, to produce an output signal of about 12MHz, which may be at an ADC (analog-to-digital converter), for exampleNot shown) at 10 MHz. The resulting signal may then be forwarded to an analysis system 32 (in this case a computer) via an interface.

In addition to the transmit antenna 26 and the receive antenna 28, the local RF coil 40 also includes a second transmit antenna 26 'and a second receive antenna 28'. The second transmitting antenna 26 'and the second receiving antenna 28' are connected to a signal processing system (not shown here) which is identical to the signal processing system to which the transmitting antenna 26 and the receiving antenna 28 are connected. The two transmit receive systems allow the patient's head 30 to be viewed from two different angles or two different depths. For example, closely different frequencies, e.g. 600+63.5MHz and 600+2x63.5MHz, may be transmitted together, i.e. the pilot tone signal PT is multiplied before mixing with the multiplied LO1 signal. Alternatively, the signals transmitted by the two antennas 26 and 26' may be far apart, for example at one of the frequencies (MHz)600+63.5, 1200+63.5, 2400+63.5 and 4800+ 63.5. In other words, the integer N by which the signal LO1 is multiplied₁Preferably an index to the base of 2 and may be between 4 and 124, in these examples one of 8, 16, 32 and 64.