Magnetic resonance imaging apparatus and signal processing method

Abstract

To expand a dynamic range of an MR signal without increasing imaging time and cost of manufacture in an MRI apparatus.SOLUTION: In an MRI apparatus, a receiver 200 for performing A / D conversion on two systems of nuclear magnetic resonance signals by a direct sampling method is equipped. The receiver 200 includes a distributor 201 for distributing the nuclear magnetic resonance signals into two systems, an attenuator 202 for attenuating an overflow signal output from the distributor, and a switch 203 for outputting signals of a same gain by switching. Digital processing means for respectively performing the A / D conversion on the signals of two kinds of gains by ADC of the same number as the nuclear magnetic resonance signal, putting the gain of the attenuated signal back, and re-synthesizing is equipped. A dynamic range is expanded by synthesizing the signals of different gains output by switching by the switch and increasing the bit number of digital data to entire sampling points.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a magnetic resonance imaging apparatus and an imaging method for having a wide dynamic range.

Background Art

[0002] In recent years, remarkable progress has been made in magnetic resonance imaging (MRI) technology, and improvements in the sensitivity of receiving coils for higher image quality and faster speed, diversification of imaging sequences, and introduction of image processing technologies using deep learning have been advanced. The magnetic resonance signal (MR signal, hereinafter also referred to as echo signal) detected by the receiving coil of MRI is subjected to appropriate signal processing and then A / D (analog-digital) conversion and image processing.

[0003] Since MRI is signal measurement in Fourier space, as shown in FIG. 3, the MR signal has the characteristic that the signal intensity is very large at the center of the Fourier space but very small in the surrounding area (the area with high spatial frequency). Furthermore, the form of the MR signal depends on the imaging method of what kind of imaging sequence and what kind of image is imaged, and it must be considered that the dynamic range becomes extremely large in the MRI apparatus.

[0004] In addition, in recent years, the increase in static magnetic field strength and the improvement of SNR (Signal-to-Noise-Ratio) in 3D imaging methods have advanced, and a high-speed and high-resolution A / D converter (hereinafter referred to as ADC) is required for accurate digitization. However, the currently available ADCs are not always satisfactory in terms of quantization resolution, sampling speed, and the price of the converter.

[0005] To address this, Non-Patent Document 1 shows a method (Dual Scan method) of acquiring data twice by changing the gain difference, which is widely adopted in the technical field of MRI imaging.

[0006] Furthermore, Patent Document 1 (Figure 7) proposes a method for artificially expanding the dynamic range by performing A / D conversion on a signal obtained by adjusting the amplitude of an MR signal using a first amplitude adjuster, and on a signal obtained by adjusting the amplitude of the difference signal between the original MR signal and the amplitude-adjusted signal using a second amplitude adjuster, and then combining them in a digital processing unit. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] R.Bein, J.Bishop, RH Henkelman. Dynamic Range Requirements for MRI. Conc. Magn. Reason. B26:28-35,2005 [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2005-270583 (Embodiment: Figure 7) [Overview of the project] [Problems that the invention aims to solve]

[0009] However, the Dual Scan method disclosed in Non-Patent Document 1 is not preferable in terms of device stability because it involves two imaging steps. In particular, MRI imaging is performed over a long period of time, so the conditions for the first and second imaging steps do not necessarily coincide. Therefore, signal synthesis is not simple, resulting in the disadvantage of image artifacts and longer imaging times.

[0010] Furthermore, the method disclosed in Patent Document 1 has the disadvantage that when processing signals from multiple channels, an ADC is required for each channel, and as the number of channels increases, a large number of ADCs are required, leading to increased complexity of the device and increased manufacturing costs.

[0011] This invention has been made in view of the above-mentioned conventional problems, and aims to be realized without increasing the size of the device or significantly increasing manufacturing costs, and to expand the dynamic range for all sampling points of the MR signal. [Means for solving the problem]

[0012] To achieve the above objectives, the present invention provides a first ADC that distributes the MR signal to two or more channels, processing one of them as is, and a second ADC that processes it with gain adjustment. For multiple MR signals input to the receiver, the timing of input to the first and second ADCs, and the timing of combining the output from the first ADC with the output from the second ADC with gain adjustment are switched. This allows ADC processing to be performed on each of the multiple MR signals with only two ADCs with different gains, virtually widening the dynamic range.

[0013] In other words, the MRI apparatus of the present invention comprises a magnetic field generating unit that generates nuclear magnetic resonance in a subject, a receiving unit having a receiving coil that receives nuclear magnetic resonance signals emitted by the subject, and a processing unit that performs processing including image reconstruction using the nuclear magnetic resonance signals received by the receiving unit. The receiving unit comprises a distributor that distributes the nuclear magnetic resonance signal into at least two signal systems, a gain adjuster that adjusts the gain of at least one of the first signal system and the second signal system, a first AD converter and a second AD converter that perform analog-to-digital conversion of the first signal system and the second signal system, whose gains have been adjusted by the gain adjuster, respectively, and a digital processing unit that returns the gain of the digitized signals, whose gains have been adjusted by the gain adjuster, to their original gains and synthesizes the two digitized signals. The digital processing unit is characterized by comprising a control unit that controls the inputs to the first AD converter and the second AD converter, and the outputs of the first AD conversion and the second AD converter.

[0014] Furthermore, the signal processing method of the present invention is a method for processing nuclear magnetic resonance signals measured by an MRI device, and includes the steps of distributing the magnetic resonance signal to two systems, performing analog-to-digital conversion on one system either as is or after gain adjustment, performing analog-to-digital conversion on the other system after gain adjustment so that it has a different gain than the first system, returning it to its original gain, and then combining the digital signal of one system with the digital signal of the other system. In this case, for a single nuclear magnetic resonance signal, the timing of the analog-to-digital conversion of one system and the timing of the analog-to-digital conversion of the other system are synchronized, and control is performed to switch between them according to the number of nuclear magnetic resonance signals to be processed. [Effects of the Invention]

[0015] According to the present invention, the signal is distributed to two systems, each with a different gain, and the signals with the same gain are switched between and output to two ADCs. By combining the signals with different gains that have been switched and output, the number of bits of digital data for all sampling points is increased. As a result, even if the number of signals to be processed increases, it is not necessary to increase the size of the equipment or the manufacturing cost. Furthermore, the dynamic range can be expanded without increasing the imaging time, making it possible to collect high-resolution image data. [Brief explanation of the drawing]

[0016] [Figure 1] A block diagram showing the overall configuration of an MRI apparatus according to an embodiment of the present invention. [Figure 2] A diagram showing an example of an imaging pulse sequence. [Figure 3] A diagram illustrating typical echo signals generated by a subject. [Figure 4] A diagram showing the configuration of a receiver according to the first embodiment of the present invention. [Figure 5] Functional block diagram of the processing circuit of the first embodiment. [Figure 6] A flowchart illustrating the operation of the switch according to the first embodiment. [Figure 7] Flowchart showing data processing after ADC according to the first embodiment. [Figure 8] Diagram for explaining the expansion of the dynamic range by bit number expansion according to the first embodiment. [Figure 9] Diagram showing Modification Example 1 of the receiving circuit according to the first embodiment. [Figure 10] Flowchart showing the operation of the switch in Modification Example 1. [Figure 11] Diagram showing Modification Example 2 of the receiving circuit according to the first embodiment. [Figure 12] Block diagram showing the configuration of the receiver according to the second embodiment of the present invention. [Figure 13] Functional block diagram of the processing circuit according to the second embodiment.

Embodiments for Carrying Out the Invention

[0017] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0018] <The First Embodiment> [[ID=!33]]

[0019] First, an overview of the MRI apparatus to which the present invention is applied will be described.

[0020] As shown in FIG. 1, the MRI apparatus 100 includes a static magnetic field generator 101 that generates a static magnetic field in an imaging space where an imaging site of a subject 105 mounted on a bed 106 is arranged, a gradient magnetic field coil 102 that applies a gradient magnetic field to the imaging space, a transmission coil 103 that irradiates the subject 105 with a high-frequency magnetic field pulse, and a reception coil 104 that receives an MR signal from the subject 105.

[0021] It should be noted that there seems to be an incorrect tag in the original text at line 33 which is "

[0019] " and it's not clear if it's a real tag or a formatting error. I've left it as is in the translation for now. If it's an error, it might need to be corrected in the original text for a more accurate translation.The MRI apparatus 100 further includes a sequencer 111 that controls the operation of the gradient magnetic field power supply 107, the high-frequency generator 108, and the receiver 200 according to a predetermined pulse sequence, a computer 300 that controls imaging via the sequencer 111 and performs calculations using MR signals, an input device 110 that serves as an interface between the apparatus and the user, a display device 301, and a storage device 109 that stores data and images necessary for imaging.

[0022] The gradient coil 102 is driven by the gradient power supply 107, and a gradient magnetic field with three axes (X, Y, Z) perpendicular to the static magnetic field is applied. The transmitting coil 103 is supplied with a high-frequency signal generated from the high-frequency generator 108, and irradiates the subject 105 with a high-frequency magnetic field pulse. The MR signal received by the receiving coil 104 is transmitted to the receiver 200. The receiver 200 performs necessary signal processing on the MR signal, such as AD conversion, and passes it to the computer 300.

[0023] The sequencer 111 operates the gradient power supply 107, the high-frequency generator 108, and the receiver 200 according to a series of pulse sequence information provided by the input device 110 and the computer 300 to collect the MR signal. There are various pulse sequences that differ depending on the imaging method, and these are stored in the storage device 109 in advance.

[0024] The user inputs control information and imaging sequence information for the MRI device via the input device 110, and operates the computer 300 and sequencer 111 based on the input information to obtain the desired image.

[0025] Computer 300 is composed of memory and a CPU and GPU, and receives digital signals from receiver 200. It performs an inverse Fourier transform on the collected data to reconstruct an image. The reconstructed image is stored in storage device 109 and displayed on display device 301 as appropriate.

[0026] Imaging with an MRI system configured in this way is performed as follows, for example, when the imaging pulse sequence is a spin echo sequence 400 as shown in Figure 2. The sequencer 111 excites the spin of the subject 105 by transmitting a high-frequency magnetic field pulse (RF) 401-1 from the transmitting coil 103 while applying a slice encoding gradient magnetic field (Gz) 402-1 from the gradient magnetic field coil 102. Next, after applying a phase encoding gradient magnetic field (Gy) 403, a predetermined time (TE / 2) has elapsed since the application of the excitation high-frequency magnetic field pulse (90° pulse), and a high-frequency magnetic field pulse (180° pulse) 401-2 that reverses the magnetization is applied together with the slice encoding gradient magnetic field 402-2. The echo signal 405, which has a maximum peak at the echo time (TE), is received by the receiving coil 104 while applying a readout gradient magnetic field (Gx) 404. The process from the application of the 90° pulse to the reception of the echo signal is repeated at a predetermined repetition time TR while changing the intensity of the phase encoding gradient magnetic field, and the number of echo signals necessary for image reconstruction is collected.

[0027] Here, the waveform of the received signal output from the receiving coil 104, as shown in Figure 3 as an example, shows that its amplitude (signal strength) increases towards the maximum peak, although there are small fluctuations, and then decreases. A characteristic of the MR signal is that the amplitude at the position of the maximum peak differs greatly from the amplitude at the position of the minimum peak, and the ratio between them (dynamic range) is large.

[0028] The receiver 200 samples the echo signal received by the receiving coil 104 at a predetermined sampling period, and converts the amplitude (signal strength) at the sampling point into a digital signal using an ADC. In this case, processing MR signals with a large dynamic range requires a high-performance ADC that can handle such signals, but the MRI device of this embodiment can handle signals with a wide dynamic range by switching and controlling a general-purpose ADC.

[0029] The following describes the specific configuration and operation of the receiver 200. For simplicity, the following explanation will describe the case where two received signals are processed. Two signals include, for example, when the receiving coil is composed of two small coils (called channels), the signals received by each channel, or when the receiving coil receives a complex signal (QD) and processes the real and imaginary parts.

[0030] As shown in Figure 4, the receiver 200 of this embodiment includes two distributors 201-1 and 201-2 corresponding to two input signals (echo signal 1 and echo signal 2), attenuators 202-1 and 202-2 connected to one output of each of the distributors 201-1 and 201-2, two ADCs (first ADC 204 and second ADC 205), switches 203-1 and 203-2 for switching the signal input to the first ADC 204 and second ADC 205 respectively, and a digital processing circuit (hereinafter simply referred to as the processing circuit) 206.

[0031] Note that when it is not necessary to distinguish which of the two signals the distributors 201-1 and 201-2, and the attenuators 202-1 and 202-2 correspond to, they are collectively referred to as distributor 201 and attenuator 202.

[0032] The distributor 201 distributes the input echo signals (echo signal 1 and echo signal 2) into two separate outputs. One output of the distributor 201 is directly input to switch 203-1, while the other output is attenuated by attenuator 202 before being input to switch 203-2. The signal directly input to switch 203-1 is called the through signal, and the signal attenuated before being input to the switch is called the attenuated signal. When it is not necessary to distinguish whether switches 203-1 and 203-2 correspond to the through signal or the attenuated signal, they are collectively referred to as switch 203.

[0033] Switch 203-1 operates in response to a control signal (switching signal) from processing circuit 206, switching between the through signal 1 of echo signal 1 and the through signal 2 of echo signal 2, and inputting them to the first ADC 204. Similarly, switch 203-2 switches between the attenuation signal 1 of echo signal 1 and the attenuation signal 2 of echo signal 2, and inputs them to the second ADC 205.

[0034] Switch 203 has a fast switching speed that takes transient response into account, relative to the sampling speed of the first ADC 204 and second ADC 205, for example, the rising edge is the sampling rate. period Approximately half of the above is used. Although not limited, as an example, if a sampling frequency of 100 MHz is used, the sampling period is 10 ns, so the rise time of the signal after switching can be about half of the sampling period, or 5 ns. This allows the two sets of data to be combined in the subsequent processing circuit 206 without losing their continuity.

[0035] The processing circuit 206 is a digital processing unit that restores the dynamic range of the attenuated signal after AD conversion, then combines it with the through signal after AD conversion to virtually expand the dynamic range, and also functions as a control unit that generates a control signal (switching signal) to control the switching of the switch. Figure 4 shows the case where the processing circuit 206 is an FPGA (Field programmable gate array), but the processing circuit 206 is not limited to an FPGA and can be composed of other programmable ICs, digital signal processors (DSPs), computers, etc.

[0036] Figure 5 shows a block diagram illustrating the functions of the processing circuit 206. As shown in the figure, the processing circuit 206 comprises a digital amplifier 2061, a combining unit 2062, and a control signal generation unit 2063. The digital amplifier 2061 extends the dynamic range of the digitally attenuated signal to its original dynamic range. The combining unit 2062 combines the through signal after AD conversion and the attenuated signal after dynamic range extension. The combined signal is sent to the computer 300 for image reconstruction and other calculations. The control signal generation unit 2063 generates a switching signal that determines the timing at which the switch 203 switches the inputs of the first ADC 204 and the second ADC 205.

[0037] Next, the operation of the receiver 200 with the above configuration will be explained.

[0038] When two signals, echo signal 1 and echo signal 2, are input to receiver 200, distributor 201 distributes each echo signal into two separate signals. Through signal 1 of echo signal 1, which is one output of distributor 201-1, and through signal 2 of echo signal 2, which is one output of distributor 201-2, are both input to switch 203-1 and, while being switched by switch 203-1, are input to the first ADC 204. The other output of distributor 201-1 and the other output of distributor 201-2 are adjusted to a predetermined gain by attenuators 202-1 and 202-2, respectively, becoming attenuated signals 1 and 2 with reduced dynamic range, which are input to switch 203-2 and, while being switched by switch 203-2, are input to the second ADC 205.

[0039] Switching between the through signal and attenuation signal output from switches 203-1 and 203-2 is handled by the processing circuit. FPGA The system is controlled by a switching signal generated by the control signal generation unit 2063 of 206. For example, as shown in Figure 6, when the switching signal is H, switch 203-1 inputs through signal 1 to the first ADC 204, and switch 203-2 inputs attenuation signal 1 to the second ADC 205. When the switching signal is L, switch 203-1 inputs through signal 2 to the first ADC 204, and switch 203-2 inputs attenuation signal 2 to the second ADC 205.

[0040] As shown in Figure 4, the first ADC 204 performs A / D conversion on the switch output of the through signal, and the second ADC 205 performs A / D conversion on the switch output of the attenuated signal that has been attenuated by the attenuator 202. Both ADCs output digital data to the processing circuit 206. As a result, the two digital signals of echo signal 1 and the two digital signals of echo signal 2 are input to the processing circuit 206 in a synchronous and alternating manner.

[0041] In the processing circuit 206, as shown in the flowchart in Figure 7, if, for example, H (High) is output as the switching signal to the switch (S11), the digital amplifier 2061 adjusts the attenuation amount of the attenuated signal 1, which has been A / D converted by the second ADC 205, to restore it to its original gain (S12). At the same time, the combining unit 2062 combines the through signal 1 input to the processing circuit 206 with the attenuated signal 1 with the adjusted attenuation amount (S13).

[0042] If, for example, L (Low) is output as the switching signal to the switch (S11), the digital amplifier 2061 adjusts the attenuation amount of the attenuated signal 2, which has been A / D converted by the second ADC 205, to restore it to its original gain (S14). At the same time, the combining unit 2062 combines the through signal 2 input to the processing circuit 206 with the attenuated signal 2 with the adjusted attenuation amount (S15). The receiver 200 then transfers the digitized signal to the computer 300, which reconstructs the image (S16).

[0043] Through gain adjustment and synthesis in digital signal processing, both echo signal 1 and echo signal 2 become digital signals with expanded bit depth. The expansion of bit depth is explained using Figure 8.

[0044] In Figure 8, signals A and B correspond to the through signal and the attenuated signal, respectively. As mentioned earlier, the dynamic range of the echo signal (MR signal) measured by receiver 200 is wide, but because there is an upper limit to the number of bits in a single ADC, the dynamic range is narrowed, and as is (i.e., the through signal), the signal becomes one in which the high-intensity portion is cut off (overflowed), as in signal A. On the other hand, in the attenuated signal, the signal intensity is attenuated, so it is compressed to a width that can be processed by the number of bits in the ADC, as in signal B, and becomes a signal that includes the portion that overflowed in signal A. However, in signal B, the fine details are leveled.

[0045] When signals A and B are digitized using an ADC with the same maximum number of bits, as shown on the right side of Figure 8, bits are assigned directly to the minute portion of the signal with a wide dynamic range (signal A), while bits are assigned to the attenuated portion of the signal (signal B) that has overflowed. In the combined signal, the number of bits is effectively expanded. Since expanding the number of bits expands the dynamic range, the combined signal of signals A and B becomes a digital signal with a virtually expanded dynamic range.

[0046] The extended dynamic range (DR) can be calculated using the following formula, where N is the upper limit bit of the ADC used, fs [Hz] is the sampling frequency, and BW [Hz] is the bandwidth. (Math 1) DR = 6.02N + 1.76 + 10log 10 (f s (2BW) For example, if N=14bit, fs=40MHz, and BW=1kHz, the DR becomes 127.3dB according to the above formula. Then, if we virtually expand to N=20bit, assuming other conditions remain the same, the DR becomes 163.4dB, which means an expansion of 36.1dB is possible.

[0047] In this way, the processing circuit 206 enables high-precision A / D conversion for all sampling points by treating the minute signal portion as a pass-through signal and assigning bits of the attenuated signal after gain adjustment to the overflow signal, for signals that have a wide dynamic range characteristic of MR signals.

[0048] The receiver 200 transmits the digitized signal, as described above, to the computer 300, which reconstructs the image. The image reconstruction and other processing in the computer 300 are the same as those in conventional MRI devices, and therefore will not be explained here.

[0049] As explained above, according to this embodiment, even signals with a wide dynamic range that are difficult to process with a normal ADC can be processed by dividing the signal portion containing minute changes and the signal portion that overflows into two separate systems. This increases the upper limit of the number of bits that was limited by a single ADC, thereby expanding the dynamic range. In addition, the receiver 200 divides multiple (two in Figure 4) input echo signals into two systems each and switches between them, performing A / D conversion on each divided signal system. Therefore, the dynamic range can be expanded without increasing the number of ADCs as in the conventional method.

[0050] <Modified form of the first embodiment> In the first embodiment, the case where two echo signals are input to the receiver 200 simultaneously was described. However, the receiver 200 can also process three or more signals by considering the switching speed of the switch 203 and the sampling speed. Processing three or more signals can be done, for example, in the case of a multi-array coil where the receiving coil is composed of three or more small receiving coils, or when processing each complex signal from two or more receiving coils.

[0051] Figure 9 shows an example configuration of receiver 200 when processing three signals (modification example 1). In Figure 9, circuits with the same function are denoted by a symbol for one representative circuit, and "-1" is added to the end to distinguish them. or "-2"As shown in the diagram, the receiver 200 is equipped with 201 distributors equal to the number of signals to be processed M (M=3 in the diagram). Two sets of switches 2031 and 2032 (two of each) are provided to switch the input to the ADC. The first set of switches 2031 is switched by switching signal 1 emitted from the processing circuit 206, and the second set of switches 2032 is switched by switching signal 2.

[0052] Similar to the first embodiment, the distributor 201 distributes the input signal to the same number of outputs as the number of signals to be processed, designating one output as a pass-through signal and sending the other outputs to the attenuator 202. As a result, the echo signal becomes a pass-through signal and an attenuated signal, respectively. The pass-through and attenuated signals of echo signal 1 and echo signal 2 are input to the first-stage switches 2031-1 and 2031-2, respectively, similar to the first embodiment. The outputs of the first-stage switches 2031-1 and 2031-2 are input to the second-stage switches 2032-1 and 2032-2. Simultaneously, the pass-through and attenuated signals of echo signal 3 are input to the second-stage switches 2032-1 and 2032-2.

[0053] Switching signals 1 and 2 switch at timings such as those shown in the upper part of Figure 10. As a result, as shown in the lower part of Figure 10, when both switching signals 1 and 2 entering switches 2031 and 2032 are H, the through signal and attenuation signal of echo signal 1 are input synchronously to the first ADC 204 and second ADC 205. If switching signal 1 is H but switching signal 2 is L, the through signal and attenuation signal of echo signal 3 are input synchronously to the first ADC 204 and second ADC 205 via switch 2032. When switching signal 1 is L, switching signal 2 is H, and switches 2031 and 2032... 2032 Through this, the through signal and attenuation signal of echo signal 2 are input synchronously to the first ADC204 and the second ADC205.

[0054] In this modified example, the control of switching signals 1 and 2 differs from that of the first embodiment. However, by simply adding a distributor according to the number of signals, ADC processing can be performed in the same way as in the first embodiment, and the same effects can be obtained.

[0055] Figure 11 shows another modified configuration (modification 2) for when the number of signals is three or more. In this modification, the number of signals distributed by the distributor 201 is increased, with one output being a pass-through signal and the other two outputs being attenuated by two attenuators (attenuator a, attenuator b). Three ADCs are provided corresponding to one pass-through signal and the attenuated signals (attenuated signal a, attenuated signal b) attenuated by the two attenuators, and two stages of switches are placed in front of each.

[0056] The first stage switch 1 (three switches) receives the through signals of echo signal 1 and echo signal 2, as well as attenuation signal a and attenuation signal b, respectively. The second stage switch 2 (three switches) receives the outputs of the first stage switches, as well as the through signal of echo signal 3, attenuation signal a and attenuation signal b, respectively.

[0057] In this configuration as in Modification 1, when the first-stage switch is H and the second-stage switch is H, the three signals distributed from echo signal 1 are input to the three ADCs, digitized, and input to the processing circuit 206. Also, when the first-stage switch is L and the second-stage switch is H, the three signals distributed from echo signal 2 are input to the three ADCs, and when the second-stage switch is L, the three signals distributed from echo signal 3 are input to the three ADCs, each.

[0058] In this modified example 2, by adding an attenuator, multiple attenuated signals with different degrees of attenuation can be obtained according to the dynamic range of the original signal, and the dynamic range can be further expanded without increasing the number of ADCs relative to the number of signals. Alternatively, even when the number of bits of the ADC is less than in the first embodiment or modified example, the number of bits can be expanded in the same way as in the first embodiment or modified example.

[0059] <Second Embodiment> In the first embodiment, one of the distributed signals was used as a pass-through signal and the other as an attenuated signal. However, in this embodiment, the dynamic range is optimized by appropriately adjusting the attenuation amount of each distributed signal using a gain adjuster.

[0060] In this embodiment as well, the overall configuration of the MRI apparatus is the same as in the first embodiment shown in Figure 1. The following description will focus on the differences from the first embodiment.

[0061] Figure 12 shows an example configuration of the receiver 200 according to the second embodiment. In this example configuration, two echo signals are input to the receiver 200, but the number of echo signals input to the receiver 200 is not limited to two.

[0062] As shown in the figure, the receiver 200 of the second embodiment includes a distributor 201 that distributes multiple input echo signals into two separate systems, and a first gain adjuster 207 and a second gain adjuster 208 connected to the respective output sides of the distributor 201. It also includes a first ADC 204 that receives inputs from two first gain adjusters 207 by switching a switch 203, and a second ADC 205 that receives inputs from two second gain adjusters 208 by switching a switch 203, similar to the first embodiment. Furthermore, each switch 203 is switched by a switching signal from the processing circuit 206.

[0063] As shown in Figure 13, the processing circuit 206 is equipped with two gain adjusters 2065 (gain adjuster a and gain adjuster b) that restore the gains adjusted by the two first gain adjusters 207 and second gain adjusters 208 to their original values, and is also equipped with a gain calculation unit 2064 that calculates the gains of the gain adjusters 207, 208 and the two gain adjusters 2065.

[0064] The gain calculation unit 2064 adjusts the adjustment values ​​of the first gain adjuster 207 and the second gain adjuster 208 to the optimal adjustment values ​​according to the dynamic range of the original MR signal. The dynamic range of the original MR signal varies depending on the imaging conditions and the object being imaged, so it can be determined, for example, using the MR signal obtained in a pre-scan before the main scan to acquire an image of the subject, and the adjustment values ​​are calculated based on this. In general, in MRI, a pre-scan is performed before the main scan to determine image processing and various conditions, so there is no need to perform a separate pre-scan for gain adjustment, and the measurement signals from the prepared pre-scan can be used. However, instead of using the pre-scan data, the adjustment values ​​may be calculated using the measurement signals obtained during the main scan.

[0065] For example, in the case of a signal with an extremely large dynamic range, the first gain adjuster 207 reduces the gain to a certain extent, i.e., attenuates it by a predetermined amount, and then the second gain adjuster... 208 The adjustment amount is set to further reduce the signal by a larger amount of attenuation (an attenuation that covers the entire signal). The amount of attenuation depends on the performance of the ADC used and the design of the analog circuit, and is not limited, but for example, the first gain adjuster 207 may be set to an attenuation that covers up to about 1 / 3 of the signal strength. It is also possible to set the adjustment value of the first gain adjuster 207 to zero, that is, to make the output from the first gain adjuster 207 a pass-through signal, and to adjust the gain only with the second gain adjuster 208. If the gain adjustment by the second gain adjuster 208 corresponds to signal attenuation, it is the same as in the first embodiment. Although it is rare to increase the gain (amplify the signal) with gain adjusters 207 and 208, it is also possible to adjust the gain. For example, if the dynamic range of the original MR signal is small and there is no signal that will overflow even without gain adjustment, that is, if it can be processed by the provided ADC, it is possible to adjust the gain of one gain adjuster without adjusting the gain of the other.

[0066] The adjustment values ​​for the two sets of first gain adjusters 207 and second gain adjusters 208 corresponding to the two input echo signals can basically be the same, but they may be changed depending on the echo signal input to the receiver 200. For example, the dynamic range of the MR signal may differ depending on the channel of the receiving coil, in which case different adjustments may be made to the signal for each channel.

[0067] The outputs of the two gain adjusters (two first gain adjusters 207 and two second gain adjusters 208) are input to switches 203-1 and 203-2, respectively. These switches 203 are switched by a switching signal from the processing circuit 206, and the outputs of the first gain adjusters are processed by the first ADC 204, the outputs of the second gain adjusters are processed by the second ADC 205, and the outputs of these ADCs are combined after gain adjustment again in the processing circuit 206, as in the first embodiment.

[0068] Although this embodiment has been described using the example of two signals input to the receiver 200, this embodiment is also capable of processing multiple (three or more) signals, as described as a modification of the first embodiment.

[0069] According to this embodiment, flexible response is possible depending on the MR signal actually measured, and, similar to the first embodiment, it is not necessary to increase the number of ADCs beyond the number of signals input to the receiver 200, and high-precision A / D conversion that can handle signals with a wide dynamic range is possible. [Explanation of symbols]

[0070] 100...MRI device 101...Static magnetic field generator 102...Gradient coil 103...Transmitting coil 104... Receiving coil 105... Subject 106...Bed 107... Gradient magnetic field power supply 108... High-frequency generator 109...Storage device 110...Input device 111... Sequencer 200...Receiver 201...Distributor 202... Attenuator 203... Switch 204...1st ADC 205...2nd ADC 206...Processing circuit (FPGA) 207...First Gain Adjuster 208...Second Gain Adjuster 300...calculator 301...Display device

Claims

1. The system comprises a magnetic field generating unit that generates nuclear magnetic resonance in a subject, a receiving unit having a receiving coil that receives the nuclear magnetic resonance signal emitted by the subject, and a processing unit that performs processing including image reconstruction using the nuclear magnetic resonance signal received by the receiving unit. The receiving unit comprises a distributor that distributes the nuclear magnetic resonance signal into at least two signal systems, a gain adjuster that adjusts the gain of at least one of the first signal system and the second signal system, a first AD converter that converts the signal of one of the two signal systems into analog-to-digital signals either as is or after gain adjustment, a second AD converter that converts the signal of the other system into analog-to-digital signals after gain adjustment so that it is different from the signal of the first system, and a digital processor that restores the digital signal of the other system after digital conversion to its original gain and then combines it with the digital signal of the first system after digital conversion. The digital processor includes a control unit that controls the inputs to the first AD converter and the second AD converter, and the outputs of the first AD converter and the second AD converter. The magnetic resonance imaging apparatus is characterized in that the control unit performs control to switch between the timings of the analog-to-digital conversion of one system and the timings of the analog-to-digital conversion of the other system for a single nuclear magnetic resonance signal, in accordance with the number of nuclear magnetic resonance signals to be processed.

2. A magnetic resonance imaging apparatus according to claim 1, The nuclear magnetic resonance signals input to the receiving unit are multiple, The magnetic resonance imaging apparatus is characterized in that the control unit performs control so that the inputs to the first AD converter and the second AD converter, and the outputs of the first AD converter and the second AD converter, are at the same timing for each nuclear magnetic resonance signal.

3. A magnetic resonance imaging apparatus according to claim 1, A magnetic resonance imaging apparatus characterized in that the signal from the first system is directly input to the first AD converter, and the signal from the second system is input to the second AD converter after gain adjustment by the gain adjuster.

4. A magnetic resonance imaging apparatus according to claim 1, The receiving unit receives multiple nuclear magnetic resonance signals, and the distributor has multiple distributors corresponding to each of the multiple nuclear magnetic resonance signals. A magnetic resonance imaging apparatus characterized by having switches between one output of each distributor and the first AD converter, and between the other output of each distributor and the second AD converter, and by switching the switches, the signals of the first system and the signals of the second system are switched for each nuclear magnetic resonance signal to perform analog-to-digital conversion.

5. A magnetic resonance imaging apparatus according to claim 2, A magnetic resonance imaging apparatus characterized in that the plurality of nuclear magnetic resonance signals are complex signals.

6. A magnetic resonance imaging apparatus according to claim 2, The magnetic resonance imaging apparatus is characterized in that the receiving coil is a multi-channel receiving coil having a plurality of small receiving coils, and the plurality of nuclear magnetic resonance signals are nuclear magnetic resonance signals received by each of the plurality of small receiving coils.

7. A magnetic resonance imaging apparatus according to claim 1, A magnetic resonance imaging apparatus characterized in that the gain adjuster is an attenuator.

8. A magnetic resonance imaging apparatus according to claim 1, The magnetic resonance imaging apparatus is characterized in that the digital processor further comprises a gain calculation unit that calculates the gain adjustment amount of the gain adjuster.

9. A magnetic resonance imaging apparatus according to claim 8, The magnetic resonance imaging apparatus is characterized in that the gain calculation unit calculates the gain adjustment amount using nuclear magnetic resonance signals acquired during pre-scanning or imaging.

10. A method for processing nuclear magnetic resonance signals measured by a magnetic resonance imaging device, The process includes distributing the magnetic resonance signal to two systems, converting one system from analog to digital as is or after gain adjustment, converting the other system from analog to digital after gain adjustment so that it has a different gain than the first system, returning it to its original gain, and then combining the digital signals of the first system and the second system. A signal processing method characterized by controlling the switching of analog-to-digital conversion timings in one system and the other system for a single nuclear magnetic resonance signal, while synchronizing these timings according to the number of nuclear magnetic resonance signals to be processed.

Images