JP4664605B2 - Magnetic resonance imaging system - Google Patents

Description

The present invention relates to a magnetic resonance imaging apparatus that captures an image of a subject using a nuclear magnetic resonance signal, and in particular, a magnetism that corrects non-uniformity in signal values of image data caused by sensitivity distribution of each receiving coil. The present invention relates to a resonance imaging apparatus .

  Conventionally, a magnetic resonance imaging (MRI) apparatus 1 as shown in FIG. 9 is used as a monitoring apparatus in a medical field (see, for example, Patent Document 1).

  The magnetic resonance imaging apparatus 1 includes an X axis in each of the gradient magnetic field coils 3x, 3y, and 3z of the gradient magnetic field coil unit 3 in an imaging region of a subject P set inside a cylindrical static magnetic field magnet 2 that forms a static magnetic field. Excites the spins in the subject P magnetically by forming a gradient magnetic field in the Y-axis and Z-axis directions and transmitting a radio frequency (RF) signal of Larmor frequency from an RF (Radio Frequency) coil 4 Is an apparatus for reconstructing an image of the subject P using a nuclear magnetic resonance (NMR) signal generated by the above-described process.

  That is, a static magnetic field is previously formed in the static magnetic field magnet 2 by the static magnetic field power source 5. Further, in response to a command from the input device 6, the sequence controller control means 7a gives a sequence, which is signal control information, to the sequence controller 8, and the sequence controller 8 is connected to each gradient magnetic field coil 3x, 3y, 3z according to the sequence. The transmitter 10 which gives a high frequency signal to the magnetic field power supply 9 and the RF coil 4 is controlled. For this reason, a gradient magnetic field is formed in the imaging region, and a high-frequency signal is transmitted to the subject P.

  At this time, the X-axis gradient magnetic field, the Y-axis gradient magnetic field, and the Z-axis gradient magnetic field formed by the gradient magnetic field coils 3x, 3y, and 3z are mainly a phase encoding (PE) gradient magnetic field, a readout (RO: readout). The gradient magnetic field is used as a gradient magnetic field and a slice encoding (SE) gradient magnetic field. For this reason, the X coordinate, Y coordinate, and Z coordinate, which are nuclear position information, are converted into the nuclear spin phase, frequency, and slice position, respectively, and the sequence is repeatedly executed while changing the phase encoding amount.

  Then, the NMR signal generated along with the excitation of the nuclear spin in the subject P is received by the RF coil 4 and given to the receiver 11 to be converted into digitized raw data. Further, the raw data is taken into the sequence controller control means 7a via the sequence controller 8, and the sequence controller control means 7a arranges the raw data in the K space (Fourier space) formed in the raw data database 7b. Then, the image reconstruction unit 7c performs Fourier transform on the raw data arranged in the K space, whereby reconstructed image data of the subject P is obtained and stored in the image data database 7d. Further, image data is appropriately given to the display device 7f by the image display means 7e and displayed.

  In such a magnetic resonance imaging apparatus 1, the RF coil 4 has a whole-body (WB) coil for transmission and a phased-array coil as a main coil for reception in order to increase the imaging speed. (For example, refer nonpatent literature 1). Since the phased array coil includes a plurality of surface coils, the imaging time can be shortened by receiving NMR signals simultaneously by each surface coil and collecting more raw data in a short time.

  However, if the RF coil 4 is composed of a phased array coil or a WB coil, the signal intensity of the image data obtained by the reconstruction process is also non-uniform depending on the non-uniformity of the sensitivity of the phased array coil or the WB coil. Sex occurs. In general, the sensitivity non-uniformity of the WB coil is sufficiently small to be negligible, but the non-uniformity of the sensitivity of the surface coil in the phased array coil as the purpose-specific coil is large and affects the image data.

  For this reason, it is necessary to correct the non-uniformity in the signal intensity of the image data due to the non-uniform sensitivity of the phased array coil.

Therefore, conventionally, a sensitivity pre-scan is performed prior to the main scan for generating an image of the subject P. Then, image data is acquired from the phased array coil and the WB coil by sensitivity pre-scanning, and the phased array is based on the signal intensity ratio (S _PAC / S _WB ) that is a divided value of the signal intensity S _PAC and S _WB of each image data. The sensitivity distribution of the coil is estimated as three-dimensional sensitivity map data, and the signal intensity unevenness of the image data is corrected using the obtained three-dimensional sensitivity map data of the phased array coil.

  That is, as shown in the flowchart of FIG. 10, in step S1, the sensitivity pre-scan execution means 7g gives the sequence for sensitivity estimation to the sequence controller control means 7a, and the sensitivity pre-scan is performed with the phased array coil and the WB coil as reception coils. Executed. Then, the WB coil image data obtained by the WB coil and the main coil image data obtained by the phased array coil are acquired as image data for estimating the sensitivity distribution of the phased array coil, and the WB coil image database 7h and the main coil are obtained, respectively. It is stored in the image database 7i. For this reason, imaging of volume data, which is three-dimensional image data, is performed twice.

Next, in step S2, the sensitivity distribution estimation means 7j obtains an estimated value of the sensitivity distribution of the phased array coil. That is, the signal intensity S _PAC of the main coil image data as shown in FIG. 11A is divided by the division processing means 7k by the signal intensity S _WB of the WB coil image data as shown in FIG. The signal intensity ratio (S _PAC / S _WB ) between the main coil image data and the WB coil image data as shown in FIG. 11C is obtained as an estimated value of the sensitivity distribution of the phased array coil.

At this time, it is necessary to prevent the division processing from being performed on an area where the signal strengths S _PAC and S _{WB of} the main coil image data and the WB coil image data are less than a threshold value, for example, less than 10% of the maximum value. Accordingly, the threshold processing of the signal strengths S _PAC and S _WB is performed by the threshold processing means 71 as pre-processing of the division processing, and the signal strengths S _PAC and S _WB in the region below the threshold are masked.

  The division processing with this threshold processing as preprocessing cancels the influence on the signal intensity of the image data due to factors other than non-uniformity of the sensitivity distribution of the phased array coil, such as image contrast, etc. Sensitivity distribution can be estimated.

  Next, the interpolation means 7m performs interpolation or extrapolation on the no-signal area, which is a data missing portion caused by threshold processing due to the presence of a lung field or the like, and the sensitivity distribution is estimated. Further, fitting processing and smoothing processing are performed. Is implemented by the smoothing processing means 7n over the entire two-dimensional region to obtain an estimated value curve of the sensitivity distribution shown in FIG.

  Similar image data processing is performed over each cross section of the entire three-dimensional region, and an estimated value of the sensitivity distribution is obtained as volume data.

  Next, in step S3, the estimated value of the sensitivity distribution of the phased array coil is stored in the sensitivity map database 7o as three-dimensional sensitivity map data.

  Next, in step S4, the image acquisition sequence is given to the sequence controller control means 7a by the main scan execution means 7p, and the main scan is executed with the phased array coil as the reception coil. Then, raw data is collected and image data is obtained by image reconstruction processing of the image reconstruction means 7c.

  Next, in step S5, the image data correction means 7q corresponds from the sensitivity map database 7o according to various conditions such as the imaging section direction in the main scan, imaging conditions such as spatial resolution, data collection conditions, and image reconstruction conditions. Cut out 3D sensitivity map data.

  In step S6, the image data correction unit 7q corrects the image data using the cut-out three-dimensional sensitivity map data. For this reason, the nonuniformity of the signal intensity of image data is improved.

  On the other hand, the sensitivity distribution of the phased array coil is estimated as sensitivity map data by post-processing from the image data itself obtained from the NMR signal received by the phased array coil, and an image is obtained using the obtained sensitivity map data of the phased array coil. A method for correcting the signal intensity unevenness of data is also used. For example, there is a method of creating image data of extremely low frequency components by executing smoothing processing on image data obtained by a phased array coil and substituting it as a sensitivity distribution.

In addition, a technique for correcting signal intensity unevenness of image data obtained from an NMR signal received by a phased array coil with reference to the signal intensity of a high-frequency signal transmitted from a WB coil (see, for example, Patent Document 2), A technique is proposed for correcting signal intensity unevenness in image data using the stored sensitivity distribution of the phased array coil and position information of the phased array coil obtained by estimation from the image data (see, for example, Patent Document 3).
Japanese Patent No. 3135592 JP 63-132645 A Japanese Unexamined Patent Publication No. 7-59750 Roemer PB, et al. The NMR Phased Array, MRM 16, 192-225 (1990)

The sensitivity distribution of the phased array coil is calculated based on the signal intensity values Sig _PAC and Sig _WB of the image data obtained from the phased array coil and the WB coil by the sensitivity pre-scan, respectively (Sig _PAC / Sig _WB ). In the method of estimating and correcting the signal intensity of image data, there is a problem that the time required for sensitivity pre-scan increases. For this reason, for example, when taking an image of the abdomen of the subject P, the breath-holding time becomes longer depending on the execution time of the sensitivity pre-scan. Also, the collection of image data using the phased array coil and the WB coil are used. There is a possibility that a positional shift (misregistration) may occur in the subject P due to the movement of the subject P during the collection of the image data. Furthermore, in order to acquire image data from both the phased array coil and the WB coil, it is necessary that the decoupling between the phased array coil and the WB coil is complete.

  On the other hand, in the method of correcting the signal intensity unevenness of the image data using the sensitivity distribution of the phased array coil estimated by post-processing from the image data itself by the phased array coil, the estimation accuracy of the sensitivity distribution of the phased array coil is low. The correction of the signal intensity of the image data becomes insufficient, and the uniformity of the corrected image data finally obtained cannot be obtained sufficiently.

  There is also a problem that it is difficult to always perform correction with sufficient accuracy for various image types. For example, when the image data is image data having a desired contrast, such as T1-weighted or T2-weighted image data, even if the sensitivity distribution is estimated by performing the smoothing process, the image data indicating the sensitivity distribution is also present. Therefore, it is impossible to substitute the image data after the smoothing process as the sensitivity distribution.

  Furthermore, it is difficult to consistently correct image data in all slices in multi-slice imaging.

The present invention has been made in order to cope with such a conventional situation, and in a shorter time, image data due to non-uniformity of the sensitivity distribution of the receiving coil with good accuracy without depending on the imaging conditions such as the image type. It is an object of the present invention to provide a magnetic resonance imaging apparatus capable of correcting the signal intensity unevenness.

A magnetic resonance imaging apparatus according to an embodiment of the present invention uses the phased array coil as a reception coil prior to a main scan for generating image data by a sequence including at least a phased array coil as a reception coil. Means for executing sensitivity pre-scan for generating sensitivity correction data for the receiving coil in the main scan; and the sensitivity correction data is obtained using only data acquired by the phased array coil in the sensitivity pre-scan as original data. means for generating, possess means for generating the image data by executing the main scan, and means for correcting, based the image data obtained by the main scan to the sensitivity correction data, the sensitivity press Proton as shooting conditions It is for setting an imaging condition at the time of photographing a degree enhanced image.

In the magnetic resonance imaging apparatus according to the present invention, the signal intensity unevenness of the image data due to the nonuniformity of the sensitivity distribution of the receiving coil can be corrected with good accuracy in a shorter time without depending on the imaging conditions such as the image type. be able to.

Embodiments of a magnetic resonance imaging apparatus according to the present invention will be described with reference to the accompanying drawings.

  FIG. 1 is a functional block diagram showing a first embodiment of a magnetic resonance imaging apparatus according to the present invention.

  The magnetic resonance imaging apparatus 20 does not show a cylindrical static magnetic field magnet 21 that forms a static magnetic field, and a shim coil 22, a gradient magnetic field coil unit 23, and an RF coil 24 provided inside the static magnetic field magnet 21. It is built in the gantry.

  In addition, the magnetic resonance imaging apparatus 20 includes a control system 25. The control system 25 includes a static magnetic field power supply 26, a gradient magnetic field power supply 27, a shim coil power supply 28, a transmitter 29, a receiver 30, a sequence controller 31, and a computer 32. The computer 32 is provided with an input device 33 and a display device 34.

  The gradient magnetic field power source 27 of the control system 25 includes an X-axis gradient magnetic field power source 27x, a Y-axis gradient magnetic field power source 27y, and a Z-axis gradient magnetic field power source 27z. The computer 32 includes an arithmetic device and a storage device (not shown), and an input device 33 and a display device 34 are provided.

  The static magnetic field magnet 21 is connected to a static magnetic field power supply 26 and has a function of forming a static magnetic field in the imaging region by a current supplied from the static magnetic field power supply 26. A cylindrical shim coil 22 is coaxially provided inside the static magnetic field magnet 21. The shim coil 22 is connected to the shim coil power supply 28, and is configured such that a current is supplied from the shim coil power supply 28 to the shim coil 22 to make the static magnetic field uniform.

  The gradient magnetic field coil unit 23 includes an X-axis gradient magnetic field coil 23 x, a Y-axis gradient magnetic field coil 23 y, and a Z-axis gradient magnetic field coil 23 z, and is formed in a cylindrical shape inside the static magnetic field magnet 21. A bed 35 is provided inside the gradient magnetic field coil unit 23 as an imaging region, and the subject P is set on the bed 35. The RF coil 24 may not be built in the gantry but may be provided near the bed 35 or the subject P.

  The gradient magnetic field coil unit 23 is connected to a gradient magnetic field power supply 27. The X axis gradient magnetic field coil 23x, the Y axis gradient magnetic field coil 23y, and the Z axis gradient magnetic field coil 23z of the gradient magnetic field coil unit 23 are respectively an X axis gradient magnetic field power source 27x, a Y axis gradient magnetic field power source 27y, and a Z axis. It is connected to the gradient magnetic field power supply 27z.

  The X-axis gradient magnetic field power source 27x, the Y-axis gradient magnetic field power source 27y, and the Z-axis gradient magnetic field power source 27z are supplied with currents supplied to the X-axis gradient magnetic field coil 23x, the Y-axis gradient magnetic field coil 23y, and the Z-axis gradient magnetic field coil 23z, respectively. In the imaging region, a gradient magnetic field Gx in the X-axis direction, a gradient magnetic field Gy in the Y-axis direction, and a gradient magnetic field Gz in the Z-axis direction can be formed, respectively.

  The RF coil 24 is connected to the transmitter 29 and the receiver 30. The RF coil 24 receives the high frequency signal from the transmitter 29 and transmits it to the subject P, and receives the NMR signal generated by the excitation by the high frequency signal of the nuclear spin inside the subject P and receives it to the receiver 30. Has the function to give.

  FIG. 2 is a detailed configuration diagram showing an example of the RF coil 24 and the receiver 30 shown in FIG.

  The RF coil 24 includes a WB coil 24a for high-frequency signal transmission and a phased array coil 24b as a main coil that is a coil for receiving NMR signals in the main scan. The phased array coil 24b includes a plurality of surface coils 24c.

  On the other hand, the receiver 30 includes a plurality of reception system circuits 30a. Each surface coil 24 c is individually connected to each reception system circuit 30 a of the receiver 30, and the WB coil 24 a is connected to the transmitter 29. However, the WB coil 24 a may be connected to the reception system circuit 30 a of the receiver 30.

  FIG. 3 is a schematic cross-sectional view showing an arrangement example of the WB coil 24a and the phased array coil 24b shown in FIG.

  For example, the surface coils 24c of the phased array coil 24b are arranged symmetrically around the Z axis that is around the cross section L including the specific region of interest of the subject P. Further, a WB coil 24a is provided outside the phased array coil 24b. The RF coil 24 transmits a high-frequency signal to the subject P by the WB coil 24a, and receives NMR signals from the cross section L including the specific region of interest in multiple channels by each surface coil 24c of the phased array coil 24b. Each receiver 30 can be provided.

  On the other hand, the sequence controller 31 of the control system 25 is connected to the gradient magnetic field power source 27, the transmitter 29, and the receiver 30. The sequence controller 31 has control information necessary for driving the gradient magnetic field power supply 27, the transmitter 29, and the receiver 30, for example, operation control information such as the intensity, application time, and application timing of the pulse current to be applied to the gradient magnetic field power supply 27. And the gradient magnetic field power source 27, the transmitter 29, and the receiver 30 are driven according to the stored predetermined sequence to drive the X-axis gradient magnetic field Gx, the Y-axis gradient magnetic field Gy, and the Z-axis gradient magnetic field. It has the function of generating Gz and high frequency signals.

  In addition, the sequence controller 31 is configured to receive raw data, which is a digitized NMR signal, from the receiver 30 and to supply the raw data to the computer 32.

  For this reason, the transmitter 29 is provided with a function of supplying a high frequency signal to the RF coil 24 based on the control information received from the sequence controller 31, while the receiver 30 is required for the NMR signal received from the RF coil 24. A function for generating raw data that is a digitized NMR signal and a function for giving the generated raw data to the sequence controller 31 by performing the signal processing and A / D conversion are provided.

  In addition, the computer 32 reads and executes a program to thereby execute a sensitivity pre-scan execution unit 36, a sensitivity pre-scan condition setting unit 37, a main scan execution unit 38, a sequence controller control unit 39, a raw data database 40, an image. It functions as a reconstruction means 41, an image data database 42, a main coil image database 43, a sensitivity distribution estimation means 44, a sensitivity map database 45, an image data correction means 46, and an image display means 47. However, the computer 32 may be configured by providing a specific circuit regardless of the program.

  The sensitivity prescan execution means 36 obtains a sequence (sensitivity estimation sequence) for executing the sensitivity prescan for obtaining the three-dimensional sensitivity map data which is the sensitivity distribution of the phased array coil 24 b from the sensitivity prescan condition setting means 37. It has a function of generating based on the received imaging conditions and a function of executing the sensitivity pre-scan by giving the generated sensitivity estimation sequence to the sequence controller control means 39.

  The sensitivity pre-scan condition setting means 37 has a function for setting the photographing condition in the sensitivity pre-scan so that the contrast is sufficiently low for obtaining the three-dimensional sensitivity map data, and the sensitivity pre-scan photographing condition is set to the sensitivity pre-scan. A function to be given to the execution means 36.

  The main scan execution unit 38 has a function of executing a main scan by giving various sequences to the sequence controller control unit 39 when executing a main scan for acquiring image data.

  The sequence controller control means 39 gives the sequence controller 31 a required sequence among the sequences received from the sensitivity pre-scan execution means 36 and the main scan execution means 38 based on information from the input device 33 or other components. Therefore, it has a function to execute sensitivity pre-scan or main scan. Further, the sequence controller control means 39 receives the raw data of each surface coil 24c of the phased array coil 24b collected from the sequence controller 31 by executing the sensitivity pre-scan or the main scan and is formed in the raw data database 40. (Fourier space).

  For this reason, each raw data for each surface coil 24 c generated in the receiver 30 is stored in the raw data database 40. That is, raw data is arranged in the K space formed in the raw data database 40.

  The image reconstruction unit 41 performs image reconstruction processing such as Fourier transform (FT) on the raw data arranged in the K space of the raw data database 40 by executing the main scan, thereby obtaining the image data of the subject P. A function of reconstructing, and a function of writing the reconstructed image data into the image data database.

  Further, the image reconstruction unit 41 uses a technique equivalent to the reconstruction process for the raw data obtained by executing the main scan on the raw data arranged in the K space of the raw data database 40 by performing the sensitivity pre-scan. It has a function of reconstructing image data of the subject P as main coil image data by performing reconstruction processing, and a function of writing the reconstructed main coil image data in the main coil image database 43.

  The sensitivity distribution estimation means 44 uses the main coil image data stored in the main coil image database 43 as sensitivity estimation data, which is the original data of the sensitivity correction data, so that the three-dimensional sensitivity map data of the phased array coil 24b is obtained. A function for creating sensitivity correction data and a function for writing the created three-dimensional sensitivity map data to the sensitivity map database 45 are provided. Therefore, the sensitivity distribution estimation unit 44 includes a threshold processing unit 44a, a region reduction unit 44b, an interpolation unit 44c, and a smoothing processing unit 44d.

  The threshold processing unit 44a has a function of performing threshold processing on the main coil image data, that is, a function of masking data in a portion where the signal intensity of the main coil image data is equal to or less than a preset threshold.

  The area reduction means 44b has a function of reducing the area of the main coil image data used for sensitivity distribution estimation and excluding a portion having a small signal intensity in the vicinity of the mask area from the data for creating the three-dimensional sensitivity map data.

  The interpolation means 44c interpolates the main coil image data for sensitivity distribution estimation by estimating and extrapolating or interpolating the three-dimensional sensitivity map data in the masked no-signal area after the area reduction processing of the main coil image data. It has the function to do.

  The smoothing processing unit 44d has a function of creating final three-dimensional sensitivity map data by performing smoothing processing on the main coil image data for sensitivity distribution estimation.

  The image data correction means 46 obtains 3D sensitivity map data according to image data acquisition conditions such as imaging conditions, data collection conditions, and image reconstruction conditions in the main scan from the 3D sensitivity map data stored in the sensitivity map database 45. It has a function of cutting out and extracting, and a function of correcting the signal intensity of the image data stored in the image data database 42 by executing the main scan using the extracted three-dimensional sensitivity map data.

  The image display means 47 has a function of giving the image data stored in the image data database 42 to the display device 34 for display.

  The magnetic resonance imaging apparatus 20 configured as described above has a sensitivity pre-scan for generating sensitivity correction data for the reception coil in the main scan image capturing as a receiving coil in the image capturing. It functions as means for executing as a receiving coil and means for generating sensitivity correction data using only the data acquired by the receiving coil in image capturing in sensitivity pre-scan as original data.

  FIG. 4 is a flowchart showing a procedure when a tomographic image of the subject P is picked up by the magnetic resonance imaging apparatus 20 shown in FIG. 1, and the reference numerals with numerals in the figure indicate the steps of the flowchart.

  First, in step S10, sensitivity pre-scan is executed. Therefore, the sensitivity pre-scan condition setting unit 37 sets the imaging condition in the sensitivity pre-scan and gives a sensitivity estimation sequence to the sensitivity pre-scan execution unit 36. Here, the imaging conditions in the sensitivity pre-scan are set so that the image obtained by reconstructing to create the three-dimensional sensitivity map data has a sufficiently low contrast.

  As the imaging conditions for low contrast, for example, T1 (longitudinal relaxation time) and T2 (lateral relaxation time) can be set by increasing the repetition time (TR) and shortening the echo time (TE). It is possible to set conditions for photographing a proton density weighted image with little influence or photographing conditions close to this condition.

  As a sensitivity estimation sequence for executing sensitivity pre-scan, for example, a fast field echo (FFE) sequence in which TE is set to a short value of about 1 to 5 ms and a flip angle is reduced to about 5 to 10 degrees is used. Use it. Furthermore, if TR is set to about 200 ms, it is possible to collect main coil image data in 20 or more slices, and a volume reflecting the sensitivity of the surface coil 24c necessary for creating three-dimensional sensitivity map data. The entire main coil image data can be acquired.

  On the other hand, imaging in the sensitivity pre-scan may be 3D imaging instead of 2D multi-slice. As the sensitivity estimation sequence for 3D imaging, for example, an FFE sequence in which TE is set to a short value of about 1-5 ms and a flip angle is reduced to about 5 degrees or less can be used. Furthermore, by setting TR to about 10 ms, necessary main coil image data can be acquired in an imaging time equivalent to 2D imaging.

  Then, the sensitivity pre-scan execution means 36 gives the sequence for sensitivity estimation to the sequence controller control means 39, and the sensitivity pre-scan is executed using only the phased array coil 24b as the reception coil (main coil) in the main scan as the reception coil. Is done. That is, prior to the main scan for acquiring image data, a sensitivity pre-scan for obtaining sensitivity map data of the phased array coil 24b is executed.

  That is, the subject P is set on the bed 35 in advance, and a current is supplied from the static magnetic field power source 26 to the static magnetic field magnet 21 to form a static magnetic field in the imaging region. Further, a current is supplied from the shim coil power supply 28 to the shim coil 22, and the static magnetic field formed in the imaging region is made uniform.

  Next, an operation command is given from the input device 33 to the sequence controller control means 39. Therefore, the sequence controller control means 39 gives a sequence for sensitivity estimation to the sequence controller 31. The sequence controller 31 drives the gradient magnetic field power source 27, the transmitter 29, and the receiver 30 in accordance with the sensitivity estimation sequence to thereby set an X-axis gradient magnetic field Gx and Y-axis gradient magnetic fields Gy, Z in the imaging region where the subject P is set. An axial gradient magnetic field Gz is formed and a high frequency signal is generated.

  At this time, the X-axis gradient magnetic field Gx, the Y-axis gradient magnetic field Gy, and the Z-axis gradient magnetic field Gz formed by the gradient coil are mainly a phase encode (PE) gradient magnetic field, a read (RO) gradient magnetic field, and a slice encode ( Used as a gradient magnetic field for SE). For this reason, regularity appears in the spin rotation direction of the nuclei inside the subject P, and the X and Y coordinates, which are two-dimensional position information in the slice formed in the Z-axis direction by the SE gradient magnetic field, are PE The phase change amount and the frequency change amount of the spin of the nucleus inside the subject P are respectively converted by the gradient magnetic field for RO and the gradient magnetic field for RO.

  A high frequency signal is given from the transmitter 29 to the WB coil 24a of the RF coil 24 in accordance with the sensitivity estimation sequence, and the high frequency signal is transmitted from the WB coil 24a to the subject P. Further, the NMR signal generated by nuclear magnetic resonance of the nuclei contained in the slice corresponding to the frequency of the high frequency signal inside the subject P is increased by each surface coil 24 c of the phased array coil 24 b which is the main coil of the RF coil 24. It is received on the channel and given to each receiver 30.

  The WB coil 24a is used only for transmitting a high-frequency signal and is not used for receiving.

  Each receiver 30 receives the NMR signal from each surface coil 24c of the phased array coil 24b, and executes various signal processing such as preamplification, intermediate frequency conversion, phase detection, low frequency amplification, and filtering. Further, each receiver 30 A / D converts the NMR signal to generate raw data which is an NMR signal of digital data. The receiver 30 gives the generated raw data to the sequence controller 31.

  The sequence controller 31 gives the raw data received from the receiver 30 to the sequence controller control means 39, and the sequence controller control means 39 arranges the raw data in the K space formed in the raw data database 40. Further, the image reconstructing means 41 performs the Fourier transform (FT) on the raw data arranged in the K space of the raw data database 40, thereby converting the volume data, which is the three-dimensional image data of the subject P, into the main coil. Reconstructed as image data and written in the main coil image database 43 as sensitivity estimation data for obtaining three-dimensional sensitivity map data of the phased array coil 24b.

  At this time, the reconstruction processing method of the raw data collected by the sensitivity pre-scan is the same method as the reconstruction processing method in the main scan. As a reconstruction processing method using the phased array coil 24b in this scan, Sum of Square processing (SoS processing) that takes the square root of the square sum of the image data obtained by each surface coil 24c, or each surface coil 24c is used. There is a reconstruction processing method that takes the sum of the absolute values of the signal intensities of the obtained image data.

  Next, in step S11, the sensitivity distribution estimation means 44 estimates the sensitivity distribution by using the main coil image data, which is volume data stored in the main coil image database 43, as sensitivity estimation data.

  FIG. 5 is a flowchart showing an example of a detailed procedure for estimating the sensitivity distribution of the phased array coil 24b in the flowchart shown in FIG. 4, and the reference numerals with numerals in the figure indicate each step of the flowchart.

  First, in step S 20, main coil image data is read from the main coil image database 43 into the sensitivity distribution estimation means 44.

  FIG. 6 is a diagram comparing the signal intensity distribution of main coil image data obtained under imaging conditions with low contrast and the signal intensity distribution of main coil image data obtained under imaging conditions with general contrast. .

  6A and 6B, the vertical axis indicates the signal value of the main coil image data, and the horizontal axis indicates the position in the cross-sectional L direction including the region of interest (ROI) in FIG. FIG. 6A shows the signal intensity distribution of the main coil image data obtained under the imaging conditions with low contrast, and FIG. 6B shows the main coil image obtained with the imaging conditions with general contrast. The signal intensity distribution of data is shown.

  As shown in FIG. 6B, in the main coil image data obtained under the imaging conditions with general contrast, the influence due to the difference in contrast is not sufficiently small, and is used as it is as data for estimating the sensitivity of the phased array coil 24b. Is difficult.

  On the other hand, as shown in FIG. 6 (a), the main coil image data obtained under the imaging conditions with low contrast has a sufficiently small influence due to the difference in contrast, and is used as it is as data for estimating the sensitivity of the phased array coil 24b. However, the error can be reduced.

  Here, the main coil image data is shown as one-dimensional data on the straight line L including the ROI. However, in practice, two-dimensional or three-dimensional imaging is performed to obtain two-dimensional or three-dimensional main coil image data. It is used as sensitivity estimation data and is subject to various processes for creating three-dimensional sensitivity map data.

  Therefore, the main coil image data obtained under the imaging conditions with low contrast is used as sensitivity estimation data, and various processes for creating three-dimensional sensitivity map data are performed.

  FIG. 7 illustrates the comparison between the data generated when the magnetic resonance imaging apparatus 20 generates the three-dimensional sensitivity map data and the data generated when the conventional magnetic resonance imaging apparatus 1 generates the three-dimensional sensitivity map data. It is a figure to do.

  FIG. 7A is a diagram showing data generated when three-dimensional sensitivity map data is created by the conventional magnetic resonance imaging apparatus 1. In the conventional magnetic resonance imaging apparatus 1, since the sensitivity pre-scan is executed using both the phased array coil 24b and the WB coil 24a as receiving coils, the main coil obtained by the phased array coil 24b shown in (a-1). Image data and WB coil image data obtained by the WB coil 24a shown in (a-2) are acquired, and both data are used as sensitivity estimation data. Then, both the main coil image data and the WB coil image data are subjected to threshold processing by comparison with thresholds ε and ε ′ set in advance, and the non-signal region portion is excluded from the sensitivity estimation data.

  Further, in the sensitivity estimation data area D1 after the threshold processing, the main coil image data is divided by the WB coil image data, thereby generating non-dimensional three-dimensional sensitivity map data shown in (a-3). Then, the three-dimensional sensitivity map data is estimated by interpolation processing such as extrapolation and interpolation over the entire region, and the three-dimensional sensitivity map data shown in (a-4) is created.

  On the other hand, in the magnetic resonance imaging apparatus 20 shown in FIG. 1, sensitivity pre-scanning is executed using only the phased array coil 24b as a receiving coil.

  FIG. 7 (b) is a diagram for explaining the problems in creating the three-dimensional sensitivity map data by the magnetic resonance imaging apparatus 20 shown in FIG. 1, and (c) avoids the problems shown in (b). It is a figure which shows the data produced | generated in the case of 3D sensitivity map data creation by the magnetic resonance imaging apparatus 20 in a procedure.

  That is, in step S21 in FIG. 5, threshold processing is performed on the main coil image data by the threshold processing means 44a. That is, as shown in FIGS. 7 (b-1) and (c-1), the data of the portion where the signal intensity of the main coil image data is less than or equal to the preset threshold value ε is masked to mask the area outside the subject P or the lungs. The portion of the no-signal area from the field is excluded from the sensitivity estimation data.

  The low-contrast main coil image data such as the proton density enhanced image can be used as sensitivity estimation data as it is without being divided by the WB coil image data in the sensitivity estimation data area D1 after threshold processing. Only the processing does not cancel the influence of the low signal intensity portion outside the region of the subject P or in the vicinity of the non-signal region from the lung field as shown in (b-2), and the value of the sensitivity estimation data is also small. Therefore, it becomes difficult to create more accurate three-dimensional sensitivity map data.

  Therefore, in step S22, the area reduction means 44b performs the area reduction process used as the sensitivity estimation data. That is, as shown in FIG. 7 (c-2), the sensitivity estimation data area D1 generally has a phenomenon in which the signal intensity becomes smaller than the other parts in the vicinity of the boundary with the mask area. Due to the reduction processing of the region D1, the region of the edge portion where the signal intensity is reduced is excluded.

  In step S23, as shown in FIG. 7C-3, the signal intensity of the main coil image data in the new sensitivity estimation data region D2 after the region reduction processing is regarded as three-dimensional sensitivity map data. Further, the three-dimensional sensitivity map data in the masked no-signal area after the area reduction processing is estimated by interpolation processing such as extrapolation or interpolation by the interpolation means 44c, and the area as shown in FIG. The entire 3D sensitivity map data is created.

  Next, in step S24, smoothing processing is performed by performing fitting such as orthogonal function expansion on the three-dimensional sensitivity map data over the entire region after interpolation. As a result, final three-dimensional sensitivity map data having more continuity is created.

  4, the three-dimensional sensitivity map data of the phased array coil 24b is stored in the sensitivity map database 45.

  Next, in step S13, the main scan execution means 38 gives an image acquisition sequence to the sequence controller control means 39, and the main scan is executed using the phased array coil 24b as a reception coil. Then, raw data is collected and image data is obtained by image reconstruction processing of the image reconstruction means 41.

  Next, in step S14, the image data correction means 46 responds from the sensitivity map database 45 in accordance with various conditions such as the imaging section direction in the main scan, imaging conditions such as spatial resolution, data collection conditions, and image reconstruction conditions. Cut out 3D sensitivity map data.

  In step S15, the image data correction unit 46 corrects the image data using the cut-out three-dimensional sensitivity map data. That is, a correction process for multiplying each signal intensity of the image data by the reciprocal of the three-dimensional sensitivity map data is executed. At this time, general error processing for correcting image data such as processing for setting the three-dimensional sensitivity map data to non-zero or case-by-case processing when the three-dimensional sensitivity map data is zero is appropriately performed.

  As a result, it is possible to obtain image data in which the influence of the nonuniformity of the signal intensity due to the sensitivity variation of the phased array coil 24b is suppressed and the image quality is improved.

  According to the magnetic resonance imaging apparatus 20 as described above, even when the sensitivity of the receiving coil varies as in the case of capturing an image using the phased array coil 24b formed of a plurality of surface coils, Since the WB coil 24a is not used as a receiving coil in the sensitivity pre-scan, it is possible to correct the signal intensity unevenness of the image data in a shorter time without depending on the imaging conditions such as the image type. In addition, it is possible to easily obtain an image with high diagnostic ability over the entire field of view with sufficient accuracy.

  Further, for example, under the above-described TR200 ms imaging condition, in the case of imaging with a 48 × 48 matrix, conventionally, an imaging time of 19.2 seconds is required to acquire data of both the WB coil 24a and the phased array coil 24b in the sensitivity prescan. What is necessary is that the magnetic resonance imaging apparatus 20 can acquire all the main coil image data necessary for creating the three-dimensional sensitivity map data in half 9.6 seconds of breath-hold imaging, The burden on the patient can be reduced by halving the imaging time.

  Further, since this method does not require photographing with the WB coil 24a, it can be executed even if the decoupling between the WB coil 24a and the phased array coil 24b is insufficient, and data between the WB coil 24a and the phased array coil 24b. It is also possible to prevent the occurrence of errors such as misalignment.

  FIG. 8 is a functional block diagram showing a second embodiment of the magnetic resonance imaging apparatus according to the present invention.

  The magnetic resonance imaging apparatus 20A shown in FIG. 8 is different from the magnetic resonance imaging apparatus 20 shown in FIG. 1 in that the computer 32 also functions as shimming imaging condition setting means 50. Since other configurations and operations are not substantially different from those of the magnetic resonance imaging apparatus 20 shown in FIG. 1, the same components are denoted by the same reference numerals and description thereof is omitted.

  The computer 32 of the magnetic resonance imaging apparatus 20A also functions as shimming imaging condition setting means 50. The shimming imaging condition setting means 50 has a function of setting imaging conditions for shimming performed to correct spatial nonuniformity of the static magnetic field and supplying the imaging conditions to the sensitivity prescan execution means 36. For this reason, the sensitivity pre-scan execution unit 36 is configured to generate a sequence based on imaging conditions for simultaneously performing the sensitivity pre-scan and shimming and give the sequence to the sequence controller control unit 39.

  In the magnetic resonance imaging apparatus 20A, shimming may be performed simultaneously with the execution of the sensitivity prescan. As an imaging sequence at this time, for example, TE can be set to 2 echoes of 4.5 ms / 9.0 ms by FFE, and the flip angle can be set to about 5 to 10 degrees. Then, the magnetic field distribution is obtained from the signal phase difference between the two echoes, shimming is performed, and data of 4.6 ms can be used for sensitivity data estimation.

  For this reason, according to the magnetic resonance imaging apparatus 20A, in addition to the effects of the magnetic resonance imaging apparatus 20, more efficient imaging can be performed.

  In the magnetic resonance imaging apparatuses 20 and 20A, not only the phased array coil 24b but also various coils for various purposes such as a head coil, various array coils, and a surface coil are used as receiving coils in the main scan, that is, the main coil of the RF coil 24. It can be a coil. Further, the RF coil 24 or the main coil may be constituted by a single coil.

  For this reason, the sensitivity map data of the WB coil 24a can be created using the WB coil 24a itself. Although the sensitivity unevenness of the WB coil 24a is smaller than that of the phased array coil 24b, there is a high possibility that the sensitivity unevenness cannot be ignored even in the WB coil 24a when the apparatus is downsized in the future. Therefore, if only the WB coil 24a is used as a receiving coil in the sensitivity pre-scan and the sensitivity map data of the WB coil 24a is created, the apparatus can be easily downsized.

  On the other hand, if the sensitivity map data is generated only from the image data obtained by the receiving coil in the main scan as the original data, a coil other than the receiving coil in the main scan is used as the receiving coil in the sensitivity prescan. May be.

1 is a functional block diagram showing a first embodiment of a magnetic resonance imaging apparatus according to the present invention. The detailed block diagram which shows an example of the RF coil and receiver which are shown in FIG. The cross-sectional schematic diagram which shows the example of arrangement | positioning of the WB coil and phased array coil which are shown in FIG. The flowchart which shows the procedure at the time of imaging the tomographic image of a subject with the magnetic resonance imaging apparatus shown in FIG. The flowchart which shows an example of the detailed procedure at the time of estimating the sensitivity distribution of a phased array coil in the flowchart shown in FIG. The figure which compared the signal intensity distribution of the main coil image data obtained on the imaging conditions used as low contrast with the signal intensity distribution of the main coil image data obtained on the imaging conditions used as general contrast. The figure explaining comparing the data produced | generated at the time of 3D sensitivity map data creation with a magnetic resonance imaging apparatus with the data produced at the time of 3D sensitivity map data creation with the conventional magnetic resonance imaging apparatus. The functional block diagram which shows 2nd Embodiment of the magnetic resonance imaging apparatus which concerns on this invention. The functional block diagram of the conventional magnetic resonance imaging apparatus. 10 is a flowchart showing a procedure for correcting signal intensity unevenness of image data by the magnetic resonance imaging apparatus shown in FIG. 9. Explanatory drawing which shows the estimation procedure of the sensitivity distribution by the magnetic resonance imaging apparatus shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 20 Magnetic resonance imaging apparatus 21 Magnet for static magnetic field 22 Shim coil 23 Gradient magnetic field coil unit 23x X-axis gradient magnetic field coil 23y Y-axis gradient magnetic field coil 23z Z-axis gradient magnetic field coil 24 RF coil 24a WB coil 24b Phased array coil 24c Surface coil 25 Control System 26 Static magnetic field power supply 27 Gradient magnetic field power supply 27x X-axis gradient magnetic field power supply 27y Y-axis gradient magnetic field power supply 27z Z-axis gradient magnetic field power supply 28 Shim coil power supply 29 Transmitter 30 Receiver 31 Sequence controller 32 Computer 33 Input device 34 Display device 35 Bed 36 Sensitivity prescan execution means 37 Sensitivity prescan condition setting means 38 Main scan execution means 39 Sequence controller control means 40 Raw data database 41 Image reconstruction means 42 Image data database 43 Main coil Image database 44 the sensitivity distribution estimating unit 44a thresholding means 44b region reduction means 44c interpolation means 44d smoothing means 45 sensitivity map database 46 the image data correction unit 47 the image displaying unit 50 shimming for photographing condition setting means

Claims (4)

Prior to the main scan in which image data is generated by a sequence including at least the phased array coil as a reception coil, the sensitivity correction data of the reception coil in the main scan is generated by using the phased array coil as a reception coil. Means for performing a sensitivity pre-scan for
Means for generating the sensitivity correction data using only the data acquired by the phased array coil in the sensitivity pre-scan as original data;
Means for generating the image data by executing the main scan;
Means for correcting the image data obtained by the main scan based on the sensitivity correction data ;
The magnetic resonance imaging apparatus according to claim 1, wherein imaging conditions for capturing a proton density weighted image are set as imaging conditions for the sensitivity prescan . 2. The magnetic resonance data according to claim 1 , wherein the sensitivity correction data is generated using data after area reduction processing of data extracted by performing threshold processing on the original data of the sensitivity correction data. Imaging device. 2. The magnetic resonance imaging apparatus according to claim 1, wherein, as the imaging conditions for the sensitivity pre-scan, an echo time is set in a range of 1 ms to 5 ms and a flip angle is set in a range of 5 degrees to 10 degrees . The magnetic resonance imaging apparatus according to claim 1 , wherein a magnetic field distribution is obtained from a signal phase difference of a plurality of echo signals obtained by the sensitivity prescan , and shimming is performed from the obtained magnetic field distribution .

Images