JP2005168868A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a MRI apparatus with a plurality of receiving coils, on each of which phase encoding is selectively operated and acquired signals are developed with a sensitivity distribution of the receiving coil by matrix calculation, which is a high-speed photographing method (parallel imaging method) aiming for a shorter imaging time, less calculation, and a better SN ratio. <P>SOLUTION: Adding echo data for the sensitivity distribution or sensitivity distributions of the receiving coil between more than two adjoining slices shares them and improves the SN ratio of the sensitivity distribution. When the same slice is redundantly imaged, obtaining a sensitivity distribution by measuring echo data for the sensitivity distribution only once and sharing it on the slice to be redundantly imaged can omit a process of obtaining the same sensitivity distribution more than once. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to aliasing artifacts on an image generated when a nuclear magnetic resonance signal (that is, echo data) is measured by thinning out a phase encoding step of a measurement space using a plurality of RF coils in a magnetic resonance imaging apparatus. The present invention relates to a technique for removing using reception sensitivity distribution data of each RF receiving coil, and more particularly, to a technique for shortening the imaging time by improving the SN ratio of the aliasing removal image and reducing the amount of aliasing removal calculation.

The magnetic resonance imaging (hereinafter abbreviated as “MRI”) device measures nuclear magnetic resonance (hereinafter abbreviated as “NMR”) signals from hydrogen, phosphorus, etc. in the subject, and the density distribution and relaxation of the nuclei. The time distribution is imaged and used for medical diagnosis.
In this MRI apparatus, as one technique for reducing the imaging time, the phase of a measurement space (hereinafter abbreviated as “k space”) using a plurality of RF reception coils (hereinafter abbreviated as “reception coil”). After measuring echo data while thinning out the encoding step, the image is reconstructed for each receiving coil, and the artifacts generated on the reconstructed image by thinning out the phase encoding step (hereinafter abbreviated as “folding”). Is known by performing a folding expansion calculation using the reception sensitivity distribution of each reception coil (hereinafter abbreviated as “sensitivity distribution”) (Non-patent Document 1). That is, the imaging time is shortened by thinning out the phase encoding step, but the adverse effect of aliasing is removed using the sensitivity distribution of each receiving coil. Such an imaging method is generally referred to as a parallel imaging method.

  In this parallel imaging method, since the sensitivity distribution of the receiving coil is used for the calculation of the folding expansion, it is necessary to acquire the sensitivity distribution of the receiving coil as accurately as possible in order to obtain a good image free from folding artifacts.

  As a method for acquiring the sensitivity distribution of the receiving coil, a method of performing sensitivity distribution measurement of the receiving coil prior to the main measurement (hereinafter abbreviated as PCM method) is common. However, since this method requires additional measurement, the measurement time is extended. For this reason, when there is a movement of the subject before the main measurement, a wrinkle occurs between the echo data for image reconstruction and the sensitivity distribution during the main measurement, and as a result, the aliasing is sufficiently removed during the aliasing calculation. Artifacts that remain unfolded in the reconstructed image are generated.

As a method of dealing with this problem, echo data for image reconstruction and echo data for sensitivity distribution are measured during this measurement by measuring echo data closely without thinning out the phase encoding step only in the low spatial frequency region of k-space. [Patent Document 1] discloses a technique for simultaneously measuring the above (hereinafter abbreviated as SCM method). The folding expansion operation is the same as the PCM method.
SENSE: Sensitivity Encoding for Fast MRI (Klaas P. Pruessmann et al.), Magnetic Resonance in Medicine 42: 952-962 (1999) Japanese Patent Laid-Open No. 2001-161657

  In the SCM method described above, several problems occur when measuring the sensitivity distribution of the receiving coil depending on the imaging conditions. For example, as in MRA (angiography), when measuring multiple thin slices and obtaining a calculated image (for example, a blood vessel image by the maximum value projection (MIP) method, volume rendering method, etc.), The SN ratio of echo data of a thin slice and an image obtained by reconstructing the echo data is very low. In the SCM method, the echo data for image reconstruction and the echo data for sensitivity distribution are measured under the same conditions, so the SN ratio of the sensitivity distribution image obtained by reconstructing the sensitivity distribution echo data is also very low. . If the sensitivity distribution of the receiving coil is obtained based on such an image with a low SN ratio, only a sensitivity distribution with a very poor SN ratio can be obtained. If this is used to perform the folding expansion processing of parallel imaging, the calculation error increases. Image quality will deteriorate.

Also, when the number of slices is very large or when measuring slices in duplicate, the SCM method requires extra time for measuring the sensitivity distribution echo data, resulting in a very long imaging time. .
Furthermore, in the SCM method, calculation of sensitivity distribution and calculation of folding expansion processing are required for each slice image, so that the amount of calculation increases and image reconstruction takes time.

  Accordingly, the present invention has been made to solve the above-described problem, and in the SCM method in parallel imaging, by obtaining a high S / N ratio of the sensitivity distribution of the receiving coil, a calculation error at the time of folding expansion is obtained. By reducing the amount of aliasing in the reconstructed image that has been expanded, it is possible to improve the signal-to-noise ratio and suppress the occurrence of artifacts due to the failure of the aliasing, thereby improving the image quality and reducing the amount of aliasing removal calculation. An object of the present invention is to provide an MRI apparatus capable of shortening the imaging time.

In order to solve the above problems, the present invention is configured as follows. That is,
In the first embodiment of the present invention, a plurality of RF receiving coils for receiving a nuclear magnetic resonance signal from a subject, and a sensitivity distribution for each RF receiving coil from the nuclear magnetic resonance signal for receiving sensitivity distribution are obtained. Means for obtaining, means for thinning out a phase encoding step of a measurement space using the plurality of RF receiving coils, measuring the nuclear magnetic resonance signal for image reconstruction, and reconstructing an image for each RF receiving coil; In a magnetic resonance imaging apparatus, comprising: means for developing folding artifacts generated on the reconstructed image using the sensitivity distribution; and means for imaging a plurality of slices of the subject.
The means for obtaining the sensitivity distribution includes calculating the sensitivity distribution in a desired slice of the plurality of slices, the nuclear magnetic resonance signal for reception sensitivity distribution or the sensitivity distribution in at least two slices of the plurality of slices. Obtained by arithmetic processing.

  In particular, in the calculation process, as described in claim 8, the reception sensitivity distribution nuclear magnetic resonance signal or the sensitivity distribution in the at least two slices is subjected to complex addition, complex addition average, or complex geometric average.

  According to this embodiment, the sensitivity distribution of the receiving coil can be obtained by increasing the SN ratio in parallel imaging. As a result, the calculation error when performing folding expansion using this is reduced, so that the SN ratio is improved in the reconstructed image in which the folding is expanded, and the occurrence of artifacts on the image due to the failure of the folding expansion can be suppressed. Therefore, the image quality can be improved.

In a preferred second embodiment of the present invention, in the MRI apparatus of the first embodiment, the means for obtaining the sensitivity distribution sequentially obtains the sensitivity distribution for each slice in the plurality of slices.
According to this embodiment, the SN ratio of the sensitivity distribution can be improved and the sensitivity distribution change between slices can be smoothed.
In addition, in the preferred third embodiment of the present invention, in the MRI apparatus of the first embodiment, the means for obtaining the sensitivity distribution discretely obtains the slice for obtaining the sensitivity distribution in the plurality of slices, The folding expansion means uses the obtained sensitivity distribution for a slice from which no sensitivity distribution is obtained.
According to this embodiment, it is possible to improve the sensitivity distribution SN ratio and reduce the sensitivity distribution calculation process.

In a preferred fourth embodiment of the present invention, in the MRI apparatus of the first embodiment, when at least one slice of the plurality of slices is imaged at least twice, the overlap measurement is performed. The number of measurements of the reception sensitivity distribution nuclear magnetic resonance signal in the slice is made less than the number of overlaps.
According to this embodiment, it is possible to reduce the number of times the reception sensitivity distribution nuclear magnetic resonance signal is measured, so that the imaging time can be shortened.

Further, in a preferred fifth embodiment of the present invention, in the MRI apparatus of the fourth embodiment, the means for obtaining the sensitivity distribution includes: a nuclear magnetic field for reception sensitivity distribution that is duplicated in the slice that has been duplicated. After complex addition of resonance signals, a sensitivity distribution in the slice is obtained, and the folding expansion unit uses the sensitivity distribution in common with the slice.
According to this embodiment, it is possible to improve the S / N ratio of the sensitivity distribution in addition to shortening the calculation processing time by reducing the calculation amount, thereby suppressing the occurrence of artifacts on the image due to the failure of the folding expansion. it can.

Further, in a preferred sixth embodiment of the present invention, in the MRI apparatus of the first to fifth embodiments, the number of slices employed for the complex addition is changed to a spatial change in the sensitivity distribution of each RF receiving coil. It is determined corresponding to
According to this embodiment, the number of slices that can be added can be set to be larger at locations where the spatial variation of the sensitivity distribution of the receiving coil is gentle, and as a result, the imaging time can be further shortened. On the other hand, at locations where the spatial variation in the sensitivity distribution of the receiving coil is steep, the number of slices to be added can be reduced to reflect the abrupt spatial variation in the sensitivity distribution. Can be suppressed.

Further, in a preferred seventh embodiment of the present invention, in the MRI apparatus of the first to fifth embodiments, the number of slices adopted for the complex addition is set to a nuclear magnetic resonance signal for reception sensitivity distribution after the complex addition. Alternatively, it is determined corresponding to the SN ratio of the sensitivity distribution obtained from this signal.
According to this embodiment, in a portion where the S / N ratio of the sensitivity distribution of the receiving coil is low, the number of slices to be added is set to be large, thereby suppressing the occurrence of artifacts on the image due to the failure of folding expansion. On the other hand, since the number of slices to be added can be reduced at locations where the SN ratio of the sensitivity distribution of the receiving coil is high, the imaging time can be further shortened as a result.

In a preferred ninth embodiment of the present invention, in the MRI apparatus of the first to eighth embodiments, the reception sensitivity distribution nuclear magnetic resonance signal is measured, and the image reconstruction nuclear magnetic resonance signal is measured. This is executed when the low spatial frequency region in the measurement space is measured.
According to this embodiment, by applying the present invention described in the first to eighth embodiments in the SCM method, an image in which the occurrence of artifacts on the image due to the failure of the folding expansion is suppressed can be quickly obtained. Can be acquired.

  According to the present invention, as described above, the SCM method in parallel imaging (the echo data is measured closely without thinning out the phase encoding step only in the low spatial frequency region of the k space, so that the image re- In the method of simultaneously measuring configuration echo data and sensitivity distribution echo data), it is possible to obtain a sensitivity distribution of the receiving coil by increasing the SN ratio. As a result, the calculation error when performing folding expansion using this is reduced, so that the SN ratio is improved in the reconstructed image in which the folding is expanded, and the occurrence of artifacts on the image due to the failure of the folding expansion can be suppressed. Therefore, the image quality of the reconstructed image that is folded back is improved.

  In addition, it is not necessary to perform phase encoding in the low spatial frequency region of k-space to measure the reception sensitivity distribution echo data of the receiving coil. The amount can be reduced. As a result, the image reconstruction time can be shortened.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiment of the invention, and the repetitive description thereof is omitted.

  First, an embodiment of an MRI apparatus to which the present invention is applied will be described. FIG. 1 shows an overall perspective view of an embodiment of a vertical magnetic field type (open type) MRI apparatus to which the present invention is applied. However, the present invention is not limited to the vertical magnetic field method, and can be applied to other horizontal magnetic field methods. This MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject. As shown in FIG. 1, various devices for inducing a NMR phenomenon in a subject and receiving NMR signals (echo data) are provided. A gantry 1 to be housed, a table 2 to place a subject, a power source for driving various devices in the gantry and a housing 3 to accommodate various control devices, and a tomographic image of the subject by processing received NMR signals It consists of a processing device 4 to be reconfigured, and each is connected by a power source / signal line 5. The gantry and the table are arranged in a shield room that shields a high-frequency electromagnetic wave and a static magnetic field (not shown), and the casing and the processing apparatus are arranged outside the shield room.

  FIG. 2 shows a block configuration diagram in which the configuration of the MRI apparatus of FIG. 1 is disassembled for each more detailed function. As shown in FIG. 2, the MRI apparatus includes a static magnetic field generation system 52, a gradient magnetic field generation system 53, a transmission system 55, a reception system 56, a signal processing system 57, a sequencer 54, and a central processing unit (CPU). 58 and an operation system 75.

  The static magnetic field generation system 52 generates a uniform static magnetic field in a direction orthogonal to the body axis in the space around the subject 51 in the vertical magnetic field method (or the body axis direction in the horizontal magnetic field method). Thus, a permanent magnet type, a normal conducting type or a superconducting type static magnetic field generating source is arranged around the subject 51. The static magnetic field generation system 52 is accommodated in the gantry 1.

  The gradient magnetic field generation system 53 includes a gradient magnetic field coil 59 wound in the three axis directions of X, Y, and Z, and a gradient magnetic field power source 60 that drives each gradient magnetic field coil 59. By driving the gradient magnetic field power supply 60 of each coil in accordance with the above command, gradient magnetic fields Gx, Gy, and Gz in the three-axis directions of X, Y, and Z are applied to the subject 51. More specifically, a slice direction gradient magnetic field pulse (Gs) is applied in one of X, Y, and Z to set the slice plane for the subject 1, and the phase encode direction gradient magnetic field is applied to the remaining two directions. A pulse (Gp) and a frequency encoding direction gradient magnetic field pulse (Gf) are applied, and position information in each direction is encoded into echo data. The gradient magnetic field coil 59 is accommodated in the gantry 1, and the gradient magnetic field power source 60 is accommodated in the housing 3.

  The sequencer 54 is a control means that repeatedly applies a high-frequency magnetic field pulse (hereinafter referred to as “RF pulse”) and a gradient magnetic field pulse in a predetermined pulse sequence, operates under the control of the CPU 58, and collects tomographic image data of the subject 51. Various commands necessary for the transmission are sent to the transmission system 55, the gradient magnetic field generation system 53, and the reception system 56. Furthermore, in the MRI apparatus of the present invention, the sequencer 54 is provided with means that can measure while changing the output of the RF pulse. The sequencer 54 is accommodated in the housing 3.

  The transmission system 55 irradiates an RF pulse to cause nuclear magnetic resonance to the nuclear spins of the atoms constituting the biological tissue of the subject 51, and includes a high frequency oscillator 61, a modulator 62, a high frequency amplifier 63, and a transmission side And a high frequency coil 64a. The high-frequency pulse output from the high-frequency oscillator 61 is amplitude-modulated by the modulator 62 at the timing according to the command from the sequencer 54, and the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 63 and then placed close to the subject 51. By supplying the high frequency coil 64a, the subject 51 is irradiated with the RF pulse. Generally, the high frequency coil 64a is accommodated in the gantry 1 and the others are accommodated in the housing 3.

The receiving system 56 detects echo data (NMR signal) emitted by the NMR phenomenon of the nuclear spin constituting the biological tissue of the subject 51, and receives the high-frequency coil 64b, the signal amplifier 65, and the quadrature detector on the receiving side. 66 and an A / D converter 67. After the NMR signal of the response of the subject 51 induced by the RF pulse irradiated from the high-frequency coil 64a on the transmission side is detected by the high-frequency coil 64b arranged close to the subject 51 and amplified by the signal amplifier 65 The signals are divided into two orthogonal signals by the quadrature phase detector 66 at the timing according to the command from the sequencer 54, converted into digital quantities by the A / D converter 67, and sent to the signal processing system 57. In general, the device group constituting the receiving system 56 is accommodated in the gantry 1.
In FIG. 2, the high-frequency coil 64a and the gradient magnetic field coil 59 on the transmission side are placed facing the subject 1 in the static magnetic field space of the static magnetic field generation system 52 into which the subject 1 is inserted. The high-frequency coil 64b on the receiving side is installed so as to face or surround the subject 51.

  The signal processing system 57 performs various data processing and display and storage of processing results, and includes an external storage device such as an optical disk 69 and a magnetic disk 68, a display 70 composed of a CRT, etc., and temporal image analysis processing and ROM (read-only memory) 71 for storing measurement programs and invariant parameters used in the execution, measurement parameters obtained in the previous measurement, echo data detected by the receiving system 56, and images used for setting the region of interest It has a RAM 72 (temporary writing / reading memory) 72 that temporarily stores and stores parameters for setting the region of interest. When data from the receiving system 56 is input to the CPU 58, the CPU 58 performs signal processing and image processing. Processing such as reconstruction is executed, and the resulting tomographic image of the subject 1 is displayed on the display 70 and recorded on the magnetic disk 68 of the external storage device. The signal processing system 57 is accommodated in the processing device 4.

The operation unit 75 inputs control information for processing performed by the signal processing system 7, and includes a trackball or mouse 73 and a keyboard 74. The operation unit 75 is disposed in the vicinity of the display 70, and the operator controls various processes of the MRI apparatus interactively through the operation unit 75 while looking at the display 70.
Currently, the radionuclide to be imaged by the MRI apparatus is a hydrogen nucleus (proton) which is a main constituent material of the subject as widely used in clinical practice. By imaging information on the spatial distribution of proton density and the spatial distribution of relaxation time in the excited state, the form or function of the human head, abdomen, limbs, etc. is imaged two-dimensionally or three-dimensionally.

  Next, echo data measurement in parallel imaging according to the present invention will be described. FIG. 3 is a diagram illustrating a state in which echo data for reconstruction of a two-dimensional image is filled in the k space, and (a) illustrates a case of normal imaging in which echo data of all phase encoding is measured. In the k space in which the measured echo data is arranged, the phase encoding direction is ky and the frequency encoding direction is kx. (B) shows the case of parallel imaging by the PCM method, and (c) shows the case of parallel imaging by the SCM method. In any of the drawings, the black portion represents actually measured one-phase encoded echo data, and the white portion represents actually measured one-phase encoded echo data (the same applies hereinafter). In the PCM method (b), the sensitivity distribution echo data is measured prior to the main measurement for measuring the image reconstruction echo data. The phase encoding step is evenly thinned out with respect to the phase encoding direction of the k space in correspondence with the ratio. The imaging time is shortened by the thinned-out amount, but aliasing occurs in the reconstructed image. In order to remove the aliasing, the sensitivity distribution of the receiving coil is obtained from the previously measured sensitivity distribution echo data, and the aliasing operation is performed on the aliased image using this sensitivity distribution (Non-patent Document 1).

  However, in this PCM method, the sensitivity distribution echo data measurement and the image reconstruction echo data measurement are performed separately in time, so that the imaging time is extended, and both echoes are caused by the movement of the subject during that time. As a result, wrinkles occur between the data, and as a result, there is an artifact in which the aliasing cannot be sufficiently removed during the aliasing operation and the aliasing remains in the reconstructed image.

  On the other hand, in the SCM method (c), the sensitivity distribution echo data and the image reconstruction echo data are measured almost simultaneously in time. In other words, by densely measuring echo data only in the low spatial frequency region in the phase encoding direction in k-space, the sensitivity distribution echo data and the image reconstruction echo data are simultaneously measured in the same imaging, and in the high spatial frequency region, Only the echo data for image reconstruction is measured by thinning out the phase encoding step according to the double speed number. As a result, the distribution of echo data in the k space becomes dense in the low spatial frequency region in the phase encoding direction, and the high spatial frequency region becomes rough according to the double speed number. As shown in (c), the echo data measured by the SCM method and arranged in the k space are the image reconstruction echo data having the same arrangement as (b) in the k space and the low spatial frequency in the phase encoding direction. It can be separated into sensitivity distribution echo data extracted from the region. Using this echo data in the low spatial frequency region in the phase encoding direction, a sensitivity distribution image with low resolution and no aliasing is reconstructed. From this sensitivity distribution image, the sensitivity distribution (the maximum pixel value of the sensitivity distribution image is set to 1). Image converted into a relative ratio). This solves the problem of the PCM method, reduces the aliasing on the reconstructed image, and shortens the entire imaging time.

Next, the present invention will be described. In the SCM method, when a plurality of slices are simultaneously imaged, the sensitivity distribution echo data of at least two slices is processed to obtain a sensitivity distribution, and this sensitivity distribution is shared and used across the slices. To perform the folding expansion operation. This arithmetic processing is processing for obtaining, for example, complex addition (hereinafter abbreviated as “addition”), addition average, or complex geometric average. Hereinafter, addition will be described as an example, but the present invention is not limited to addition.
In the addition of the sensitivity distribution echo data over a plurality of slices, for example, the sensitivity distribution echo data of a plurality of slice positions that are adjacent to each other across the slice position for which the sensitivity distribution is to be obtained are added. Thereby, even when the slice thickness is thin and the SN ratio of the echo data is low, the sensitivity distribution is smoothed to improve the SN ratio.

  If the spatial distribution of the sensitivity distribution is gradual, slices for which the sensitivity distribution is calculated from the sensitivity distribution echo data are thinned out. Or, even if it is obtained by interpolation calculation using successive sensitivity distributions, an error that causes a particular problem does not occur. Furthermore, when the image reconstruction echo data of the same slice is measured repeatedly, the sensitivity distribution echo data is measured less than the number of times of overlap. When the sensitivity distribution echo data is measured more than once in the same slice, the sensitivity distribution is calculated after adding the measured sensitivity distribution echo data, and the obtained sensitivity distribution is duplicated. Commonly used when reconstructing images from the echo data for construction. Thereby, the calculation amount for obtaining the sensitivity distribution is reduced.

  In the embodiment shown in FIG. 2 of the MRI apparatus to which the present invention is applied, the above processing is executed as follows, for example. That is, the high-frequency coil 64b on the receiving side is composed of a plurality of receiving coils, and the measurement control of the image reconstruction echo data and the sensitivity distribution echo data is performed by the sequencer 54, and the measured image reconstruction echo data and the sensitivity distribution are measured. The echo data is stored in the magnetic disk 58, the echo data is read into the RAM 72, various processing programs are read from the ROM 71 or the magnetic disk 58, the CPU 58 adds the echo data for sensitivity distribution, calculates the sensitivity distribution, The distribution interpolation processing and the image reconstruction including the folding expansion are performed. The obtained sensitivity distribution for each slice and the reconstruction image subjected to the folding expansion are stored again in the magnetic disk 58, and the reconstructed image is displayed on the display 70. Is displayed.

  Next, a first embodiment of the present invention will be described. In the first embodiment, when obtaining the sensitivity distribution of the receiving coil, the sensitivity distribution echo data is acquired from the echo data closely measured in the low spatial frequency region of the k space between adjacent slices, and addition is performed. Then, a sensitivity distribution that is commonly used among a plurality of slices is obtained, and a folded image expansion operation in each slice is performed using this.

  FIG. 4 shows an example of this embodiment. In this embodiment, the number of receiving coils is two and three adjacent slices are imaged. However, the present invention is not limited to this, and the number of receiving coils and the number of slices are each at least two. I just need it. Three adjacent slices 401 to 403 are imaged based on the SCM method, and echo data from each receiving coil is measured for each slice. As a result, two sets of echo data groups are measured for each slice, and the arrangement of the measured echo data groups in the k space is 411 to 413, respectively. Sensitivity distribution echo data in the low spatial frequency region is acquired from these echo data groups, added by addition processing 421, and collected into sensitivity distribution echo data 431 for each receiving coil, and received from this sensitivity distribution echo data 431. A sensitivity distribution 441 for each coil is obtained. Also, the image reconstruction echo data 451 to 453 are acquired from the echo data groups 411 to 413 for each reception coil. Then, the images 471 to 473 of the slices 401 to 403 are obtained from the sensitivity distribution 441 for each reception coil and the image reconstruction echo data 451 to 453 by the folding expansion calculation 460. That is, the image 471 of the slice 401 is obtained from the image reconstruction echo data 451 and the sensitivity distribution 441 by the folding expansion calculation 460. Similarly, the image 472 of the slice 402 is obtained from the echo data 452 for image reconstruction and the sensitivity distribution 441 by the folding expansion calculation 460, and the slice 403 of the slice 403 is obtained from the echo data 453 for image reconstruction and the sensitivity distribution 441 by the folding expansion calculation 460. Get image 473.

In the embodiment shown in FIG. 2 of the MRI apparatus to which the present invention is applied, each process in the above-described embodiment is performed by the sequencer 54 for controlling the echo data measurement, adding the echo data for sensitivity distribution and calculating the sensitivity distribution. The CPU 58 performs calculation of image reconstruction including folding and unfolding.
In the first embodiment, there are several methods for determining the number of slices to which the echo data for sensitivity distribution should be added, as described below, but the present invention is not limited to these. Also, these methods can be applied to other embodiments described below.

  The first method for determining the number of slices is a method in which a relational expression for determining the number of slices to be added is determined in advance from the three-dimensional sensitivity distribution of the receiving coil, and the number of slices satisfying the relational expression is obtained. That is, the approximate sensitivity distribution of the receiving coil used for parallel imaging is found by prior measurement. The absolute value of the spatial change rate in the slice direction s in the sensitivity distribution (S (r)) of this receiving coil | ∂S (r) / ∂s | and the relative position of the slice with respect to the receiving coil (r (x, y, The relational expression for obtaining the upper limit (Nmax) of the number of slices that can be added corresponding to z)) and the slice thickness (W) can be expressed, for example, by the following expression (1).

ここで、Tは予め設定された閾値である。この（1）式に基づいて、事前に求めておいた受信コイルの大凡の感度分布と、撮像の都度設定されるスライス方向sと受信コイルに対するスライスの相対位置rとに対応して、加算できるスライス枚数の上限を求め、加算できるスライス枚数を求めた上限値以下に設定する。
なお、上記事前に求めておいた受信コイルの大凡の感度分布を、実際の撮像時に求めた感度分布で随時更新し、この更新された受信コイルの感度分布を用いて上記（1）式に基づいて、加算できるスライス枚数の上限を求めても良い。

Here, T is a preset threshold value. Based on this equation (1), it is possible to add according to the approximate sensitivity distribution of the receiving coil obtained in advance, the slice direction s set for each imaging, and the relative position r of the slice with respect to the receiving coil. The upper limit of the number of slices is obtained, and the number of slices that can be added is set to be equal to or less than the obtained upper limit.
The approximate sensitivity distribution of the receiving coil obtained in advance is updated as needed with the sensitivity distribution obtained at the time of actual imaging, and based on the above equation (1) using the updated sensitivity distribution of the receiving coil. Thus, the upper limit of the number of slices that can be added may be obtained.

The second method of determining the number of slices is a method of determining the number of slices that can be added so that the difference between the maximum value and the minimum value of the sensitivity distribution at an arbitrary point in the slice direction is equal to or less than a predetermined threshold value. That is, based on the sensitivity distribution for each slice, at a given point (x, y) in the slice plane, the difference between the maximum value and the minimum value of the sensitivity distribution in the slice direction (Δ (x, Slices are added as long as y)) is below a predetermined threshold.
In this method, as in the first method, the sensitivity distribution for each slice may be obtained using the approximate sensitivity distribution of the receiving coil obtained in advance, or for each slice obtained during actual imaging. Sensitivity distribution may be used. However, when calculating the sensitivity distribution for each slice during actual imaging, the amount of calculation of the sensitivity distribution increases. In the case of a thin slice, the SN ratio of the sensitivity distribution decreases, so the sensitivity distribution is affected by noise. there is a possibility.

  In one embodiment shown in FIG. 2 of the MRI apparatus to which the present invention is applied, the number of slices to be added in the first and second methods is, for example, an approximate sensitivity distribution of the receiving coil stored in the magnetic disk 58, and the RAM 72. The data is calculated by the CPU 58 together with the slice position, slice direction, and slice thickness data set by the operation unit 75.

  A third method for determining the number of slices is a method in which the operator directly designates the number of slices. While viewing the reconstructed image, the operator designates the number of slices to be added when obtaining the sensitivity distribution. For example, if the reconstructed image is folded due to an increase in the number of additions, this is caused by an error in the sensitivity distribution, so the number of additions is reduced. On the other hand, if the reconstructed image is folded due to a decrease in the number of additions, this is caused by a decrease in SN of the sensitivity distribution, so the number of additions is increased.

In one embodiment shown in FIG. 2 of the MRI apparatus to which the present invention is applied, the designation of the number of slices to be added in the third method of determining the number of slices is performed by, for example, reconstructing an image displayed on the display 70 by the operator. While watching, the addition number is designated by the trackball or the mouse 73 or the keyboard 24.
The value of the number of slices determined by any of the first to third methods described above can be automatically reflected when imaging the same spatial position thereafter, and unnecessary calculations can be avoided. It is.
In any of the first to third slice number determination methods, for example, the sensitivity distribution changes spatially near the center of the receiving coil, so that the number of slices that can be added can be set larger. As a result, the imaging time can be further shortened. On the other hand, since the sensitivity distribution changes greatly at a location away from the center of the receiving coil, the number of slices that can be added decreases.

Next, the addition of the sensitivity distribution echo data will be described. There are several methods for this addition processing as described below, but the present invention is not limited to these methods.
In the first method of addition, sensitivity distribution echo data is added over the number of slices within the upper limit of the number of slices to be added among the slices adjacent to the slice for which the sensitivity distribution is obtained. Then, this addition processing is sequentially performed for each sheet over a plurality of slices. That is, for each slice, the sensitivity distribution echo data of a certain number of consecutive slices is added to the slice to obtain the sensitivity distribution of the slice. This process is sequentially performed while moving slices one by one. Thereby, it is possible to improve the SN ratio of the sensitivity distribution and smooth the sensitivity distribution change between slices.

FIG. 5 shows an embodiment of the first method. FIG. 5 shows a case in which the echo data for sensitivity distribution of five slices before and after are added. However, the present invention is not limited to this, and there may be at least two. When obtaining the sensitivity distribution of the slice 502, sensitivity distribution echo data of adjacent slices 501 to 503 are added. Similarly, when obtaining the sensitivity distribution of the slice 503, the echo data for the sensitivity distribution of the adjacent slices 502 to 504 is added, and when obtaining the sensitivity distribution of the slice 504, the adjacent slices Add echo data for sensitivity distribution from 503 to 505.
In the second method of addition, a slice for which a sensitivity distribution is obtained is made discrete, and the obtained sensitivity distribution is used in common in slices that are adjacent to the slice and for which a sensitivity distribution is not obtained. Thereby, it is possible to improve the S / N ratio of the sensitivity distribution and reduce the sensitivity distribution calculation process.

  FIG. 6 shows an embodiment of the second method. FIG. 6 shows the case where the echo data for sensitivity distribution of the five slices that follow each other is added. However, the present invention is not limited to this, and there may be at least two. The sensitivity distribution of the slice 602 is obtained by adding the echo data for sensitivity distribution of the adjacent slices 601 and 603, which are used in common by the slices 601 to 603. This eliminates the need to directly obtain the sensitivity distribution of the slices 601 and 603. Next, the sensitivity distribution of the slice 604 is obtained by adding the sensitivity distribution echo data of the adjacent slices 603 and 605, and this is commonly used by the slices 604 to 605. This eliminates the need to directly determine the sensitivity distribution of the slice 605. In the example of FIG. 6, an example is shown in which the sensitivity distribution echo data of the slice for which the sensitivity distribution is obtained is not added, but it may be added.

In one embodiment shown in FIG. 2 of the MRI apparatus to which the present invention is applied, the CPU 58 performs the addition processing in the first and second methods of addition.
Note that, in both the first and second methods relating to addition described above, the addition target is sensitivity distribution echo data. However, after obtaining the sensitivity distribution from the sensitivity distribution echo data, addition between the sensitivity distributions may be performed. . Further, even when the arithmetic processing is addition average or complex geometric average, the same processing can be applied if addition is replaced with addition average or complex geometric average in the first and second methods.

  The above describes the first embodiment in which the sensitivity distribution echo data is added between adjacent slices to obtain the sensitivity distribution. However, when overlapping imaging at the same slice position is performed, the sensitivity distribution echo data is repeatedly measured. A second embodiment of the present invention that avoids and shortens the imaging time will be described next.

In MRA, which is commonly performed, when some of the slices are imaged redundantly, applying the first embodiment results in redundant measurement of sensitivity distribution echo data at the same slice position. As a result, imaging becomes redundant. Thus, in the second embodiment, when the same slice position is imaged repeatedly, the sensitivity distribution echo data in the slice is measured to be less than the number of overlapping imaging. In other words, imaging is provided to thin out measurement of sensitivity distribution echo data. In addition, if the number of times the sensitivity distribution echo data is measured for the same slice is two or more, a sensitivity distribution is created after adding the sensitivity distribution echo data, and this slice position is used using this sensitivity distribution. All the images that are duplicated and captured are folded and expanded.
In the embodiment shown in FIG. 2 of the MRI apparatus to which the present invention is applied, the portions where each process of the second embodiment is performed are the same as those in the case of the first embodiment.

  An example of the second embodiment is shown in FIG. In FIG. 7A, slices 701 to 704 are continuously captured in the first imaging 700, slices 711 to 714 are continuously captured in the second imaging 710, and slices 721 to 724 are continuously captured in the third imaging 720. In this example, slices 703 and 711, slices 704 and 712, slices 713 and 721, and slices 714 and 722 are the same slice. However, the present invention is not limited to this case. In other words, the slices 711 to 714 in the second imaging 710 are duplicated. In such a case, the sensitivity distribution echo data of the slice 711 in the second imaging 710 is diverted from that of the slice 713 in the first imaging 700, and the sensitivity distribution echo data is not measured when the slice 711 is imaged. Similarly, the sensitivity distribution echo data of the slice 712 in the second imaging 710 is diverted from that of the slice 714 in the first imaging 700, and the sensitivity distribution echo data of the slice 713 in the second imaging 710 is the third imaging 720. The sensitivity distribution echo data of the slice 714 in the second imaging 710 is diverted from the slice 721 in the second imaging 710, and the sensitivity distribution echo data in the imaging of the slices 712 to 714 is diverted from that of the slice 722 in the third imaging 720. Measurement is not performed. In this way, the sensitivity distribution echo data of the same slice is diverted in successive imaging without measuring the sensitivity distribution echo data in the second imaging 710.

On the other hand, in the first imaging 700 and the third imaging 720, imaging similar to that in the first embodiment is performed, and sensitivity distribution echo data is measured as necessary. The arrangement of echo data in the k space in each of the above imaging is shown in (b) and (c). (B) shows the arrangement of the echo data measured in the first and third imaging in the k space, and both the image reconstruction echo data and the sensitivity distribution echo data are measured. On the other hand, (c) shows the arrangement of the echo data measured in the second imaging in the k space, only the image reconstruction echo data is measured, and the measurement of the sensitivity distribution echo data is omitted. . Accordingly, the imaging time for the second imaging is shortened.
In one embodiment shown in FIG. 2 of the MRI apparatus to which the present invention is applied, for example, measurement control of the reconstruction echo data and sensitivity distribution echo data is performed by the sequencer 57, and each data is stored in the magnetic disk 58. The diversion processing of the sensitivity distribution is performed by the CPU 58.

  Furthermore, when the first to third imaging is repeated twice or more, it is not necessary to measure the sensitivity distribution echo data for each slice in the first and third imaging, and the sensitivity distribution echo data is measured. May be thinned out. When the sensitivity distribution echo data of the same slice is measured a plurality of times (less than the number of times of imaging), the sensitivity distribution is obtained after adding the sensitivity distribution echo data measured up to the previous time for each slice. Thereby, since the time for measuring the sensitivity distribution echo data can be reduced, the entire imaging time can be shortened.

Next, a description will be given of a third embodiment of the present invention in which the measurement of sensitivity distribution echo data is omitted, and the sensitivity distribution in the slice is obtained by interpolation processing from the sensitivity distributions of the preceding and succeeding slices.
FIG. 8 shows an example of the third embodiment. In FIG. 8, slices 801 to 803 are continuously imaged, and sensitivity distribution echo data 811 and 813 are measured in slices 801 and 803, respectively, and sensitivity distributions 821 and 823 are obtained, respectively. A case where only the configuration echo data 812 is measured and the sensitivity distribution 822 is not directly obtained is shown. However, the present invention is not limited to this case, and there may be at least two adjacent slices.

  In such a case, the sensitivity distribution 822 of the slice 802 is obtained by interpolation processing from the sensitivity distributions 821 and 823 of the slices 801 and 803 that follow each other. Any of the known interpolation processes can be used for this interpolation process. For example, simple linear interpolation can be used. Using the sensitivity distribution 822 obtained by the interpolation processing, the image is reconstructed by performing the folding expansion processing in the slice 802.

On the other hand, the slices 801 and 803 that have directly obtained the sensitivity distribution by measuring the sensitivity distribution echo data are subjected to folding expansion processing from the obtained sensitivity distributions 821 and 823 to respectively perform image reconstruction.
In one embodiment shown in FIG. 2 of the MRI apparatus to which the present invention is applied, this interpolation processing is performed by the CPU 58. In other words, the sensitivity distributions 821 and 823 of the slices 801 and 803 stored in the magnetic disk 68 are read into the RAM 72, and interpolation processing is performed by the CPU 58 to obtain the sensitivity distribution 822 of the slice 802. Process.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an overall perspective view of an embodiment of a vertical magnetic field type (open type) MRI apparatus to which the present invention is applied. The figure which shows the block structure which decomposed | disassembled the structure of the MRI apparatus of FIG. 1 for every more detailed function. The figure which shows arrangement | positioning in k space of echo data. (A) is a figure which shows arrangement | positioning of normal imaging | photography, (b) is a figure which shows arrangement | positioning by PCM method, (c) is a figure which shows arrangement | positioning by SCM method. FIG. 3 is a diagram showing an example of the first embodiment of the present invention. The figure which shows the 1st addition method of sensitivity distribution echo data in creation of sensitivity distribution Shows the second method of adding echo data for sensitivity distribution in creating sensitivity distribution FIG. 10 is a diagram showing an example of the second embodiment of the present invention. (A) shows the relationship between slices in three imaging operations, (b) shows the arrangement of echo data measured in the first and third imaging in k-space, and (c) shows the measurement in the second imaging The arrangement | positioning in k space of echo data to be performed. The figure which shows one Example of the 3rd Embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Gantry, 2 ... Table, 3 ... Case, 4 ... Processing apparatus, 51 ... Subject, 52 ... Static magnetic field generation system, 53 ... Gradient magnetic field generation system, 54 ... Sequencer, 55 ... Transmission system, 56 ... Reception system 57 ... Signal processing system, 58 ... Central processing unit (CPU), 59 ... Gradient magnetic field coil, 60 ... Gradient magnetic field power supply, 61 ... High frequency transmitter, 62 ... Modulator, 63 ... High frequency amplifier, 64a ... High frequency coil (transmission) Coil), 64b ... high frequency coil (receiver coil), 65 ... signal amplifier, 66 ... quadrature detector, 67 ... A / D converter, 68 ... magnetic disk, 69 ... optical disk, 70 ... display, 71 ... ROM, 72 ... RAM, 73 ... trackball or mouse, 74 ... keyboard

Claims (9)

A plurality of RF receiving coils for receiving a nuclear magnetic resonance signal from a subject; means for obtaining a receiving sensitivity distribution for each RF receiving coil from the nuclear magnetic resonance signal for receiving receiving sensitivity distribution; and the plurality of RFs Means for measuring the nuclear magnetic resonance signal for image reconstruction by thinning out the phase encoding step of the measurement space using a reception coil, and reconstructing an image for each RF reception coil; and generated on the reconstructed image Means for developing a folding artifact using the reception sensitivity distribution; means for imaging a plurality of slices of the subject;
In a magnetic resonance imaging apparatus comprising:
The means for obtaining the reception sensitivity distribution is configured to obtain the reception sensitivity distribution in a desired slice of the plurality of slices, the reception sensitivity distribution nuclear magnetic resonance signal in at least two slices of the plurality of slices, or the reception. A magnetic resonance imaging apparatus characterized in that a sensitivity distribution is obtained by arithmetic processing.   2. The magnetic resonance imaging apparatus according to claim 1, wherein the means for obtaining the reception sensitivity distribution sequentially obtains the reception sensitivity distribution for each slice in the plurality of slices.   2. The magnetic resonance imaging apparatus according to claim 1, wherein the means for obtaining the reception sensitivity distribution discretizes slices for obtaining the reception sensitivity distribution in the plurality of slices, and the folding expansion means obtains the reception sensitivity distribution. A magnetic resonance imaging apparatus characterized in that the obtained reception sensitivity distribution is used for no slice.   2. The magnetic resonance imaging apparatus according to claim 1, wherein when at least one slice of the plurality of slices is imaged at least twice more than once, the reception and reception sensitivity distribution nuclear magnetic resonance in the slice that has been measured twice. A magnetic resonance imaging apparatus characterized in that the number of signal measurements is less than the number of overlaps.   5. The magnetic resonance imaging apparatus according to claim 4, wherein the means for obtaining the reception sensitivity distribution is obtained by performing the arithmetic processing on the nuclear magnetic resonance signal for the reception reception sensitivity distribution that has been subjected to the overlap measurement in the slice that has been subjected to the overlap measurement. A magnetic resonance imaging apparatus characterized in that a reception sensitivity distribution is obtained, and the folding expansion means uses the reception sensitivity distribution in common for the slices.   6. The magnetic resonance imaging apparatus according to claim 1, wherein the number of slices employed in the calculation processing is determined in accordance with a spatial change in reception sensitivity distribution of each RF reception coil. Magnetic resonance imaging device.   6. The magnetic resonance imaging apparatus according to claim 1, wherein the number of slices employed in the calculation process is an SNR of a reception sensitivity distribution obtained from the nuclear magnetic resonance signal for reception reception sensitivity distribution after the calculation process or the signal. The magnetic resonance imaging apparatus is determined in accordance with   8. The magnetic resonance imaging apparatus according to claim 1, wherein the arithmetic processing is performed by performing a complex addition, a complex addition average, or a complex geometry on the reception reception sensitivity distribution nuclear magnetic resonance signal or the reception sensitivity distribution in the at least two slices. A magnetic resonance imaging apparatus characterized by averaging.   9. The magnetic resonance imaging apparatus according to claim 1, wherein the measurement of the reception magnetic resonance signal for receiving reception sensitivity distribution is performed in a low spatial frequency region in the measurement space when measuring the nuclear magnetic resonance signal for image reconstruction. A magnetic resonance imaging apparatus, which is executed at the time of measurement.

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