JP3112926B2 - Magnetic resonance imaging - Google Patents

Description

The present invention relates to a magnetic resonance imaging apparatus, and in particular, to collecting magnetic resonance signal data in real time using an arbitrary gradient magnetic field waveform. More particularly, the present invention relates to a magnetic resonance imaging apparatus that realizes high-speed switching of a gradient magnetic field, particularly a read gradient magnetic field.

(Technique of the Invention) Magnetic resonance imaging (MRI) is well known,
When a group of nuclei having a unique magnetic moment is placed in a uniform static magnetic field, it utilizes the phenomenon of resonantly absorbing the energy of a high-frequency magnetic field rotating at a specific frequency,
This is a method of visualizing chemical and physical microscopic information of a substance.

In this magnetic resonance imaging, data acquisition time is much longer than in other medical image diagnostic apparatuses such as an ultrasonic diagnostic apparatus and an X-ray CT. Therefore, there is a problem that artifacts occur due to movements such as respiration of the subject, and it is difficult to visualize a moving heart or vascular system. Further, since the imaging time is long, the pain given to the subject is great. Therefore,
As a method of reconstructing an image at high speed in magnetic resonance imaging, an echo planar method by Mansfield, an ultrafast Fourier method by Hutcison, and the like have been proposed.

In a pulse sequence for image data collection by the ultrafast Fourier method (also called multiple echo Fourier method), a 90 ° high-frequency pulse for selective excitation is applied as a high-frequency magnetic field, and at the same time, a gradient magnetic field for slicing is applied to the slice plane. After selectively exciting the magnetization of the slice, a 180 ° high-frequency pulse is further applied, and then the readout gradient magnetic field is switched in a direction parallel to the slice plane at high and negative speeds and applied at the same time. A phase encoding gradient magnetic field is applied in a direction orthogonal to the read gradient magnetic field in a pulsed manner every time the read gradient magnetic field is switched.

According to such a pulse sequence, there is a time at which the phase of the magnetization in the slice plane is aligned each time the read gradient magnetic field is switched, so that a large number of echo signal trains are observed as magnetic resonance signals. These echo signal trains are collected as image data within a time period in which the magnetization in the slice plane is relaxed by the relaxation phenomenon of the transverse magnetization, thereby enabling ultra-high-speed imaging.

In the ultrafast imaging method such as the above-described ultrafast Fourier method, it is necessary to increase the gradient magnetic field strength of the readout gradient magnetic field by about 5 to 10 times compared to the normal imaging method, and if the switching speed is not increased, No. For this reason, if the gradient coil power supply is controlled in a rectangular manner as in the case of the normal imaging method, a very large current capacity is required at the rising portion of the rectangularity for driving the gradient coil, and the high-speed switching of the readout gradient magnetic field is difficult. Extremely difficult. Further, since most of the power supply capacity is required in the rising portion, the power supply capacity is wasted in other portions.

As a method for solving such a problem, it is conceivable to control the gradient coil power supply with an arbitrary control waveform such as a sine wave having a gradual change with time to switch a high-intensity readout gradient magnetic field at high speed. ing. However, this causes a new problem as follows.

FIG. 12 shows the relationship between the waveform of the read gradient magnetic field Gr and the phase kx. As shown in FIG. 12 (a), when the waveform of the readout gradient magnetic field Gr is a rectangular wave, by performing sampling at equal intervals in time, data points at equal intervals in the phase space are obtained as magnetic resonance signal data. Obtainable.
On the other hand, if the waveform of the read gradient magnetic field Gr is a sine wave as shown in FIG. 12 (b), for example, if sampling is performed at regular intervals in time, only data points at irregular intervals in the phase space will be obtained. I can't get it. Even if the waveform of the read-out gradient magnetic field Gr is not a sine wave, if the waveform is an arbitrary waveform other than a rectangular wave, the arrangement of data points on the phase space will be unequally spaced for temporally equally spaced sampling as well. . However, in order to use the 2D-FFT method (two-dimensional fast Fourier transform) as an image reconstruction method, data points arranged at equal intervals are required.

Data points on the phase space of the magnetic resonance signal data can be equally spaced by obtaining data points at an equal interval on the phase space by interpolating the data obtained by the equally-spaced sampling or by equalizing the data points on the phase space. Two methods are conceivable in which sampling is performed at irregular intervals on the time axis so as to form intervals. Of these, the former method of combining interpolation with equal-interval sampling can be realized only by changing the software in the current system, and is an advantageous method in terms of cost. However, in this method, since preprocessing such as interpolation is included, there is a problem in increasing the speed when real-time MRI or the like is realized. On the other hand, the latter uneven sampling on the time axis is considered to require new hardware for realization, but it is an advantageous method for speeding up image reconstruction processing in real-time MRI etc. .

(Problems to be Solved by the Invention) As described above, when ultra-high-speed imaging is realized, the gradient coil power supply for the read-out gradient magnetic field is controlled by an arbitrary waveform such as a sine wave other than a rectangular wave, so that high-speed imaging is achieved. It is effective to switch the strong readout gradient magnetic field at high speed. When controlling the read gradient magnetic field with an arbitrary waveform such as a sine wave, in order to arrange the data points in the phase space of the magnetic resonance signal data at equal intervals, sampling at irregular intervals that does not require preprocessing is performed. Although it is considered preferable for real-time MRI and the like, an effective method for specifically realizing such uneven sampling has not yet been considered.

The present invention realizes unequally spaced sampling of magnetic resonance signal data that is effective when controlling a read gradient magnetic field with an arbitrary waveform such as a sine wave, thereby enabling high-speed imaging without requiring preprocessing. It is intended to provide a device.

[Constitution of the Invention] (Means for Solving the Problems) The present invention applies a high-frequency magnetic field and a gradient magnetic field to a subject placed in a uniform static magnetic field in accordance with a predetermined pulse sequence, and obtains the subject from within the subject. The magnetic resonance signal is collected by sampling using a sampling pulse to collect magnetic resonance signal data, a magnetic resonance imaging apparatus that performs image reconstruction, by switching the readout gradient magnetic field,
In the magnetic resonance image signal using the so-called high-speed imaging method, which collects all the magnetic resonance signal data necessary for image reconstruction within the time when the transverse magnetization in the slice plane relaxes, the time of the gradient magnetic field waveform which is an arbitrary waveform The main point is that a time interval during which the integral value changes by a substantially constant amount is obtained, and unequally spaced sampling pulses for collecting magnetic resonance signal data are generated at this time interval.

More specifically, a high-frequency magnetic field and a gradient magnetic field for slicing, reading, and phase encoding are applied to a subject placed in a uniform static magnetic field in accordance with a predetermined pulse sequence, and a predetermined magnetic field in the subject is applied. A magnetic resonance imaging apparatus that collects magnetic resonance signal data by sampling a magnetic resonance signal obtained from a slice plane using a sampling pulse, and performs image reconstruction, and switches a read-out gradient magnetic field to obtain a slice. In a magnetic resonance imaging apparatus that collects all magnetic resonance signal data necessary for image reconstruction within a time when the in-plane transverse magnetization is relaxed, a waveform of a read gradient magnetic field is obtained, and the time of the read gradient magnetic field waveform is obtained. The time interval during which the integral value changes by a substantially constant amount is obtained and stored for each combination of the readout gradient magnetic field waveform and the slice plane, and stored during this time. In a configuration for generating the non-equidistant sampling pulses for magnetic resonance signal data acquisition.

To determine the gradient magnetic field waveform, the gradient magnetic field waveform may be directly measured, or the gradient magnetic field waveform may be determined by calculation from the control waveform of the gradient magnetic field. When the latter calculation is implemented by hardware, for example, voltage-frequency conversion may be used.

The sampling pulse generating means stores, for example, sampling timing data set in advance with a high time resolution, sequentially compares this data with output data of a counter that counts the time resolution clock,
The sampling pulse is generated when both match, or the sampling timing data is stored as a bit string with a high time resolution, and the contents are sequentially read out with a clock having the above time resolution, and the bit data exists. In this case, a sampling pulse is generated.

The arbitrary gradient magnetic field control waveform can be obtained, for example, by reading an arbitrary waveform previously stored as digital data in a storage device at a variable reading speed and performing D / A conversion.

(Operation) The phase of the magnetic resonance signal data is proportional to the time integral of the readout gradient magnetic field waveform. Therefore, if the magnetic resonance signal is sampled and the magnetic resonance signal data is collected at time intervals during which the time integration value of the gradient magnetic field waveform changes by a substantially constant amount, the interval between the data points in the phase space becomes the waveform of the gradient magnetic field. It is constant regardless of

As described above, in the present invention, in a real-time magnetic resonance imaging apparatus using ultra-high-speed imaging such as an echo planar method or an ultra-fast Fourier method, it is necessary to change the readout gradient magnetic field from a conventional rectangular wave to an arbitrary waveform. Unequal interval sampling becomes possible.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing a configuration of a magnetic resonance imaging apparatus according to one embodiment of the present invention. In FIG. 1, a static magnetic field magnet 1 is driven by an excitation electrode 2 to apply a uniform static magnetic field to a subject 5 (for example, a human body). The excitation power supply 2 is controlled by a sequence controller 9 as shown by a broken line as necessary. The gradient coil group 3 is driven by a gradient coil power supply 4 controlled by a sequence controller 9, and includes two orthogonal directions r and e in a desired cross section (slice plane) of interest of the subject 5 and a direction s perpendicular thereto. , A gradient magnetic field whose magnetic field strength changes respectively is applied. In this embodiment, the gradient magnetic field applied in the direction s perpendicular to the slice plane is a slice gradient magnetic field Gs, the gradient magnetic field applied in one direction r in the slice plane is a read gradient magnetic field Gr, The applied gradient magnetic field is described as a phase encoding gradient magnetic field Ge. These directions are not necessarily the xyz coordinates where the center axis of the static magnetic field magnet 1 is the z axis, and the gradient magnetic fields Gx, G generated in the x, y, z directions, respectively.
It does not coincide with the directions of y and Gz. That is, the gradient magnetic fields Gr, Ge,
In order to generate Gs, it is necessary to apply the gradient magnetic fields Gx, Gy, and Gz at an appropriate intensity.

Under the control of the sequence controller 9, a high-frequency magnetic field generated from the probe 6 by a high-frequency signal from the transmission unit 7 is applied to the subject 5. In the present embodiment, the probe 6 includes a transmitting coil for a high-frequency signal,
Although shared with a receiving coil for receiving magnetic resonance signals related to various nuclei in the subject 5, separate transmitting and receiving coils may be provided.

The magnetic resonance signal (echo signal) received by the probe 6 is amplified and detected by the receiving unit 8, and then sent to the data collecting unit 10 under the control of the sequence controller 9. The data collection unit 10 receives the magnetic resonance signal extracted via the reception unit 8, performs unequal interval sampling with an A / D converter under the sequence controller 9 as described below, digitizes and collects, The resonance signal data (echo signal data) is sent to the data processing unit 11.

The data processing unit 11 is controlled by the computer 12, and performs image reconstruction processing by Fourier transform on the echo signal data input from the data collection unit 10 to obtain image data. The computer 12 also controls the sequence controller 9. The image data obtained by the data processing unit 11 is supplied to the image display 14 and displayed as an image. The computer 12 and the image display 14 are controlled by the console 13.

In this embodiment, in order to realize the above-mentioned non-uniform sampling, a magnetic field sensor 15 for detecting a read gradient magnetic field Gr generated by the gradient coil group 3 and a data collecting unit
A sampling interval setting unit 16 for determining a sampling interval in the A / D converter in 10 is provided. The sampling interval setting unit 16 will be described later in detail.

FIG. 2 shows an example of a pulse sequence for acquiring image data of a slice plane in the subject 5, and includes a high-frequency magnetic field RF and gradient magnetic fields Gs, Gr, and Ge for slicing, reading, and phase encoding. And magnetic resonance signal (echo signal)
Each waveform of Sig. Is shown. This pulse sequence is
It is controlled by the sequence controller 9.

First, in order to selectively excite the magnetization in the slice plane of interest in the subject 5 first, a 90 ° high-frequency pulse for selective excitation as a high-frequency magnetic field RF (to rotate only the magnetization in a predetermined slice region by 90 °). At the same time as applying the slice gradient magnetic field Gs,
A specific slice plane in the subject 5 is selectively excited. Note that the slice gradient magnetic field Gs is inverted after the application of the 90 ° high-frequency pulse for selective excitation to make the phases of magnetization uniform. Next, a high-frequency pulse of 180 ° is applied in order to cancel the effect of the spatial non-uniformity of the static magnetic field. At this time, the slice gradient magnetic field Gs is not applied.

One direction of the xy plane orthogonal to the slice direction z, for example, x
In the direction, a read gradient magnetic field Gr (= Gx) of an arbitrary waveform (sine wave in the example in the figure) is repeatedly switched and applied so as to alternately reverse the positive and negative directions, and at the same time, in another direction of the xy plane, for example, in the y direction. Gradient magnetic field for phase encoding Ge (= G
y) is applied in pulse form. Thereby, a magnetic resonance signal (echo signal) Sig. From inside the subject 5 is obtained. These echo signals are digitized after phase detection, echo signal data is collected, Fourier-transformed and image reconstruction is performed, thereby generating slice plane image data. In order to collect the magnetic resonance signal Sig. For the readout gradient magnetic field Gr of the arbitrary waveform, the sequence controller 9 shown in FIG. 1 generates sampling pulses at the sampling interval set by the sampling interval setting unit 16 and the data collection unit 10. To supply.

FIG. 3 is a block diagram showing a configuration of a main part of the sequence controller 9 related to the gist of the present invention and a detailed configuration of the sampling interval setting unit 16. The sequence controller 9 includes a clock generator 21 for generating an original clock pulse having an interval (time resolution) of Δtq, and a frequency divider 22 for dividing the original clock pulse to obtain a clock pulse having an interval (time resolution) of Δtu. And a gradient magnetic field control waveform generator 23 driven by a clock pulse from the frequency divider 22, a sampling timing storage 24, and a sampling pulse generator 25.

The gradient magnetic field control waveform generator 23 is a circuit that generates a control waveform for independently controlling the gradient magnetic field waveforms in the three orthogonal directions x, y, and z into arbitrary waveforms, and is configured, for example, as shown in FIG. You. The gradient magnetic field control waveform generator 23 shown in FIG.
Is a clock generator 31 activated by a sequence start signal, and an event that is driven by a clock pulse from the clock generator 31 and generates digital data indicating the gradient magnetic field strength at each time of Gx, Gy, and Gz. It has memories 32, 33, 34 and D / A converters 35, 36, 37 for converting output data of the event memories 32, 33, 34 into analog values. Event memory 32, 33,
The reading of 34 and the D / A conversion are performed. D / A converters 35, 36,
The output of 37 is input to the gradient coil power supplies 41, 42, and 43 as a gradient magnetic field control waveform, and is supplied to each gradient coil after current amplification. Thereby, a gradient magnetic field having an arbitrary waveform can be generated from each gradient coil of the gradient coil group 3. Further, according to this configuration, by adjusting the gradient magnetic field strength set in the event memories 32, 33, 34, the gradient magnetic fields Gr, Ge, Gs
Can be arbitrarily set, and a slice plane to be imaged can be arbitrarily selected.

On the other hand, the sampling interval setting section 16 comprises a sampling interval calculating section 26 and a sampling interval database 27 as shown in FIG. The sampling interval calculation unit 26 is, for example, the magnetic field sensor 15 shown in FIG.
From the gradient magnetic field waveform obtained through the data collection unit 10, the time interval at which the time integral value of the gradient magnetic field waveform changes by a constant value is calculated as the sampling interval. Also,
The sampling interval database 27 accumulates unequally-spaced sampling interval data corresponding to combinations of various waveforms such as sine waves and trapezoids of gradient magnetic fields, intensities, and slice plane directions (cross-sectional directions).

As shown in FIG. 12 (b), when the readout gradient magnetic field has an arbitrary waveform, even if the magnetic resonance signals are sampled at regular intervals on the time axis, the data points on the phase space remain at regular intervals. do not become. In order for the intervals of data points to be equal in the phase space, it is necessary to collect data at time intervals each time the integrated value of the gradient magnetic field waveform reaches a certain value. Considering the sine wave of FIG. 12 (b) as an example, in the case of equal-interval sampling, the data interval on the phase space becomes maximum near the maximum amplitude of the gradient magnetic field waveform. In order to achieve this, it is necessary to reduce the sampling interval in the vicinity, and to increase the sampling interval as the amplitude of the gradient magnetic field waveform decreases. For this purpose, it is necessary to increase the sampling speed at the maximum amplitude portion of the gradient magnetic field waveform. However, the required sampling speed is the same as the switching speed and π in the case of a rectangular wave under the same image conditions.
/ 2 times, no technical difficulties.

In addition, since the time resolution in the case of realizing the unequal interval sampling affects the coordinate position shift in the phase space, it is desired that the time resolution is as high as possible in order to prevent image artifacts. A time resolution several tens of times the sampling interval Δtu in the case of a wave is required. On the other hand, if the time resolution is increased, the word length for expressing the event time becomes longer, so that the scale of the hardware becomes larger and it becomes difficult to increase the speed. Therefore, it is necessary to comprehensively determine the time resolution from the S / N of the obtained image. This time resolution is the third
The time interval Δtq of the clock pulse output from the clock generator 21 shown in FIG.
Has a precision of nq = Δtu / Δtq times the time interval Δtu of the clock pulse from the frequency divider 22 for driving the clock pulse.

In order to realize unequal interval sampling determined by the above conditions, the sampling interval setting unit 16 in FIG.
The sampling intervals corresponding to the arbitrary slice plane and the arbitrary gradient magnetic field waveform are set so that the integrated values of the gradient magnetic field waveforms become equal as described above.

Here, a method of estimating the gradient magnetic field waveform will be discussed. The control waveform output from the gradient magnetic field control waveform generator 23 in FIG. 3 and the actual gradient magnetic field waveform generated from the gradient coil are slightly different due to the influence of the non-ideal state such as the gradient coil inductance component and eddy current. . Also, by adjusting the gradient magnetic field strength of Gx, Gy, Gz for selection of an arbitrary slice plane, the current value applied to each gradient coil is determined by the vector direction determined when realizing a constant gradient magnetic field strength by vector synthesis. Differently, the influence of non-ideal states such as phase inductance and eddy current also differs depending on the difference of each current value. In the case of a rectangular gradient magnetic field, data sampling is performed after the gradient magnetic field waveform rises and becomes a uniform gradient magnetic field, so the effect of the difference between the control waveform and the actual magnetic field waveform on the data interval in the phase space Is relatively small. On the other hand, if the gradient magnetic field is controlled with an arbitrary waveform, the control waveform and the actual magnetic field waveform do not match, so if the unequal sampling interval is calculated using the control waveform as it is, an error will occur in the position of the data point. Occurs.

Therefore, in the present embodiment, when the sampling interval is calculated by the sampling interval calculation unit 26 in FIG. 3, prior to the integration calculation, the gradient magnetic field waveform is actually measured in the data collection unit 10 via the magnetic field sensor 15, or Then, the magnetic field waveform is accurately estimated from the control waveform, and then the unequal sampling interval is calculated and the sampling interval data is output. In order to obtain the actual gradient magnetic field waveform, it is sufficient to directly observe the magnetic field via the magnetic field sensor 15 with high precision as described above. In this case, a system for measuring a magnetic resonance signal, in particular, a data collection unit 10, a data processing unit 11, and a computer 12 can be used for measuring the gradient magnetic field waveform. As another method for obtaining the gradient magnetic field waveform, if the relationship between the gradient magnetic field control waveform and the actual gradient magnetic field waveform is linear, the gradient magnetic field control waveform is read from the control waveform storage unit 51 as shown in FIG. There is a method in which the magnetic field waveform calculator 52 calculates the gradient magnetic field waveform using this linear relationship. For this purpose, a transfer function may be obtained in advance, and this may be convolution-integrated in the magnetic field waveform calculator 52 to obtain a gradient magnetic field waveform corresponding to an arbitrary control waveform.

Hereinafter, a specific configuration method of each unit in FIG. 3 will be described in detail.

Regarding the sampling interval calculator 26 The sampling interval calculator 26 calculates the magnetic field waveform Gtu (k) (k = 0,..., N−1) measured or calculated as described above.
From the sampling interval in data collection: Δtu), the sampling interval is calculated by, for example, the algorithm shown in FIG. First, the number of samples is set to nq in order to match the required time resolution (Δtq) of uneven sampling.
Multiply and supplement the data points by interpolation (step 61). However, as described above, nq = Δtu / Δtq.

Next, the integral value of the gradient magnetic field waveform: ΣGtp (i) (i = 0,
.., N · nq−1, sampling interval: Δtq) are calculated (step 62).

Next, i is determined so that abs (ΣGtq (i) −j · ks) is minimized, where ks is the resolution in the phase space, and unequally-spaced sampling event times: ts (j) (j = 0, ..., N-1)
Are sequentially set (steps 63 to 69). That is, first, in step 63, after setting i = 0 and j = 0, i becomes N · nq
It is checked whether it is smaller than (step 64). here,
If i <N · nq, then in step 65, abs (ΣGtq
It is checked whether (i) −j · ks) is smaller than εps, and ab
If s (ΣGtq (i) −j · ks) <εps, ts (j) =
After tq (i), j is incremented by one (step 67), and i is further incremented by one (step 67).
68). Further, in step 65, abs (ΣGtq (i) -j
If ks) <εps, jump to step 68. After step 68, the process returns to step 64, and the same processing is repeated. When i ≧ N · nq in step 64,
The calculation of the sampling interval ends.

FIG. 7 is a block diagram showing an example in which the sampling interval calculator 26 is realized by hardware. The output of the magnetic field sensor 15 is directly input to the sampling interval calculator 26 via the path shown by the broken line in FIG. The integrating circuit 71 for integrating the output of the magnetic field sensor 15 and the comparator 72 constitute a V / F converter (voltage-frequency converter) 73, and output a signal having a frequency corresponding to the output voltage of the magnetic field sensor 15. Reference voltage Vr for comparison given to comparator 72
ef corresponds to the resolution: ks in the phase space of the magnetic resonance signal data. The AND circuit 74 calculates the logical product of the output of the comparator 72 and the output of the clock generator 75, and the output of the AND circuit 74 drives the sampling event time generation circuit 76. The clock generator 73 generates a reference clock for quantizing the output of the comparator 72. By appropriately setting the time interval Δtq of the reference clock and the reference voltage Vref, unequally spaced sampling pulses are generated. Then, the reference clock from the clock generator 74 is counted by the counter 77, and combined with this count value, the sampling event time at irregular intervals: ts (j)
Make the settings for

The sampling interval calculator 26 shown in FIG.
By using, the speed can be increased as compared with the method of calculating the sampling interval by software processing as shown in FIG. Therefore, especially the sampling interval database
The setting may be directly performed in the sampling timing storage unit 24 by the scan without passing through the.

Regarding the sampling interval database 27 The sampling interval database 27 stores the sampling interval data obtained in advance by the sampling interval calculator 26 in a database corresponding to a plurality of combinations of arbitrary slice planes and arbitrary gradient magnetic field waveforms,
The data is transferred to the sampling timing storage unit 24 in the sequence controller 9 as needed. By preparing the sampling interval database 27, it is not necessary to calculate the sampling interval every time imaging is performed, and the processing speed can be increased.

If the sampling interval data can be prepared immediately by actual measurement or calculation, a method of preparing a database every time, performing a prescan every measurement, and directly setting the data in the sampling timing storage unit 24 is also applicable.

Sampling timing storage unit 24 The sampling timing storage unit 24 in the sequence controller 9 is a memory capable of high-speed reading, and holds, for example, the sampling interval data transferred from the sampling interval database 27 as described above. The sampling pulse generator 25 generates sampling pulses at irregular intervals in real time using the sampling interval data. The generated sampling pulse is sent to the data collection unit 10 and used for sampling in A / D conversion. As a result, the data collection unit 10 obtains data points at equal intervals in the phase space, so that the data processing unit 11 may perform the same image configuration processing as in the related art.

Regarding the sampling pulse generating unit 25, the time resolution of irregularly spaced sampling: Δtq9 is a time resolution for data collection: Δtq9 in order to reduce artifacts caused by the displacement of data points in the phase space.
It is required that tu be as small as one tenth or more. It is generally difficult to generate such high-speed non-uniform sampling pulses on-line. Therefore, in the present embodiment, the sampling pulse generation circuit 25 is provided in the sequence controller 9 and the irregular-interval sampling timing is set at high speed from the irregular-interval sampling timing data set offline in the sampling timing storage unit 24 as described above. A pulse is generated.

FIG. 8 is a block diagram showing a configuration example of the sampling pulse generator 25, which comprises a clock generator 81, an event counter 82, and a comparator 83. Event counter
The 82, the comparator 83, and the sampling timing storage unit 24 need a word length of at least m bits in order to represent the event time. However, 2 ^m > N · nq.

Before the sequence is executed, the data of the sampling timing is transferred from the sampling interval database 27 or the prescan data to the sampling timing storage unit 24. This data has time resolution: Δtq,
This is m-bit binary data, and an event time from the input of the A / D start pulse is set.

FIG. 9 is a timing chart showing the operation of FIG. 8, in which the A / D start pulse starts the clock generator 81 to generate a clock with a time interval Δtq. With this clock, one of the input data P0 to Pm
Is an event counter consisting of a binary binary counter
Updated sequentially by 82. As the other input data Q1 to Qm of the comparator 83, the first data in the sampling timing storage unit 24 storing the data of the sampling timing ts is given. Comparator 83
An A / D sampling pulse (unequal interval sampling pulse) is generated when both input data P0 to Pm and Q1 to Qm match.
At the same time as supplying the data to the data collection unit 10, the reading counter in the sampling timing storage unit 24 is incremented so that the next data in the sampling timing storage unit 24 is read. Hereinafter, by repeating the same operation, irregular sampling pulses are generated.

FIG. 10 is a block diagram showing another example of the configuration of the sampling pulse generator 25. The clock generator 91 generates a clock with a time interval Δtq, and the clock generator 91 generates the clock with the data read from the sampling timing storage 24. It comprises an AND circuit 92 for taking a logical product. This is a configuration corresponding to the case where the word length of the sampling timing storage unit 24 is 1 bit and the number of data is l, where l> N · nq.

Before the sequence is executed, data is transferred from the sampling interval database 27 or the prescan data to the sampling timing storage unit 24. This data represents the time axis of the event time, and the time resolution is Δtq. When this data is “0”, no sampling pulse is generated, and when “1”, a sampling pulse is generated.
It is developed from the sampling interval database 27 or the event time of the prescan data.

FIG. 11 is a time chart showing the operation of FIG. 10,
The A / D start pulse activates the clock generator 91 to generate a clock with a time interval Δtq. Reading is performed sequentially from the sampling timing storage unit 24 by this clock, and when the read value is “1”, a sampling pulse is generated via the AND circuit 92.

[Effects of the Invention] As described above, according to the present invention, magnetic resonance signal data is collected by sampling magnetic resonance signals at time intervals such that the time integral of the gradient magnetic field waveform changes by a substantially constant amount. Accordingly, even if a waveform other than a rectangular wave such as a sine wave is used as the gradient magnetic field waveform, the intervals between data points on the phase space can be made equal.

Therefore, it is possible to realize a real-time MRI by ultra-high-speed imaging such as an echo planar method or an ultra-fast Fourier method, without requiring a pre-processing such as a method of combining interpolation at regular intervals.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the configuration of a magnetic resonance imaging apparatus according to one embodiment of the present invention, FIG. 2 is a diagram showing a pulse sequence for acquiring image data in the embodiment, and FIG. 4 is a block diagram showing a configuration of a sampling pulse generator in FIG. 4, FIG. 5 is a block diagram showing another example of a gradient magnetic field waveform measuring method in the embodiment, and FIG. FIG. 2 is a diagram showing an algorithm when the sampling interval calculation unit in FIG. 2 is realized by software, FIG. 7 is a block diagram showing an example in which the sampling interval calculation unit in FIG. 2 is realized by hardware, and FIG. FIG. 9 is a block diagram showing a configuration example of a sampling pulse generator in FIG. 2, FIG. 9 is a time chart for explaining the operation of FIG. 8, and FIG. FIG. 11 is a block diagram showing another example of the configuration of the loose generating unit, FIG. 11 is a time chart for explaining the operation of FIG. 10, and FIG. 12 shows the relationship between the read gradient magnetic field waveform Gr and the phase kx of the magnetic resonance signal. FIG. 1 ... Static magnetic field magnet 3 ... Gradient coil group 6 ... Probe 9 ... Sequence controller 10 ... Data collection unit 15 ... Magnetic field sensor 16 ... Sampling interval setting unit 23 ... Gradient magnetic field control waveform generation unit 24 ... … Sampling timing storage unit 25… Sampling pulse generation unit 26… Sampling interval calculation unit 27… Sampling interval database

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Claims (2)

(57) [Claims] 1. A high-frequency magnetic field and a gradient magnetic field are applied to a subject placed in a uniform static magnetic field in accordance with a predetermined pulse sequence, and a magnetic resonance signal obtained from inside the subject is sampled using a sampling pulse. A magnetic resonance imaging apparatus that collects magnetic resonance signal data and performs image reconstruction by switching the readout gradient magnetic field so that the image reconstruction can be performed within a time period in which transverse magnetization in the slice plane is relaxed. In a magnetic resonance imaging apparatus which collects all necessary magnetic resonance signal data, a gradient magnetic field means for generating a gradient magnetic field having an arbitrary waveform; a means for obtaining a waveform of a gradient magnetic field generated by the gradient magnetic field means; Means for determining a time interval during which the time integration value of the determined gradient magnetic field waveform changes by a substantially constant amount, and sampling at the time interval And a sampling pulse generating means for generating a pulse. 2. A high-frequency magnetic field and a gradient magnetic field for slicing, reading, and phase encoding are applied to a subject placed in a uniform static magnetic field in accordance with a predetermined pulse sequence.
A magnetic resonance imaging apparatus for collecting magnetic resonance signal data by sampling a magnetic resonance signal obtained from a predetermined slice plane in a subject using a sampling pulse and performing image reconstruction, wherein the readout gradient magnetic field is used. In the magnetic resonance imaging apparatus that collects all the magnetic resonance signal data necessary for the image reconstruction within the time when the transverse magnetization in the slice plane is relaxed, a gradient magnetic field for reading an arbitrary waveform is generated. Reading gradient magnetic field generating means; means for obtaining a waveform of the reading gradient magnetic field generated by the reading gradient magnetic field generating means; and a time integration value of the reading gradient magnetic field waveform obtained by the means being substantially constant. Storage means for obtaining and storing a time interval between changes for each combination of the waveform of the read gradient magnetic field and the slice plane; And a sampling pulse generating means for generating the sampling pulse at the time interval stored by the storage means.