JP3748678B2 - Magnetic resonance imaging device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance signal acquisition method and a magnetic resonance imaging apparatus, and more particularly to magnetic resonance that collects a plurality of magnetic resonance signals sufficient to generate a slice of a tomogram based on a magnetic resonance signal generated by one excitation. The present invention relates to a signal acquisition method and a magnetic resonance imaging apparatus that generates an image based on magnetic resonance signals collected in this manner.
[0002]
[Prior art]
As an example of a method of collecting a plurality of magnetic resonance signals sufficient to generate a slice image of a slice based on a magnetic resonance signal generated by one excitation, a single shot fast spin echo (single shot fast spin echo) ) The law is known. This is because a plurality of magnetic resonance signals sufficient to generate one slice of tomographic image by applying a plurality of 180 ° pulses and a phase encode gradient magnetic field per one excitation by 90 ° pulse. Is intended to be collected.
[0003]
In order to collect spin echo signals with a good signal-to-noise ratio (SNR) when the single shot first spin echo method is executed in a low and medium magnetic field with a static magnetic field strength of 0.5T or less, for example, A single shot / first spin echo method pulse sequence is repeatedly executed a plurality of times, and a plurality of sets of signals obtained are averaged. The number of repetitions of the pulse sequence is called NEX (number of excitation). Usually, NEX is selected to be about 2 to 3.
[0004]
At this time, the repetition of the pulse sequence is performed at a predetermined repetition time TR including a waiting time for longitudinal relaxation of the spin. The waiting time is set to a time during which the spin having a longitudinal relaxation time T1 of about 1800 mS is sufficiently relaxed, for example, about 10 seconds. If it takes 1 second to execute the pulse sequence of the single shot first spin echo method once, the repetition time TR is set to 11 seconds, for example.
[0005]
[Problems to be solved by the invention]
The time required for the scan is NEX times the repetition time TR. When the repetition time TR is 11 seconds, if NEX is 2, the scan time is 22 seconds. That is, the scanning sequence is not completed unless 22 seconds have elapsed after the start of scanning, and the imaging apparatus cannot proceed to the next step.
[0006]
For this reason, for example, in a scan in which signal acquisition is completed when the 2NEX pulse sequence is completed in 1 second, the subsequent 10-second waiting time is wasted time.
[0007]
The present invention has been made to solve the above-described problems, and an object of the present invention is to realize a magnetic resonance signal acquisition method and a magnetic resonance imaging apparatus that acquire a magnetic resonance signal having a good SNR without wasting time. is there.
[0008]
[Means for Solving the Problems]
(1) The first present invention that solves the above-described problem is a magnetic resonance signal collection that collects a plurality of magnetic resonance signals sufficient to generate a tomographic image of one slice based on a magnetic resonance signal generated by one excitation. A method of repeating the acquisition of the magnetic resonance signal a plurality of times at a predetermined repetition time per slice, and ending the signal acquisition sequence immediately after collecting the magnetic resonance signal corresponding to the final round of the repetition. Features.
[0009]
(2) The second aspect of the present invention that solves the above problems includes a static magnetic field forming unit that forms a static magnetic field in a space that accommodates a subject, a gradient magnetic field forming unit that forms a gradient magnetic field in the space, and the space. A high-frequency magnetic field forming means for forming a high-frequency magnetic field at one time, a measuring means for measuring a magnetic resonance signal from the space, a gradient magnetic field forming means, the high-frequency magnetic field forming means, and the measuring means are controlled to perform one excitation. A scan sequence for collecting a plurality of magnetic resonance signals sufficient to generate a tomographic image of one slice based on the magnetic resonance signal generated in step 1 is repeated a plurality of times at a predetermined repetition time per slice, and the magnetism corresponding to the last round of the repetition is performed. Control means for ending the scan sequence immediately after collecting the resonance signal, and an image based on the magnetic resonance signal measured by the measurement means Image generating means for generating, characterized in that it comprises a.
[0010]
(Function)
In the present invention, when the collection of magnetic resonance signals is repeated a plurality of times at a predetermined repetition time per slice, the scan sequence is immediately terminated after collecting the magnetic resonance signals corresponding to the final iteration. Thereby, the remaining time of the repetition time is excluded from the scan time, and the scan time is shortened.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The present invention is not limited to the embodiment.
[0012]
FIG. 1 shows a block diagram of a magnetic resonance imaging apparatus. This apparatus is an example of an embodiment of the present invention. An example of an embodiment relating to the apparatus of the present invention is shown by the configuration of the apparatus. An example of an embodiment related to the method of the present invention is shown by the operation of the present apparatus.
[0013]
(Constitution)
The configuration of this apparatus will be described. As shown in FIG. 1, a substantially cylindrical static magnetic field generator 2 forms a uniform static magnetic field in its internal space. The static magnetic field generator 2 is an example of an embodiment of the static magnetic field forming means in the present invention.
[0014]
A gradient coil (coil) portion 4 and a body coil (body coil) portion 6 each having a substantially cylindrical shape are arranged inside the static magnetic field generating portion 2 so as to share a central axis. The subject 8 is carried into the internal space of the body coil unit 6 by carrying means (not shown).
[0015]
A gradient driving unit 10 is connected to the gradient coil unit 4. The gradient coil unit 4 and the gradient driving unit 10 are an example of an embodiment of a gradient magnetic field forming unit in the present invention. The gradient drive unit 10 gives a drive signal to the gradient coil unit 4 to generate a gradient magnetic field. There are three types of gradient magnetic fields that are generated: a slice gradient magnetic field, a read out gradient magnetic field, and a phase encoding gradient magnetic field.
[0016]
A transmission unit 12 is connected to the body coil unit 6. The body coil section 6 and the transmission section 12 are an example of an embodiment of the high-frequency magnetic field forming means in the present invention. The transmission unit 12 applies a drive signal (RF (radio frequency) pulse) to the body coil unit 6 to generate an RF magnetic field, thereby exciting the spin in the body of the subject 8.
[0017]
A magnetic resonance signal generated by the excited spin is detected by the body coil unit 6. A receiving unit 14 is connected to the body coil unit 6. The receiving unit 14 is configured to receive a signal detected by the body coil unit 6.
[0018]
An analog-to-digital converter 16 is connected to the receiver 14. The body coil section 6, the receiving section 14, and the analog / digital conversion section 16 are an example of an embodiment of the measuring means in the present invention. The analog / digital converter 16 converts the output signal of the receiver 14 into a digital signal. The analog / digital converter 16 is connected to a computer 18.
[0019]
The computer 18 receives a digital signal from the analog / digital converter 16 and stores it in a memory (not shown). A data space is formed in the memory. This data space constitutes a two-dimensional Fourier space. The computer 18 reconstructs an image of the subject 8 by performing two-dimensional inverse Fourier transform on the data in the two-dimensional Fourier space. The computer 18 is an example of an embodiment of image generation means in the present invention.
[0020]
A controller 20 is connected to the computer 18. The control unit 20 is an example of an embodiment of control means in the present invention. The control unit 20 is connected to the gradient driving unit 10, the transmission unit 12, the reception unit 14, and the analog / digital conversion unit 16. The control unit 20 controls the gradient driving unit 10, the transmission unit 12, the reception unit 14, and the analog / digital conversion unit 16 based on a command given from the computer 18.
[0021]
A display unit 22 and an operation unit 24 are connected to the computer 18. The display unit 22 displays various information including a reconstructed image output from the computer 18. The operation unit 24 is operated by an operator and inputs various commands and information to the computer 18.
[0022]
(Operation)
The operation of this apparatus will be described. First, the single shot first spin echo method will be described. FIG. 2 shows an example of a pulse sequence by the single shot fast spin echo method.
[0023]
In FIG. 2, the horizontal axis represents time, and the vertical axis represents signal intensity. (A) is an RF pulse signal (90 ° pulse and 180 ° pulse), (b) is a slice gradient magnetic field signal (slice gradient), (c) is a readout gradient magnetic field signal (leadout gradient) and dephase ( dephase) Gradient magnetic field signal (dephase gradient), (d) a phase encode gradient magnetic field signal (phase encode gradient), and (e) a spin echo signal.
[0024]
The sequence (a) of the RF pulse shows the operation of the body coil unit 6 and the transmission unit 12. Slice gradient sequence (b), readout gradient and dephase gradient sequence (c), and phase encode gradient sequence (d) show the operation of gradient coil section 4 and gradient drive section 10.
[0025]
FIG. 3 conceptually shows the spin behavior. In the figure, x ′, y ′ and z ′ indicate three coordinate axes perpendicular to each other in the rotating coordinate system. Hereinafter, the operation by the single shot first spin echo method will be described with reference to FIGS.
[0026]
As shown in FIG. 2, spin excitation is performed by a 90 ° pulse at time t1. At this time, a slice gradient is simultaneously applied. As a result, as shown in FIG. 3A, the spin directed in the z ′ direction is tilted by 90 ° and directed in the y ′ direction.
[0027]
Next, a dephase gradient is applied for a predetermined time at time t2. As a result, the spin phase is dispersed (dephased) as shown in FIG.
Next, at time t3, spin inversion is performed by a 180 ° pulse (180 ° y pulse). At this time, a slice gradient is simultaneously applied. As a result, as shown in FIG. 3C, the spin rotates 180 ° around the y ′ axis.
[0028]
Next, a phase encode gradient is applied at time t4, and spin phase encode is performed. Also, the application of the readout gradient starts. The spin phase change continues during the application period of the readout gradient, and the dispersed phase converges as shown in FIG.
[0029]
At time t5 in the middle, the integrated value of the readout gradient becomes equal to the integrated value of the dephase gradient, and the spin phases are aligned as shown in FIG. At this point, the peak of the first spin echo signal occurs.
[0030]
After time t5, as shown in (f) of FIG. 3, the phase is dispersed by the continuation of the phase change of the spin, and the echo signal is attenuated.
After applying the phase encode gradient, the echo signal is read out during the readout gradient application period. Reading of the echo signal is performed by the system of body coil unit 6-receiving unit 14-analog / digital converting unit 16-computer 18. The same applies hereinafter.
[0031]
FIG. 4 shows the spin echo signal in this period when the time axis is enlarged. However, the spin echo signal is shown as a conceptual diagram rather than an accurate waveform diagram. As shown in the figure, the amplitude of the spin echo signal gradually increases from the reading start time t4 ′ to t5 and reaches a peak, and the amplitude attenuates from the time t5 to the reading end time t6.
[0032]
Returning to FIG. 2, a phase encode gradient having a reverse polarity is applied at the end time t6 of reading the spin echo signal. As a result, the phase encoding amount is pulled back to zero.
[0033]
At time t7, the second 180 ° pulse is applied, and spin reinversion is performed. At this time, a slice gradient is applied. Note that the peaks at both ends of the waveform of the slice gradient are the FID (free induction decay) signal spoil after 180 ° pulse and stimulated echo (killer pulse I) for imaging. It is.
[0034]
Thereafter, similarly to the first time, the phase encoding of the spin by the phase encoding gradient and the reading of the spin echo signal by the readout gradient are performed. However, the intensity of the phase encode gradient is different from the first. As a result, a second spin echo signal having a different phase encoding amount is obtained.
[0035]
In the same manner, spin echo signals having different phase encoding amounts are sequentially obtained by applying the 180 ° pulse, slice gradient, phase encode gradient, and readout gradient each time. These spin echo signals are collected in the memory of the computer 18.
[0036]
Accompanying the reading of such a spin echo signal, data collection proceeds along a predetermined trajectory (trajectory) in the two-dimensional Fourier space. This is shown in FIG. In FIG. 5, k is a two-dimensional Fourier space. This is also called k-space. kx and ky are two coordinate axes orthogonal to each other in the two-dimensional Fourier space k, where kx is a frequency axis (lead-out axis) and ky is a phase axis (phase encode axis).
[0037]
The data collection trajectory trj starts from, for example, kx = -100 and ky = 100. The unit of coordinates is%. kx = -100 corresponds to the left end of the spin echo signal shown in FIG. ky = 100 is the phase encoding amount of the first spin echo echo signal. This is determined by the phase encoding gradient at time t4.
[0038]
As the spin echo signal is read during the readout period (collection of spin echo data), the trajectory trj reaches kx = 100 along the arrow. The point of kx = 0 in the middle corresponds to the time t5, and the peak value is collected here.
[0039]
The trajectory is lowered by one step due to the second phase encoding at time t8, and data collection for the next spin echo is performed during the next readout period, and the trajectory trj is kx = −100 along the arrow in the second row. To kx = 100.
[0040]
Similarly, data is collected in the two-dimensional Fourier space k along the kx axis while sequentially transitioning downward along the ky axis at each phase encoding. As a result, data for filling the entire two-dimensional Fourier space k is collected.
[0041]
Alternatively, data collection is performed only by filling a half of the two-dimensional Fourier space k corresponding to, for example, the upper half of the ky axis. This is preferable in that the number of 180 ° pulse applications is halved and the scan time is shortened.
[0042]
An image is reconstructed by the computer 18 based on the data in the two-dimensional Fourier space k. Image reconstruction is performed by two-dimensional inverse Fourier transform. If data collection is limited to filling only half of the two-dimensional Fourier space k, a so-called half fourie method for reconstructing an image from half of the two-dimensional Fourier space k is used. .
[0043]
When NEX is 2 or more, the pulse sequence of FIG. 2 is repeated NEX times per slice. The repetition time TR of the pulse sequence is, for example, 11 seconds including a waiting time of 10 seconds until a spin whose longitudinal relaxation time T1 is about 1800 mS recovers 99.6%.
[0044]
In order to increase the scanning efficiency, a plurality of slices with different positions are sequentially scanned while waiting for relaxation of the spin, that is, a multi-slice scan is performed. In that case, the pulse sequence shown in FIG. 2 is executed for each slice.
[0045]
FIG. 6 shows a conceptual diagram of a multi-slice scan with NEX of 2 or more. As shown in the figure, n-slice scans are sequentially executed from the first slice to the n-th slice. The execution time of the pulse sequence per slice is acttr. Acttr is, for example, 1 second.
[0046]
The time from the first scan (1NEX) of the first slice to the start of the second scan (2NEX) of the same slice is the relaxation waiting time T. The relaxation waiting time T is TR-acttr, for example, 10 seconds.
[0047]
When the number of slices n is small, the 1NEX pulse sequence of the nth slice ends in the middle of the relaxation waiting time T, as shown in FIG. On the other hand, when the number of slices n increases, the end point of the 1NEX pulse sequence of the nth slice exceeds the relaxation waiting time T as shown in FIG.
[0048]
Next, the operation of this apparatus when performing multi-slice scanning with multi-NEX will be described. FIG. 7 shows a flow diagram of the operation of the apparatus. This operation is performed under the control of the control unit 20.
[0049]
As shown in the figure, in step 700, the value of NEX is set to m. Here, for example, m = 2. As the value of m, a standard value prepared in advance according to the scan site is used. Alternatively, the operator may set a desired value each time.
[0050]
Next, in step 702, the count value i of a counter (not shown) is reset to zero.
Next, in step 704, 1 is added to the count value i. The count value i indicates the number of times (NEX) of scanning. Now, by adding 1 to 0, the first scan, that is, 1NEX scan is shown.
[0051]
Next, in step 706, the first slice is scanned.
Next, in step 708, the second slice is scanned.
Thereafter, scanning is sequentially performed for each slice, and in step 720, the nth slice (last slice) is scanned.
[0052]
Next, in step 722, it is determined whether or not the count value i is equal to m.
Now, since i = 1 and not equal to m = 2, the result is No, and the process branches to step 724.
[0053]
Next, in step 724, it is determined whether or not acttr × (n−1)> T.
This determination determines whether the magnitude relationship between the scan time acttr × (n−1) of n−1 slices and the relaxation waiting time T corresponds to (a) or (b) of FIG. Is.
[0054]
In the case of No, it corresponds to (a) of FIG. 6, and the process branches to step 726 and waits for relaxation, that is, waits until the relaxation waiting time T elapses. Then, after the relaxation waiting time T has elapsed, the process returns to step 704.
[0055]
Note that, during the standby period, for example, if only the slice gradient is generated in the same manner as during the execution of the pulse sequence, the operator feels as if the device is operating due to the sound generated by the gradient coil unit 4. It is preferable that the standby state is not mistaken for a stopped state or a failure state (hung up) of the apparatus.
[0056]
In the case of Yes, it corresponds to (b) of FIG. 6, and since the relaxation waiting time T has already passed, the process returns to step 704 without waiting for relaxation.
Next, in step 704, 1 is added to the count value i, so that i = 2. Thereafter, in steps 706 to 720, 2NEX scans are sequentially performed on the first slice to the nth slice.
[0057]
Next, in step 722, it is determined whether i = m. Since i = 2, it is Yes and the process branches to step 728.
Next, in step 728, the pulse sequence is terminated. Thus, for example, as shown in FIG. 8A, when the time point when the scan of the 2NEX nth slice (the last slice) is within the relaxation waiting time T, wait until the relaxation waiting time T elapses. Immediately terminate the pulse sequence.
[0058]
In this way, the waiting for relaxation is terminated by ending the pulse sequence, and unnecessary waiting time is eliminated. Therefore, the scan time is shortened by the amount that the waiting for relaxation is terminated.
[0059]
In particular, as shown in FIG. 8B, for example, when a single slice is scanned with NEX being 2, the relaxation waiting time T in 2NEX is completely eliminated, so that the effect of shortening the scanning time is great. That is, when the waiting for relaxation is not performed as in the prior art, for example, the scan time which took 22 seconds is shortened to 12 seconds, and is almost halved. This is particularly effective when the subject 8 is breath-hold scanned.
[0060]
Next, in step 730, the image of each slice is reconstructed based on the spin echo data collected as described above. In image reconstruction, the SNR of the collected data is improved by averaging the 1NEX data and the 2NEX data for each slice in advance.
[0061]
Next, in step 730, the reconstructed image is displayed as a visible image on the display unit 22.
The above is an example using the single shot first spin echo method, but the magnetic resonance signal acquisition is not limited to the single shot first spin echo method. For example, the echo planer method should be used. Also good. In that case, when NEX performs scanning of 2 or more, it is possible to eliminate wasted time in the same manner as described above.
[0062]
In addition, although an example of a so-called horizontal magnetic field type magnetic resonance imaging apparatus in which the direction of the static magnetic field is parallel to the body axis of the subject has been described, the present invention is directed to the direction of the static magnetic field perpendicular to the body axis of the subject. It can also be implemented by a so-called vertical magnetic field type magnetic resonance imaging apparatus.
[0063]
FIG. 9 shows a block diagram of a perpendicular magnetic field type magnetic resonance imaging apparatus. As shown in the figure, the static magnetic field generator 2 forms a uniform static magnetic field in its internal space. The static magnetic field generation unit 2 includes a pair of magnetic generators (not shown) such as permanent magnets, which are opposed to each other in the vertical direction with a gap therebetween, and form a static magnetic field (vertical magnetic field) in the facing space. Yes. The magnetic generator is not limited to a permanent magnet, but may be a normal conductive magnet or a superconductive magnet.
[0064]
The static magnetic field generating unit 2 is provided with gradient coil units 4 and 4 ′ and transmission coil units 5 and 5 ′, respectively, which are similarly opposed to each other in the vertical direction while maintaining an interval.
A body coil portion 6 having a substantially cylindrical shape is disposed in the static magnetic field space between the transmission coil portions 5 and 5 '. The central axis of the body coil unit 6 is orthogonal to the direction of the static magnetic field. A subject 8 is carried into a generally cylindrical space formed inside the body coil section 6 by a carrying means (not shown). The body axis of the subject 8 is orthogonal to the direction of the static magnetic field.
[0065]
A gradient driving unit 10 is connected to the gradient coil units 4 and 4 ′. The gradient drive unit 10 generates a gradient magnetic field by giving drive signals to the gradient coil units 4 and 4 '. Three types of gradient magnetic fields are generated: a slice gradient magnetic field, a read gradient magnetic field, and a phase encoding gradient magnetic field.
[0066]
A transmission unit 12 is connected to the transmission coil units 5 and 5 ′. The transmission unit 12 applies a drive signal to the transmission coil units 5 and 5 ′ to generate an RF magnetic field, thereby exciting the spin of specific atoms in the body of the subject 8.
[0067]
The body coil section 6 detects a magnetic resonance signal generated by the excited spin in the subject 8. A receiving unit 14 is connected to the body coil unit 6. The receiving unit 14 is configured to receive a signal detected by the body coil unit 6.
[0068]
The configuration is the same as that of the apparatus shown in FIG. With such a configuration, an imaging operation similar to that of the apparatus shown in FIG. 1 is performed.
[0069]
【The invention's effect】
As described above in detail, according to the present invention, when collecting a plurality of magnetic resonance signals sufficient to generate a tomographic image of one slice based on the magnetic resonance signals generated by one excitation, the magnetic resonance signals are collected. Since the signal acquisition sequence is terminated immediately after acquiring the magnetic resonance signal corresponding to the final repetition of the repetition at a predetermined repetition time per slice, the magnetic resonance with good SNR can be achieved without wasting time. A magnetic resonance signal collecting method and a magnetic resonance imaging apparatus for collecting signals can be realized.
[Brief description of the drawings]
FIG. 1 is a block diagram of an exemplary apparatus according to an embodiment of the present invention.
FIG. 2 is a diagram illustrating an example of a pulse sequence executed by an apparatus according to an embodiment of the present invention.
FIG. 3 is a conceptual diagram of spin behavior when a spin echo method is executed.
FIG. 4 is a conceptual diagram of a spin echo signal read by an example apparatus according to an embodiment of the present invention.
FIG. 5 is a conceptual diagram of data collection into a two-dimensional Fourier space by an apparatus according to an example of an embodiment of the present invention.
FIG. 6 is a conceptual diagram when multi-slice scanning is performed with multi-NEX by the apparatus according to the example of the embodiment of the present invention;
FIG. 7 is a flowchart of the operation of the apparatus according to the example of the embodiment of the present invention.
FIG. 8 is a conceptual diagram when a multi-slice scan is performed with NEX set to 2 by the apparatus according to the embodiment of the present invention.
FIG. 9 is a block diagram of a vertical magnetic field type magnetic resonance imaging apparatus.
[Explanation of symbols]
2 Static magnetic field generating unit 4, 4 ′ Gradient coil unit 5, 6 ′ Transmitting coil unit 6 Body coil unit 8 Subject 10 Gradient driving unit 12 Transmitting unit 14 Receiving unit 16 Analog / digital conversion unit 18 Computer 20 Control unit 22 Display unit 24 Operation part

Claims (2)

A static magnetic field forming means for forming a static magnetic field in a space for accommodating a subject;
A gradient magnetic field forming means for forming a gradient magnetic field in the space;
High-frequency magnetic field forming means for forming a high-frequency magnetic field in the space;
Measuring means for measuring magnetic resonance signals from the space;
By controlling the gradient magnetic field forming means, the high-frequency magnetic field forming means, and the measuring means, a plurality of magnetic resonance signals that are sufficient to generate one slice of tomographic image are collected based on the magnetic resonance signals generated by one excitation. A control unit that repeats the scan sequence a plurality of times at a predetermined repetition time per slice, and immediately ends the scan sequence after collecting the magnetic resonance signals corresponding to the final round of the repetition;
An image generation means for generating an image based on the magnetic resonance signal measured by the measurement means. The magnetic resonance imaging apparatus according to claim 1,
The magnetic resonance imaging apparatus, wherein the scan sequence is based on a single shot first spin echo method.

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