JPH02149251A - MR imaging method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] [Industrial application field]

This invention relates to an MR imaging method that uses nuclear magnetic resonance (NMR) to create images, and in particular, to an ultrahigh-speed imaging method such as echo plechie imaging.
Regarding R imaging method.

[Conventional technology]

Ultrahigh-speed imaging methods, such as echo plechie imaging, differ from conventional MR imaging methods such as spin warp methods, in that ultrahigh-speed imaging methods, such as echo plechie imaging, encode different amounts of phase in multiple echo signals generated by a single excitation. , it is possible to obtain the information necessary to create an image in an extremely short time compared to conventional methods. These ultra-high-speed imaging methods appear to be particularly useful when imaging areas with a lot of movement (such as the heart and upper abdomen), and also enable real-time imaging with MR imaging methods, which was previously impossible. It is considered to be a very important imaging method with unique characteristics.

[Problem to be solved by the invention]

However, in ultrahigh-speed imaging methods, multiple echo signals are generated with one excitation, and each of them is subjected to different amounts of phase encoding to obtain the information necessary to create an image. The plurality of echo signals generated by excitation are affected by the transverse relaxation time T2 of the object and the non-uniformity of the magnetic field, and the signal strength of the echo signals generated later becomes smaller. As a result, there is a problem in that even if image reconstruction is performed using data obtained from these echo signals as is, a correct image cannot be obtained. Of course, a gradient magnetic field system (gradient magnetic field coil and gradient magnetic field power supply) with ultra-high-speed switching characteristics and extremely large gradient magnetic field capability that allows the temporal attenuation of the echo signal train due to the influence of the transverse relaxation time T2 etc. to be ignored is used. Although this problem does not occur, it is actually impossible. An object of the present invention is to provide an MR imaging method that can correct the temporal attenuation of the signal intensity of an echo signal train in an ultrahigh-speed imaging method to obtain a correct image.

[Means to solve the problem]

In order to achieve the above object, in the MR imaging method according to the present invention, an excitation signal is applied once, and then a readout gradient magnetic field is sequentially reversed to generate multiple echo signals. In an ultra-high-speed imaging method having a pulse sequence in which a gradient magnetic field for phase encoding is applied, data is collected by performing the above pulse sequence without applying the gradient magnetic field for phase encoding, and each The feature is that the signal strength of each echo signal is determined, and the signal strength of each echo signal obtained by the above-mentioned normal pulse sequence is corrected in relation to the above-mentioned signal strength.

[For production]

In an ultrahigh-speed imaging method that generates multiple echo signals with a single excitation and applies different amounts of phase encoding to each of them to obtain the information necessary to create an image,
The intensity of the echo signal train is attenuated over time due to the transverse relaxation time T2 of the object and the non-uniformity of the magnetic field. Here, before, during, or after performing the pulse sequence of the conventional ultrahigh-speed imaging method for the above-mentioned object, this ultrahigh-speed imaging method, which is completely the same except that the phase encode amount is set to zero, When a pulse sequence is performed, the intensity of each echo signal is not affected by the gradient magnetic field for phase encoding, and is faithful only to the temporal attenuation of the echo signal intensity caused by the transverse relaxation time T2 of the object, magnetic field inhomogeneity, etc. It will depend on. Therefore, by capturing the intensity of each echo signal from the data collected with this pulse sequence, the signal intensity that differs for each echo signal due to the transverse relaxation time T2 of the object, magnetic field inhomogeneity, etc. can be converted to the same signal intensity. A correction coefficient for alignment can be found. Therefore, by applying this correction coefficient to the intensity of each echo signal obtained by the normal ultra-high-speed imaging method, variations in the intensity of each echo signal due to transverse relaxation time T2, magnetic field inhomogeneity, etc. are corrected, and the same signal is This means that the strength can be adjusted. Therefore, since the signal intensities of all the echo signals are uniform, accurate MR and MR images can be obtained by subjecting the data thus obtained to two-dimensional Fourier transformation.

【Example】

Next, an embodiment of the present invention will be described with reference to the drawings. Figures 1(a) and (b) show an embodiment in which the present invention is applied to an ultrahigh-speed imaging method using the field echo method, and Figure 1(a) shows T2 relaxation of each echo signal and magnetic field isolation. FIG. 1(b) shows a pre-scan for obtaining intensity attenuation information based on uniformity, etc., and FIG. 1(b) shows an actual imaging scan of the subject. The pulse sequence shown here is performed, for example, by an MR imaging system as shown in FIG. The subject 1 is placed in a static magnetic field generated by the static magnetic field generator 2, and a pre-scan is first performed in order to obtain intensity attenuation information of each echo signal according to instructions from the host computer 3. At this time, the waveform generation circuit 31
A predetermined waveform is generated, and a high-frequency signal from the high-frequency oscillation circuit 33 is amplitude-modulated in the amplitude modulation circuit 32 according to this waveform, and then transmitted through the power amplifier 34 to the transmitting coil 35.
and a predetermined excitation pulse is given to the subject 1. At the same time, the waveform generation circuit 27 supplies a Gz power supply (a power supply for generating a gradient magnetic field Gz whose magnetic field strength is inclined in the Z direction; the same applies to the Gx power supply and Gy power supply described later).
s> Send a signal with a predetermined waveform to 26, and connect the Gz coil (Z
A current is applied to a coil (the same applies to the Gx coil and Gy coil, which will be described later) 23 for generating a gradient magnetic field Gz whose magnetic field strength is inclined in a direction, to generate a gradient magnetic field Gz as shown in Fig. 1(a). A specific slice plane of the subject 1 in the Z direction is selectively excited. Thereafter, the waveform generation circuit 27 sends a signal with a predetermined waveform to the Gx power supply 24, and current is applied to the Gx coil 21 so that the gradient magnetic field Gx is sequentially reversed as shown in FIG. 1(a). Ru. Then, as shown in FIG. 1(a), echo signals A, B, and C are generated. D, E, F, . . . occur sequentially. Since these echo signals are generated when the gradient magnetic field Gx is applied, the position signal in the X direction is encoded in frequency. This echo signal is received by the receiving coil 41, and the preamplifier 4
2 and is sent to the detection circuit 43. In the detection circuit 43, the high frequency signal from the high frequency oscillation circuit 33, which was used as a carrier wave in the amplitude modulation circuit 32, is used as a reference signal, and quadrature detection is performed. The signal obtained by the detection is sent to the A/D conversion circuit 44, where it is sampled by a sampling pulse as shown in FIG. 1(a), and converted into a digital signal I\. This digital data is taken into the host computer 3. In addition, in this pre-scan, no signal is sent to the G, y power supply 25, and no current is passed through the Gy coil 22.
The gradient magnetic field Gy is kept at zero as shown in FIG. 1(a). A plurality of echo signals sequentially generated in such a pulse sequence have an intensity corresponding to the transverse relaxation time T2 of the subject 1.
It is attenuated due to magnetic field inhomogeneity, magnetic field inhomogeneity, etc. Furthermore, in this case, since the phase encoding gradient magnetic field Gy is set to zero, the intensity of each echo signal faithfully represents the attenuation due to the transverse relaxation time T2 of the subject I, magnetic field inhomogeneity, and the like. Therefore, these echo signals are sampled and the data is loaded into the host computer 3, and the host computer 3 captures the signal strength of each of them.The strength of the first echo signal A is used as a reference, and the following echo signals B, C, D
. ... is calculated, and these values are stored in the memory device 4 as correction coefficients for each of the second and subsequent echo signals. Next, a normal pulse sequence is performed based on instructions from the host computer 3, and actual imaging of the subject 1 is performed. At this time, in addition to each operation explained in the pre-scan above, a signal with a predetermined waveform is sent from the waveform generation circuit 27 to the Gy power supply 25, and as shown in FIG. 1(b), the phase A gradient magnetic field Gy pulse for encoding is applied to encode position information in the Y direction. Then, data is taken into the host computer 3 using a sampling pulse for each echo signal. The host computer 3 multiplies the data representing the intensity of the second and subsequent echo signals by the correction coefficient for each echo signal stored in the memory device 4 described above. As a result, a large number of echo signals A, B. The intensities of C, D, . Therefore, by correcting the intensities so that they are the same, the signal intensities in the data required for image reconstruction, which were different for each echo signal, are now all made the same. An accurate image can be reconstructed by performing two-dimensional Fourier transformation on the corrected data. Furthermore, since the correction coefficient for each echo signal determined for a certain subject 1 is valid for that subject 1, prescanning only needs to be performed once until the subject 1 is replaced. In addition, in the above embodiment, the value obtained by dividing the intensity of the first echo signal by the intensity of the second and subsequent echo signals is used as the correction coefficient. The value obtained by dividing each can also be used as a correction coefficient.

【Effect of the invention】

According to the MR imaging method of the present invention, it is possible to correct the attenuation of the intensity of each echo signal due to the transverse relaxation time T2 of the object, magnetic field inhomogeneity, etc. in the ultrahigh-speed imaging method.
Therefore, the conditions imposed on the gradient magnetic field system are relaxed, and accurate images can be obtained by ultrahigh-speed imaging even under such relaxed conditions.

[Brief explanation of the drawing]

FIGS. 1(a) and 1(b) are time charts showing a pulse sequence according to an embodiment of the present invention, and FIG. 2 is a block diagram of an MR imaging system used in the embodiment. Gx... Gradient magnetic field for reading (for frequency encoding), ay... Gradient magnetic field for phase encoding, Gz... Gradient magnetic field for slice plane selection, 1... Subject, 2... Static magnetic field generator , 3... host) computer, 4...
Memory device, 21.22.23... Static magnetic field coil, 2
4.25.26... Static magnetic field power supply, 27.31... Waveform generation circuit, 32... Amplitude modulation circuit, 33... High frequency oscillation circuit, 34 Power amplifier, 35... Transmission coil, 4 ]...Receiver, Il, 42...Preamplifier,
43...Detection circuit, 44...A/D conversion circuit.

Claims (1)

[Claims] (1) Apply an excitation signal once, then sequentially invert the readout gradient magnetic field to generate multiple echo signals, and apply a phase encoding gradient magnetic field in advance of each of these multiple echo signals. In an ultrahigh-speed imaging method having a pulse sequence, the above-mentioned pulse sequence is performed without applying the above-mentioned phase encoding gradient magnetic field, data is collected, the signal intensity of each echo signal is determined from the data, and the above-mentioned normal An MR imaging method characterized in that the signal intensity of each echo signal obtained in the pulse sequence is corrected in relation to the signal intensity.