JPH04329929A - Mr imaging system - Google Patents

Abstract

PURPOSE:To obtain a datum of an NMR signal with a uniform and best S/N ratio regardless of the position of slicing by performing a sequence for adjustment to each slicing beforehand to adjust an amplification factor in power and receiving systems of an RF pulse for excitation at each slicing beforehand based on datum. CONSTITUTION:When a surface coil is used as a receiving antenna 95, sensitivity thereof lowers quickly according to distance causing a change in the intensitly of a received NMR signal according to each position of slicing. Then, an A/D conversion output of an NMR signal received for each slicing is monitored with a computer 99 in a sequence for adjustment to adjust an amplification factor of a frequency circuit 96 for receiving at each slicing so that it is inputted into an A/D converter 97 as such with the optimum size, thereby making the S/N ratio of the received signal the best for each slicing as well.

Description

[Detailed description of the invention]

0001

[Industrial Field of Application] This invention relates to nuclear magnetic resonance (NMR).
The present invention relates to an MR imaging apparatus that performs imaging using the following methods, and particularly relates to an MR imaging apparatus suitable for performing a multi-slice method for obtaining a large number of images.

0002

2. Description of the Related Art When performing an MR imaging sequence, it is necessary to tilt the magnetization of spins by a desired angle (flip angle) by irradiating and exciting a high-frequency magnetic field pulse (RF pulse). Therefore, the power of the excitation RF pulse is adjusted so that the flip angle is exactly 90 degrees for the 90 degree pulse for excitation, and the power is such that the flip angle is exactly 180 degrees for the 180 degree pulse. . Also, regarding the reception of NMR signals, the S/N
In order to improve the ratio, it is necessary to receive with an optimal amplification degree, and for this purpose, the amplification degree of the receiving system is adjusted.

0003

However, there is a problem in that the power of the excitation RF pulse is often not optimal for achieving a desired flip angle in an actual slice. In other words, since the high-frequency magnetic field of the actual excitation antenna is spatially non-uniform,
If the position of the slice differs, the effective power applied to that slice will differ, and for example, the flip angle that should have been adjusted to be 90 degrees may instead be adjusted to 80 degrees or 100 degrees. As a result, especially when performing a multi-slice imaging sequence, the RF pulse is adjusted to the same power for each slice, resulting in the inconvenience that the MR number and contrast differ for each slice.

[0004]Also, even though the amplification degree of the receiving system is adjusted, conventionally it is a uniform adjustment, so it cannot cope with fluctuations in signal strength due to spatial non-uniformity in the sensitivity of the receiving antenna. There is a problem. In particular, when obtaining multi-slice images using a surface coil as a receiving antenna, the sensitivity of this surface coil varies greatly depending on the slice position, so the magnitude of the received NMR signal varies greatly depending on the slice, making it difficult to select the optimal receiving system. Therefore, it is impossible to obtain the best S/N ratio for any slice.

In view of the above, it is an object of the present invention to provide an MR imaging apparatus that can provide an excitation RF pulse of optimal power to a slice at any position, and can optimize the degree of amplification of a received signal. The purpose is to

[0006]

[Means for Solving the Problems] In order to achieve the above object, the MR imaging apparatus according to the present invention performs an adjustment sequence before performing an imaging sequence, and adjusts excitation RF pulses based on the data obtained there. The feature is that the power and the amplification degree of the receiving system are adjusted for each slice, which allows the excitation RF pulse of the optimum power to be given to the slice at any position, and the amplification degree of the received signal to be optimized. The reconstructed image of each slice can have a uniform level and accurate contrast.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. In this embodiment, as shown in FIG. 1, a predetermined excitation waveform is generated from a sequencer 91 controlled by a computer 99, which is sent to an excitation high frequency circuit 92, and a high frequency signal of a predetermined frequency is generated along the above waveform. Amplitude modulated. This high frequency signal is amplified by a high frequency power amplifier 93, and the excitation antenna 9
4 to a subject (human body) 90.

On the other hand, the NMR signal generated by the subject 90 is received by a receiving antenna 95 and sent to a receiving high frequency circuit 96.
The signal is amplified and detected. The detected signal is sent to the A/D converter 9
At step 7, the data is converted into digital data and sent to an image reconstruction device 98, where the image is reconstructed by arithmetic processing such as Fourier transformation.

In this embodiment, a multi-slice imaging sequence using the spin echo method is performed as shown in FIG. That is, 90 degree pulses 11, 21, . . . , 180 degree pulses 12, 22, . . . are sequentially applied to slices 1, 2, .
,... is generated. At this time, although not shown, gradient magnetic field pulses for slice selection, phase encoding gradient magnetic field pulses for adding position information, and frequency encoding gradient magnetic field pulses are also applied (note that these gradient magnetic field pulses are also applied in Figure 1). (Generation system omitted). Then, after performing an excitation/reception sequence using the same amount of phase encoding for a large number of slices, the excitation/reception sequence is performed again for a large number of slices by changing the amount of phase encoding, which is repeated at a repeated time interval. T
Repeat with R. This allows data in multiple slices to be collected in one imaging sequence and images of those slices to be reconstructed.

[0010] The receiving antenna 95 here uses a surface coil, and is designed to image coronal cross sections at slices 1, 2, 3, . . . in the waist region of a human body as shown in FIG. FIG. 3 shows a sagittal image of the lower back of a human body taken with a receiving antenna for the spine.

[0011] When performing a multi-slice imaging sequence in this manner, the power of the RF pulse is adjusted for each slice so that the magnetization is accurately tilted by 90 degrees and 180 degrees in each slice. That is, the subject 90
Prior to performing the imaging sequence for the target object, an adjustment sequence is performed to selectively irradiate each slice of the actual subject 90 with a 90 degree pulse and a 180 degree pulse, receive the NMR signal, and transmit the image to the A/D converter 97. The computer 99 monitors the data output from the
The computer 99 connects the sequencer 9 so that the signal can be obtained.
1 to adjust the excitation waveform. This makes it possible to apply an RF pulse that tilts exactly a predetermined flip angle to any slice of the multi-slice.

Furthermore, when a surface coil is used as the receiving antenna 95, the sensitivity of the surface coil decreases rapidly depending on the distance.
,... changes greatly depending on the position of each slice. In the case shown in Figure 3, slice 1
The signal strength is the weakest at slice 4, and the signal strength is strongest at slice 4. Therefore, when using a 16-bit A/D converter 97, for example, in order to effectively utilize the input range corresponding to the 16 bits, the receiving high-frequency circuit 96 is adjusted according to the received NMR signal strength. The amplification degree is adjusted. That is, the computer 99 monitors the A/D conversion output of the NMR signal received for each slice in the adjustment sequence, and outputs the A/D conversion output as a signal of the optimum size.
The receiving high frequency circuit 96 is inputted to the D converter 97.
The computer 99 adjusts the degree of amplification for each slice. This makes the S/N ratio of the received signal the best for each slice.

[0013] After the power of the excitation RF pulse has been adjusted to the optimum value for each actual slice and the reception amplification degree has been adjusted to the optimum value for each slice, the 90 imaging sequences are performed, so that each image of the multislice has the same image level and accurate contrast, and each image is obtained with the best signal-to-noise ratio.

[0014]

As described above with respect to the embodiments, according to the MR imaging apparatus of the present invention, the power of the excitation RF pulse is adjusted according to the actual slice, so that accurate control of the flip angle is possible. It is possible to make the image level uniform and have the correct contrast for any slice. Furthermore, since the amplification degree of the received signal is adjusted for each actual slice, reception with the best S/N ratio is possible for any slice.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an MR imaging apparatus according to an embodiment of the present invention.

FIG. 2 is a time chart showing a time series of high-frequency signals obtained by the multi-slice method.

FIG. 3 is a diagram showing the positional relationship between a receiving antenna, a subject, and a slice.

[Explanation of symbols]

Claims (1)

[Claims] 1. Means for obtaining data by performing a sequence of RF pulse excitation for adjustment and NMR signal reception in each slice prior to an imaging sequence, and a means for obtaining data by performing a sequence of excitation of an excitation RF pulse and reception of an NMR signal in each slice, and determining the power of the excitation RF pulse and the amplification degree of the reception system based on the data. 1. An MR imaging apparatus characterized by comprising: means for adjusting for each slice.