JPH05277084A - Mr imaging method - Google Patents

Abstract

PURPOSE:To achieve higher effect of presaturation by the removal of artifact and a high signal attributed to blood stream or the like by performing a reselective excitation for an adjacent slice plane finished with the collection of data immediately before the selective excitation for individual slice planes to make the adjacent slice surface T1 be saturated. CONSTITUTION:RF pulses 01 and 02 for exciting presaturation areas P1 and P2 are applied to a specimen. A selection excitation pulse 11 for a slice plane 1 is applied to generate an echo signal. The collection of data ends for the slice plane 1 and an RP pulse 12 is applied to excite the slice plane 1 of light selectively immediately before a photographing sequence starting from a 90 deg. pulse 21 pertaining to the slice plane 1 begins. The photographing sequence starting from 90 deg. pulses 11, 21... and the application of selective excitation pulses 12, 22... are repeated for the slice planes 1, 2, 3... RF pulses 01 and 02 are applied and likewise. the photographing sequence and the application of the selective excitation pulse are repeated for the slice planes 1, 2, 3....

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for imaging by utilizing the NMR (nuclear magnetic resonance) phenomenon.

[0002]

2. Description of the Related Art Conventionally, when performing MR imaging of the abdomen or neck, a presaturation method in which a region other than a desired slice plane is excited in advance in order to remove an artifact due to blood flow from a thick blood vessel is known. May be used.

On the other hand, one scan, that is, an imaging sequence of excitation / data acquisition of a number corresponding to the reconstructed image matrix is repeated while changing the amount of phase encoding.
A multi-slice method is used to obtain a larger number of images in a scan for obtaining one image. This means that the imaging sequence of excitation / data acquisition is performed for multiple slice planes within one repetition time. It is necessary to collect data for multiple slice planes in the time required for one scan. You can

[0004]

However, when the presaturation method is applied to the multi-slice method, there is a problem that a sufficient artifact removing effect cannot be obtained.

That is, in the multi-slice method, since many slice planes are sequentially excited within one repetition time, the presaturation region for removing blood flow artifacts is located outside the multi-slice region occupied by the many slice planes. Only can be set. Therefore, the relatively slow blood flow moving in the multi-slice region cannot be presaturated, so that the blood flow influences and hinders diagnosis. Especially when an imaging sequence having a long TE (echo time) is adopted, blood flow may cause artifacts or the blood flow portion may remain in the image as a high signal portion.

In view of the above, it is an object of the present invention to provide an MR imaging method improved so that the effect of presaturation can be sufficiently obtained in the multi-slice method.

[0007]

In order to achieve the above object, in the MR imaging method according to the present invention, a presaturation sequence is performed prior to each imaging sequence in multislices having different phase encoding amounts, and At the end of the imaging sequence for each slice in the imaging sequence, the selective excitation is performed again for the slice plane where the imaging sequence was performed, and the slice plane is saturated. This selective excitation again is for the slice plane adjacent to the slice plane for which the imaging sequence is performed next, and therefore is a presaturation sequence performed immediately before for the slice plane for performing the imaging sequence. Thus, immediately before selective excitation for each slice plane, reselective excitation is performed for adjacent slice planes for which data collection has been completed,
Since this adjacent slice surface is in the T1 saturation state, the blood flow flowing in from this adjacent slice surface does not generate a large NMR signal, and blood flow artifacts and high signals are removed.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a time chart showing a pulse sequence according to an embodiment of the present invention, and FIG. 2 is a block diagram schematically showing an MR device which performs the above pulse sequence.

First, referring to FIG. 2, the main magnet 101 is for generating a static magnetic field, and a gradient magnetic field is applied by the gradient magnetic field coil 102 so as to be superposed on the static magnetic field. The space to which the static magnetic field and the gradient magnetic field are applied is an examination space in which the subject 103 is placed. An excitation RF pulse is applied to the subject 103.
03 generated and the N generated in the subject 103.
An RF coil 104 is attached for receiving MR signals.

The gradient magnetic field power supply 1 is provided in the gradient magnetic field coil 102.
05 is connected and electric power for generating the gradient magnetic field is supplied. R
A transmission power amplifier 107 and a preamplifier 110 are connected to the F coil 104 via a switch 106. The switch 106 is switched to the transmission power amplifier 107 side during excitation and switched to the preamplifier 110 side during reception. The transmission power amplifier 107 has a signal generator 1
An RF signal obtained by modulating the carrier signal from 09 with a modulation signal having a predetermined waveform in the transmission circuit 108 is sent. A receiving circuit 111 is connected to the preamplifier 110, and phase detection of the received signal is performed using the signal from the signal generator 109 as a reference signal. The detected signal is the A / D converter 11
2 is sampled, converted into digital data, and loaded into the computer 113.

The computer 113 controls the modulation signal waveform of the excitation RF pulse in the transmission circuit 108, determines the frequency of the signal generator 109, and determines the sampling timing of the A / D converter 112. In addition, the gradient magnetic field power source 10
5 is controlled to arbitrarily program the timing, waveform, intensity, etc. of the gradient magnetic field pulse. Further, processing for reconstructing an image from the collected digital data is performed. The display device 114 displays a reconstructed image or the like.

In such an MR device, the sequence shown in FIG. 1 is performed. Here, as shown in FIG. 3, many slice planes 1, 2, 3, ... Are data-collected by the multi-slice method and these images are reconstructed. The area occupied by the large number of slice planes 1, 2, 3, ... Is called a multi-slice area, and the adjacent areas P1 and P2 on both sides of the multi-slice area are called presaturation areas.

First, as shown in FIG. 1A, RF for selectively exciting the presaturation regions P1 and P2, respectively.
Pulses 01, 02 (which may be 90 ° pulses, for example) are generated from the RF coil 104 and applied to the subject 103. Due to the selective excitation, although not shown, the gradient magnetic field coil 1 together with these RF pulses 01 and 02
From 02, a gradient magnetic field in a direction perpendicular to the slice planes 1, 2, 3, ... Is applied in a pulse form.

Next, as shown in FIG. 1A, the slice plane 1
Then, the imaging sequence is started by adding the selective excitation pulse 11 for the echo signal, and an echo signal is generated as shown in B of FIG. Although the spin echo method or the like can be adopted for this imaging sequence, the field echo method is used here. That is, the excitation pulse 11 is a 90 ° pulse.
The imaging sequence by the field echo method includes, in addition to the 90 ° pulse, a gradient magnetic field pulse for slice selection for selectively exciting the slice plane 1, a gradient magnetic field pulse for phase encoding, a gradient magnetic field pulse for readout and frequency encoding (Fig. Is omitted).

The echo signal generated in the subject 103 is received by the RF coil 104, input to the receiving circuit 111 via the switch 106 and the preamplifier 110, and detected. This detected signal is sampled by the A / D converter 112 during the sampling period indicated by C in FIG. 1 and converted into digital data.

Data collection for slice plane 1 is completed in this way, and 90 ° pulse 2 for the next slice plane 2.
Immediately prior to the start of the imaging sequence starting at 1, an RF pulse 12 (which may be, for example, a 90 ° pulse) is selectively applied which excites the previous slice plane 1. In order to selectively excite in this way, although not shown, a gradient magnetic field pulse in a direction perpendicular to the slice plane 1 is applied together with the RF pulse 12.

The data acquisition for this slice plane 2 is completed, and the 90 ° pulse 31 for the next slice plane 3 is obtained.
An RF pulse 22 (which may be, for example, a 90 ° pulse) that selectively excites the previous slice plane 2 is applied immediately before the start of the imaging sequence starting from. In order to selectively excite in this way, a gradient magnetic field pulse in a direction perpendicular to the slice plane 2 is applied together with the RF pulse 22, although not shown.

The slices 1, 2, 3, ... Of the imaging sequence starting from such 90 ° pulses 11, 21, ... And the selective excitation pulses 12, 22 ,.
Repeat for all. In this case, the phase encoding amount is the same in the imaging sequence for each slice plane. When this is finished, RF for selectively exciting the presaturation regions P1 and P2 again
Pulses 01 and 02 are added, and 90 ° pulses 11 and 2 are also added.
The imaging sequence starting from 1, ... (It has a phase encoding amount different from the previous phase encoding amount) and the selective excitation pulse 12, 22 ,. (The phase encoding amount is the same for each of these slice planes).

When the above operation is repeated until the phase encoding amount differs by the number corresponding to the reconstructed image matrix, the data necessary for the image reconstruction is collected for each of the slice planes 1, 2, 3, .... it can.

Here, the presaturation region P1,
In the slice plane 1 and the like adjacent to P2, the adjacent region is selectively excited in advance to be in the T1 saturated state, so that the blood flow and the like flowing therein does not generate a large signal in the imaging sequence of the slice plane 1 and the like.

Further, since the slice plane located in the middle of the multi-slice region, for example, the slice plane 4, is distant from the presaturation regions P1 and P2 in terms of distance, saturated blood flow or the like does not flow in so much. In other words, the presaturation in the presaturation regions P1 and P2 is not very effective. On the other hand,
Blood flow or the like may flow into the slice plane 4 from another slice plane adjacent to it in the multi-slice region, for example, the slice plane 3, and this is not excited by the RF pulses 01 and 02 for presaturation. However, immediately before the imaging sequence for the slice plane 4 is started, an RF pulse for selectively exciting the slice plane 3 is applied after the imaging sequence for the adjacent slice plane 3 before the slice plane 4 is finished. 4 becomes an RF pulse for presaturation, and a signal suppressing effect on the blood flow and the like flowing from the slice surface 3 is produced. Therefore, it is possible to remove the artifacts due to the relatively slow blood flow moving in the multi-slice region, or to reduce the high signal due to the blood flow.

When a multi-slice image of the abdomen is actually obtained by this method, the blood flow in the inferior vena cava and the main portal vein are determined by setting the excitation order of a large number of slice planes from the bottom to the top of the body. The signals of blood flow flowing from the bottom to the top such as the blood flow in the inside are reduced, and it becomes possible to distinguish between the blood flow in the portal vein and the liver tumor invading the portal vein in the images of each slice plane (conventional By the method, they were indistinguishable due to high signal in both parts).

[0023]

As described above, according to the MR imaging method of the present invention, artifacts due to a relatively slow blood flow moving in a multi-slice region or the like, which cannot be removed by simply applying the presaturation method to the multi-slice method, can be eliminated. The signal can be removed, and the effect of presaturation is enhanced.

[Brief description of drawings]

FIG. 1 is a time chart showing a pulse sequence according to an embodiment of the present invention.

FIG. 2 is a block diagram of an MR device used in the embodiment.

FIG. 3 is a schematic diagram showing a positional relationship between a slice plane and a presaturation region in the example.

[Explanation of symbols]

 01, 02 Presaturation RF pulse 11, 21, 31 Imaging RF pulse 12, 22, Re-excitation RF pulse 101 Main magnet 102 Gradient field coil 103 Subject 104 RF coil 105 Gradient field power supply 106 Switch 107 Transmitting power amplifier 108 Transmission circuit 109 Signal generator 110 Preamplifier 111 Reception circuit 112 A / D converter 113 Computer 114 Display device

Claims (1)

[Claims] 1. A presaturation region adjacent to a multi-slice region occupied by a desired plurality of slice planes is selectively excited, and then a first slice plane is selectively excited to collect data with a certain phase encoding amount for the slice plane. After that, the slice plane is selectively excited again, and then the second slice plane is selectively excited, so that selective excitation, data collection, and
The sequence of reselective excitation is repeated for each slice plane, and when this sequence is completed for the last slice plane, the presaturation region is selectively excited again, and then the first to the other slice planes with different phase encoding amounts are selected. An MR imaging method characterized by repeating the sequence of the selective excitation, data acquisition, and reselective excitation, and repeating the selective excitation of the presaturation region and the above sequence in each slice for a predetermined number of phase encode amounts. ..