JPH04288141A - Nuclear magnetic resonance imaging - Google Patents

Abstract

PURPOSE:To achieve faster photographing with a quicker measuring time of longitudinal relaxation time T1, lateral relaxation time T2 and a magnetization Mo value used for a calculation value image of a magnetic resonance imaging. CONSTITUTION:RE (high frequency) pulse for excitation is applied to a spin system in vivo by a repetition frequency TR and an inclined magnetic field for slice selection, an inclined magnetic field for phase encoding and the like are applied. The TR is made shorter sufficiently than the T1 and T2 and a spin causes a normal precession motion at the TR. Under such a condition, a echo (FED) signal is detected from the elapse time t10 and t20 after the generation of the RF pulse calculated starting from the generation of the RF pulse and the retrograde time r1t and t2t before the generation of the RF pulse calculated starting from the moment of the subsequent RF pulse. A hypercomplex equation with T1, T2 and the Mo value as unknown number is computed from echo signal values S10 and S20 and S2t and S1t detected.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to nuclear magnetic resonance (NMR) imaging, and particularly to a technique for measuring longitudinal relaxation time T1, transverse relaxation time T2, and spin magnetization Mo, which is suitable for high-speed imaging.

0002

[Prior Art] NMR imaging devices are used in the biological field, etc., and their image forming elements include longitudinal relaxation time T1, transverse relaxation time T2, etc. that occur when the spin system of a living body is excited with an RF pulse (high frequency pulse). This is an important parameter.

Conventionally, spin echo method (SE method) and inversion recovery method (IR method) have been used to measure longitudinal relaxation time T1 and transverse relaxation time T2, but in order to measure relaxation, RF pulse Since it was necessary to lengthen the repetition period TR, the measurement time took a long time.

[0004] Considering the above, recently Japanese Patent Application Laid-Open No. 1983-1
As disclosed in Japanese Patent No. 49953, the steady-state precession state (SSFP) of the spin of the imaging region is
Detect the echo signal (FID signal) of e-Free Precession, and calculate the transverse relaxation time T from this detected value.
2 has been proposed to speed up the measurement, such as finding it from a calculation formula.

The invention disclosed in the above publication makes the repetition period TR of the RF pulse sufficiently shorter than the relaxation time, and the signals S+ and S which appear immediately before and immediately before the generation of the RF pulse are
The transverse relaxation time T2 is calculated from the calculation formula regarding the ratio of -.

[0006]

As described above, in the past, even if the relaxation time was calculated by capturing the SSFP signal, only the transverse relaxation time T2 was calculated. By the way, in order to improve the precision of magnetic resonance imaging images, it is necessary to change both the longitudinal relaxation time T1 and the transverse relaxation time T2, or in addition to them, the magnetization M.
It is necessary to synthesize the calculated value image using o. Therefore, it is desirable to calculate not only T2 but also T1 and Mo at high speed.

The present invention has been made in view of the above points, and its purpose is to calculate T1, T2, Mo values, etc. using signals in the steady state precession state (SSFP) to improve magnetic resonance imaging. The purpose is to achieve high-speed photography.

[0008]

[Means for Solving the Problems] In order to achieve the above object, the present invention basically attempts to calculate T1, T2 or, in addition, the Mo value from the echo signal value in the steady state precession state (SSFP). For this purpose, we focused on the fact that it is not sufficient to sample the echo signal value one by one just before and after the excitation pulse as in the above-mentioned Japanese Patent Application Laid-Open No. 149953/1982, and we will do the following. We propose a new magnetic resonance imaging method.

That is, in a nuclear magnetic resonance imaging method in which at least a longitudinal relaxation time T1 and a transverse relaxation time T2 are obtained as two-dimensional or three-dimensional image forming elements by exciting (magnetic resonance) a spin system in a specific part of the body, An RF pulse for spin excitation is applied to a specific region at a repetition period TR that is sufficiently shorter than the relaxation times T1 and T2, and the spins in the imaging region undergo steady precession (SSFP) at each period TR.
occurs, and for each period TR, two elapsed times t1O,
Set t2O and two retrograde times t1t and t2t before the RF pulse generation starting from the next RF pulse generation time,
Magnetic resonance echo signal (FID signal) values S1O, S2O and S2t, S1t with these two time settings, a total of four time settings.
are sampled, and these signal values S1O, S1t, S
From 2O and S2t, a multidimensional equation including T1 and T2 as unknowns is established to calculate T1 and T2.

0010

[Operation] When an excitation RF pulse is applied to the spins to be measured at a repetition period TR that is sufficiently shorter than the relaxation times T1 and T2, steady precession by the spins occurs during the time period TR.

In this state, as shown in FIG. 2, two elapsed times t1O and t2O starting from the time when the RF pulse is generated and two retrograde times t starting from the time when the next RF pulse is generated.
1t and t2t are set in advance, and the magnetic resonance echo signal (FID signal) value S is set at each of these times.
1O, S2O, S2t, and S1t are sampled. Echo signal values S1O and S1t, S2O and S2t in this case
and have similar signal values. Echo signal value SO (SO is a generic term for S1O and S2O) and St (St is S1t and S2t)
) can be expressed by the following formula.

0012

[Math 1]

TR: Repetition period tO: Elapsed time tt from the time of RF pulse generation during period TR; Retrograde time from the time of RF pulse generation during period TR T1: Longitudinal relaxation time, T2: Transverse relaxation time α; Flip angle , Mo; magnetization E1=exp(-TR/T1),
E2=exp(-TR/T2) Then, apply equation (1) to S1O and S2O, and apply equation (2) to S1t and S2
By applying this to t, it becomes possible to establish a multidimensional equation with T1, T2, and Mo as unknowns, and T1, T2, and Mo are calculated. Note that an example of the multidimensional equation is explained in the embodiment.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be explained with reference to the drawings.

FIG. 1 shows a sequence of RF pulses and gradient magnetic fields related to an NMR apparatus for carrying out this embodiment, and FIG. 2 shows signals used in the algorithm of this embodiment.

In FIG. 1, 101 is the repetition period TR
The RF pulse output by the RF pulse is used to excite (magnetic resonance) the spin system of a specific part of the body that is being imaged. The RF pulse has a period TR that is sufficiently shorter than the longitudinal relaxation time T1 and transverse relaxation time T2 that occur when spins are excited.
It is made to occur repeatedly.

Reference numeral 102 denotes a slice selection gradient magnetic field for selecting a slice portion when performing tomography, and is applied so that the spin phase becomes zero (initial state).

Reference numeral 103 denotes a phase encoding gradient magnetic field that gives a phase change to the excited spins of the slice portion, and divides the region of the slice portion. 104 is a gradient magnetic field for returning the phase to the initial state after data acquisition of the echo signal, and 105 is a gradient magnetic field for frequency encoding.

Reference numerals 106, 107, 108, and 109 are sampling command signals from a control device (not shown). When the sampling command signals are output, the echo signal (FID signal) value at that time is detected.

As shown in FIG. 2, the sampling command consists of two elapsed times t1O and t2O after the generation of the RF pulse, calculated from the time of generation of the RF pulse, and the next RF.
The signal is set to be output at two retrograde times t1t and t2t before the RF pulse is generated starting from the pulse generation time. Here, the length of time is t1O = t1t and t2O = t2t based on each starting point, and if taken at the time point in the forward direction, t1t = TR - t1
O, t2t=TR-t2O.

[0021] Therefore, in this embodiment, the relaxation time T1,
By repeatedly irradiating the spins in the imaging region with RF pulses with a period TR that is sufficiently shorter than T2, the spins are caused to undergo steady precession. During the time period (period TR) of this steady precession state, as shown in FIG.
At O, t2O, t2t, and t1t, the echo signal values at that time are detected, and these echo signal (FID) values S1O,
The longitudinal relaxation time T1, the transverse relaxation time T2, and the magnetization Mo as image elements are calculated from S2O, S2t, and S1t using a three-dimensional equation described below without actually measuring them.

A series of steps from spin excitation by the RF pulse 101 to echo signal measurement and T1, T2, and Mo value calculation are repeated while changing the phase encode gradient magnetic field.

[0023] Here, the arithmetic expressions for calculating the T1, T2, and Mo values will be explained.

[0024]

[Math 2]

[0025]

[Math 3]

[0026]

[Math 4]

[0027]

[Math 5]

In this embodiment, as described above, T1, T2
, Mo values are calculated by the CPU, these T1, T2
, a specific repetition period TR and echo time TE from the Mo value.
and two-dimensional or three-dimensional calculated value images at the flip angle α are synthesized.

[0029]

As described above, according to the present invention, the repetition period TR of the RF pulse for spin excitation can be made sufficiently shorter than the longitudinal and transverse relaxation times T1 and T2, and the steady precession that occurs in this situation can be reduced. T based on the state echo signal value (signal strength)
Since the Mo value is calculated as 1, T2 or in addition to this, the signal measurement time can be significantly shortened and high-speed photography can be achieved.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing a sequence of RF pulses and gradient magnetic fields in a magnetic resonance imaging method according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing sample states of echo signals used in the algorithm of the above embodiment.

[Explanation of symbols]

101...RF pulse, 102...Gradient magnetic field for slice selection, 103...Gradient magnetic field for phase encoding, 106, 107
, 108, 109...Sampling command signal.

Claims (3)

[Claims] Claim 1. A nuclear magnetic resonance imaging method that excites (magnetic resonance) a spin system in a specific part of the body and obtains at least a longitudinal relaxation time T1 and a transverse relaxation time T2 as two-dimensional or three-dimensional image forming elements, The relaxation time T1
, T2, an RF (radio frequency) pulse for spin excitation is applied to a specific region at a repetition period TR that is sufficiently shorter than T2, and the spins in the imaging region undergo steady precession (SSFP) at each period TR.
;Steady-State-Free Prece
ssion), and for each period TR, two elapsed times t1O and t2O after the RF pulse generation starting from the RF pulse generation time and two retrograde times before the RF pulse generation starting from the next RF pulse generation time. t1t, t2t
and the magnetic resonance echo signal (FID signal) values S1O, S2O and S
2t and S1t, and these signal values S1O
, S1t, S2O, and S2t to calculate T1 and T2 by establishing a multidimensional equation including T1 and T2 as unknowns. 2. A step in which a spin system in a specific part of the body is excited (magnetic resonance) by an RF pulse, a step in which a phase encoding gradient magnetic field is applied to the excited spins to cause a phase change, and a step in which the spin system in a specific part of the body is excited (magnetic resonance) by an RF pulse; echo of resonance (F
ID) A step of detecting and measuring a signal, repeating the steps from excitation to echo signal measurement while changing the phase encoding gradient magnetic field to obtain a two-dimensional or three-dimensional echo signal, and using this signal. In the nuclear magnetic resonance imaging method for forming images, the repetition period TR of the RF pulse applied to the spins is made sufficiently shorter than the longitudinal relaxation time T1 and the transverse relaxation time T2, and the spins in the imaging region are In the step of causing steady precession and detecting and measuring the echo signal, R
Two elapsed times t1O and t2O after the RF pulse is generated starting from the time when the F pulse is generated, and two retrograde times t1t before the RF pulse is generated starting from the time when the next RF pulse is generated.
A total of four echo signal values S1O, S at the timing of t2t
2O, S2t, and S1t are detected and measured, and a total of 4 of these are detected and measured.
A nuclear magnetic resonance imaging method characterized in that at least a longitudinal relaxation time T1 and a transverse relaxation time T2 are calculated as two-dimensional or three-dimensional image elements from two signal values using a multidimensional equation. 3. The multi-dimensional equation is a three-dimensional equation that includes magnetization Mo of the spin system as an unknown in addition to T1 and T2, and a specific repetition period TR, echo time TE, and flip A nuclear magnetic resonance imaging method characterized by combining calculated images at an angle α.