The present invention relates to a magnetic resonance imaging (MRI) system for imaging, by means of computed tomography (CT), the distribution of the magnetic resonance (MR) of the specific atomic nucleus existing in a specified cross section of an object (e.g., the density distribution of the nuclear spin), thereby to form an MR image.
The MRI system for a diagnosis obtains tomograms consisting of the distribution images of the MR data at the specific position of the object. The radiography principle in an example of the MRI system will now be described.
In this system, a sufficiently uniform static magnetic field H.sub.0 is applied to object P as shown in FIG. 1 and a gradient magnetic field G.sub.z is further applied to object P due to a pair of gradient coils C.sub.1a and C.sub.1b in addition to static magnetic field H.sub.0. The direction of line of magnetic force of static magnetic field H.sub.0 is parallel to the Z axis shown in the diagram. The magnetic force of gradient magnetic field G.sub.z is parallel to the Z axis shown in the diagram and this gradient magnetic field has a linearly ascending gradient with respect to the direction of the Z axis (namely, the magnetic field intensity gradually differs with respect to the Z axis). The magnetic gradient of gradient magnetic field G.sub.z is such that, for example, the magnetic field intensity of substantially the central portion regarding the direction of the Z axis is zero and the directions of lines of magnetic forces before and after the central portion are opposite and at the same time the magnetic field intensity gradually increases. The synthetic magnetic field of static magnetic field H.sub.0 and gradient magnetic field G.sub.z also has the magnetic gradient which is gradient with respect to the Z-axis direction, so that the magnetic field contour planes where the Z axis perpendicularly crosses are formed due to the synthetic magnetic field. Namely, in this case, the specific magnetic field contour plane (this magnetic field contour plane consists of the plane where the Z axis perpendicularly crosses) corresponds to the specific magnetic field intensity and substantial central portion regarding the Z-axis direction corresponds to the intensity of magnetic field of H.sub.0.
The atomic nucleus resonates with static magnetic field H.sub.0 at an angular frequency of .omega..sub.0 shown by the following expression. EQU .omega..sub.0 =.gamma.H.sub.0 ( 1)
In expression (1), .gamma. is a magnetogyric ratio which is peculiar to the atomic nucleus and is determined depending on the kind of atomic nucleus.
In addition to static magnetic field H.sub.0 and gradient magnetic field G.sub.z, rotating magnetic field H.sub.1 of the angular frequency .gamma..sub.0, functions to resonate only the specific atomic nucleus, is applied to object P as a pulse through a pair of transmitter coils C.sub.2a and C.sub.2b provided in a probe head. This rotating field H.sub.1 is called an "excitation pulse". Rotating field H.sub.1 substantially selectively acts on only the portion of the X-Y plane shown in the diagram (which is selectively determined with respect to the Z-axis direction) due to the gradient magnetic field G.sub.z. Thus, the MR phenomenon occurs in only the specific slice portion S (although it is the plane portion, it actually has a certain thickness), from which tomograms are obtained.
Due to the occurrence of the MR phenomenon, a free induction decay (FID) signal is detected through a pair of receiver coils C.sub.3a and C.sub.3b provided in the probe head. This FID signal is subjected to a Fourier transformation, so that a single spectrum is derived with respect to the rotating frequency of the specific atomic nuclear spin. To obtain the tomogram, the projection data of slice portion S regarding multi directions in the X-Y plane is necessary. Therefore, after the MR phenomenon was caused by exciting slice portion S, as shown in FIG. 2, a gradient magnetic field G.sub.xy having a magnetic gradient which is linear in the direction of the X' axis (coordinate axis which is rotated by an angle of .theta. from the X axis) is allowed to act on magnetic field Hn (by a coil or the like (not shown)). Thus, magnetic field contour lines E.sub.1 to E.sub.n in slice portion S (X-Y plane) of object P become straight lines which perpendicularly cross the X' axis. The rotating frequencies of the specific atomic nuclear spins on magnetic field contour lines E.sub.1 to E.sub.n are expressed by expression (1) mentioned above. For convenience of explanation, in this case, magnetic field contour lines E assume E.sub.1 to E.sub.n and it is possible to consider that signals D.sub.1 to D.sub.n as the FID signal are caused by the magnetic fields on those magnetic field contour lines E.sub.1 to E.sub.n, respectively. Amplitudes of signals D.sub.1 to D.sub.n can be proportional to densities of the specific atomic nuclear spins on magnetic field contour lines E.sub.1 to E.sub.n which pierce slice portion S, respectively. However, the FID signal which is actually observed becomes the composite FID signal FID of which all of D.sub.1 to D.sub.n were added. This composite FID signal FID is subjected to the Fourier transformation, so that projection data (one-dimensional image) PD of slice portion S onto the X' axis is obtained. The X' axis is rotated in the X-Y plane (for example, a gradient magnetic field G.sub.x for providing a magnetic gradient with regard to the X direction is caused by a pair of gradient coils, and at the same time a gradient magnetic field G.sub.y for providing a magnetic gradient with respect to the Y direction is caused by another pair of gradient coils, and these gradient magnetic fields G.sub.x and G.sub.y are synthesized to form a gradient magnetic field G.sub.xy, then a synthetic ratio of gradient magnetic fields G.sub.x and G.sub.y is changed, thereby making it possible to realize the rotation of gradient magnetic field G.sub.xy). Due to this, the projection data regarding each direction in the X-Y plane can be derived in a manner similar to the above. The MR image can be synthesized by an image reconstruction process using those projection data.
Although there is a case where the FID signal due to the magnetic resonance is observed by directly detecting the FID signal itself, there is also a case where the magnetic resonance is excited so as to generate a spin echo signal relative to the FID signal and this spin echo signal is detected, thereby enabling observation of the FID signal.
A 90.degree. pulse and/or a 180.degree. pulse is used as an exciting pulse in order to obtain the FID signal or spin echo signal. The 90.degree. pulse is the exciting pulse for causing such a magnetic resonance as to rotate the magnetic moment of the spin system by an angle of 90.degree. from the direction parallel to the magnetic field to the direction which is normal thereto due to the resonance. Similarly, the 180.degree. pulse is the exciting pulse for causing such a magnetic resonance as to rotate the magnetic moment of the spin system by an angle of 180.degree.. The 90.degree. pulse is mainly used to obtain the sole FID signal. Both of the 90.degree. and 180.degree. pulses are frequently used to obtain the spin echo signal.
The conventional diagnostic MRI system encounters a problem. Even if the condition to generate the 90.degree. or 180.degree. pulse has been preset, the power condition of the pulse will inevitably change since, when the object if put into the transmitter coils, the quality factor (Q) and resonance frequency (f.sub.0) of the coils alter depending on attributes of the object, for example, the shape. Thus, the angle of inclination of the magnetic moment of the spin system is deviated from the set value of 90.degree. or 180.degree. when the exciting pulse is applied. Consequently, picture quality features, e.g., contrast, an S/N ratio, or the like of the MR image which are obtained from the FID signal or spin echo signal, vary depending on the attributes of the object.