The present invention relates to a spin exciting method, a magnetic resonance imaging method, and a magnetic resonance imaging system. More particularly, the present invention relates to a spin exciting method, a magnetic resonance imaging method, and a magnetic resonance imaging system for performing magnetic resonance imaging according to a fast spin echo (FSE) technique combined with an inversion recovery (IR) technique.
In a magnetic resonance imaging (MRI) system, a subject of imaging is carried into a bore of a magnet system, that is, an imaging space in which a static magnetic field is created. Magnetic field gradients and a radio-frequency magnetic field are applied to the subject in order to excite spins in the subject. Consequently, a magnetic resonance signal is induced, and an image is reconstructed based on the signal received.
A sequence of exciting spins so as to induce a magnetic resonance signal and receiving the signal is repeated at predetermined intervals of a repetition time TR. The TR is often set to a time long enough for the excited spins to recover to exhibit an original longitudinal magnetization. When an imaging time must be shortened, the TR is set to a short time and spins are forcibly recovered. The forcible recovery of spins is achieved with additional excitation. This technique is referred to as fast recovery.
Japanese Examined Patent Publication No. 4-21488 describes that the fast recovery is combined with the IR. In short, as shown in FIG. 7, spins are turned 180xc2x0 with application of a 180xc2x0 pulse and thus brought to an inversion. Thereafter, when a predetermined inversion time TI has elapsed, a 90xc2x0 pulse is applied in order to turn the spins 90xc2x0. A free induction decay (FID) signal that is induced accordingly is then acquired.
Thereafter, when a half of an echo time TE has elapsed, a xe2x88x92180xc2x0 pulse is applied in order to reverse the spins. Thereafter, when a half of the TE has elapsed, a xe2x88x9290xc2x0 pulse is applied in order to turn the spins xe2x88x9290xc2x0 and a 180xc2x0 pulse is then applied in order to reverse the spins. Thus, the fast recovery of the spins is achieved.
U.S. Pat. No. 6,054,853 describes that the fast recovery is combined with the FSE. In short, as shown in FIG. 8, a 90xc2x0 x pulse is applied in order to excite spins and turn them 90xc2x0 with respect to an x axis. Thereafter, when a half of an echo space esp has elapsed, a 180xc2x0 y pulse is applied in order to reverse the spins with respect to a y axis. Thereafter, when the esp has elapsed, the 180xc2x0 y pulse is applied in order to reverse the spins again with respect to the y axis. When the echo space esp has elapsed, the 180xc2x0 y pulse is applied in order to reverse the spins again with respect to the y axis. Consequently, a spin echo is acquired during the echo space esp between applications of the 180xc2x0 y pulse.
When a half of the echo space esp has elapsed since the last application of the 180xc2x0 y pulse, a xe2x88x9290xc2x0 x pulse is applied in order to turn the spins xe2x88x9290xc2x0, and a 180xc2x0 x pulse is applied in order to reverse the spins. Thus, the fast recovery of the spins is achieved.
According to the technology described in the Japanese Examined Patent Publication No. 4-21488, the fast recovery employing the xe2x88x9290xc2x0 pulse and 180xc2x0 pulse is achieved through unselective excitation that is not intended to select a slice. This disables multiple slice imaging that interleaves pulse sequences like the foregoing one and involves a plurality of slices.
According to the related art described in the U.S. Pat. No. 6054853, the fast recovery employing the xe2x88x9290xc2x0 x pulse and 180xc2x0 is performed through selective excitation. However, a selected slice is not perfectly square. It is not easy to properly achieve the fast recovery using the two selective excitation pulses.
Moreover, the number of reversals of spins stemming from application of the 180xc2x0 y pulse is an odd value. If the degree of the reversal of spins stemming from application of the 180xc2x0 y pulse has an error, the spins are not restored to exactly face along the x-y plane. Therefore, the succeeding fast recovery is achieved imperfectly.
Therefore, an object of the present invention is to realize a spin exciting method, a magnetic resonance imaging method, and a magnetic resonance imaging system for properly performing fast recovery during magnetic resonance imaging in which the fast spin echo technique combined with the inversion recovery technique is implemented.
(1) In one aspect of the present invention intended to solve the aforesaid problems, there is provided a spin exciting method for producing an image using a magnetic resonance signal induced by spins in a subject being imaged according to the fast spin echo technique combined with the inversion recovery. Specifically, a 180xc2x0 pulse is applied in order to excite spins. Thereafter, when a first time has elapsed, a first 90xc2x0 x pulse is applied in order to excite the spins. Thereafter, when a second time has elapsed, a 180xc2x0 y pulse is applied in order to excite the spins. Thereafter, when a third time that is double the second time has elapsed, the 180xc2x0 y pulse is applied an odd number of times in order to sequentially excite the spins. Thereafter, when the second time has elapsed, a second 90xc2x0 x pulse is applied in order to excite the spins.
(2) In another aspect of the present invention intended to solve the aforesaid problems, there is provided a magnetic resonance imaging method for producing an image using a magnetic resonance signal induced by spins in a subject being imaged according to the fast spin echo technique combined with the inversion recovery technique. Specifically, a 180xc2x0 pulse is applied in order to excite spins. Thereafter, when a first time has elapsed, a first 90xc2x0 x pulse is applied in order to excite the spins. Thereafter, when a second time has elapsed, a 180xc2x0 y pulse is applied in order to excite the spins. Thereafter, when a third time that is double the second time has elapsed, the 180xc2x0 y pulse is applied an odd number of times in order to sequentially excite the spins. Thereafter, when the second time has elapsed, a second 90xc2x0 x pulse is applied in order to excite the spins. A spin echo is read during the third time, and an image is produced based on the spin echo.
(3) In another aspect of the present invention intended to solve the aforesaid problems, there is provided a magnetic resonance imaging system for producing an image using a magnetic resonance signal induced by spins in a subject being imaged according to the fast spin echo technique combined with the inversion recovery technique. The magnetic resonance imaging system includes a spin exciting means, an echo reading means, and an image producing means. The spin exciting means excites spins with application of a 180xc2x0 pulse. Thereafter, when a first time has elapsed, the spin exciting means applies a first 90xc2x0 x pulse to excite the spins. Thereafter, when a second time has elapsed, the spin exciting means applies a 80xc2x0 y pulse to excite the spins. Thereafter, when a third time that is double the second time has elapsed, the spin exciting means applies the 180xc2x0 y pulse an odd number of times to excite the spins sequenlially. Thereafter, when the second time has elapsed, the spin exciting means applies a second 90xc2x0 x pulse to excite the spins. The echo reading means reads a spin echo during the third time. The image producing means produces an image according to the spin echo.
In the aspects of the present invention set forth in paragraphs (1) to (3), a 180xc2x0 pulse is applied in order to excite spins. Thereafter, when the first time has elapsed, a first 90xc2x0 x pulse is applied in order to excite the spins. Thereafter, when the second time has elapsed, a 180xc2x0 y pulse is applied in order to excite the spins. Thereafter, when the third time that is double the second time has elapsed, the 180xc2x0 y pulse is applied an odd number of times in order to excite the spins. Thereafter, when the second time has elapsed, a second 90xc2x0 x pulse is applied in order to excite the spins. The spins are reversed an even number of times due to the applications of the 180xc2x0 y pulse, and therefore returned to exactly face along the xy plane. Consequently, the spins are accurately recovered with the subsequent application of the 90xc2x0 x pulse.
Moreover, the inversion recovery is performed with application of the 90xc2x0 x pulse alone. Therefore, imperfect fast recovery stemming from the employment of two selective excitation pulses as conventionally will not take place.
In order to successfully achieve fast recovery of spins whose relaxation time is relatively long, the second 90xc2x0 x pulse should preferably be a +90xc2x0 x pulse.
In order to achieve imaging while enhancing a magnetic resonance signal induced by spins whose relaxation time is relatively long, the first time should preferably be shorter than the polarity recovery time for spins that are used for imaging.
In order to successfully achieve fast recovery of spins whose relaxation time is relatively short, the second 90xc2x0 x pulse should preferably be a xe2x88x9290xc2x0 x pulse.
In order to achieve imaging while enhancing a magnetic resonance signal induced by spins whose relaxation time is relatively short, the first time should preferably be longer than the polarity recovery time for spins that are used for imaging.
In order to produce a tomographic image, excitation should preferably be selective excitation all the time.
In order to achieve multiple slice imaging, a series of excitations starting with excitation initiated with application of the 180xc2x0 pulse and ending with excitation initiated with application of the second 90xc2x0 x pulse should preferably be started at successive time instants within the first time with slices changed sequentially.
According to the present invention, there are provided a spin exciting method, a magnetic resonance imaging method, and a magnetic resonance imaging system for making it possible to properly achieve fast recovery during magnetic resonance imaging in which the fast spin echo technique combined with the inversion recovery technique is implemented.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.