Method and system for improving resolution of images in magnetic resonance imaging

In the magnetic resonance imaging, magnetic resonance (MR) signals generated from a predetermined region of a subject are detected by using the selective excitation method. The estimation of the subject is performed by executing the repetition solution in accordance with the spectral data of detected MR signal and region data representing the region of the subject. The MR image having the high resolution than the conventional MR image is obtained.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and system for improving 
resolution of images in magnetic resonance imaging (MRI). 
2. Description of the Related Art 
Magnetic resonance (MR) is a phenomenon exhibited by atomic nuclei which 
are placed in a static magnetic field and have a nonzero magnetic moment 
whereby the atomic nuclei absorb and emit electro-magnetic energy only at 
specific frequencies through their resonance. The atomic nuclei resonate 
at an angular frequency .omega.o (.omega.o=2.pi..nu., .nu.=Larmor 
frequency) given by 
EQU .omega.o=.gamma.Ho (1) 
where .gamma. is the gyromagnetic ratio inherent in each type of nucleus 
and Ho is the strength of the applied static magnetic field. 
MRI apparatus utilizing the magnetic resonance phenomenon have been in wide 
use for medical diagnosis. An MRI apparatus detects MR signals after the 
resonance absorption and processes the MR signals to obtain diagnostic 
information, for example, slice images of a subject resulting from the 
density of nuclei, longitudinal relaxation time, transverse relaxation 
time, flow and chemical shifts, noninvasively. 
The diagnostic information may be acquired by exciting the whole body of 
the subject placed in the static magnetic field. In practice, however, the 
diagnostic information is obtained by exciting only a specific portion of 
the subject because of limitations on the arrangement of the MRI apparatus 
and clinical demands for acquired images. 
A high resolution version of MR images will now be discussed. The MR 
signals observed by the MRI correspond to points on the K space of a 
subject (see FIG. 1A), namely, on a frequency region (Fourier plane) 
having a readout direction and a phase encoding direction as shown in FIG. 
1B. In FIG. 1B, the end of the frequency region corresponds to high 
frequency components in the subject. Hence, to estimate the original 
subject at high resolution, in other words, to improve resolution of 
reconstructed MR images, the MRI device must acquire MR data over a wide 
frequency range. For example, as shown in FIG. 2, images reconstructed 
from MR data within a narrow frequency range (hatching portion) blur. 
The position of data in the K space is proportional to an integral amount 
of the strength of a gradient magnetic field applied to spins in the 
subject. To observe the high frequency components of the original data, a 
large integral amount of gradient magnetic field is needed. To increase 
the integral amount of the gradient magnetic field, the strength or 
application time period of the gradient magnetic field must be increased. 
When the application time period of the gradient magnetic field is 
increased, the strength of MR signals decrease because of the effect of 
the transverse relaxation time. Further, echo time is an important 
parameter relating to induction of the MR signals and thus increase of the 
application time is limited. For this reason, the strength of the gradient 
magnetic field is increased in order to increase the integral amount of 
the gradient magnetic field. As a result, the high frequency components of 
the original data are observed, and it is possible to realize a high 
resolution version of the MR images. However, this leads to the necessity 
of a large capacity power supply source for generating the gradient 
magnetic field. This power supply source is not a practical apparatus from 
the standpoint of the arrangement of the MRI apparatus. 
An MRI apparatus is according desired which can improve resolution of 
images without the need of a large-capacity power supply source. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide a method of 
improving resolution of images without the need of a large-capacity power 
supply source and a system for executing the method. 
According to one aspect of the present invention, there is provided a 
system for estimating a magnetic resonance signal, the system comprising: 
static field generating means for generating a static field; gradient 
field generating means for generating gradient fields; transmitting and 
receiving means for transmitting an excitation pulse to a subject and for 
receiving a magnetic resonance signal generated in the subject; control 
means for controlling the static field generating means, the gradient 
field generating means, and the transmitting and receiving means in 
accordance with a predetermined sequence; setting means for setting a 
region of the subject in accordance with the predetermined sequence, and 
estimating means for estimating the magnetic resonance signal received by 
the transmitting and receiving means in accordance with the set region. 
According to another aspect of the present invention, there is provided a 
method for estimating a magnetic resonance signal, the method comprising 
the steps of: setting a pulse sequence to obtain a subject region; 
canceling a transverse magnetization component in a region around the 
subject region after an excitation pulse is applied to the region around 
the region in accordance with the pulse sequence; acquiring a magnetic 
resonance signal generated by applying the excitation pulse to the subject 
region in accordance with the pulse sequence; and estimating the acquired 
magnetic resonance signal by the subject.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of the present invention will be described with reference to 
the drawings hereinafter. 
First, the principle of ultra-resolution will be described. In 
ultra-resolution, if an original image f is represented by a finite 
function, then the Fourier transform F of the original image will be 
represented by an analytic function. Here, a "finite function" means a 
continuous function whose non-zero-valued region is included in a closed 
interval. In an analytic function, if its values are found on an open 
interval, then, the Fourier transform F based on the original image f can 
be estimated through analytic continuation. 
When an original subject as shown in FIG. 3A is supposed, a frequency 
spectrum for the original subject, as shown in FIG. 3B, will be 
represented by an analytic function. If the original subject is estimated 
on the basis of only frequency components within a frequency range A 
(shown in FIG. 3C) in the frequency spectrum shown in FIG. 3B, high 
frequency components would be lost as shown in FIG. 3D. The analytic 
continuation may be applied to the analytic function. Therefore, if the 
analytic continuation for frequency components within a frequency range B 
of FIG. 3C is performed, the original subject can be estimated accurately 
as shown in FIG. 3E. 
As practical algorithms for the above analytic continuation, the repetition 
method is widely used because of its easiness of operations. That is, 
according to "Ultra--Resolution----the Principle and Algorithm-", by 
Shigeru Ando, and Yasuhiro Doi, Measurement and Control, vol. 22, No. 10, 
1983, as shown in FIG. 4, spectrum data is read over a finite band 
[-.OMEGA./2, .OMEGA./2] in process 1 and then a fast Fourier transform 
(FFT) is performed in process 3 via process 2. Subsequently, in process 4, 
region data about a range [31 W/2, W/2] of a subject is obtained by a 
method to be described later. The region data is applied to the result of 
the FFT operation obtained by process 3 and thus components outside the 
range of the subject are set to zero in process 5. 
After the completion of an IFFT (inverse fast Fourier transform) operation 
in process 6, the result of operation in process 6 is processed to 
coincide with the observed spectrum within a passband [-.OMEGA./2, 
.OMEGA./2 ] in process 2. That is, the waveform of the spectrum within the 
passband is processed to coincide with the originally observed spectrum 
while the spectrum waveform outside the passband is left. The above 
processes (3.fwdarw.5.fwdarw.6.fwdarw.2.fwdarw.3.fwdarw.. . . ) are 
repeated and data in process 5 is acquired as estimated data. 
To apply the principle of ultra-resolution to the MRI technique, a method 
of observing a spectrum and a method of determining the existence region 
of a subject are needed. Since the MRI apparatus is inherently a spectrum 
observation apparatus, the spectrum can readily be observed. Hence the 
method of determining the existence region of the subject is left to be 
solved. The localizing method disclosed in U.S. Pat. No. 4,737,714 is used 
as the method of determining the existence region of the subject. 
In accordance with the localizing method, the outside of a predetermined 
local region is magnetically nonexisted by the application of gradient 
magnetic fields, and MR signals generated from the local region only are 
acquired. That is, the local region is defined as the existence region of 
the subject. 
In the localizing method, in order to obtain a tomogram image in a 
predetermined position of a subject under examination, a static magnetic 
field is generated in the Z axis and the subject is placed in the static 
magnetic field. The magnetization is created in the direction of the Z 
axis. Subsequently magnetic fields used for specifying the direction of 
the magnetization and the position of a slice of the subject are added to 
the static magnetic field. In the following the rotating coordinate system 
(X', Y' , and Z') is used 
In the rotating coordinate system, to cause the magnetization to tip 
90.degree. in the minus direction of the X' axis, an RF pulse (90.degree. 
pulse) is applied in the direction of Y'. At the same time a slicing 
gradient field Gy is added. The RF pulse includes two frequencies f1 and 
f2. That is, in FIGS. 5A through 5D and FIG. 6, when the center frequency 
of the RF pulse applied to the subject P in order to excite a region 
including the local portion is f0, the RF pulse includes frequencies f1 
and f2 to select regions 31 and 32 (hatched portions in FIG. 6) on both 
sides of the local portion. In this case, f1 and f2 are center frequencies 
and the excitation widths are determined by .DELTA.f1 and .DELTA.f2. The 
reason why an RF pulse with different frequencies is used to select a 
predetermined region as described above will be apparent from the 
following equations. 
EQU .omega.o=.gamma.Ho (2) 
EQU fo=(.gamma./2.pi.).multidot.Ho (3) 
The slicing gradient field Gy is applied only during a predetermined time 
interval .tau.1 and subsequently a gradient field (referred to as a 
spoiler SP) having a sufficiently great strength is applied during a 
predetermined time interval .tau.2. As a result, the transverse 
magnetization component disappears. 
The regions 31 and 32 have slice thicknesses .DELTA.t1 and .DELTA.t2, in 
the direction of the Y axis, respectively, .DELTA.t1 and .DELTA.t2 are 
given by 
EQU .DELTA.t1=.DELTA.f1/((.gamma./2.pi.).multidot.Gy) (4) 
EQU .DELTA.t2=.DELTA.f2/((.gamma./2.pi.).multidot.Gy) (5) 
Next, to select regions 33 and 34 shown in FIG. 7, a 90.degree. RF pulse is 
applied in the X' direction so as to tip the magnetization in the Y' 
direction and, at the same time, a slicing gradient field Gx is applied. 
At that time, an RF pulse having two different frequencies f3 and f4 
(frequency bandwidths of .DELTA.f3 and .DELTA.f4) on both sides of center 
frequency f0 is applied and the gradient field Gx is made to have the 
slicing field strength in the interval .tau.1 and the spoiler SP in the 
interval .tau.2. Therefore, the magnetizations in regions 33 and 34 which 
had once be excited will disappear. 
Moreover, as shown in FIG. 8, after the lapse of a predetermined time, an 
RF pulse with a center frequency of f0 (band of .DELTA.f0) is applied in 
the direction of the Z axis to selectively excite center region 35 and a 
gradient magnetic field Gz is applied. Afterward, MR signals are detected 
by a series of pulses used for a normal imaging (not shown). At this time, 
since the magnetizations in other regions have disappeared, the regions 
outside the local portion S1 are made nonexistent and thus only MR signals 
produced from local portion S1 are acquired. With the selective excitation 
method, a portion from which MR signals are generated specifies the 
substantial existence region of a subject on an image. The method can thus 
determine the existence region of the subject in advance. 
The MRI method having both the ultra-resolution and the selective 
excitation methods was theoretically considered hereinbefore. Next a 
specific embodiment of the present invention will be described. 
As shown in FIG. 9, coil system 13 into which a subject can be disposed is 
made of a normal conductive or a super-conductive material or the like and 
includes static field coil 1 (a shim coil for correcting a static field is 
sometimes added) for generating a static magnetic field, gradient magnetic 
field coil 2 for generating gradient magnetic fields for adding position 
information of a portion in which MR signals are induced and RF coil 3 for 
transmitting RF pulses to the subject and for detecting the MR signals 
induced within the subject. The present MRI system further includes static 
field control unit 4 for supplying a current to static field coil 1, 
transmitter 5 for transmitting the RF pulses, receiver 6 for receiving the 
MR signals, gradient magnetic field power sources 7, 8, 9 for supplying a 
current to coils 2 for generating gradient magnetic fields in the 
directions of the X, Y, and Z axes, sequence controller 10 for controlling 
a pulse sequence for local excitation as shown FIGS. 5A through 5D, 
computer system 11 for controlling sequence controller 10 and for 
processing the MR signals received by receiver 6 and display unit 12 for 
displaying the result of the MR signal processing. 
As shown in FIG. 10, computer system 11 includes CPU 21 for controlling the 
whole system via data bus 27, image memory 22 for storing signals output 
from receiver 6 via interface 20, image reconstruction processor 23 for 
reconstructing two-dimensional images and the like in accordance with data 
stored in image memory 22, ultra-resolution processor 26 for 
ultra-resolution processing, interface 25 and region setting circuit 24 
for setting the region of a subject to be processed using 
ultra-resolution. Ultra-resolution processor 26 includes functional blocks 
of spectral coincidence block 26a, FFT operation block 26b, 
external-region-zero-set block 26c and IFFT operation block 26d. 
The operation of the present system will be described with reference to a 
flowchart shown in FIG. 11. 
In step S1, the region of an object is determined by a pulse sequence of 
the selective excitation method. In step S2, MR signals are input from 
receiver 6 to ultra-resolution processor 26. 
In step S3, the MR signals are subjected to FFT and, in step S4, an error 
is acquired by examining the waveform of the transformed MR signals. In 
step S5, the error obtained in step S4 is compared with a preset allowable 
error. 
When the error obtained in step S5 is larger than the allowable error, the 
MR signals are set to zero except for MR signals corresponding to the 
region of the subject in step S6. The MR signals corresponding to the 
region of the subject are subjected to IFFT in step S7 and subsequently 
the MR signals input in step S1 are adapted to the transformed MR signals 
within the pass band in step S8. After step S8 is completed, step S3 and 
the subsequent steps are performed again. 
If the obtained error is below the allowable error in step S5, then the 
transformed MR signals are transferred to interface 20 in step S9. 
The subject is estimated by the above ultra-resolution process. A two 
dimensional image is acquired by using two dimensional FFT and IFFT. 
With such an arrangement, by placing the subject in the static field and 
operating sequence controller 10, the pulse sequence shown in FIGS. 5A 
through 5D is carried out. As a result, transmitter 5 is driven to 
transmit RF pulses through RF coil 3, gradient magnetic field power 
sources 7, 8, 9 are driven to apply gradient fields Gx, Gy and Gz through 
gradient field coil 2, and the MR signals from the local portion of the 
subject are detected by RF coil 3. 
In the embodiment of the present invention described herein, the MR data is 
acquired by repeating the local exciting pulse sequence shown in FIGS. 5A 
through 5D a predetermined number of times. The local region is set in 
region setting circuit 24 by executing the pulse sequence. The MR data 
obtained through the pulse sequence is input into ultra-resolution 
processor 26 to carry out the processes shown in FIG. 11 and is then 
stored in image memory 22 via data bus 27. Furthermore, MR images are 
constructed by image-reconstruction processor 23 and then displayed by 
display unit 12. 
Although the preferred embodiment of the present invention has been 
disclosed and described, it is apparent that other embodiments and 
modifications are possible. 
According to the present embodiment, the MR signals can be acquired only 
from a local portion by the selective excitation method for specifying the 
existence region of a subject, namely, the region for imaging. The MR 
signal processing is performed by using the ultra-resolution method on the 
basis of region data representing the region of the subject obtained by 
the selective excitation method. Therefore, the present invention permits 
high resolution imaging by MR signals from a narrow region, without 
large-capacity power sources for the gradient magnetic fields and the 
like. Further, a small-capacity gradient field power source can now be 
used without lowering the resolution. It is possible to realize high 
resolution imaging in a short echo time. The ultra-resolution process may 
be performed after the acquired MR data is stored in the image memory. A 
three dimensional MR image may be reconstructed by a combination of the 
ultra-resolution method and the selective excitation method.