Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1998-09-24
2001-04-24
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000, C324S318000
Reexamination Certificate
active
06222365
ABSTRACT:
TECHNICAL FIELD
The present invention is related to a magnetic resonance imaging (MRI) apparatus, and more specifically, is related to a technique for imaging an image with high resolution under a low readout gradient magnetic field strength.
BACKGROUND ART
A conventional examining apparatus with using a magnetic resonance, namely a conventional magnetic resonance apparatus (will be simply referred to as an “examining apparatus” hereinafter) is made of an arrangement, for example, as shown in FIG.
26
.
In
FIG. 26
, reference numeral
2601
indicates a magnet for generating a static magnetic field, reference numeral
2602
represents a gradient magnetic field generating coil for generating a gradient magnetic field, and reference numeral
2603
shows an object under examination. This object
2603
under examination is set within the static magnetic field generating magnet
2601
and the gradient magnetic field generating
2602
.
Also, a sequencer
2604
sends a command to a gradient magnetic field power supply
2605
and also a radio frequency pulse generator
2606
so as to generate a gradient magnetic field and an RF (radio frequency) magnetic field. This RF magnetic field is applied via a probe
2607
to the object
2603
under examination.
On the other hand, a signal produced from the object
2603
under examination is received by the probe
2607
and then is detected by a receiver
2608
.
The detected signal is supplied to a computer
2609
in which a signal process operation such as an image reconstruction is carried out. The process result is displayed on a display
2610
. It should be noted that the signals and the measuring conditions may be stored in a storage medium
2611
, if required.
In the case that uniformity of the static magnetic field must be adjusted, a shim coil
2612
is used. The shim coil
2612
is constituted by a plurality of channels to which currents are supplied from a shim power supply
2613
. While the uniformity of the static magnetic field is adjusted, the currents flowing through the respective coils are controlled by the sequencer
2604
. At this time, the sequencer
2604
sends a command to the shim power supply
2613
in order to produce from the shim coil
2612
, such an additional magnetic field capable of correcting nonuniform static magnetic fields.
It should also be noted that the sequencer
2604
normally controls the respective apparatuses in such a way that these apparatuses are operated at the preprogrammed timing and strengths. Among these programs, such a program for especially describing the RF magnetic field, the gradient magnetic field, and the timing/strengths of the signal receptions is referred to as a “pulse sequence”.
Next, the imaging sequential operation with employment of the examining apparatus shown in
FIG. 26
will now be summarized with reference to the spin echo method corresponding to a typical pulse sequence indicated in FIG.
27
.
The object
2603
under examination is set within the static magnetic field, and while a slice gradient magnetic field
201
is applied, a magnetic excitation radio frequency magnetic field (RF) pulse
202
is applied, so that a magnetic resonance phenomenon is induced in a certain slice within a target.
Next, a phase encode gradient magnetic pulse
204
for applying positional information along a phase encode direction to a phase of magnetization is applied, and a 180-degree pulse
205
is applied. Thereafter, while a readout magnetic field pulse
206
for applying positional information along a readout direction is applied, a magnetic resonance signal (echo)
203
is measured.
To measure data required to acquire 1 image, the above-described sequential operation is repeatedly performed to measure a plurality of echoes. At this time, since several seconds are required in order that the once excited magnetization is returned to the equilibrium condition, normally, a waiting time period equal to several seconds is needed after the echo measurement is completed until next excitation.
In general, the sampling point numbers of the echo are usually 64 to 512 per 1 echo, and a total number of echoes to be measured is 64 to 256.
After a measurement is accomplished, echoes are arranged on a frequency space (K space, measuring space) of an image, as shown in FIG.
28
. Then, an image is reconstructed by executing a 2-dimensional Fourier transform to thereby acquire a tomographic image. A matrix number of the image becomes (sampling point of single echo)×(echo number) of this time.
A field of view “Wx” along the readout direction, and a pixel size “&Dgr;Wx” may be expressed by the below-mentioned formulae (1) and (2), assuming now that the strength of the readout gradient magnetic field is “Gx”, the sampling rate (sampling interval) is “&Dgr;t”, and the sampling point is “N”;
&Dgr;
Wx=
1/(&ggr;×
Gx×&Dgr;t×N
) (1)
Wx=&Dgr;Wx×N
(2)
In these formulae, symbol “&ggr;” represents a gyromagnetic ratio of an atom under measurement. As to proton which is normally imaged, this gyromagnetic ratio is equal to approximately 42.5759 MHz/T.
As apparent from these formulae (1) and (2), the following methods are conceivable as the microscopy method for acquiring images with high resolution. That is, either the sampling rate &Dgr;t or the gradient magnetic field Gx is increased, or both the sampling rate &Dgr;t and the gradient magnetic field Gx are increased. Alternatively, the sampling point number N is increased.
In a general-purpose microscopy, resolution is increased by increasing the gradient magnetic field. Normally, such a gradient magnetic field having a very strong magnetic field strength, e.g., on the order of 100 to 1000 mT/m.
In this case, since the measuring time of the echoes is not prolonged, the attenuation of the signal strength caused by relaxations of the magnetization, and also the adverse influence caused by the nonuniform static magnetic field can be suppressed, and therefore, the deterioration in the image quality can be reduced, as compared with another microscopy method in which a sampling rate and a sampling point number are increased.
The Inventors of the present invention could find out the below-mentioned problems by considering the above-explained prior art microscopy methods.
To perform the microscopy in the clinical MRI apparatus corresponding to the conventional magnetic resonance imaging apparatus directed to measure the human body, this clinical MRI apparatus is required to be installed within a room such as an examining room having a limited space. Also, a better linearity is required over a wide area having a diameter of approximately 40 cm, and furthermore the gradient magnetic field having such a strong magnetic field of on the order of 100 to 1000 mT/m must be produced.
However, although such a strong gradient magnetic field can be produced in a compact magnetic resonance imaging apparatus for an analysis purpose, there is such a problem that this strong gradient magnetic field could not be produced in the wide area having the diameter of approximately 40 cm.
Also, there is another problem that since the magnetic field is rapidly changed in connection with the generation of such a strong gradient magnetic field, the adverse influence given to the human body, namely the load loaded to the human body is not negligible.
There is such a research example that the magnetic resonance imaging was carried out for several tens of minutes under the low gradient magnetic field. However, even when this magnetic resonance imaging is tried to be applied to the clinical purpose, it is impossible to realize such a magnetic resonance imaging due to a limitation of imaging time.
It should be understood that the gradient magnetic field strength which can be produced in the presently available clinical MRI apparatus is selected to be on the order of 30 mT/m at maximum.
DISCLOSURE OF INVENTION
An object of the present invention is to provide a magnetic resonance imaging apparatus and also a magnetic resonance imaging method
Hirata Satoshi
Ochi Hisaaki
Okajima Ken'ichi
Taniguchi Yo
Hitachi Medical Corporation
Mattingly Stanger & Malur, P.C.
Oda Christine
Shrivastav Brij B.
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