Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2001-07-30
2002-09-03
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S315000, C324S322000
Reexamination Certificate
active
06445183
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a magnetic resonance diagnosing apparatus (MRI), and more specifically, to a medical MRI apparatus suitably for measuring a temperature distribution.
BACKGROUND ART
Very recently, MRIs may constitute very important diagnosing means for diseases as diagnostic imaging apparatus capable of drawing tissues under superior conditions in conjunction with X-ray CT. Furthermore, MRIs are not only used in diagnosing purposes, but also are developed as such a technique (Interventional MR, namely IVMR) which is applied to guides for catheters and laser fibers when low invasive medical treatments are carried out. As one of these MRI applications, a temperature distribution of a tissue is detected. This MRI application is attractively known as such a means for monitoring curing conditions of a diseased portion in a real time manner while a laser ablation and/or a focused ultrasonic ablation is carried out, during which a tissue of such a diseased portion as tumor and hernia is burned out so as to be cured.
Among parameters for defining a magnetic resonance signal (MR signal), such a parameter indicative of a temperature dependent characteristic involves the spin density “&rgr;”, longitudinal relaxation time “T1”, the transverse relaxation time “T2”, the diffusion coefficient of water, the chemical shift “94” of water proton, and the like (see J. C. Hindman, J. Chem. Phys. Volume 44, page 4582, 1966). Among these parameters, it is known that the reliability of chemical shift of water proton is high, in view of less dependent characteristics to the factors except for the temperature.
As a utilization method of a chemical shift, a method for employing a phase map is more effective, since spatial resolution is high and measurement time is short (see JP-A-5-253192 “A Precise and Fast Temperature Mapping Method Using Water Proton Chemical Shift” Y. Ishihara, A. Calderon et al., Abstracts of the Society of Magnetic Resonance Medicine, 11th Annual Meeting, Berlin, p. 4803 (1992)).
This method is performed as follows: That is, while using the sequence having the chemical shift sensitive characteristic such as the gradient echo (GrE) method, a change in the chemical shifts occurred before and after the temperature change is detected as the phase difference of the MR signal. The frequency shift of water proton caused by the temperature is equal to 0.01 ppm/° C., and the phase difference “&Dgr;ø is expressed by the following formula (1):
&Dgr;&phgr;=2&pgr;·&Dgr;&dgr;·&ggr;
Bo·TE
(1)
In this formula (1), symbol &Dgr;ø shows the phase difference in the pixel of interest, symbol &Dgr;&dgr; represents a change in the chemical shifts of water proton in this pixel of interest, symbol “&ggr;” denotes the gyromagnetic ratio, symbol Bo shows the static magnetic field strength, and symbol TE indicates the echo time. These symbols are similarly applied to the below-mentioned descriptions.
Further, the temperature difference &Dgr;T is calculated from this phase difference &Dgr;ø based upon the following formula (2):
Δ
T
(
x
,
y
)
=
Δφ
(
x
,
y
)
2
π
·
γ
Bo
·
TE
·
α
(
2
)
In this formula (2), symbol “&agr;” indicates the temperature dependent characteristic of the chemical shift of water proton, i.e., [0.01 ppm/° C.]. This symbol is similarly applied to the below-mentioned explanations.
The measuring precision of the temperatures by this method may depend upon both the S/N ratio of the signal and the stability of the hardware, and is on the order of +1° C. to −1° C.
In the conventional phase map formation by employing the GrE method sequence, it is practically difficult to form the phase map within the short time, since the phase encode loop must be repeated along one direction of the space in the 2-dimensional measurement, and the dual phase encode loop must be repeated along two directions of the space in the 3-dimensional measurement. As one example, assuming now that the echo time TE=20 ms, the repetition time TR=30 ms, and the phase encode step number is 64 by way of the highspeed GrE method, approximately 2 seconds are required to form the image. Moreover, in order to execute the slice encode by 16 steps, 32 seconds are required. In a temperature measurement executed in IVMR, a temperature change of a diseased portion, which is caused by a focused ultrasonic medical treatment and the like must be monitored in the real time mode. Also, it is desirable to image several sheets of images per 1 second, and also it is preferable to display a 3-dimensional temperature distribution in combination with these images. However, as previously explained, in accordance with the conventional GrE method, these operations could not be realized.
Therefore, an object of the present invention is to provide such an MRI apparatus capable of forming/displaying a temperature distribution image within a very short time period.
DISCLOSURE OF THE INVENTION
To solve the above-explained problem, in accordance with the present invention, as a radio-frequency magnetic field used to excite water proton, a series of radio-frequency pulses (will be referred to as a “burst wave” hereinafter) is employed which is constituted by a plurality of sub-pulses. Also, such a gradient magnetic field echo is produced which owns a higher phase sensitivity characteristic than that of a spin echo. As a result, a phase map can be formed in very high speeds, and also a temperature distribution can be displayed in very high speeds.
It should be understood that as the highspeed imaging sequence with employment of the burst wave, the burst method is known (JP-B-6-34784). An MR image diagnosing apparatus of the present invention is featured by that while a sequence is executed which is made by modifying the sequence of this burst method, a gradient magnetic field echo is produced to which a phase rotation proportional to a chemical shift is applied, and thus, both a phase map and a temperature map may be acquired.
In other words, an MRI apparatus, according to the present invention, is featured by such a magnetic resonance image diagnosing apparatus comprising: magnetic field generating means for generating a static magnetic field, a gradient magnetic field, and a radio-frequency (RF) magnetic field in a space where an object under examination is located; detecting means for detecting a magnetic resonance signal produced from the object under examination; means for reconstructing an image based upon the detected magnetic resonance signal; display means for displaying thereon an image; and control means for controlling each of the means, wherein:
the control means executes the below-mentioned sequences, namely:
1) a burst wave is applied as the radio-frequency magnetic field, and at the same time, the gradient magnetic field along at least one direction is applied;
2) a gradient magnetic field in the same direction as that of said gradient magnetic field is applied as a readout-operating gradient magnetic field, and an MR (magnetic resonance) signal is detected as a gradient magnetic field echo;
3) when the burst wave is applied, or the magnetic resonance signal is detected, such a gradient magnetic field which phase-encodes the magnetic resonance signal is applied;
4) the detected magnetic resonance signal is Fourier-transformed, and a phase distribution is calculated based upon both a real part and an imaginary part of complex data of the Fourier-transformed magnetic resonance signal; and
5) an image is constructed from the phase distribution or a temperature distribution obtained from the phase distribution, and the constructed image is displayed on the display means.
In this case, a burst wave will be referred to as “a series of RF pulses” which is constituted by employing a plurality of sub-pulses “p” as indicated in FIG.
3
A. When a burst wave on a time axis is Fourier-transformed, a series of pulse stream having the same pulse number as that of th
Shimizu Hiromichi
Watanabe Shigeru
Antonelli Terry Stout & Kraus LLP
Hitachi Medical Corporation
Lefkowitz Edward
Shrivastav Brij B.
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