Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-01-03
2001-09-18
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000, C324S322000
Reexamination Certificate
active
06291996
ABSTRACT:
The present invention relates to an apparatus for and a method of determining values of relaxation parameters, T
1
and T
2
, for use with a resonance technique in which a resonance response signal is produced in response to excitation.
The invention is suitable for use with Nuclear Magnetic Resonance (NMR) techniques, and in particular, although not exclusively, with Magnetic Resonance Imaging (MRI) techniques, but may also be used in Nuclear Quadrupole Resonance, Electron Spin Resonance, and other such techniques.
The present invention claims priority from UK Patent Application No. 9717183.9, filed Aug. 13, 1997, whose disclosure is incorporated herein by reference.
NMR is a technique in which a radio frequency (rf) excitation pulse is applied to a sample in the presence of a magnetic field, and echo or other signals resulting from the excitation of protons (that is hydrogen atoms) in the sample are subsequently detected. In MRI, the magnitudes of the echo signals are used to construct an image. The concentration of hydrogen atoms will generally correspond to the water density in the sample. MRI is therefore widely used for imaging tissue in the human body.
The echo signals will have various parameters associated with them, such as the spin-lattice relaxation time, T
1
and the spin-spin relaxation time, T
2
. These parameters are properties that are inherent to the material being sampled. The relaxation parameters T
1
and T
2
are often used to provide contrast in images, because the typical variation in these parameters may be much larger than that in straightforward proton density. For example, the difference in the proton concentration between normal and abnormal tissue in the human body may only be a few percent, while the difference between the relaxation parameters may be much higher. Grey and white areas in the brain may also have very different T
1
and T
2
values.
A technique for the measurement of T
1
for imaging was proposed by Mansfield et al in Physical Medical Biology, vol 31, no. 2, pp 113-124. Mansfield et al proposed performing two EPI experiments using different delay times, and analyzing the data resulting from the two experiments to obtain values of T
1
. An alternative T
1
imaging technique was investigated by Kay et al in Magnetic Resonance in Medicine vol. 22, pp 414-424. Kay et al used a Look-Locker excitation pulse sequence to acquire a set of proton density images, and used a fitting procedure to derive values of T
1
in order to obtain a T
1
map.
A technique for T
2
imaging was proposed by Poon et al in Journal of Magnetic Resonance Imaging, vol 4, pp 701-708. Poon et al proposed an excitation pulse sequence which was tailored specifically for T
2
imaging. Thirty two echoes were acquired and values of T
2
calculated from them.
The prior art techniques for determining values of T
1
use different types of excitation pulse sequence to those for determining values of T
2
. Hence, the prior art techniques suffer from the problem that it would be very inefficient to determine both T
1
and T
2
, since two different experiments would have to be set up and run separately. This would be time consuming and would make inefficient use of expensive resources.
In one aspect of the present invention, the problems in the prior art are sought to be overcome by providing an apparatus for determining values of relaxation parameters, T
1
and T
2
, for use with a resonance technique in which a resonance response signal is produced in response to excitation of a sample, the apparatus comprising means for determining values of both T
1
and T
2
of the sample from said response signal.
Hence, the values of the relaxation parameters T
1
and T
2
are measured simultaneously. Therefore only one experiment need be done to obtain values of both T
1
and T
2
, resulting in improved efficiency.
As used herein, the term “resonance response signal” may connote a single such signal or a plurality of individual such signals generated, for example, by a plurality of excitation pulses.
If, for example, a series of excitation pulses is applied to a sample, under certain conditions the resultant resonance response signal will exhibit a transient behaviour which will be oscillatory in nature. It has been discovered pursuant to the present invention that this transient behaviour contains information from which the values of T
1
and T
2
may be derived. Therefore, in a preferred embodiment of the invention, the means for determining values of T
1
and T
2
may comprise means for determining the values of T
1
and T
2
from the transient behaviour of the response signal. This can provide an efficient technique for determining the values of T
1
and T
2
.
The means for determining the values of T
1
and T
2
may be arranged to determine their values by fitting an equation to the transient behaviour of the response signal, to enable efficient determination of their values.
For example, the determining means may comprise means for fitting the Bloch equation to the transient behaviour of the response signal. This may yield accurate quantitative values directly with no need for further calibration of the apparatus.
Alternatively, the determining means may comprise means for fitting a (typically empirical) equation to the transient behaviour of the response signal to thereby obtain a set of fit parameters, and means for mapping the set of fit parameters to values of T
1
and T
2
using predetermined calibration values. This technique may require less processing and therefore be faster.
The means for determining the values of T
1
and T
2
may be adapted to also determine values of the flip angle, &thgr;. This can provide useful additional information.
Another aspect of the invention provides an apparatus for resonance testing the sample, comprising:
means for applying the excitation;
means for detecting the response signal; and
the means as aforesaid for determining the values of T
1
and T
2
.
The means for detecting the response signal may further comprise means for acquiring a set of image data, and may in particular comprise means for acquiring image data after each of at least two (and preferably all) of the excitation pulses to thereby obtain a set of image data; indeed a number of sets of image data may be obtained. The means for acquiring image data may be adapted to acquire such data using a single or multiple scan technique, such as Echo Planar Imaging, Echo Volumar Imaging, a Spiral Scan method or a 2-Dimensional Fourier Transformation technique.
The means for determining the values of T
1
and T
2
may be arranged to perform a pixel-by-pixel fitting procedure through the set of image data. Alternatively, the means for determining the values of T
1
and T
2
may comprise means for determining average values of groups of pixels and means for performing a fitting procedure to the average values through the set of image data.
The means for applying the excitation may comprise means for applying a sequence of excitation pulses, and, if so, the apparatus may further comprise means for applying a gradient magnetic field after each pulse, the field being the same for at least two of said pulses. In the preferred embodiment, the same gradient evolution (and hence imaging module) is used for preferably all of the pulses.
The apparatus may comprise means for calculating the optimum number of repetitions of the excitation pulses for the best measurement of T
2
and T
1
, and the means for applying excitation may be adapted to apply this number of pulses.
The apparatus may further comprise means for determining an appropriate number of repetitions of the excitation pulses for the determination of T
2
and T
1
, and wherein the excitation applying means is adapted to apply said appropriate number of excitation pulses.
Again, the apparatus may further comprise means for determining an appropriate time interval between the excitation pulses for the determination of T
2
and T
1
, and the means for applying excitation may be adapted to apply pulses with said appropriate time interval.
T
Glover Paul Martin
Hill Richard John
BTG International Limited
Nixon & Vanderhye
Oda Christine
Shrivastav Brij B.
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