Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2002-02-25
2003-08-12
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
Reexamination Certificate
active
06605943
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of mapping proton transverse relaxation time constants, or functions thereof, in a target subject to localized movement, such as abdominal tissue, using nuclear magnetic resonance imaging.
BACKGROUND OF THE INVENTION
Magnetic resonance imaging is an imaging modality that has been developed to elucidate the internal structure of essentially diamagnetic bodies through exploitation of the phenomenon of nuclear magnetic resonance. From a semi-classical standpoint, in a static magnetic field, nuclear isotopes that have a nuclear magnetic moment experience a torque which causes the moments to precess around the axis of the field at a frequency that is proportional to the magnitude of the magnetic moment and the magnitude of the applied field. Further, the orientations of the nuclear magnetic moments are quantised in a limited number of spin states. Thus, for a particular nuclear species, the moments or spins precess at the same frequency in random phase around the direction of the field but in different equilibrium populations of spin states. If a radio frequency (RF) pulse is applied with a frequency that matches the precessional frequency of a certain nuclear species, the populations of the spin states will be perturbed from their equilibrium values. Further, the spins acquire a certain level of phase coherence in that they precess in some measure of synchrony with each other. After the RF pulse is removed, the spins return to their equilibrium populations by two relaxation processes for which a magnetic resonance signal having the same frequency as the RF pulse can be detected. One of the relaxation processes involves the return of the spins to their equilibrium population values, called spin-lattice or longitudinal relaxation, for which the relaxation rate is characterised by the longitudinal relaxation time constant T
1
. The other relaxation process is one in which the spins lose their phase coherence, called spin-spin or transverse relaxation, for which the relaxation rate is characterised by the transverse relaxation time constant T
2
. For a particular nuclear species, the relaxation rates can vary greatly according to the chemical environment surrounding each isotope, on both the molecular and macro-molecular scale. The outstanding image contrast that can be achieved by MRI is a function of the variation in these relaxation rates, coupled with the variations in nuclear density that occur throughout the body being imaged.
Although there are a number of nuclear species for which magnetic resonance can be observed, the hydrogen proton is of the greatest relative sensitivity. Consequently, it is the nuclear species around which magnetic resonance imaging has been developed. The hydrogen proton is also the most abundant nuclear species within the human body, with approximately two thirds of the body hydrogen contained in water molecules and the remainder found in fat and protein. The hydrogen proton thus makes an ideal probe for anatomical imaging. The remarkable level of soft tissue contrast that can be obtained by MRI is a result of the variation in hydrogen proton density and relaxation times for different tissues, and the perturbation of these times in various disease states.
Magnetic resonance images are constructed by varying the magnetic field strength in three dimensions throughout the subject or target to be examined. The variations in magnetic field result in precessional frequency changes of the nuclear species at various points in space, and thus enable the discrimination of magnetic resonance signals from different spatial locations. A map of signal intensities can then be constructed to obtain a magnetic resonance image. Depending on the manner in which the RF pulse is applied to cause magnetic resonance, the images that result can either be predominantly T
1
-weighted or T
2
-weighted. In T
1
-weighted images, the image intensities predominantly reflect the progression of spin-lattice relaxation, the extent of which depends on when the magnetic resonance signals are acquired. In T
2
-weighted images, the intensities essentially reflect the progression of spin-spin relaxation.
Typically, the relaxation time constant for a given region of interest over the image sequence is determined through the fitting of an equation to the measured signal intensities that describes the return of the hydrogen protons to their equilibrium spin states. For relaxation processes modeled in this fashion, the relaxation time constant that is determined is essentially an average of each and every relaxation time constant for each and every hydrogen proton within the region of interest. However, within any given region of interest, there may be particular populations of hydrogen protons that do not necessarily neighbour in space but which neighbour in terms of the chemical and physical environments which the protons experience, and which are thus characterised by their own distinct relaxation times. For example, the population of hydrogen protons found in fat will have distinctly different relaxation times from the population of hydrogen protons found in extra-cellular water. Thus, depending on the number of images acquired at different measurement times, a number of relaxation processes may be determined within the one region of interest for different populations of hydrogen protons. For the number of relaxation processes that are desired to be resolved, the equation that describes the return of the hydrogen protons to their equilibrium spin states is separately summed for each distinct population. For transverse relaxation, where the average relaxation process is characterised by a single exponential decay term involving the relaxation time constant and the measurement time, the relaxation time constant calculated is typically referred to as that for single (or mono-) exponential decay. When two or more transverse relaxation processes are being modeled, a number of exponential decay terms are summed, and the resulting equation is referred to as one of multi-exponential decay. When only two transverse relaxation processes are being modeled, the equation is one for double (or bi-) exponential decay. In this instance, it is common to refer to fast and slow relaxation components of hydrogen protons, ie: a population of hydrogen protons that undergo fast relaxation back to their equilibrium spin rates, and a population of hydrogen protons that experience slow relaxation.
For a sequence of either T
1
- or T
2
-weighted images acquired at different measurement times, the relaxation time constants of the dominant relaxation process can be theoretically determined over the entire image. However, the calculation of a map of T
1
or T
2
relaxation times constants is complicated when the region to be examined is affected by some form of localized movement. This arises, for example, in imaging of the abdomen, where the regular, repetitive motion of breathing results in image intensity perturbations across the image. The existence of breathing artefacts over the region of interest makes the calculation of both accurate and complete T
1
or T
2
maps infeasible. To date, the successful generation of relaxation time maps has only been reported in those cases where sample movement is not a factor, as in non-medical applications of materials research and NMR microscopy, and in imaging of the brain, where pulsatile and respiratory affects may be considered to be negligible. The successful generation of accurate and complete relaxation time maps over the abdomen or other targets subject to a similar extent of localized movement has not been demonstrated.
The image intensity perturbations caused by localized movement arise as a result of the method of image construction. To obtain magnetic resonance images of sufficient intensity for both qualitative and quantitative analysis, the image signal intensities must be sufficiently above the background image noise. To obtain such intensities, repeated measurements must be performed over the same
Clark Paul
St. Pierre Timothy
Edell Shapiro & Finnan LLC
Inner Vision Biometrics Pty Ltd
Lefkowitz Edward
Shrivastav Brij B.
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