Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2003-05-12
2004-11-09
Shrivastav, Bril B. (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000
Reexamination Certificate
active
06815952
ABSTRACT:
BACKGROUND OF THE INVENTION
Diffusion tensor imaging (DTI) has a number of important applications including characterizing the effect of ischemic attacks and predicting the connectivity of the brain. Despite their clinical significance, diffusion images can suffer severe distortion due to the rapid switching of magnetic field gradients. This switching induces eddy currents in conductive materials (such as Faraday screens, RF coils, main magnet windings, and shim coils) within the field, which in turn generate induced magnetic fields that decay over time. The decay of these magnetic fields can be described as a series of exponentials with relatively long time constants (typically tens or hundreds of milliseconds)
1,2
. The induced time varying fields contain two components: a field gradient opposing the applied gradient, and a shift in the main magnetic field B
0
(t). This leads to unwanted phase dispersion of the net magnetization, which results in poor excitation of spins, imperfect rephasing of echoes, loss of signal and image distortion. Depending upon the direction of the eddy current relative to the imaging plane, the image can be sheared, scaled or translated in the phase encoding direction. In diffusion tensor imaging, the diffusion gradient amplitudes are often significantly larger than the imaging gradients and are often applied in several directions simultaneously. This leads to a complicated combination of shearing, scaling and translation.
In view of the above mentioned aspects of prior art it is the underlying purpose of the present invention to introduce an eddy current compensated and optimized imaging sequence which achieves improved imaging through optimization of signal to noise ratios of the detected signal while avoiding distortions in imaging due to magnetic fields associated with eddy currents.
SUMMARY OF THE INVENTION
The object of the invention is achieved with a method of eddy current compensated diffusion imaging using magnetic resonance in which a spin echo signal is obtained in a readout time window by excitation of a nuclear resonance signal using a first radio-frequency pulse. The first radio-frequency pulse is refocused using at least one second radio-frequency pulse and one third radio-frequency pulse. Gradient fields are applied, those fields having a direction and strength and being activated by means of gradient pulses, the gradient pulses located between each of the radio-frequency pulses and prior to the readout window. To generate an echo at the correct position within the readout time window, the totality of the gradient pulses has a gradient time integral between a time of said excitation and the center of k
x
(or center of k-space if there is a number of echoes for one excitation pulse) which is equal to zero. In this sequence, gradient pulses are applied with two-fold purpose:
1) Generation of spin echo image: These gradients are referred to as imaging gradients, applied for frequency encoding, at least one direction of phase encoding, and slice encoding. A number of echoes can be generated for one excitation RF pulse by using alternating frequency encoding gradients for each echo (echo-planar readout).
2) Generation of diffusion weighting: The diffusion gradient pulses have a polarity, which is alternated between successive gradient pulses. Although the totality of the gradient pulses having a gradient time integral between a time of said excitation and of the center of k
x
(or k-space if there is a number of echoes for one excitation pulse) is equal to zero, at least two of the gradient pulses have differing gradient time integrals in order to reduce problems due to stimulated echoes. In a subsequent method step in accordance with the invention, the gradient direction is changed and the previous steps are repeated to evenly distribute gradient direction vectors over a sphere.
The use of diffusion gradient pulses having alternate polarities in the manner described above and having a total gradient time integral of zero with at least two of the gradient pulses having differing gradient time integrals, provides a diffusion imaging method which is effective in avoiding distortions in the image due to eddy current production. By combining these features with a systematically changed gradient direction such that the gradient vectors are evenly distributed over a sphere, imaging distortions due to directional differences in the diffusion tensor are avoided.
In a preferred embodiment of the invention, time locations of the radio-frequency pulses, the time to the center of kx or k-space as well as the time of gradient pulses are adjusted to maximize diffusion parameter signal to noise ratio and to minimize eddy current field distortion at the center of k
x
or k-space. The signal to noise ratio tends to decrease with increasing time duration of the pulse sequence. However, later readout time windows are further removed from possible eddy current distortions caused by the switching of the gradient fields. Therefore, a compromise must be made between th e need for good signal to noise ratio while avoiding eddy current field distortions. By balancing these two conflicting requirements, an optimized pulse sequence can be obtained.
In a preferred improvement in this latter embodiment, the time locations are iteratively and systematically varied to obtain a relative maximum in the signal to noise ratio and a relative minimum in eddy current field distortions. This variation takes advantage of a two-dimensional correlation between the time dependences of the signal to noise ratio and eddy current distortion to optimize the pulse sequence. By iteratively and systematically varying the time locations of the radio frequency pulses and the gradient fields relative to the readout time window an optimized sequence can be obtained.
In a preferred embodiment, the time locations are analyzed as a function of echo times, defined by th e locations of the RF refocusing pulses. This embodiment has the advantage of taking into consideration the relationship between the echo time and the transverse relaxation time and their effects on the signal to noise ratio. The RF refocusing pulse time in spin echo sequences defines the echo time and therefore affects the overall time duration of the pulse sequence and the associated signal to noise ratio at readout.
In a further improvement, the time locations are analyzed as a function of gradient field durations. The gradient field durations affect the integral of the diffusion gradient field and therefore the overall strength of diffusion related signals while also directly influencing the overall time duration of the diffusion sequence and therefore the associated signal to noise ratios at readout.
In a further improvement, the time locations are analyzed as a function of an eddy current decay time. In this manner, an additional parameter influencing the time dependence of eddy currents is taken into consideration.
In a preferred embodiment of the invention, time locations are analyzed as a function of a number of measurements. The overall number of measurements taken to determine the diffusion tensor influences the signal to noise ratio at readout. Consideration of the number of measurements in defining the time locations for the gradient and RF excitation pulses permits an improved iterative optimization of these parameters.
In an associated improvement, the time locations are analyzed as a function of a gradient time integral. As previously mentioned, the overall time duration of the sequence as well as the strength of the gradient fields leading to observable diffusion effects directly depend on the gradient time interval. Taking this integral into consideration in determining the details of the pulse sequence therefore leads to improved signal to noise ratios and associated image quality.
In an improved method, the time locations are analyzed as a function of diffusivity. Clearly the overall image quality depends not only on the time duration of the pulse sequence and the other parameters mentioned above, but also
Griffin Mark Philip
McMahon Katie Louise
Rose Stephen Edward
Shrivastav Bril B.
The University of Queensland
Vincent Paul
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