Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2001-01-10
2003-10-07
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000, C324S300000
Reexamination Certificate
active
06630828
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention is magnetic resonance imaging (“MRI”) methods and systems and more particularly a method and apparatus for rapidly acquiring MRI data from a portion of an imaged object.
MRI Imaging
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B
0
) the individual nuclei in the tissue attempt to align their magnetic moments with this polarizing field but as a result of nuclear spin, precess about it in random order at their characteristic Larmor frequency. The Larmor frequency is dependent on the strength of the magnetic field and on the properties of a particular nucleus as represented by a magnetogyric constant &ggr;. Nuclei which exhibit this phenomenon are referred to as “spins”.
By convention, the polarizing field B
0
is considered to lie along a z axis of a Cartesian coordinate system. The procession of the nuclei in the polarizing field B
0
creates a net magnetic moment M
z
in the direction of the polarizing field. Individual spins have magnetic moments that are perpendicular to the z axis in the transverse or x-y plane, however, the random orientation of the spins cancels any net transverse magnetic moment.
In MRI imaging, a radio frequency signal is applied in the x-y plane near the Larmor frequency to tip the net magnetic moment into the x-y plane so that it rotates at the Larmor frequency. The practical value of this phenomenon resides in the signal which is then emitted by the excited spins termed the NMR signal (“nuclear magnetic resonance”). In simple systems, the excited spins induce an oscillating sine wave in a receiving coil which may be the same coil used to excite the spins. The amplitude of this signal decays as a function of the homogeneity of the magnetic field caused by atomic scale interaction between the spins or “spin-spin” relaxation and the engineering limitations of producing a truly homogenous polarizing field B
0
. This decay is caused by a loss of phase coherence in the spins and is commonly referred to as T*
2
relaxation. Second decay mechanism is the gradual return of the magnetic moments of the individual spins to a longitudinal direction aligned with the polarizing field B
0
. This is termed T
1
relaxation and in most substances of medical interest is much longer than T
2
relaxation.
An image of a patient may be obtained by evaluating the NMR signal contributed by different spins at different locations in the patient's tissue. A pulse sequence using gradient magnetic fields encodes location information on the spins in the form of the phase and frequency. The encoded spin signal may then be separated to produce an image.
A wide variety of pulse sequences is known. For example, the spin warp or spin echo technique is described in “Spin Warp NMR Imaging And Applications To Human Whole-Body Imaging” by W. A. Edelstein et al.,
Physics in Medicine and Biology,
vol. 25, pp. 751-756 (1980); the steady state free precession (“SSFP”) technique including gradient refocused acquired steady state pulse sequences (“GRASS”) as described in U.S. Pat. No. 4,665,365 and contrast enhanced fast imaging (SSP-ECHO) described in “Rapid Fourier Imaging Using Steady State Free Precision”, R. C. Hawks and S. Patz,
Magnetic Resonance in Medicine
4, pp. 9-23 (1987); and echo planer imaging (“EPI”) is described in an article by Peter Mansfield (J. Phys. C. 10: L55-L58, 1977). These descriptions of pulse sequences are hereby incorporated by reference.
Cartesian MRI Acquisition
In a representative spin echo pulse sequence, a z-axis gradient and a narrow-band, radio frequency excitation pulse may be applied to a patient, for example, so that only spins in a “slice” to the patient perpendicular to the z-axis are excited. An x-gradient field may then be applied to cause the spins at one side of the slice to precess faster than spins on the other side of the slice. In this manner, the spins have been given a frequency encoding that allows them to be distinguished along the x-axis.
This NMR signal of the spins at different frequencies is acquired for a period of time and digitized to provide a first row of data that may be stored in an array in a reconstruction computer. The number of dimensions of the array and the number of elements in the array define a k-space well known to those in the art. The NMR signals must be sampled at a rate at least twice the frequency of the highest frequency component of the NMR signal (the Nyquist sampling rate) so as to prevent the introduction of aliasing artifacts.
Additional NMR signals are then collected for this slice with the same x-gradient but with a progressively increased y-axis gradient field. The y-axis gradient serves to phase encode the spins in the y-direction. Each successive NMR acquisition with a different y-axis gradient forms a successive row in the k-space array in the computer.
Once k-space has been filled, a two dimensional Fourier transform may be made of the k-space data to produce the desired image. Generally it is known to band limit the NMR signal to eliminate the contribution of spins beyond certain spatial ranges in the frequency encoding x-axis direction limiting the amount of k-space data somewhat. Such band limiting cannot be performed in the phase encoding direction, however, until after the data is fully acquired and therefore is of little value in reducing the data acquisition time.
Radial MRI Acquisition
In an alternative method of data acquisition, the k-space data is filled not by rows and columns but by a series of radial projections about a point within k-space. This acquisition technique is analogous to the acquisition of data in an x-ray computed tomography (“CT”) machine and allows the data to be reconstructed into an image by CT-type algorithms including filter back projection.
MRI Angiography
In MRI angiography, images of the blood vessels are obtained. For contrast-enhanced applications in which contrast materials such as gadolinium compounds are injected into a peripheral vein, the acquisition of k-space data must be carefully coordinated with the arrival of contrast so as to prevent an unfavorable variation in the weighting of the k-space data. The availability of a high speed imaging technique would be helpful in this regard since it would permit a series of images to be obtained throughout the passage of contrast.
In contrast enhanced MRI, two images, one before the introduction of a contrast medium into the vessels and one after the introduction of the contrast medium, may be obtained and subtracted. The subtracted image reveals information about the bloodflow through the vessels allowing the detection of obstructions and the like. Structures other than flowing blood are similar in the two images and thus substantially reduced in contrast.
The timing of the acquisitions of the two MRI images is crucial to providing a high contrast image. Normally there is a time delay between the introduction of the contrast medium into the patient and its time of arrival at the region of the vessel of interest.
Ideally the first image should be concluded immediately before the arrival of the contrast medium so as to provide an accurate comparison image and the second image begun immediately after the arrival of the contrast medium so as to be complete before the contrast medium dissipates The time consuming process of acquiring an image of a patient and the difficulty of monitoring the progress of the contrast medium, make production of a high quality contrast enhanced MRI image a difficult task.
Acquisition Speed
It would be advantageous to be able to acquire images of higher resolution more quickly. This is important when the available imaging time is limited by the passage of injected contrast material or by respiratory motion. In Cartesian acquisitions image spatial resolution is proportional to imaging time, so any reduction in acquisition time produces images of reduced spatial resolution. Within the context of Cartesian imaging, some investigators have developed methods to image reduced field of
Mistretta Charles A.
Peters Dana C.
Lefkowitz Edward
Quarles & Brady LLP
Shrivastav Brij B.
Wisconsin Alumni Research Foundation
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