Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1998-11-12
2002-11-05
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C324S201000, C600S410000
Reexamination Certificate
active
06477398
ABSTRACT:
BACKGROUND
This invention relates to method and apparatus for imaging a body. In particular, the invention provides a magnetic susceptibility image of an animate or inanimate body. Many imaging techniques exploit some natural phenomenon which varies from tissue to tissue, such as acoustic impedance, nuclear magnetic relaxation, or x-ray attenuation to provide a contrast image of the tissue. Alternatively, some imaging techniques add a substance such as a positron or gamma ray emitter to the body to construct an image of the body by determining the distribution of the substance. Each imaging technique possesses characteristics which result in certain advantages relative to other imaging techniques. For example, the short imaging time of x-ray contrast angiography reduces motion artifacts. In addition, the high resolution of x-ray contrast angiography renders this technique superior to many prior known imaging techniques for high resolution imaging of veins and arteries. However, x-ray contrast angiography is invasive, requires injection of a noxious contrast agent, and results in exposure to ionizing radiation. Thus, it is not typically employed except for patients with severe arterial or venous pathology.
Nuclear Magnetic Imaging (NMR) which is commonly called magnetic resonance imaging (MRI) entails magnetizing a transverse tissue slice with a constant primary magnetic field in a direction perpendicular to the slice, and further magnetizing the slice by applying a gradient in the plane of the slice. A radiofrequency pulse excites selected nuclei of the slice. The excited nuclei relax and emit energy, i.e., radio signals, at frequencies corresponding to local magnetic fields determined by the gradient. A Fourier analysis of the emitted signals provides the signal intensity at each frequency, thereby providing spatial information in one dimension. Repeating the excitation of the nuclei and obtaining the Fourier spectrum of the emitted signals, as the gradient rotates in the plane of the slice, provides a two-dimensional image.
MRI is of primary utility in assessing brain anatomy and pathology. But long NMR relaxation times, a parameter based on how rapidly excited nuclei relax, have prevented NMR from being of utility as a high resolution body imager. The most severe limitation of NMR technology is that for spin echo imaging n, the number of free induction decays (“FIDs”), a nuclear radio frequency energy emitting process, must equal the number of lines in the image. A single FID occurs over approximately 0.1 seconds. Not considering the spin/lattice relaxation time, the time for the nuclei to reestablish equilibrium following an RF pulse, which may be seconds, requires an irreducible imaging time of n times 0.1 seconds, which for 512×512 resolution requires approximately one minute per each two dimensional slice. This represents a multiple of 1500 times longer that the time that would freeze organ movements and avoid image deterioration by motion artifact. For example, to avoid deterioration of cardiac images, the imaging time must not exceed 30 msec. A method for speeding NMR imaging flips the magnetization vector of the nuclei by less than 90 degrees onto the x-y plane, and records less FIDs. Such a method, known as the flash method, can obtain a 128×128 resolution in approximately 40 seconds. Another technique used to decrease imaging time is to use a field gradient and dynamic phase dispersion, corresponding to rotation of the field gradient, during a single FID to produce imaging times typically of 50 msec. Both methods produce a decreased signal-to-noise ratio (“SNR”) relative to spin echo methods. The magnitude of the magnetization vector which links the coil is less for the flash case because the vector is flipped only a few degrees into the xy-plane. The echo-planar technique requires shorter recording times with a concomitant increase in bandwidth and noise. Both methods compensate for decreased SNR by increasing the voxel size with a concomitant decrease in image quality. Physical limitations of these techniques render obtaining high resolution, high contrast vascular images impractical.
Thus, it is an object of the invention to provide high resolution multi-dimensional images of tissue.
It is another object of the invention to provide multi-dimensional magnetic susceptibility images of an object.
It is yet another object of the invention to provide high resolution multi-dimensional images of the cardiopulmonary system.
It is yet another object of the invention to provide a magnetic susceptibility image of a body.
SUMMARY OF THE INVENTION
These and other objects of the invention are attained by providing an apparatus for obtaining a multi-dimensional susceptibility image of a body. The apparatus includes a radiation source for magnetizing the body with a magnetic component of a first radiation field. The apparatus also includes a first detector for measuring the magnetic component of the first radiation field in the absence of the body in a volume to be occupied by the body. The apparatus further includes a source for applying a second radiation field to the body, to elicit a third radiation field from the body. A second detector senses this third radiation field, and produces a signal that a reconstruction processor employs to create the magnetic susceptibility image of the body.
One practice of the invention provides a method for determining the distribution of radiation within a magnetized body, emanating from the body in response to an excitation radiation. The method includes the steps of measuring the emanated radiation over a three-dimensional volume by an array of detectors, and uniquely correlating each frequency component of the detected radiation with locations within the body producing that frequency component.
The invention is in part based on the realization that matter having a permeability different from that of free space distorts a magnetic flux applied thereto. This property is called magnetic susceptibility. An object, herein called a phantom, can be considered as a collection of small volume elements, herein referred to as voxels. When a magnetic field is applied to the phantom, each voxel generates a secondary magnetic field at the position of the voxel as well as external to the phantom. The strength of the secondary magnetic field varies according to the strength of the applied field, the magnetic susceptibility of the material within the voxel, and the distance of the external location relative to the voxel. For example, U.S. Pat. No. 5,073,858 of Mills, herein incorporated by reference including the references therein, teaches that the net magnetic flux at a point extrinsic to a phantom to which a magnetic field is applied, is a sum of the applied field and the external contributions from each of the voxels. The '858 patent further teaches sampling the external flux point by point and employing a reconstruction algorithm, to obtain the magnetic susceptibility of each voxel from the sampled external flux.
Unlike the '858 patent that relies on a static response from a magnetized body to determine the magnetic susceptibility of the body, a preferred practice of the invention elicits a radiative response from a magnetized body by subjecting the body to a resonant radiation field. One embodiment of the present invention generates a three-dimensional magnetic susceptibility image of an object including a patient placed in a magnetic field from a three-dimensional map of a radio frequency (RF) magnetic field external to the patient, induced by subjecting selected nuclei of the body to a resonant RF field. Application of an RF pulse to the body causes the body to emit the RF magnetic flux external to the body. A Fourier transform of this external flux produces its frequency components (“Larmor frequencies”). Each Larmor frequency determines the magnetic susceptibility of the voxels of the body producing that Larmor frequency. Further, the intensity variation of the external RF field over a three-dimensional volume of space is
Lateef Marvin M.
Mantis Mercader Eleni
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