Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2001-03-15
2003-05-06
Vo, Hieu T. (Department: 3747)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C324S309000
Reexamination Certificate
active
06560477
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of imaging and, more particularly, to the utilization of magnetic resonance imaging to image joint spaces.
BACKGROUND OF THE INVENTION
Previous techniques utilized to image internal structures of interest in subjects are well known in the art. Generally, prior art techniques utilize computed axial tomographic X-rays, also known as CT scanning, or magnetic resonance imaging (MRI) to afford an internal view of particular regions in subjects undergoing imaging procedures. These prior art techniques may be adequate for image construction of tissue matter such as bone, muscle, brain, spinal cord, veins, arteries and nerves, as well as other tissues. However, particular regions of interest, namely the joint regions and spaces, are not as amenable to prior art techniques of imaging, particularly tissue selective imaging.
Joint spaces are found in such medically important sites as the knee joint, intervertebral discs, the shoulder joint and the hip joint, as well as other joints. These four listed joints, taken together, are the subject of various pathologies that affect millions of patients and require over one million surgical repair procedures each year in the United States.
Despite the enormous medical and economic impact of various joint diseases, there has not been any prior art methods that are amenable to generalized automated use for the imaging of joints that result in unobstructed images or three-dimensional views. Typically, the prior art techniques provide joint images wherein particular angles of viewing the joint are impeded or obstructed by bone, marrow, fat and blood vessels, for example. Prior art methods are typically comprised of operator driven techniques wherein the general image of the joint and surrounding tissue is observed and imaged. The operator then uses a computer-input device to outline and identify components of interest in the joint region in each of a series of successive image slices taken of the joint region. Surrounding tissues that are components of the joint region that are not necessarily of interest are then deleted and the remaining elements of the joint region, in the series of image slices, are computationally stacked and projected. As those skilled in the art can appreciate, this method is a painstakingly slow and subjective process.
The use of X-rays to evaluate joint regions is well known in the art. This typically entails the direct, invasive injection of joints of interests with various X-ray contrast materials. However, dense bone and the injected contrast material often display similar effects on the resultant X-ray image, and as a result, this technique is only of limited use. Further, the cartilage components of the joint may be difficult to view. Therefore, the need for novel and improved techniques and methods of joint imaging will be well appreciated by those skilled in the art of medical imaging.
One approach of particular interest that has been used to image physiological structures is magnetic resonance imaging (MRI). By way of introduction, MRI involves the exposure of tissue to a variety of different magnetic and radio-frequency (rf) electromagnetic fields. The response of the tissue's atomic nuclei to the fields is then processed to produce an image of the tissue.
More particularly, the tissue is initially exposed to a polarizing magnetic field. In the presence of this field, nuclei exhibiting magnetic moments (hereinafter referred to as spins) will seek to align themselves with the field. The nuclei precess about the polarizing field at an angular frequency (hereinafter referred to as the Larmor frequency) whose magnitude depends upon both the field's strength and the magnetogyric constant of the specific nuclear species involved.
Although the magnetic components of the spins cancel each other in a plane perpendicular to the polarizing field, the spins exhibit a net magnetic moment in the direction of the polarizing field. By applying an excitation field perpendicular to the polarizing field and at a frequency near the Larmor frequency, the net magnetic moment can be tilted. The tilted magnetic moment includes a transverse component, in the plane perpendicular to the polarizing field, rotating at the Larmor frequency. The extent to which the magnetic moment is tilted and, hence, the magnitude of the net transverse magnetic moment, depends upon the magnitude and duration of the excitation field.
An external return coil is used to sense the field associated with the transverse magnetic moment, once the excitation field is removed. The return coil, thus, produces a sinusoidal output, whose frequency is the Larmor frequency and whose amplitude is proportional to that of the transverse magnetic moment. With the excitation field removed, the net magnetic moment gradually reorients itself with the polarizing field. As a result, the amplitude of the return coil output decays exponentially with time.
Two factors influencing the rate of decay are known as the spin-lattice relaxation coefficient T
1
and the spin-spin relaxation coefficient T
2
. The spin-spin relaxation coefficient T
2
represents the influence that interactions between spins have on decay, while the spin-lattice relaxation coefficient T
1
represents the influence that interactions between spins and fixed components have on decay. Thus, the rate at which the return coil output decays is dependent upon, and indicative of, the composition of the tissue.
By employing an excitation field that has a narrow frequency band, only a relatively narrow band within a nuclear species will be excited. As a result, the transverse magnetic component and, hence, return coil output, will exhibit a relatively narrow frequency band indicative of that band of the nuclear species. On the other hand, if the excitation field has a broad frequency band, the return coil output may include components associated with the transverse magnetic components of a greater variety of frequencies. A Fourier analysis of the output allows the different frequencies, which can be indicative of different chemical or biological environments, to be distinguished.
In the arrangement described above, the contribution of particular spins to the return coil output is not dependent upon their location within the tissue. As a result, while the frequency and decay of the output can be used to identify components of the tissue, the output does not indicate the location of components in the tissue.
To produce such a spatial image of the region of tissue, gradients are established in the polarizing field. The direction of the polarizing field remains the same, but its strength varies along the x, y, and z axes oriented with respect to the tissue. By varying the strength of the polarizing field linearly along the x-axis, the Larmor frequency of a particular nuclear species will also vary linearly as a function of its position along the x-axis. Similarly, with magnetic field gradients established along the y-axis and z-axis, the Larmor frequency of a particular species will vary linearly as a function of its position along these axes.
As noted above, by performing a Fourier analysis of the return coil's output, the frequency components of the output can be separated. With a narrow band excitation field applied to excite a select nuclear species, the position of a spin relative to the xyz coordinate system can then be determined by assessing the difference between the coil output frequency and the Larmor frequency for that species. Thus, the MRI system can be constructed to analyze frequency at a given point in time to determine the location of spins relative to the magnetic field gradients and to analyze the decay in frequency to determine the composition of the tissue region at a particular point.
The generation and sensing of the fields required for proper operation of an MRI system is achieved in response to the sequential operation of, for example, one or more main polarizing field coils, polarizing gradient field coils, rf ex
Greenberg & Traurig, LLP
The Regents of the University of California
Vo Hieu T.
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