Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1998-11-12
2002-08-20
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S421000, C600S422000, C324S318000, C324S319000, C324S320000, C324S322000
Reexamination Certificate
active
06438402
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
BACKGROUND OF THE INVENTION
The field of the invention is magnetic resonance imaging (MRI) and, in particular, local coils for use in magnetic resonance angiography (MRA).
A. MRI Imaging
In MRI, a uniform magnetic field Bo is applied to an imaged object along the z-axis of a Cartesian coordinate system fixed with respect to the imaged object. The effect of the magnetic field Bo is to align the object's nuclear spins along the z-axis.
In response to a radio frequency (RF) excitation signal of the proper frequency oriented within the x-y plane, the nuclei precess about the z-axis at their Larmor frequencies according to the following equation:
&ohgr;=&ggr;
B
0
(1)
where &ohgr; is the Larmor frequency, and &ggr; is the gyromagnetic ratio which is a constant and a property of the particular nuclei. The component of the nuclear spins aligned with the x-y plane is termed the transverse magnetization.
The rate of decay of the transverse magnetization differs for different tissues and hence may be used to distinguish among tissue in an MRI image. Hydrogen, and in particular the nucleus (protons) because of its relative abundance in biological tissue and the properties of its nuclei, is of principle concern in such imaging. The value of the gyromagnetic ratio g for protons is 4.26 kHz/gauss and therefore in a 1.5 Tesla polarizing magnetic field Bo, the resonant or Larmor frequency of protons is approximately 63.9 MHz.
In a typical imaging sequence for an axial slice, the frequency of the RF excitation signal is centered at the Larmor frequency of the protons and applied to the imaged object at the same time as a magnetic field gradient Gz is applied. The gradient field Gz causes only the nuclei, in a slice with a limited width through the object along an x-y plane, to be excited into resonance.
After the excitation of the nuclei in this slice, magnetic field gradients are applied along the x and y axes. The gradient along the x-axis, Gx, causes the nuclei to precess at different frequencies depending on their position along the x-axis, that is, Gx spatially encodes the precessing nuclei by frequency. The y axis gradient, Gy, is incremented through a series of values and encodes the y position into the rate of change of phase of the precessing nuclei as a function of gradient amplitude, a process typically referred to as phase encoding.
A weak nuclear magnetic resonance generated by the precessing nuclei may be sensed by the RF coil and recorded as an NMR signal. From this NMR signal, a slice image may be derived according to well known reconstruction techniques. An overview of NMR image reconstruction is contained in the book “Magnetic Resonance Imaging, Principles and Applications” by D. N. Kean and M. A. Smith.
B. Angiography
The delay between the RF excitation and the recording of the NMR data may be used to detect and measure the flow of blood in blood vessels and thereby to detect obstructions and to distinguish the blood vessels from stationary tissue as demarcated by the flowing blood.
Such flow measurement may be made most simply by selectively exciting the spins in a given location and measuring the transverse magnetization of the spins at a downstream location a short while later. Examples of this “time of flight” technique are described in U.S. Pat. Nos. 3,559,044; 3,191,119; 3,419,793 and 4,777,957, hereby incorporated by reference. A variation of this technique notes the change of transverse magnetization in the region excited by the RF pulse. Examples of this method are described in U.S. Pat. Nos. 4,574,239; 4,532,474; and 4,516,582; also incorporated by reference. A third technique measures flow by making use of the fact that spins moving in a gradient magnetic field experience a phase shift. This technique is described in U.S. Pat. Nos. 4,609,872 and 5,281,916, hereby incorporated by reference.
C. Local Coils
The quality of the image produced by MRI techniques is dependent, in part, on the strength of the NMR signal received from the precessing nuclei. For this reason, it is known to use an independent RF receiving coil placed in close proximity to the region of interest of the imaged object in order to improve the strength of this received signal. Such coils are termed “local coils” or “surface coils”. The smaller area of the local coil permits it to accurately focus on NMR signals from the region of interest. Further, the RF energy of the field of such a local coil is concentrated in a smaller volume giving rise to improved signal-to-noise ratio in the acquired NMR signal.
The signal-to-noise ratio of the NMR signal may be further increased by employing a coil that is sensitive to RF energy along both of a pair of mutually perpendicular axes. This technique is generally known as quadrature detection and the signals collected are termed quadrature signals.
The outputs of the quadrature coil pairs are combined so as to increase the strength of the received signal according to the simple sum of the output signals corrected for phase shift from the coils. The strength of the uncorrelated noise component of these signals, however, will increase only according to the square root of the sum of the squares of the noise components. As a result, the net signal-to-noise ratio of the combined quadrature signals increases by approximately {square root over (2)} over the signal-to-noise ratio of the individual signal.
The quadrature orientation of the two coils introduces a 90° phase difference between the NMR signals detected by these coils. Therefore, combining the outputs from the two quadrature coils, to achieve the above described signal-to-noise ratio improvement, requires that one signal be shifted to have the same phase as the other signal so that the amplitudes of the signals simply add.
Such phase shifting and combining is typically accomplished by means of a hybrid network. Hybrid networks are four-port networks known in the art and having the property that when the four ports are properly terminated, energy input to two of the ports, with the proper relative phase angles, will be combined at one of the remaining two ports. The antenna coils are attached to two of the ports and the output lead is attached to a third port and produces the sum of the signals from the antenna coils, one being shifted so that they add in-phase. The remaining uncommitted port is connected to a termination resistor.
As used herein, the term quadrature coil and quadrature signal, will refer to the detecting of the NMR signal along multiple axes and combining the signals so collected, with the appropriate phase shifts to produce a signal of improved signal-to-noise ratio.
1. Volumetric Local Coils
One method of constructing a local coil is the “bird cage” construction in which two conductive loops are spaced apart along a common longitudinal axis and interconnected by a series of regularly spaced longitudinal connectors. The impedance of the loops and of the longitudinal conductors is adjusted so that the coil may be excited into resonance by a rotating transverse magnetic field at the Larmor frequency. A quadrature signal may be obtained by monitoring the current through two longitudinal conductors spaced at 90° around the periphery of the loops. Such coils are described in detail in U.S. Pat. Nos. 4,680,548, 4,692,705, 4,694,255 and 4,799,016.
The use of volumetric local coils of conventional bird cage or other quadrature design may be undesirably constraining to the patient who must be surrounded by the relatively small volume of the coil. The use of a conventional volumetric coil for angiographic imaging of the lower extremities would require threading the patient's feet through a relatively long tubular structure—a procedure that may be difficult or impossible for many patients.
For this reason it is known to produce an angiographic coil having flexible side panels supporting the coils, the side panels being folded around the supine patient after the patient is centered on the coil. See U.S. Pat. N
Bluma Janette A.
Hashoian Ralph
IGC-Medical Advances, Inc.
Lateef Marvin M.
Lin Jeoyuh
Quarles & Brady LLP
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