Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-09-29
2003-06-10
Shaw, Shawna J (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C324S318000
Reexamination Certificate
active
06577888
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to magnetic resonance imaging (MRI) systems, and particularly to the radio-frequency (RF) coils used in such systems.
MRI utilizes hydrogen nuclear spins of the water molecules in organic tissue, which are polarized by a strong, uniform, static magnetic field of a magnet (named B
0
—the main magnetic field in MRI physics). The magnetically polarized nuclear spins generate magnetic moments in the tissue. The magnetic moments point in the direction of the main magnetic field in a steady state, and produce no useful information if they are not disturbed by any excitation.
The generation of a nuclear magnetic resonance (NMR) signal for MRI data acquisition is accomplished by exciting the magnetic moments with a uniform RF magnetic field (named B
1
—the excitation field). The B
1
field is produced in the imaging region of interest by an RF transmit coil which is driven by a computer-controlled RF transmitter with a power amplifier. During excitation, the nuclear spin system absorbs magnetic energy, and it's magnetic moments precess around the direction of the main magnetic field. After excitation, the precessing magnetic moments will go through a process of free induction decay, releasing their absorbed energy and returning to the steady state. During free induction decay, NMR signals are detected by the use of a receive RF coil, which is placed in the vicinity of the excited volume of the tissue. The NMR signal is the secondary electrical voltage (or current) in the receive RF coil that has been induced by the precessing magnetic moments of the tissue. The receive RF coil can be either the transmit coil itself, or an independent receive-only RF coil. The NMR signal is used for producing MR images by using additional pulsed magnetic gradient fields, which are generated by gradient coils integrated inside the main magnet system. The gradient fields are used to spatially encode the signals and selectively excite a specific volume of the human body. There are usually three sets of gradient coils in a standard MRI system, which generate magnetic fields in the same direction of the main magnetic field, varying linearly in the imaging volume.
In MRI, it is desirable for the excitation and reception to be spatially uniform in the imaging volume for better image uniformity. In a standard MRI system, the best excitation field homogeneity is usually obtained by using a whole-body volume RF coil for transmission. The whole-body transmit coil is the largest RF coil in the system. A large coil, however, produces lower signal-to-noise ratio (SNR or S/N) if it is also used for reception, mainly because of its greater distance from the signal-generating tissue being imaged. Since a high signal-to-noise ratio is desirable in MRI, special-purpose coils are used for reception to enhance the S/N ratio from the tissue volume of interest.
It is desirable for specialty RF coil to have the following functional properties: high S/N ratio, good uniformity, high unloaded quality factor (Q) of the resonance circuit, and high ratio of the unloaded to loaded Q factors. In addition, the coil device may be mechanically designed to facilitate tissue sample (e.g., human body, animal, or other organic tissue) handling and comfort, and to provide a protective barrier between the tissue and the RF electronics. Another way to increase the SNR is by quadrature reception. In this method, NMR signals are detected in two orthogonal directions, which are in the transverse plane or perpendicular to the main magnetic field. The two signals are detected by two independent individual coils which cover the same volume of interest. With quadrature reception, the SNR can be increased by up to 2 over that of the individual linear coils.
In MRI and Magnetic Resonance Angiography (MRA), a neurovascular RF coil is used head, neck/c-spine and vascular imaging without repositioning the sample (e.g., a human patient). The coverage of a neurovascular coil, depending on the usable imaging volume (e.g., a sphere of 45 to 50 cm in diameter), may be about 48 cm (from the top of the head to the aortic arch). It is desirable for the performance, i.e., signal-to-noise ratio (SNR) and image uniformity, of a neurovascular coil to be comparable to a conventional head coil for head imaging and to a stand-alone neck coil for neck/c-spine imaging. For vascular imaging, it is desirable for a neurovascular coil to be able to provide homogeneous images for coverage of the blood vessels from the circle of Willis to the aortic arch for most of the patient population.
To cover the head and neck with a single RF coil, an asymmetric birdcage coil design has been used. In this design, the anterior and posterior parts of a conventional birdcage (Hayes, U.S. Pat. No. 4,692,705) head coil are extended further over the neck and chest regions to cover these regions. The asymmetric birdcage coil is operated in quadrature mode for head and neck imaging.
To further extend the coverage to the aortic arch, a quadrature RF coil has also been implemented by (Misic, et al., U.S. Pat. No. 5,517,120) for neurovascular imaging and spectroscopy of the human anatomy. This neurovascular coil utilizes multiple horizontal conductors and end conductors to distribute the current such that two orthogonal magnetic modes, i.e., one horizontal field and one vertical field, are created by the coil to achieve the quadrature detection of magnetic resonance signal. Mechanically, the neurovascular coil is separated into two shells: an upper shell for the anterior conductors and a lower shell for the posterior conductors. These two shells are connected by a hinge at the middle of the top end of the head coil mechanical housing.
The development of array coil technology (Roemer, et al., U.S. Pat. No. 4,825,162) allows one to image a large field-of-view (FOV) while maintaining the SNR characteristic of a small and conformal coil. Using this concept, a two channel (four linear coils) volume array coil for magnetic resonance angiography of the head and neck has been built. The first channel is a four bar quadrature head coil consisting of two linear coils. Two Helmholtz type coils form the second channel for covering the neck and chest. The two Helmholtz type coils are arranged such that the magnetic fields generated by them are diagonally oriented and perpendicular to each other (i.e., a quadrature coil pair). The quadrature neck coil is attached to the quadrature head coil. Each of the two Helmholtz type neck coils overlaps with the head coil to minimize the inductive coupling between the head and neck coils, i.e., the neck coils are critically coupled to the head coil, to reduce the noise correlation caused by the cross-talk between the head and the neck coils.
A split-top, four channel, birdcage type array coil has also been developed (Srinvasan, et al., U.S. Pat. No. 5,664,568; U.S. Pat. No. 5,602,479) for head, neck and vascular imaging. This split-top head and neck coil consists of a birdcage head coil and two distributed type (flat birdcage type) coils: one for the anterior neck-torso and the other for the posterior neck-torso. The quadrature signal obtained with the head coil is separated into two channel. The anterior and posterior neck-torso coils form the other two channels. The housing of the head and neck coil is divided into two parts: the lower housing for the posterior one half of the head coil and the posterior neck-torso coil and the upper housing for the anterior one half of the head coil and the anterior neck-torso coil. The upper housing is removable, i.e., a split top. The upper housing is secured to the lower housing with a latch during imaging. The inductive coupling between the neck-torso coils and the head coil is minimized by overlapping the neck-torso coils with the head coil.
It is known that significant gains in SNR (about 30%) can be achieved by using two short overlapping decoupled birdcage coils to cover the whole field-of-view compared to a single birdcage coil covering the same field-of-view. Converging t
Chan Pei Hsuon
Gyori Matthias
Zou Mark Xueming
Pearne & Gordon LLP
Shaw Shawna J
USA Instruments, Inc.
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