Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-11-09
2003-07-08
Van, Quang T. (Department: 3742)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C324S318000
Reexamination Certificate
active
06591128
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the art of magnetic resonance. It finds particular application to magnetic resonance imaging receiver coil systems having detachable, relocatable, and/or interchangeable sections. It will be appreciated, however, that the present invention is also applicable to the examination of other portions of the human anatomy and to the imaging or spectroscopic examination of non-human subjects or other objects, materials, and so forth.
Conventionally, magnetic resonance imaging systems generate a strong, uniform, static magnetic field in a free space between poles or in a bore of a magnet. This main magnetic field polarizes the nuclear spin system of an object to be imaged placed therein. The polarized object then possesses a macroscopic magnetic moment vector pointing in the direction of the main magnetic field. In a superconducting main annular or bore magnet assembly, the static magnetic field B
0
is generated along a longitudinal or z-axis of the cylindrical bore.
To generate a magnetic resonance signal, the polarized spin system is excited by applying a magnetic resonance signal or radio frequency field B
1
perpendicular to the z-axis. The frequency of the magnetic resonance signal is proportional to the gyromagnetic ratio &ggr; of the dipole(s) of interest. The radio frequency coil is commonly tuned to the magnetic resonance frequency of the selected dipole of interest, e.g., 64 MHZ for hydrogen dipoles
1
H in a 1.5 Tesla magnetic field.
Typically, a radio frequency coil for generating the magnetic resonance signal is mounted inside the bore surrounding the sample or patient/subject to be imaged. In a transmission mode, the radio frequency coil is pulsed to tip the magnetization of the polarized sample away from the z-axis. As the magnetization precesses around the z-axis back toward alignment, the precessing magnetic moment generates a magnetic resonance signal which is received by the radio frequency coil in a reception mode.
For imaging, a magnetic field gradient coil is pulsed for spatially encoding the magnetization of the sample. Typically, the gradient magnetic field pulses include gradient pulses pointing in the z-direction but changing in magnitude linearly in the x, y, and z-directions, generally denoted G
x
, G
y
, and G
z
, respectively. The gradient magnetic fields are typically produced by a gradient coil which is located inside the bore of the magnet and outside of the radio frequency coil.
Conventionally, when imaging the torso, a whole body radio frequency coil is used in both transmit and receive modes. By distinction, when imaging the head, neck, shoulders, or an extremity, the whole body coil is often used in the transmission mode to generate the uniform excitation field B
1
and a local coil is used in the receive mode. Placing the local coil surrounding or close to the imaged region improves the signal-to-noise ratio and the resolution. In some procedures, local coils are used for both transmission and reception. One drawback to local coils it that they tend to be relatively small and claustrophobic.
One type of local frequency coil is known as the “birdcage” coil. See, for example, U.S. Pat. No. 4,692,705 to Hayes. Typically, a birdcage coil is cylindrical and comprises a pair of circular end rings which are bridged by a plurality of equi-spaced straight segments or legs. Birdcage head coils are capable of providing a high signal-to-noise ratio (SNR) and achieving readily homogeneous images. Birdcage coils are widely used for functional MRI (fMRI) and other applications.
Birdcage coils, however, are not without their disadvantages. Since, generally, the SNR and thus image quality increases with decreasing distance between the receiver coil and the volume being imaged, birdcage coils are generally designed so that they will be located very close to the subject's head, particularly since fMRI applications require the ability to extract small signals (e.g., reported to be as low as about 2-5% at 1.5 T). As the name implies, birdcage coils are also closed or cage-like in nature and thus restrict access to the subject's face and head. This results in a lack of space for placement of stimulation devices that would be desirable for fMRI experiments. Stimulation devices are devices constructed to stimulate a specific neural function of a subject, the response to which is sought to be observed through imaging the appropriate region of the brain. Such stimulators may emit, for example, mechanical, electrical, thermal, sound, or light signals designed to stimulate the neural activity of interest. The neural activity is induced by sensory stimuli, such as visual, auditory, or olfactory stimuli, taste, tactile discrimination, pain and temperature stimuli, proprioceptive stimuli, and so forth. Since the birdcage design is close fitting and not particularly open in nature, many such stimulation experiments must be performed in a manner that is suboptimal, if at all. For example, the use of a birdcage coil might preclude, due to space constraints, the use of an auditory stimulation device, such as a headphone set. Likewise, since bars are placed over the face, and in some instances directly over the eyes, birdcage coils are particularly disadvantageous for eye-tracking experiments or other visualization experiments.
Another problem with birdcage coils is that the design limits access to the patient, e.g., for therapeutic, physiological monitoring, and patient comfort purposes. Access may be needed, for example, to monitor physiological functions, such as oxygen levels, or to perform interventional medicine or use life-support devices, such as ventilator tubes, tracheotomy tubes, etc., while imaging a patient. Drug delivery, contrast agent delivery, and delivery of gases such as anesthetizing gases, contrast-enhancing gases, and the like, also require patient access. Also, it is also often desirable to enhance patient comfort through the use of patient comfort devices. However, the proximity of the axial segments to one another and to the head of the patient impairs such practices.
Yet another problem with birdcage head coils is their claustrophobic effect on patients. Many pediatric and adult patients already experience claustrophobic reactions when placed inside the horizontal bore of a superconducting magnet. Placement of a close-fitting head coil having anterior legs which obstruct the direct view of the patients further adds to their discomfort. Attempts to reduce the discomfort have been made, for example, through the use of illumination inside the magnet bore, air flow, and the use of reflective mirrors. Although claustrophobic reactions and discomfort are sometimes reduced somewhat by such measures, claustrophobia can still be problematic.
Birdcage coils are circularly polarized. Removing or altering the spacing of the legs adjacent the face alters the symmetry and can degrade performance.
Other types of localized coils include a phased array of smaller surface coils. In this manner, a greater SNR (that increases in proportion to the number of elements) than birdcage design can be achieved. For fMRI applications, flexible coil arrays can be wrapped around the head. However, these so-called flex-wrap designs are lacking in the spatial openness necessary for stimulation studies, interventional imaging, and the accommodation of therapy devices. Furthermore, it is difficult to achieve uniform placement of coils, both as between different subjects and for repeat studies of the same subject.
The present invention provides a new and improved localized RF coil that overcomes the above-referenced problems and others.
SUMMARY OF THE INVENTION
In one aspect, the present invention provides an radio frequency (RF) coil system for magnetic resonance imaging of one or more regions of a subject. The coil system includes a first coil section comprising one or more conductive coils in a first nonconductive housing, and a second coil section comprising one or more conductive coils in a second nonconduc
Burl Michael
Carlon John T.
Reden Laura M.
Wu Dee H.
Koninklijke Philips Electronics , N.V.
Van Quang T.
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