Abstract:
An RF coil construction ( 40, 40′ ) includes removable, relocatable, and/or detachable sections ( 42, 44 ) that are inherently decoupled. The sections can be relocated, removed, or exchanged with sections having different coil sizes or coil configurations, allowing the coil configuration to be tailored to a desired imaging procedure and region of the brain. The coil construction provides space for stimulation devices and adjusting patient access and comfort. Since the operator can select coil removal or placement to reduce the amount of data outside the region of interest, the coil construction can also reduce scanning and reconstruction time, reduce artifacts, and provide increased temporal resolution and image throughput.

Description:
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 γ 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&#39;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&#39;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 nonconductive housing, wherein the first and second coil sections are configured to be inherently decoupled or have minimal coupling. The coil system further comprises one or more fasteners removably and movably joining the housings of the first and second coil sections. 
     In a further aspect, the present invention provides a magnetic resonance imaging system comprising a main field magnet for generating a temporally constant magnetic field along a main field axis and an RF coil system. The RF coil system includes a first coil section configured for maximum or predominant field sensitivity along a first axis perpendicular to the main magnetic field axis, a second coil section configured for maximum sensitivity along a second axis perpendicular to the first and main magnetic field axes, and a fastening system for selectively fastening the first and second coil sections on opposite sides of a region of interest for quadrature reception of resonance signals emanating from the region of interest. 
     In yet a further aspect, the present invention provides a magnetic resonance method comprising the steps of establishing a polarizing magnetic field in a region of interest; exciting resonance of selected dipoles in the region of interest to generate magnetic resonance signals; and concurrently receiving the magnetic resonance on one side of the region of interest with a first linear coil having a maximum sensitivity along a first axis orthogonal to the polarizing magnetic field, and on an opposite side of the region of interest with a second linear coil having a maximum sensitivity along a second axis orthogonal to both the polarizing magnetic field and the first axis. 
     One advantage of the present invention is that it increases spatial openness around the subject. 
     Another advantage resides in its ability to easily select the desired coverage. 
     Another advantage resides in improved accommodation for stimulation devices, such as the type used for fMRI experiments, and coil placement or removal options to maximize patient comfort. 
     Another advantage resides in improved accommodation of patient comfort devices. 
     Another advantage of the present invention is that detaching coil sections still permit the remaining coil sections to be operational. 
     Another advantage of the present invention is that it accommodates life support or therapeutic devices such as ventilator tubing, tracheotomy tubes, immobilization collars, etc. 
     Another advantage of the present invention is that it provides detachable and/or relocatable coil sections matched to fMRI experiments, such as auditory or visual fMRI experiments. In addition to providing openness in the space around the subject&#39;s head that matches the requirements of the particular fMRI procedure, data acquisition throughput is increased in that the region of interest can be tailored to the appropriate region of the brain, i.e., the region or regions containing the neural activity of interest. 
     Another advantage of the present invention is that aliasing can be reduced by reduction of coverage of the excitation area. 
     Yet another advantage of the present invention is that coil concentration can be increased for areas of interest or extended to areas not well covered by the current coil designs. 
     Another advantage is that switching between different fMRI experiments and/or different stimulation equipment, such as between vision and auditory experiments, can be more readily performed. 
     Still another advantage resides in its ability to monitor relatively small signals, such as in blood oxygen level dependent contrast (BOLD) studies. 
     Yet another advantage of the present invention is that it allows a technologist to readily position and lock non-imaging devices. 
     Still another advantage of the present invention is that it is readily adaptable to time-saving techniques where temporal resolution is desired. 
     Still another advantage of the present invention is its adaptability to subjects having different body shapes and sizes, including subjects for whom the conventional head coil designs might provide an ill fit. 
     Yet another advantage of the present invention is that it is can also be used for interventional imaging. 
     Another advantage is that it allows addition, removal, and exchanging of coils. 
     Other advantages include the improved physiological monitoring, improved drug and contrast agent delivery, and improved delivery of gases such as anesthetizing gases or contrast-enhancing, e.g., hyperpolarized, gases. 
     Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
     FIG. 1 is a diagrammatic illustration of a magnetic resonance imaging system apparatus including a coil construction in accordance with the present invention; 
     FIGS. 2 and 3 are functional block diagram illustrating alternative embodiments of data acquisition circuitry for use with the RF coil system in accordance with the present invention; 
     FIG. 4 illustrates a first exemplary embodiment of a coil construction according to the present invention; 
     FIG. 5 shows the coil conductors of the embodiment of FIG. 4; 
     FIG. 6 illustrates a manner in which the movable coil section of FIG. 5 can be relocated; 
     FIG. 7 illustrates a second exemplary embodiment of a coil construction according to the present invention; 
     FIG. 8 shows the coil conductors of the embodiment of FIG. 7; 
     FIG. 9 illustrates an exemplary manner in which the movable coil section of FIG. 7 can be relocated; and 
     FIG. 10 illustrates a coil system in accordance with the present invention comprising multiple interchangeable coil sections. 
     FIG. 11 illustrates an alternative embodiment of a coil section of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a plurality of primary magnetic coils  10  generate a uniform, temporally constant magnetic field B 0  along a longitudinal or z-axis of a central bore  12 . In a preferred superconducting embodiment, the primary magnet coils are supported by a former  14  and received in a toroidal helium vessel or can  16 . The vessel is filled with helium to maintain the primary magnet coils at superconducting temperatures. The can  16  is surrounded by a series of cold shields  18  which are supported in a vacuum dewar  20 . Of course, annular resistive magnets, open magnets, and the like are also contemplated. 
     A whole body gradient coil assembly  30  includes x-, y-, and z-gradient coils mounted along the bore  12  for generating gradient magnetic fields, G x , G y , and G z  along the x, y, and z axes, respectively. Preferably, the gradient coil assembly is a self-shielded gradient coil that includes primary x, y, and z-coil assemblies  32  plotted in a dielectric former and secondary x, y, and z-coil assemblies  34  that are supported on a bore defining cylinder of the vacuum dewar  20 . A whole body radio frequency coil  36  is mounted inside the gradient coil assembly  30 . A whole body radio frequency shield  38 , e.g., copper mesh, is mounted between the whole body RF coil  36  and the gradient coil assembly  30 . 
     An insertable radio frequency head coil system  40  is removably inserted into the bore of an examination region defined about an isocenter of the magnet  10 . In the illustrated embodiment, the insertable radio frequency coil system  40  comprises front or face coil section  42  and rear or back coil section  44 . The front coil section  42  and the rear coil section  44  are shown aligned in opposing, facing relation defining a volume sized to receive a subject&#39;s head. The front coil is configured for maximum sensitivity to radio frequency signals along a first axis perpendicular to the main field or z-axis, e.g., the vertical axis. The rear coil is configured for maximum sensitivity to signals along an axis perpendicular to the first axis and the main field axis, e.g., the horizontal axis. In this manner, the front and rear coils are magnetically isolated and achieve quadrature detection. 
     An operator interface and control station  50  includes a human-readable display  52 , such as a video, CRT, CCD, LCD, active matrix monitor, or the like, and one or more operator input devices including a keyboard  54 , a mouse  56  or other pointing device, such as a trackball, track pad, joystick, light pen, touch-screen overlay, and the like. A computer control and reconstruction module  58  includes hardware and software for enabling the operator to select among a plurality of preprogrammed magnetic resonance sequences that are stored in a sequence control memory of a sequence controller  60 . The sequence controller  60  controls gradient amplifiers  62  connected with the gradient coil assembly  30  for causing the generation of the G x , G y , and G z  gradient magnetic fields at appropriate times during the selected gradient sequence. A digital transmitter  64  causes a selected one of the whole body and insertable radio frequency coils to generate B 1  radio frequency field pulses at times appropriate to the selected sequence. 
     In certain embodiments, the coil construction  40  is employed as both a transmitter and receiver coil. The use of coil construction  40  for transmission and receiving is particularly advantageous for imaging methods which employ pre-excitation or presaturation pulses prior to the imaging portion of the pulse sequence, such as flow tagging angiographic methods, fat saturation methods, and the like. 
     Resonance signals received by the coil construction  40  are demodulated by a data acquisition circuitry  66  and stored in a data memory  68 . A reconstruction or array processor  70  performs a two- or three-dimensional inverse Fourier transform, or other known transform, to reconstruct a volumetric image representation that is stored in an image memory  72 . A video processor  74  under operator control converts selected portions of the volumetric image representation into slice images, projection images, perspective views, or the like as is conventional in the art for display on the video monitor  52 . 
     Referring now to FIG. 2, in a preferred embodiment of the data acquisition electronics  66 , the signal from each of n individual RF coils in the head coil system  40  is amplified by a corresponding one of n individual preamplifiers  80   a ,  80   b , . . . ,  80   n , where n is the number of receiver coils in head coil system  40 . The individual amplified signals are demodulated by n individual receivers  82   a ,  82   b , . . . ,  82   n  and fed to an array of A/D converters including n individual A/D converters  84   a ,  84   b , . . . ,  84   n . A digital combiner  90  processes, weights, and combines the individual digital signals using standard digital signal processing techniques. The operator can also control the combiner  90  to divide the signals to be reconstructed to a plurality of related images. Alternatively, the signals from the coils can be digitized and then demodulated with digital receivers. 
     The number of receiving channels depends on the particular MRI system and thus, it will be recognized that it is not necessary that the number of receiver channels be equal to the number of RF coils. For example, the signals from a plurality of coils may be multiplexed or otherwise combined in analog or digital fashion with appropriate combining circuitry as necessary in light of the number of receiver channels available on the imaging system employed. 
     Referring now to FIG. 3, there appears a block diagram illustrating an alternative embodiment of data acquisition circuitry  66 . The 90° out of phase analog signals received by the front and rear RF coils of head coil system  40  are combined by a conventional quadrature coil combiner  80 , which typically phase shifts and adds the received signals. The resulting combined signal is supplied to a receiver  82 . Receiver  82  demodulates the combined signal and an analog-to-digital converter  84  digitizes the signal to numerical data representative of the magnetic resonance signals. The data thus produced is stored in the data memory  68 . 
     Referring now to FIG. 4, there is shown a first exemplary embodiment of the head coil construction  40  according to the present invention. The front coil section  42  and the rear coil section  44  are constructed such that when arranged in facing relation, as depicted, they are inherently decoupled. As used herein, the terms “inherently decoupled” and “intrinsically isolated” describe coils or coil arrays that exhibit little or no mutual inductance. While complete decoupling is desirable, it will be recognized that complete decoupling is often a condition that cannot be met. Therefore, the terms “inherently decoupled” and “intrinsically isolated” are not intended to preclude small amounts of coupling that are acceptable to the operation of the coils. In the preferred embodiment, the decoupling is achieved by designing the front and rear coils to be linearly polarized along orthogonal axes, although other decoupling techniques are also contemplated. 
     The inherent decoupling enables the front coil section  42  to be freely moved with respect to the rear coil section  44 , or to be removed altogether, without the need to retune either coil system. Also, the front coil section  42  is exchangeable with alternate coil systems having field sensitivity in the same direction, for example, having different sizes, coil configurations, and so forth, without the need to retune the rear coil system  44 . 
     The front coil section  42  of head coil construction  40  comprises a housing  100  constructed of a nonconductive material enclosing conductive RF coils and one or more fasteners or fastening systems  104 . The fasteners  104  are depicted as elongate in the z-direction allowing front coil  42  to be removed and/or removably replaced at a plurality of, and preferably any number of, positions along the z-axis. The manner in which the front coil  42  can be freely positioned with respect to rear coil  44  is illustrated more clearly in FIG.  6 . Fasteners  104  are preferably hook and loop fasteners, such as Velcro® or similar material. Other fastener types include, but are not limited to, removably attachable adhesive material, one or more clamps or latches, straps, snap fit fasteners, fasteners forming a sliding engagement between the coil halves, such as a guide pin, guide roller, guide rib, and so forth engaging a complimentary groove. A means for reproducing a given set up can optionally be provided, such as a scale or other markings or indicia on the housings  100  and  102 . Where a sliding engagement between the coil halves is employed, they can optionally be positionable in a plurality of predetermined positions, for example, by providing one or more resilient or spring biased protrusions engaging a series of complimentary openings or depressions on the fastener  104  or housing halves  100  or  104 . In still further embodiments, handles can be provided on the detachable/relocatable coil section housings to facilitate coil section movement, removal, and replacement. 
     The coils of the head coil construction  40  are illustrated in FIG.  5 . In the embodiment shown, the front coil section  42  comprises a pair of overlapping saddle or loop coils  106  and  108 , which are overlapped and positioned for minimum mutual inductance to form a phased array. Other coils which have maximum sensitivity in the vertical direction are also contemplated. The rear coil section  44  comprises a pair of overlapping butterfly coils  114  and  116  arranged in a phased array. Other coils with maximum sensitivity in the horizontal direction, such as double-D coils, are also contemplated. 
     The coils are built with a conductive material, including but not limited to copper, aluminum, silver, or other conductive material. The coils can be built, for example, by laminating a nonconductive substrate with copper or other conductive foil, depositing a layer of copper or other metal onto a nonconductive substrate, and so forth. The coils can include conventional RF coil circuit components such as capacitors and so forth as appropriate to tune or match the coils as is known to those skilled in the art. 
     Referring now to FIGS. 7 and 8, there is shown a head coil construction  40 ′ according to a second exemplary embodiment of the present invention. The head coil  40 ′ comprises a front coil section  42 ′ and a rear coil section  44  arranged in facing relation. Again, coil configurations which are inherently decoupled have been selected. 
     The front coil section  42 ′ in this embodiment is freely moveable and removable with respect to rear coil section  44  without the need to retune either coil system. Also, the front coil section  42 ′ is exchangeable with alternate coil systems having field sensitivity in the same direction, for example, having different sizes, coil configurations, and so forth, without the need to retune the rear coil system  44 . A head coil system that comprises a plurality of differently configured front coil sections  42  and  42 ′, and the manner of their interchangeability, is illustrated in FIG.  10 . 
     Referring again to FIGS. 7 and 8, front coil section  42 ′ of the head coil construction  40 ′ comprises a housing  100 ′ constructed of a nonconductive material enclosing a conductive RF coil  106 ′ and one or more fasteners  104 , as detailed above by way of reference to FIG.  4 . Again, fasteners  104  allow removal of the front coil  42 ′ and/or removable placement of front coil  42 ′ in a plurality of positions along the z-direction in a manner analogous to that shown in FIG.  6 . Likewise, coil system  42 ′ can also be rotated, e.g., 180 degrees, as illustrated in FIG.  9 . This facilitates placing the front coil close to the region of interest (ROI) to optimize sensitivity to signals from that region, but displaced from sources of artifacts such as moving eyes, metal dental work, and the like. Further, the fasteners can be used to attach other equipment in addition to or instead of the front coil section  42 ′. 
     The coils of the head coil construction  40 ′, illustrated in the embodiment of FIG. 8, include a front coil section  42 ′ comprising a single loop saddle coil  106 ′ and the rear coil section  44 ′, comprising a pair of overlapping butterfly coils  114  and  116  arranged in a phased array. The coils are built with a conductive material and can include additional circuit components as described above by way of reference to FIG.  5 . 
     The use of horizontal field coils in the rear coil is particularly advantageous when imaging the base of the brain, which is in close proximity to the rear coil. When other areas are of primary interest, the front coil can be the horizontal field coil and the rear coil can be the vertical field coil. 
     In preferred embodiments, in addition to being physically displaceable, removable, and/or interchangeable, it is particularly advantageous that the front and rear coil systems of the present invention are electronically individually and selectively removable from the circuit. Likewise, it is particularly advantageous, when either or both of front and rear coil sections are multi-coil systems, that individual coils thereof be electronically individually and selectively removable or replaceable. 
     In certain embodiments, the rear coil  44  includes a coil or a phased array of coils having a maximum sensitivity in a horizontal direction and a coil or phased array of coils having a maximum or predominant sensitivity in the vertical direction. An exemplary embodiment of such a rear coil arrangement is illustrated in FIG. 11, which includes overlapping butterfly coils  114  and  116 , and which further includes overlapping loop coils  120  and  122 . In operation, when a front coil (e.g.,  42 ,  42 ′)is present, the loop coils  120  and  122  are electronically removed from the circuit. However, when the front coil is removed in accordance with this teaching, coils  120  and  122  can be engaged and the rear coil alone can be used alone to provide quadrature detection. This configuration is advantageous for imaging the cervical spine and the back of the head, and for fMRI applications requiring access to the subject&#39;s face. The use of the rear coil section illustrated in FIG. 11 alone in a non-quadrature mode is also contemplated, e.g., wherein a front coil section is removed and wherein coils  120  and  122  are electronically removed. 
     In operation, the RF coil construction in accordance with this teaching allows the MRI operator to perform anatomical imaging of the entire head followed by facile change in coil configuration, e.g., coil or coil set removal (either physical or electronic), adjustment of the front coil or coil set placement, exchange of coil sections for stimulation devices, comfort devices, or alternate coil sets. Calibration, landmark adjustment of the coil, and repeatable position of stimulation devices can also be improved through positioning. 
     In certain embodiments, the entire coil system of the present invention is used for anatomical imaging of the entire head or the head and neck of the subject. The ability to slide or relocate the coil sections improve the adaptability in the coverage, accommodating a wide range of patient profiles, such as patient size, length of neck, kyphotic subjects, and other body types. 
     The coil system in accordance with this teaching can also be used with any other coils or coils sets with which it is decoupled. For example, a head coil system in accordance with this teaching can be operated with a spinal imaging array for imaging the central nervous system. Other coils that can be used with the coil system of the present invention include thyroid coils, cardiac coils, or other local coils, such as a coil for imaging trauma sites, and so forth. 
     Functional images may require only a portion of the region of interest to be acquired. Thus, during fMRI studies, one or more coils are moved or removed (either physically or electronically) to more closely tailor the imaging field of view (FOV) to match the region of interest during fMRI (e.g., the region appropriate to the brain response sought to be observed). This improves imaging throughput and accuracy while also improving access for placement of stimulation devices for use in fMRI experiments. In especially preferred embodiments, stimulation devices or patient comfort devices are connected, e.g., using fasteners  104 , or otherwise, or placed into positions vacated by the detached coil sections. Comfort devices are helpful in achieving patient cooperation, e.g., in the patient cooperating to remaining still. Comfort devices which may be employed with the present invention include, for example, audio and/or visual devices for the presentation of music, movies, television, and so forth. Advantageously, the comfort devices are exchangeable with removable coil section, for example, occupying a space vacated by the removal of a coil section. Coil sections integrating such comfort devices are also contemplated. 
     In certain embodiments, a visual stimulation fMRI experiment is performed using the coil system in accordance with the present invention. The front coil section is moved to a position that does not obstruct the subject&#39;s field of vision (and optionally electronically removed) or physically removed. In a preferred aspect, the imaged region comprises the occipital lobe and cerebellum regions of the subject, and the imaged region does not include the subject&#39;s eye region. 
     The coil construction of the present invention can also be modified to integrate stimulation devices, e.g., on or within the coil housing. For example, a combined stimulation device/RF coil section can include, for example, an auditory stimulation device such as audio speakers (e.g., in a headphone-like configuration within the coil section), optical displays, and the like. 
     It will be recognized that the present invention is not limited to the above-described embodiments and that the invention is also applicable to other coil types. For example, the front coil loops are not limited to the configurations shown, and can comprise planar and non-planar loops (circular, square, rectangular, elliptical) and phased arrays thereof, Helmholtz coils, and the like. Likewise, the rear coil section can comprise a single butterfly coil or a phased array of more than two butterfly coils, ladder coils, double-D coils, and the like. For optimal signal-to-noise ratio, the front and rear coil systems have a quadrature relationship. 
     Likewise, although the invention has been shown and described herein primarily by way of reference to a moveable, detachable, and/or interchangeable front coil section that is particularly suited for fMRI experiments requiring access to the facial region, other arrangements are contemplated as well. For example, the present invention can be readily adapted so as to provide access to the subject&#39;s ears, for example, through the use of removable coils or through the use of a coil configuration and housing having openings or cutaway regions allowing access to the ears. 
     The description above should not be construed as limiting the scope of the invention, but as merely providing illustrations of some of the presently preferred embodiments of this invention. In light of the above description and examples, various other modifications and variations will now become apparent to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims. Accordingly, the scope of the invention should be determined solely by the appended claims and their legal equivalents.