Mitsuaki Arakawa

RF coil coupling for MRI with tuned RF rejection circuit using coax shield choke

Undesirable RF coupling via the outside of an outer coaxial cable conductor to/from RF coils in a magnetic resonance imaging apparatus is minimized by employing a parallel resonance tuned RF choke in the circuit. The choke is realized by forming a short coiled section of the coaxial cable with a lumped fixed capacitance connected in parallel thereacross and a conductive tuning rod positioned within the center of the coiled section so as to trim the parallel resonant frequency to the desired value.

Switchable MRI RF coil array with individual coils having different and overlapping fields of view

An array of plural magnetic resonance imaging (MRI) RF coils is provided having different and overlapping fields of view. Controllable switches are connected with each individual coil of the array and are capable of selectively conditioning any one of the coils for individual usage in an MRI procedure. Either mechanical or electrical (e.g., PIN diode) switching control may be utilized. Preferably, controllable electrical switches are located at points having approximately zero RF potential. Distributed capacitance is also preferably employed for reducing terminal inductance, preventing the establishment of spurious magnetic fields and facilitating the use of electrical switching diodes and/or varactor capacitance elements. Such distributed capacitances are also dimensioned so as to cause the terminal inductance of each coil to be within the tuning/matching range of a common tuning/matching RF circuit.

Self-cancelling RF receive coil used to decouple MRI transmit/receive RF coils

In addition to the usual winding of an MRI RF receive coil, a second, opposite sense, winding is connected to the same pair of RF output terminals and linked to at least part of the same space as the first winding. One or more serially connected RF switches in the second winding selectively connect it in circuit only during transmission of NMR RF nutation pulses. Under these conditions, any transmitted RF fields linked to the first winding are also linked to the second winding. Accordingly, any induced RF currents flowing in the receive coil windings produce self-cancelling effects in the tissue being imaged (thereby reducing possible distortion of the desired transmit fields being used for NMR nutation purposes).

Tomography Of Hydrogen With Nuclear Magnetic Resonance (NMR), And The Potential For Imaging Other Body Constituents

Nuclear magnetic resonance (NMR) imaging has already attracted a great deal of attention because it is non-hazardous and because the intrinsic NMR contrast appears to be considerably higher than x-ray contrast. On the other hand, the in-vivo NMR characteristics of normal tissues are not well understood, and no reliable data exist on the potential for differentiating normal from abnormal tissues, or benign from malignant lesions.

Apparatus and method for decoupling MRI RF coil from selected body portions using passive components

In an MRI system, a passive conductive RF decoupling structure is disposed about a portion of the volume to be imaged but at a location which is proximate a sub-volume from which MRI RF responses are to suppressed. The passive decoupling structure may be a sheet of conductive material or a shorted loop of conductive material (preferably having a gap in conductivity bridged by RF bypass capacitance so as to suppress lower frequency eddy currents otherwise caused by changing magnetic gradient fields in the MRI system).

Open design and flat cross sectional RF transmit coil for transverse magnet based MRI systems

MRI has become a widely used and indispensable component of diagnosis, and over 5000 units are operational world wide. As cost containment becomes a priority, the utilization of MRI becomes limited by its high acquisition, siting and operating costs. Because cost reduction is a major determinant of system design, attention was paid to reducing the cost of each component. One such component is the RF power transmitter used to excite the hydrogen nuclei in the body. The decision was made to provide a separate transmitter coil, as opposed to the alternative of transmitter/receive coils. A separate transmitter coil reduces the cost and complexity of the many RF coils (10 to 12) used for reception. To minimize RF power transmitter cost the coil needs to be as small as possible, consistent with the need to accommodate the largest receiver coils in use. This led to design described here, where the transmitter coil has an arch shape with maximum width of 71 cm and height above the bed pallet of 43 cm. This coil permits imaging of patients that could not fit any available superconducting magnet MR imager. Nevertheless, even though the patient population that can be imaged in the ACCESS (Toshiba America MRI, Inc., South San Francisco) system has expanded, experience demonstrated that there was a significant population component that exceeded even the aperture provided by the arch-shaped transmitter. Also, there was a benefit in placing extra axial anatomy in the center of magnet. In large subjects the transmitter coil impedes the necessary lateral displacement. This led to the exploration of an alternative transmitter configuration that would have a minimal impact on patient access. Because the volume of such a coil would be large and its conductor would be close to the magnet structures, such a coil would require increased RF transmitter power inputs to achieve equivalent power delivery to the patient. The design of the open transmitter sought to minimize this unavoidable increase.<<ETX>>

Potential Hazards In Nuclear Magnetic Resonance (NMR) Imaging: Heating Effects Of Changing Magnetic Fields And rf Fields On Small Metallic Implants

To test if changing magnetic fields and RF fields used in NMR imagers could induce electrical currents in surgical clips and prostheses capable of causing localized tissue heating, we exposed steel surgical clips, copper wire clips, and hip prostheses to fields greater than those used in our NMR imager. Results indicate that no significant heating should be expected to occur from implanted surgical clips during exposure to our NMR imager. The heating of larger metallic implants should be further investigated.

MRI Receivers For Reduced Time Acquisition

Simultaneous acquisition of MRJ signals from a receiver coil with a uniform spatial response and a coil with a linearly weighted spatial response allows for standard Fourier imaging with onehalf the normal number of phase encoded cycles. This reduces minimum scan time in a given se quence by 50% v d can be used very effectively in many fast MRI techniques. We describe the construction of a dual quadrature receiver coil for the implementation of reduced time acquisition on a commercial MRJ system. Example images using the receivers illustrate signal to noise and artifact level of the algorithm