Radio-frequency system-课版-10.pptVIP

Radio-frequency system;Radio-frequency coils RF coils send and receive radio waves in a fashion similar to a radio antenna. The term “coil” is used rather than “antenna” when the source and receiver are very close, as is the case in MRI. When pulses are sent into the patient, the coil is the source and the patient is the receiver. ;When the MR signal is read out, the patient is the source, and the coil is the receiver. In either case, the source and receiver are within a few coil diameters of each other. 1. Sending and Receiving Coils The sending coil of an MRI system is usually built into the bore of the magnet. ;Precise positioning of the patient within the sending coil is necessary to ensure that the anatomy of interest appears in the image. In most imaging units it is difficult to visualize patient position after the patient has entered the bore of the magnet. Therefore, a positioning system is used that involves the use of external markers or a patient table indexing scale.;Under some circumstances, the use of separate sending and receiving coils may be eliminated. The same coil may send and receive radio waves in the same fashion as radio antennas may be both senders and receivers. There is some interest in eliminating the in-bore sending coil to provide more room in the bore to help alleviate claustrophobic effects in patients. ;Alternately, deletion of the in-bore coil would allow a reduction in bore size that could resolve some of the design problems inherent in MRI magnet construction. The RF pulse produced in a small surface coil is less uniform than provided by an in-bore solenoidal sending coil. It is possible to overcome this limitation with special pulsing techniques. ;Positioning of the coil is an important determinant of performance. Surface coils, for example, are often positioned on the patient prior to entry of the patient into the magnet bore. A fundamental limitation in the sensitivity of a receiver coil is the presence of spurious electrical s