April 28, 2012Qs 1.The use of surface coil was introduced in 1980. Surface coil consist of either one or two copper loops of wire. They are positioned under the patient’s organ close to the location of interest. Surface coils are placed in a way that the major component of B1 RF field that is generated by the coil is orthogonal to B0. They can be used for both RF transmission and detection of NMR signals (Lean et. al, 2007 pp 552). One challenge with surface coils is the failure to be precise with the localization.
This is caused by the in homogeneity of the RF fields of the surface coil. Surface coils also have a challenge of inhomogeneous transverse magnetic field which is associated with localisation and assessment of VOI. Surface coils have an advantage as they reduce the noise levels. Orbital surface coils have a small field of view. Surface coil location is a simple way of achieving a good signal to noise ratio short measurement time. MRI surface coils are normally placed on the skin of the patient and they obtain images of tissues which are in proximity to the coil.
They have very high sensitivity and thus can produce a much larger signal to noise ratio than what is obtained with whole volume coils. Small surface coils can be used in diffuse metabolic effects of superficial structures for studies. Surface coil performance can be improved if combined by adiabatic pulses (Katz-BrullR, Lavin, & Lenkinski, 2002, pp. 1199). The sensitivity of the surface coil decreases with the distance from the coil. As a result of Lorentz reciprocity sensitivity of the coil is directly proportion to the magnetic field intensity which is created inside the body by the coil for a standard value of the coil current.
One disadvantage of surface coils is that it suffers from inadequate spatial localization and it also has non uniform radio frequency excitation. Decrease in sensitivity with increased distance from the coil makes the coil useless in obtaining images for those tissues that are located deeper in the body than the dimensions of the coil. Surface coils are preferred due to their better reception of signal. This is because they are more sensitive to signal that are at close proximity in the body of interest.
In areas where the body tissues that are being imaged are close, they produce a very clear image. However, they amplify noise signals too efficiently. This is advantageous because the sensitivity of noise signals that are far from the area of interest is reduced. In some cases a body coil can be used as a receive coil. If these two coils are different the latter have to be detuned and the preamplifier blocked to prevent burning from the larger transmit signal.
Transmit coil is separated from gradient coils by use of an RF shield. RF power that is received from the amplifier is in the form of two signals with a 900 phase difference. The RF transmits coil functions to convert power into circularly RF magnetic field.