Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

image acquisition. Eddy currents are signals that are opposite of the induced currentproduced by transverse magnetization in the RF coil, and reduce the overallsignal.The proximity of the receiver coil to the volume of interest affects the coil qualityfactor, but there are trade-offs with image uniformity. Body receiver coils positionedin the bore of the magnet have a moderate quality factor, whereas surfacecoils have a high quality factor. With the body coil, the signal is relatively uniformacross the FOV; however, with surface coils, the signal falls off abruptly near theedges of the field, limiting the useful imaging depth and resulting in nonuniformbrightness across the image.Magnetic Field StrengthMagnetic field strength influences the SNR of the image by a factor of B1.0 to B 1.5.Thus, one would expect a three- to fivefold improvement in SNR with a 1.5T magnetover a O.5T magnet. Although the gains in the SNR are real, other considerationsmitigate the SNR improvement in the clinical environment, including longerTl relaxation times and greater RF absorption, as discussed previously.Cross-excitation occurs from the nonrectangular RF excitation profiles in the spatialdomain and the resultant overlap of adjacent slices in multislice image acquisitionsequences. This saturates the spins and reduces contrast and the contrast-tonoiseratio. To avoid cross-excitation, interslice gaps or interleaving procedures arenecessary (see Artifacts, below).Image acquisition and reconstruction algorithms have a profound effect on SNR.The various acquisition/reconstruction methods that have been used in the past andthose used today are, in order of increasing SNR, point acquisition methods, lineacquisition methods, two-dimensional Fourier transform acquisition methods, andthree-dimensional Fourier transform volume acquisition methods. In each of thesetechniques, the volume of tissue that is excited is the major contributing factor toimproving the SNR and image quality. Reconstruction filters and image processingalgorithms will also affect the SNR. High-pass filtration methods that increase edgedefinition will generally decrease the SNR, while low-pass filtration methods thatsmooth the image data will generally increase the SNR at the cost of reduced resolution.15.5 ANGIOGRAPHY AND MAGNETIZATION TRANSFERCONTRASTSignal from blood is dependent on the relative saturation of the surrounding tissuesand the incoming blood flow in the vasculature. In a multislice volume, repeatedexcitation of the tissues and blood causes a partial saturation of the spins, dependenton the Tl characteristics and the TR of the pulse sequence. Blood outside of