Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

camera heads that revolve about the patient (Fig. 22-1). The use of multiple cameraheads permits the use of higher resolution collimators, for a given level of quantummottle in the images, than would a single head system. However, the use ofmultiple camera heads imposes considerable technical difficulties on the manufacturer.It places severe requirements on the electrical and mechanical stability of thecamera heads. Furthermore, the Y-offsets and X- and Y-magnification factors of allthe heads must be precisely matched throughout rotation. Early multihead systemsfailed to achieve acceptance because of the difficulty of matching these factors.Today's multihead systems are more mechanically and electronically stable than earliersystems, providing high-quality tomographic images for a variety of clinicalapplications.Multihead scintillation cameras are available in several configurations. Doubleheadcameras fixed in a 180-degree configuration are good for head and bodySPECT and whole-body planar scans. Triple-head, fixed-angle cameras are good forhead and body SPECT, but less suitable for whole-body planar scans because of thelimited widths of the crystals. Double-head, variable-angle cameras are highly versatile,capable of head and body SPECT and whole-body planar scans in the 180-degree configuration and cardiac SPECT in the 90-degree configuration. (The usefulportion of the NaI crystal does not extend all the way to the edge of a camerahead. If the two camera heads are placed at an angle of exactly 90 degrees to eachother, both heads cannot be close to the patient without parts of the patient beingoutside of the fields of view. For this reason, one manufacturer provides SPECTacquisitions with the heads at a 76-degree angle to each other.)PerformanceSpatialResolutionThe spatial resolution of a SPECT system can be measured by acquiring aSPECTstudy of a line source, such as a capillary tube filled with a solution of technetium(T c) 99m, placed parallel to the axis of rotation. The National Electrical ManufacturersAssociation (NEMA) specifies a cylindrical plastic water-filled phantom, 22cm in diameter, containing three line sources (Fig. 22-8, left) for measuring spatialresolution. The full-width-at-half-maximum (FWHM) measures of the line sourcesare determined from the reconstructed transverse images (Fig. 22-8, right). A rampfilter is used in the filtered backprojection, so that the filtering does not reduce thespatial resolution. The NEMA spatial resolution measurements are primarily determinedby the collimator used. The tangential resolution for the peripheral sources(typically 7 to 8 mm FWHM for low-energy high-resolution and ultra-high-resolutionparallel-hole collimators) is much superior to the central resolution (typically9.5 to 12 mm). The tangential resolution for the peripheral sources is better thanthe radial resolution (typically 9.4 to 12 mm) for the peripheral sources.These FWHMs measured using the NEMA protocol, while providing a usefulindex of ultimate system performance, are not necessarily representative of clinicalperformance, because these spatial resolution studies can be acquired using longerimaging times and closer orbits than would be possible in a patient. Patient studiesmay require the use oflower resolution (higher efficiency) collimators than the oneused in the NEMA measurement to obtain adequate image statistics. In addition,the filters used before backprojection for clinical studies have lower spatial fre-