Single Photon Emission Computed Tomography (SPECT) images of the brain can be performed using either multidetector or rotating gamma camera systems. Each imaging system has its own advantages; the choice of equipment depends on the level of utilization and on the purposes for which the technique will be applied.
SPECT images are generated using gamma cameras or ring-type imaging systems that record photons emitted by tracers trapped in the brain. SPECT results in better image quality than planar (2-D) imaging because focal sources of activity are not superimposed upon each other; hence the signal-to-noise-ratio (i.e. the contrast between the target and the background activity) becomes greatly increased.
The high collection efficiency of the multidetector system makes rapid scanning of an entire slice possible. The primary advantage of this system is its high sensitivity, resulting in high spatial resolution and rapid imaging of the organ. As a result, SPECT perfusion images of the brain can be obtained with a spatial resolution of 10 mm FWHM (full width at half maximum) in the plane of the slice. The multidetector system would, therefore, be the preferred instrument for studies requiring high spatial resolution, regional quantification, or rapid sequential imaging.
The rotating gamma camera approach is preferable for routine clinical imaging because of its availibility and because it can be used for other types of the tomographic and nontomographic imaging. The major constraint on rotating tomographic gamma camera tomography is sensitivity. The low sensitivity on each tomographic slice is compensated for by the fact that the gamma camera collects volumetric information obtained with multidetector system.
Improvements in camera designs, in collimator design (e.g. the slant hole, long bore, or the fan beam collimators) and in reconstruction algorithms have substantially improved the quality of SPECT perfusion images using the Anger type gamma camera. Satisfactory tomographic imaging has been achieved with the rotating gamma camera using all brain perfusion agents.
With the rotating gamma camera, data is collected from multiple views obtained as the sodium iodide detector rotates about the patient's head. Since spatial resolution and image quality are dependant on the total number of primary, unscattered photons recorded by the detector, gamma cameras have been designed with multiple detectors to improve instrument sensitivity.
Three and four-head cameras allow a marked improvement in spatial resolution (6 to 10 mm FWHM) compared with 14 to 17 mm for single head systems without increasing the examination time.
Special purpose ring-type imaging systems are also designed to maximize the amount of detector recording activity from the brain. They use multiple detectors or a single sodium iodide ring and collect activity simultaneously from either a single or multiple slices (multidetector systems) or from all regions of the brain (annular detectors). Special purpose systems produce high quality images with a spatial resolution of 5 to 6 mm FWHM.
The volume imaging capacity of most SPECT systems permits reconstruction at any angle, including the axial, coronal and sagittal planes, or at the same angle of imaging obtained with CT or MRI to facilitate image comparisons. SPECT images can be merged with MRI and CT, creating a single image that combines anatomy and physiology (morphological and functional correlation). three dimentional surface and volume rendered images add perspective and facilitate the localization and sizing of lesions.
Desirable features and limitations of camera systems.