A Single Photon Dynamic Computer Assisted Tomograph (DCAT) for Imaging Brain Function in Multiple Cross Sections

Abstract

A single-photon emission tomograph was designed and built for regional studies of brain blood flow using 133Xe or 127Xe. The dynamic computer assisted tomograph (DCAT) features fast rotation of a square array of detectors enabling the collection of data in 5 s, simultaneous recording from 3 cross-sectional layers and a field of view the size of the human head. The 64 NaI(Tl) scintillation detectors are arranged in 4 banks of 16. Each detector consists of a 14 .times. 2 .times. 1.3 cm NaI (Tl) crystal fitted with 3 photomultiplier tubes with 1 dimensional positioning capability. Three brain slices (4 cm apart) are examined simultaneously by fitting each crystal with 3 exchangeable focusing lead collimators. Three collimators were designed: high resolution dynamic mode collimator for the intracarotid Xe injection method, a complete set of projections gathered over a 180.degree. rotation in 5 s with 1.7 cm FWHM [field width at half maximum] resolution at the center of image; high sensitivity dynamic mode collimator for performing Xe inhalation studies, a complete set of projections gathered over a 360.degree. rotation in 10 s with a resolution at the center of the image of 2.5 cm FHWM; and high resolution static mode collimator for 360.degree. rotation and resolution at the image center of 1.1 cm. Angular sampling is either 41/2.degree. or 9.degree.. By interlacing offset detectors, linear sampling is reduced to 1/4 or 1/2 the interdetector distance. The calculated high resolution dynamic mode sensitivity of the DCAT is 170,000 counts/s per .mu.Ci per ml (3 slices) for a 20 cm diameter phantom filled with 99mTc for a resolution of 1.7 cm .times. 2.3 cm at the image center. The image is reconstructed using either an iterative technique or a filtered back-projection algorithm, with the inclusion of attenuation correction in either case. Early results demonstrate the feasibility of single-photon computed tomography for measuring brain blood flow.