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

Abstract

A single-photon emission tomograph has been 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 sec, simultaneous recording from three cross-sectional layers, and a field of view the size of the human head. The 64 Nal(Tl) scintillation detectors are arranged in four banks of 16. Each detector consists of a 14 x 2 x 1.3 cm Nal (Tl) crystal fitted with three photomultiplier tubes with one-dimensional positioning capability. Three brain slices (4 cm apart) are examined simultaneously by fitting each crystal with three exchangeable focusing lead collimators. Three collimators have been designed: (a) high resolution dynamic mode collimator for the intracarotid xenon injection method, a complete set of projections gathered over a 180° rotation in a period of 5 sec with 1.7 cm FWHM resolution at the center of image: (b) high sensitivity dynamic mode collimator for performing xenon inhalation studies, a complete set of projections gathered over a 360° rotation in 10 sec with a resolution at the center of the image of 2.5 cm FHWM: and (c) high resolution static mode collimator for 360° rotation and resolution at the image center of 1.1 cm. Angular sampling is either 4 1/2° or 9°. 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/sec/μ Ci/ml (three slices) for a 20 cm diameter phantom filled with 99mTc for a resolution of 1.7 cm x 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.

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