Coded Aperture Emission CT Using M-Array

Abstract

Three dimensional distribution of radiation emitters is measured by a simple method using a random-coded aperture and a 2-dimensional (area) sensor. A radiation emitter casts the shade of the aperture onto the area sensor. The area sensor yields 2-dimensional image data. The depth from the aperture and the shift of an emitter in a plane parallel to the aperture modulate the magnitude and the shift of the shade, respectively. Demodulation from the output (image data) of the area sensor is conducted for every plane at some incremental distance from the aperture by cross-correlating the image data with the coded aperture pattern magnified according to the distance. The aperture was coded by a pseudo-random pattern (M-array composed of maximum-length sequence), whose auto-correlation function is approximately a Dirac's delta function. A maximum of the cross-correlation function indicates the existence of a radiation emitter. The procedure of cross-correlating with the magnified M-array is repeated for a number of planes at different distance from the aperture. Thus 3-D distribution of radiation emitter was measured.
The spatial resolution in the plane parallel to the aperture was about 5mm, and that in the direction of depth was about 50mm (at the depth of 300mm) for an object with 70×70×140mm³. The M-array we used was composed of 8-tuple m-sequence (period is 255), and the dimension of the aperture made of a 3mm thick lead plate was about 200×180mm² which includes 4×4 periods of M-arrays. The hole has 2mm diameter, and the pitch of the holes was 3mm.