Spatial Resolution of Shapes in Gamma Camera Imaging Using an Exact Formula for Solid Angle of View

Spatial Resolution of Shapes in Gamma Camera Imaging Using an Exact Formula for Solid Angle of View

S. Zimeras (University of the Aegean, Greece)
DOI: 10.4018/ijsbbt.2012010104
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Abstract

Modeling of the tomographic weights usually incorporates angle of view, decay and attenuation. A usual assumption is that the gamma camera is a long way from the object, this leads to the approximation that the angle of view subtending the front of the collimator tube is the same as that subtending the back (Weir & Green, 1994). However, if the distance between camera and subject is small then this approximation may not be good enough and artifacts may be produced. In this paper, the derivations of the angle of view and the solid angle of view are analyzed considering distances when the camera is close to the body. The solid angle is explained by an appropriate formula which is defined as the exact solid angle of view. The effect of the solid angle of view for different distances is examined. A comparison between exact solid angle and the approximate solid angle formula of Weir and Green (1994) is presented. Finally the effects of the two formulae on reconstruction are studied using simulated data from circular gamma camera rotation systems. Comparison between the resolution could be achieved by applying multiresolutio-multigrid approaches.
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Physics Of Gamma Camera

Gamma camera imaging is a modern medical diagnostic technique, based to study function rather than form. The patient is injected with or inhales an appropriate drug, which became concentrated in the organ of interest. Photons emission occurs in the organ at a rate ranging spatially according to the concentration and measurements of this concentration can be made by counting emitted photons in the gamma camera. The physical and operational details of the gamma camera are described by Larsson (1980). The gamma camera rotated about an axis through the patient to collect a sequence of projections from the body at numerous equally spaced angles. The session begins with the gamma camera directly above and facing the patient. In this potion, the camera detects and records photons leaving the patient’s body in a certain time period after 20 seconds. The camera then rotates about an axis parallel to the table, stops after 64 degrees and collects counts at the new angle. This process is repeated until the gamma camera return to its original position directly above the patient. The majority of emissions are never recorded by the system since their path is not towards the camera. The camera has a lead parallel-hole collimator which further limits the number of recorded emissions by only emitted photons whose path is nearly exactly perpendicular to the surface of the camera. Figure 1 demonstrates the basic component of a gamma camera and the various paths that an emitted photon can take: (1) photon emitted away from the camera, (2) photon emitted and scattered away from the camera, (3) photon absorbed inside the subject, (4) photon emitted away from the camera but scattered through collimator, (5) photon emitted directly through the collimator, (6) photon directed towards camera, but at an angle preventing it from passing through collimator.

Figure 1.

Detection of photons in the gamma camera: (1) photon emitted away from the camera, (2) photon emitted and scattered away from the camera, (3) photon absorbed inside the subject, (4) photon emitted away from the camera but scattered through collimator, (5) photon emitted directly through the collimator, (6) photon directed towards camera, but at an angle preventing it from passing through collimator

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