Pinhole SPECT vs. Cone-Beam SPECT
In single photon emission computed tomography (SPECT), pinhole and cone-beam collimators are used for small object imaging. It is accepted that a pinhole collimator should be used if the object is very small, and the cone-beam collimator should be used if
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Abstract. In single photon emission computed tomography (SPECT), pinhole and cone-beam collimators are used for small object imaging. It is accepted that a pinhole collimator should be used if the object is very small, and the cone-beam collimator should be used if the object is larger. This paper presents criteria to determine which collimation geometry is more advantageous in terms of spatial resolution and detection sensitivity.
1 Introduction SPECT (single photon emission computed tomography) is a medical imaging technique that creates images of the distribution of radiopharmaceuticals inside the patient (or other objects) [1]. The radiation source is injected into the patient. The gamma photons emitted from the patient body are acquired by a gamma camera outside the patient. The camera rotates around the patient (see Figure 1), and the computer processes the projections acquired by the camera from different views and produces a three-dimensional reconstructed image. The SPECT images are important in understanding of human organ’s biological processes.
Fig. 1. A SPECT system that has two gamma cameras. A collimator is mounted on each camera. The two cameras rotate with the system gantry during data acquisition. D. Zhang (Ed.): ICMB 2008, LNCS 4901, pp. 240–247, 2007. © Springer-Verlag Berlin Heidelberg 2007
Pinhole SPECT vs. Cone-Beam SPECT
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Due to the fact that the gamma ray energies are much higher than those of visible lights, the conventional lens does not function in a gamma camera. Instead, the “selective” collimators are used in gamma cameras to determine which photons are to be received and which photons are to be rejected. Parallel-hole collimators are commonly used for patient imaging. Pinhole collimators are routinely used for small animal imaging and for patient thyroid imaging. Cone-beam collimators are sometimes used for patient brain imaging and cardiac imaging [2]. Recently, multi-pinhole collimators have been applied to patient cardiac imaging as well [3]. The purpose of this paper is to compare the pinhole and cone-beam collimators and provide criteria to determine which collimator should be used in a particular imaging application.
2 Theory Figure 2 illustrates a pinhole collimator and a cone-beam collimator. In this paper, we assume that these two systems have the same focal-length f and same distance b from the focal point to the point-of-interest (POI). The goal of this paper is to compare these two systems by placing a small object at the POT position and requiring that they give the identical spatial resolution (or detection sensitivity) on the detectors. Since we fix the resolution (or the sensitivity) of the two systems, the superior system is the one that provide larger detection geometric efficiency (or the better resolution). Larger detection sensitivity means that more gamma photons can be detected and this results in lower Poisson noise in the data. Better resolution means smaller objects (e.g., lesions) can be resolved. We use the equations from [4] to derive the result
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