Preliminary Monte Carlo study of coded aperture imaging with a CZT gamma camera system for scintimammography
Introduction
Much current research in Scintimammography (SM) imaging is based on dedicated gamma cameras equipped with Low-Energy High-Resolution (LEHR) collimators [1], [2]. This has gained favour as previous work [3] with clinical large scintillation cameras found it difficult to gain close proximity to the breast. However, the utilised fraction of the emitted photon flux is poor. These together limit both the quality and the diagnostic value of the observed images. As an alternative to collimator-based system we have focused on Modified Uniformly Redundant Arrays (MURAs) [4] Coded Aperture (CA) methods for use in SM, as these have a proven record for successfully imaging point-like target objects in astronomy, which is similar to the imaging objective in SM. Such CA patterns have an open area of up to 50%, thus making good use of the emitted photons. Object magnification, via displacement of the camera away from the target also eases the intrinsic spatial resolution requirements of the imaging technology. Combining this technique with a Cadmium Zinc Telluride (CZT) camera provides a compact, high detection efficiency imaging system with better scatter rejection properties compared to conventional scintillation-based cameras. This should assist in rejecting the relatively large out-of-field scatter flux emanating from non-specific tracer uptake. The performance of a MURA-CZT camera system for breast tumour imaging is evaluated by calculating lesion contrast and Full-Width-Half-Maximum (FWHM) under a variety of imaging situations.
Section snippets
Methods
MCNPX code [5] is used with its physics simulation capabilities to model the imaging system. The simulation consists of tracing the path of gamma photons (99mTc isotropic sources emitting 140 keV photons) through tissue equivalent scattering material, through the CA until detected in the CZT camera. Thus, the energy, position, and direction of the gammas that successfully reach the face of the detector are saved in a list-mode file.
We also simulate the statistical uncertainty in position read
Correcting for non-specific uptake
Lesions of 8 and 10 mm were placed in the centre of the field of view at a depth of 3 cm from the breast surface. Each simulation consisted of approximately 109 photon histories. The TBR of each lesion was varied, along with the complexity of the imaging geometry. Fig. 2 shows profiles through a 10 mm lesion in air, in cold breast equivalent material, and then with a warm background with a TBR of 100:1. This clearly shows the effects of gamma ray emission from volumetrically distributed background
Conclusion
We have described the application of a MURA CA imaging system in SM based on a CZT gamma camera. MURAs are attractive because of their high transmission, and because displacing the gamma camera away from the breast relaxes the camera resolution requirements.
These preliminary results have shown that with the use of appropriate shielding and a background subtraction technique, it is possible to visualise small lesions down to a TBR of 20:1. However, the use of improved energy resolution using CZT
Acknowledgements
M. Alnafea gratefully acknowledges financial support from King Saud University, Riyadh, Saudi Arabia. We wish to thank D. Mahboub and M.I. Saripan for their valuable suggestions.
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