Accelerated Monte Carlo simulation for scatter correction in SPECT

Single Photon Emission Tomography (SPECT) is often used in the clinical practice to image the distribution of photon-emitting pharmaceuticals in the patient. From this distribution, functional information can be obtained (e.g. perfusion and metabolic processes). To assess the viability of myocardial tissue using SPECT, one perfusion measurement is acquired with the patient in rest and one measurement after exercise. In dual-isotope SPECT, Tl-201 can be used for the rest acquisition and Tc-99m for the stress acquisition. Both acquisitions take about 20 minutes each. In simultaneous dual-isotope SPECT, the rest and stress image are measured in one single acquisition of 20 minutes. This has the advantage of strongly reduced study time which reduces patient discomfort and increases patient throughput and very important, enables perfect registration between rest and stress image. However, simultaneous dual-isotope SPECT is hampered by photon down-scatter; the Tc-99m photons used for the stress image can be detected into the rest Tl-201 image, degrading the quality of the rest image. Accurate correction for photon down-scatter is therefore mandatory. Correction can be achieved by calculating the spatial distribution of down-scattered photons. Monte Carlo simulation is often regarded as a very accurate method to calculate photon scatter, however it is computational very demanding (slow) and therefore unsuited for clinical use. In this thesis methods are proposed to accelerate Monte Carlo simulation of down-scatter by combining Monte Carlo calculations with analytical simulation methods, resulting in acceleration factors of more than one-thousand. Application of the accelerated Monte Carlo method on clinical acquired SPECT data proved excellent down-scatter correction in acceptable computation times. In conclusion, the achieved acceleration of Monte Carlo now makes accurate correction for down-scatter possible, which realizes the use of the simultaneous dual-isotope SPECT protocol with the many advantages this protocol has to offer.