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

Quantitative Evaluation of Scintillation Camera Imaging Characteristics of Isotopes Used in Liver Radioembolization

Scintillation camera imaging is used for treatment planning and post-treatment dosimetry in liver radioembolization (RE). In yttrium-90 (90Y) RE, scintigraphic images of technetium-99m (99mTc) are used for treatment planning, while 90Y Bremsstrahlung images are used for post-treatment dosimetry. In holmium-166 (166Ho) RE, scintigraphic images of 166Ho can be used for both treatment planning and post-treatment dosimetry. The aim of this study is to quantitatively evaluate and compare the imaging characteristics of these three isotopes, in order that imaging protocols can be optimized and RE studies with varying isotopes can be compared.Phantom experiments were performed in line with NEMA guidelines to assess the spatial resolution, sensitivity, count rate linearity, and contrast recovery of 99mTc, 90Y and 166Ho. In addition, Monte Carlo simulations were performed to obtain detailed information about the history of detected photons. The results showed that the use of a broad energy window and the high-energy collimator gave optimal combination of sensitivity, spatial resolution, and primary photon fraction for 90Y Bremsstrahlung imaging, although differences with the medium-energy collimator were small. For 166Ho, the high-energy collimator also slightly outperformed the medium-energy collimator. In comparison with 99mTc, the image quality of both 90Y and 166Ho is degraded by a lower spatial resolution, a lower sensitivity, and larger scatter and collimator penetration fractions.The quantitative evaluation of the scintillation camera characteristics presented in this study helps to optimize acquisition parameters and supports future analysis of clinical comparisons between RE studies

Monte Carlo simulated primary, scatter and collimator penetration fractions for the experimental set-up with the line source centered in 20 cm PMMA.

<p>Monte Carlo simulated primary, scatter and collimator penetration fractions for the experimental set-up with the line source centered in 20 cm PMMA.</p

Schematic overview of the spatial resolution measurement of the line-source centered in 20 cm PMMA.

<p>Shown are the camera, including the collimator (A), a stack of 20 PMMA slabs of size 40×40×1 cm (B), the location of the line-source (C), and the patient bed (D). The line-source to collimator-face distance is 11 cm.</p

Sensitivity in counts per second per unit activity.

<p>Sensitivity in counts per second per unit activity.</p

Radioisotope characteristics and measurement settings.

a<p>E_βmax represents the maximum energy and abundance of the beta particles.</p>b<p>E_γ represents the gamma photon energy and abundance.</p>c<p>E_win represents the lower and upper limits of the energy window.</p

Collimator characteristics.

<p>All collimators have hexagonal hole shapes.</p

Measured and simulated LSF of the line-source centered in 20 cm PMMA.

<p>(A) LSF of the ^99mTc line-source and VXGP collimator, (B) ¹⁶⁶Ho and HEGP, (C) ⁹⁰Y 120–250 keV and HEGP, and (D) ⁹⁰Y 50–250 keV and MEGP. Data is plotted on semi-logarithmic scale, showing good agreement between the measurements (data points) and simulations (blue solid line). The intensity is normalized to the total number of counts in the ROI. Contributions of primary, scattered, and penetrated photons are shown in green, light blue, and red, respectively.</p

Contrast recovery as a function of sphere diameter.

<p>QH is the recovery of sphere-to-background contrast in the measurement, as compared to the true contrast in the phantom.</p