Characterization of scatter and penetration using Monte Carlo simulation in 131I imaging

Abstract

In 131I SPECT, image quality and quantification accuracy are degraded by object scatter as well as scatter and penetration in the collimator. The characterization of energy and spatial distributions of scatter and penetration performed in this study by Monte Carlo simulation will be useful for the development and evaluation of techniques that compensate for such events in 131I imaging.

Methods: First, to test the accuracy of the Monte Carlo model, simulated and measured data were compared for both a point source and a phantom. Next, simulations to investigate scatter and penetration were performed for four geometries: point source in air, point source in a water-filled cylinder, hot sphere in a cylinder filled with nonradioactive water, and hot sphere in a cylinder filled with radioactive water. Energy spectra were separated according to order of scatter, type of interaction, and gamma-ray emission energy. A preliminary evaluation of the triple-energy window (TEW) scatter correction method was performed.

Results: The accuracy of the Monte Carlo model was verified by the good agreement between measured and simulated energy spectra and radial point spread functions. For a point source in air, simulations show that 73% of events in the photopeak window had either scattered in or penetrated the collimator, indicating the significance of collimator interactions. For a point source in a water-filled phantom, the separated energy spectra showed that a 20% photopeak window can be used to eliminate events that scatter more than two times in the phantom. For the hot sphere phantoms, it was shown that in the photopeak region the spectrum shape of penetration events is very similar to that of primary (no scatter and no penetration) events. For the hot sphere regions of interest, the percentage difference between true scatter counts and the TEW estimate of scatter counts was <12%.

Conclusion: In 131I SPECT, object scatter as well as collimator scatter and penetration are significant. The TEW method provides a reasonable correction for scatter, but the similarity between the 364-keV primary and penetration energy spectra makes it difficult to compensate for these penetration events using techniques that are based on spectral analysis.

FIGURE 1

Measured and simulated radial PSFs (A) and energy spectra (B) for point source in air.

FIGURE 2

Measured (Meas) and simulated (Sim) FOV and ROI energy spectra for hot sphere phantom with no background activity.

FIGURE 3

Geometric (A) penetration (B), and scatter (C) components of image and components of radial PSF (D) for point source in air simulation.

FIGURE 4

Simulated separated energy spectra for point source in water. Curves are scattered and unscattered photons (I), all scattered photons (II), first-order scattered photons (III), second-order scattered photons (IV), and third-order scattered photons (V).

FIGURE 5

Total, primary, scatter, and penetration components of energy spectrum corresponding to ROIs indicated by shaded regions in inset. (A) Hot sphere ROI for phantom with no background activity. (B) Cold ROI for phantom with no background activity. (C) Hot sphere ROI for phantom with background activity. (D) Background ROI for phantom with background activity.

FIGURE 6

Spectra of 284-, 364-, and 637 + 723-keV γ-rays separated according to type of interaction for ROI indicated in Figure 5C.

FIGURE 7

Scatter spectrum, scatter and penetration spectrum, and trapezoid approximation (approx) in photopeak window for ROI indicated in Figure 5A.