Multi-energy, single-isotope imaging using stacked detectors

Abstract

We investigated a scheme for concurrently detecting low- and high-energy emissions from (123)I with a stacked silicon double-sided strip detector (DSSD) and modular scintillation camera (Modcam) from the FastSPECT II design. We sequentially acquired both low- and high-energy emission images of an (123)I object with a prototype DSSD and a Modcam. A sandwich aperture increases spatial resolution in the low-magnification DSSD image via a smaller pinhole diameter and allows a higher magnification image on the Modcam. Molybdenum, the insert material, efficiently stops 20-30 keV photons due to its ∼20 keV K-edge. Theoretically, less than 10% of 159 keV photons interact in 0.035 cm thick sheet of molybdenum, while this thickness stops virtually all ∼30 keV photons. Thus, photons from both energy regions will be incident upon their respective detectors with little cross talk. With a multi-pinhole collimator, we can decode multiplexed images on the Modcam by making use of the lower-magnification DSSD image. This approach can provide an increase in system sensitivity compared to single-detector configurations. Using MCNP5 we examined the potential benefits and drawbacks of stacked detectors and the sandwich aperture for small-animal pinhole SPECT via the synthetic-collimator method. Simulation results encourage us to construct the novel aperture and use it with our new DSSDs designed for mounting in a transmission configuration.

Fig. 1

Modcam image of the two line sources (left). Axes are in millimeters. The right figure shows a line profile through the prototype DSSD image (counts vs. strip number (50 mm strip pitch)). At 2 × and 0.9 × magnifications, line projections are ∼7 mm (A) and ∼3.2 mm (B) apart.

Fig. 2

The inset in the right figure is a 0.035 cm thick, 0.03 cm diameter molybdenum pinhole, whereas the larger pinhole is a 1.0 cm thick tungsten collimator with a 0.1 cm pinhole diameter. The image on the right is a magnified section of the bottom middle region of the figure on the left. The thin central object is the 1.0 mm thick DSSD and upper objects are the Modcam components.

Fig. 3

Line profiles through on- (right) and 20° off-pinhole-axis point spread functions on the Modcam with and without the molybdenum insert and the DSSD. Counts were normalized to the peak count of the simulation without the insert and DSSD.