A Silicon SPECT System for Molecular Imaging of the Mouse Brain

Abstract

We previously demonstrated the feasibility of using silicon double-sided strip detectors (DSSDs) for SPECT imaging of the activity distribution of iodine-125 using a 300-micrometer thick detector. Based on this experience, we now have developed fully customized silicon DSSDs and associated readout electronics with the intent of developing a multi-pinhole SPECT system. Each DSSD has a 60.4 mm × 60.4 mm active area and is 1 mm thick. The strip pitch is 59 micrometers, and the readout of the 1024 strips on each side gives rise to a detector with over one million pixels. Combining four high-resolution DSSDs into a SPECT system offers an unprecedented space-bandwidth product for the imaging of single-photon emitters. The system consists of two camera heads with two silicon detectors stacked one behind the other in each head. The collimator has a focused pinhole system with cylindrical-shaped pinholes that are laser-drilled in a 250 μm tungsten plate. The unique ability to collect projection data at two magnifications simultaneously allows for multiplexed data at high resolution to be combined with lower magnification data with little or no multiplexing. With the current multi-pinhole collimator design, our SPECT system will be capable of offering high spatial resolution, sensitivity and angular sampling for small field-of-view applications, such as molecular imaging of the mouse brain.

Fig. 1

A photograph of SiliSPECT, a dual-headed SPECT with two silicon DSSDs stacked in each camera head.

Fig. 2

The stacked acquisition geometry (left) and the projection images (right) on stacked detectors at two orthogonal camera views. The magnification on the front detectors (upper images) is 0.6 and the back detectors (lower images) 1.6. The phantom consists of a pair of brachytherapy seeds with a total activity of 63 μCi. The acquisition time was 87 minutes.

Fig. 3

Schematic of focused pinhole systems on each collimator plate and the overlap region created with two collimators (left). A single axial slice of a 3D sampling map (Metzler et al. 2002).

Fig. 4

Left: The collimator system of SiliSPECT is characterized by small pinhole opening angle and short source-collimator normal distances. Under these geometric conditions, the geometric sensitivity profile (right) becomes a function of pinhole opening angle and collimator thickness, and can be made very narrow. At larger source-collimator distances and pinhole opening angle the geometric sensitivity fall-off follows the conventional formulation (dashed line).

Fig. 5

Left: At small pinhole opening angles the geometric sensitivity profile can be varied with the collimator thickness, whereas with larger pinhole angles (right) the collimator thickness does not impact the geometric sensitivity, which follows the conventional term (cos³(θ).d³/16h²).

Fig. 6

The total system sensitivity (including a dual-headed camera with stacked, one mm thick silicon detectors) for a pair of collimators (each 127 pinholes) and a focused pinhole system compared to an unfocused pinhole system.

Fig. 7

Simulation study with SiliSPECT. The phantom projections were acquired at two different magnifications, M₁ = 1, M₂ = 1.5 (top). The phantom consists of microstructure activity hot spots of diameters between 60–250 μm (lower, left). A slice from simulated data reconstructed using the MLEM algorithm is shown in the lower right.