Most dedicated small-animal SPECT systems use gamma cameras based on photomultiplier tubes (PMTs). The PMTs detect the light generated when emitted gamma photons hit a scintillation crystal, with pinhole collimation employed to increase the system's spatial resolution. But according to Jan Heemskerk of the University Medical Center Utrecht in the Netherlands, combining pinhole collimation with an intrinsically high-resolution detector such as a CCD array could ultimately provide a definitive high-performance small-animal gamma camera (Phys. Med. Biol. 54 3003).

Heemskerk and colleagues at UMC Utrecht and Delft University of Technology are working to develop such a SPECT system. The big advantage of using CCDs to detect scintillation signals is their superior spatial resolution. CCDs have demonstrated resolution as good as 45 µm, compared with 1.5 mm for a PMT-based gamma camera. Furthermore, says Heemskerk, a CCD-based camera will be far more compact and robust.

"We are looking to develop a modular, ultrahigh-resolution gamma detector for small-animal SPECT," he told medicalphysicsweb. "The idea is to employ a fairly large number - around 75 - of these cameras in an optimized geometry with a focused multi-pinhole design. This system would require compact ultrahigh-resolution cameras that are capable of detecting the position and energy of individual gamma photons."

Clever coating
Using a CCD detector in combination with micro-columnar CsI(Tl) (thallium-doped caesium iodide) scintillation crystals can deliver a spatial resolution of better than 100 μm. However, the crystal's low capture efficiency means that it's not ideal for small-animal imaging. To increase the detection efficiency of their gamma camera, the Dutch researchers replaced the micro-columnar crystal with a continuous CsI(Tl) crystal. And to alleviate the degrading effects of using a thick crystal, they topped it with a reflective coating to enhance its light output.

Ray-tracing simulations indicated that the use of a micro-machined retro-reflector comprising 75 µm tetrahedra would significantly improve the light output, enhancing the signal by 90% in some cases. The relative number of reflected photons varies with the depth-of-interaction (DOI) of the gamma photons, with the largest gain seen for scintillations close to the top of the crystal. The researchers used a multi-scale detection algorithm that can estimate the DOI to correct for this variation when determining the energy values.

Next, the team fabricated a tetrahedral retro-reflector and examined the detection capabilities of a gamma camera with and without the reflective coating. The camera comprised a 1.5x1.5 cm, 2-mm-thick CsI(Tl) crystal (from Scionix of the Netherlands), read-out by an electron-multiplying CCD. The crystal was irradiated by a Co-57 (122 keV) source through a 30-mm-wide slit.

The reflective coating improved the camera's intrinsic energy resolution by 32%, from 50% for the uncoated crystal to 34% with the retro-reflector. The retro-reflector also improved spatial resolution by about 4%, resulting in a (full-width at half-maximum) resolution of 159 µm. The full-width at tenth-maximum was improved by 13%. In particular, scintillations occurring relatively close to the top of the crystal could be localized much more accurately.

The signal-to-noise ratio (number of counts within the area irradiated by the slit divided by the number of false positive counts) was also enhanced - increasing from 16.4 to 19.3 when the retro-reflector was in place. This 17.5% improvement was attributed to the inclusion of more true positive counts, as well as a reduced background level due to the better energy resolution.

To further develop the CCD-based gamma camera, Heemskerk and colleagues are now working on three key tasks: improving the photon-count algorithm used to the detect the individual scintillations in each frame; optimizing the scintillation crystal with regard to light yield, stopping power, and spectral matching to the CCD; and investigating the performance of newly developed CCDs with a larger active area and improved noise performance.

Related articles in PMB
A micro-machined retro-reflector for improving light yield in ultra-high-resolution gamma cameras Jan W T Heemskerk et al Phys. Med. Biol. 54 3003
Multi-scale algorithm for improved scintillation detection in a CCD-based gamma camera Marc A N Korevaar et al Phys. Med. Biol. 54 831
Front-illuminated versus back-illuminated photon-counting CCD-based gamma camera: important consequences for spatial resolution and energy resolution Jan W T Heemskerk et al Phys. Med. Biol. 52 N149
Optimizing multi-pinhole SPECT geometries using an analytical model M C M Rentmeester et al Phys. Med. Biol. 52 2567