Multi-Isotope Capabilities of a Small-Animal Multi-Pinhole SPECT System

Relative (black) and absolute (red) energy resolution of NanoSPECT/CT^PLUS detector expressed as exponential (solid lines) and inverse square root law (dotted lines). Energy resolution is suitable for single-, dual- and triple-isotope studies above 75 keV. For energies below, resolution is restricted by nonproportional light yield of scintillator NaI(Tl) and hampers separation of multiple isotopes.

Count rate performance (A) and pileup spectra (B) of NanoSPECT/CT^PLUS detector. Count rate loss is low for single-, dual- and triple-isotope experiments, as sensitivity of pinhole collimation barely allow 400 kcps to be exceeded (gray area). For triple-isotope studies with high total activities, pileup may affect quantitative accuracy and image quality of high-energy radioisotopes.

Photon penetration fractions for different collimators (A) and multi-pinhole projections of RU collimator (B). Given penetration fractions partly involve collimator scatter and, in case of ²⁰¹Tl, tungsten x-ray fluorescence. Photon penetration increases background counts in planar projections and therefore degrades image quality and signal-to-noise ratio of both single- and multi-isotope reconstructions.

Spectra (A) and spectral component fractions (B) for ^99mTc phantoms of different sizes. Three energy windows were used to estimate fractions of tungsten x-ray fluorescence F_T (I), Compton scatter F_C (II), and full-energy peak F_P (III). Scatter fraction increases only slightly for rodent-sized objects and therefore is caused mainly by hardware components. Disproportionately rising Compton maxima at 100 and 135 keV indicate change in detected scatter angles. Backward scatter from hardware is gradually overcome by forward scatter from measured object. Corresponding energy shift forces increase of tungsten x-ray fluorescence.

Small-animal Jaszczak phantom (A), uniformity phantom (B), and line profiles of single- and triple-isotope mixtures (C). Compared with single-isotope acquisitions, multiple superimposed isotopes degrade image quality slightly in terms of geometric distortion, background level, and overall noise. Multiple superimposed isotopes tend to degrade uniformity because of enhanced activity distributions at phantom edges.