Ultra-low-dose total body mouse SPECT with a focusing conical pinhole collimator

Objectives To develop and evaluate a new ultra-high-sensitivity mouse SPECT collimator that can be used for both focused and total body imaging, and extremely low dose or fast dynamic imaging.

Methods The collimator consists of a lead cylinder with 54 conical pinholes of 2 mm diameter. The field of view can be extended up to total body imaging using the scanning focus method [Vastenhouw & Beekman, J.Nucl. Med. 2007, pp487-493]. The collimator was mounted in the triangular stationary detector set-up of the U-SPECT+ system (MILabs, The Netherlands). Images were obtained from the list-mode data using pixel-based OSEM reconstruction with model-based resolution recovery through use of an accurate PSF model during reconstruction. The peak geometric sensitivity along axial and transaxial directions was measured with a ^99mTc point source. Reconstructed image resolution was characterized for 30 minute scans with 0.5, 1.5 and 10 MBq ^99mTc in a Jaszczak resolution phantom (rod diameter equals rod separation). In addition, 30 minute 0.25 MBq ^99mTc-MDP total body SPECT scans were acquired.

Results The peak sensitivity of the collimator reaches 13080 cps/MBq. The achievable resolution in terms of rod visibility was 1 mm for all resolution phantom scans. Despite the very low dose of only a quarter MBq, mouse total body scans show many detail: e.g. in the skull area the nasal, zygomatic, frontal, and parietal bones can be distinguished, ilium and sacrum can be seen separately, and in the kidneys medulla structures are clearly discerned from the renal cortex.

Conclusions We successfully developed a novel collimator including reconstruction software that allows extremely low dose mouse SPECT. This opens new avenues for carrying out fast dynamic SPECT or SPECT with tiny amounts of tracer, which can be beneficial for e.g. efficient screening of new tracers (e.g. when tracer’s labeling efficiency is low) or for receptor imaging with low receptor density.