Performance comparison of ultra-ultra high resolution collimators to low-energy high resolution collimators

Objectives We compared the performance of a unique ultra-ultra high resolution (UUHR) collimator to that of a low-energy high resolution (LEHR) collimator. The UUHR collimator was designed to accommodate SPECT scanning in a biosafety level-4 (BSL-4) environment where a containment tube spanning the BSL-4 area and the imaging room limits the SPECT orbit.

Methods We measured the system spatial resolution (FWHM) and the collimator sensitivity following the NEMA performance standard. Reconstructed SPECT image resolution was determined using three 99mTc point sources inside the containment tube. In addition, we acquired SPECT images of a Jaszczak resolution phantom (Data Spectrum, Hillsborough NC) containing hot spheres to measure recovery coefficients.

Results Resolution degradation at increasing distance from the collimators was less for UUHR collimators than for LEHR collimators. With UUHR collimators, we achieved a similar resolution at 35 cm as with LEHR collimators at 20 cm. Collimator sensitivity at 10 cm was 56.4 cpm/µCi (UUHR) and 168 cpm/μCi (LEHR). At a 33-cm radius orbit, the SPECT resolution of a central point source was 10.6 mm (transaxial) and 11.2 mm (axial) with UUHR collimators, whereas resolution deteriorated to 17.3 mm (transaxial) and 18.8 mm (axial) with LEHR collimators. The two largest of six total groups of rods in the resolution phantom were resolved in UUHR-collimated images but not in the LEHR-collimated images. Recovery coefficients for a 25-mm diameter sphere were 1.02 (UUHR collimators) and 0.72 (LEHR collimators).

Conclusions By using UUHR collimators, we achieved a SPECT resolution of about 10-12 mm FWHM in our BSL-4 setting, which is similar to the resolution typically obtained in clinical settings using LEHR collimators. However, the improvement in spatial resolution with UUHR collimators comes at the price of reduced sensitivity compared to LEHR collimators.

Research Support This work was performed under Battelle Memorial contract HHSN272200200016I with NIAID