Performance characteristics of a new pixelated portable gamma camera

To evaluate and characterize the performance of a new commercially available pixelated portable gamma camera Ergo (Digirad, Poway, CA).

The authors evaluated a pixelated portable gamma camera system, Ergo, that consists of 11 520 elements of 3 × 3 mm(2) CsI(Tl) crystals that are 6-mm thick and are coupled to silicon photodiodes. The detector element has a size of 3.31 × 3.24 mm(2). The gamma camera performance was evaluated for both low-energy all-purpose (LEAP) and low-energy high-resolution (LEHR) collimators. The flood-field uniformity for (99m)Tc and (201)Tl was assessed using fillable uniform flood phantoms. Energy spectra were acquired for (99m)Tc, (111)In, (201)Tl, and (67)Ga to evaluate energy linearity and energy resolution. Spectral fits were performed to calculate the photopeak energies and resolutions. The pixel size and multiwindow spatial registration (MWSR) was evaluated by measuring mixed (99m)Tc and (201)Tl point sources placed at known distances apart. The system's sensitivity was measured according to the National Electrical Manufacturer's Association (NEMA) NU1-2007 standards for both LEAP and LEHR collimators as a function of distance from the collimator surface (5, 10, 15, 20, 25, 30, and 40 cm). The system resolution without scatter was measured for both LEAP and LEHR using (99m)Tc-filled capillary tubes located at 0, 2, 4, 6, 10, and 12 cm away from the surface of the collimator. As a measure of the spatial resolution, the full width at half maximum (FWHM) at a given distance was calculated from the presampling line spread function (LSF), constructed from the line profiles of the capillary tubes at the same distance. As a comparison, the FWHM at 10 cm away from LEHR and LEAP collimators was also calculated from linear interpolation as described by NEMA NU-1 2007 and from fitting the profiles to a Gaussian-plus-constant model.

All isotope-collimator pairs demonstrated good flood-field uniformity with an integral uniformity of ≤5% and a differential uniformity of ≤3%. The system demonstrated excellent energy linearity with maximum discrepancy of measured keV from true keV of <1%. The energy resolution of the (99m)Tc 140-keV photopeak was 7.4%. The image pixel size was measured as 3.23 × 3.18 mm(2), and the MWSR was within 0.3 mm (or ~10% of the nominal pixel size). The system sensitivity at 10 cm was 112.6 cps/MBq (249.9 cpm/μCi) for LEAP and 63.1 cps/MBq (140.1 cpm/μCi) for LEHR. The system spatial resolution varied linearly with distance from the collimator and the FWHM were measured to be 7.2 and 8.9 mm at 10 cm for LEHR and LEAP, respectively.

Herein, the authors describe detailed performance evaluation procedures of a new pixelated portable gamma camera system, which can also be applied to evaluate other pixelated gamma camera system. Spatial resolution assessment in near-field imaging condition offers a unique challenge where the measured FWHM is highly dependent on relative position between the capillary tube and the detector element. The evaluations of the Ergo gamma camera suggest suitable clinical imaging performance. This portable gamma camera has a high (LEAP) planar sensitivity, high energy and spatial resolutions that are comparable to other available gamma cameras, and it exhibits superior count rate performance that is linear up to tens of millions count per second. The Ergo imaging performance, however, can still be improved, for example, by optimizing collimator design for near field imaging.