Comparison of compact gamma cameras with 1.3 mm and 2.0 mm quantized elements for dedicated emission mammotomography

In an effort to image smaller breast lesions, two compact gamma cameras with different intrinsic NaI(Tl) pixel sizes are evaluated for use in the application specific emission tomography system for mammotomographic emission imaging. Comparison measurements were made with two scintillator arrays with 1.3×1.3×6mm³ or 2.0×2.0×6mm³ elements on exactly the same set of PMTs, electronics and control/processing hardware. Uniformity, sensitivity and energy resolution were assessed with flood field phantoms. Spatial resolution measurements included: a ^99mTc (140 keV) activity filled capillary tube imaged in planar mode from 1 - 10 cm distance; two such tubes separated by 2 cm were also imaged with simple circular tomography from 3 - 7 cm radii-of-rotation (RORs); and a ^99mTc filled mini-cold rod phantom was imaged at 5 cm ROR with a simple circular orbit. Finally, a freely suspended and uniformly filled 950 ml breast phantom containing four fillable lesions (4 - 10 mm dia) was imaged with a lesion-to-uniform-background activity concentration ratio of 15:1, using simple and complex 3D orbits and minimal RORs. The measured sensitivity varied by the crystal fill-factor; uniformity had <4% variability; and mean energy resolutions of each camera were ≤12% full-width at half-maximum (FWHM). The planar spatial resolutions correspond to calculated values, with smaller pixels yielding 2 - 13% better resolution with decreasing distance, corroborating the change from collimator-limited to intrinsic resolution with decreasing separation distance; tomographic results ranged from 3.2 - 5.2 mm FWHM at 3 - 7cm, with nominally better contrast-resolution for the smaller pixel camera. Consistent with signal detection characteristics for these measurement conditions, quantitative SNRs and contrasts from lesion imaging with the uniform breast background illustrate better overall performance under nearly all conditions and for all lesions for the larger pixel camera.