Performance of a PSPMT based detector for scintimammography

In breast scintigraphy, compact detectors with high intrinsic spatial resol ution and small inactive peripheries can provide improvements in extrinsic spatial resolution, efficiency and contrast for small lesions relative to l arger conventional cameras. We are developing a pixelated small field-of-vi ew gamma camera for scintimammography. Extensive measurements of the imagin g properties of a prototype system have been made, including spatial resolu tion, sensitivity, uniformity of response, geometric linearity and energy r esolution. An anthropomorphic torso phantom providing a realistic breast ex it gamma spectrum has been used in a qualitative study of lesion detectabil ity. A new type of breast imaging system that combines scintimammography an d digital mammography in a single upright unit has also been developed. The system provides automatic co-registration between the scintigram and the d igital mammogram, obtained with the breast in a single configuration. Intrinsic spatial resolution was evaluated via calculation of the phase-dep endent modulation transfer unction (MTF). Measurements of extrinsic spatial resolution, sensitivity and uniformity of response were made for two types of parallel hole collimator using NEMA (National Electrical Manufacturers Association) protocols. Geometric linearity was quantified using a line inp ut and least squares analysis of the measured line shape. Energy resolution was measured for seven different crystal types, and the effectiveness of o ptical grease coupling was assessed. Exit gamma spectra were obtained using a cadmium zinc telluride based spectrometer. These were used to identify a ppropriate radioisotope concentrations for the various regions of an anthro pomorphic torso phantom, such that realistic scatter conditions could be ob tained during phantom measurements. For prone scintimammography, a special imaging table was constructed that permits simultaneous imaging of both bre asts, as well as craniocaudal views. A dedicated breast imaging system was also developed that permits simultaneous acquisition and superposition of p lanar gamma images and digital x-ray images. The intrinsic MTF is nonstationary, and is dependent on the phase relations hip between the signal and the crystal array matrix. Averaged over all phas es, the MTF is approximately 0.75, 0.57 and 0.40 at spatial Frequencies of 1.0, 1.5 and 2.0 cycles per cm, respectively The phase averaged line spread function (LSF) has a FWHM value of 2.6 mm, Following uniformity correction s, the RMS deviations in flood images are only slightly greater than is pre dicted from counting statistics. Across an 80 mm section of the active area , the differential linearity is 0.83 mm and the absolute linearity 2.0 mm, Using an anthropomorphic torso phantom with detachable breasts, scatter rad iation similar to that observed exiting the breast of scintimammography pat ients was observed. It was observed that scattered gamma rays can constitut e the majority of the radiation incident on the detector, but that the scat ter-to-primary ratio varies significantly across the field of view, being g reatest in the caudal portion of the breast, where scatter from the liver i s high. Using a lesion-to-breast concentration ratio of 6:1, a 1.0 cm(3) si mulated breast lesion was detectable in lateral images obtained with both t he developmental camera and with a clinical camera, while a 0.35 cm(3) lesi on was detectable in neither. Utilization of the dual x-ray transmission, g amma emission breast imaging system greatly increases the conspicuity of sc intimammographic lesions relative to prone imaging, as well as greatly faci litating the localization and identification of structures in the gamma ima ge. The prototype imaging gamma detector exhibits spatial resolution superior t o that of conventional cameras, and comparable uniformity of response and g eometric linearity. Because of light losses in the crystals, the energy res olution is inferior to that of single crystal NaI(T1) cameras. Concentratio n ratios for an anthropomorphic phantom providing realistic scatter conditi ons and lesion imaging tasks have been identified. A dedicated system for d ual modality breast imaging has been developed that provides improved scint igraphic breast imaging relative to the established prone scintimammography techniques.