Optimizing orientation of SPECT and cone beam CT detectors through quantification of cross contamination in a dual modality mammotomography system

A compact, dual modality SPECT/CT system for dedicated 3D breast imaging is in development. A major consideration in determining the relative placement of the SPECT and CT detectors is minimizing cross contamination. The transmission contamination of the emission image is investigated here using a 16×20 cm² quantized CZT SPECT camera located at various orientations relative to the 20×25 cm² CsI(TI) CT detector, with a pendant, water-filled breast phantom placed in the common field-of-view. This arrangement is repeated with and without ^99mTc emission radioactivity in the breast, with the breast laterally offset from the central ray of the X-ray cone-beam in a half-cone beam configuration. Planar transmission projections use a quasi-monochromatic (35 keV mean energy, 20% FWHM) X-ray beam, with an exposure equivalent to 1/360, 1/180, 1/90 of the total exposure of dual view, screening mammography; these fractional exposures are used due to the requirement of multiple images necessary for CT data acquisition and ∼360 projection exposures are expected. Images and energy spectra acquired from the SPECT projections are used to quantify contamination from the X-ray beam. With the offset CT acquisition geometry, initial results show X-ray scatter contamination of the emission camera is limited to photons well below the 140 keV photopeak. Pulse pile-up is not a concern, with the instantaneous X-ray scatter flux. Transmission contamination in the energy windowed emission image is negligible (<0.5%) for any SPECT camera position (polar orientation and azimuthal contamination) and various measured breast sizes (325 - 1500 mL). Optimal placement of the detectors is found to be more a function of physical constraints than X-ray transmission contamination.