While contemporary clinical single photon emission computed tomography (SPECT) scanners are limited to a horizontal axis of rotation about the patient, the compact application specific emission tomography (ASET) system overcomes physical proximity limitations of clinical scanners and allows for fully three-dimensional movement in an inverted hemisphere about the pendant breast through combined variable radius of rotation (ROR) and polar and azimuthal angular positioning. With these three degrees of freedom, the ASET can provide trajectories that satisfy Orlov's sampling criterion while maintaining a small ROR, necessary to minimize resolution degradation. One class of orbits investigated here consists of combinations of circular orbits and up to 60 arcs. Orbits are evaluated for image quality, including sampling and resolution characterization, utilizing both cold disk and cold rod emission phantoms, and quantitated contrasts and signal-to-noise ratios (SNRs) of small lesions located in a breast phantom with and without additional torso phantom backgrounds. The reconstructed cold disk and cold rod results indicate that all tested orbits in this class completely sample the volume and provide near equal resolution recovery. Furthermore, results from all orbits yielded higher contrasts and/or SNRs for the breast lesions compared with simple vertical-axis-of-rotation acquisitions, and nearly an order of magnitude better than those from uncompressed planar imaging.Index Terms-Breast imaging, mammotomography, single photon emission computed tomography, three-dimensional orbits.

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Parallel-beam tilted-head single photon emission computed tomography (TH-SPECT) was previously implemented on a SPECT system for its potential to image breast lesions and nearby axilla of seated, upright women. All TH-SPECT reconstructions will contain artifacts since the tilted orbit does not satisfy the Orlov sampling criteria. However, it is not clear which reconstruction method, if any, is better suited for TH-SPECT data. Here a geometric derivation of the ramp filter for tilted parallel-beam geometries is presented. A filtered backprojection (FBP) algorithm, using this filter, was then implemented and compared with an iterative ordered subsets expectation maximization (OSEM) algorithm, using TH-SPECT data. A breast scan at various tilt angles was simulated and used to generate a noise versus bias study for both methods. Contrast and signal-to-noise ratio (SNR) values, as well as axial elongation present in all TH-SPECT reconstructions, were characterized using a mini-Defrise disk phantom placed inside a fillable breast phantom and imaged from 0 to 15 head tilt. A fillable breast phantom containing lesions was also imaged with a system dedicated to prone breast SPECT from 0 to 30 to evaluate the effects of incomplete sampling due to greater tilt angles. FBP noise versus bias studies indicated a greater increase in bias with tilt angle compared to OSEM reconstructions. At small tilt angles about the mini-Defrise disk phantom, poorer contrasts were obtained with FBP compared to OSEM at similar noise levels. All reconstructions of the fillable breast phantom indicated axial elongation at greater tilt angles, although FBP reconstructions displayed an increase in stretching distortions of the breast. OSEM SNR and contrast values were higher at all degrees of tilt. In conclusion, measured results indicate OSEM TH-SPECT reconstruction provides better contrast and SNR values and may offer better shape and uniform activity distribution of the breast compared to FBP methods.Index Terms-Breast imaging, filtered backprojection (FBP), ordered subsets expectation maximization (OSEM), Orlov, single photon emission computed tomography (SPECT).