Initial Investigation of Circle-Plus-Arc Orbit Variants with a Dedicated Emission Mammotomograph
A dedicated emission mammotomograph, uniquely capable of imaging in a hemisphere, is utilized to image the breast by way of complex acquisition orbits, completely sampling most of the breast. Orbits such as the circle-plus-arc (CPA) in particular provide for extended viewing times in a specific portion of the breast. This is advantageous in that as a diagnostic rather than screening imaging tool, a long duration of the camera trajectory may be distinctly positioned in the quadrant where the lesion is suspected. However, orbit modifications that distribute the scan time more evenly over the breast may also be effective. As the trajectories conform to the shape of the breast, including dynamic radius-of-rotation control, resolution degradation is minimized. However, for both the arc and the circular portion of the orbit, increased background contamination may result since the line-of-sight of the single photon camera can view unscattered primary radiation, particularly from cardiac and hepatic sources. The effect of azimuthal position of the arc, as well as the degree of the arc, on image quality is investigated by measurements of lesion signal-to-noise ratios and contrasts. Additionally, because the resolution characteristics of the CPA are not uniform throughout breast quadrants, the implementation of a modified orbit, in the shape of a 3D cloverleaf is investigated through measurements of the breast phantom alone with lesions of varying radioactive concentrations, and with additional torso backgrounds. These initial results indicate that polar tilt does affect image quality, with low polar tilt showing little variation in SNRs and contrasts, while increasing tilt decreases SNRs and contrasts by ∼2x when torso background is present. Considering the effect of azimuthal arc location, θ=45° yields the highest SNRs and contrasts for the locations at which SNR was evaluated. Both cloverleaf orbits yield comparable SNRs and contrasts to the CPA orbits.
