Molecular Breast Imaging Using a Clinical Cardiac SPECT Scanner
Using a clinical, dedicated breast SPECT system for low dose 3D molecular breast imaging (MBI) with Tc-99m-sestamibi could be beneficial as functional imaging follow-up to suspicious mammograms. The feasibility of using a readily available, high sensitivity, multi-CZT-detector, clinical cardiac SPECT camera for low dose fully-3D MBI using geometric and anthropomorphic phantoms is investigated.
A stationary, multi-view 19-CZT camera (GE NM530c) using 4.7mm pinhole collimators was utilized for all physical measurements. Reconstructed SPECT spatial resolution and quality field uniformity were evaluated with a planar Cartesian array phantom developed from machineable Styrofoam. Over 100 wells were formed within and extending beyond the reconstructed field of view (FOV) and were filled with 99mTc-pertechnetate point sources. This planar array was stepped through the camera’s FOV; reconstructed images allow the evaluation of the 3D field uniformity. Fillable anthropomorphic breast phantoms (470 to 1730 mL) with inserted fillable lesions (10 mm diameter), attached to a torso phantom, were imaged using different lesion-to-background activity concentrations (from [3:1] to [20:1]), and total acquisition times (30-360 sec) meant to evaluate whether lower injected dose or shorter acquisition times are feasible. To mimic a radiopaque table, a lead sheet and pliable vinyl-coated lead apron, both with a cut-out were placed around the exposed breast of the torso phantom.
The planar array demonstrated good linearity (1.852.7 mm) throughout the FOV. Breast lesion results varied, with a visible 9mm diameter lesion having contrast of at least 0.59 and CNR of 1.18 at [7:1] concentration ratio for all breast sizes, consistent with the expected biological uptake of the radiotracer. Shielding the breast induced a streaking artifact indicating that additional shielding prevents adequate sampling and introduces image artifacts rather than enhance image quality.
While optimization would improve the system for 3D MBI, uncompressed, prone MBI using the current cardiac SPECT scanner is possible due to the spatial accuracy, contrast and CNR characteristics of the reconstructed images for the entire breast volume. There is no need to shield the camera from out-of-field background activity. Measured characteristics of simulated lesions indicate that 1 cm lesions with expected biological uptake should be readily visible in humans.
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