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Is SPECT or CT based attenuation correction more quantitatively accurate for dedicated breast SPECT acquired with non-traditional trajectories?

Attenuation correction is necessary for SPECT quantification. There are a variety of methods to create attenuation maps. For dedicated breast SPECT-CT imaging, it is unclear if either a SPECT- or CT-based attenuation map would provide the most accurate quantification and whether or not segmenting the different tissue types will have an effect on the quantification. For these experiments, 99mTc diluted in methanol and water filled geometric and anthropomorphic breast phantoms was imaged with a dedicated dual-modality SPECT-CT breast scanner. SPECT images were collected using a compact CZT camera with various 3D acquisition trajectories including vertical and 30° tilted parallel beam, and complex sinusoidal trajectories. CT images were acquired using a quasi-monochromatic x-ray source and CsI(T1) digital flat panel detector in a half-cone beam geometry. Measured scatter correction for SPECT and CT were implemented. To compare photon attenuation correction in the reconstructed SPECT images, various volumetric attenuation maps were derived from 1) uniform SPECT, 2) uniform CT, and 3) segmented CT, populated with different attenuation coefficient values. Comparisons between attenuation masks using phantoms consisting of materials with different attenuation values show that at 140 keV the differences in the attenuation between materials do not affect the quantification as much as the size and alignment of the attenuation map. The CT-based attenuation maps give quantitative values 30% below the actual value, but are consistent. The SPECT-based attenuation maps can provide within 10% accurate quantitative values, but are less consistent.