Development of Fully-3D CT in a Hybrid SPECT-CT Breast Imaging System

This work describes initial measurements with the CT subsystem of the assembled, fully-3D, hybrid SPECT-CT system for dedicated breast imaging. The hybrid system, designed for clinical breast imaging, consists of fully-flexible SPECT and CT subsystems, with each capable of 3D acquisition motions. The SPECT subsystem employs a 16 × 20 cm² CZT detector with 2.5 mm pixellation, is capable of viewing into the chest wall in addition to imaging the complete breast volume, and has been extensively reported elsewhere. The polar tilting capability of the CT subsystem has marked improvement in volumetric sampling while eliminating cone beam artifacts due to the fully-3D acquisitions. The CT subsystem can also view into the chest wall, while delivering <5 mGy total dose, compared with a simple circular orbit breast CT. The CT subsystem consists of a 0.4 mm focal spot x-ray tube with a rotating 14° W-anode angle, and a 40 × 30 cm² CsI(Tl) flat panel imager having 127 micron pixellation and 8.0 mm bezel edge, placed on opposing ends of the completely suspended gantry. A linear stage mechanism is used to tilt the suspended CT gantry up to ±15° in the polar directions about the 3D center of rotation; the SPECT system is nestled inside the suspended CT gantry, oriented perpendicular to the CT source-detector pair. Both subsystems rest on an azimuthal rotation stage enabling truncated spherical trajectories independently for each. Several simple and more complex 3D trajectories were implemented and characterized for the CT subsystem. Imaging results demonstrate that additional off-axis projection views of various geometric phantoms and intact cadaveric breast, facilitated by the polar tilting yield more complete breast-volume sampling and markedly improved iteratively reconstructed images, especially compared to simple circular orbit data. This is the first implementation of a hybrid SPECT-CT system with fully-3D positioning for the two subsystems, and could have various applications in diagnostic breast imaging.