Abstract
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Introduction: There is a unique advantage to imaging radiopharmaceuticals which reveal biological processes and detect molecular function changes during the early stages of breast cancer. However, conventional gamma camera and SPECT technologies are subject to intrinsic constraints of mechanical collimation, achieving only suboptimal resolution and detection efficiency. To overcome the constraints, we propose a single-photon emission breast tomography (SC-SPEBT) design based on a new self-collimation concept, optimize the design’s configuration, and evaluate its imaging performance.
Methods: Self-collimation is a concept we have recently introduced that employs sensitive detectors as collimators themselves. In this work, the concept is implemented through a new detector architecture that is an assembly of sparsely distributed scintillators in a grid pattern, gamma rays from an image object reach the crystals on the far side of the detector through the pathways or collimations created by the crystals that are closer to the image object.
The SC-SPEBT design consists of 6 detector panels that form a hexagon with a 180-mm (Φ) × 160-mm (L) cylindrical field of view (FOV). Each panel comprises 10 modules stacked in the axial direction. Each module consists of a GAGG(Ce) scintillator assembly masked by a tungsten plate attached on the side facing the FOV. Most scintillators are sparsely distributed over a 16×12 grid pattern with a step size of 6.72 mm in both tangential (T) and radial (R) directions, only the ones on the outmost row (farthest from FOV) have no gaps between them. Each scintillator is a cuboid with a size of 3 mm (T) × 20 mm (axial, A) × 3 mm (R), and is coupled to 2 readout components at its axial ends. The expected axial direction resolution on each scintillator is 4 mm. The tungsten plate consists of slit-apertures that are parallel to the transverse (TR-) plane and evenly distributed in the axial direction.
In this work, we assume a preselected detector geometrical configuration as described above and focus on optimizing the width and number of slits on the tungsten plate, and the plate movement step-size and range in the axial direction during imaging. To identify a preferred set of parameters, we assess the resolution-to-noise trade-off of each configuration using covariance and local impulse response derived contrast recovery coefficient as figures of merit.
To evaluate the transverse resolution, axial resolution, detection efficiency, and lesion detectability of the system design, we perform analytical simulation studies of a planar hot-rod, a Defrise disk, a uniform cylindrical source, and a contrast phantom. The lesion detectability is performed with a total activity of 0.6 MBq in the contrast phantom, to simulate a total-body activity of 300 MBq in a typical planar molecular breast imaging study, and an acquisition time of 10 min.
Results: The optimal configuration of the metal plate is having twenty 2-mm slit apertures and moving in the axial direction with a fixed 2-mm step size over a 10-mm range. The average detection efficiency in the FOV is 4.8%. With enhanced sampling in the axial direction, the planar hot-rod phantom study shows that the best achievable transverse image resolution is 2.5 mm in noise-free condition and 3.0 mm with a total activity of 3 MBq in 10 min acquisition time. The best achievable axial image resolution is 3.0 mm. With a criterion of contrast to noise ratio > 3, the lesions of 3.0 mm, 4.0 mm, and 5.0 mm can be identified at a contrast condition of 20:1, 10:1, and 5:1 respectively.
Conclusions: We have proposed an SC-SPEBT design, optimized a few of its geometrical and movement parameters, and assessed its imaging performance. The imaging system shows a resolution of 3-mm, detection efficiency of 4.8%, and improved lesion detectability as compared to published planar molecular breast imaging results.