Examination of aperture layout designs for an adaptive-stationary multi-pinhole brain-dedicated SPECT system

Organ specific multi-pinhole (MPH) SPECT imaging could potentially improve the sensitivity/resolution trade-off and image quality (IQ), while facilitating the use of a variety of imaging-agents, thereby addressing diagnostic, quantitative, and research clinical needs.

Investigate through simulation six different MPH aperture-layout designs, plus variations in projection multiplexing (MUX) and truncation, for a prototype brain-dedicated MPH SPECT system, named AdaptiSPECT-C, to understand tradeoffs for such choices and guide selection of an optimal design for construction of the actual AdaptiSPECT-C system.

The prototype AdaptiSPECT-C system investigated herein employs 25 MPH gamma-camera modules arranged in three rings to image a 21 cm diameter spherical volume-of-interest (VOI). With a focal point (FP) to center of detector distances of 38.7 cm, the pinhole aperture diameters were constrained to provide a calculated spatial resolution of 8 mm at the FP. Variations in the number of pinhole (PH) apertures, FP to aperture distance, PH layout, temporal changes in MUX, and extent-of-truncation of the projection images were investigated. Designs of the aperture layouts were used to create inputs for GATE and analytic simulations of a sphere phantom with uniform Tc-99 m activity filling the VOI, to assess MUX, detector utilization, and uniformity in reconstructed slices. We investigated axial and angular sampling using customized-spherical Defrise and Derenzo phantoms. Finally, we assessed reconstructed IQ and activity quantification in reconstructions of analytic simulations of the XCAT digital anthropomorphic phantom with activity and attenuation distributions mimicking clinical-SPECT brain-perfusion imaging. For each phantom, comparison was also made to imaging with a dual-headed SPECT system with low-energy high-resolution (LEHR) parallel-hole (Vertex high resolution [VXHR]) collimators.

Sensitivity at the FP (SENS) for a Tc-99 m source in air calculated relative to a clinical dual-headed SPECT system with VXHR collimators was 2.7x higher for a single aperture with no MUX or truncation, increased to 5.7x for five apertures with limited VOI truncation and MUX, and decreased to 2.5x with 13 apertures with limited MUX. For the spherical tub phantom, limited truncation did not impact uniformity, MUX decreased it, and temporal shuttering of projections helped lessen this impact. Visually, the 6.4 mm rods were generally well differentiated for the single central apertures. For designs with four or more apertures, all the 4.8 mm rods were well differentiated visually. Projection images of the XCAT phantom acquired for an imaging time that would result in the minimum clinically recommended count-level for brain perfusion imaging with parallel-hole collimators, showed low MUX of the brain structures for all of the MPH aperture layout designs. The best reconstructions for the XCAT phantom, both visually and quantitatively, were obtained with the design using 4- or 5-PH-apertures for the aperture-layout design that included MUX and some truncation of imaging.

We determined for a prototype brain-dedicated MPH SPECT employing 25 camera modules in three rings with different PH layout designs imaging a 21 cm diameter spherical VOI, that a system with five apertures per module provided the best SENS, and IQ of the XCAT brain phantom, both visually and numerically.