Submillimeter total-body murine imaging with U-SPECT-I

Recently, we launched a stationary SPECT system (U-SPECT-I) dedicated to small-animal imaging. A cylinder with 75 gold micropinhole apertures that focus on a mouse organ was used to maximize the detection yield of gamma-photons. Image resolutions of approximately 0.45 and 0.35 mm could be achieved with 0.6- and 0.3-mm pinholes, respectively. Here, we present a combined acquisition and reconstruction strategy that allowed us to perform full-body imaging with U-SPECT-I.

The bed was stepped in the axial and transaxial directions so that the pinholes collected photons from the entire animal (scanning focus method, or SFM). Next, a maximum-likelihood expectation maximization algorithm exploited all projections simultaneously to reconstruct the entire volume sampled. The memory required for image reconstruction was dramatically reduced by using the same transition submatrix for each of the bed positions. This use of the same submatrix was possible because the submatrix acted on subvolumes that were shifted during reconstruction to match the corresponding location of the focus.

In all cases, SFM clearly improved on the method that involves stitching separate reconstructions of subvolumes obtained from the different bed positions. SFM suffered less from noise, streak artifacts, and improper background activity. In a mouse-sized phantom containing a capillary-resolution insert, sets of radioactively filled capillaries as small as 0.45 mm separated by 0.45 mm could be distinguished. Total-body mouse bone imaging using (99m)Tc-hydroxymethylene diphosphonate showed that uptake in very small structures, such as parts of the vertebral processes, could be distinguished.

In addition to providing ultra-high-resolution images of mouse organs, focusing SPECT pinhole systems are also suitable for submillimeter-resolution total-body imaging of mice.