We present a new rodent SPECT system (U-SPECT-II) that enables molecular imaging of murine organs down to resolutions of less than half a millimeter and high-resolution total-body imaging. Methods: The U-SPECT-II is based on a triangular stationary detector set-up, an XYZ stage that moves the animal during scanning, and interchangeable cylindric collimators (each containing 75 pinhole apertures) for both mouse and rat imaging. A novel graphical user interface incorporating preselection of the field of view with the aid of optical images of the animal focuses the pinholes to the area of interest, thereby maximizing sensitivity for the task at hand. Images are obtained from list-mode data using statistical reconstruction that takes system blurring into account to increase resolution. Results: For 99m Tc, resolutions determined with capillary phantoms were smaller than 0.35 and 0.45 mm using the mouse collimator with 0.35-and 0.6-mm pinholes, respectively, and less than 0.8 mm using the rat collimator with 1.0-mm pinholes. Peak geometric sensitivity is 0.07% and 0.18% for the mouse collimator with 0.35-and 0.6-mm pinholes, respectively, and 0.09% for the rat collimator. Resolution with 111 In, compared with that with 99m Tc, was barely degraded, and resolution with 125 I was degraded by about 10%, with some additional distortion. In vivo, kidney, tumor, and bone images illustrated that U-SPECT-II could be used for novel applications in the study of dynamic biologic systems and radiopharmaceuticals at the suborgan level. Conclusion: Images and movies obtained with U-SPECT-II provide high-resolution radiomolecule visualization in rodents. Discrimination of molecule concentrations between adjacent volumes of about 0.04 mL in mice and 0.5 mL in rats with U-SPECT-II is readily possible.

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Block-iterative image reconstruction methods, such as ordered subset expectation maximization (OSEM), are commonly used to accelerate image reconstruction. In OSEM, the speed-up factor over maximum likelihood expectation maximization (MLEM) is approximately equal to the number of subsets in which the projection data are divided. Traditionally, each subset consists of a couple of projection views, and the more subsets are used, the more the solution deviates from MLEM solutions. We found for multi-pinhole single photon emission computed tomography (SPECT) that even moderate acceleration factors in OSEM lead to inaccurate reconstructions. Therefore, we introduce pixel-based ordered subset expectation maximization (POSEM), which is based on an alternative subset choice. Pixels in each subset are spread out regularly over projections and are spatially separated as much as possible. We validated POSEM for data acquired with a focusing multi-pinhole SPECT system. Performance was compared with traditional OSEM and MLEM for a rat total body bone scan, a gated mouse myocardial perfusion scan and a Defrise phantom scan. We found that POSEM can be operated at acceleration factors that are often an order of magnitude higher than in traditional OSEM.

In a standard 18 daN force-controlled compression protocol, the authors observed an average pressure of 21.3 kPa±54% standard deviation for CC compressions and 14.2 kPa±32% for MLO compressions. Women with smaller breasts endured higher pressures and experienced more pain, as indicated by a significant negative correlation (ρ=-0.19, p<0.01) between contact area and pain score. Multivariate regression showed that contact area is a strong and significant predictor for severe pain (ORNRS≥7 (CC)=0.10/dm2, p<0.05), as is the case with any pain already present before compression (ORNRS≥7 (CC)=1.61 per NRS-point, p<0.05). Model estimations showed that mammographic breast compression with a standardized pressure of 10 kPa, corresponding with normal arterial blood pressure, may significantly reduce the number of severe pain complaints with an average increase in breast thickness of 9% for small breasts and 2% for large breasts. For an average 16.5% thickness difference in prior-current mammogram pairs, the authors found no differences in image quality and AGD CONCLUSIONS: Model estimations and an observer study showed that pressure-controlled mammographic compression protocols may improve standardization and reduce discomfort with limited effects on image quality and AGD.

Pressure-standardized compressions resulted in AGD values and a retake proportion similar to force-standardized compressions, while pain was significantly reduced.