U-SPECT-I: a stationary molecular imaging system for small animals with 0.1 micro-litre resolution

A novel SPECT system, U-SPECT-I, was designed and constructed at Utrecht University for the ultra high-resolution imaging of small laboratory animals. The goal was to devise a system with superb resolution and sensitivity that could be used for the imaging of murine organs. Methods: In the center of a triple detector SPECT system, a cylindrical collimator is placed containing 75 gold micro-pinhole apertures. The detectors are divided into sub-cameras, by taking care of that each pinhole projects on a limited area. Radiation shielding prevents overlapping of the projections. The resulting mini pinhole cameras focus to the center of the field-of-view (FOV). The pinholes are arranged in 5 rings of 44 mm diameter. The high number of focusing cameras result in an excellent sensitivity for a small field-of-view that contains a mouse heart, brain, part of the spine or a tumor. SPECT images are reconstructed using Maximum Likelihood Expectation Maximization (ML-EM) Resolution recovery is performed during reconstruction using measured and interpolated point spread function tables. Results: The peak absolute sensitivity measured with a point source is 0.22 % and remains higher than 0.12 % in the central 12 mm of the central plane. Images of a resolution phantom clearly show 0.5 mm capillaries separated by 0.5 mm. A sufficient number of different projection angles for reconstructing images of mouse organs can be readily acquired without any detector or collimator movement. Images of a mouse spine show Tc-99m-hydroxy-methylene diphosfate uptake down to the level of parts of vertebral processes. Bone uptake of the tracer is clearly separated from tiny structures like the (inter-)vertebral foramen. Myocardial perfusion in the left and right ventricular wall, and in structures as small as papillary muscles can be observed in Tc-99m tetrofosmin images. Conclusions: This is the first SPECT scanner to use a highly focused multiple gold micro-pinhole collimation, dedicated to the imaging of mouse organs, at a higher resolution than can be reached with state-of-the-art small animal PET. The combination of imaging characteristics of this prototype system (resolution and sensitivity) opens up new possibilities for the study of animal models. It allows differentiation between the uptake of various structures of about one micro-liter. The system can be transformed from clinical triple detector SPECT system to the U-SPECT-I setup within ten minutes. This means that our new animal imaging set-up is both flexible and cost-effective.