Abstract
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Introduction: Collimators are, besides the detectors, the most crucial elements of a SPECT system as they define sensitivity, spatial resolution and field of view. The specific choice of a collimator mainly depends on the imaging task. In preclinical collimator design, the focus lies on increasing the spatial resolution, because of the small size of the animals. However, an increased sensitivity, combined with a sub-mm spatial resolution results in a collimator that offers many advantages.
To start, shorter scan times can be used, which facilitates high-throughput screening of compounds. Or, it allows the use of less radiotracer for the same imaging time, offering the possibility to evaluate low-capacity receptor systems and organs with low uptake or to evaluate new compounds with (initial) low radiochemical yields or expensive precursors. It also enables visualization of fast dynamic processes and it allows longitudinal follow up of compounds, like antibodies, with slow clearance. Lastly, it results in a lower dose for the operator and subjects in a longitudinal study. Combining all these factors results in a reduced study cost. However, a collimator that combines both a high resolution and sensitivity is often bulky, large and difficult to handle.
Here, we have developed a light-weighted, high sensitivity mouse collimator with a peak sensitivity of 1.25% and a spatial resolution of 0.9 mm. The collimator has a large FOV that allows to scan a full body mouse with an acceptable scan time and administered radiotracer dose. The sub-MBq capabilities of this collimator were quantitatively evaluated using a mouse phantom. In addition, in vivo measurements were done to demonstrate the image quality.
Methods: A high sensitivity mouse collimator was designed and produced using selective laser melting of tungsten powder. The collimator consists of 7 segments each containing 4 loftholes with an aperture diameter of 2.5 mm. In order to optimize the sampling, apertures are positioned at a different location on each segment.
A phantom study was performed on the SPECT-CT platform (Y- and X-CUBE, MOLECUBES, Belgium) by using a mouse phantom (BioEmtech, Greece), that was filled with low 99mTc concentrations. The liver, bladder, and tumor compartments were filled with respectively 0.2, 0.5, and 0.7 MBq/cc. A 30 min full body SPECT acquisition was performed, followed by a general purpose CT acquisition for attenuation correction and anatomical correlation. The data was reconstructed using an MLEM reconstruction algorithm, 500 µm voxel size and 100 iterations. An energy window of 20% around the 141 keV energy peak was chosen. Regions of interest (ROIs) were drawn in PMOD v4.2 and the image derived values (MBq/cc) for each organ were compared to those from the dose calibrator to evaluate quantification.
To confirm the image quality, a mouse was injected with 0.5 MBq 99mTc-DMSA. A 15 and 30 minute SPECT acquisition were done, followed by a general purpose CT acquisition, and the data was reconstructed as described above.
Results: The ROIs and resulting values are shown in Table 1 (Supplemental data). Quantitative accuracy is less than 5%. Figure 1A,B shows a SPECT-CT acquisition of 30 and 15 minutes.
Conclusions: In the current study, a high sensitivity collimator with a sensitivity of 1.25% and sub-mm spatial resolution was evaluated in vivo. The collimator allows researchers to evaluate organs or tumors with low radiotracer uptake. It was demonstrated that it is possible to detect low activity concentration with quantitative accuracy. This shows that this collimator has a broad application range and allows high throughput imaging, reduces radiotracer/imaging cost and operator dose, and enables the evaluation of fast in vivo processes. It also allows the researcher to evaluate organs or tumors with low radiotracer uptake. Thanks to the light weight, it can be easily handled, thus allowing the researcher to easily switch between collimators regularly, depending on the application.
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