Preclinical Molecular Imaging is a fundamental tool in development and evaluation of theranostic radiopharmaceuticals, and in the characterization of animal models of human diseases in, e.g., Oncology, Cardiology or Neurology. As most studies are performed in mice or rats, spatial resolution around 1.5 mm is desirable. Regarding imaging of single photon emitting radioisotopes, as is the case of several theranostic radiopharmaceuticals (e.g., 177Lu, 225Ac, 161Tb), planar or tomographic images can be obtained. Perhaps, the most expensive piece of the imaging device is the detector and its maintenance, so, the option of having access to the spare time of a clinical instrument will be considered.

To implement upgrading strategies to obtain high spatial resolution planar and tomographic images of small animal organs using a clinical gamma camera, in a timeframe compatible with an experimental routine, in energies between 140 and 360 keV.

Making planar images using parallel collimators seems an appropriate option, if there are no overlapping structures. However, spatial resolution can be inappropriate. To improve it, a small diameter, lead-based pinhole collimator was used to introduce a 5 to 7 image magnification factor, which is appropriate for most animal models. Static and dynamic studies were carried out. Tomographic images can be obtained by acquiring a set of pinhole-collimated planar projections. In this case, images are acquired as a dynamic protocol, while the target rotates in front of the collimator. An Arduino-controlled step motor, synchronized with the image recording, was used for this purpose. A set of 40 projections in 360 deg, with an integration time of 30 sec each, was considered. In this case, developing a reconstruction image software tool was necessary, as clinical cameras are not prepared for tomography with pinhole collimators. Here, we present implementation and examples of these strategies, based on Monte Carlo simulations and real images of phantoms and animals.

By using a pinhole collimator, planar images of mice were obtained, reaching a spatial resolution better than 2 mm, appropriate to identify non-overlapping individual organs. A sequence of images with 60 secs each one was used to register the kinetics of 99mTc-DISIDA during the first 30 minutes after injection. Tomographic images of individual organs of small animals were obtained, with a spatial resolution of 1.5 mm, using 99mTc-labelled radiopharmaceuticals. Images of phantoms filled with 177Lu and 67Ga were produced, showing that a similar spatial resolution can be reached. Based on Monte Carlo simulations, it was verified that by using a 4-pinhole collimator, the acquisition time or the injected activity can be reduced by a factor 3 without compromising image quality.

In this work, we show affordable and simple upgrading strategies for a clinical gamma camera, to produce planar and tomographic images of small animal organs. High spatial resolution can be obtained in both cases using pinhole collimators, in a timeframe compatible with the experimental routine. The solution can be extended to energies higher than that of the 99mTc, such as those found in theranostic radiopharmaceuticals, and can be improved by using multipinhole collimators.