Whole animal imaging.

Translational research plays a vital role in understanding the underlying pathophysiology of human diseases, and hence development of new diagnostic and therapeutic options for their management. After creating an animal disease model, pathophysiologic changes and effects of a therapeutic intervention on them are often evaluated on the animals using immunohistologic or imaging techniques. In contrast to the immunohistologic techniques, the imaging techniques are noninvasive and hence can be used to investigate the whole animal, oftentimes in a single exam which provides opportunities to perform longitudinal studies and dynamic imaging of the same subject, and hence minimizes the experimental variability, requirement for the number of animals, and the time to perform a given experiment. Whole animal imaging can be performed by a number of techniques including x-ray computed tomography, magnetic resonance imaging, ultrasound imaging, positron emission tomography, single photon emission computed tomography, fluorescence imaging, and bioluminescence imaging, among others. Individual imaging techniques provide different kinds of information regarding the structure, metabolism, and physiology of the animal. Each technique has its own strengths and weaknesses, and none serves every purpose of image acquisition from all regions of an animal. In this review, a broad overview of basic principles, available contrast mechanisms, applications, challenges, and future prospects of many imaging techniques employed for whole animal imaging is provided. Our main goal is to briefly describe the current state of art to researchers and advanced students with a strong background in the field of animal research.

Influence of the angle of incidence on the sensitivity of gamma camera based PET.

Thicker crystals have been used to increase the detection efficiency of gamma cameras for coincidence imaging. This results in a higher detection probability for oblique incidences than for perpendicular incidences. As the point sensitivity at different radial distances is composed of coincidences with different oblique incidences, the thickness of the crystal will have an effect on the sensitivity profiles. To correct this non-uniform sensitivity, a sensitivity map is needed which can be measured or calculated. For dual- or triple-head gamma camera based positron emission tomography (PET) a calculated sensitivity map is preferable because the radius and the head orientation often change between different acquisitions. First, these sensitivity maps are calculated for 2D acquisitions by assuming a linear relationship between the detection efficiency and the crystal thickness. The 2D approximation is reasonable for gamma cameras with a small axial acceptance angle. The results of the 2D approximation show a good agreement with the results of Monte Carlo simulations of different realistic gamma camera configurations. For dual-head gamma cameras the influence on the sensitivity profile is limited. Greater variation of the sensitivity profile is seen on three-headed gamma cameras and correction of this effect is necessary to obtain uniform reconstruction. To increase the sensitivity of gamma cameras, axial collimators with larger acceptance angles are used. To obtain a correct sensitivity for these cameras a sensitivity calculation in 3D is needed. For a fixed camera position the sensitivity is obtained by integrating the detection efficiency over the solid angle formed by the voxel and the intersection of the first detector with the projection of the second detector on the plane of the first detector. The geometric sensitivity is obtained by averaging this for all camera angles. The values obtained show a good agreement with the Monte Carlo simulations for different points in the field of view. Both 2D and 3D sensitivity profiles show the highest influence of the detector thickness on the radial profiles of the U-shape configuration. Taking the detector thickness into account also has an influence on the axial profiles. This influence is maximal in the centre where more oblique coincidences are present. This method is not limited to gamma camera based PET scanners but can be used to calculate the sensitivity of any PET camera with continuous detector blocks.