Technologies & Services

Positron Emission Tomography (PET) is an imaging system that images high-energy γ-rays emitted from within the subject. The source of imaging for PET comes from the injected positron-emitting biological molecules or other types of radiotracers, e.g. glucose analog [¹⁸F]FDG (fludeoxyglucose) or radiolabeled antibodies. As the radioisotope decays, it emits positrons, which annihilate with nearby electrons found naturally in the body. This produces two γ-rays ~180° apart that are detected by scintillation crystals in the PET detector ring. This allows individual annihilation events to be localized within the body, and the data set is reconstructed to produce 3-dimensional images of the biodistribution of the injected radiotracer within the subject. Non-invasive, in vivo functional imaging with microPET allows both serial and longitudinal studies to be conducted in the same animal.

X-Ray Computed Tomography (CT) uses X-ray imaging to produce cross sectional and volumetric images of the subject. X-rays are projected through the body onto an array of detectors that measure the X-ray attenuation along the X-ray source and detector. The source and detector are then rotated around the subject to make multiple views at different angles of rotation. Using computer reconstruction algorithms, an estimate of absolute density at each point (volume element, “voxel”) in the body is computed. Thus, the CT image is a computed image of the measured X-ray attenuation from all projects. MicroCT is a miniaturized version of the conventional CT imaging system. The MicroCT currently available at Porter preclinical imaging facility is a Viva CT 40 scanner from Scanco Medical.

Historically, optical imaging as the first imaging device uses visible light as source of imaging but with very limited tissue penetration. The most common optical imaging modalities used for in vivo small animal imaging are the fluorescence and bioluminescence imaging systems. Fluorescence imaging gets its source for imaging from fluorochromes injected inside the subject that are excited by an external light source and emitting light of a different wavelength in response. Traditional fluorochromes include GFP, RFP, and their many other mutants. Recently, near-infrared dyes and infrared fluorescent proteins (700 nm-800 nm) are used due to low autofluorescence and deeper tissue penetration at these wavelengths. Bioluminescence imaging, on the other hand, is based on light generated by chemiluminescent enzymatic reactions such as luciferase. In both fluorescence and bioluminescence, the light signals are captured by Charged Coupled Device (CCD) cameras cooled up to -95 °F to be extremely light sensitive.

Photoacoustic imaging is a technique that combines optical illumination, optical absorption, and ultrasound detection to map the location of absorbers within light scattering media with high sensitivity and spatial resolution. A short pulse of electromagnetic radiation induces a milli-Kelvin rise in temperature within the absorber resulting in a thermal expansion. The rapid heating and cooling results in an acoustic wave that is emitted from the absorber that may be detected at the tissue surface by acoustic receivers. The depth of the absorber may be determined by the time of arrival of the thermo-acoustic signal at the tissue surface. A typical photoacoustic imaging system employs a laser source that generates pulses of light <10 nsec in duration with pulse energy in the 1-30 mJ range. Acoustic receivers are typically wideband and have center frequencies in the 1-20 MHz range.

Ultrasound imaging systems work through the generation of sound waves from transducers into the subject. As the sound waves propagate through tissue, they are reflected back and picked up by the transducer, and can then be translated into 2D and 3D images. Ultrasound systems for small animals are specifically developed with higher frequencies ranging from 15 MHz to 80 MHz compared to clinical ultrasound system with frequencies range from 3-15 MHz. In addition, contrast agents in the form of microbubbles, which have different acoustic properties from that of tissues can be introduced into animal systems to highlight structure such as vasculature or be targeted towards specific receptors.