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MicroCAT: Restraint-Free Animal Imaging

A multidisciplinary team of scientists is developing 3-D imaging techniques using single-photon emission computed tomography (SPECT) scanners to study small animals without anesthesia.

Project

SPECT Without Restraint

SPECT scanners create pictures by tracing the paths of two detected gamma rays to determine the originating collision point, a technique called electronic collimation. Imaging an unrestrained animal using SPECT scanners requires a highly sensitive gamma imager capable of recording such a “gamma movie” with short time exposures. This requires more-sensitive scanners than are currently available.

Coded aperture imaging, a technique used in astrophysics, could provide the needed higher resolution. Coded apertures work on the same principle as cameras that use a pinhole instead of a lens to let light onto film. Rather than a single pinhole, a coded aperture has a matrix of openings that are decoded when the image is processed. These apertures allow a single point emitting energy from radioisotopes to be sampled by two or more detector elements. This method is at least twice as sensitive as conventional collimators.

Tools and techniques being developed will measure volume using high-resolution SPECT images of mice and rats. These volumetric images will be matched with previously acquired microcomputed tomography (microCT) images. The microCT images will serve as a base for the development of an atlas of anatomic images corresponding to different positions of an unrestrained animal (straight or bent, standing or lying down). In principle, this atlas will permit 3D localization of the animal’s organs.

During the imaging session when the animal is awake, the mouse is placed in a small tube, like a burrow, where it can easily roam. An optical laser tracking system, being developed at ORNL, records the animal’s motion and pose in a minimally confined space. This positional information establishes a nonvarying anatomical presentation of the animal to which the nuclear imaging data is mapped. Software developed by ORNL and Jefferson Laboratory performs computation-intensive calculations to align all maps.

Using these techniques and others, high-resolution, high-sensitivity SPECT detectors could substantially increase the sensitivity of small-animal and human SPECT systems.

Potential Impact

Recent imaging research has focused on PET systems because of their high resolution and sensitivity. If sufficient resolution can be achieved, SPECT has the advantages of greater radiotracer-compound availability and potential for longer-term studies through the use of longer-lived tracers. Researchers and clinicians hope eventually to achieve much of the functionality currently available in PET using less-expensive SPECT technology that does not require a nearby cyclotron to produce tracers.

The new motion-correction technology will help children and patients with neurological or other disorders who have trouble keeping still during the sometimes-lengthy procedures.

Another important result of the new systems will be the ability to image mice several times in a series of experiments that follow the course of disease. In the past, mice had to be dissected to evaluate disease progression.

Taken together, new imaging-technology improvements have great potential for enhancing the usefulness and quality of small-animal imaging studies—and moving improvements closer to the clinic.

Related Websites

Small Animal Imaging at Jefferson Lab

Animal Tracking for Micro-SPECT–CT IMaging at ORNL

Small Animal Imaging Resource Program at Johns Hopkins

News Articles

"Detector Technology Aids Development of Cystic Fibrosis Therapy," Jefferson Lab News, August 11, 2004. Small-animal imager allows cystic fibrosis gene therapy researchers to image gene function in live mice.

"Lab is Working to Build a Better Mouse Camera," Daily Press, November 5, 2001

"Imager May One Day Scan Moving Animals," United Press International, July 11, 2001