Figure 1: Compound image from a Compton camera showing the positions of three different radioisotopes, zinc (red), strontium (blue) and iodine (green), in a live mouse. (This work was completed in compliance with Japan#sc#s ethical standards for experiments on live animals.)
Reproduced from Ref.1 © 2008 by permission of The Royal Society of Chemistry (RSC)
Doctors and scientists can visualize specific biological processes in living creatures by monitoring radioactive tracer molecules. So far, imaging techniques have largely been limited to seeing one tracer molecule at a time, which is unlikely to provide the full picture of complex functions or diseases.
Now Shuichi Enomoto, Shinji Motomura and co-workers at the RIKEN Molecular Imaging Research Program in Kobe and Wako have produced images of three radioactive isotopes at the same time in a live mouse1. The researchers adapted a gamma-ray imaging device called a semiconductor Compton camera, which was originally developed for gamma-ray astrophysics.
“We had been working on research and development of ‘multitracer’ technology,” explains Motomura. “A multitracer contains radioisotopes of various chemical elements, so that many elements and their interactions can be observed by one experiment. Later we proposed realizing multiple molecular imaging with a semiconductor Compton camera.”
The Compton camera consists of two detectors made from intermeshed strips of germanium, and can probe a wide range of gamma ray energies. “An extremely pure crystal of germanium can work as a radiation detector with high energy resolution,” explains Motomura. “Two sets of germanium electrodes are arranged in strips at right angles, so that the gamma-ray energy and hit positions can be detected.”
To test their modified Compton camera for biological imaging, the researchers chose three common radioactive tracers—isotopes of iodine, strontium and zinc—and injected them into an eight-week-old male mouse. The mouse was anaesthetized and scanned for 12 hours, producing both 2D and 3D images. The three tracers were distinguished by identifying their different emission energy peaks, and could be represented together in images by allocating three different colors: red, green and blue (Fig. 1).
All the tracers collected in areas where they would normally be expected: zinc tends to accumulate in the liver or in tumors, while strontium collects in the bones and iodine is taken up into the adrenal and thyroid glands. The researchers observed similar concentrations and distributions of the tracers every 3 hours over the 12-hour scanning period, implying a fast and long-lasting imaging capability.
The researchers believe their results show great promise for the Compton camera in biological imaging. At present these germanium-based detectors are very expensive, but there could be strong demand in future, once the researchers improve their equipment to provide higher resolution images in a shorter time.