A team of scientists at the National Institute of Standards and Technology (NIST), Gaithersburg, MD, has demonstrated the first calibration system for positron emission tomography (PET) scanners directly tied to national measurement standards. Better calibrations of the machines, they say, potentially can increase the accuracy of their diagnostic images by several times.
The new calibration capability can be used to fine-tune PET scanners that are used to find cancers as well as to track the progress of treatments, among other diagnostic applications. The technology will be used to ensure the accuracy of some of the newest scanners on the market. (See Figure 1)
NIST’s technique, developed over a few years, is used to calibrate devices called “phantoms,” built specifically for PET scanners. Phantoms are commonly used to check medical imaging devices such as X-ray scanners. Typically, they are simply blocks of materials known to respond to, for example, X-rays in a consistent, known manner similar to how human tissues respond. PET phantoms are more complicated though, because the scanners work by detecting radioactive materials injected in the patient.
The PET phantoms are lightweight hollow cylinders about the size of a two-liter soda bottle. They contain a small amount of radioactive germanium, which glows brightly in a PET scanner’s readout. Measuring its brightness each day provides a benchmark for scanner sensitivity that allows medical technicians to compare patient scans taken on different days more effectively.
The NIST phantoms will be the first ones commercially available that can trace calibration directly to NIST standards. The team’s method measures the phantom’s germanium content directly, eliminating some known sources of error. The phantoms are designed specifically for scanners that combine PET and magnetic resonance imaging (MRI). PET and MRI are potentially an ideal combination for diagnosis because PET reveals spots where cancer likely exists, and MRI provides the basic picture of a patient’s body so the PET scan can be oriented precisely.
The researchers say that this kind of quantitative work can reveal differences of 5 percent in tumor size, rather than the 20 percent they look for now, which could help doctors make decisions more quickly concerning whether a treatment or a clinical trial is working out.
For more information, visit www.nist.gov .