Positron emission tomography, or PET, is a medical imaging technique that detects gamma rays emitted by a radioactive tracer that is introduced into the body. Three-dimensional images of tracer concentration are constructed using computer algorithms. With PET-CT technology, a computed tomography (CT) scan is performed at the same time using the same medical imaging machine.
The radioactive tracer used in PET imaging is called fluorodeoxyglucose 18F, or F-FDG. It is a glucose analog with fluorine-18 substituted for part of the normal glucose molecule. Uptake of F-FDG is correlated with certain types of tissue metabolism, notably by cancer metastases.
PET-CT medical imaging is extremely useful in diagnostic processes, but patients are exposed to both the radiation from the F-FDG and from the CT imaging. Researchers are finding new ways to reduce patient radiation exposure while still obtaining high quality images. Three techniques for reducing patient radiation exposure include:
• Determining when peak kilo-Voltage (kVp) of a CT machine can be reduced
• Determining when F-FDG dose to the patient can be reduced
• Using PET with magnetic resonance imaging (MRI) rather than CT
Ensuring Best PET-CT Image Quality
Image noise, which lowers image quality, has an inverse relationship to x-ray beam energy. As tube current or voltage decreases, image noise increases. Radiologists must determine the acceptable range of image quality and must determine the minimum radiation dose that is needed to create images of acceptable quality. Though many PET-CT devices by default use 140 peak kilo-voltage (kVp), 80 to 120 kVp can sometimes be used to reduce radiation dosage while still producing high quality medical imaging.
Optimal F-FDG Dosing with Larger Patients
A research team in the Netherlands studied optimized dose regimens for PET-CT imaging and found that a linear correlation between F-FDG dose and body mass was not ideal. Using a linear model for determining F-FDG dose resulted in image degradation for obese patients. These researchers found that a quadratic relationship between F-FDG dose and body mass resulted in higher quality of medical imaging using PET-CT. Image quality was more consistent when using the quadratic method of determining F-FDG dose, further refining knowledge of how to optimize F-FDG for superior image quality.
PET-MRI Instead of PET-CT
Researchers in Germany recently carried out studies on how to reduce F-FDG dose while maintaining image quality using a PET-MRI machine rather than PET-CT. Rather than use human subjects, they used a “PET phantom” that mimicked the shape of the human torso and contained spongy material with radiation attenuating properties similar to that of lung tissue. Within the phantom, hollow spheres were filled with either F-FDG or plain water. By reducing F-FDG dosage and using MRI rather than CT imaging, the goal was to create high quality medical images while minimizing exposure to radiation.
Researchers acquired images at time intervals corresponding to reductions in F-FDG activity and assessed contrast, background variability, and signal-to-noise ratio under a range of conditions. Experienced readers rated image quality from best to worst for randomized series of images. Readers placed images of comparable image quality together. By decreasing F-FDG dosage and lengthening image acquisition times, researchers created images with very little noticeable difference at different time points.
In other words, researchers were able to maintain image quality with lower levels of tracer by increasing the acquisition time of the MRI images. The result was high quality images at much lower radiation dose than with traditional PET-CT medical imaging.
PET-CT technology has revolutionized medical imaging. In one exam, radiologists can bring together metabolic and anatomic information, depicted in high quality images in a short amount of time. The main drawback to PET-CT is that patients are exposed to two sources of radiation, but radiology researchers are learning when it’s appropriate to reduce dosage of the F-FDG tracer and when peak kilo-Voltage of the CT device can be reduced. Additionally, the use of PET-MRI as an alternative to PET-CT is being explored, and this technology could reduce patient radiation exposure even more without compromising medical imaging quality.
Knowing the best medical imaging technology to use in a given situation requires advanced knowledge, skill, and experience. The radiologists who own and operate SteleRAD have over 40 years of experience in radiology, including radiology subspecialties like nuclear radiology. SteleRAD’s Board-certified radiologists deliver timely and accurate interpretations for hospitals, physician groups, and imaging centers, with an unmatched commitment to excellence. To learn more, contact SteleRAD online or call us at 954-358-5250.