Positron emission tomography (PET) uses a camera and radioactive tracer to examine bodily organs. The tracers are generally special forms of glucose that tend to collect in cells that metabolize a lot of glucose, like cancer cells. As the tracer travels through the body, it gives off positrons, and the camera detects them and turns them into a computer image. PET images are not as detailed as other imaging techniques because they’re made to show the location of the tracer.
A tracer called [18F]fluorodeoxyglucose, shortened to 18F-FDG, or simply FDG, is commonly used in evaluating the spread of cancer with PET. In the past, PET has been combined with computed tomography (CT) imaging so that not only the tracer distribution but anatomical details could be seen when looking for cancer metastases. A newer technique is the combination of PET with magnetic resonance imaging (MRI) for the same purpose.
PET/CT in Cancer Staging
The combination of PET and CT is better than PET or CT used alone because the combination minimizes the limitations of the individual imaging techniques. PET/CT is used in staging or restaging certain tumors, and can be used to detect recurrence of cancer in asymptomatic patients who have equivocal findings on traditional medical images, or who have rising tumor marker levels. PET/CT is also valuable in monitoring response to cancer therapy and allowing modification to therapy where needed. In around one-quarter of patients who receive PET/CT, the imaging combination leads to changes in the course of cancer management.
PET/MRI in Cancer Staging
Some cancers are anatomically defined better by MRI than by CT, because MRI offers better soft-tissue contrast. Liver cancer is one cancer that is better defined by MRI, and is being studied in comparisons of PET/MRI with PET/CT. Multi-parameter MRI with liver-specific tracer agents allow clinicians to differentiate between certain types of benign and malignant tumors – tumors which may be indeterminate on a CT or ultrasound image. Furthermore, PET/MRI is able to detect lesions that are smaller than the resolution of PET or CT imaging, and this can affect the course of disease management.
Which Detects Liver Metastases Better?
Researchers in Germany and Spain selected 32 patients with solid liver malignancies to undergo both PET/CT and PET/MRI of the liver. Then, two radiologists assessed both data sets for lesion characteristics (benign, malignant, or undetermined), conspicuousness of lesions, and diagnostic confidence. They found that PET/MRI had higher accuracy, sensitivity, and negative predictive value when compared to PET/CT. Moreover, PET/MRI showed up lesions more conspicuously and enabled detection of metastases that would not show up on PET alone. The researchers concluded that PET/MRI offered higher diagnostic accuracy in detecting liver metastases.
The Future of PET/MRI in Cancer Staging
For PET/MRI to become mainstream in imaging of cancer, larger scale clinical research needs to be done to answer specific questions about imaging versus diagnosis. Another issue that limits the use of PET/MRI is the fact that MRI scan times are typically long, and studies have not yet determined the optimal MRI sequences to combine with PET for the best information. Logistical issues like workflows, time management, and cost must also be addressed, because PET/MRI scanners are expensive, and since PET/MRI scans last longer than PET/CT scans, time to return on investment must be determined and considered.
Detecting Smaller Liver Metastases to Maximize Treatment Options
For people with metastasized liver cancer, the detection of new lesions when they’re newer and smaller can affect clinical disease management. Therefore, these patients may have excellent reason to prefer PET/MRI to PET/CT, but more studies are needed to learn exactly how PET/MRI works better and how it affects the course of treatment.
Nuclear medicine specialists, like Charles C. Cole, MD of SteleRAD, know how to combine nuclear imaging with CT and MRI imaging to produce the specific types of images that allow cancer specialists to stage cancers and plan out the best possible courses of treatment. All the owners of SteleRAD are Board-certified radiologists with experience covering all subspecialties, and they assist hospitals, imaging centers, and medical practices throughout South Florida in providing the highest quality radiology services to patients. If you would like to know more about how SteleRAD’s radiologists can help your facility, we encourage you to call 954-358-5250 or contact us online at any time.