Positron Emission Tomography combined with Computed Tomography (PET/CT)

What is PET/CT?

PET/CT is a modality within Nuclear Medicine which combines a PET scanner and a CT scanner within a single gantry.

Nuclear Medicine is considered ‘Functional Imaging’; it obtains images of how the body functions.  Most often, Functional Imaging requires the injection of a radiopharmaceutical (radioactive drug) designed to look for specific functions in the body, such as metastatic cancer.  Functional Imaging can often detect abnormalities in the body much sooner than other imaging techniques.  It can detect an abnormality prior to it becoming an invasive tumor.

Cancer Imaging

The majority of PET scanning is done to detect cancer.  For cancer detection, a patient is injected with Fluorodeoxyglucose (FDG).  FDG is a glucose analog ‘tagged’ with radioactive fluorine (F18).  If metastatic cancer is present, the cancer cells metabolize the glucose analog and the F18 gets trapped in the cancer cells.  As F18 decays, it emits (thus Emission) radiation (thus Positron).  The PET scanner detects the emitted radiation and generates a three-dimensional image of the internal structures of the body (thus Tomography).  If present, metastatic cancer will be displayed on the image as a ‘Hot Spot’.

A biopsy will likely need to be taken to confirm diagnosis, but a PET/CT scan can aid the physician in locating the best location to biopsy.  PET/CT scanning is a non-invasive way to stage, assess treatment response, and re-stage cancer with a high level of accuracy.  It can be used to stage know cancer to determine the best course of treatment.  It can be used to assess treatment response to chemotherapy and other forms of treatment to better guide a patient’s course of treatment.  It can also be used to re-stage cancer to determine if more treatment is needed or if the patient has reached partial or complete remission.

PET/CT FDG Procedure

A PET/CT scan is a rather simple non-invasive procedure.There are special preparation instructions for all patients and more specific instructions for patients with diabetes.These instructions can be found in the Patient Resources section of this website.

While seated or lying comfortably, the patient is injected with FDG.There are no known contraindications to FDG, as it acts within the body similarly to glucose.Patients with diabetes can be injected with FDG as it has little effect on blood-glucose levels.The FDG circulates within the body typically for one hour prior to image acquisition.The patient is then positioned on the imaging table as comfortably as possible.Image acquisition time is typically around 25 minutes for most patients on most scanners.The patient is typically released with no necessary precautions, because F18 decays quickly with a radioactive half-life of just under 110 minutes.

Of the 25 minutes, 5 minutes is spent acquiring a non-diagnostic CT scan.The CT scan in PET/CT scanning typically uses low radiation dosing and non-optimal positioning for diagnostic CT acquisition.The purpose of the CT in PET/CT scanning is only to provide density information necessary for the reconstruction of diagnostic PET images, and to aid in determining the exact location of any ‘Hot Spots’ found on the PET images.This is why physicians may order both PET/CT and CT scans at or near the same time.We do not charge for the CT portion of a PET/CT scan.

The PET scan accounts for the remainder of the scanning time, and acquires images when the table is not moving.The patient lies as still as possible while the scanner detects the radiation emitted from their body in approximately 14cm segments typically referred to as ‘Bed Steps’.It typically takes 7 Bed Steps to scan most patients.

PET/CT scanning is typically done from the ‘Eyes to Thighs’.Eyes to Thighs scanning is typically referred to as a ‘Whole Body’ scan as most cancers typically do not metastasize beyond the inguinal lymph nodes, which are in the patient’s groin area.If a patient has Melanoma, another cancer known to metastasize beyond the Eyes to Thighs, or known cancer beyond the Eyes to Thighs; the physician may order an ‘Entire Body’ scan.Entire Body scanning is unnecessary and not recommended for patients who do not have a specific need for it.Also, we do not recommend performing ‘Limited’ field-of-view scanning, such as scanning the chest only.Since the patient is injected with the majority of the radiation they are exposed to, it is best to image all the areas that particular cancer has a potential to metastasize to.

Of note: the radiation exposure from a PET/CT scan is similar to that of one whole body diagnostic CT scan.The potential of side effects from the radiation exposure from a PET/CT scanning is extremely low to nearly non-existent.Consult your physician, radiologist, or technologist to address any concerns.

Of note: PET/CT is not the imaging modality of choice for the imaging of brain lesions.Normal brain cells metabolize glucose as their energy source and the entire brain displays as a Hot Spot.Unless the physician is imaging a large highly metastatic lesion, most brain lesions will not be easily visible on a PET/CT scan.

Once the CT and PET image acquisitions are complete, the scanner processes the data and creates images which can be viewed on a computer screen.The PET images are overlaid on the CT images, which is typically referred to as ‘Fused’.These images are sent to a physician, typically a radiologist.The radiologist can see any potential Hot Spots on the PET images and know their exact location within the body due to the CT images.The radiologist then sends a report of his/her findings to the patient’s referring physician.The patient receives the results from their referring physician.

Other PET/CT Imaging Procedures offered by FRMI:

  • FDG Metabolic Brain Imaging
    • To determine the cause of dementia
    • For the diagnosis of Alzheimer’s
    • For seizure localization
  • FDG Cardiac Imaging
    • To diagnose myocardial viability
  • NaF (Sodium Fluoride) Bone Scanning
    • For imaging of metastatic bone lesions
  • Fluciclovine F18 (Axumin)
    • For imaging of prostate cancer