A Positron Emission Tomography (PET) scan provides information about metabolic processes in the body. It uses a radioactive tracer, most commonly Fluorodeoxyglucose (FDG), which is a sugar analog detected by the scanner. The term “FDG uptake” refers to the amount of this tracer absorbed by cells, highlighting areas of different metabolic activity.
The Cellular Process of FDG Uptake
All cells in the body use glucose as a primary energy source. FDG is a glucose analog, meaning it is structurally similar and taken up by cells through the same transport proteins. Once inside the cell, both glucose and FDG undergo phosphorylation by an enzyme called hexokinase.
This phosphorylation step is where the paths of glucose and FDG diverge. For glucose, this is the first step in a long metabolic pathway to release energy. However, the phosphorylated form of FDG has a slightly different chemical structure and cannot be further metabolized, effectively trapping the radioactive molecule inside the cell.
Cells that are more metabolically active, such as rapidly dividing cancer cells, have a higher energy demand. They express more glucose transporters and have higher hexokinase activity, leading them to absorb significantly more FDG than less active cells. This metabolic trapping allows PET scans to visualize these areas of high cellular activity.
Pathologic Uptake in Medical Imaging
“Pathologic uptake” refers to FDG accumulation associated with a disease process, most notably cancer. Many malignant tumors have a high rate of metabolic activity and consume large amounts of glucose. This increased metabolism results in significant FDG uptake, causing tumors to appear as bright spots on a PET scan, sometimes before anatomical changes are visible on other imaging.
FDG PET scans are used for the initial diagnosis and staging of many cancers, helping to determine the extent of the disease by identifying the primary tumor and any spread to lymph nodes or distant organs. The intensity of the FDG uptake, quantified using the Standardized Uptake Value (SUV), can offer insights into a tumor’s aggressiveness.
Beyond initial diagnosis, these scans are also used to monitor the effectiveness of cancer treatments. A decrease in FDG uptake in a tumor after a course of chemotherapy or radiation therapy can indicate that the treatment is successfully killing cancer cells. Conversely, the appearance of new areas of high uptake may signal disease progression. This imaging is also used for surveillance after treatment is complete to detect any potential cancer recurrence.
Physiologic and Benign Causes of Uptake
High FDG uptake is not exclusive to cancer, as numerous non-cancerous conditions can cause similar findings on a PET scan. Understanding these physiologic and benign causes is important for accurate interpretation. These instances of uptake occur in tissues that are metabolically active or responding to a non-malignant process.
Normal physiologic uptake is consistently seen in certain organs. The brain is a highly metabolic organ that relies on glucose for energy and therefore shows intense FDG uptake. The heart muscle also demonstrates variable uptake, and because FDG is cleared through the urinary system, the kidneys and bladder will appear bright.
Inflammation and infection are common benign causes of increased FDG accumulation. Inflammatory cells involved in the immune response are highly active and consume significant glucose. This can lead to high uptake in areas of arthritis, recent surgical wounds, or dental infections.
Brown adipose tissue (BAT), or brown fat, is another cause of benign uptake. This tissue’s function is to generate heat, a process that requires high glucose consumption. BAT is often found in the neck and supraclavicular regions and can be activated when a patient feels cold.
Recent physical activity can lead to increased FDG uptake in skeletal muscles. Muscles that have been exercised have a higher metabolic rate as they repair and replenish energy stores. The pattern of uptake often corresponds to specific muscle groups, which helps differentiate it from pathologic causes.
Ensuring an Accurate Scan
Proper patient preparation is necessary to minimize physiologic FDG uptake and enhance the accuracy of a PET scan. Reducing this “background noise” helps radiologists more clearly identify any pathologic activity.
Key patient instructions include:
- Fasting for several hours (typically 4-6) before the scan to lower blood glucose and insulin levels.
- Following a low-carbohydrate, high-protein diet in the 24 hours before the scan to reduce background uptake in normal tissues.
- Avoiding strenuous exercise for at least 24 hours to prevent increased FDG uptake in muscles.
- Staying warm before and during the scan to prevent the activation of brown adipose tissue (BAT).
- Informing the medical staff about any recent illnesses, injuries, or surgeries, as these can be sources of inflammation.