What Does FDG Mean on a Pet Scan? Understanding FDG PET/CT in Veterinary Medicine

18F-fluorodeoxyglucose (FDG) PET/CT imaging is increasingly vital in veterinary medicine, particularly for diagnosing and managing cancer in pets. Integrating metabolic information from the FDG component with anatomical details from the CT scan, this modality offers a comprehensive view of structural, physiological, and biochemical abnormalities in animals. While PET/CT scans are well-established in human oncology, their application and interpretation in veterinary patients are gaining significant traction. This article will detail what FDG signifies in a pet scan, how FDG PET/CT works for pets, and why it’s a powerful tool in modern veterinary diagnostics.

Understanding FDG: The Key to Metabolic Imaging

FDG, or 18F-fluorodeoxyglucose, is a radioactive tracer that is structurally similar to glucose, a type of sugar that is the primary source of energy for cells. In essence, FDG is a glucose analog used in Positron Emission Tomography (PET) scans. But what does this mean for your pet?

Cells that are highly active, such as cancer cells or cells in inflamed tissues, typically require more energy than normal cells. They consume glucose at a higher rate. FDG leverages this characteristic. When FDG is injected into your pet, it travels through the bloodstream and is absorbed by cells that are using glucose for energy.

However, unlike normal glucose, FDG is slightly modified. Once inside the cell, it gets trapped. The radioactive component of FDG, fluorine-18 (18F), emits positrons, which are detectable by the PET scanner. Areas in the pet’s body with high FDG uptake indicate regions of increased glucose metabolism. This is why FDG is so valuable in identifying metabolically active tissues, such as tumors, inflammation, or infection.

How FDG PET/CT Scans Work for Pets

A PET/CT scan combines the functional imaging of PET with the detailed anatomical imaging of Computed Tomography (CT). Here’s a simplified breakdown of the process when your pet undergoes an FDG PET/CT scan:

Preparation: Just like in human PET scans, proper patient preparation is crucial for optimal results in pets. This may involve fasting your pet for a certain period to ensure stable glucose levels and potentially administering medication to minimize muscle uptake of FDG, which can sometimes obscure results. Veterinary specialists will provide specific instructions based on your pet’s condition and the area being scanned.
FDG Injection: A small dose of FDG is injected intravenously into your pet. The amount is carefully calculated based on your pet’s weight.
Uptake Period: There’s a waiting period, usually around 30-90 minutes, after the FDG injection. During this time, the FDG circulates throughout your pet’s body and is taken up by metabolically active cells. It’s important for your pet to remain calm and still during this period to ensure even distribution of FDG.
Scanning: Your pet is then positioned in the PET/CT scanner. The PET scanner detects the positrons emitted by the FDG, creating a 3D image of FDG distribution in the body. Simultaneously, the CT scanner acquires detailed anatomical images.
Image Fusion and Interpretation: Specialized software merges the PET and CT images, creating a fused PET/CT image. This fusion is critical as it overlays the metabolic information (from PET, showing FDG uptake) onto the anatomical information (from CT, showing organ structures). Veterinary radiologists or nuclear medicine specialists then interpret these fused images. Areas of high FDG uptake, appearing as “hot spots” on the PET scan, are analyzed in conjunction with the CT images to understand their location and nature.

Figure 1: The PET window intensity is adjusted so that the liver appears light to mid-grey on the grey scale, corresponding to flecks of green in the liver on the rainbow colour scale. Despite the difference in SUVmax of the liver secondary to differences in weights of the two patients (a and b), the liver intensity this appears the same in both patients.

Why is FDG PET/CT Useful in Veterinary Medicine?

FDG PET/CT offers unique advantages in veterinary diagnostics, especially in oncology and inflammatory conditions:

Cancer Detection and Staging: Cancer cells are typically highly metabolic and avidly consume glucose. FDG PET/CT is excellent at detecting tumors, assessing their extent (staging), and identifying metastases (spread to other parts of the body). This is particularly helpful in cancers that are not easily visualized with other imaging modalities or when assessing the overall spread of cancer.
Distinguishing Cancer Recurrence from Treatment Effects: After cancer treatment (like chemotherapy or radiation), it can be challenging to differentiate between scar tissue or treatment-related inflammation and actual cancer recurrence using CT or MRI alone. FDG PET/CT can help by showing if an area of concern is still metabolically active (likely recurrence) or not (likely scar tissue).
Monitoring Treatment Response: FDG PET/CT can be used to evaluate how well a cancer treatment is working. A decrease in FDG uptake in tumors after therapy suggests a positive response, while persistent or increased uptake may indicate treatment resistance or disease progression.
Identifying the Best Biopsy Site: In cases where a biopsy is needed to confirm a diagnosis, FDG PET/CT can guide veterinarians to the most metabolically active areas within a tumor. These areas are more likely to yield a diagnostic sample and represent the most aggressive parts of the disease.

Figure 2: This patient presented with suspected metastatic nasopharyngeal cancer. Initial workup with endoscopic ultrasound and biopsy of the subcarinal node was non-diagnostic with necrotic tissue. FDG PET/CT demonstrates very intense uptake at all sites with lower uptake in the subcarinal node, only evident when widening the PET window. The findings suggest a different tumour biology at this site with necrosis. When feasible, we recommend biopsy of the most FDG-avid lesion which likely represents the site of most aggressive disease and least likely to be non-diagnostic. In summary, the PET study windowed narrowly is primed for sensitivity whereas a wider window enables superior characterisation.

Diagnosis of Inflammatory Conditions: Beyond cancer, FDG PET/CT can also be valuable in diagnosing and monitoring inflammatory diseases in pets. Inflamed tissues also exhibit increased glucose metabolism and FDG uptake, helping to identify the location and extent of inflammation.

Interpreting FDG PET/CT Scans: What Veterinarians Look For

Veterinary specialists interpreting FDG PET/CT scans follow a systematic approach to ensure accurate diagnosis. Key aspects of interpretation include:

Technical Adequacy: First, they assess if the scan is technically sound, ensuring proper FDG administration, uptake time, and image quality.
Image Windowing and Display: Correct image display settings are critical. Veterinarians adjust the PET image “window” to optimally visualize both normal physiological FDG uptake and areas of potentially abnormal high uptake. Using color scales, like the “rainbow” scale, helps differentiate uptake intensities, with higher uptake areas often displayed in warmer colors (reds, oranges) and lower uptake in cooler colors (blues, greens).

Figure 3: Patient with metastatic colorectal carcinoma and hepatic metastasis. The fused image is presented in different colour scales. We recommend using the “rainbow” scale owing to the superior tumour-to-liver contrast compared to other commonly used colour maps.

Review Sequence: Veterinarians often start by reviewing the Maximum Intensity Projection (MIP) images, which provide a “gestalt” overview of FDG distribution throughout the body. They then systematically review images in different planes (axial, coronal, sagittal) and correlate PET findings with CT anatomy.
Standardized Uptake Value (SUV): SUV is a semi-quantitative measure of FDG uptake. While not absolute, it helps in assessing the intensity of FDG accumulation in different tissues. Veterinarians use SUV measurements to compare uptake in lesions to normal background tissues and to monitor changes over time.
Pattern Recognition: Experience is crucial in interpreting PET/CT scans. Veterinarians learn to recognize patterns of FDG uptake associated with different diseases, differentiating malignant patterns (like spherical tumor uptake) from benign or inflammatory patterns (like linear uptake along tissue planes).
Correlation with CT Findings: The anatomical information from CT is essential. Veterinarians carefully correlate areas of FDG uptake with corresponding structures on CT to understand the anatomical context of metabolic activity. For instance, FDG uptake in a lymph node seen on CT might suggest metastasis.

Figure 4: Patient with diffuse large B cell lymphoma. On the standard windowing, no abnormality is readily identified in the brain (a coronal & axial slice, b MIP image). By increasing the upper SUV threshold, abnormal uptake becomes readily becomes visible (c MIP image, d coronal & axial slice). This corresponded to a MRI abnormality which was not reported prospectively but identified following targeted review after the PET scan. Changing the PET window so that abnormalities can be identified above physiologic brain activity should be a routine component of image review.

Limitations and Considerations

While FDG PET/CT is a powerful tool, it’s important to acknowledge its limitations in veterinary medicine:

Physiological FDG Uptake: Normal tissues, like the brain, liver, and bladder, also take up FDG, which is physiological uptake. Veterinarians must differentiate this normal uptake from abnormal uptake in disease. Careful patient preparation, like fasting, helps minimize uptake in certain organs.
False Positives and False Negatives: Inflammatory conditions can cause increased FDG uptake, leading to false positives for cancer. Conversely, some slow-growing or less metabolically active cancers may not show significant FDG uptake, resulting in false negatives. Correlation with CT and other clinical information is crucial to minimize these issues.
Availability and Cost: PET/CT technology is not as widely available in veterinary medicine as CT or MRI. It’s typically found in specialized veterinary centers. The cost of PET/CT scans can also be higher than other imaging modalities.
Species-Specific Differences: FDG uptake patterns and normal ranges can vary slightly between different animal species. Veterinary specialists experienced in PET/CT interpretation in specific species (dogs, cats, etc.) are essential for accurate diagnoses.

Conclusion

In veterinary medicine, when a report mentions “FDG uptake on a PET scan,” it signifies areas in your pet’s body that are using glucose at a higher rate than normal. This metabolic information, especially when combined with detailed anatomical CT images, provides invaluable insights for diagnosing, staging, and managing cancer and certain inflammatory conditions in pets. While not a perfect test, FDG PET/CT represents a significant advancement in veterinary imaging, offering a deeper understanding of disease processes at a metabolic level and ultimately contributing to better care for our animal companions.

This technology continues to evolve, and as veterinary expertise in PET/CT grows, we can expect even wider applications and improved diagnostic accuracy, leading to better outcomes for pets facing complex medical challenges.