PET/CT of the Spine¶
Definition¶
Positron emission tomography/computed tomography (PET/CT) combines functional metabolic imaging (PET) with anatomic CT in a single study. The most commonly used radiotracer is ¹⁸F-fluorodeoxyglucose (FDG), a glucose analog that accumulates in metabolically active cells. In spine imaging, PET/CT is primarily used for oncologic evaluation and certain inflammatory/infectious conditions.
Technique¶
Procedure¶
- Patient preparation: fasting for 4–6 hours; blood glucose should be <200 mg/dL
- FDG injection: 10–15 mCi administered intravenously
- Uptake phase: 60 minutes of quiet rest (to minimize physiologic muscle uptake)
- Imaging: combined PET and CT acquisition from skull base to mid-thigh (standard oncologic protocol) or focused on the spine
Standardized Uptake Value (SUV)¶
- SUV is a semi-quantitative measure of FDG uptake, normalized to body weight and injected dose
- Higher SUV indicates greater metabolic activity
- SUVmax (maximum SUV within a region of interest) is the most commonly reported value
- SUV values must be interpreted in clinical context — infection, inflammation, and post-surgical changes can all increase FDG uptake
Indications¶
Oncologic¶
- Metastatic disease: detection, staging, and restaging of osseous and soft tissue metastases
- Treatment response: assessing response to chemotherapy or radiation (decreasing SUV = response)
- Primary bone tumors: staging and monitoring (especially lymphoma, Ewing sarcoma)
- Unknown primary: identifying the primary tumor in patients presenting with vertebral metastases
Non-Oncologic¶
- Infection: FDG-PET/CT is highly sensitive for spinal infection (discitis-osteomyelitis, epidural abscess) and can detect infection when MRI is equivocal or contraindicated
- Sarcoidosis: detection of active spinal/paraspinal disease
- Differentiating benign from malignant compression fractures — malignant fractures typically show higher FDG uptake, though overlap exists with acute benign fractures
Advantages Over Bone Scan¶
| Feature | Bone Scan | PET/CT |
|---|---|---|
| Detects lytic lesions | Poor (may be cold) | Excellent (FDG uptake regardless of osteoblastic response) |
| Spatial resolution | Low | Higher (CT component) |
| Anatomic correlation | Limited (planar) or SPECT/CT | Built-in CT correlation |
| Soft tissue evaluation | Limited | Detects soft tissue/epidural disease |
| Specificity | Low | Higher (with SUV quantification and CT correlation) |
| Myeloma detection | Poor | Excellent |
Clinical Pearl
PET/CT is superior to bone scan for detecting lytic osseous metastases, including multiple myeloma, renal cell carcinoma, and thyroid cancer. These tumors often cause purely lytic destruction without significant osteoblastic response, making them invisible on conventional bone scan. PET/CT detects them based on their metabolic activity rather than bone reaction. For blastic metastases (prostate cancer), both modalities are effective, though sodium fluoride (NaF) PET/CT is emerging as the most sensitive technique.
Limitations¶
- Cost — significantly more expensive than bone scan or MRI
- Availability — not available at all centers
- False positives — FDG uptake occurs in infection, inflammation, post-surgical changes, degenerative disease, and normal marrow activation (after chemotherapy or G-CSF)
- False negatives — low-grade tumors with minimal metabolic activity may not accumulate FDG
- Radiation exposure — combined PET and CT dose (~20–25 mSv)
- Hyperglycemia — elevated blood glucose competes with FDG for cellular uptake, reducing sensitivity
Key Points¶
- PET/CT combines metabolic (PET) and anatomic (CT) information in a single study
- FDG-PET/CT is the primary oncologic imaging modality for metastatic staging and treatment response
- Superior to bone scan for lytic metastases, myeloma, and soft tissue disease
- Also useful for spinal infection when MRI is equivocal or contraindicated
- SUV quantification provides objective assessment of metabolic activity
- False positives from infection, inflammation, and post-treatment changes limit specificity
References¶
- Patel PY, Dalal I, Griffith B. [18F]FDG-PET evaluation of spinal pathology in patients in oncology: pearls and pitfalls for the neuroradiologist. AJNR Am J Neuroradiol. 2022;43(3):332-340. https://pmc.ncbi.nlm.nih.gov/articles/PMC8910786/
- Cheung H, Yechoor A, Behnia F, Behrad Abadi A, Khodarahmi I, Soltanolkotabi M, Shafiei M, Chalian M. Common skeletal neoplasms and nonneoplastic lesions at 18F-FDG PET/CT. RadioGraphics. 2022;42(1):250-267. https://pubmed.ncbi.nlm.nih.gov/34919467/
- Panagiotidis E, Lam K, Mistry A, Seshadri N, Vinjamuri S. Skeletal metastases and benign mimics on NaF PET/CT: a pictorial review. AJR Am J Roentgenol. 2018;211(1):W64-W74. https://pubmed.ncbi.nlm.nih.gov/29702023/
- Farolfi A, Calderoni L, Mattana F, Mei R, Telo S, Fanti S, Castellucci P. Current and emerging clinical applications of PSMA PET diagnostic imaging for prostate cancer. J Nucl Med. 2021;62(5):596-604. https://pubmed.ncbi.nlm.nih.gov/33712536/
- Sanli Y, Garg I, Kandathil A, Kendi T, Zanetti MJB, Kuyumcu S, Subramaniam RM. Neuroendocrine tumor diagnosis and management: 68Ga-DOTATATE PET/CT. AJR Am J Roentgenol. 2018;211(2):267-277. https://pubmed.ncbi.nlm.nih.gov/29975116/
- Prodromou ML, Ziakas PD, Poulou LS, Karsaliakos P, Thanos L, Mylonakis E. FDG PET is a robust tool for the diagnosis of spondylodiscitis: a meta-analysis of diagnostic data. Clin Nucl Med. 2014;39(4):330-335. https://pubmed.ncbi.nlm.nih.gov/24445277/