11.2 Bone and Musculoskeletal Imaging
Key Takeaways
- Bone scintigraphy uses Tc-99m MDP/HDP (~20–30 mCi adult) with delayed imaging at about 2–4 hours; hydrate and void to clear soft tissue and bladder
- Three-phase (flow, blood pool, delayed) and four-phase (add 24-hour) studies help separate cellulitis from osteomyelitis and characterize complex MSK infection/trauma
- SPECT and SPECT/CT refine spine and complex anatomy; NaF-18 PET/CT is a high-sensitivity bone-seeking PET alternative when available
- Indications include metastases, occult fracture, infection, RSD/CRPS patterns, and prosthesis workups—always document surgery, trauma, and hardware
- Normal variants (growth plates, asymmetric uptake, urine contamination) are high-yield traps for false-positive “lesions”
11.2 Bone and Musculoskeletal Imaging
Quick Answer: Tc-99m MDP/HDP maps osteoblastic activity (~20–30 mCi, image 2–4 h). Use limited, whole-body, 3-phase, or 4-phase protocols; add SPECT/SPECT-CT. NaF PET is the PET bone alternative. Know indications, prep, phase roles, and normal variants.
Agents and Physiology (Procedure Focus)
Tc-99m medronate (MDP) and oxidronate (HDP) bind bone mineral by chemiadsorption to hydroxyapatite where osteoblastic activity and blood flow are high. Procedure questions assume you already know the chemistry (Domain III) and now execute the right protocol for the clinical question.
| Agent | Role in MSK imaging |
|---|---|
| Tc-99m MDP/HDP | Standard planar/SPECT bone scan |
| F-18 sodium fluoride (NaF) | PET/CT bone imaging—high bone-to-background, rapid kinetics |
Protocol Menu
Limited Bone Scan
Spot views of a region (e.g., hands, feet, lumbar spine) when disease is localized. Still inject standard activity unless pediatric/weight-based rules apply; image the area of interest plus comparison views as ordered.
Whole-Body Bone Scan
Anterior and posterior head-to-toe passes (plus spots as needed) for metastatic surveys and multifocal disease. Typical adult dose ~20–30 mCi (740–1110 MBq) IV; delayed imaging ~2–4 hours. Hydrate; void immediately before scanning; remove metal; note pacemakers/ports.
Three-Phase Bone Scan
Ordered for infection, trauma, and complex regional pain patterns when blood flow and soft-tissue phases matter.
| Phase | Timing | What it shows (tech-level) |
|---|---|---|
| Flow (dynamic) | Immediate dynamic frames over ROI during/after bolus | Regional perfusion |
| Blood pool (soft tissue) | ~5–10 min (immediate static) | Hyperemia / soft-tissue inflammation |
| Delayed (bone) | ~2–4 h | Osteoblastic bone reaction |
Interpretation roles for the technologist: obtain a true bolus over the correct region; keep the patient still; label left/right; do not “skip flow” if ordered for osteomyelitis vs cellulitis. Physician reads patterns (e.g., all three phases hot in osteomyelitis vs soft-tissue-only in cellulitis)—your job is complete, timed, quality phases.
Four-Phase Bone Scan
Adds a ~24-hour delayed set. Used when soft-tissue clearance is poor (renal failure, edema) or when separating persistent soft-tissue activity from true bone uptake in difficult infection cases. Schedule the return visit and reinforce hydration/voiding.
SPECT and SPECT/CT
SPECT improves contrast for spine, skull base, and joints. SPECT/CT adds anatomic correlation for hardware, facets, and small bones—critical for specificity after trauma or surgery. Position to include the clinical site; minimize motion; document pain-limited positioning.
NaF-18 PET/CT
NaF is taken up in bone proportional to blood flow and osteoblastic activity. Typical adult activity is often ~5–10 mCi class (protocol-specific); uptake is rapid with imaging commonly ~30–60+ minutes after injection. PET/CT provides tomographic whole-skeleton surveys with high sensitivity for osteoblastic mets. Prep still includes hydration and voiding; follow PET timing and CT parameters per SOP.
Indications Map
| Clinical question | Typical approach |
|---|---|
| Osseous metastases (prostate, breast, lung, etc.) | Whole-body MDP/HDP ± SPECT; or NaF PET |
| Osteomyelitis vs cellulitis | Three-phase ( ± 4-phase, WBC/other as ordered) |
| Occult / stress fracture | Limited or whole-body delayed ± SPECT |
| RSD / CRPS | Three-phase periarticular pattern evaluation |
| Prosthesis pain | Three-phase ± complementary infection imaging |
| Paget, metabolic surveys | Whole-body delayed |
Patient Preparation
- Explain procedure length and need to hold still; screen pregnancy.
- Encourage oral hydration after injection unless fluid-restricted.
- Void before delayed imaging; catheterize if ordered for pelvic clarity.
- Remove jewelry and metal; document recent surgery, fractures, dental work, and injections (extravasation can cause soft-tissue hot spots).
- For flow phase: place camera over ROI before injection; secure IV; inject as a bolus.
Normal Variants and Interpretation Traps
| Finding | Trap / teaching point |
|---|---|
| Pediatric growth plates | Symmetric intense metaphyseal uptake—normal, not “mets” |
| Asymmetric uptake | Degenerative joint disease, weight-bearing stress, or prior trauma—correlate clinically |
| Skull suture / sinus activity | Can mimic lesions on planar views—SPECT/CT helps |
| Bladder / urine contamination | Pelvic false lesions—wash skin, change garments, post-void images |
| Injection site / extravasation | Soft-tissue focus away from skeleton |
| Renal failure | High soft-tissue background; consider delayed 24-h views |
| Flare phenomenon | Temporary increase in met intensity after therapy—clinical context |
| Cold lesions | Purely lytic non-reactive disease may be photopenic |
Extraosseous uptake (kidneys, stomach free Tc, liver-spleen colloid contamination) is a QC/pathology differential covered with agents—but on procedure day you still recognize and document it and notify the interpreting physician.
Three-Phase: Tech Quality Checklist
- Correct FOV for symptomatic region (include comparison side when useful).
- True dynamic flow—not a delayed “blood pool only” when flow was ordered.
- Identical positioning for blood-pool and delayed spots when possible.
- Whole-body or additional spots if multifocal disease is possible.
- SPECT/CT when ordered for spine/complex hardware.
Protocol Snapshot Table
| Protocol | When used | Key timing |
|---|---|---|
| Limited spots | Focal symptoms | Delayed 2–4 h |
| Whole-body | Staging / multifocal | Delayed 2–4 h |
| 3-phase | Infection / trauma / CRPS | Flow + pool + 2–4 h |
| 4-phase | Problem clearance / complex infection | Add ~24 h |
| SPECT/CT | Localization | After delayed planar or dedicated |
| NaF PET/CT | High-end bone survey | ~30–60+ min post-injection |
Bone procedure mastery is right phases + clean delayed images + variant awareness—that is what CNMT clinical items reward.
A three-phase bone scan is ordered for possible osteomyelitis of the foot. Which description correctly matches the phases the technologist must acquire?
Which patient-preparation step most improves soft-tissue clearance and reduces bladder artifact on a delayed Tc-99m MDP whole-body bone scan?
Intense symmetric metaphyseal uptake at the knees in a 10-year-old on a bone scan most likely represents: