12.3 Hematopoietic and Bone Marrow Imaging
Key Takeaways
- Tc-99m sulfur colloid maps functional reticuloendothelial marrow (and liver/spleen); normal adult marrow activity is predominantly axial (spine, pelvis, proximal humeri/femurs) with limited distal appendicular uptake
- Marrow imaging supports oncology (marrow expansion, replacement, radiocolloid shift) and infection workups when paired with labeled WBC or bone scintigraphy
- Combined In-111 (or Tc-99m) WBC + sulfur colloid marrow studies distinguish osteomyelitis (WBC uptake without congruent marrow activity) from expanded or reinstalled marrow (congruent WBC and colloid)
- Marrow expansion into distal extremities, colloid shift to spleen/marrow in cirrhosis, and photopenic marrow defects from tumor or fibrosis are high-yield patterns
- Traps: free Tc stomach/thyroid activity, wrong particle size, interpreting isolated bone-scan hyperemia as infection without cell/marrow correlation, and expecting sulfur colloid to label active osteoblasts like MDP
12.3 Hematopoietic and Bone Marrow Imaging
Quick Answer: Tc-99m sulfur colloid → RES phagocytosis maps functional marrow (plus liver/spleen). Adults: axial skeleton dominates. Use marrow scans for expansion/replacement questions and—critically—with labeled WBCs to separate osteomyelitis from marrow packing. Sulfur colloid is not MDP; it does not bind hydroxyapatite osteoblasts.
Why Image Marrow?
Bone marrow nuclear studies answer distribution and function questions that plain radiographs miss:
- Is marrow expanded into distal long bones (stress response, hemoglobinopathy, marrow-stimulating states)?
- Is a photopenic defect tumor, fibrosis, radiation, or infarction replacing RES cells?
- Does a hot focus on WBC scan represent infection or merely active/reconstituted marrow after surgery or prosthesis workup?
- Is there colloid shift (increased spleen/marrow, decreased liver) suggesting hepatic RES dysfunction (classic cirrhosis teaching)?
MRI and FDG PET often dominate oncology staging, but CNMT still tests sulfur-colloid marrow logic and WBC + marrow congruence for infection.
Radiopharmaceutical and Mechanism
| Item | Teaching points |
|---|---|
| Agent | Tc-99m sulfur colloid (unfiltered for RES/marrow; do not confuse with filtered lymphocolloid) |
| Mechanism | Particles phagocytized by Kupffer cells, splenic macrophages, and marrow RES |
| Normal hierarchy | Liver > spleen > bone marrow activity in healthy adults |
| Typical adult activity | Often ~8–10 mCi IV for marrow-focused protocols (liver-spleen doses may be ~4–6 mCi—follow order) |
| Imaging | Planar whole-body or spot marrow fields shortly after injection (distribution is rapid) |
Particle-size trap: filtered small-particle colloid is for lymphoscintigraphy, not standard marrow mapping. Kit boiling/prep errors change particle size and biodistribution. Free Tc-99m adds stomach/thyroid activity that is not marrow.
Normal Marrow Distribution
| Age / state | Expected pattern |
|---|---|
| Healthy adult | Axial: vertebrae, pelvis, sternum, ribs; proximal humeri/femurs; little distal extremity marrow |
| Child | More extensive appendicular marrow (age-dependent conversion to fat) |
| Marrow expansion | Activity extends into distal femurs, tibiae, skull—seen in chronic anemia, marrow stimulants, some myeloproliferative states |
| Radiation ports | Regional photopenia where RES destroyed |
| Tumor replacement | Focal or diffuse photopenic defects (nonspecific without correlation) |
Remember: sulfur colloid shows RES-containing marrow, not every hematopoietic stem-cell niche identical to MRI fat-fraction maps. Correlation prevents overcalling.
Oncology-Related Uses
Although less common as a first-line staging tool today, marrow scintigraphy still teaches:
| Pattern | Possible significance |
|---|---|
| Focal cold defects | Metastatic replacement, avascular necrosis, fibrosis |
| Diffuse decreased marrow | Extensive replacement, prior wide-field radiation, marrow toxins |
| Peripheral expansion | Compensatory hematopoiesis |
| Colloid shift | Hepatic dysfunction with relatively increased spleen/marrow uptake |
Correlate with chemo, G-CSF, and radiation history—growth factors expand marrow and complicate FDG and colloid reads.
Infection Workups: Marrow vs WBC vs Bone Scan
Suspected osteomyelitis—especially with prostheses, neuropathic feet, or post-surgical bone—often needs more than a three-phase bone scan.
Three-phase bone scan (reminder)
Tc-99m MDP/HDP reflects blood flow, blood pool, and osteoblastic activity. Hyperemia and delayed uptake occur in infection and in sterile arthritis, fracture healing, and loose hardware—sensitive, not specific.
Labeled leukocytes
In-111 oxine WBCs or Tc-99m HMPAO WBCs migrate to infection. After orthopedic surgery, WBCs also accumulate where marrow was redistributed or remains active—creating false-positive “infection” if read alone.
Dual-study rule (highest yield in this section)
Perform WBC imaging + Tc-99m sulfur colloid marrow imaging (often sequential or dual-isotope with In-111 WBC):
| WBC focus | Sulfur colloid at same site | Interpretation teaching |
|---|---|---|
| Increased | Absent / photopenic (no congruent marrow) | Favors osteomyelitis |
| Increased | Congruent increased marrow activity | Favors marrow packing / expanded marrow, not infection |
| None | Normal or expanded marrow | No leukocyte-avid infection pattern |
This congruence logic is repeatedly tested: discordant WBC-positive / marrow-negative = infection pattern; matched WBC and marrow = marrow.
Practical workflow notes
Label cells with rigorous patient ID and reinject only the donor; image WBC at protocol delays (Tc HMPAO earlier; In-111 ~4 and 24 h); match marrow projections; use SPECT/CT for hardware. Neuropathic foot bone scans are often diffusely positive—add WBC ± marrow.
Comparison Table for Exam Cramming
| Modality | What it maps | Strength | Limitation |
|---|---|---|---|
| Sulfur colloid marrow | RES marrow distribution | Expansion/replacement; WBC congruence | Not osteoblast-specific; liver/spleen dominate counts |
| MDP/HDP bone scan | Osteoblastic turnover + flow | Sensitive for bone reaction | Nonspecific for infection |
| Labeled WBC | Neutrophil-avid inflammation | Infection localization | Marrow and surgical uptake mimic infection |
| Ga-67 (legacy) | Iron-analog inflammation/tumor | Selected FUO/spine contexts | Bowel activity; slow; largely replaced |
| FDG PET/CT | Glucose metabolism | Oncology marrow lesions; some infection | Uptake in marrow stimulation, recent treatment |
Technical and Interpretation Traps
| Trap | Result |
|---|---|
| Calling every distal marrow activity “metastasis” | Misses benign expansion physiology |
| Reading WBC without marrow after arthroplasty | False-positive osteomyelitis |
| Expecting sulfur colloid to behave like MDP | Wrong mechanism and wrong indications |
| Free Tc or colloid impurities | Stomach/thyroid or altered RES map |
| Ignoring liver-spleen saturation / colloid shift | Misjudge relative marrow intensity |
| Pediatric pattern applied to adults | Overcalls normal childhood appendicular marrow |
When Is Marrow Imaging Still Ordered?
Problem-solving infection (WBC congruence around hardware), selected oncology expansion/replacement questions when MRI/PET are unavailable, and liver-spleen/colloid-shift studies that display marrow.
Bottom line: memorize axial adult marrow, recognize expansion, and apply the WBC–colloid congruence rule.
What is the normal adult distribution pattern on a Tc-99m sulfur colloid bone-marrow map?
On a combined labeled-WBC and sulfur-colloid marrow study around a prosthesis, which pattern most favors osteomyelitis?
Which statement correctly contrasts Tc-99m sulfur colloid marrow imaging with Tc-99m MDP bone scintigraphy?