11.2 Skeletal System Imaging & 3-Phase Bone Scans

Skeletal System Imaging

Skeletal imaging, commonly referred to as a bone scan, is one of the most frequently performed procedures in nuclear medicine. It is highly sensitive for detecting a wide range of osseous pathologies, including metastatic disease, primary bone tumors, osteomyelitis, stress fractures, and avascular necrosis. Unlike plain radiography, which requires a substantial change in bone density (often 30-50% mineral loss) to show a lesion, nuclear medicine bone scans detect changes in bone metabolism and blood flow, allowing for much earlier detection of disease.

Radiopharmaceuticals and Mechanism of Localization

The radiopharmaceuticals of choice for skeletal imaging are Technetium-99m labeled diphosphonates, specifically:

• Tc-99m MDP (Methylene Diphosphonate)
• Tc-99m HDP (Hydroxymethylene Diphosphonate)

Both agents localize in the bone via a process called chemisorption. When injected intravenously, the diphosphonate compounds chemically bond to the hydroxyapatite crystals that make up the mineral matrix of the bone. The degree of uptake is primarily dependent on two factors:

1. Osteoblastic Activity: Areas of active bone turnover and repair (such as a healing fracture or an osteoblastic metastasis) will accumulate significantly more radiopharmaceutical.
2. Regional Blood Flow: The tracer must be delivered to the site; therefore, hyperemic areas will show increased delivery and subsequent uptake.

The standard adult dose is typically 20 to 30 mCi (740-1110 MBq) injected intravenously. Following injection, the radiotracer rapidly clears from the blood pool and localizes in the skeleton. Approximately 50% of the injected dose is retained in the skeleton, while the remaining 50% is excreted by the kidneys into the urine. Because of this renal excretion pathway, the urinary bladder wall is typically the critical organ (the organ receiving the highest radiation dose). Patients are instructed to drink plenty of fluids and void frequently to minimize radiation exposure to the bladder.

The 3-Phase Bone Scan

While a standard whole-body bone scan only involves imaging 2 to 4 hours post-injection, many clinical indications require a more dynamic evaluation of blood flow and tissue pooling. This is where the 3-Phase Bone Scan is utilized. It is particularly valuable for differentiating between soft tissue infection (cellulitis) and bone infection (osteomyelitis).

Phase 1: Blood Flow (Radionuclide Angiography)

Phase 1 captures the initial arterial delivery of the radiotracer to the area of interest.

• Acquisition: Immediately upon bolus injection, dynamic images are acquired over the specific region of interest (e.g., a painful foot), typically at a rate of 1 to 3 seconds per frame for about 60 seconds.
• Purpose: Demonstrates regional arterial perfusion and identifies areas of focal hyperemia (increased blood flow).

Phase 2: Blood Pool (Tissue Phase)

Phase 2 captures the tracer as it equilibrates in the extracellular fluid space, before significant skeletal uptake has occurred.

• Acquisition: Static images are acquired immediately following the flow phase, usually between 1 to 5 minutes post-injection. The image is acquired for a set time (e.g., 5 minutes) or a set number of counts (e.g., 500,000 counts).
• Purpose: Demonstrates relative capillary dilation and soft tissue pooling. Increased pooling indicates soft tissue inflammation or hypervascularity.

Phase 3: Delayed Skeletal Phase

Phase 3 is the standard bone scan image, acquired after the tracer has had time to bind to the hydroxyapatite matrix and clear from the soft tissues.

• Acquisition: Static or whole-body sweep images are acquired 2 to 4 hours post-injection. The patient must empty their bladder immediately prior to imaging to prevent the bladder activity from obscuring the pelvic bones.
• Purpose: Demonstrates the actual rate of bone turnover and osteoblastic activity.

(Note: A "4th Phase" is occasionally performed at 24 hours for patients with poor renal function or severe peripheral vascular disease to allow for better background clearance.)

Clinical Interpretation: Osteomyelitis vs. Cellulitis

The 3-Phase Bone Scan is the gold standard in nuclear medicine for differentiating osteomyelitis from cellulitis.

• Cellulitis: An infection of the soft tissues. Because there is inflammation, blood flow and soft tissue pooling will be increased. However, the bone itself is not infected. Therefore, the scan will show increased flow (Phase 1) and increased blood pool (Phase 2), but the delayed bone phase (Phase 3) will show either normal or mildly, diffusely increased uptake (due to the adjacent hyperemia), but no focal, intense bony abnormality.
• Osteomyelitis: An infection of the bone itself. This condition provokes a strong osteoblastic repair response. The scan will show focal, intensely increased activity in all three phases: increased focal flow, increased focal blood pool, and intensely increased, focal uptake in the delayed skeletal phase.

The Role of SPECT and SPECT/CT

Planar whole-body imaging provides an excellent overview, but it suffers from the superimposition of overlying structures, particularly in complex anatomical areas like the spine, pelvis, and skull.

Single Photon Emission Computed Tomography (SPECT) solves this by acquiring 3D tomographic slices, allowing the physician to isolate specific structures (e.g., determining whether increased uptake in a vertebra is located in the vertebral body or the posterior elements).

Modern SPECT/CT hybrid systems combine the functional data of the bone scan with the high-resolution anatomical data of a low-dose CT scan. This fusion imaging drastically improves diagnostic specificity. For example, a hot spot in the spine on a planar scan might be ambiguous. SPECT/CT can definitively prove that the hot spot corresponds exactly to a benign osteophyte (bone spur) seen on the CT, instantly ruling out a metastasis.