Nuclear Medicine: Skeletal System Quiz

Nuclear Medicine: Skeletal System

• Bone scans provide earlier diagnoses and reveal more lesions compared to radiographic procedures.
• Technetium-labeled diphosphonates, especially methylene diphosphonate, are common radiopharmaceuticals for skeletal imaging.
• Diphosphonates have rapid renal excretion, leading to a high target-to-non-target ratio within 2 to 3 hours of injection. Approximately 50% to 60% of the activity localizes in bone.
• Maximal skeletal uptake occurs around 5 hours post-injection.
• The biologic half-life of most diphosphonates is about 24 hours.
• Avoiding air injection during phosphate radiopharmaceutical preparation is crucial to prevent technetium oxidation, which impairs phosphate tagging.
• Administering the radiopharmaceutical more than 4 hours after preparation may result in gastric and thyroid visualization on bone scans due to free pertechnetate.
• Technetium accumulation in bone is influenced by blood supply (a fourfold increase in blood flow enhances bone uptake by 30% to 40%), capillary permeability, local acid-base balance, fluid pressure within bone, hormones, vitamins, quantity of mineralized bone, and bone turnover.
• Bone scans aid in diagnosing osteomyelitis, septic arthritis, metabolic bone disease, unexplained musculoskeletal pain, and suspected non-accidental injury, tumors (primary or secondary).
• Bone scans also help in the detection and follow-up of metastatic disease, differentiation between osteomyelitis and cellulitis, assessing bone viability (infarction or avascular necrosis), evaluating fractures that are difficult to assess on x-rays (stress fractures), assessing prosthetic joint integrity for infection or loosening, and determining biopsy site.
• Routine planar scans typically involve intravenous injection of 10 to 20 mCi (370 to 740 MBq) of technetium diphosphonate.
• Imaging is done 2 to 4 hours after injection. The injection site should be away from any suspected osseous pathology.
• For suspected osteomyelitis or cellulitis, a three-phase study (radionuclide angiogram, initial blood pool image, and routine images at 2-3 hours) is performed.
• In rare cases, a four-phase study (additional images 18-24 hours post-injection) may be necessary for patients with renal failure due to poor soft-tissue clearance.
• Following IV injection, MDP circulates in the vascular system briefly before equilibrating to the extravascular space.
• MDP rapidly accumulates in bone and is excreted via the urine.
• Approximately half the administered dose is eliminated within 4 hours creating a high bone-to-background activity ratio, excluding cases of poor renal function.
• Good oral hydration and bladder emptying are crucial before scanning.
• Two to four hours after injection, whole-body imaging (using a mobile table) is performed using either a dual-head gamma camera (simultaneous anterior and posterior views) or a single-head gamma camera (spot views).

Imaging Technique

• Oblique views of the sternum and ribs, lateral views of the lower legs and pelvis are often part of the study.
• Magnification views may enhance visualization of hands, wrists (adults), and hip joints (pediatric patients) using selective pinhole or high-resolution collimators.
• Increased vascularity and permeability contribute to early MDP accumulation in bone tumors, healing trauma, inflammatory, and infected conditions.

Normal Appearances in Bone Scans

• In a normal adult, individual bones are visible and symmetrical.
• Minor soft tissue uptake, particularly in obese patients, is usually observed.
• Kidneys and the urinary bladder should be identifiable.
• Increased activity is often observed around epiphyseal plates in a 15-year-old.
• Focal maxillary/mandibular alveolar ridge activity could stem from dental disease.
• Increased lower cervical spine activity could reflect degenerative changes or spinal lordosis.
• Increased activity in the knees, a common occurrence in elderly patients, could signal arthritis.
• Increased activity in the sternum, sternoclavicular joints, acromioclavicular joints, shoulders, iliac crests, and hips are also frequently seen.
• The bones of the thoracic spine, scapulae, and sacroiliac joints are well visualized.

Abnormal Appearances in Bone Scans

• Asymmetric osseous activity warrants suspicion.
• Focal space-occupying lesions causing photopenic defects in the renal cortex should be investigated.
• Asymmetric renal activity is possible due to accumulation in extra-renal regions.
• Additional images may be obtained if urinary tract obstruction is suspected.

Distinguishing Issues

• Extravasation of radiopharmaceutical at the injection site can lead to lymph node visualization.
• Metastatic disease is characterized by multiple focal areas of intense MDP uptake often localized in the axial skeleton but can show asymmetry.
• Increased activity over the course of examinations may indicate worsening metastases (size, number, intensity of uptake).
• A bone lesion visible in a bone scan that isn't visible in a standard x-ray should lead to further suspicion.

Bone Scan Comparison with Radiography

• Localized demineralization (30-50%) is needed for a lytic lesion to appear in a radiograph.
• Bone scans detect metastatic lesions earlier when compared to radiographic studies.
• Radiographic skeletal surveys have a 50% false-negative rate for certain tumor types. - Bone scanning for common neoplasms may have a low false-negative rate (~2%)

Indications for Bone Scans in Cancer Patients

• Bone scans are highly effective in detecting metastases in patients with bone pain (about 80% of patients with known neoplasms who have bone pain).
• In asymptomatic patients with suspected metastases, bone scan use should be considered for tumors with a tendency towards bone metastasis (e.g., breast, lung, prostate), however, bone scan use may not be cost-effective for tumors with a low propensity for bone metastasis (e.g., colon, cervix, uterus, head, neck).
• Follow-up bone scans in advanced breast and prostate cancer patients require cautious interpretation due to the potential of a "flare phenomenon." The first 3 months of chemotherapy can lead to an increased uptake as tumors heal; if this is not clinically correlated with improvement, it may give a false impression of either new or worsening metastasis.
• Bone lesions arising 6 months or more after treatment indicate disease progression.

Malignant Bone Tumors

• Osteosarcoma, often diagnosed earlier in its pulmonary rather than bone metastasis stage. However, aggressive chemotherapy has changed the natural history of Osteosarcoma, now showing osseous metastasis in ~20% of cases, thus making follow-up scans now essential.
• Ewing sarcoma is a moderately uncommon primary bone tumor that often affects the pelvis, ribs, and femurs, and in ~11% of cases, it displays osseous metastasis.

Benign Osseous Neoplasms

• Malignant lesions initially accumulate large amounts of radiopharmaceutical.
• An early blood-pool image may be helpful in detecting benign lesions because they often show minimal or no increased uptake.
• Osteoid osteomas are an exception to the rule. They demonstrate high uptake on delayed scans. Other benign lesions such as Hemangiomas show minimal uptake.

Trauma

• Fractures invisible on traditional radiography (x-rays) may be identified with CT, MRI, or radionuclide bone scanning.
• Fractures that occur in occult locations are best identified with MRI.
• Bone scanning is preferable for identifying multi-focal injuries suspected.

Fractures (Bone Scan Appearance)

• Acute fracture phase (3-4 weeks): Diffuse, generalized increase in radionuclide activity around fracture site.
• Subacute phase (2-3 months): localized and intense activity at the fracture site.
• Healing phase (longer periods): gradual decline in radiotracer intensity.
• Pelvic and spine fractures may show elevated activity in the first few days in ~30% of cases.
• All recent axial skeleton and long bone fractures are discernible by 14 days.
• Skull fractures typically don't show increased activity.
• Rib fractures frequently demonstrate increased activity in consecutive ribs; however, it can be difficult to distinguish from a metastasis.
• Occult hip fractures take ~3 days to appear on a bone scan which may prompt use of MRI for earlier detection.

Stress Fractures

• Difficulty in visualizing on plain radiographs (7-10 days).
• Radionuclide bone scans are often positive at presentation, enabling early diagnosis and treatment.

Osteomyelitis, Cellulitis, and Septic Arthritis

• Early bone involvement from inflammation is often undetectable on radiographs.
• MRI offers excellent spatial resolution but is cost-prohibitive for patients with infected metallic prostheses.
• Diagnosis with radionuclides tends to show increased activity in both soft tissues and bony regions.

Radiopharmaceuticals for Osteomyelitis

• 99mTc-diphosphonate
• Indium-111 or 99mTc-labeled leukocytes
• 18F-FDG, gallium-67 citrate (less common)
• Differentiating osteomyelitis from cellulitis using a 99mTc diphosphonate bone scan typically involves a three-phase approach including a radionuclide angiogram, immediate blood pool image, and routine images taken 2-3 hours after injection.

Cellulitis vs Osteomyelitis

• Cellulitis often shows increased blood flow and soft tissue activity on early images, with decreasing activity on subsequent scans; no significant focus of bone activity is observed on delayed scans.
• Osteomyelitis tends to show increased blood flow and blood pooling with bone activity that becomes more intense and focal on delayed scans.

Septic Arthritis

• Bone scan activity is increased in all phases.
• Differentiating septic arthritis from osteomyelitis is usually indicated by diffusely increased activity on both sides of the joint (compared to focal activity on one side with osteomyelitis).

Metabolic Bone Disease (Osteoporosis)

• Osteoporosis typically involves reduced bone density and an increased fracture risk, common in elderly women.
• Vertebral compression fractures are a hallmark of osteoporosis.
• The classic appearance in an osteoporosis bone scan is a vertebral body that has reduced height with intense linear MDP uptake crossing its width.
• Sacral insufficiency fracture (a common osteoporosis fracture) reveals visible vertical, linear increased uptake through the sacral ala that is bridged by horizontal uptake across the sacral body, creating a pathognomonic "H" sign.
• Bone densitometry is often used for assessing bone density.