Approach to Musculoskeletal Imaging

Approach to Musculoskeletal Imaging

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Skeletal scintigraphy is one of the most common procedures performed in a nuclear medicine department. Patients are referred for skeletal pain, trauma, vascular compromise of bone, malignancy, infection, arthritis, and orthopedic surgery/hardware. Imaging can be performed as a 3-phase protocol, limited area, whole-body, &/or SPECT/CT. The 2 most common radiopharmaceuticals used are Tc-99m methyl diphosphonate (MDP) and Tc-99m hidroxydiphosphonate (HDP). Both are bone-seeking agents that undergo physicochemical adsorption (chemisorption) to the hydroxyapatite structure of bone. Therefore, more uptake occurs in bone-forming pathology or growth regions of immature bone in the presence of osteoblasts. Uptake depends on blood flow to the bone and extraction efficiency (bone turnover).

During the 24 hours following IV injection of Tc-99m MDP, ~ 50% of the dose is retained in the skeleton, and ~ 50% is excreted in the urine. Clearance from the blood is rapid, with ~ 10% of the injected dose remaining at 1 hour and < 5% at 2 hours. This allows for early imaging with high bone-to-background ratio. Normal uptake is homogeneous and most significant in the axial and proximal appendicular skeleton. Faint visualization of the kidneys is normal.

Detailed patient history should include history of trauma, presence of hardware, bone surgeries, prior radiation/chemotherapy, prior bone scans, and correlative imaging. Laboratory values regarding serum tumor markers and markers of bone turnover should be reviewed. Physical examination may be useful in the evaluation of extremity ulcers to correlate location of ulcer and findings on bone scan.

Patient preparation is important. Generally, drinking ~ 16 oz of water between radiopharmaceutical injection and imaging is recommended for better target-to-background activity. Prior to imaging, any removable metallic objects must be removed. Patient should be asked to void immediately prior to whole-body or pelvic imaging to reduce activity in the urinary bladder.

Delayed-phase whole-body or limited area scans typically occur 2-4 hours after radiopharmaceutical injection. Consider a longer time delay in patients with poor renal function or dehydration. Low-energy, high-resolution parallel hole collimators are preferred while pinhole or diverging collimators are helpful for imaging smaller osseous structures. SPECT/CT allows for 3D imaging and improved localization of abnormalities with CT.

F-18 NaF PET/CT bone scan has reemerged as a method to identify osseous metastatic disease, including localization and determination of extent of disease. The resolution and pharmacokinetics are far greater than Tc-99m-based bone agents and the total exam time is shorter. However, the radiation exposure to the patient is higher. The mechanism of uptake is incorporation of fluoride ions into the bone matrix in sites of newly mineralizing bone, such as during growth, infection, malignancy (primary or secondary), after trauma, or during inflammation. During shortages of Tc-99m, F-18 PET/CT bone scan offers an alternative for skeletal imaging.

Imaging Protocols

Whole-Body Imaging

The utility and advantage of bone scans over radiographs or MR is their ability to screen the entire skeletal system for disease. In patients with a known malignancy, a whole-body bone scan is performed in the anterior and posterior projections. Additional static images of the calvarium in lateral projections and images to fully include the upper extremities are often performed. Whole-body imaging is also important in looking for polyostotic disease in patients with fibrous dysplasia, Paget disease, histiocytosis, enchondromas, or osteochondromas. Similar utility is shown for multifocal joint-centered symptoms, screening all involved joints.

Limited Bone Scan

Planar regional images have better resolution than whole-body examinations. These limited bone scans are often used to address a very specific clinical question. For example, thoracic static views can be obtained to diagnose an occult rib fracture. To address a specific sclerotic lesion on a radiograph or CT, a limited bone scan could be performed to assess the uptake of tracer associated with the lesion. However, a whole-body bone scan would be more useful to exclude a multifocal aggressive osseous process.

3-Phase Bone Scan

The 3-phase bone scan offers sensitivity and specificity. The 1st phase addresses the degree of blood flow to the region of interest while the 2nd phase evaluates the degree of hyperemia in the soft tissues and adjacent bone. Delayed phase determines the amount of bone formation occurring. One of the most common indications for a 3-phase bone scan is the evaluation of osteomyelitis in the setting of a diabetic foot ulcer, prior orthopedic fixation, or joint replacement. Noninfectious indications include evaluation and staging of complex regional pain syndrome and evaluating for avascular necrosis.

Pinhole Collimator Images

A pinhole collimator is indicated if very high-resolution images of a specific area are needed to evaluate small structures, such as wrist bones or pediatric hips for slipped capital epiphysis. The magnification of the object increases as the pinhole collimator is moved closer to the structure being imaged.


SPECT imaging allows for better image contrast and precise lesion localization. In complex anatomical structures, such as the spine, SPECT allows for greater accuracy in the determination of the source of abnormal activity in addition to lesion detection and characterization. SPECT is an added procedure rather than a stand-alone procedure in that whole-body images or limited bone scan are performed initially followed by SPECT imaging. SPECT/CT has even more advantage of lesion localization but should be used judiciously in the context of radiation exposure, particularly in pediatric patients.

May 7, 2023 | Posted by in CARDIOVASCULAR IMAGING | Comments Off on Approach to Musculoskeletal Imaging
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