How to order a computed tomography (CT) scan for a patient with a soft mass on the dorsal aspect of the foot and pain, who has extensive spinal hardware and is unable to undergo magnetic resonance imaging (MRI)?

CT Imaging for Dorsal Foot Soft Tissue Mass When MRI is Contraindicated

Order CT of the foot with and without IV contrast, as CT is an appropriate alternative imaging modality when MRI is contraindicated, and contrast enhancement is essential for characterizing soft tissue masses and guiding management decisions. 1, 2

Imaging Protocol Specifications

CT Protocol Details

• Order CT foot without and with IV contrast to maximize diagnostic information 1, 2
• Precontrast images are essential because distinguishing subtle calcification from enhancement may be difficult or impossible without baseline non-contrast sequences 1, 2
• Postcontrast images allow visualization of tumor margins, extent, neurovascular involvement, and identification of viable tumor tissue 2, 3
• Request multiplanar reconstructions, as CT's multiplanar capability is ideally suited to depict the character of the interface between a soft-tissue mass and adjacent osseous cortex 1

Why CT is Appropriate in This Scenario

• CT is the study of choice when MRI is contraindicated or not feasible due to factors such as pacemakers, large body habitus, or in your case, extensive spinal hardware 1
• Although CT lacks the soft tissue specificity afforded by MRI in many cases, it provides appropriate staging data and anatomic information 1
• CT is particularly useful for assessment of mass mineralization in areas where osseous anatomy is complex, such as the deep soft tissues of the foot 1
• CT demonstrates soft tissue masses in all cases and can provide specific diagnoses in approximately 80% of benign masses 4

Initial Imaging Considerations

Start with Plain Radiographs First

• Obtain plain radiographs of the foot before CT, as radiographs remain the most appropriate initial imaging study for any soft tissue mass 1, 2, 5
• Radiographs can identify mineralization patterns, osseous involvement, and mass effect that may be diagnostic or guide subsequent imaging 2, 5
• Radiographs demonstrate positive findings in 62% of soft tissue masses, including calcification, bone involvement, and intrinsic fat 5
• If radiographs have already been obtained and are non-diagnostic, proceed directly to CT 1

Critical Advantages of Contrast Enhancement

Why Both Non-Contrast and Contrast Phases Matter

• Non-contrast images establish baseline attenuation values and allow identification of calcification, hemorrhage, and fat content 1, 2, 4
• Contrast-enhanced images reveal enhancement patterns that help differentiate viable tumor from necrotic regions, identify vascular lesions, and characterize the mass 2, 3
• Contrast enhancement allows better visualization of the mass's relationship to surrounding neurovascular structures 2, 3
• This information is crucial for selecting optimal biopsy sites if needed and for surgical planning 2, 3

Diagnostic Capabilities and Limitations

What CT Can Accomplish

• CT is superior to radiography in detecting zonal patterns of mineralization, essential for diagnosing conditions like early myositis ossificans 1
• CT allows distinction of ossification from calcification and identification of characteristic patterns of mineralization 1
• CT can provide specific diagnoses for hematomas, synovial cysts, myositis ossificans, fatty tumors, aneurysms, and abscesses based on attenuation values and enhancement patterns 4
• CT delineates the extent of masses and demonstrates their relation to surrounding structures, which is helpful for planning surgical excision or percutaneous biopsy 4

Limitations to Acknowledge

• CT cannot reliably distinguish benign from malignant soft tissue masses based on imaging characteristics alone 6, 4
• CT has lower soft tissue contrast resolution compared to MRI, making characterization of some lesions more challenging 1, 7
• Specific attenuation values are generally not helpful in differentiating between various types of tumors, except for lipomas, cysts, and some other specific entities 6, 4

Common Pitfalls to Avoid

Critical Mistakes in Ordering

• Do not order CT without contrast only, as this significantly limits diagnostic accuracy for soft tissue mass characterization 2, 3
• Do not skip precontrast images, as you will lose the ability to distinguish calcification from enhancement 1, 2
• Do not proceed to biopsy before obtaining cross-sectional imaging, as this violates fundamental principles of orthopedic oncology and can lead to sampling error and contamination of tissue planes 5, 3

Clinical Decision Points

• If the mass is >5 cm, deep-seated, or demonstrates concerning features on CT (heterogeneous enhancement, aggressive characteristics, bone invasion), refer to a specialized sarcoma center before biopsy 5, 3
• If CT demonstrates a simple cyst, lipoma, or other clearly benign entity with characteristic features, further imaging may not be necessary 4
• If CT findings remain indeterminate and clinical suspicion for malignancy is high, consider alternative MRI-compatible techniques or referral to a center with expertise in imaging patients with extensive hardware 1

Alternative Considerations

Ultrasound as Complementary Study

• Ultrasound may be useful as an adjunct for superficial masses on the dorsum of the foot, particularly for confirming fluid content of suspected ganglion cysts or identifying acute tenosynovitis 1
• However, ultrasound has limited utility for deep masses and should not replace CT in this scenario 1