What type of radiographic study is indicated for a 64-year-old woman with four days of right foot swelling, pain, a red spot, and a bruised sensation?

Plain Radiographs of the Right Foot

Order plain radiographs (X-rays) of the right foot as the initial imaging study for this 64-year-old woman with foot swelling, pain, and erythema. 1

Rationale for Plain Radiographs First

The ACR Appropriateness Criteria (2022) explicitly state that radiographs should be used for the initial evaluation of musculoskeletal infections, including osteomyelitis, septic arthritis, and soft tissue infection 1. This represents the highest quality guideline evidence available for this clinical scenario.

What Plain Films Accomplish

Plain radiographs serve multiple critical functions in this presentation:

• Exclude fractures and tumors as alternative causes of swelling and pain 1
• Identify soft tissue swelling, joint effusion, gas, and foreign bodies that may indicate infection 1
• Detect bony changes including erosions, periosteal reaction (acute osteomyelitis), or bone sclerosis (chronic osteomyelitis) 1
• Provide anatomic overview essential for interpreting subsequent advanced imaging if needed 1

Important Caveat About Timing

In early acute osteomyelitis (<14 days), radiographs may be normal or show only mild soft tissue swelling 1. Since this patient has only 4 days of symptoms, plain films may not reveal bony changes even if osteomyelitis is present. However, they remain the mandatory first step and can identify other important pathology 2.

When to Proceed to Advanced Imaging

If radiographs are negative or indeterminate and clinical suspicion remains high:

• MRI without or with IV contrast becomes the next appropriate study for suspected osteomyelitis or soft tissue infection 3, 1
• MRI demonstrates excellent soft tissue contrast and high sensitivity to marrow abnormalities 3
• MRI can identify alternative diagnoses including soft tissue infection, abscess, tumor, neuropathic arthropathy, and fracture 3

Ultrasound may be considered for superficial soft tissue assessment if available, particularly for evaluating fluid collections that might require aspiration 1.

What NOT to Order Initially

The guidelines explicitly state insufficient evidence to support initial use of:

• CT of the foot 1
• MRI as first-line imaging 1
• 3-phase bone scan 1

These modalities have roles in specific scenarios but should not bypass plain radiographs as the initial study.

Clinical Context Matters

While the presentation (swelling, pain, erythema, bruised sensation) raises concern for infection, the differential diagnosis includes:

• Cellulitis or soft tissue infection
• Trauma/occult fracture
• Gout or other inflammatory arthritis
• Venous insufficiency
• Tumor

Plain radiographs help narrow this differential efficiently and cost-effectively before proceeding to more expensive or time-consuming studies 1.