For an otherwise healthy adult who heard an acute pop in the knee while walking and now has persistent pain, limited range of motion, and a normal X‑ray, what specific non‑contrast MRI study and protocol should be ordered?

MRI Knee Without IV Contrast

For your patient with an acute pop, ongoing pain, limited mobility, and negative X-ray, order a standard MRI knee without IV contrast. 1

Specific MRI Protocol

• Order: MRI knee without intravenous contrast 1, 2
• This is the American College of Radiology's recommended next imaging study after negative radiographs when internal derangement (meniscal or ligamentous injury) is suspected following acute trauma 1, 2
• Do not order MR arthrography or MRA – these are not routinely used for acute trauma evaluation and add unnecessary complexity and cost 1

Why This Specific Study

The "pop" your patient heard is highly suggestive of a structural injury, most commonly:

• Meniscal tear – the McMurray test has 97% specificity (though only 52% sensitivity), and joint line tenderness has 75% sensitivity for meniscal pathology 3, 4
• Anterior cruciate ligament (ACL) tear – the Lachman test is more sensitive and specific than the drawer sign for ligamentous tears 3
• Other ligamentous injuries – physical examination is moderately sensitive (74-87%) and highly specific (92-95%) for cruciate and meniscal damage 3

MRI without contrast provides superb soft-tissue resolution with multiplanar imaging capability, proven highly accurate for detecting meniscal injuries, ligamentous tears, and bone marrow contusions that may not appear on radiographs 1, 5

Standard MRI Sequences Included

A routine knee MRI protocol without contrast typically includes:

• Coronal, sagittal, and axial planes 1
• T1-weighted, T2-weighted, and proton density sequences 1
• Fat-suppressed sequences to highlight bone marrow edema and fluid 1

These sequences are sufficient to evaluate all internal structures without requiring intravenous contrast 1, 2

When to Consider Alternative Imaging

CT is NOT the appropriate next study unless you specifically suspect an occult fracture that requires better bony characterization 1. CT shows 100% sensitivity for tibial plateau fractures versus 83% for radiographs, but it has low sensitivity for soft-tissue injuries 1. Given your patient's presentation with a "pop" and limited mobility, soft-tissue injury is far more likely than occult fracture 1, 4.

Common Pitfalls to Avoid

• Don't order MRI as initial imaging – radiographs must come first to rule out fracture and satisfy evidence-based practice guidelines 1, 2
• Don't add IV contrast routinely – it adds cost, time, and potential adverse effects without improving diagnostic accuracy for traumatic internal derangement 1
• Don't order MR arthrography – this is reserved for specific indications like evaluating cartilage defects or labral pathology in chronic conditions, not acute trauma 1
• Avoid the temptation to "wait and see" – with ongoing pain and limited mobility after negative X-rays, MRI is appropriate now rather than delaying further 2, 6

Clinical Context

Your patient meets the criteria for MRI because they have:

• Acute trauma with audible "pop" (suggests structural disruption) 4
• Persistent pain despite negative radiographs 1, 2
• Limited mobility (functional impairment suggesting significant internal derangement) 2, 7

The American College of Radiology specifically designates MRI knee without IV contrast as "usually appropriate" (highest rating) for this exact clinical scenario 1, 2. Physical examination findings like effusion, positive McMurray test, or Lachman test would further support this decision, but are not required given the acute traumatic mechanism and ongoing symptoms 2, 3, 4.