What is the preferred imaging study for severe low back pain with red‑flag features suggesting nerve compression, infection, tumor, or fracture?

Best Imaging for Severe Low Back Pain with Red Flags

MRI lumbar spine without IV contrast is the preferred initial imaging study for severe low back pain with red-flag features suggesting nerve compression, infection, tumor, or fracture. 1

Clinical Context and Red Flag Assessment

When severe low back pain presents with red-flag features, prompt imaging is critical because delayed diagnosis significantly worsens outcomes for serious conditions like cauda equina syndrome, spinal infection, malignancy, or unstable fractures. 1 The American College of Radiology 2021 guidelines provide clear direction based on the specific red flag present.

Primary Imaging Recommendation by Clinical Scenario

For Suspected Nerve Compression or Cauda Equina Syndrome

MRI lumbar spine without IV contrast is the definitive initial study because it accurately depicts soft-tissue pathology, assesses vertebral marrow, and evaluates spinal canal patency with high sensitivity. 1 A prospective study demonstrates that urgent MRI assessment should be performed in all patients presenting with new-onset urinary symptoms in the context of low back pain or sciatica. 1

• Recent advances include a single 3-D heavily T2-weighted fat-saturated sequence protocol that serves as a rapid, highly sensitive tool for evaluating cauda equina syndrome in emergency departments. 1
• MRI detects multifocal deficits and progressive neurologic deficits better than any other modality. 1

For Suspected Infection

MRI lumbar spine without and with IV contrast is the preferred study due to its high sensitivity and specificity for spinal infection. 1 The addition of IV contrast with fat suppression is invaluable for identifying epidural and paraspinal abscesses and distinguishing abscess from phlegmon. 1

• MRI detects infection before bone destruction becomes evident on CT or radiography. 1
• CT lumbar spine without IV contrast is less sensitive and specific but can be obtained to evaluate associated osseous abnormalities like pathologic fracture or bony destructive change. 1

For Suspected Malignancy or Metastatic Disease

MRI lumbar spine without and with IV contrast is superior for localizing disease (intramedullary, intradural-extramedullary, and extradural) and assessing lesion extent. 1 Both bony/marrow involvement and neural compression from epidural tumor are visualized with high spatial resolution. 1

• History of cancer is the only red flag with proven increased probability of finding spinal malignancy on systematic review. 1
• MRI offers greater specificity than bone scan with comparable sensitivity and provides superior anatomic detail. 1
• Radiography requires at least 50% bone erosion before changes become noticeable, making it inadequate for early detection. 1

For Suspected Fracture

The imaging approach depends on fracture mechanism and patient risk factors:

For osteoporotic or minor trauma fractures: Initial evaluation with plain radiography (anteroposterior and lateral views) is useful in patients with history of osteoporosis or steroid use. 1 Upright radiographs provide functional information about axial loading, and flexion-extension views can evaluate spine stability. 1

For determining fracture acuity or malignant vs. benign compression fractures: MRI lumbar spine without IV contrast is superior because it demonstrates bone marrow edema (indicating acute fracture), spinal canal compromise from displaced/retropulsed fractures, and distinguishes pathologic from benign fractures. 1 Convex posterior vertebral body border, extension into posterior elements, and abnormal marrow signal suggest pathologic fracture. 1

If malignancy, infection, or inflammation is suspected as the fracture etiology: MRI lumbar spine without and with IV contrast should be obtained. 1

Alternative Imaging Modalities

CT Lumbar Spine

CT without IV contrast can evaluate osseous integrity (pathologic fracture, bony destructive change) when tumor is involved, but intradural and spinal cord pathologies are poorly depicted. 1 CT is inferior to MRI for soft tissue and neural structure evaluation. 2

CT Myelography

CT myelography assesses spinal canal/thecal sac patency and neural foramen but requires lumbar puncture for intrathecal contrast injection, making it less desirable. 1 It has safety advantages for patients with MRI-incompatible implanted devices and can be useful when significant metallic hardware artifact degrades MRI quality. 1

Critical Timing Considerations

Prompt work-up with MRI is mandatory when severe or progressive neurologic deficits are present or serious underlying conditions are suspected, as delayed diagnosis and treatment result in poorer outcomes. 1 Do not delay imaging for a trial of conservative therapy when red flags are present. 1, 3

Common Pitfalls to Avoid

• Do not order plain radiography alone for suspected nerve compression, infection, or early malignancy—it lacks sensitivity for these conditions. 1
• Do not use MRI with IV contrast alone as an initial study; interpretation requires correlation with standard noncontrast sequences. 1
• Avoid routine advanced imaging in uncomplicated acute low back pain without red flags, as it does not improve outcomes and identifies abnormalities poorly correlated with symptoms. 1, 4
• Be aware that MRI findings (such as bulging disc without nerve root impingement) are often nonspecific and common in asymptomatic individuals. 1, 4