When should I use computed tomography (CT) versus magnetic resonance imaging (MRI) for diagnostic imaging?

CT vs MRI: Choosing the Right Imaging Modality

MRI is the preferred modality for soft tissue evaluation, spinal cord pathology, joint assessment, and detecting subtle brain lesions, while CT excels at evaluating bone detail, acute trauma, calcifications, and when rapid imaging is essential. 1, 2

General Principles for Modality Selection

When to Choose MRI First

MRI should be your first choice for soft tissue masses, as it has become the technique of choice for detecting and characterizing these lesions due to superior soft-tissue contrast and multiplanar capabilities. 1 The American College of Radiology specifically states that literature does not support using MRI as the initial examination for a soft-tissue mass only when radiographs haven't been obtained first—but once basic imaging is done, MRI is preferred for further characterization. 1

• Brain and CNS pathology: MRI is more sensitive than CT for detecting small white matter lesions, subtle findings adjacent to the calvarium or skull base, and abnormalities in the posterior fossa and brainstem. 2, 3
• Spinal cord and nerve root evaluation: MRI detects soft tissue injuries including ligamentous disruption, spinal cord injury, and bone marrow edema that CT cannot visualize. 1
• Musculoskeletal soft tissue tumors: MRI demonstrates sensitivity of 97.1% and specificity of 93.3% compared to ultrasound's 85.7% and 83.3% respectively. 4
• Acute stroke/TIA: MRI detects acute ischemic lesions in 39% of TIAs versus only 8% with CT, and 86% of minor strokes versus 18% with CT. 5
• Inflammatory arthritis: MRI allows detection of synovitis, tenosynovitis, bone marrow edema, and early erosions with greater sensitivity than clinical examination or radiography. 6

When to Choose CT First

CT is the preferred initial modality for acute trauma, bone detail assessment, and when rapid imaging is critical for immediate management decisions. 1, 3

• Acute trauma to head or spine: CT is preferred for initial evaluation after acute trauma to identify fractures and hemorrhage rapidly. 3
• Suspected fractures in ankylosed spine: CT is typically the preferred modality due to technical difficulties with MRI in the acute posttraumatic setting, including patient immobility, motion artifact, and occasional lack of bone marrow edema at fracture sites. 1
• Bone mineralization and calcification: CT is the optimal imaging method to characterize soft-tissue mineralization, distinguish ossification from calcification, and identify characteristic patterns like the zonal pattern of myositis ossificans. 1
• Complex bony anatomy: CT is particularly useful when assessing mass mineralization in areas where osseous anatomy is complex or obscured, such as the skull base, temporal bone, and paraspinal regions. 1
• Chest, abdomen, and pelvis: CT is generally preferred for these body regions. 3

Specific Clinical Scenarios

Soft Tissue Masses

For deep or nonspecific soft tissue masses, obtain radiographs first, then proceed directly to MRI for characterization—CT serves only as an adjunct when MRI is contraindicated or to assess specific calcification patterns. 1

• Radiographs remain best suited for initial assessment to identify mineralization. 1
• MRI has superior soft-tissue contrast for tumor staging, showing muscle, bone, joint, and neurovascular involvement. 1
• CT is useful as a complementary study when MRI is contraindicated (pacemakers, large body habitus) or when distinguishing subtle calcification from enhancement. 1
• Critical pitfall: CT lacks the specificity of MRI for soft tissue characterization but can provide adequate staging data when MRI is not feasible. 1

Suspected Spinal Pathology in Children

In pediatric back pain with red flags and negative radiographs, MRI without contrast is the next appropriate step for most scenarios, while CT is reserved for specific osseous questions or when MRI is unavailable. 1

• MRI has superior soft tissue resolution for detecting infection, tumor, and spinal cord pathology. 1
• CT has increased sensitivity for nondisplaced fractures and spondylolysis compared to radiography. 1
• However, CT is less sensitive than MRI for detecting stress injuries involving the pars interarticularis without lysis, which is frequently seen in pediatric patients. 1
• When to add CT: If there is a question of calcifications after MRI, limited CT images of the area of interest can be obtained, potentially using dual-energy technique to avoid multiple scans. 1

Inflammatory Ear Disease

For complicated otitis externa or media with suspected intracranial extension, obtain CT temporal bone with IV contrast first to assess bone erosion and mastoid involvement, then add MRI if intracranial complications are suspected. 1

• CT temporal bone provides high spatial resolution to assess for coalescent mastoiditis and bone erosion. 1
• MRI is the preferred method over CT for intracranial complications due to higher sensitivity and specificity. 1
• Algorithm: Start with CT temporal bone with contrast → if intracranial complications suspected clinically or on CT, proceed to MRI head without and with contrast. 1

Primary Bone Tumors

MRI is generally the preferred modality for staging bone tumors, but CT provides complementary information about cortical bone destruction and mineralized matrix. 1

• MRI is superior to CT in detecting cortical bone destruction (in 4.5% of cases), marrow involvement (25%), soft-tissue involvement (31%), joint involvement (36.4%), and neurovascular invasion (15.3%). 1
• CT is superior to MRI for detecting cortical bone destruction in 13.6% of cases and neurovascular involvement in 7.7%. 1
• Practical approach: If both modalities are available, MRI is preferable, but CT provides valuable complementary data about bone detail and calcification patterns. 1

Axial Spondyloarthritis with Suspected Fracture

In patients with ankylosed spine and suspected fracture, CT is typically preferred initially, but MRI provides complementary diagnostic utility as some fractures are better detected on each modality. 1

• CT and MRI have similar sensitivities for fracture detection, with some fractures better detected on CT and others on MRI. 1
• In the setting of neurologic deficit, MRI should be performed to detect ligamentous disruption or spinal cord injury. 1
• Technical difficulties with MRI in acute trauma (patient immobility, motion artifact, spinal deformity) make CT typically preferred initially. 1

Key Contraindications and Limitations

MRI Limitations

• Pacemakers and certain metal implants (absolute contraindications). 1, 2
• Large body habitus preventing patient positioning. 1
• Severe claustrophobia. 2
• Inability to remain immobile during examination. 1
• Cannot reliably identify mineralization—this is MRI's inherent limitation. 1

CT Limitations

• Radiation exposure, particularly concerning in pediatric and young patients. 1, 7
• Poor soft tissue contrast compared to MRI. 1, 3
• Less sensitive for detecting small white matter lesions, subtle contusions, and early marrow changes. 2, 5

Critical Pitfalls to Avoid

• Never assume a normal CT excludes all significant intracranial pathology—studies show MRI can detect additional findings in up to 27% of patients with normal CT. 2
• Don't order CT after negative MRI without new symptoms—this exposes patients to unnecessary radiation without additional diagnostic value. 2
• Don't rely solely on radiographs to rule out navicular fractures or other high-risk stress fractures—proceed directly to MRI when clinical suspicion is high. 8
• Don't use CT as initial imaging for soft tissue masses—it is not typically ordered for this purpose and MRI is superior. 1
• Don't forget that distinguishing subtle calcification from enhancement on CT may be difficult without precontrast images—obtain at least some precontrast images when this distinction matters. 1