What will a Computed Tomography (CT) scan show that a Magnetic Resonance Imaging (MRI) won't?

CT vs MRI: Unique Diagnostic Capabilities of CT

CT is superior to MRI for detecting and characterizing calcification, evaluating acute hemorrhage, assessing cortical bone destruction, and providing rapid imaging in emergency settings or when MRI is contraindicated.

Calcification Detection and Characterization

CT excels at identifying mineralization patterns that MRI cannot detect:

• CT is the optimal imaging method to characterize soft-tissue mineralization, allowing distinction between ossification and calcification and identification of characteristic mineralization patterns 1
• CT can identify the zonal pattern of mineralization essential for diagnosing early myositis ossificans, while radiography and MRI remain nonspecific 1
• MRI cannot detect calcium in mediastinal masses or other tissues, whereas CT provides superior sensitivity to dystrophic calcification in neoplasms 1
• Scattered calcifications in enlarged lymph nodes from old granulomatous disease are not detectable by MRI but are readily visible on CT 2

Cortical Bone Evaluation

CT provides superior assessment of bone cortex:

• CT better defines destruction of cortical bone compared to MRI, with CT being superior to MRI in detecting cortical bone destruction in 13.6% of patients in one prospective study 1
• The multiplanar capability of CT is ideally suited to depict the character of the interface between soft-tissue masses and adjacent osseous cortex for assessing cortical remodeling or invasion 1
• CT gives a better picture of the destruction of cortical bone and provides greater detail of bone structures, potentially showing effects of tumors on the skull 1, 3

Acute Hemorrhage Detection

In emergency settings, CT has critical advantages:

• CT is ideal for rapid assessment of acute intracranial hemorrhage, ventriculomegaly, and shunt-related issues 1
• CT is recommended in the acute or immediate postoperative setting when rapid evaluation is needed 1
• CT and MRI show equal ability to identify acute intracerebral hemorrhage, but CT is more practical in emergency situations 1

Practical Clinical Advantages

CT offers important logistical benefits:

• CT has shorter acquisition time and sedation is generally not needed, making it preferable when rapid imaging is required 1
• CT is the study of choice in patients for whom MRI is contraindicated or not feasible due to pacemakers, large body habitus, or unsafe implants 1
• MRI was not feasible in 20% of acute stroke patients due to contraindications, impaired consciousness, hemodynamic compromise, or agitation, with 73% of ineligible patients having intracerebral hemorrhage 1

Specific Anatomic Applications

Mediastinal Imaging

• CT can identify associated aneurysms, arteriovenous malformations, or tumors when performed with contrast 1
• CT angiography may provide additional detail in patients with suspected vascular abnormalities 1

Skull Base and Complex Anatomy

• CT is particularly useful in assessment of mass mineralization in areas where osseous anatomy is complex or obscured 1
• CT may reveal bony defects within the foramen cecum or intracranial calcifications in frontonasal dermal sinus tracts, complementing MRI's soft tissue evaluation 1

Important Caveats

While CT has these specific advantages, clinicians should recognize its limitations:

• CT has limited soft tissue contrast compared to MRI, restricting evaluation of soft tissue masses, marrow involvement, and neurovascular structures 1
• Small lung nodules may be better detected on CT than MRI due to respiratory motion and partial-volume averaging on MRI 2
• CT involves radiation exposure, which is a consideration particularly in pediatric patients 1

The key principle: Use CT when calcification detection, cortical bone detail, acute hemorrhage assessment, or rapid/emergency imaging is the priority; use MRI for superior soft tissue characterization and detailed anatomic staging 1.