What is the difference between Computed Tomography (CT) and Magnetic Resonance Imaging (MRI)?

CT vs MRI: Key Differences in Clinical Practice

CT and MRI serve complementary but distinct roles in diagnostic imaging, with CT excelling at rapid acquisition, cortical bone evaluation, and calcification detection, while MRI provides superior soft tissue contrast and is preferred for most tumor staging and neurological applications. 1, 2

Fundamental Technical Differences

CT (Computed Tomography):

• Uses ionizing radiation (X-rays) to create cross-sectional images based on tissue density variations 1
• Average radiation dose of 3 mSv (equivalent to approximately 150 chest x-rays) 1
• Scan time typically <5 minutes 1
• Larger scanner bore, better tolerated by claustrophobic patients 1

MRI (Magnetic Resonance Imaging):

• Uses strong magnetic fields and radio waves to excite hydrogen atoms, generating images based on tissue relaxation properties 1
• No ionizing radiation exposure 1
• Scan time typically exceeds 30 minutes 1
• Contraindicated with certain implantable devices (pacemakers, some neurostimulators) 1

Clinical Advantages of CT

Speed and Accessibility

• CT is the modality of choice when rapid imaging is essential, particularly in emergency settings, acute trauma, and unstable patients 2, 3
• More readily available and costs considerably less than MRI 1
• Sedation generally not needed due to shorter acquisition time 2

Cortical Bone and Calcification

• CT is superior for detecting cortical bone destruction, outperforming MRI in 13.6% of patients in prospective studies 1, 2
• Optimal for characterizing soft-tissue mineralization patterns, distinguishing ossification from calcification 2
• Better defines fine bony details and identifies microcalcifications that MRI cannot detect 2, 4
• Essential for detecting subtle matrix mineralization in bone tumors (demonstrated in 85% of telangiectatic osteosarcomas) 1

Acute Hemorrhage Detection

• Ideal for rapid assessment of acute intracranial hemorrhage, though MRI shows equal ability to identify acute intracerebral hemorrhage 2, 5
• More practical in emergency situations where speed is critical 2
• CT remains standard for initial evaluation after acute head or spine trauma 3

Practical Considerations

• Preferred when MRI is contraindicated due to pacemakers, large body habitus, or unsafe implants 2
• In acute stroke patients, MRI was not feasible in 20% due to contraindications, impaired consciousness, or hemodynamic compromise 2

Clinical Advantages of MRI

Soft Tissue Characterization

• MRI provides superior soft tissue contrast, making it the preferred modality for tumor staging and evaluation of soft tissue masses 1, 2
• Better at evaluating marrow involvement (25% superior to CT), soft tissue involvement (31% superior), joint involvement (36.4% superior), and neurovascular structure invasion (15.3% superior) 1
• Superior for defining tumor length and demonstrating involvement of muscle compartments 1

Tumor Detection and Staging

• MRI is generally considered the preferred imaging modality for staging bone tumors 1
• Most sensitive modality for detection of spine tumors, followed by FDG-PET/CT and CT 1
• Detects subclinical tumors not evident with other modalities 1
• Contrast-enhanced MRI shows 61% sensitivity and 95% specificity for differentiating benign from malignant cartilaginous tumors 1

Neurological Applications

• Preferred for CNS evaluation, spine disk disease, and major joints 3
• Superior for detecting nasopharyngeal tumors and cranial nerve abnormalities due to improved sensitivity to skull base abnormalities and perineural spread 1
• Detects significantly more intraparenchymal lesions in pediatric traumatic brain injury (34% vs 15% with CT), particularly in abusive head trauma cases (43% vs 11%) 6

Chronic Hemorrhage and Microbleeds

• More accurate than CT for detecting chronic intracerebral hemorrhage and microbleeds (detected in 49 patients on MRI vs 0 on CT in one study) 5

Clinical Decision Algorithm

Choose CT when:

• Rapid imaging is required (emergency, trauma, unstable patient) 1, 2
• Evaluating cortical bone destruction or calcification patterns 2
• MRI is contraindicated or not feasible 2
• Assessing acute hemorrhage in emergency settings 2
• Evaluating chest, abdomen, or pelvis 3

Choose MRI when:

• Staging known or suspected tumors 1
• Evaluating soft tissue masses, marrow involvement, or neurovascular structures 1
• Assessing CNS pathology, spine disk disease, or major joints 3
• Detecting nasopharyngeal tumors or skull base abnormalities 1
• Radiation exposure is a concern (pediatric patients, pregnancy) 1

Important Caveats

CT Limitations:

• Limited soft tissue contrast restricts evaluation of soft tissue masses and neurovascular structures 2
• Radiation exposure is a consideration, particularly in pediatric patients 2
• Cannot detect calcium in certain contexts where MRI's tissue characterization is superior 2

MRI Limitations:

• Motion artifact from breathing and patient movement degrades image quality during long scan times 1
• Cannot detect calcifications 2
• Contraindicated with certain implantable devices 1
• More expensive and less readily available 1
• Claustrophobic patients may not tolerate the examination 1

Contrast Considerations:

• Intravenous contrast should be used with both modalities unless contraindicated (contrast allergy, renal insufficiency) 1
• Contrast improves lesion characterization, maps lesion borders, and identifies small primary sites 1
• MRI gadolinium contrast can cause nephrogenic systemic fibrosis in severe renal insufficiency 1