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

MRI vs CT Scan: Key Differences and Clinical Applications

MRI offers superior soft tissue contrast compared to CT, making it the preferred imaging modality for evaluating soft tissues, while CT excels at visualizing bone detail and is faster with fewer contraindications.

Fundamental Differences

• Basic Technology:

  • MRI uses magnetic fields and radio waves to create images without ionizing radiation 1
  • CT uses X-rays (ionizing radiation) to create cross-sectional images 1

• Tissue Contrast:

  • MRI provides superior soft tissue contrast, making it better for visualizing relationships between tumors and surrounding soft tissues, bone marrow, and neural structures 1, 2
  • CT better defines cortical bone destruction and calcifications 1

• Acquisition Time:

  • CT scans are much faster (typically <5 minutes) compared to MRI (often >30 minutes) 1
  • CT is generally more easily tolerated by patients due to shorter scanning time and larger scanner bore 1

Clinical Strengths of MRI

• Superior for:

  • Soft tissue evaluation with better contrast resolution 1, 3
  • Bone marrow involvement (25% better than CT) 1
  • Soft tissue involvement (31% better than CT) 1
  • Joint involvement (36.4% better than CT) 1
  • Neurovascular structure invasion (15.3% better than CT) 1
  • Spine lesions (highest sensitivity for tumor detection) 1
  • Tumor length definition and muscle compartment involvement 1

• Specialized Applications:

  • Advanced sequences like diffusion-weighted and chemical shift imaging help differentiate benign from malignant bone tumors 1
  • Contrast-enhanced MRI can provide additional information for preoperative evaluation of primary bone tumors 1
  • Better detection of perineural spread and skull base abnormalities 1

Clinical Strengths of CT

• Superior for:

  • Detecting cortical bone destruction (13.6% better than MRI in some cases) 1
  • Evaluating fine bony details and microcalcifications 4
  • Identifying patterns of mineralization (e.g., distinguishing ossification from calcification) 1
  • Acute trauma evaluation, especially for head and spine 5
  • Characterizing soft-tissue mineralization 1

• Practical Advantages:

  • More readily available and less expensive than MRI 1
  • Faster acquisition time (minutes vs. 30+ minutes for MRI) 1
  • Fewer contraindications (can be used with implanted medical devices) 1

Comparative Performance in Specific Scenarios

• Bone Tumors:

  • MRI and CT were judged equivalent in most categories (63-82% of the time) 1
  • In a study of 409 biopsy-proven tumors, MRI showed better sensitivity than CT for tumor detection 1
  • For spine lesions specifically, MRI was the most sensitive modality, followed by FDG-PET/CT, then CT 1

• Soft Tissue Masses:

  • MRI has become the technique of choice for detecting and characterizing soft tissue masses 1
  • CT remains important when MRI is contraindicated or not feasible 1

• Head and Neck Evaluation:

  • Both are effective, but CT is often preferred initially due to availability, cost, and shorter scanning time 1
  • MRI is preferred for suspected nasopharyngeal tumors or when cranial nerve abnormalities are present 1

Common Pitfalls and Considerations

• MRI Limitations:

  • Motion artifacts related to breathing and patient movement 1
  • Contraindicated with certain implantable medical devices (pacemakers, neurostimulators) 1
  • Claustrophobia can be a significant barrier 1
  • Longer acquisition times may be problematic for unstable patients 1

• CT Limitations:

  • Exposure to ionizing radiation (average dose of 3 mSv, equivalent to approximately 150 chest x-rays) 1
  • Dental artifacts may obscure visualization of certain areas 1
  • Less effective for soft tissue characterization 1, 3

• Contrast Considerations:

  • For both modalities, intravenous contrast should generally be used unless contraindicated 1
  • Renal insufficiency may preclude contrast administration in both CT and MRI examinations 1
  • MRI contrast (gadolinium) is associated with nephrogenic systemic fibrosis in patients with severe renal insufficiency 1

Decision Algorithm for Selecting Imaging Modality

1. Choose MRI first when evaluating:

   • CNS and spine for disk disease 5
   • Major joints and soft tissues of extremities 5, 3
   • Suspected syringomyelia or spinal dysraphism 2
   • Soft tissue masses requiring characterization 1
   • Bone tumors requiring staging 1

2. Choose CT first when evaluating:

   • Head and spine after acute trauma 5
   • Chest, abdomen, and pelvis 5
   • Fine bony details and microcalcifications 4
   • Patients with contraindications to MRI 1
   • Unstable patients requiring rapid imaging 1