Which has less radiation exposure, Computed Tomography (CT) or Magnetic Resonance Imaging (MRI)?

MRI Has No Radiation Exposure While CT Uses Ionizing Radiation

MRI has no radiation exposure, while CT scans expose patients to ionizing radiation that carries a small but real risk of radiation-induced malignancy. 1

Radiation Exposure Comparison

• MRI uses magnetic fields and radio waves to generate images, resulting in zero radiation exposure to patients 1
• CT scans use X-rays (ionizing radiation) to create images, with a typical organ dose of approximately 20 mSv per scan 1
• The radiation from a single CT scan is equivalent to approximately 100-200 chest X-rays 1
• Approximately 0.4% of all cancers in the United States may be attributable to radiation from CT studies 1

Health Risks Associated with Radiation Exposure

• The FDA estimates that a CT examination with an effective dose of 10 mSv may be associated with an increased chance of developing fatal cancer for approximately 1 in 2,000 patients 2
• The BEIR VII lifetime risk model predicts that with the same low-dose radiation, approximately 1 in 1,000 individuals will develop cancer 2
• Children have higher rates of malignancy and a longer life span in which radiation-induced malignancies can develop 1
• Patients with chronic conditions requiring repeated imaging (such as Crohn's disease) have significantly higher cumulative radiation exposure, with 15.5% of patients in one study having a cumulative dose exceeding 75 mSv (associated with a 7.3% increase in cancer mortality risk) 1

Clinical Considerations When Choosing Between MRI and CT

Advantages of MRI:

• No radiation exposure makes it ideal for children, young adults, and patients requiring repeated imaging 1
• Better soft tissue contrast and characterization 1
• Superior for detecting small lesions in certain contexts (e.g., small HCCs, proximal or superficial small bowel lesions) 1
• Potential for reduced overall healthcare costs by avoiding radiation-induced cancers 1

Advantages of CT:

• Faster acquisition time (rapid test) 1
• More widely available in many healthcare settings 1
• Often easier to interpret 1
• Less affected by patient factors like claustrophobia or implanted metal devices 1
• May be preferred in acute trauma settings 3

Specific Clinical Scenarios

• For inflammatory bowel disease monitoring: MRI enterography and ultrasound are preferred over CT to limit radiation exposure 1
• For liver disease evaluation: MRI shows diagnostic advantage over CT, particularly for small lesions (<2 cm) 1
• For cancer staging: PET/MRI offers comparable staging to PET/CT with a 79.6% reduction in radiation exposure (3.6 mSv vs 17.6 mSv) 4
• For testicular cancer follow-up: MRI with diffusion-weighted imaging is as effective as CT for detecting retroperitoneal metastases without radiation exposure 5

Potential Risks of MRI

• Contraindicated in patients with certain metallic implants or devices 1
• Potential for claustrophobia and anxiety in some patients 1
• Longer scan times compared to CT 1
• Sedation may be required for some patients, particularly children, which carries its own risks 1
• Gadolinium contrast agents may cause rare adverse reactions and are contraindicated in patients with severe renal disease 1

Practical Recommendations

• For patients requiring repeated imaging over time (e.g., chronic disease monitoring), MRI should be preferred when possible to minimize cumulative radiation exposure 1, 2
• In emergency settings where rapid imaging is essential, CT may be more appropriate despite the radiation exposure 1, 3
• For young patients, particularly children and adolescents, every effort should be made to use MRI or other non-ionizing radiation techniques when clinically appropriate 1
• When CT is necessary, use of low-dose protocols can help reduce radiation exposure while maintaining diagnostic quality 6