MRI (magnetic resonance imaging) and CT (computed tomography) are two common medical imaging techniques used to visualize internal body structures. Both methods provide important diagnostic information to help doctors identify and treat medical conditions. However, one key difference between MRI and CT is the use of ionizing radiation. CT scans involve exposure to x-ray radiation, while MRI scans do not use ionizing radiation. This leads to an important question for patients and providers – is MRI more radiation than CT?
MRI and Radiation
MRI does not use ionizing radiation. Instead, MRI uses strong magnetic fields and radio waves to generate images of the inside of the body. The magnetic field aligns hydrogen atoms in the body, while radio waves cause these atoms to produce faint signals that are used to create cross-sectional MRI images.
While MRI does not use ionizing radiation, the strong magnets require careful safety screening. MRI is generally not performed on patients with metal implants or shrapnel near vital organs. The magnetic pull can cause metal objects to move and disrupt imaging. MRI also cannot be used to image patients with pacemakers or certain metal implants. Still, when performed properly, MRI imaging itself is considered very safe with no ionizing radiation risk.
CT and Radiation Exposure
Unlike MRI, CT scans do involve exposure to ionizing radiation from x-rays. In CT imaging, an x-ray beam is rotated around the body area being imaged. Detectors measure the amount of radiation absorbed by different tissues. These transmission recordings are digitized and reconstructed by computer to generate cross-sectional images.
Because ionizing radiation is used to generate the images, CT scans result in much higher radiation exposure compared to MRI scans. Exposure levels vary based on the type of CT scan:
Head CT
- Radiation dose: 1-3 mSv
Chest CT
- Radiation dose: 5-7 mSv
Abdomen/Pelvis CT
- Radiation dose: 8-15 mSv
Coronary Artery CT
- Radiation dose: 12 mSv
Thus, the radiation exposure from a single CT scan ranges from 1 to 15 mSv depending on the type of study. This level of radiation exposure from a single CT scan is very small. However, it is significantly greater than zero radiation from MRI.
Lifetime Risks from CT Radiation
While the radiation risk from a single CT scan is low, risks can accumulate from multiple CT studies over a lifetime. Exact risk levels are debated, but most experts estimate the additional lifetime cancer risk at:
- 1 head CT – 1 in 1000 risk
- 1 chest CT – 1 in 250 risk
- 1 abdomen CT – 1 in 330 risk
Thus, multiple CT scans over a lifetime can increase overall cancer risk, especially for children with developing bodies. However, the benefits of needed CT imaging for accurate diagnosis generally outweigh the small radiation risks. Still, unnecessary CT scans should be avoided when possible.
MRI vs CT Radiation Summary
In summary:
- MRI does not use ionizing radiation, while CT does expose patients to x-ray radiation.
- A single CT scan exposes patients to 1-15 mSv depending on body part imaged.
- MRI has no radiation risk and is the preferred imaging test when both MRI and CT can provide the diagnostic information needed.
- However, the radiation risk from any single CT scan is very small and justified when CT is necessary for diagnosis.
- Multiple CT scans over a patient’s lifetime does increase cumulative radiation exposure and slightly increase cancer risk.
- Unneeded CT scans should be avoided to reduce unnecessary radiation exposure.
So in direct comparison, MRI involves zero radiation while CT does expose patients to low-level ionizing radiation. However, the radiation levels from a single necessary CT scan are justified by the diagnostic benefit. MRI avoids radiation exposure altogether, making it the preferred imaging test when either MRI or CT can provide the clinical information needed.
Differences between MRI and CT Imaging
While radiation exposure is a key difference, MRI and CT scans have other important differences in how they image the body:
Soft Tissue Imaging
- MRI provides excellent soft tissue contrast and is ideal for neurological imaging of the brain and spinal cord.
- MRI is also preferred for joint and musculoskeletal imaging as it images bone marrow, cartilage, ligaments and tendons very well.
- CT provides less soft tissue contrast resolution and is not ideal for imaging small or complex neurological structures.
Bone Imaging
- CT provides excellent visualization of bone structures.
- MRI can also image bones but CT is generally preferred, especially if fractures are suspected.
Blood Vessel Imaging
- CT angiography (CTA) is commonly used to image blood vessels, particularly around the heart.
- MR angiography (MRA) can also be performed but CTA is generally the preferred modality for vascular imaging.
Speed of Imaging
- CT scans are very fast, with imaging times under 1 minute in most cases.
- MRI scans take significantly longer, often up to 30 minutes or more depending on the body part imaged.
Implants and Metal
- CT can be performed on patients with metal implants, pacemakers, and prosthetics that may preclude MRI.
- MRI poses risks for patients with metal in or near vital organs and cannot be performed in these cases.
| Imaging Factor | MRI | CT |
|---|---|---|
| Radiation Exposure | None | Low exposure |
| Soft Tissue Imaging | Excellent | Limited |
| Bone Imaging | Adequate | Excellent |
| Blood Vessel Imaging | Adequate with MRA | Preferred with CTA |
| Imaging Time | Longer (up to 30+ minutes) | Very fast (under 1 minute) |
| Metal Implants | Often unsafe | Usually safe |
Guidelines for Ordering MRI or CT Scans
Given the differences in MRI and CT capabilities, here are general guidelines on when each imaging modality is most appropriate:
MRI Recommended For:
- Brain/neurological disorders
- Spinal cord injuries or disorders
- Soft tissue pathology (muscle, ligaments, tendons)
- Joint abnormalities
- Cancer staging
- Breast imaging
- Liver/biliary imaging
- Female pelvic imaging
- Best initial study when either CT or MRI could work
CT Recommended For:
- Chest imaging (lungs, heart, great vessels)
- Abdominal disorders
- Urinary tract imaging
- Appendicitis
- Abdominal injuries or bleeding
- Fractures or bone disorders
- Sinus/facial disorders
- Quick imaging when speed is critical
- Suspected blood clots
- Metal implants that preclude MRI
Following these basic guidelines will help providers select the best imaging test for each patient while minimizing unnecessary radiation exposure from CT scans.
Conclusion
In conclusion, MRI avoids ionizing radiation and is preferred over CT whenever it can provide the clinical information needed. However, the radiation exposure from a single necessary CT scan is very low and justified by the benefit of an accurate diagnosis. CT is still required in many situations where MRI cannot be performed or does not provide optimal imaging. By understanding the radiation risks along with the strengths of each modality, providers can make informed imaging decisions that maximize diagnostic benefit while minimizing radiation exposure. Choosing MRI over CT when both tests are equally applicable is the best way to avoid unnecessary radiation given that MRI has zero radiation risk.
