Magnetic resonance imaging (MRI) and computed tomography (CT) scans are two important imaging techniques used in the medical field. Both provide detailed images of internal body structures and are painless, noninvasive procedures. However, there are some key differences between what MRIs and CT scans can detect.
Brief Overview of MRI and CT Scans
An MRI machine uses powerful magnets and radio waves to produce detailed cross-sectional images of the body. The strong magnetic field aligns protons in the body’s cells, and when the radio waves are applied, the protons are stimulated to produce signals that are then translated into images. An MRI can detect subtle changes in body tissues and is especially useful for imaging soft tissues like the brain, muscles and organs. It can help identify and diagnose tumors, bleeding, infections and nerve damage.
A CT or CAT scan combines multiple X-ray images taken from different angles to produce cross-sectional images of the bones and tissues inside your body. CT scans are often faster than MRI scans and are the preferred choice for examining bone injuries and lung and chest problems. They can show internal injuries and bleeding quickly.
Differences in What They Detect
One of the main differences between MRI and CT scans is the types of body tissues they best detect:
- MRIs are better for imaging soft tissues.
- CTs are better for visualizing bone injuries, lung lesions and issues related to blood vessels.
Below is a more in-depth look at what each type of scan can detect:
MRI Detection Capabilities
MRIs produce very detailed images of soft body tissues and organs like the brain, heart, liver, kidneys, ligaments, tendons and cartilage. Specific things an MRI can detect include:
- Brain and spinal cord injuries: MRIs can detect brain tumors, bleeding, swelling, and nerve damage. They can also detect spinal injuries and alignment issues.
- Heart problems: MRIs can assess heart structure, function, and blood flow. They can detect coronary artery disease and heart muscle problems.
- Liver diseases: MRIs can detect and characterize liver tumors, fibrosis, fatty liver disease, cirrhosis, and iron overload.
- Kidney disorders: MRIs can detect kidney tumors, obstruction, infections, and evaluate kidney blood flow and function.
- Joint and musculoskeletal problems: MRIs can detect bone marrow edema, sports injuries like pulled muscles and ligament tears, herniated disks, arthritis, and vertebral problems.
- Breast cancer: Breast MRI is more sensitive than mammography for detecting tumors.
- Uterine and ovarian problems: Pelvic MRI can detect fibroids, endometriosis and gynecologic cancers.
- Prostate cancer: Multi-parametric prostate MRI is used for detecting and staging prostate cancer.
CT Scan Detection Capabilities
CT scans are ideal for visualizing bones, blood vessels and organs like the lungs. Specific things a CT scan can effectively detect include:
- Bone fractures and joint injuries: CTs detect bone fractures or injuries that may not show up on X-rays. CTs are also useful for guiding joint injections and biopsies.
- Internal bleeding and blood vessel problems: CT angiography can detect aneurysms, blockages and bleeding in blood vessels anywhere in the body.
- Lung disorders: CT chest scans obtain excellent views of the lungs and are used to detect lung cancer, infections, COPD, bronchitis, pneumonia and pulmonary embolism.
- Brain hemorrhages and strokes: CT scans can quickly detect brain bleeding and distinguish between hemorrhagic and ischemic stroke types.
- Spinal injuries: CT myelography images the spinal cord and is useful for detecting disc herniations, pinched nerves, spinal fractures and tumors pressing on the spinal cord.
- Cancer: CT scans are commonly used to detect cancers in the lungs, liver, adrenal glands, kidneys and digestive tract organs.
- Appendicitis: CT is highly accurate for diagnosing appendicitis and detecting complications like rupture.
Soft Tissue vs. Bone Imaging
One of the major differences between MRI and CT scans comes down to imaging soft tissues versus denser bony structures. MRIs use strong magnetic fields and radio waves which interact with protons in soft tissues to generate images. This allows an MRI to differentiate subtle variations between soft tissues. CT scans measure how x-rays pass through tissues – denser structures like bone absorb more x-rays. This makes CT scans ideal for visualizing fractures, degenerative changes in bones and joints and detecting calcifications.
| Tissue Type | Best Suited Imaging Test |
|---|---|
| Brain | MRI |
| Muscles | MRI |
| Heart | MRI |
| Liver | MRI |
| Kidneys | MRI |
| Joints | MRI |
| Spinal cord | MRI |
| Bones | CT scan |
| Blood vessels | CT scan |
| Lungs | CT scan |
As shown in the table, MRI is generally more appropriate for imaging and characterizing diseases in soft tissue organs, while CT excels at assessing disorders related to bone and lung problems.
Differences in Image Production
Not only do MRI and CT scans detect different things, they also work in fundamentally different ways to generate images. This results in some key differences in the image quality produced:
- MRIs provide excellent soft tissue contrast. The high level of detail allows doctors to better evaluate the health of organs and detect small masses or irregularities.
- CTs produce very detailed images of bony structures. Tiny bone fractures and subtle bone erosion from arthritis can be detected.
- MRIs take longer – often 30 minutes or more for imaging. CTs are much faster, sometimes taking just minutes for image acquisition.
- CT images have lower tissue contrast between fluid, soft tissue and bone. MRIs better distinguish between these structures.
- CT scans involve exposure to ionizing radiation from X-rays. MRIs use strong magnetic fields and radio waves but do not involve harmful radiation.
Uses of MRI vs. CT Scans
Because of the different strengths of MRI and CT scans, they tend to be used in slightly different circumstances. Some common uses of each type of test include:
Typical Uses of MRI Scans
- Examining joints for injuries
- Detecting spinal cord injuries like disc herniations
- Evaluating nerve compression syndromes like carpal tunnel
- Diagnosing multiple sclerosis and other neurological disorders
- Checking for liver tumors and metastasis
- Finding prostate cancer
- Detecting uterine fibroids
- Diagnosing torn ligaments and tendons
Typical Uses of CT Scans
- Diagnosing lung cancer and infections
- Detecting pulmonary embolism
- Identifying bone fractures
- Guiding biopsies and needle aspirations of suspicious areas
- Diagnosing kidney stones
- Detecting acute appendicitis
- Scanning the sinuses and mastoids to find infections
- Searching for bleeding and blood clots in the brain shortly after a stroke
Advantages of MRI Over CT Scan
While both MRI and CT provide valuable medical imaging, MRI offers some advantages over CT scanning. These include:
- Greater soft tissue contrast – MRI produces very detailed soft tissue images, allowing for early detection of tumors, tears, inflammation and other abnormalities.
- No radiation exposure – Unlike CT scans, MRI does not use ionizing radiation so there is no risk of radiation damage over multiple scans.
- Earlier detection of disease – The excellent soft tissue differentiation of MRI allows finding disorders at very early stages before they may be evident on CT scans.
- Helpful for implant evaluations – MRI avoids x-rays and can safely scan individuals with medical implants to check for complications or loosening.
- Multiplanar imaging capabilities – MRI allows obtaining images in various planes without repositioning the patient.
- Blood vessel imaging – MR angiography can evaluate blood vessels and detect aneurysms or narrowing.
- Functional imaging – Advanced MRI techniques like functional MRI (fMRI) provide images of active areas of the brain.
Advantages of CT Over MRI
While MRI scans have definite advantages in soft tissue imaging, CT scans also offer benefits in certain situations. These include:
- Quick scan times – CT scans are very fast, allowing critically ill patients to be imaged.
- Lower cost – CT scanners are more widely available and CT scans are cheaper than MRI.
- Bone visualization – CT accurately images small bone details that may not be seen on MRI.
- Use with implants – CT avoids artifacts from metallic implants which can limit MRI.
- Open design of scanner – CT scanners are open for anxious or claustrophobic patients.
- Guiding interventions – CT is used to guide biopsies, contrast injections and ablation procedures.
- Recommended in some cases – CT is superior for imaging lung disorders, vascular disease, appendicitis, kidney stones.
Risks and Contraindications
MRI and CT scans are generally very safe procedures. However, there are some risks and contraindications to consider with each modality:
MRI Risks and Contraindications
- Not safe for individuals with metallic implants like pacemakers or defibrillators.
- Claustrophobia from being inside an enclosed scanner.
- Possible adverse reactions to contrast agents.
- Loud noises from the MRI may require ear protection.
- Can dislodge metallic foreign bodies.
CT Scan Risks and Contraindications
- Exposure to ionizing radiation, especially risky for children and pregnant women.
- Reactions to intravenous contrast material.
- Increased cancer risk from multiple scans due to cumulative radiation exposure.
- Can miss very small soft tissue abnormalities detected by MRI.
Conclusion
Both MRI and CT provide detailed views inside the body and are critical tools for making medical diagnoses. The choice between having an MRI versus CT scan depends on the specific body part being evaluated and the type of disorder suspected. MRIs excel at imaging soft tissues and detecting subtle inflammation, tumors, nerve impingements and joint injuries. CT scans are preferred for assessing lung diseases, diagnosing appendicitis, finding kidney stones and imaging bone fractures or degenerative joint disease. Talk to your doctor about whether MRI or CT scanning is more appropriate for your situation.