MRI vs CT Scan

What It Is

A CT (computed tomography) scanner rotates an x-ray source around the body and uses a computer to build cross-sectional images. An MRI scanner uses a magnetic field and radio wave pulses to detect signals from hydrogen atoms in tissues. Both produce slice images that radiologists review.

Because the two methods measure different physical properties, they show tissues differently. CT is very good at showing bone, air-filled lungs, and acute bleeding, and it is widely available and quick. MRI gives strong contrast between different soft tissues and can use many specialized sequences, but it takes longer and is more sensitive to motion.

Neither test is "better" overall. Clinicians and radiologists choose the test that best answers a specific question, and sometimes both are used.

Common Terminology

Ionizing radiation

High-energy radiation, such as x-rays used in CT, that can affect cells; doses are kept as low as reasonably achievable.

Soft tissue contrast

How well an imaging test distinguishes between different soft tissues; MRI generally excels at this.

Iodinated contrast

The iodine-based contrast agent commonly used in CT.

Gadolinium contrast

The contrast agent commonly used in MRI.

Hounsfield units

The scale CT uses to describe tissue density, appearing in some CT reports.

Signal intensity

How bright or dark a tissue appears on MRI, often described in MRI reports.

Why the Test Is Done

General examples of when each test is commonly used include:

• CT is often used in emergencies, such as after major injury or when quickly checking for bleeding in the brain.
• CT is commonly used to look at the lungs, kidney stones, and complex bone fractures.
• MRI is commonly used to evaluate the brain, spinal cord, and nerves in detail.
• MRI is frequently chosen for joints, ligaments, tendons, and cartilage.
• Both tests may be used for organs in the abdomen and pelvis, and for evaluating masses, depending on the question.

What the Results Generally Describe

Reports from both tests are written by radiologists, but they use different descriptive language:

• CT reports often describe density, using words like hypodense, hyperdense, or isodense.
• MRI reports often describe signal, using words like hyperintense, hypointense, or isointense on specific sequences.
• Both may describe enhancement after contrast, size measurements, and comparisons with prior studies.
• Both typically end with an "Impression" summarizing key findings.
• Results from either test are interpreted by a radiologist and the ordering clinician in context.

How the Test Is Done

In general, the experience differs in these ways:

• CT: the person lies on a table that moves through a short, open ring; many scans take only a few minutes.
• MRI: the person lies inside a longer tube or bore; scans usually take 15 to 60 minutes or more.
• MRI is loud, so hearing protection is provided; CT is relatively quiet.
• Both may use contrast given through an IV, and some CT exams use contrast taken by mouth.
• Both require staying still for clear images.

Preparation and What to Expect

General safety and preparation differences include:

• MRI requires careful screening for metal, implants, and devices; CT generally does not have this concern.
• CT involves a radiation dose, which is a consideration for children, pregnancy, and repeated scans.
• Contrast-related screening differs: CT contrast screening focuses on iodine reactions and kidney function, and MRI contrast screening on gadolinium and kidney function.
• MRI may be harder for people with claustrophobia or difficulty lying still for long periods.
• Preparation such as fasting depends on the specific exam for either test.

When Results Are Discussed With a Clinician

People often discuss these topics with a clinician when an imaging test is recommended:

• Why one test was chosen over the other for the specific question.
• Whether radiation exposure is a consideration and how it is minimized.
• Whether implants or devices affect which test can be done.
• Whether contrast is needed and which type.
• Whether further imaging with the other test may be useful after the first results.

Questions to Ask a Healthcare Professional

General questions people often find useful to bring to an appointment:

• Why is this type of scan being recommended rather than the other?
• How much radiation does a CT scan involve compared with other exposures?
• Would an MRI or CT give more useful information for this question?
• Does either test need contrast in this case?
• How long will each test take?
• Might both tests be needed?

Sources

This guide was written in plain language from the following authoritative references.

1. Diagnostic Imaging — MedlinePlus (U.S. National Library of Medicine)
2. Computed Tomography (CT) - Body — RadiologyInfo.org (RSNA and ACR)
3. Magnetic Resonance Imaging (MRI) - Body — RadiologyInfo.org (RSNA and ACR)
4. Computed Tomography (CT) — U.S. Food and Drug Administration (FDA)

Related Guides

• TestsMRI ScanGeneral guide to MRI scans: how magnetic resonance imaging works, why it is used, what the scan is like, and safety basics.
• TestsCT ScanGeneral guide to CT (computed tomography) scans: how they work, why they are done, contrast, radiation, and what the exam is like.
• TestsMRI Contrast (Gadolinium)General information about gadolinium-based contrast agents used in MRI: why they are used, how they are given, and known safety considerations.
• TestsPreparing for an MRIGeneral overview of how people commonly prepare for an MRI, including safety screening, metal and implants, clothing, and what happens on the day.
• Medical TermsMRI TerminologyPlain-English explanations of common MRI terms, from tesla and T1/T2 to sequences, coils, and contrast. General reference only.