CT scan or MRI — your doctor ordered one, but do you know what to expect? From how each machine works to prep requirements and what each scan does best, here's everything you need to know before you go.
In this article:
- CT scans use radiation to produce fast, detailed images — making them ideal for emergencies; MRIs use magnets and radio waves for more detailed imaging of complex structures.
- The right scan depends on your medical needs: CT excels at speed and bone/trauma imaging, while MRI offers superior detail for soft tissue, nerves and the brain.
- Prep requirements differ significantly between the two — especially around metal removal, fasting and implanted devices.
Medical imaging has come a long way since 1895, when German physicist Wilhelm Conrad Röntgen produced the very first X-ray, an image of his wife’s hand. Fast-forward to today. The routine X-ray still has its place, but imaging technology has grown ever more sophisticated. Depending on diagnostic and therapeutic goals, today’s doctors can prescribe traditional X-rays, computed tomography (CT), or magnetic resonance imaging (MRI).
How CT Scans and MRIs Work
The difference between a CT scan and an MRI is a common source of confusion. A CT (computed tomography) scan, also known as a CAT (computerized axial tomography) scan, uses ionizing radiation, just like traditional X-rays. During a CT scan, you lie on a motorized table that moves through the scanner’s circular opening. As the scanner rotates around you, it emits low-dose X-ray beams from various angles. A computer processes these beams into detailed, cross-sectional images of your body. These can be read individually or assembled into a 3D rendition of bones, organs, blood vessels or soft tissue.
An MRI scanner creates images using magnets and radio waves. During an MRI, you lie on a table that slides into a cylindrical scanner. The scanner creates a powerful magnetic field that aligns all the hydrogen atoms in your body in the same direction — much like the Earth’s magnetic field affects a compass. The MRI machine then emits radio waves that disrupt this new alignment. The atoms absorb some of the energy from the radio waves. When the technician turns off the radio waves, the hydrogen atoms return to their aligned position with the scanner’s magnet, emitting tiny signals as they move back into place. The MRI machine translates these signals into images.
MRI and CT — Compare and Contrast
Best Uses
Both MRI and CT scan bones, organs and soft tissues. MRI results are more detailed, allowing for better visualization of intricate structures, while CT results are quicker, making the technology an emergency room staple.
MRI scans yield remarkably detailed imaging. They can detect things like tiny tumors, small strokes, spinal disc issues, compressed nerves or issues with blood vessels. MRI scans are also used to detect brain and spinal cord abnormalities, diagnose joint problems or look for changes that indicate a cancer may have spread.
CT scans are especially useful in the emergency room because they can provide a rapid, accurate snapshot to help the medical team arrive at a diagnosis and care plan. This makes CT scans the go-to imaging mode to detect broken bones, pulmonary embolisms, internal bleeding or brain bleeds. CT scans also have their uses outside the ER, where they can help evaluate abdominal pain, identify bowel blockages, diagnose kidney stones and more.
Contrast Dyes
Both MRI and CT sometimes require contrast dyes to enhance image clarity of complex structures such as the blood vessels or intestines. The dye used for MRIs is based on the rare-earth metal gadolinium, while the dye used for CT scans contains iodine. If you are given a CT scan with contrast dye, you may be asked to wait to make sure that you feel OK after the exam. Be sure to drink lots of fluids to help your kidneys remove the dye from your body.
Radiation Exposure
MRI scans don’t use radiation. CT scans use low-dose radiation to produce images, which can be a risk factor for pregnant women, children and teens, people who are allergic to iodine, and those with chronic conditions that require repeat scans, such as IBS, kidney stones or certain cancers.
Duration
CT scans are generally completed in just a few minutes. MRI scans usually range from 15 to 30 minutes, depending on the area being examined.
Noise
MRI machines produce loud, rhythmic noises that can be quite startling. The noises are caused by the rapid vibration of the machine's internal metal coils switching electrical currents on and off to create magnetic fields. Patients getting an MRI are provided with ear protection, including foam earplugs to plug the ear canal and headphones or earmuffs worn over the earplugs for extra protection. MRI-safe headphones allow you to hear music or talk to the technologist conducting the exam. In contrast, the CT scanning process is relatively quiet — you will hear a soft whirring or buzzing sound during the exam.
Prep
| PREP ITEM | MRI | CT SCAN |
|---|---|---|
| Fasting | Not required unless contrast or sedation are planned. Some abdominal imaging requires 4-6 hours fasting. | 2–4 hours without food are required if using IV contrast. |
| Water/Clear liquids | Usually allowed | Usually allowed |
| Removal of jewelry and metal items | Critical. Very important. Remove ALL jewelry and piercings, hairpins, hearing aids, underwire bras, belts, coins, phones, etc. | Remove any metal around the scanned area. |
| Clothing | No hidden metal fibers, zippers, or snaps. A hospital gown is usually worn. | Loose clothing or hospital gown. |
| Implants/Devices | "Some devices are not MRI-safe, and it’s critical to report pacemakers, aneurysm clips, cochlear implants, stimulators, etc. Modern dental fillings are non-magnetic, but you should tell the technician about crowns, bridges, implants, and older dental work." | "It’s important to report any implanted devices, but they don’t generally interfere with CT scans." |
The above chart describes basic prep for MRIs and CT scans. However, prep changes significantly in situations such as:
- Cases when sedation or anesthesia is necessary
- Pediatric scans
- Cardiac imaging
- Abdomen/pelvis studies
- Emergency imaging
- Contrast-enhanced exams
Just What the Doctor Orders
The choice between a CT scan and an MRI depends on your medical needs, the urgency to obtain imaging, the issue the scan is intended to detect and your doctor’s recommendation. Learn more about MRI and CT services at El Camino Health.
This article first appeared in the June 2026 edition of the HealthPerks newsletter.
