Introduction
MRI (magnetic resonance imaging) and CAT (computed axial tomography) scans are two common medical imaging techniques used to generate detailed pictures of the inside of the human body. Both utilize different technologies to create cross-sectional images that allow doctors to see inside organs, tissues, bones and other internal body structures (MRI Safety).
An MRI scanner uses powerful magnetic fields and radio waves to produce detailed images, while a CAT scan uses multiple X-ray beams rotated around the body to obtain image data that is processed by a computer (CT Patient Safety And Care). Both techniques are critical diagnostic tools, but have key differences in their mechanism, risks, and applications.
How They Work
An MRI (magnetic resonance imaging) and a CT (computed tomography) scan are two medical imaging techniques that create detailed images of the inside of the human body. However, they use very different methods to generate these images.
An MRI scanner utilizes strong magnetic fields and radio waves to produce images. The patient lies inside a large tube that contains a powerful magnet. This magnetic field aligns the protons in the body’s cells. Short bursts of radio waves are then sent to the area of the body being examined. The protons absorb the energy and release it in a specific pattern picked up by receivers in the scanner. A computer analyzes all of this information to construct a detailed picture.
In contrast, a CT or CAT scan uses multiple X-ray beams and detectors rotating around the body to obtain image “slices.” The X-rays pass through the body and are absorbed in different amounts depending on the density of the structures they pass through. The detectors measure this absorption and a computer compiles the information into cross-sectional images of the internal structures.
So in summary, MRIs use magnets and radio waves while CAT scans rely on X-rays to generate diagnostic images.
Radiation Exposure
One of the main differences between CAT scans and MRIs is in their use of radiation. CAT scans, also known as CT scans, utilize ionizing radiation to generate images. The X-rays pass through the body and are detected by sensors, creating cross-sectional views of the internal structures. While the radiation exposure from a single CAT scan is relatively low, it still carries a small risk, especially for patients who require multiple scans over time.
In contrast, MRIs do not use any ionizing radiation. They rely on powerful magnetic fields and radio waves to excite hydrogen atoms in the body’s water molecules. When the atoms relax back into alignment, they emit signals that are used to construct detailed images. This makes MRIs an extremely useful diagnostic tool when frequent imaging is required, as there is no cumulative exposure risk. MRIs are considered very safe, even for pregnant women and children.
Risks
Both MRI and CT scans carry some risks that patients should be aware of.
Risks of MRI
Some of the main risks of MRI scans include:
- Feeling anxious or claustrophobic during the scan due to the enclosed space of the MRI machine. Some patients may require medication to relax.
- Possible issues with medical implants. The strong magnets can disrupt pacemakers, insulin pumps, and certain metal implants in the body. Patients with implants may not be able to get an MRI.
- In rare cases, patients can have an allergic reaction to the contrast dye used in some MRI scans.
Overall, MRI scans are considered very safe when proper precautions are taken. The MRI machine does not emit any radiation.
Risks of CT scans
The primary risk of CT scans is exposure to ionizing radiation from the X-rays used to generate images. According to Healthline, the amount of radiation from a single CT scan is 100 to 500 times greater than a standard X-ray. While the risk from one CT scan is minimal, repeated CT scans over time can increase cancer risk.
Some other potential CT scan risks include:
- Allergic reactions to contrast dye, though less common than with MRI contrast agents.
- Small chance of developing cancer later in life from the radiation exposure.
CT scans require radiation exposure, so doctors weigh the benefits against the small potential risks.
Safety Precautions
Both MRI and CT scans require safety precautions to ensure patients are not harmed during the imaging process. According to the RadiologyInfo.org article “MRI Safety” (https://www.radiologyinfo.org/en/info/safety-mr), patients undergoing an MRI will be screened for any metal implants, devices, or objects that could cause issues during scanning. Patients may be required to change into MRI-safe clothing and must remove all jewelry or metal items before entering the MRI room.
The UC Davis Health article “MRI Safety” (https://health.ucdavis.edu/radiology/mymri/mymri-safety.html) explains some key safety practices for MRIs. The MRI magnet is always on, so no metal objects are allowed in the room to avoid them becoming dangerous projectiles. Staff will confirm no metal items enter the MRI room. Patients may receive earplugs or headphones to protect hearing from the loud noises during scanning. Patients are monitored at all times and can communicate with staff during the procedure. Emergency equipment is available if needed.
For CT scans, there are fewer safety restrictions compared to MRI. However, RadiologyInfo.org’s “Safety in X-Ray, Interventional Radiology and Nuclear Medicine Procedures” (https://www.radiologyinfo.org/en/info/safety-xray) notes that pregnant patients should only undergo a CT scan if it is an emergency, as radiation exposure can harm a developing fetus. Staff take steps to minimize radiation exposure as much as possible.
Patient Populations
Certain patient populations are better suited for either MRI or CAT scans depending on their condition and needs. For example, pregnant patients and children are generally advised to get an MRI over a CAT scan due to the radiation exposure from CAT scans (Smith-Bindman, 2008). MRI scans utilize magnets rather than radiation, making them a safer choice for developing fetuses and young children.
Patients with pacemakers, insulin pumps, aneurysm clips, or any metal implants are advised against getting an MRI since the magnets can interfere with or dislodge the devices. These patients are better candidates for CAT scans. Additionally, claustrophobic patients may find the enclosed MRI machine distressing, so a CAT scan could be preferable.
For imaging soft tissues like the brain and abdominal organs, an MRI generally provides more detailed images than a CAT scan. But for imaging bones, joints, and lung abnormalities, a CAT scan is often the preferred choice (Smith-Bindman, 2019). The physician will determine which modality is most appropriate based on the clinical indication and patient factors.
Image Quality
MRI and CT scans produce different kinds of images. CT scans use X-rays to create detailed pictures of organs, bones, and other tissues. The images show cross-sectional “slices” of the body. CT scans provide excellent spatial resolution and are better for viewing bones, blood vessels, and organs like the lungs. However, they have low contrast resolution and don’t distinguish between different types of soft tissue very well.
MRIs use radio waves and strong magnets to generate images. The images have excellent contrast resolution and show subtle differences between soft tissues. This makes MRI better for imaging the brain and spinal cord, muscles, ligaments, the heart and cancer.1 MRIs can detect small lesions, tumors, tears or abnormalities that may not be visible on CT scans. However, MRIs have lower spatial resolution than CT scans.
In summary, CT scans are preferred for examining bones, blood vessels and organs, while MRIs provide more detailed images of soft tissue structures. The choice depends on the clinical need and area being imaged.
Cost
MRIs generally cost more than CT scans. According to Healthimages, the average MRI costs around $2,000, while the average CT scan is about $1,200. DocPanel notes that MRIs can range from $1,200 to $4,000, whereas CT scans may be half the cost. Overall, CT scans tend to be the more affordable imaging option.
Availability
CT scanners are more widely available than MRI scanners. According to a 2008 study, there were approximately 9 times more CT scanners than MRI scanners in the United States (Ginde et al., https://pubmed.ncbi.nlm.nih.gov/18783491/). CT scanners are present in most hospitals and imaging centers, whereas MRI scanners are less common and tend to be located at larger medical facilities and hospitals. The higher availability of CT makes it easier for patients to access this type of imaging. However, MRI availability has increased in recent years as the technology becomes more cost-effective. Still, MRI access remains more limited, which can lead to longer wait times for appointments.
Conclusion
When comparing the safety of MRI and CT scans, there are a few key differences to consider. MRI scans do not use any ionizing radiation, making them a safer choice, especially for pregnant women and children. CT scans do expose patients to radiation, albeit a low dose, so they carry a slightly higher long-term cancer risk. However, CT scans are generally faster than MRI, requiring less time inside the scanner. For claustrophobic patients or young children who may have trouble holding still, CT can be preferable. MRI scans involve very strong magnets, so they are contraindicated for patients with metal implants, pacemakers, or shrapnel. Overall, MRIs are considered the safer choice from a radiation exposure standpoint, but CT scans do play an important diagnostic role in many clinical scenarios. The ordering provider should weigh the benefits against the small radiation risks for each individual patient.
