Female patient undergoing CT scan test

MRI or CT Scan: What's the Difference?

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CT scans (also known as CAT scan) and MRIs are two of the most commonly performed imaging techniques to help doctors diagnose, and sometimes treat, injury or disease. A CT scan (computed tomography) employs x-rays while MRI (magnetic resonance imaging) uses a magnetic field and radiofrequency pulses; both techniques are able to provide a look at the internal structure of the body.

 

Given that each test seemingly provides similar results, you might wonder why your doctor would choose one instead of the other.

 

MRI

Doctor standing in front of an MRI machine

 

As the name suggests, MRIs use magnets to generate images. These aren’t your everyday refrigerator magnets though. MRIs use huge superconductive magnets which have a greater magnet field strength thousands of times stronger than that of the earth’s. MRIs do not use ionizing radiation and are able to image the brain, spine, organs, bones/joints, and soft tissues quite nicely. So MRI must be better than CT right? The answer is not so simple.

 

The machinery needed for MRI is more expensive and the superconductive magnet requires significant safety precautions so they are not as readily available as CT. Additionally, the process of producing an MR image is inherently slower and the patient must be able to remain completely still. MRIs can take up to 30 minutes, sometimes longer, depending on the body part and the number of pictures needed to be acquired. Therefore MRI is often not the best choice in the setting of emergencies, bleeding, trauma, and in patients who are claustrophobic or are unable to remain still for long periods of time.   

 

Because MRIs use a powerful magnet, patients who have internal metal, medical devices, or certain piercings may not be eligible for an MRI. Internal devices such as defibrillators, pacemakers, stimulators, or shunts may be affected by the magnetic field and cause the device to malfunction (of note medical device companies are currently designing devices which are MRI safe, several of which are already on the market). Piercings and metallic foreign bodies can heat up in the magnet resulting in burns. Most radiology departments complete an extensive screening questionnaire to identify any potential hazards. 

 

If you have a medical device be sure to save the manufacturers card and information so it can be fully reviewed by radiology staff prior to your MRI. If your device is not safe for the MRI, the radiologist may recommend a different type of imaging. Additionally, be sure to remove piercings and alert the radiology staff to any metal which may be on or inside your body.

 

CT Scans

Doctor standing next to CT scanner

 

CT scans are readily available at most hospitals and can be obtained rapidly, so are commonly used in emergency situations. In cases where a CT scanner hasn’t been made available, a mobile CT scanner can be requested for these emergencies. The patient lies on the table which quickly moves through the CT tube and images are obtained.

 

CT scans can produce images of the bones, chest, internal organs, digestive tract, and blood quite well so is commonly used to diagnose head injuries, abnormalities in the chest or abdomen, fractures, or internal bleeding.  In the nonemergent setting, CT scans can be used for imaging heart and lung disease, abdominal abnormalities, bones/joints, and cancer.

 

CT scan of healthy lungs

 

Additionally, because CT scans provide an assessment of patients anatomy, surgeons may opt to obtain a CT scan for surgical planning (like a road map).

 

CT Scans Expose Patients to Radiation

Human graph of average radiation exposure

 

Now let’s address the elephant in the room, radiation. CT scans use x-rays which pass through your body to generate images. X-rays are a type of ionizing radiation, which means they have enough energy to create ions of the material they pass through. When cells inside the body (i.e. the patient) are exposed to ionizing radiation, the DNA inside can become damaged and the cell can then become cancerous. 

 

Don’t worry too much. You are actually exposed to low levels of radiation that exist naturally in the environment every day and generally speaking your cells do a pretty good job of DNA repair. For comparison sake, an average person is exposed to 3 mSv of radiation annually just from the environment, while an average CT scan is between 2-8 mSv. The good (and bad) news is that there is no radiation dose or threshold point which one will develop cancer, but most scientists agree that the risk of cancer increases as radiation dose increases.

 

When you and your doctor decide for you to get a CT scan, you are deciding that the risks of the radiation are less than the disease or injury you are looking to diagnose. 

 

Conclusion

CTs and MRIs each have their own unique risks and benefits which need to be applied to an individual patient to answer a clinical question. When used appropriately both have proven to be a safe and effective means of diagnostic imaging. If your doctor recommends you receive a CT or MRI, feel free to ask why they ordered one versus the other. Also, be sure to direct any additional questions to the radiology personnel prior to your exam.

Disclaimer: Please note the information provided in this article is a broad generalization and is designed for educational purposes only. This article does not constitute medical advice. It is recommended you consult with your physician with concerns or questions related to your specific medical condition and for additional recommendations.  

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