If you are looking to find out more information about medical imaging tools such as MRI and CT scans, you have reached the right place. Read on to find an in-depth comparison of the methods used in the diagnosis and treatment of diseases.

At a glance

Medical imaging is vital in the diagnosis and prognosis of many diseases and injuries. Diagnostic tools such as MRI and CT scans are used to help diagnose and plan treatments for patients. While there are several differences between the types of medical imaging, the main difference between these two diagnostic imaging techniques is that an MRI uses strong magnetic fields to take images, while a CT scan uses X-rays. Continue reading to learn about more differences between the two medical imaging tools.

Basics of MRI

Magnetic Resonance Imaging (MRI) is a type of medical imaging tool that uses strong magnetic fields and different frequencies of radio waves to produce detailed images of different internal body structures.¹ An MRI machine is a large tube that you lie in which will examine almost any part of your body such as blood vessels, internal organs, the brain, bones, and joints. It does not use ionising radiation which avoids the associated carcinogenic risk.²

MRI works by strong magnetic fields aligning atoms in your body in the same direction. Radio waves are then used to move these atoms out of the original position. When these radio waves are turned off, the atoms return to their original position sending back radio signals. These radio signals are sent to a computer to convert them into an image of the part of the body being examined.³

Basics of CT scans

Unlike MRI scans which use magnetic fields and radio waves, Computed Tomography (CT) or CAT scans use X-rays to obtain detailed pictures of inside your body.⁴ Like an MRI scan, you will be lying down whilst the machine scans you, however, you may be given a nonradioactive dye containing iodine to provide more detail in the CT scan pictures. These scans are used to create images of your bones and soft tissues. As the CT scan uses X-rays to produce precise images, it uses ionising radiation which has been identified to affect DNA structure and function by causing injury.⁵

CT scans work by using a motorised X-ray source which combines images from different angles around the body. These images are then sent to a computer which constructs a 2D image ‘slice’ of the patient.⁶ Image slices can then be stacked together by the computer to generate a 3D image of the patient which shows the skeleton, organs, and any abnormalities that the physician is trying to identify.

What do they show?

Both MRI and CT scans show a precise account of different internal structures of your body.

As the imaging tools give us different types of information within a scan, it is hard to compare if one is better than the other. Generally, CT scans provide more spatial resolution, whilst MRIs provide more contrast resolution. This means that CT scans are good at differentiating where each structure ends and where another one starts and MRIs are better at showing us the difference between parts of the body.⁷

Reasons for getting an MRI scan could include

• Soft tissue problems
• Spinal problems
• Nerve issues
• Inflammation

Reasons for getting a CT scan could include

• Emergency diagnoses e.g. strokes
• Tumours
• Cancer development
• Fractured bones
• Blood clots

 Scanning procedure

The procedure for both scans is very similar. They are both performed by radiographers who control the machine from a separate room, and they require you to lie flat on a bed whilst you are moved into the scanner. You must stay as still as you can to ensure accurate images are taken. On some occasions, the radiographer may ask you to hold your breath during the scan. Throughout the scan, you can contact the radiographer via an intercom system if you need their assistance.

One of the differences between the two diagnostic tools is that a CT scan is much quieter than an MRI. As MRI scans are noisy, the radiographer will provide you with headphones or earplugs to reduce the disturbance. Another difference is that for a CT scan, you may be given an injection of a nonradioactive dye to help with the imaging. Lastly, the duration times will vary per scan. CT scans tend to last for around 10 minutes, whereas MRI scans can take much longer, some lasting up to 90 minutes.^1,4

Pros and cons of MRI and CT scans

While both of these scans are painless and non-invasive, there are some disadvantages that needs to be considered.

MRI advantages

1. No ionizing radiation exposure: unlike CT scans which use X-rays, MRI does not involve ionising radiation. This makes it a safer option, especially for patients who need repeated or long-term observation
2. Superior soft tissue visualization: MRI provides great levels of contrast resolution which makes it ideal for imaging structures like the brain, muscles, and organs
3. Functional imaging capability: Functional MRI can assess brain activity by monitoring changes in blood flow which provides insights into neurological functions
4. No risk of radiation-induced cancer: Due to the absence of ionising radiation, MRI scans do not pose a risk of radiation-induced cancer. Studies have identified that there is a link between using ionising radiation in CT scans and cancer⁵

MRI disadvantages and risks

1. Longer scan times: MRI scans tend to take longer than other imaging tools such as CT scans
2. Louder: MRI scans generally are louder than other methods of medical imaging like CT scans, which can cause irritation to patients. Although this can be overcome by using the headphones or earbuds which the radiographer will provide
3. Metal implants: Patients with metallic implants or devices such as pacemakers or cochlear implants will not be able to use MRI machines
4. Claustrophobia: due to the enclosed space, this can cause distress to patients who have a fear of enclosed spaces

CT scan advantages

1. Faster imaging: CT scans are the fastest type of medical imaging which makes them useful in emergency situations
2. Better for bone and dense tissue imaging: Due to the CT scanner taking multiple images of your body at different angles, it provides an excellent insight into your bones which is superior in detecting fracture and bone abnormalities
3. Metal implants: CT scans are less affected by metallic implants, making them a suitable alternative for patients with implants of prosthetics

CT scan disadvantages and risks

1. Ionizing radiation exposure: As CT scans use X-rays, it will expose patients to ionising radiation which may pose a risk to their health
2. Risk of intravenous dye: Sometimes a dye composed of iodine will be injected into the patient for better visualisation of intricate components. Although this injection is painless, patients may form a reaction in response to this
3. Overuse concerns: Due to its quick speed and availability, there’s a concern about overusing CT scans, leading to unnecessary radiation exposure for patients⁹

Summary

CT scans and MRI scans are both vital in the diagnosis and treatment of injuries and diseases. While MRIs provide great insights into soft tissue imaging with few health risks, CT scans excel in bone and dense tissue visualisation, especially in urgent medical situations. However, they both come with their drawbacks which should be considered with your healthcare provider on a case-by-case basis.