People often get CT scans and MRIs confused. They are both types of medical imaging that allow us to see into the human body without the need for a scalpel (really cool, I know), but they often serve different purposes. Both types of scans produce detailed pictures of the anatomy being imaged but there typically is an imaging method of choice for most indications.
In this blog post, I will discuss the differences between CT scans and MRI scans and explain why a patient might need one or the other. So, let’s get started!
CT Scan vs MRI Scan
What is a CT Scan?
A computed tomography (or CT) scan (also referred to as a computerized axial tomography or CAT scan) is a type of imaging using x-rays that produces cross-sectional images of the body.
A CT scanner is essentially a large x-ray machine that consists of a donut-like ring of x-ray generators and detectors (typically 16-64 detectors), referred to as a “gantry.” The gantry rotates around a patient, taking images from multiple different angles while the gantry rotates. Fancy computer algorithms take information from all of the images and process them into a stack of images for clinicians to review.
Since CT uses x-rays to create images, the CT images are essentially a density map of the patient. Density measurements are referred to as “Hounsfield Units” (HU), named after English electrical engineer Godfrey Hounsfield, credited with inventing CT technology for which he shares a Nobel Prize with Allan MacLeod Cormack. Bonus fun fact: Hounsfield created the CT scanner while working at EMI (Electric and Musical Industries), the record label of a small band called “The Beatles.” So if you or any of your family members ever bought a Beatles album back in the 1960s and 1970s, thank you for helping fund the creation of the CT scanner!
CT scans are often used to diagnose problems with bones, muscles, and organs. They can also be used to guide procedures such as biopsies and even surgeries.
A typical CT scan can scan the entire length of a patient in under a minute with most scans taking less than 30 seconds. The more detectors a CT scanner has and the stronger the power source, the faster it can scan.
It is also worth noting that x-rays are a form of ionizing radiation, a dangerous form of radiation that can negatively affect biological tissues, such as DNA. X-rays carry a very low (though non-zero) risk of developing cancer in the future and therefore we try to limit a patient’s radiation dose whenever possible.
What is an MRI?
Magnetic resonance imaging (MRI) scanners use strong magnets (we’re talking 30,000-60,000x that of Earth’s magnetic field) and radio waves to create detailed images of the inside of the body. MRI scans produce images with higher soft tissue resolution than CT scans and can be used to diagnose problems with the brain, spine, and other organs.
MRI technology relies on very strong magnets and sophisticated computers utilizing complex physics to produce images of patients. MRIs take much longer than CTs with protocols (scan time) generally ranging from 10-60 minutes. This is dependent on the type of anatomy being scanned and whether or not contrast is administered.
Both MRI and CT scans are obtained by having patients lie on a table that enters a donut-like structure where the imaging takes place. Since MRI is so much slower than CT, the patient will be placed in the magnet (similar in appearance to a CT gantry) for the duration of the scan, and some patients may become claustrophobic.
One important thing to remember is that the MRI machine is always on and therefore there is a constant magnetic field. Because of this, there are strict guidelines and policies to ensure patient safety as anything magnetic that enters the room will become a projectile and rapidly accelerate toward the magnet.
Why Would I Need a CT Scan or MRI?
There are many reasons why patients undergo CT or MRI scans, from evaluating a patient for traumatic injuries (car accident), evaluating patients with pain (abdominal pain, chest pain, headache, arm or leg pain following an injury), abnormal blood laboratory values, or the diagnosis and follow-up of cancers.
CT scans may be used as screening exams as well. Examples include lung cancer screening, colon cancer screening, and coronary artery calcium score screening. The scans are performed with a low-dose technique, exposing patients to a very small dose of radiation.
Doctors/physicians and other providers (physician assistants and nurse practitioners) always make a risk vs. benefit assessment before ordering any study/lab or prescribing any treatment. They only order imaging exams when they believe the benefit outweighs any potential risk. And when in doubt, providers can always call their local radiologist for a consult.
How Do CT Scans and MRI Scans Differ?
CT scans use x-rays to create images of your body, essentially creating a density map of the body. CT scans are fast with modern scanners capable of scanning a body part in a matter of seconds.
MRI scans produce images by using strong magnets and radio waves to image protons in the body (which we have 10^28 of – that’s 10 with 28 0s after it!). MRI scanners have no dangerous (ionizing) radiation and therefore no increased cancer risk.
Both CT scans and MRI scans frequently benefit from IV contrast agents (which can make you feel hot all over for a few minutes) and sometimes oral contrast agents as well. Contrast is generally helpful when evaluating patients undergoing significant trauma or anything that may be infectious, inflammatory, or malignant (cancer). IV contrast is named after what it does – it adds contrast to what is being imaged. It makes pathology like cancers more visible and easier to detect.
While CT has excellent spatial resolution (ability to make out small objects), its soft tissue contrast resolution is quite poor. This is because water and soft tissue density are so close that it’s difficult to tell them apart – hence we give contrast.
An MRI scan produces images with better contrast between soft tissues than CT. For example, it’s easier to tell soft tissue structures apart from one another, such as gray and white matter in the brain or endometrium from myometrium in the uterus.
Let’s cover some common examples of when CT and MRI are appropriate for various organ systems.
Brain and Spine Imaging
CT Scans
CT scans are the gold standard in the evaluation for fractures or head bleeds. Non-contrast CT of the head and CT angiogram of the head and neck play a critical role in the stroke pathway. These exams rule out intracranial hemorrhage (head bleed) and evaluate the blood vessels supplying the brain to assess for potential causes of stroke.
CT scans are also preferred in the evaluation for fractures or malalignment of the spine and play an incredibly important role in the setting of trauma.
MRI Scans
CT scans and MRI scans play a complementary role to one another in neuroradiology, as in other subspecialties. MRI scans are preferred when evaluating for stroke (more sensitive than CT in the acute setting), mass/malignancy, infection, inflammatory disorders, and when there is concern for ligamentous injury of the spine.
Cardiothoracic Imaging
CT dominates in cardiothoracic radiology and is the workhorse in the evaluation of pulmonary nodules, cancer staging, evaluating for pulmonary emboli and aortic dissection/acute aortic injury, and assessing the lungs for entities such as pneumonia, interstitial lung disease (ILD), and more.
CT and MRI have a complementary role in cardiac imaging with calcium score screening and coronary CTA (assessing for coronary artery disease) off