CT Scan vs MRI: What’s the Difference?

CT Scan vs MRI - What's the Difference? - BrettMollard.com
Learn about the differences between a CT scan vs MRI scan and when you might need one or the other.

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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) offering excellent evaluation of the coronary arteries. Cardiac MRI, on the other hand, allows for a functional assessment of the heart, including the evaluation of arrhythmias, valvular abnormalities, masses, and viability in the setting of myocardial infarction.

Both CT and MRI can be used to evaluate and follow up on thoracic aortic aneurysms.

Abdomen and Pelvis Imaging

 

Both CT scans and MRI scans play an important role in imaging internal body parts within the abdomen and pelvis. CT makes up the bulk of imaging exams in these parts of the body with MRI reserved for working up indeterminate masses in various organs, staging various cancers (rectal, prostate, cervical, liver, kidney), following up cystic pancreatic lesions, and further evaluating abnormalities of the female pelvic organs that remain unanswered by ultrasound (ultrasound is the gold standard for evaluating the uterus, endometrium, ovaries, and pregnancies).

Patients with difficulty breathing that are unable to perform consistent breath holds make obtaining high-quality MRI scans difficult and multiphase CT exams may be preferred for these individuals.

CT is preferred in the work-up of hematuria with non-contrast CTs performed to evaluate for renal stones in younger patients and patients with flank pain and CT urograms (CTU) performed in patients with risk factors for bladder cancer.

Musculoskeletal Imaging

 

Similar to the above, and you’re probably seeing a pattern here, MRI and CT serve different purposes with an MRI preferred for evaluating masses, joints, and joint infections and CT preferred for evaluating fractures. Both MRI and CT produce high-quality detailed images of the vasculature of the extremities, though CT angiography (CTA) is more commonly performed.

The Patient Experience

 

Now that we’ve discussed some of the differences between CT and MRI scans, let’s talk about the actual patient experience.


Process for Computed Tomography (CT)

 

– Schedule an appointment and receive instructions on how to prepare for the exam (may require not eating or drinking anything for several hours before your scan and/or arriving 1-2 hours early to drink oral contrast before an exam – mostly for CT scans).

– Arrive at the imaging center, check in with front desk staff, and fill out any necessary forms.

– Change into a medical gown. A peripheral intravenous (IV) catheter will be placed if your examination requires IV contrast.

– You will be taken into the CT scan room and lie on a table that will slide into the CT scanner.

– The technologist will slide you into the scanner and prepare to scan you. This requires taking some very low-dose preliminary images to set up the actual scan.

– You will be informed of the intravenous contrast injection (if having a contrast-enhanced CT) and you will be given breath hold instructions. The table will then slide through the CT machine as the CT scanner takes pictures of your body from different angles. You will hear the scanner rotating as you move through the scanner.

– The majority of your time will be spent preparing for the scan. The CT scan itself generally takes less than 30 seconds. Some exams may require more than one scan.

Yay – you’re done and can go on with your day! You are NOT radioactive after having a CT scan.

Process for Magnetic Resonance Imaging (MRI)

 

– Similar to getting a CT scan, you will check-in and fill out the necessary paperwork and change into a gown.

– You will be screened for the possibility of any internal metal (devices such as pacemakers and spinal cord stimulators, coronary stents, shrapnel, any medical implant, etc.) and undergo MRI safety clearance. A radiologist may be required to clear images of your eyes (orbits) and prior imaging if you are unsure if you have metal in your body. All external metal and electronic devices will be removed and stowed in a locker with your clothes.

– You will lie on a table that slides into the MRI machine. Headphones will typically be provided (and hopefully you can choose your music of choice!) as well as a squeeze device to alert the technologist if you are experiencing any issues during the scan.

– The scan will commence and you will be given various breathing instructions throughout the scan to avoid blurry images from breathing movement (just like motion causes blurring in photography).

– The machine will make loud noises throughout the scan (hence the headphones and frequently music) – this is normal.

– The scan usually takes 10 to 60 minutes, depending on the type of scan you are having.


Contrast Dye

 

Sometimes, an injection of contrast dye is used during a CT or MRI scan. The contrast helps produce more detailed images of pathology and helps detect abnormalities. The injection is usually given through an IV in your arm.

The contrast injection can make you feel warm for a short time. Some people also experience a metallic taste in their mouths. These side effects are normal and should go away quickly.

After the scan is complete, the IV is removed and you are sent home to (hopefully) enjoy the rest of your day.

The CT or MRI scanner sends images to be read by a radiologist. The radiologist will look for any problems and send a report to your provider or call your provider to discuss any urgent or emergent concerns. Your provider will discuss the results of the scan with you at your next appointment or call you if there’s anything urgent that requires immediate attention.

Are CT and MRI Scans Safe?

 

Yes, getting a CT or MRI scan is safe.

A CT scan does use ionizing radiation to create x-ray images, which carry a very low, though non-zero, risk of developing future cancer (typically 20 or more years in the future).

Your provider (doctor, physician assistant, or nurse practitioner) only orders imaging exams and other tests after performing a risk vs. benefit analysis to ensure the test is indicated. The radiologists reading the scans will have pre-built protocols that minimize the radiation dose to what is necessary for a diagnostic exam.

MRI scans utilize non-ionizing radiation (radio waves) and are safe in patients that have passed MRI safety clearance.

The injection of contrast for both MRI and CT pose slight risks including extravasation (contrast leaks out into the soft tissues surrounding the IV) and an allergic-like reaction to the contrast. Both are quite uncommon and typically very mild. A small number of people may have an allergic reaction to the contrast that typically consists of hives (rash) or rarely difficulty breathing or anaphylaxis (a life-threatening condition). While incredibly rare, a radiologist or other provider is generally on-site specifically to treat these reactions when they rarely occur. Patients will occasionally experience side effects (not allergic reactions) like nausea.

Contrast may potentially negatively impact kidney function in patients with chronic kidney disease (CKD) stage 4 or 5. (eGFR <30 mL/min/1.73m2) These patients will generally require IV hydration before and following the CT to help protect their kidneys. This issue does not exist with MRI contrast.

You should tell your doctor if you are pregnant or think you might be pregnant. Radiation does pose potential risks to the fetus and should be minimized whenever possible. There are also theoretical risks associated with MRI (due to the strong magnetic field and radio waves used), particularly in the early first trimester. Generally, only the bare minimum of sequences necessary to make a diagnosis is used in pregnant patients requiring an MRI. MRI contrast is also avoided in pregnancy due to theoretical risks to the fetus. Other medical imaging methods such as ultrasound should be considered whenever possible.

Overall, both CT and MRI are relatively safe procedures with incredibly high diagnostic value.

CT Scan Risks

 

You will be exposed to a small amount of radiation that carries a very small risk of developing future cancer from the radiation exposure, typically 20 or more years following the exposure. The risk is incredibly low, but not zero.

MRI Scan Risks

 

– People with pacemakers or other metal implants cannot have an MRI because the magnet can interfere with the device unless an electrophysiology cardiologist is available to turn the device off during the procedure and check the device following the procedure.

– There is a small risk of burns from the MRI scanner. This is why we remove all metal from a patient and tell patients not to cross their legs or arms.

– You may be claustrophobic and feel anxious during the scan.

– MRI machines make very loud noises. Headphones and music help, but you will still hear the loud MRI noises in the background.

Can I Choose Which Scan to Get?

 

If your doctor orders a CT or MRI scan, you may be able to choose which test to get depending on your insurance coverage.

Ultimately, however, it is up to your doctor to order the type of scan that is most appropriate for diagnosing your medical problem. For example, if your doctor suspects you have appendicitis, they are most likely to order a CT scan as it is readily available and can rapidly produce pictures of the appendix and identify complications from appendicitis.

If you are pregnant or think you might be pregnant, tell your doctor before getting a CT or MRI scan. You may be able to choose an ultrasound or MRI instead of a CT scan.

If you have metal in your body, such as a pacemaker or artificial heart valve, make sure to tell your doctor. The metal could interfere with the images from a CT scan. You might not be able to get an MRI scan because the magnet can interfere with your device.

If you need an MRI and are easily claustrophobic, you can ask if there is an open MRI scanner nearby. An open MRI scanner places you between two pancake-like magnets rather than within a big donut-shaped magnet.

If you are unable to hold your breath consistently and repetitively for 15 seconds at a time, you may find MRI scanners difficult and result in poor image quality, limiting the ability to make a definitive diagnosis.

Conclusion

 

CT and MRI scans are excellent imaging modalities in medicine and frequently have a complementary role to one another.

CT scans are generally a little cheaper and more widely available, however, will result in exposure to harmful radiation. They are a fast and reliable way of imaging the body and making important clinical diagnoses.

MRI scans produce a more detailed image of the soft tissues. MRI scans are excellent for evaluating the brain, spine, and joints as well as in the work-up of incidental findings, particularly unexpected masses within the body. It is not infrequent for CT reports to recommend MRI scans to better evaluate incidental findings to figure out if they have any clinical significance or can be safely ignored.

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