If you've ever been injured or had a trauma, chances are, you've had to do a CT scan or an MRI. Advanced imaging techniques allow doctors to see inside the body without cutting the body open. They are a crucial component of modern medicine, and they make it easier for doctors to assess minute damages and problems within all organs and bodily tissues.
So, without further ado, let's discuss the technology behind CT scans and MRIs and how they are applied to modern medicine!
- First, we will discuss what a CT scan and an MRI are.
- Then, we will learn how images of CT scans and MRIs are interpreted, and we will also elaborate on the potential risks of undergoing a CT scan or MRI.
- After, we will review the difference between a CT scan and an MRI.
- Lastly, we will talk about how CT scans and MRIs can affect patients with claustrophobia.
Definition of CT Scan and MRI
Let's start by looking at the definition of computed tomography (CT) scan.
The computed tomography (CT) scan (also known as computed axial tomography or CAT scan) is an imaging procedure where a machine projects a beam of X-rays toward a patient while it rotates around the body, relaying information to a computer which then generates images of different views of the selected region of the body.
With a CT scan, the viewer can better understand the depth and shape of the structures being analyzed than with typical X-ray images. Additionally, CT scans can simultaneously show bone, soft tissue, and blood vessels.
For this reason, a CT scan has different uses, such as:
To evaluate stroke, abdominal tumors, and abscesses.
Assess patients with acute traumatic injuries (say, from a motor vehicle accident).
Help visualize tumor and assess its size, location, and potential interaction with nearby tissues and organs when cancer is suspected.
Diagnose abnormalities in bone and soft tissues (such as muscles, tendons, and cartilage), especially when X-rays, physical exams, or other exams do not produce a definitive diagnosis.
Test for Alzheimer's disease and other causes of cognitive decline in adults with cognitive impairment.
Now, let's look at what magnetic resonance imaging (MRI) is.
Magnetic resonance imaging is a medical imaging technique used to produce images of the inside of the human body. MRI allows doctors to obtain information on soft tissue (such as internal organs in humans) and determine the extent of a patient's injury, illness, or tumor growth.
To create high-quality visual images of organs and tissues within the body, MRIs combine a magnetic field with computer-generated radio waves.
You can use the MRI scan to diagnose many types of cancers, concussions, broken bones, and other structural injuries. Doctors typically use MRI scans to determine whether a patient needs surgery, and it allows doctors to plan exactly where to operate in the case of surgery.
MRI vs. CT Scan Images
Now take we know what MRIs and CT scans mean and some of their uses, let's take at what their images look like and how doctors interpret them.
Interpreting CT scans requires a systematic approach. First, the radiologist must know the patient's history. A patient with a history of CT scans or other imaging procedures can compare previous reports with the latest scan to come up with a more accurate interpretation.
After viewing the images, they should check their orientation. Orienting the image correctly allows them to easily identify anatomical structures and abnormalities.
Anatomical structures are identified by how bright or dark they appear in the image, which corresponds to their density. The density of the tissue is proportional to the attenuation of x-rays passing through it.
Low-attenuation tissues, such as lungs and fat cells, appear dark on a CT scan due to their low density.
Muscles, livers, and bones are denser and have a higher attenuation, so they appear as bright patches on CT scans.
The radiologist would then write a report based on their interpretations. A copy of this report will be added to the patient's medical file, and a copy will be sent to the physician who ordered it. Based on this information, the doctor would decide on the best way to treat the patient.
A radiologist is a specialist who has had training in the use of imaging equipment and cross-sectional image interpretation.
Figure 2. CT scan of a calcaneocuboid joint with calcaneal fracture.
The images produced by magnetic resonance imaging (MRI) look similar to those from a CT scan. When conducting an MRI, the technician can isolate the area of interest and only generate images of that given area.
If a doctor suspects that you may have a tumor deep within your brain, they may order a coronal section of your brain to view some deep brain structures, such as the hippocampus.
Now, if the doctor suspects that you may have a tumor in between the two hemispheres of your brain, they may order a sagittal section of your brain image to see the structures located in the middle of your brain.
On the other hand, if a doctor wants to look within a specific structure of the brain, they may order a transverse section.
MRIs are interpreted visually, meaning that they usually show structural damage in different colors. For instance, a bleed within the brain and body will be darker than the rest of the examined structure, while a tumor will appear lighter than the examined structure.
Similarly, lesions associated with multiple sclerosis (MS) will produce white spots in the brain and spinal cord due to the loss of myelin, which is the insulation that protects your neurons!
Figure 3. MRI imaging showing brain lesions.
MRI types
A special type of MRI is known as a functional MRI (fMRI).
A functional MRI is used to produce images of blood flow to certain areas of the body. This method is primarily used in the brain to diagnose bleeds and identify strokes and other traumatic brain injuries.
fMRIs can also be used to determine which areas of the brain control which actions. For example, If you speak in the fMRI machine, it will develop a bright image in the speech-producing part of your brain!
In addition to the fMRI, there are other types of MRIs used by healthcare providers:
Cardiac MRIs provide visualization of the heart and are often used for patients with heart failure or cardiac tumors.
Magnetic Resonance Angiography (MRA) measures blood flow through arteries.
Magnetic Resonance Venography (MRV) measures blood flow through veins.
These MRIs are used only if traditional MRIs cannot provide an accurate/detailed visualization of a person's illness, such as a tumor in a deep organ or reduced blood flow in a certain portion of the body.
CT scan vs. MRI radiation
There is a tiny amount of radiation involved in a computed tomography (CT) scan, which is well within the limits of what is considered to be safe.
In spite of CT scans exposing patients to more radiation than other x-ray treatments (like chest x-rays and mammograms), the cancer risk remains low.
If a patient undergoes multiple CT scans within a short period, the risks increase.
Furthermore, radiation affects adults and children differently. Due to their growing bodies and the fast rate at which their cells divide, children are more sensitive to radiation than adults.
Unlike CT scans, magnetic resonance imaging (MRI) uses no radiation!
Instead, MRIs use a strong magnetic field and radio waves to generate images.
While an MRI can produce necessary insight into what is going on in the body, there are risks associated with getting an MRI.
Since MRIs use powerful magnets, any metal implant in your body will disqualify you from being eligible for getting an MRI since your implant will most likely get ripped from your body.
If you have a pacemaker, you cannot get an MRI under any circumstances since the magnet can rip the pacemaker from your body and also inhibit its function.
It is not recommended to get an MRI when pregnant because the effects of exposing a fetus to magnetic fields are not well understood.
MRI vs. CT Scan Contrast
Now that we know what MRIs and CT scans are, and how doctors read their images, let's talk about contrast agents (or contrast dyes), that might be used during the procedure. The contrast dye is typically administered through an intravenous (IV) line.
A contrast dye may be injected into the patient before the test to help specific structures show up better on the images.
To optimize CT scans, contrast dyes should be able to opacify (turn opaque) blood vessels, organs, and tissues without altering physiology. Iodinated (iodine-based) contrast agents are the most common contrast agents used in CT scans because the iodine molecule absorbs X-rays, producing contrast!
For MRIs, the most common contrast agents used are gadolinium-based agents. Similar to CT scan agents, gadolinium allows doctors to see your organs and tissues more clearly, making diagnosis easier.
CT Scan vs. MRI for Abdomen
As an example, let's talk about how a patient should prepare for a CT scan or MRI of the abdomen. When a doctor suspects that something might be wrong with a patient's abdomen, the doctor might order a CT scan or MRI with contrast to check the abdomen and its organs for:
Tumors or lesions
Injuries and bleeding
Infections
Blood vessel problems
Unexplained pain
Blockages
If a CT scan with contrast has been ordered, you will need to avoid food or drinks (except water) for at least four hours before the procedure. When you arrive to the exam room, the doctor will inject the contrast dye to your arm using an IV line.
Sometimes, however, they might give you a liquid contrast to drink instead of the injection.
The next step will be to lie down on a narrow table that slides into a large, circular opening of the scanning machine. The scanner will begin to rotate around you, allowing the X-rays to pass through your body for a small period of time. The X-rays absorbed by your body tissues will be found by the scanner and sent to the computer, creating an image.
For an MRI, the process is similar. Before the MRI scan, the doctor will inject the contrast dye and leave the tube on your arm for the duration of the procedure.
In some cases, they will also inject a muscle relaxant to help reduce some movement and improve image quality.
After, you will be asked to lie down on the MRI scanner, which looks like a short cylinder open at both ends. Then, a light piece of equipment called a coil will be placed on top of your abdomen to help collect information when making the images. When the procedure is finished, the radiologist will review your images.
Pros and Cons of CT scan vs. MRI
Now, let's look at the pros and cons related to computed tomography (CT) scans and magnetic resonance imaging (MRI).
CT Scan
CT scans use X-ray to create images of bones, soft tissues, and blood vessels from different angles. They are usually ordered by a doctor to evaluate trauma, chest, bone fractures, changes in soft tissue that might indicate the spread of a disease, pain of unknown origin, and the function of the heart, kidneys, or liver.
A CT scan procedure is less expensive and considered quicker than MRIs. CT scans are typically 10-20 minutes due to the radiation being able to penetrate the body fairly easily.
In terms of risks, CT scans are not recommended for young children or pregnant women because CT scans use a small amount of radiation.
CT scans are often combined with PET scans, another imaging procedure. In a positron emission tomography (PET) scan, a radioactive substance is administered to the patient, and a machine detects energy released by the substance in the organ or tissue under study.
PET scans provide information about the metabolic pathways active within tissues and cells, while CT scans provide anatomical images of organs and structures. A PET scan can also reveal blood flow, chemical composition (for instance, oxygen levels in a particular tissue), and cancerous cell activity.
MRI Scan
MRI scans make use of magnets to capture extremely sensitive, detailed images of soft tissue in the body. MRIs help doctors diagnose tumors or cysts, diseases such as in the liver and pancreas, certain types of heart problems and blood vessel blockage, and even pelvic pain in women.
An MRI can last from 15 minutes to more than an hour if multiple areas and angles are needed to make a diagnosis. During the duration of the MRI, the patient must lay still in order for the images to be of high resolution.
Since MRIs do not use radiation, they are safer for pregnant women and children. However, MRIs should never be used by patients that have metal implants in their body or pacemakers as the magnet could rip them out of the body.
CT Scan vs. MRI Claustrophobia
CT scans and MRIs both require patients to be enclosed inside the machine, which is why they may be triggering for claustrophobic patients.
The term claustrophobia refers to an extreme fear of being confined to a small space.
Patients with claustrophobia must be given a sedative and must have a person talking to them while they get the scan because the patient must be completely still to capture a clear image.
CT scans may be less triggering to claustrophobic patients since the machine is more open, and the procedure lasts between 10-25 minutes.
MRIs, on the other hand, are less open and can take anywhere from 25 minutes to 2 hours which is why they may not be an adequate option for patients with severe claustrophobia.
CT Scan vs. MRI Difference
To finish off, let's make a table showing the difference between a CT scan and an MRI.
Computed Tomography Scan (CT) | Magnetic Resonance Imaging (MRI) |
Uses X-rays (ionizing radiation) | Uses a magnetic field and radio waves (no radiation) |
| Quick procedure, and usually lasts 10-15 min | Procedure takes longer, and can last up to an hour. |
| Less expensive | More expensive |
Uses iodine-based contrast agents | Uses gadolinium-based contrast agents |
Commonly used to evaluate trauma, fractures/broken bones, changes in soft tissue, fever, or pain of unknown origin, function of the heart. | Commonly used for the diagnosis and monitoring of tumors and cysts, certain types of heart problems, some diseases such as liver and pancreas disease. |
CT Scan vs MRI - Key takeaways
- CT scans are diagnostic imaging procedures in which a machine projects a beam of X-rays toward a patient while rotating around them. As they rotate, the machine relays information to a computer, which generates different views of the body.
- MRI scans produce highly detailed images of the inside of the human body. Magnetic fields and computer-generated radio waves are used in MRIs.
- MRIs use magnetic fields and computer-generated radio waves to generate images, whereas CT scans use a small amount of radiation.
- Due to the possible risks due to radiation, CT scans are often not recommended for pregnant women or young children.
- Patients with pacemakers or metal implants should never have MRIs since the magnet could rip them out.
References
- National Institute of Biomedical Imaging and Bioengineering (NIBIB), Computed Tomography (CT). Accessed 7 Sept. 2022.
- Hospital for Special Surgery Radiology, CT Scan (Computed Tomography ‘CAT Scan’), 26 July 2022.
- Johns Hopkins Medicine, CT Scan Versus MRI Versus X-Ray: What Type of Imaging Do I Need?, 6 Sept. 2022.
- Northwell Health, CT Scan - Imaging, Accessed 8 Sept. 2022.
- MedlinePlus Medical Encyclopedia, CT Scan, Accessed 8 Sept. 2022.
- National Cancer Institute, Computed Tomography Scan, NCI Dictionary of Cancer Terms, Accessed 8 Sept. 2022.
- The Editors of Encyclopaedia Britannica, Computed Tomography, Encyclopedia Britannica, Accessed 8 Sept. 2022.
- David C Preston, MD, CT Basics, Accessed 8 Sept. 2022.
- National Cancer Institute, Computed Tomography (CT) Scans and Cancer Fact Sheet, 14 Aug 2019.
- P Whiting, FRCA FFICM, N Singatullina, FRCA EDIC, JH Rosser, FRCA FFICM, Computed tomography of the chest: I. Basic principles, BJA Education, December 2015.
- Figure 2: CT scan of calcaneocuboid joint with calcaneal fracture (https://commons.wikimedia.org/wiki/File:CT_scan_of_calcaneocuboid_joint_with_calcaneal_fracture.jpg) by Cerevisae (https://commons.wikimedia.org/wiki/User:Cerevisae) licensed by CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en).
- Figure 3: MRI imaging showing brain lesions (https://commons.wikimedia.org/wiki/File:Brain_Lesions.png) by Elsevier/Academic Radiology licensed by CC0 1.0 Universal (https://creativecommons.org/publicdomain/zero/1.0/deed.en).
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