CT Scan Versus MRI - Uses, Procedure, and Difference

Introduction:

CT or computed tomography scan is an imaging technique that shows hard tissue structures such as bones clearly. With the use of contrast materials in CT, the soft tissue structures can appear clearly in the images. MRI or magnetic resonance imaging is also an imaging technique that uses strong magnetic fields to diagnose structural and functional abnormalities of the body's internal structures.

What Are the Uses of CT Scans?

CT is useful in the diagnosis of the following conditions:

• Cause for unexplained pain.

• Muscle disorders include muscular dystrophy (a group of diseases that cause muscular weakness).

• Bone and joint disorders such as bone tumors (cancer), osteoporosis (a condition in which bone becomes weak and thin), complex fractures (severe fractures), and spinal cord injuries.

• Guiding tool in surgery and biopsy (a procedure that collects some tissues from the body which will later be viewed under a microscope).

• Determine the effectiveness of the treatment in cancer, such as chemotherapy (anti-cancer drugs used to kill cancer cells) and radiotherapy (radio waves used to kill cancer cells).

• Heart diseases such as coronary artery disease (narrowing or damage to the coronary artery).

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• Vascular diseases such as blood clots, blockages in blood vessels, and atherosclerosis (narrowing of blood vessels by fat or cholesterol deposits).

• Lung diseases such as pleural effusion (collection of fluid between the lung tissues), emphysema (damage to the air sacs in the lungs causes breathing difficulty), and lung cancer.

• Abdominal disorders such as cancer of the liver, kidneys, or pancreas.

• Causes of hematuria (presence of blood in the urine).

• Brain disorders such as brain cancer, hemorrhage (bleeding from damaged blood vessels), brain calcification, and internal bleeding.

What Are the Uses of MRI?

MRI is useful in the diagnosis of diseases of various systems in the body. Some of them are:

• Breast cancer.

• Brain disorders such as stroke (damage to the brain cells from reduced blood supply), multiple sclerosis (a disease that affects the communication between the brain and other body parts), or other conditions affecting the brain and spinal cord.

• Blood clots, blockages in blood vessels, and aneurysms (bulging of the blood vessels due to the weakness of the vessel wall).

• Ligament problems, joint and tendon problems.

• Inflammatory bowel diseases such as Crohn's disease (a disease in which the digestive tract becomes swollen) and ulcerative colitis (inflammation of the lining of the large intestine and rectum).

• Abnormalities of the liver, kidney, pancreas, uterus, ovaries, prostate, and spleen.

• Spinal cord disorders.

• Tumors (cancer) of bone and soft tissues.

• Detects the extent of damage caused by heart diseases.

• Metastasis (cancer spread) to bones and brain.

How Is a Computed Tomography Scan Taken?

In a CT scan, you are asked to wear the hospital gown and lie on your back on the narrow table in the scanner. Your technician may use a pillow or straps to hold you in a required position. Sometimes you may require contrast materials which depend on the type of exam. The contrast materials may be swallowed or injected through an intravenous line (IV). The technician uses the remote control from a separate room to move the table into the CT machine. The table slowly moves through the scanner for scanning. You will go through the machine several times.

A narrow beam of X-ray circles around the part of the body. It provides a series of images from different angles. A computer uses these images to create cross-sectional images called slices (2D images). This process is repeated to take many slices. The computer arranges these slices to get detailed (3D) images of the organs, blood vessels, and bones. You may be asked to hold your breath for a few seconds to get better images. Any motion, including breathing and body movements, may lead to artifacts in the images.

How Is the MRI Taken?

During a scan, you may be asked to lie on the table that will be moved into the scanner. Sometimes, they may keep a pillow or blankets to make you comfortable. You may be provided with earphones to block the loud noise heard from the scanner during the imaging process. Your technician will communicate with you from a separate room through an intercom. Once you are ready, the MRI machine produces a strong magnetic field around you, and radio waves are directed to the target area to produce the images of the required area. You may be asked not to move during the procedure as it may affect the image quality. The final images will appear on the computer screen.

How Is the CT Scan Different From MRI?

Although both CT scans and MRI are imaging techniques, there are some differences between them in their structure, need, and efficiency. They are:

• CT scan is more cost-effective than an MRI scan.

• CT scan requires less time to complete the imaging process, usually about 15 minutes. Sometimes, you may require a contrast agent in this procedure to get better images. However, MRI requires more time to complete the imaging process, which is about an hour or longer.

• During MRI, you can hear a loud noise from the scanner. But CT scan does not produce any sound during imaging.

• MRI produces more detailed and sharp images of soft tissues such as ligaments. In CT, the soft tissues sometimes hide the views of abnormalities. Therefore, a CT scan is more effective in diagnosing blood vessel abnormalities, trauma, and cancer staging than MRI.

• CT scan produces ionizing radiation, but MRI does not produce any radiation. Ionizing radiation may damage your DNA and lead to cancer. But the risks are very less. The risks of developing fatal cancer in CT scans are about one in 2000. The risk may increase with every CT scan in a lifetime. Ionizing radiation is more harmful to children as they are growing. You should talk with your doctor about CT's benefits and potential risks before the scan.

• MRI scan has some risks for patients having metal objects in their body. Some of them are pacemakers (a small device that regulates the heartbeat), artificial joints, intrauterine devices (a small T-shaped device used in birth control) [IUD], dark tattoos, aneurysm clips (metal clips used to close the bleeding site), eye implants. As MRI produces a strong magnetic field, the magnetic field pulls the iron-based materials during imaging. It may displace the aneurysm clips, resulting in severe bleeding that can cause death.

• As an MRI machine is a large closed tube, it makes some patients afraid of the procedure and produces anxiety (claustrophobia). But CT is a donut-shaped machine that is not completely closed and makes the patient comfortable during imaging.

• Larger people cannot fit into an MRI machine; in such cases, a CT scan is advised because of its open design.

• Due to radiation exposure, CT is not advisable in pregnancy. However, MRI without contrast can be taken during pregnancy as it does not show any proven risks.

Conclusion:

Though CT and MRI are effective diagnostic tools, doctors should choose them wisely based on the patient's medical conditions and considering all the factors. Because of its short scanning time, a CT scan can be used in emergency medical conditions such as internal bleeding and stroke. In addition, CT is the first line of choice in cancer staging. MRI is recommended for soft tissue imaging as it provides clarity and detailed images of the soft tissue structures.

The health provider should consider the principle of ALARA (as low as reasonably achievable) during imaging, which means that we should minimize radiation exposure or try to avoid them in unnecessary conditions. Before choosing the scan, the physician should keep the benefits and risks of each of them in their mind.