CT scan : Definition, Purpose, Work, Prepare, Risks, Side Effects

What is a CT Scan ?

• Computed tomography, also known as a CT or CAT scan, is a type of diagnostic medical imaging test.
• It provides several images or photos of the inside of the body, similar to standard x-rays.
• CT scan cross-sectional images can be reformatted in multiple planes. They are even capable of producing three-dimensional images.
• These images can be viewed on a computer monitor, printed on film or with a 3D printer or copied to a CD or DVD. 
• CT images of internal organs, bones, soft tissue and blood vessels are more detailed than traditional x-rays, especially of soft tissues and blood vessels.
• Radiologists can more easily diagnose problems such as cancer, cardiovascular disease, infectious disease, appendicitis, trauma and musculoskeletal disorders by using specialized equipment and expertise to create and interpret CT scans of the body. 

Purpose of a CT Scan

Your doctor may advise you to have a CT scan :

• Identify muscle and bone disorders such as bone tumors and fractures.
• Determine the exact location of a tumor, infection or blood clot.
• Surgery, biopsies and radiation therapy are examples of procedures that can be guided.
• Cancer, heart disease, lung nodules and liver masses are just a few of the diseases and conditions that can be detected and monitored.
• Keep track of the efficacy of specific treatments, such as cancer treatment.
• Detect internal injuries and bleeding.

How does CT work ?

• Unlike a traditional x-ray, which employs a fixed x-ray tube, a CT scanner employs a motorized x-ray source that rotates around the circular opening of a donut-shaped structure known as a gantry.
• The patient lies on a bed that slowly moves through the gantry while the x-ray tube rotates around the patient, shooting narrow beams of x-rays through the body.
• CT scanners use special digital x-ray detectors, which are located directly opposite the x-ray source, instead of film.
• The detectors pick up the x-rays as they leave the patient and send them to a computer.
• The CT computer uses sophisticated mathematical techniques to construct a 2D image slice of the patient every time the x-ray source complete one full rotation.
• The thickness of the tissue represented in each image slice varies depending on the CT machine used, but it typically range between 1-10 mm.
• When a full slice is finished, the image is saved and the motorized bed is incrementally moved forward into the gantry.
• The x-ray scanning procedure is then repeated in order to generate another image slice. This process is repeated until the desired number of slices is obtained. 
• Image slices can be displayed individually or stacked together by the computer to produce a 3D image of the patient that shows the skeleton, organs and tissues as well as any abnormalities that the physician is attempting to identify.
• This method has several advantages, including the ability to rotate the 3D image in space or to view slices in succession, which makes it easier to pinpoint the exact location of a problem.

What is a Contrast CT Scan ?

• Dense substances, such as bones, are easily visible in a CT scan. Soft tissues, on the other hand, do not show up as well.
• They may appear blurry in the image. To make them stand out, you may need to use a special dye known as a contrast material.
• They absorb x-rays and appear white on scans, highlighting blood vessels, organs and other structures.
• Iodine or barium sulphate are the most common contrast materials. These drugs could be delivered to you in one of three ways : 

1. Injection : The drugs are injected into a vein directly. This is done to enhance the visibility of your blood vessels, urinary tract, liver or gallbladder in the image.
2. Orally : Drinking a liquid containing contrast material can improve scans of your digestive tract, which is the path food takes through your body.
3. Enema : When your intestines are being scanned, contrast material can be inserted into your rectum.

• You'll need to drink plenty of fluids after the CT scan to help your kidneys remove the contrast material from your body.

What should I do to prepare for a CT Scan ?

• When you make an appointment for a computed tomography angiography (CTA) or virtual colonoscopy with Johns Hopkins radiology, you will be given specific instructions.

1. Precautions : If you are pregnant, please consult your doctor before scheduling the exam. Other possibilities will be discussed with you and your doctor.
2. Clothing : You may be asked to change into a patient gown. If this is the case, a gown will be provided for you. Personal belongings will be stored in a locker. Please remove all piercings and leave all jewelry and valuables at home.
3. Contrast Media : CT scan are most commonly performed with and without contrast media. The contrast media helps the radiologist see images of the inside of the body more clearly.
4. Allergy : If you have an allergic reaction to any contrast media, please notify the access center representative when scheduling your CT scan. If you have a severe or anaphylactic reaction to any contrast media in the past, you will not be given IV contrast. If you have previously experienced mild to moderate reactions, you will most likely need to take medication prior to the CT scan.
5. Eat/Drink : If your doctor has ordered a CT scan without contrast, you can eat, drink and take your medications before your exam. If your doctor has ordered a CT scan with contrast, you should not eat anything three hours before the scan. It is recommended that you drink clear liquids. You may also take any medications prescribed to you prior to your exam.
6. Diabetics : Three hours before the scan, diabetics should eat a light breakfast or lunch. Depending on your oral diabetes medication, you may be asked to stop taking it for 48 hours after the CT scan. 
7. Medication : All patients can continue to take their medications as usual.

Your doctor may request additional preparation based on your medical condition.

What happens during the test ?

A specially trained radiographer will perform the test in a scanning room. There may also be a nurse present.

Pre-Test

• After signing in and confirming your insurance information on the day of the test, you may be asked to sign a consent form stating that you understand the purpose and risks of the procedure. After that, you'll be led to a changing room to change out of your clothes.
• If you are having a traditional CT scan, you are now ready to enter the scanning room. However, if your doctor has ordered that your test be performed with a contrast agent, you will need to make the following preparations : 

1. If an IV contrast agent is ordered, you will be positioned on the scanning table and an IV line will be inserted into a vein, usually in the arm or groin, before a contrast agent is injected. The agent may be injected directly into the joint or lower spine in some cases. You may feel a brief flush or have a metallic taste in your mouth. Depending on the part of the body being scanned, you may have to wait for several minutes or more in a reclined or prone position. The IV line is left in place until the end of the scan.
2. If you are given an oral contrast agent, you will be asked to drink either a chalky or watery substance before entering the scanning room. Depending on the area of the body being examined, you may have to wait 30-60 minutes before the scan can begin. Inform the nurse or radiographer if you feel nauseated or distressed in any way.
3. If a rectal contrast agent is requested, you will be positioned on the scanning table and your rectum will be lubricated. An enema tube will be inserted into your colon to gradually fill it with the contrast agent. You may be given a shot of bus Copan to help with muscle spasms. A balloon is then inflated at the tip of the tube to prevent leakage and is kept there untie the scan is finished. 

• You may be asked to lie on your back side or stomach depending on the nature of the investigation. The table can be raised or lowered and straps and pillows can be used to keep you in place and still during the test.
• While staying still during the scan is essential, newer multi-detector CT systems are quick and easy, reducing the amount of time you need to hold your position.
• If you are accompanying your child, you must wear a protective apron to reduce your exposure to radiation.
• You will remain in the control room with the technologist during the scan but will be able to communicate with your child vis the two way speaker.

Throughout-Test

• When it is time to start, the technician will communicate with you through the speaker. At first, the motorized table will move quickly in and out of the scanner. This is done to ensure that the table is in proper starting position and that the scan covers the entire body part under investigation.
• You will also notice special light lines projected onto your body to ensure that you stay on track. The table will then move slowly through the scanner from then on.
• As the x-ray emitters produce an array of beams, the gantry will spin around you. The beams will pass through your body and be detected by detectors.
• In some cases, the technician may be able to make you feel more at ease without interfering with the imaging.

Post-Test

• When the scans are finished, the radiographer will double-check the images to ensure they are clearly visible.
• If an IV contrast agent is ordered, the IV line will be removed and the puncture wound will be bandaged.
• If you are given an oral contrast agent, you will be given a glass of water and advised to drink plenty of fluids.
• If a rectal contrast agent is requested, the solution will be withdrawn from the colon via the enema tube. After the tube has been removed, you will be led to the restroom to expel the remainder into the toilet.
• In most cases, you should be able to change back into your clothes and drive yourself home or to work.

After-Test

• Most iodine based contrast agents have a half-life of two to four hours, implying that they are completely eliminated from your body within a day or two.
• If you were given a barium solution, you may experience temporary constipation and chalky stools for a day or two.
• The radiation from the CT scan will not be retained in the body and you will not endanger anyone you touch, kiss and stand close to.
• Call your doctor if you have any unusual symptoms, such as fever, chills, vomiting, shortness of breath or a rapid heartbeat, regardless of the type of CT you had.

CT scan Risks

• Although an abdominal CT is a relatively safe procedure, there are some risks.
• This is especially true for children, who are more vulnerable to radiation than adults. Your child's doctor will only order a CT scan as a last resort, after other tests have failed to confirm a diagnosis.
• Allergic Reactivity : If you are allergic to the oral contrast, you may develop a skin or itchiness. A life threatening allergic reaction is also possible, but it is uncommon.
• Inform your doctor if you have any medication sensitivities or kidney problems. If you are dehydrated or have a preexisting kidney problem, IV contrast increases your risk of kidney failure.
• Birth Flaws : Because radiation exposure during pregnancy increase the risk of birth defects, it is critical to notify your doctor if you are or may be pregnant. As a precaution, your doctor may recommend another imaging test, such as an MRI or an ultrasound instead.
• Cancer risk is slightly increased : During the test, you will be exposed to radiation. The amount of radiation used is greater than that of an x-ray. As a result, an abdominal CT scan increases your risk of cancer slightly.

CT scan Side Effects

• The most common cause of abdominal CT scan side effects is a reaction to any contrast used. In the majority of cases, they are mild.
• Barium contrast can cause the following side effects :
• Abdominal cramping
• Diarrhea
• Nausea or vomiting
• Constipation
• Iodine contrast can cause the following side effects :
• A rash on the skin or hives
• Itchiness
• Headache
• If you are given either type of contrast and experience severe symptoms, contact your doctor or go to the emergency room immediately. These are some of the symptoms :
• Difficulty breathing
• Rapid heart rate
• Swelling of the throat or other body part