What is a CT Scan & Why is it Done?|Procedure, Risks, Duration of CT Scan

What is a CT Scan?

CT scan (also known as CAT scan, computerized tomography scan, computerized axial tomography scan) is an imaging test, which uses a combination of x-ray images and computer to generate cross-sectional images of the body. CT scan was developed by Dr. Alan Cormack and Sir Godfrey Hounsfield; and in 1979 they both were jointly awarded the Nobel Prize for their work. Today the CT scan has become an important tool for diagnosing and detecting many medical conditions.

In CT scan, a combination of a series of x-ray images are taken from various angles of the body and computer processing is used to produce cross-sectional images or slices of the blood vessels, bones and soft tissues within the body. CT scan helps generating more detailed information than X-rays. CT scan is especially beneficial in quick examination of people who may have sustained internal injuries from injury/trauma or car accidents. A CT scan helps in visualizing almost all the parts of the body. This test also helps in diagnosing any injury or disease and is beneficial in planning the type of treatment the patient needs (medical, radiation or surgical treatment). CT is scan is also one of the most commonly done imaging tests. This is a safe, noninvasive and well-tolerated test.

What is a CT Scan?

What Is Meant By Cross-Sectional Images? How Does CT Scan Help By Generating Cross-Sectional Images?

A good description of how the CT scan helps in visualization of the body is how, if a person who wants to look inside a loaf of bread, does so by slicing the bread loaf. Similarly, images produced by CT scan helps the doctor to visualize the inside of the body where cross-sectional pictures or images (slices) of the body are taken. CT scans are commonly used to assess the internal structures of the brain, spine, neck, chest, abdomen, sinuses and pelvis.

CT scan as mentioned before generates cross-sectional images of the body which appears as the body as opened up and allows the doctor to look at it from the inside. CT scan uses x-rays and computer to produce images of the part of the body, which is being scanned. CT scans are better in evaluating soft tissues and abdominal organs to help detect subtle abnormalities, which may not be seen on regular x-rays.

CT scan, when it was invented, had completely changed the way a disease was diagnosed as the doctors could see certain injuries, or diseases, which previously could only be seen when the patient was cut open either during surgery or autopsy. CT scan provides images, which are highly detailed of different parts of the body.

Why is CT Scan Done?

CT scan is often done for further assessment of an abnormality that was initially detected or suspected during another test, such as an ultrasound or x-ray. CT scan is also done to assess the spread of cancer in a patient. A CT scan can be done to study all parts of the body, which include chest, abdomen, arms, legs, pelvis; internal organs such as heart, lungs, liver, intestines, pancreas, bladder, kidneys and adrenal glands. CT scan also helps in studying and in close inspection of the bones, blood vessels and the spinal cord.

CT scan of the Brain or Head is done to assess the internal structures of the brain to look for an area of bleeding, mass, stroke or any abnormality in the blood vessel. CT scan of the head can also be done to look at the skull.

CT scan of the Neck or neck CT is done to look at the soft tissues of the neck and to study a mass or lump in the neck. CT scan of the neck can also be done to look for lymph nodes enlargement or enlarged glands.

CT scan of the Chest (Thorax) is done to further study an abnormality or a problem that is detected on a plain chest X-ray. Chest CT can also be done to look for enlarged lymph nodes, problems with the heart, aorta, esophagus, lungs and tissues present in the chest. Some of the common problems which can be detected or diagnosed with the help of chest CT scan include pulmonary aneurysm, pulmonary embolism, lung infection, lung cancer and to check the spread of the cancer from lungs to other parts of the body.

CT scan of Sinuses is done to diagnose sinus diseases, as well as to detect obstruction or narrowing in the sinus drainage pathway.

CT scan of the Abdomen and Pelvis is done to look at the abdominal and pelvic organs, which include liver, spleen, pancreas, kidneys, and adrenal glands along with also looking at the gastrointestinal tract. CT scan of pelvis and abdomen is also done to look for the cause of pain or to closely look at an abnormality that has been detected on another test, such as ultrasound. Pelvic CT scan in a woman consists of looking at ovaries, uterus and fallopian tubes. Pelvic CT scan in a man consists of looking at the seminal vesicles and the prostate gland. Some of the common problems, which can be detected or diagnosed with an abdominal or pelvic CT scan, include abscesses, cysts, aneurysm, infection, tumors, foreign objects, enlarged lymph nodes, bleeding in the abdomen, inflammatory bowel disease, diverticulitis, appendicitis, ectopic pregnancy etc.

CT scan of the Spine is commonly done to diagnose a herniated disc or spinal stenosis (narrowing of the spinal canal), especially in patients who have arm, neck, leg, and/or back pain. Spinal CT scan is also done to detect fracture or a break in the spine.

CT scan of the Urinary Tract System consists of looking at the kidneys, ureters and bladder. This type of CT scan is known as CT Urogram or CT KUB and helps in detecting bladder stones, kidney stones and urinary tract blockage.

CT scan of the Arm or Leg is done to look for problems such as fracture etc. in the shoulder, wrist, elbow, hand, knee, hip, ankle, or foot.

What are the Risks of CT Scan?

CT scan is a safe, non-invasive and a very low-risk procedure. The patient gets exposed to radiation while undergoing the CT scan, but this radiation is of a safe level. Some of the potential risks or concerns with a CT scan consist of:

Allergic reaction to the Contrast Dye: The primary potential risk with a CT scan is when a contrast dye is used which is given by injection. This contrast dye is given to help distinguish abnormal tissues and normal tissues more clearly. The dye also helps in differentiating blood vessels from other structures, such as lymph nodes. Just as a person can develop an allergic reaction to a medication, similarly they can also develop serious allergic reaction to a contrast dye used in CT scan. Patients who are at an increased risk for an allergic reaction from a contrast given during a CT scan need to undergo special pretreatment and the test needs to be done in a hospital setting. Patients having previous history of a severe allergic reaction to other medications or of contrast reaction or if they suffer from emphysema or asthma or have severe heart disease are at a higher risk for a having an allergic reaction from a contrast dye used in CT scan. Other than the allergic reaction, the intravenous contrast dye used in CT scan can also potentially damage kidneys, especially if the patient has preexisting mild kidney disease. For this reason, the patient is told to drink plenty of fluids so that the dye gets easily flushed out of their system.

Leaking of the Dye: Whenever an injection is given into a vein, there is a risk of leakage of the contrast outside of the vein and beneath the skin. In case of leakage of large amount of contrast dye under the skin, there can be break down of the skin; however, this happens rarely.

Breastfeeding: If the patient is breastfeeding then she should consult her doctor about it. According to experts, there is very less amount of dye that passes into the breast milk. If necessary, the patient can store some of the breast milk before the CT scan and use it the following day.

Diabetes & Metformin: If the patient is diabetic and takes metformin then the contrast dye in the CT scan can cause some problems. The patient will be told when to stop taking metformin and when to start taking it again or about any dose adjustments which are needed.

Cancer: There is a very small risk of getting cancer from certain types of CT scans. This risk is higher in children, young individuals, and those people who need to undergo various other radiation tests.

What Should One Do To Prepare For A CT Scan?

Patients who are supposed to have a contrast injection during CT scan should not eat or drink anything for some hours before the CT scan is done, as the contrast dye can cause stomach upset. An IV is inserted into the patient’s arm just before the scan so that contrast can be passed to the body via the IV.

It is also important to drink an oral contrast agent, which contains dilute barium before majority of the CT scans of pelvis and abdomen are done. Dilute barium helps in clearly identifying the gastrointestinal tract, which consists of stomach, small and large intestine, helps in detecting abnormalities present in any of these organs, and also helps in differentiating these structures from other structures present inside the abdomen.

CT Scan Procedure: What Happens During The CT Scan?

Majority of the CT scans are done as an outpatient procedure and do not need hospitalization. Patient can go home after the test is done. The CT scanner projects x-rays throughout the area of the body, which needs to be scanned. With each rotation of the scanner, there is generation of a thin slice of the area or the organ, which gets scanned. All these slices or images are saved on the computer and can later be printed. The contrast dye can also be placed into other parts of the body such as a joint or the rectum in order to visualize those areas better. Sometimes, the patient needs to drink the dye, which helps in easy and clearer visualization of the organs and structures.

The radiologist or a technician is present in an adjacent room and observes the patient through a large glass window. The patient can interact with the radiologist via a two-way intercom if need be.

What Does The CT Scanner Look Like?

The CT scanner looks like a big donut and it has a narrow table present in its center. During MRI scan, the patient is placed inside the tunnel of the scanner and can feel claustrophobic; whereas in CT scan, the patient rarely feels claustrophobic due to the openness of the doughnut shape of the CT scanner. During a CT scan, the patient is told to lie on his/her back on the table. This table then passes through the center of the machine. Depending on the part of the body being scanned, the patient can pass through the scanner, either head first or feet first. For some scans, such as CT scan of the middle ear and the sinuses, the patient has to lie on his/her stomach and pass through head first.

What Does The Patient Need To Do During A CT Scan?

The patient needs to remain immobile or motionless during the process of the CT scan, which usually consists of a few minutes. The scanner moves around the patient taking images and patient has to lie motionless. In some cases, the patient is told to hold their breath for about 20 seconds. Patient should not wear any metal during the CT scan and has to remove anything metallic, such as jewelry before the CT scan.

What Should Patient Wear During A CT Scan?

The type of clothing of the patient also depends on the nature of the study that is being done. If the CT scan of a head is being done, then patient can remain in his/her normal street clothes during the scan. However, patient needs to wear a hospital gown if the CT scan is done of the chest, abdomen or pelvis.

Does The Patient Need Any Sedative Before The CT Scan?

Patient usually does not need any type of sedation, as the machine is quiet and patient only hears a quiet whirr or a click as the table and scanner move during the procedure. In some cases, the patient can get anxious or nervous and may require a sedative to relax before undergoing CT scan.

What Is The Duration Of The CT Scan?

The entire procedure of the CT scan includes set-up of the machine, the scan itself, looking at the images, and removal of the IV if required and it takes about 15 to 45 minutes depending on the part of the body being scanned. The actual scan itself takes only a few minutes but the whole process of setting up the scan till the end takes about 30 to 45 minutes on an average.

How Does The Patient Feel During The CT Scan?

A mentioned previously, CT scan is a well-tolerated test. The patient does not feel any pain during CT scan. The table, on which the patient is lying, is felt as cool and hard. The room where the CT scan takes place may also be cool. Patient may find it difficult to lie still during the test and may fidget or get restless; however, it is important to make no movement during this test. Some patients tend to feel nervous inside the CT scanner and may need to be given a sedative to help them relax. If an IV dye is used, then the patient may feel a pinch or a sting where the IV needle has been started. The dye can make the patient feel warm and flushed. It can also give the patient a metallic taste in the mouth. There are some patients who may feel nauseous or may get a headache. It is important to tell the doctor how you feel.

What Happens After The CT Scan Is Done?

After the completion of the CT scan, the IV of the contrast injection is removed from the arm. Patient is observed for any ill effects from the contrast injection or the scan itself. Rarely, patient may be given sedation and will be kept in the hospital for sometime before being sent home and that too only after they are fully awake and alert; but then also patient needs a caretaker to drive him/her safely to home.

Who Interprets the CT Scan?

A radiologist, who is a doctor specialized in interpreting various imaging studies, interprets or analyzes the CT scan. After which the results of the CT scan are then forwarded to the patient’s consulting doctor.

What Are The Symptoms Of The Allergic Reaction To The Contrast Dye Given During CT Scan?

If the patient experiences an allergic reaction to the contrast dye, then it usually always occurs immediately. It is very rare for the patient to have a reaction after leaving the hospital. In cases where the patient feels that he/she is developing a delayed reaction to the contrast dye, then they should immediately consult the hospital where they have had the CT scan.

Symptoms of an allergic reaction to the contrast dye given during CT scan consist of difficulty in breathing, itching and difficulty in swallowing. If there is leakage of contrast under the skin, then the skin would appear red with pain and swelling. Patients usually will be told to come the next day to the hospital to get their skin checked. CT scan itself does not consist of any side effects; however, if the patient is supposed to have multiple CT scans, then it is important to discuss this with their physician regarding the degree of radiation exposure from the various CT scans.

What Other Purposes Is CT Scan Used For?

Other than looking or generating cross-sectional images of the internal organs and tissues, a CT scan can also be done to ascertain that a particular procedure is done in the correct manner. For example, CT scan can be done to help guide the doctor when passing the needle during a tissue biopsy. With the help of the CT scan, the doctor can see where to place the needle and in which direction to pass the needle to collect the sample for biopsy. CT scan can also be done to guide the proper needle placement for drainage of an abscess. CT scan is also done to determine the staging of the cancer by assessing the extent of the cancer.

What Questions Does The Doctor Ask The Patient Before A CT Scan?

The doctor or the radiologist needs to ask the patient some questions before getting the CT scan done:

  • Whether the patient is pregnant?
  • If the patient is allergic to any medicines, including iodine dyes?
  • Does the patient have diabetes?
  • If patient takes metformin, as the dose needs to be adjusted a day before and after the CT scan.
  • Does the patient have a heart condition?
  • Whether the patient has asthma, multiple myeloma or kidney problems?
  • Has the patient had an x-ray using barium contrast material in the last 4 days, as barium shows up on the films and makes it difficult for the images to be viewed clearly.
  • If the patient is claustrophobic or tends to become nervous in confined spaces, as the patient needs to lie still inside the CT scanner and may need a sedative to help relax if he/she is anxious or nervous.
  • If the patient can arrange someone to take him/ her home after the CT scan, especially if a sedative is administered before the CT scan.
  • If the patient is supposed to have CT scan of the abdomen, then patient is told not to eat any solid foods the night before the CT scan. In some CT scans, patient may also be given an enema or a laxative before the test.

What Do The Results Of The CT Scan Mean? What Does A Normal/Abnormal CT Scan Look Like?

Results of CT scan may be available on the same day or within a day or two.

Normal CT scan: The blood vessels and organs are normal in shape, size, and location. There is no blockage in any blood vessels. There are no foreign objects such as glass fragments or metal visualized. A normal CT scan will not consist of any growths (tumors), infection or inflammation. There is no bleeding seen within any of the structures or nor are there any collections or pockets of fluid present.

Abnormal CT scan: In an abnormal CT scan, the organ visualized appears too small or too large. There can be damage or infection of the organ. Abscesses or cysts may also be present. There can also be presence of foreign objects (such as glass fragments or metal) visualized in the images of the CT scan. If the patient has gallstones or kidney stones, then they can be seen clearly on the CT scan. If any tumors are present in the lungs, colon, liver, ovaries, bladder, kidneys, pancreas or adrenal glands then they can be easily seen. CT scan of the chest can display pulmonary embolism/aneurysm, infection or fluid in the lungs if present. Blockage if present in the bile ducts or intestines can be seen. Abdominal CT scan can reveal diverticulitis or inflammatory bowel disease, if present. One or more blood vessels can be blocked. There can be enlargement of lymph nodes seen in an abnormal CT scan. Fracture, tumors, or infection can be visualized in the arm or leg or other regions.

What Factors Affect The CT Scan?

The following factors can stop the patient from undergoing a CT scan or can change the results of the CT scan

  • If the patient is pregnant, then she should not undergo a CT scan unless advised by her doctor.
  • If the patient had recently undergone a test in which barium was used, then it can change the results of the CT scan, as barium shows up on the CT scan and can interfere with the visualization of the structures within the body.
  • If the patient is not able to lie still or motionless during CT scan, then it also interferes with the result of the test. In such cases, patient may be given a sedative to relax.
  • If there are metal objects present in the body, such as metal implants in joint replacements or surgical clips. These will prevent a clear view of the part of the body being scanned.

Revolution of CT Scan

CT scanners were first installed in 1974 and since them have undergone vast improvement in terms of comfort of the patient and also duration of the scan, where it takes very little time when compared to before, along with producing higher-resolution images, which help in achieving a more definite diagnosis. A good example is small tumors or nodules, which can be missed on an x-ray, but can be seen clearly on a CT scan.

What Are Special Or The Latest CT Scanners And What Are Their Benefits Over Old CT Scanners?

Spiral (helical) CT scanners and multi-slice (or multi-detector) CT scanners are special and latest CT scanners. Most of the modern scanners are multi-slice scanners. These special CT scanners help in:

  • Taking better and clearer pictures of the organs and blood vessels, so that additional imaging tests are not needed.
  • The modern CT scans consume less time, i.e. they scan and provide the images in lesser time than the previous scanners.
  • The results of the CT scan are usually compared to the results of PET (positron emission tomography) scan, which is used to help find cancer. There are some latest scanners, which can do both the scans at the same time.
  • An electron beam CT scan is another special type of CT scan, which helps in detecting coronary artery disease and atherosclerosis. An electron beam CT scan is faster than a regular CT scan and it can also take good pictures of the coronary arteries while the heart is beating. Electron beam CT scans are not widely available. Multi-slice CT scan is another type of CT scanner, which is almost as fast as electron beam CT scanners and is more easily available.
  • A CT angiogram can show two- and three-dimensional images of the heart and blood vessels.
  • A spiral CT scan helps in faster scanning of the lungs than a regular CT. this scan is often recommended to screen for lung cancer in patients who are above the age of 55 and who have a high risk of lung cancer.
  • Coronary calcium scans utilize CT scan to detect the initial signs of coronary artery disease. Coronary calcium scans are however, not recommended for routine screening.
  • MRI scan can give different information than a CT scan regarding certain conditions.

Points to Ponder

  • The results of the CT scan can be different from other types of imaging tests, such as ultrasound scans; as the CT scan provides a different view of the body from different angles.
  • An ultrasound scan does not use dangerous radiation and can give results that are similar to that of a CT scan. If the patient is concerned about the radiation exposure in the CT scan, then should ask the doctor whether an ultrasound scan can be done instead of a CT scan.
  • Children who need to have CT scan need specific instructions regarding this test, as the child may get scared during the test or may hold his/ her breath during the CT scan.
  • If the child is very young and does not lie still or is scared, then the doctor may give the child a sedative to help him/her relax. Whether there is any risk to the child regarding the radiation exposure during the CT scan should also be consulted.

Also Read:

Pramod Kerkar, M.D., FFARCSI, DA
Pramod Kerkar, M.D., FFARCSI, DA
Written, Edited or Reviewed By: Pramod Kerkar, M.D., FFARCSI, DA Pain Assist Inc. This article does not provide medical advice. See disclaimer
Last Modified On:February 12, 2018

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