CT – Computed Tomography Scan

What is Computerized Tomography Scan (CT)?

Computed tomography (also known as CT scan or CAT scan) is a diagnostic test in which images of the body are generated with the help of a computer. The patient’s body is exposed to x-rays and a computer is used to provide cross-sectional images or slices of the body. Theses slices are known as tomographic images.

Conventional CT scan – The scan is taken slice by slice and the scan stops after each capture. This procedure requires the patient to hold his breath periodically. The inability on the part of the patient to do can result in movement artifact.

Spiral/helical CT scan – A constant scan which is taken in a spiral fashion. The time taken to procure images is much less and the scanned images are contiguous.

The first commercial CT scanner was invented by Sir Godfrey Hounsfield.

What are the indications of a CT scan?

A CT scan can be used to visualize any part of the body. The indications of a CT scan are numerous and depend upon the site or organ of the body that needs to be pictured. The common indications are-

CT head
– Tumor
– Trauma
– Cerebrovascular accident
– Hydrocephalus
– Seizures
– Unexplained headache
 
CT Chest
– Tumor
– Trauma
– Chronic obstructive pulmonary disease
– Interstitial lung disease
– Ischemic heart disease
– Pneumoconiosis
 
CT Abdomen
– Tumor
– Trauma
– Abdominal bleed
– Cirrhosis
– Vascular disorders
– Appendicitis
– Pancreatitis
– Renal calculus

Are there any contra-indications for CT scan?

– As radiation is employed in the procedure of CT scanning- pregnant females, particularly in the first trimester, should avoid undergoing it.

– Intravenous contrast media (IVCM) is sometimes used in the procedure of CT scanning. Patients allergic to IVCM should not undergo it.

– Intravenous contrast media should not be administered to patients with an impaired renal function as it may induce renal failure in them.

– Intravenous contrast media should not be used in patients suffering from myasthenia gravis, pheochromocytoma, and hyperthyroidism due to the fear of precipitating a flare-up in them.

My doctor has ordered a CT scan for me. What are the precautions that I need to take?

First, you need to inform your physician about your medical history, any medicines that you may be taking and any allergy that you might have. You also need to inform your physician if you are pregnant.

What are the risks involved?

A CT scan is a safe, non-invasive (unless you are injected with a dye) and a risk-free procedure. Over the years, its benefits have outweighed any risks involved which may include-

– Allergy to the dye used.

– You will be exposed to radiation briefly during the procedure. This dose is higher than that used in conventional x-ray imaging. All radiations are harmful. However, the dose of radiation used during a CT scan is very low.

– You may feel claustrophobic inside the ‘gantry’ of a CT scanner.

What is a CT contrast agent?

A contrast media is used to help visualize the organs and structures which are not seen very well normally. The contrast material blocks X-rays and appears white on tomographic images. The contrasts used during a CT scan maybe-

Intravenous contrast media (IVCM)– Used when gall bladder, urinary tract or blood vessels need to be imaged. You may experience a sense of flushing as well as a metallic taste in your mouth.

Oral Contrast Media-This is used when the organs of the gastrointestinal tract need to be visualized. As it is given orally, you may find its taste unpleasant.

Rectal contrast media-This is used in order to visualize the intestines. As the dye is administered rectally (enema), you may experience bloating and abdominal discomfort.

Can children undergo a CT scan?

Yes. It is safe for children to undergo a CT scan. The radiologist uses a lower dose of radiations after adjusting it according to the age and weight of the child. However, children below the age of 2 years should not be administered ion-based contrast media.

You may need to explain the procedure to your child to allay any fears that he or she may have. Sometimes, the radiologist recommends a sedative to keep the child calm and still. Any movement during the procedure may hamper with the diagnosis by generating blurred images.

Do I need to fast before a CT scan?

Your doctor may ask you to fast for a few hours prior to the scan, especially if an abdominal CT scan is to be performed.

How do I need to prepare for a CT scan?

– Wear comfortable, loose-fitting clothes on the day of the scan. In the hospital, you may be asked to remove your clothes and given a gown to wear.

– Remove all metallic objects that you may be wearing including eyeglasses, dentures and any prosthesis.

– Abstain from eating or drinking a few hours before the CT scan.

– Do not breastfeed for 48 hours after the test, in case you have been administered IVCM.

How is a CT scan carried out?

– You will be escorted to a room where the CT machine is installed.

– You may be administered a contrast media prior to this (the route varies, depending on the area to be imaged).

– You will be made to lie on a table (the scanner) that is moved inside the main doughnut-shaped body of the CT machine called ‘gantry’. A motorized x-ray source and a digital x-ray detector are located opposite each other in the ‘gantry’.

– Contraptions may be used to hold your body and head in place to avoid any movement.

– The x-ray tube will shoot multiple narrow beams of x-rays at your body from different angles and as the x-rays leave your body, the detectors pick them up.

– After every rotation of the x-ray source, the CT computer produces 2-D images of your body in the form of cross-sectional slices.

– These ‘slices’ are stacked together to generated 3D images of the internal organs with a high resolution.

– The process is repeated until the desired number of ‘slices’ is produced.

– During the entire procedure, you will be alone in the CT room. The technician/radiologist performing the scan will be in an adjacent room.

– He will be able to see you and communicate with you via an intercom.

– In the case of a young child or in the cases of a critically ill patient, an attendant/nurse may be allowed to stay with the patient in the CT room after putting on a lead apron as a shield against the radiation hazard.

– The actual procedure of scanning takes less than 30 seconds. However, the entire process may take half an hour or more.

When are the results out?

The radiological interpretation of a CT scan is available within a few minutes in the case of an emergency. Results of a routine scan carried out on an outpatient basis may take 1-2 days to be conveyed to the patient.

Who interprets the results?

A radiologist interprets the results of your CT scan. The findings are conveyed to your attending physician who correlates the findings with your clinical/physical examination.

What is the cost of undergoing a CT scan in India?

In private hospitals and diagnostic centers, the cost of a CT scan may range from 1500 to 15000 depending upon the area/areas examined. The cost is much lower in government hospitals.

What is a Cardiac CT scan?

It is a specialized CT scan. It provides detailed images of the heart and its blood vessels. The level of calcification and the risk of developing atherosclerosis can be depicted.

What are the benefits and limitations of a Computed tomography scan?