CT scan also known as computed tomography scan, makes use of computer-processed combinations of many X-ray measurements taken from different angles to produce cross-sectional (tomographic) images (virtual “slices”) of specific areas of a scanned object, allowing the user to see inside the object without cutting. Other terms include computed axial tomography (CAT scan) and computer aided tomography.Digital geometry processing is used to further generate a three-dimensional volume of the inside of the object from a large series of two-dimensional radiographic images taken around a single axis of rotation.Medical imaging is the most common application of X-ray CT. Its cross-sectional images are used for diagnostic and therapeutic purposes in various medical disciplines.
There are essentially two types of CT scans:
- Conventional CT scan – the scan is taken slice by slice and after each slice the scan stops and moves down to the next slice – eg, from the top of the abdomen down to the pelvis. This requires patients to hold their breath to avoid movement artefact.
- Spiral/helical CT scan – this is a continuous scan which is taken in a spiral fashion. It is a much quicker process and the scanned images are contiguous.
According to the resolution
- High-resolution CT scans (HRCT) are often used to accurately assess the lungs for inflammation and scarring.
- In multislice CT (MSCT) scan, a higher number of tomographic slices allow for higher-resolution imaging.
According to the body parts imaging
1. CT Scanning of the Head
- locating skull fractures and brain damage in patients with head injuries.
- detecting a blood clot or bleeding within the brain shortly after a patient exhibits symptoms of a stroke.
- determining the extent of bone and soft tissue damage in patients with facial trauma, and planning surgical reconstruction.
- detecting and localizing bleeding in a patient with sudden severe headache who may have a ruptured or leaking aneurysm.
- detecting some brain tumors.
- diagnosing diseases of the temporal bone on the side of the skull, which may cause hearing problems.
- illuminating enlarged brain cavities (ventricles) in patients with hydrocephalus.
- determining whether inflammation or other changes are present in the paranasal sinuses.
- planning radiation therapy for cancer of the brain or other tissues.
- guiding the passage of a needle used to obtain a tissue sample (biopsy) from the brain.
- non-invasively assessing for aneurysms or arteriovenous malformations through a technique called CT angiography.
- detecting diseases or malformations of the skull.
- abnormalities of the bones of your skull
- arteriovenous malformation, or abnormal blood vessels
- atrophy of brain tissue
- birth defects
- brain aneurysm
- hemorrhage, or bleeding, in your brain
- hydrocephalus, or fluid buildup in your skull
- infections or swelling
- injuries to your head, face, or skull
- mensional imaging of the skull and brain structures.
2. CT Scanning of the Heart
The main forms of cardiac CT scanning are
- Coronary CT angiography (CTA) – the use of CT to assess the coronary arteries of the heart. The subject receives an intravenous injection of radiocontrast and then the heart is scanned using a high speed CT scanner, allowing radiologists to assess the extent of occlusion in the coronary arteries, usually in order to diagnose coronary artery disease.
- Coronary CT calcium scan – also used for the assessment of severity of coronary artery disease. Specifically, it looks for calcium deposits in the coronary arteries that can narrow arteries and increase the risk of heart attack. A typical coronary CT calcium scan is done without the use of radiocontrast, but it can possibly be done from contrast-enhanced images as well.
3. CT Scanning of the Lungs
- Pulmonary angiogram – CT pulmonary angiogram is a medical diagnostic test used to diagnose pulmonary embolism. It employs computed tomography and an iodine predicated contrast agent to obtain an image of the pulmonary arteries.
4.CT Scanning of the Abdominal and pelvic
- CT imaging of the abdomen is an examination that uses x-rays to visualize several types of tissue with great clarity, including organs such as the liver, spleen, pancreas and kidneys. Using specialized equipment and expertise to create and interpret CT scans of the lower gastrointestinal (GI) tract, the colon and rectum, an experienced radiologist can accurately diagnose many causes of abdominal pain, such as an abscess in the abdomen, inflamed colon or colon cancer, diverticulitis and appendicitis.
- CT is an accurate technique for the diagnosis of abdominal diseases. Its uses include diagnosis and staging of cancer, as well as follow up after cancer treatment to assess response. It is commonly used to investigate acute abdominal pain.
5.CT Scanning of the Spine
- CT scanning of the spine is a type of x-ray examination that uses a scanner to obtain multiple images of the spinal column, as well as three-dimensional images if needed. CT images are far more detailed than those obtained by a conventional x-ray unit. In addition, CT is a very useful diagnostic method because it can display and distinguish many different types of tissue in the same region, including bone, muscle, soft tissue and blood vessels. The bony structure of the spinal vertebrae is clearly and accurately shown by CT scanning, as are the intervertebral disks and, to some degree, the spinal cord.
Some of the common uses of the procedure include
- To detect or rule out spinal damage in patients who have been injured.
- The spine before and after surgery.
- To detect various types of tumors in the vertebral column, including those that have spread there from another area of the body. Some tumors that arise elsewhere are first identified by finding deposits of malignant cells (metastases) in the vertebrae; prostate cancer is an example.
- To detect narrowing of the spinal canal, vertebral fracture, infection, or degenerative disease such as arthritis, CT of the spine may provide important information when carried out by itself or in addition to magnetic resonance imaging (MRI). One of the most common causes of spinal pain that may be diagnosed by CT is a herniated intervertebral disk.
- CT is often used to image complex fractures, especially ones around joints, because of its ability to reconstruct the area of interest in multiple planes. Fractures, ligamentous injuries, and dislocations can easily be recognized with a 0.2 mm resolution.
A special dye called a contrast material is needed for some CT scans, to help highlight the areas of your body being examined. The contrast material blocks X-rays and appears white on images, which can help emphasize blood vessels, intestines or other structures.
A contrast material might be given to you:
- By mouth –If your esophagus or stomach is being scanned, you may need to swallow a liquid that contains contrast material. This drink may taste unpleasant.
- By injection –Contrast agents can be injected through a vein in your arm to help your gallbladder, urinary tract, liver or blood vessels stand out on the images. You may experience a feeling of warmth during the injection or a metallic taste in your mouth.
- By enema –A contrast material may be inserted in your rectum to help visualize your intestines. This procedure can make you feel bloated and uncomfortable.
- one of the fastest and most accurate tools for examining the chest, abdomen and pelvis because it provides detailed, cross-sectional views of all types of tissue.
- used to examine patients with injuries from trauma such as a motor vehicle accident.
- performed on patients with acute symptoms such as chest or abdominal pain or difficulty breathing.
- often the best method for detecting many different cancers, such as lymphoma and cancers of the lung, liver, kidney, ovary and pancreas since the image allows a physician to confirm the presence of a tumor, measure its size, identify its precise location and determine the extent of its involvement with other nearby tissue.
- an examination that plays a significant role in the detection, diagnosis and treatment of vascular diseases that can lead to stroke, kidney failure or even death. CT is commonly used to assess for pulmonary embolism (a blood clot in the lung vessels) as well as for aortic aneurysms.
- invaluable in diagnosing and treating spinal problems and injuries to the hands, feet and other skeletal structures because it can clearly show even very small bones as well as surrounding tissues such as muscle and blood vessels.
- Tumors and masses
- Trauma and diffuse axonal injuries
- Neurodegenerative disorders and dementias
- Inflammatory conditions
- Congenital abnormalities
- Cranial neuropathies
- Fetal brain
- Hemorrhage, or bleeding into the brain or spinal cord
- Subdural hematoma (an area of bleeding just under the dura mater, or covering of the brain)
- Herniation or degeneration of discs of the spinal cord
- Help plan surgeries on the spine, such as decompression of a pinched nerve or spinal fusion
- Degenerative diseases, multiple sclerosis, hypoxic encephalopathy (dysfunction of the brain due to a lack of oxygen), or encephalomyelitis (inflammation or infection of the brain and/or spinal cord)
- Venous malformations
- Suspected hydrocephalus
Suspected intracranial hematoma
Vascular occlusive disease and aneurysm evaluation
In pediatric patients, CT imaging is often used to evaluate:
- kidney tumors
- congenital malformations of the heart, kidneys and blood vessels
- cystic fibrosis
- complications of acute appendicitis
- complications of pneumonia
- inflammatory bowel disease
- severe injuries
Radiologists and radiation oncologists often use the CT examination to:
- quickly identify injuries to the lungs, heart and vessels, liver, spleen, kidneys, bowel or other internal organs in cases of trauma.
- guide biopsies and other procedures such as abscess drainages and minimally invasive tumor treatments.
- plan for and assess the results of surgery, such as organ transplants or gastric bypass.
- stage, plan and properly administer radiation treatments for tumors as well as monitor response to chemotherapy.
- measure bone mineral density for the detection of osteoporosis.
Contraindications for the use of iodinated contrast media:
- Allergy to iodine
- Toxic goiter of the thyroid
- Planned radioiodine treatment of thyroid cancer
Contraindications against the use of iron-based contrast media in patients:
- Children below 2 years of age
- Pregnant women
- Over 60 years of age
- Complications after the previous administration of a contrast medium
- Acute and chronic circulatory and respiratory failure
- Hepatic and renal failure (also dialyzed patients)
- Asthma and pulmonary oedemas
- Insulin-dependent diabetes
- Convulsions of cerebral etiology
Preparation for a CT scan
If you are having a computed tomography angiography (CTA) or virtual colonoscopy with Johns Hopkins radiology, you will be given specific instructions when you make your appointment.
Precautions – If you are pregnant or think you may be pregnant, please check with your doctor before scheduling the exam. Other options will be discussed with you and your doctor.
Clothing – may be asked to change into a patient gown. If so, a gown will be provided for you. A locker will be provided to secure personal belongings. Please remove all piercings and leave all jewelry and valuables at home.
Contrast media – CT scans are most frequently done with and without a contrast media. The contrast media improves the radiologist’s ability to view the images of the inside of the body.
- Some patients should not have an iodine-based contrast media. If you have problems with your kidney function, please inform the access center representative when you schedule the appointment. You may be able to have the scan performed without contrast media or have an alternative imaging exam.
- You will be asked to sign a consent form that will detail the risks and side-effects associated with contrast media injected through a small tube placed in a vein called an intravenous (IV) line.
- The most common type of CT scan with contrast is the double contrast study that will require you to drink a contrast media before your exam begins in addition to the IV contrast. The more contrast you are able to drink, the better the images are for the radiologist to visualize your digestive tract. Please inform the access center representative when you schedule your CT scan if you have had an allergic reaction to any contrast media. IV contrast will not be administered if you have had a severe or anaphylactic reaction to any contrast media in the past. If you had mild to moderate reactions in the past, you will likely need to take medication prior to the CT scan. These plans will be discussed with you in detail when you schedule your exam. Any known reactions to a contrast media should be discussed with your personal physician.
Eat /drinks –If your doctor ordered a CT scan without contrast, you can eat, drink and take your prescribed medications prior to your exam. If your doctor ordered a CT scan with contrast don,t eat any thing before 3 hours to your CT scan. You are encouraged to drink clear liquids. You may also take your prescribed medications prior to your exam.
DIABETICS – Diabetics should eat a light breakfast or lunch three hours prior to the scan time. Depending on your oral medication for diabetes, you may be asked to discontinue use of the medication for 48 hours after the CT scan. If you have a CT scan with Johns Hopkins radiology, detailed instructions will be given following your examination.
MEDICATION – All patients can take their prescribed medications as usual.
Based on your medical condition, your doctor may request other specific preparation.
What happens during a CT scan?
Generally, a CT scan follows this process
- You may be asked to change into a patient gown. If so, a gown will be provided for you. A locked will be provided to secure all personal belongings. Please remove all piercings and leave all jewelry and valuables at home.
- If you are to have a procedure done with contrast, an intravenous (IV) line will be started in the hand or arm for injection of the contrast media. For oral contrast, you will be given a liquid contrast preparation to swallow. In some situations, the contrast may be given rectally.
- You will lie on a scan table that slides into a large, circular opening of the scanning machine. Pillows and straps may be used to prevent movement during the procedure.
- The technologist will be in another room where the scanner controls are located. However, you will be in constant sight of the technologist through a window. Speakers inside the scanner will enable the technologist to communicate with and hear you. You may have a call button so that you can let the technologist know if you have any problems during the procedure. The technologist will be watching you at all times and will be in constant communication.
- As the scanner begins to rotate around you, X-rays will pass through the body for short amounts of time. You will hear clicking sounds, which are normal.
- The X-rays absorbed by the body’s tissues will be detected by the scanner and transmitted to the computer. The computer will transform the information into an image to be interpreted by the radiologist.
- It will be important that you remain very still during the procedure. You may be asked to hold your breath at various times during the procedure.
- If contrast media is used for your procedure, you may feel some effects when the media is injected into the IV line. These effects include a flushing sensation, a salty or metallic taste in the mouth, a brief headache, or nausea and/or vomiting. These effects usually last for a few moments.
- You should notify the technologist if you feel any breathing difficulties, sweating, numbness or heart palpitations.
- When the procedure has been completed, you will be removed from the scanner.
- If an IV line was inserted for contrast administration, the line will be removed.
Side Effects of CT Scan
People who are allergic to the iodinated contrast used in CT may have some of the following symptoms:
- nausea and/or vomiting;
- a skin rash or hives;
- sneezing or watering eyes;
- dizziness and/or a headache;
- gagging or feeling of suffocation, or swelling of the inside of the throat or mouth;
- change in blood pressure.
After the test, you should be able to eat and drink as normal and resume regular activities.
Results of CT Scan
A CT scan can show
- a change in the shape, size or structure of tissues and organs
- These changes could be due to injury or disease.
- a mass or lesion that could be a tumor or caused by another disease
- A CT scan cannot always tell the difference between a cancerous (malignant) tumor and a non-cancerous (benign) tumor.
- the tumor’s approximate shape, size, and location
- if the tumor is pressing on other organs or tissues
- enlarged lymph nodes
- blood vessels that feed a tumor
- cancer spread (metastasis)
- a tumor’s response to cancer treatment
- A CT scan can be used to determine if the tumor has gotten smaller, stayed the same or grown after treatment.
- Viewing a CT scan, an experienced radiologist can diagnose many causes of abdominal pain or injury from trauma with very high accuracy, enabling faster treatment and often eliminating the need for additional, more invasive diagnostic procedures.
- When pain is caused by infection and inflammation, the speed, ease and accuracy of a CT examination can reduce the risk of serious complications, such as those caused by a burst appendix or an infected fluid collection and the subsequent spread of infection.
- CT scanning is painless, noninvasive and accurate.
- A major advantage of CT is its ability to image bone, soft tissue and blood vessels all at the same time.
- Unlike conventional x-rays, CT scanning provides very detailed images of many types of tissue as well as the lungs, bones, and blood vessels.
- CT examinations are fast and simple; in emergency cases, they can reveal internal injuries and bleed quickly enough to help save lives.
- CT has been shown to be a cost-effective imaging tool for a wide range of clinical problems.
- CT is less sensitive to patient movement than MRI.
- CT can be performed if you have an implanted medical device of any kind, unlike MRI.
- CT imaging provides real-time imaging, making it a good tool for guiding minimally invasive procedures such as needle biopsies and needle aspirations of many areas of the body, particularly the lungs, abdomen, pelvis and bones.
- A diagnosis determined by CT scanning may eliminate the need for exploratory surgery and surgical biopsy.
- No radiation remains in a patient’s body after a CT examination.
- X-rays used in CT scans should have no immediate side effects.
There is no conclusive evidence that radiation at small amounts delivered by a CT scan causes cancer. Large population studies have shown a slight increase in cancer from much larger amounts of radiation, such as from radiation therapy. Thus, there is always concern that this risk may also apply to the lower amounts of radiation delivered by a CT exam. When a CT scan is recommended by your doctor, the expected benefit of this test outweighs the potential risk from radiation. You are encouraged to discuss the risks versus the benefits of your CT scan with your doctor or radiologist, and to explore whether alternative imaging tests may be available to diagnose your condition.
- The effective radiation dose for this procedure varies. See the Safety page for more information about radiation dose.
- Women should always inform their physician and x-ray or CT technologist if there is any possibility that they are pregnant. See the Safety page for more information about pregnancy and x-rays.
- CT scanning is, in general, not recommended for pregnant women unless medically necessary because of potential risk to the fetus in the womb.
- Manufacturers of intravenous contrast indicate mothers should not breastfeed their babies for 24-48 hours after contrast medium is given. However, both the American College of Radiology (ACR) and the European Society of Urogenital Radiology note that the available data suggest that it is safe to continue breastfeeding after receiving intravenous contrast. For further information please consult the ACR Manual on Contrast Media and its references.
- The risk of serious allergic reaction to contrast materials that contain iodine is extremely rare, and radiology departments are well-equipped to deal with them.
Depending on the purpose of the investigation, there are standardized protocols for time intervals between intravenous radiocontrast administration and image acquisition, in order to visualize the dynamics of contrast enhancements in different organs and tissues. The main phases thereof are as follows:
|Phase||Time from injection||Time from bolus tracking
||Targeted structures and findings|
|Non-enhanced CT (NECT)||–||–||
|Pulmonary arterial phase||6-13 sec||–||
|Pulmonary venous phase||17-24 sec||–|
|Early systemic arterial phase||15-20 sec||immediately||
|Late systemicarterial phase
Sometimes also called “arterial phase” or “early venous portal phase”
|35-40 sec||15-20 sec||
|Pancreatic phase||40-50 sec||20-30 sec||
|Hepatic (most accurate) or late portal phase||70-80 sec||50-60 sec||
|Nephrogenic phase||100 sec||80 sec||
|Systemic venous phase||180 sec||160 sec||
Sometimes called “wash out phase” or “equilibrium phase”