Computed Tomography (CT) Scan | Johns Hopkins Medicine

What You Need to Know

• A CT scan is a diagnostic imaging exam that uses X-ray technology to produce images of the inside of the body.
• A CT scan can show detailed images of any part of the body, including the bones, muscles, organs and blood vessels.
• CT scans can also be used for fluid or tissue biopsies, or as part of preparation for surgery or treatment.
• CT scans are frequently done with and without contrast agent to improve the radiologist’s ability to find any abnormalities.

What is a CT scan?

Computed tomography is commonly referred to as a CT scan. A CT scan is a diagnostic imaging procedure that uses a combination of X-rays and computer technology to produce images of the inside of the body. It shows detailed images of any part of the body, including the bones, muscles, fat, organs and blood vessels.

CT scans are more detailed than standard X-rays. In standard X-rays, a beam of energy is aimed at the body part being studied. A plate behind the body part captures the variations of the energy beam after it passes through skin, bone, muscle and other tissue. While much information can be obtained from a regular X-ray, a lot of detail about internal organs and other structures is not available.

In CT, the X-ray beam moves in a circle around the body. This allows many different views of the same organ or structure and provides much greater detail. The X-ray information is sent to a computer that interprets the X-ray data and displays it in two-dimensional form on a monitor. Newer technology and computer software makes three-dimensional images possible.

CT scans may be performed to help diagnose tumors, investigate internal bleeding, or check for other internal injuries or damage. CT can also be used for a tissue or fluid biopsy.

Why are CT scans sometimes ordered with contrast?

CT scans may be done with or without contrast. Contrast refers to a substance taken by mouth or injected into an IV line that causes the particular organ or tissue under study to be seen more clearly. Contrast examinations may require you to fast for a certain period of time before the procedure. Your doctor will notify you of this prior to the procedure.

You will need to let your doctor know if you have ever had a reaction to any contrast media and/or any kidney problems. A reported seafood allergy is not considered to be a contraindication for iodinated contrast. If you have any medical conditions or recent illnesses, inform your doctor.

How do I prepare for a CT scan?

If you are having a computed tomography angiography (CTA) or a virtual colonoscopy, 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.

CLOTHING: You may be asked to change into a patient gown. If so, a gown will be provided for you. Please remove all piercings and leave all jewelry and valuables at home.

CONTRAST MEDIA: Contrast may be indicated for your exam. The contrast media improves the radiologist’s ability to find structures that are abnormal and understand normal anatomy better.

• Some patients should not have an iodine-based contrast media. If you have problems with your kidney function, please inform your radiologist in advance. We may be able to perform the scan without the contrast media or find an alternate imaging exam.
• The most common type of CT scan with contrast is the double-contrast study, which 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.

ALLERGY: Please inform the access center representative when you schedule your 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. Mild to moderate reactions warrant a plan that includes taking medication prior to the CT examination. 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/DRINK: If your study was ordered without contrast, you can eat, drink and take your prescribed medications prior to your exam.

If your doctor orders a CT scan with contrast, do not eat anything three hours prior to your CT scan. We encourage you to drink clear liquids. You may also take your prescribed medications prior to your exam.

DIABETICS: People with diabetes 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 examination. Detailed instructions will be given following your examination.

MEDICATION: All patients can take their prescribed medications as usual.

The Newest CT: Faster Than a Heartbeat

A new CT scanner will change the management and outcomes of coronary artery disease. The CT scan captures images of the heart between beats, leading to clearer images to help detect anatomic and functional characteristics of coronary artery disease. This technology can also be used to help replace invasive tests to determine which patients with chest pain need intervention.

Read more

What are the risks of a CT scan?

If you are pregnant or think you may be pregnant, you should notify your health care provider.

The amount of radiation dose used in a CT scan is small. You may want to ask your doctor about the amount of radiation used during the CT procedure and the risks related to your particular situation.

If you are claustrophobic or tend to become anxious easily, tell your doctor ahead of time. You may be prescribed a mild sedative to take before the procedure to make you more comfortable.

What happens during a CT scan?

CT scans may be performed on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your physician’s practices. Generally, CT scans follow this process:

• You 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 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 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.
• 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 is 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 contrast 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 have 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.
• While the CT procedure itself causes no pain, having to lie still for the length of the procedure might cause some discomfort or pain, particularly in the case of a recent injury or invasive procedure, such as surgery. The technologist will use all possible comfort measures and complete the procedure as quickly as possible to minimize any discomfort or pain.

What happens after a CT scan?

• If contrast media was used during your procedure, you may be monitored for a period of time for any side effects or reactions to the contrast, such as itching, swelling, rash or difficulty breathing.
• If you notice any pain, redness and/or swelling at the IV site after you return home following your procedure, you should notify your doctor, as this could indicate an infection or other type of reaction.
• There is typically no special type of care required after a CT scan. You may resume your usual diet and activities unless your doctor advises you differently.
• Your doctor may give you additional or alternate instructions after the procedure, depending on your particular situation.

What are some of the advances in CT technology?

Advances in CT technology include:

• High-resolution CT: This type of CT scan uses very thin slices (less than 0. 1 inches), which are effective in providing greater detail in certain conditions, such as lung disease.
• Helical or spiral CT: During this type of CT scan, both the patient and the X-ray beam move continuously, with the X-ray beam circling the patient. The images are obtained much more quickly than with standard CT scans. The resulting images have greater resolution and contrast, providing more detailed information. Multidetector-row helical CT scanners may be used to obtain information about calcium buildup inside the coronary arteries of the heart.
• Ultrafast CT (also called electron beam CT): This type of CT scan produces images very rapidly, thus creating a type of “movie” of moving parts of the body, such as the chambers and valves of the heart. This scan may also be used to obtain information about calcium buildup inside the coronary arteries of the heart, but the helical scanners are much more common.
• Computed tomographic angiography (CTA): Angiography (or arteriography) is an X-ray image of the blood vessels. A CT angiogram uses CT technology rather than standard X-rays or fluoroscopy to obtain images of blood vessels — for example, the coronary arteries of the heart.
• Combined positron emission tomography and CT (PET/CT): The combination of CT and positron emission tomography technologies into a single machine is referred to as PET/CT. PET/CT combines the ability of CT to provide detailed anatomy with that of PET to show cell function and metabolism in order to offer greater accuracy in the diagnosis and treatment of certain types of diseases, particularly cancer. PET/CT may also be used to evaluate conditions such as epilepsy, Alzheimer’s disease and coronary artery disease.

Computed Tomography (CT)

• What is a computed tomography (CT) scan?
• How does CT work?
• When would I get a CT scan?
• What is a CT contrast agent?
• Are there risks?
• What are examples of NIBIB-funded projects using CT?

What is a computed tomography (CT) scan?

The term “computed tomography,” or CT, refers to a computerized x-ray imaging procedure in which a narrow beam of x-rays is aimed at a patient and quickly rotated around the body, producing signals that are processed by the machine’s computer to generate cross-sectional images, or “slices. ” These slices are called tomographic images and can give a clinician more detailed information than conventional x-rays. Once a number of successive slices are collected by the machine’s computer, they can be digitally “stacked” together to form a three-dimensional (3D) image of the patient that allows for easier identification of basic structures as well as possible tumors or abnormalities.

How does CT work?

A CT machine. Credit: iStock

Unlike a conventional x-ray—which uses a fixed x-ray tube—a CT scanner uses a motorized x-ray source that rotates around the circular opening of a donut-shaped structure called a gantry. During a CT scan, 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. Instead of film, CT scanners use special digital x-ray detectors, which are located directly opposite the x-ray source. As the x-rays leave the patient, they are picked up by the detectors and transmitted to a computer.

Each time the x-ray source completes one full rotation, the CT computer uses sophisticated mathematical techniques to construct a two-dimensional image slice of the patient. The thickness of the tissue represented in each image slice can vary depending on the CT machine used, but usually ranges from 1-10 millimeters. When a full slice is completed, the image is stored and the motorized bed is moved forward incrementally into the gantry. The x-ray scanning process is then repeated to produce another image slice. This process continues until the desired number of slices is collected.

Image slices can either be displayed individually or stacked together by the computer to generate a 3D image of the patient that shows the skeleton, organs, and tissues as well as any abnormalities the physician is trying to identify. This method has many advantages including the ability to rotate the 3D image in space or to view slices in succession, making it easier to find the exact place where a problem may be located.

When would I get a CT scan?

CT scans can be used to identify disease or injury within various regions of the body. For example, CT has become a useful screening tool for detecting possible tumors or lesions within the abdomen. A CT scan of the heart may be ordered when various types of heart disease or abnormalities are suspected. CT can also be used to image the head in order to locate injuries, tumors, clots leading to stroke, hemorrhage, and other conditions. It can image the lungs in order to reveal the presence of tumors, pulmonary embolisms (blood clots), excess fluid, and other conditions such as emphysema or pneumonia. A CT scan is particularly useful when imaging complex bone fractures, severely eroded joints, or bone tumors since it usually produces more detail than would be possible with a conventional x-ray.

What is a CT contrast agent?

CT scan of the abdomen. Credit: iStock

As with all x-rays, dense structures within the body—such as bone—are easily imaged, whereas soft tissues vary in their ability to stop x-rays and therefore may be faint or difficult to see. For this reason, contrast agents have been developed that are highly visible in an x-ray or CT scan and are safe to use in patients. Contrast agents contain substances that can stop x-rays and are therefore more visible on an x-ray image. For example, to examine the circulatory system, an intravenous (IV) contrast agent based on iodine is injected into the bloodstream to help illuminate blood vessels. This type of test is used to look for possible obstructions in blood vessels, including those in the heart. Oral contrast agents, such as barium-based compounds, are used for imaging the digestive system, including the esophagus, stomach, and gastrointestinal (GI) tract.

Are there risks?

CT scans can diagnose possibly life-threatening conditions such as hemorrhage, blood clots, or cancer. An early diagnosis of these conditions could potentially be lifesaving. However, CT scans use x-rays, and all x-rays produce ionizing radiation. Ionizing radiation has the potential to cause biological effects in living tissue. This is a risk that increases with the number of exposures added up over the life of an individual. However, the risk of developing cancer from x-ray radiation exposure is generally small.

CT images of the heart and coronary artery. Credit: iStock

A CT scan in a pregnant woman poses no known risks to the baby if the area of the body being imaged isn’t the abdomen or pelvis. In general, if imaging of the abdomen and pelvis is needed, doctors prefer to use exams that do not use radiation, such as magnetic resonance imaging (MRI) or ultrasound. However, if neither of those can provide the answers needed, or there is an emergency or other time constraint, CT may be an acceptable alternative imaging option.

In some patients, contrast agents may cause allergic reactions, or in rare cases, temporary kidney failure. IV contrast agents should not be administered to patients with abnormal kidney function since they may induce a further reduction of kidney function, which may sometimes become permanent.

Because children are more sensitive to ionizing radiation and have a longer life expectancy, they have a higher relative risk for developing cancer from such radiation compared with adults. Parents may want to ask the technologist or doctor if their machine settings have been adjusted for children.

What are examples of NIBIB-funded projects using CT?

Imaging for acute ischemic stroke: Stroke, which can have lasting neurological injuries, is also a leading cause of death worldwide. To mitigate damage to the brain, patients may receive endovascular treatment, where the clot blocking the blood supply is either removed or dissolved. However, identifying patients who will benefit from endovascular therapy, such as those with only a small volume of irreversibly injured brain tissue, remains challenging, and time is a critically important factor for a successful clinical outcome.

Patient inside of a CT machine. Credit: iStock

NIBIB-funded researchers have developed an image reconstruction technique to more efficiently triage patients who present with symptoms of a stroke. This CT-based method can be used to rule out the presence of a hemorrhage; to find the site of the blood clot; and to identify the extent of damaged brain tissue. Such a technique could significantly shorten the time from the diagnosis of a stroke to the start of endovascular therapy, and could also guide the endovascular treatment. Following evaluation in animal models, researchers plan to validate this CT imaging technique in human studies.

Accounting for metal implants in CT imaging: Metal objects, such as implants and prostheses, can introduce ‘artifacts’ that may appear as streaks or shadows on a CT scan. These artifacts can obscure anatomical structures or affect calculations necessary for planning radiation therapy. While techniques exist to reduce such artifacts, they do not fully mitigate the artifacts and may even introduce new ones. In this project, NIBIB-funded researchers have developed an algorithm to reduce metal artifacts in CT imaging, without requiring knowledge of the implant material. The researchers plan to optimize their algorithm and then evaluate their technique as a potential method to improve radiation therapy planning for prostate cancer among those with hip protheses.

Leveraging CT images to guide treatments for COVID-19 and beyond: Artificial intelligence is increasingly being used with medical imaging, such as CT, to help improve diagnoses and guide treatment decisions. By using medical images and patient outcomes, clinicians can “train” machine learning-based technologies to recognize patterns and predict responses. During the COVID-19 pandemic, NIBIB created a collaborative imaging initiative called the Medical Imaging and Data Resource Center (MIDRC). This initiative collected and analyzed thousands of CT images from patients with COVID-19 for the development of artificial intelligence and machine learning tools to guide the treatment and monitoring of the disease. These datasets contribute to the development of algorithms for detection, prognosis, and optimization of therapy in acute COVID-19 patients and have the potential to contribute to the understanding of Post-Acute Sequelae of SARS-CoV-2 infection (PASC, otherwise known as “Long COVID”). Further, this initiative paves the way for new tools that leverage imaging for other medical conditions, such as cancer, liver disease, or other infectious diseases, among others.

For more information about CT, watch our video here.

Updated June 2022

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Computed tomography

Clinic

What is CT?
Computed tomography (CT) is an examination that uses X-rays to obtain detailed images of internal organs and structures.

INDICATIONS FOR CT

As a screening test – in the following conditions:

• Headache
• Head injury without loss of consciousness
• Syncope
• Exclusion of lung cancer. In the case of using computed tomography for screening, the study is done in a planned manner.

For emergency diagnosis – emergency computed tomography:

• Severe injuries aorta)
• Suspicion of some other acute injuries of the hollow and parenchymal organs (complications of both the underlying disease and as a result of ongoing treatment)

Computed tomography for routine diagnostics

• Most of the CT examinations are performed routinely, on the referral of a doctor, for the final confirmation of the diagnosis. As a rule, before performing a CT scan, simpler studies are done – X-ray, ultrasound, tests, etc. .

preoperative CT images are used in hybrid operating rooms during surgical procedures.

ABSOLUTE AND RELATIVE CONTRAINDICATIONS

Without contrast:

• Pregnancy
• Body weight over the maximum for device

With contrast:

• Allergy to contrast agent
• Renal failure
• Severe diabetes mellitus
• Pregnancy (teratogenic exposure to X-ray radiation)
• Severe general condition of the patient
• Body weight over the maximum for the device
• Thyroid diseases
• Multiple myeloma

Contrast agents
To improve the differentiation of other organs from each other, as well as normal and pathological structures, various methods of contrast enhancement are used (most often, using iodine-containing contrast agents).
The two main types of contrast agent administration are oral (a patient on a specific regimen drinks a solution of the contrast agent) and intravenous (performed by medical personnel). The main purpose of the first method is to contrast the hollow organs of the gastrointestinal tract; the second method allows assessing the nature of the accumulation of the contrast agent by tissues and organs through the circulatory system. Methods of intravenous contrast enhancement in many cases make it possible to clarify the nature of the detected pathological changes (including quite accurately indicate the presence of tumors, up to the assumption of their histological structure) against the background of the soft tissues surrounding them, as well as to visualize changes that are not detected during normal (“native” ) research.
In turn, intravenous contrast can be performed in two ways: “manual” intravenous contrast and bolus contrast.
In the first method, the contrast is injected manually by an X-ray technician or procedural nurse, the time and speed of administration are not regulated, the study begins after the injection of a contrast agent. This method is used on “slow” devices of the first generations; in MSCT, the “manual” administration of a contrast agent no longer corresponds to the significantly increased capabilities of the method.
With bolus contrast enhancement, the contrast agent is injected intravenously with a syringe-injector at a set rate and time of injection of the substance. The purpose of bolus contrast enhancement is to delimit the phases of contrast enhancement. Scan times vary between machines, at different contrast agent injection rates, and across patients; on average, at a drug injection rate of 4-5 ml/sec, scanning starts approximately 20-30 seconds after the start of injection of the contrast injector, and arterial filling is visualized (arterial phase of contrast enhancement). After 40–60 seconds, the device rescans the same area to highlight the portal-venous phase, in which the contrasting veins are visualized. A delayed phase is also distinguished (180 seconds after the start of administration), in which the excretion of the contrast agent through the urinary system is observed.

CT GROUP EQUIPMENT:

• multislice computed tomograph (16 slices) “Brilliance 16 Power”, “Philips” (Holland)
• multislice computed tomograph tomograph (40 slice) “SOMATOM Definition AS 40” “” Siemens” (Germany)
• multislice computed tomograph (128 slices, slice thickness from 0.6 mm) SOMATOM Definition AS+ (Siemens)

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Computed tomography (CT): what is it, why is it prescribed

What is CT? This is a fast and accurate diagnostic method based on the use of X-rays with minimal radiation exposure. The method of computed tomography (CT) allows you to get a layered image of any area of ​​human body tissues with a slice thickness of 0.5 mm to 10 mm, in just a few minutes.

What is CT? This is a fast and accurate diagnostic method based on the use of X-rays with minimal radiation exposure. The method of computed tomography (CT) allows you to get a layered image of any area of ​​human body tissues with a slice thickness of 0. 5 mm to 10 mm, in just a few minutes.

If your doctor has ordered a CT scan, you can make an appointment here

Attention! Diagnostics is available in the cities: Voronezh, Kursk, Tver, Khabarovsk; for patients in other cities, we perform MRI and other methods of radiation diagnostics

Examination by CT gives better contrast in the image of soft tissues in comparison with the X-ray method of examination.

Read the material on the topic: CT, MRI and ultrasound – an alternative or an addition?

The CT method is widely used in medicine and allows for qualitative studies of almost the entire human body. A special uniqueness of the method of computed tomography (CT) is the ability to study hollow organs – for example, the lungs, stomach and intestines. Also, when conducting computer diagnostics, bone structures are perfectly visible. This is one of the main differences between CT and MRI and ultrasound.

CT or MRI of the small intestine? What to choose? Read here

With the help of multislice computed tomography, it is possible to assess the localization and prevalence of the pathological process in a patient, even in an extremely serious condition, which allows the doctor to quickly make the right decision in a matter of minutes. In emergency situations, the method of computed tomography has priority over ultrasound diagnostics or magnetic resonance imaging.

For the diagnosis of the abdominal organs, kidneys, adrenal glands, small pelvis, soft tissues, as well as angiography (examination of blood vessels), the heart, for better visualization, CT is performed with the introduction of intravenous contrast.

The restriction to the introduction of a contrast agent is the presence of renal and hepatic insufficiency in a patient, and the results of a biochemical blood test (creatine, urea, ALT, AST) are also taken into account.

The only contraindications for computed tomography (CT) of the chest and other parts of the body due to radiation exposure are pregnancy and young children.