Computed Tomography
Computed Tomography
Definition
Computed tomography scanning, also called CT scan, CAT scan, or computerized axial tomography, is a diagnostic tool that provides views of internal body structures using x rays. In the field of mental health, a CT scan may be used when a patient seeks medical help for symptoms that could possibly be caused by a brain tumor. These symptoms may include headaches, emotional abnormalities, or intellectual or memory problems. In these cases, a CT scan may be performed to “rule out” a tumor, so that other tests can be performed in order to establish an accurate diagnosis.
Purpose
CT scans are used to image bone, soft tissues, and air. Since the 1990s, CT equipment has become more affordable and available. CT scans have become the imaging exam of choice for the diagnoses of most solid tumors. Because the computerized image is sharp, focused, and three-dimensional, many structures can be better differentiated (visualized) when compared with standard x rays.
Common indications for CT scans include:
- Sinus studies. The CT scan can show details of sinusitis, bone fractures, and the presence of bony tumor involvement. Physicians may order a CT scan of the sinuses to provide an accurate map for surgery.
- Brain studies. Brain CT scans can detect hematomas (blood clotted mass), tumors, strokes, aneurysms (a blood vessel that ruptures), and degenerative or infected brain tissue. The introduction of CT scanning, especially spiral CT, has helped reduce the need for more invasive procedures such as cerebral angiography (inserting a wire through an artery to where it will reach brain vessels for visualization in real time).
- Body scans. CT scans of the chest, abdomen, spine, and extremities can detect the presence of tumors, enlarged lymph nodes, abnormal collection of fluid, and vertebral disc disease. These scans can also be helpful in evaluating the extent of bone breakdown in osteoporosis.
- Heart and aorta scans. CT scans can focus on the thoracic (chest) or abdominal aorta to locate aneurysms and other possible aortic diseases. A newer type of CT scan, called electron beam CT, can be used to detect calcium buildup in arteries. Because it is a new technology, it is not yet widely used and its indications are not yet well-defined.
- Chest scans. CT scans of the chest are useful in distinguishing tumors and in detailing accumulation of fluid in chest infections.
Precautions
Pregnant women or those who could possibly be pregnant should not have a CT scan, particularly a full body or abdominal scan, unless the diagnostic benefits outweigh the risks. If the exam is necessary for obstetric purposes, technologists are instructed not to repeat films if there are errors. Pregnant patients receiving a CT scan or any x ray exam away from the abdominal area may be protected by a lead apron; most radiation, known as scatter, travels through the body, however, and is not totally blocked by the apron.
Contrast agents are often used in CT exams, though some types of tumors are better seen without it. Patients should discuss the use of contrast agents with their doctor, and should be asked to sign a consent form prior to the administration of contrast. One of the common contrast agents, iodine, can cause allergic reactions. Patients who are known to be allergic to iodine or shellfish should inform the physician prior to the CT scan; a combination of medications can be given to such patients before the scan to prevent or minimize the reaction. Contrast agents may also put patients with diabetes at risk of kidney failure, particularly those taking the medication glucophage.
Description
Computed tomography, is a combination of focused x-ray beams and the computerized production of an image. Introduced in the early 1970s, this radiologic procedure has advanced rapidly and is now widely used, sometimes in the place of standard x rays.
CT equipment
A CT scan may be performed in a hospital or outpatient imaging center. Although the equipment looks large and intimidating, it is very sophisticated and fairly comfortable. The patient is asked to lie on a gantry, or narrow table, that slides into the center of the scanner. The scanner looks like a doughnut and is round in the middle, which allows the x-ray beam to rotate around the patient. The scanner section may also be tilted slightly to allow for certain cross-sectional angles.
CT procedure
The gantry moves very slightly as the precise adjustments for each sectional image are made. A technologist watches the procedure from a window and views the images on a computer screen. Generally, patients are alone during the procedure, though exceptions are sometimes made for pediatric patients. Communication is possible via an intercom system.
It is essential that the patient lie very still during the procedure to prevent motion blurring. In some studies, such as chest CTs, the patient will be asked to hold his or her breath during image capture.
Following the procedure, films of the images are usually printed for the radiologist and referring physician to review. A radiologist can also interpret CT exams on the computer screen. The procedure time will vary in length depending on the area being imaged. Average study times are from 30 to 60 minutes. Some patients may be concerned about claustrophobia (a feeling of being “closed in”) but the width of the “doughnut” portion of the scanner is such that many patients can be reassured of openness. Doctors may consider giving sedatives to patients who have severe claustrophobia or difficulty lying still (such as small children).
The CT image
While traditional x-ray machines image organs in two dimensions, often resulting in organs in the front of the body being superimposed over those in the back, CT scans allow for a more three-dimensional effect. CT images can be likened to slices in a loaf of bread. Precise sections of the body can be located and imaged as cross-sectional views. The screen before the technologist shows a computer’s analysis of each section detected by the x-ray beam. Thus, various densities of tissue can be easily distinguished.
Contrast agents
Contrast agents are often used in CT exams and in other radiology procedures to illuminate certain details of anatomy more clearly. Some contrasts are natural, such as air or water. A water-based contrast agent is sometimes administered for specific diagnostic purposes. Barium sulfate is commonly used in gastroenterology procedures. The patient may drink this contrast or receive it in an enema. Oral or rectal contrast is usually given when examining the abdomen or cells, but not when scanning the brain or chest. Iodine is the most widely used intravenous contrast agent and is given through an intravenous needle.
If contrast agents are used in the CT exam, these will be administered several minutes before the study begins. Patients undergoing abdominal CT may be asked to drink a contrast medium. Some patients may experience a salty taste, flushing of the face, warmth or slight nausea, or hives from an intravenous contrast injection. Technologists and radiologists have the equipment and training to help patients through these minor reactions and to handle more severe reactions. Severe reactions to contrast are rare, but do occur.
Newer types of CT scans
The spiral CT scan, also called a helical CT, is a newer version of CT. This type of scan is continuous in motion and allows for the continuous re-creation of images. For example, traditional CT allows the technologist to take slices at very small and precise intervals one after the other. Spiral CT allows for a continuous flow of images, without stopping the scanner to move to the next image slice. A major advantage of spiral CT is the ability to reconstruct images anywhere along the length of the study area. Because the procedure is faster, patients are required to lie still for shorter periods of time. The ability to image contrast more rapidly after it is injected, when it is at its highest level, is another advantage of spiral CT’s high speed.
Electron beam CT scans are another newer type of CT technology that can be used to detect calcium buildup in arteries. These calcium deposits are potential risk factors for coronary artery disease. Electron beam CT scans take pictures much more quickly than conventional CTs, and are therefore better able to produce clear images of the heart as it pumps blood. Because it is a newer and expensive test, electron beam CT scanning is not widely used.
Some facilities will have spiral, electron, and conventional CT available. Although spiral is more advantageous for many applications, conventional CT is still a superior and precise method for imaging many tissues and structures. The physician will evaluate which type of CT works best for the specific exam purpose.
Preparation
If a contrast medium is administered, the patient may be asked to fast for about four to six hours prior to the procedure. Patients will usually be given a gown (like a typical hospital gown) to be worn during the procedure. All metal and jewelry should be removed to avoid artifacts on the film. Depending on the type of study, patients may also be required to remove dentures.
Aftercare
Generally, no aftercare is required following a CT scan. Immediately following the exam, the technologist will continue to watch the patient for possible adverse contrast reactions. Patients are instructed to advise the technologist of any symptoms, particularly respiratory difficulty. The site of contrast injection will be bandaged and may feel tender following the exam.
Risks
Radiation exposure from a CT scan is similar to, though higher than, that of a conventional x ray. Although this is a risk to pregnant women, the risk for other adults is minimal and should produce no effects. Severe contrast reactions are rare, but they are a risk of many CT procedures.
Normal results
Normal findings on a CT exam show bone, the most dense tissue, as white areas. Tissues and fat will show as various shades of gray, and fluids will be gray or black. Air will also look black. Intravenous, oral, and rectal contrast appear as white areas. The radiologist can determine if tissues and organs appear normal by the sensitivity of the gray shadows.
Abnormal results
Abnormal results may show different characteristics of tissues within organs. Accumulations of blood or other fluids where they do not belong may be detected. Radiologists can differentiate among types of tumors throughout the body by viewing details of their makeup.
Sinus studies
The increasing availability and lowered cost of CT scanning has lead to its increased use in sinus studies, either as a replacement for a sinus x ray or as a follow-up to an abnormal sinus radiograph. The sensitivity of CT allows for the location of areas of sinus infection, particularly chronic infection. Sinus tumors will show as shades of gray indicating the difference in their density from that of normal tissues in the area.
Brain studies
The precise differences in density allowed by CT scan can clearly show tumors, strokes, or lesions in the brain area as altered densities. These lighter or darker areas on the image may indicate a tumor or hematoma within the brain and skull area. Different types of tumors can be identified by the presence of edema (fluid), by the tissue’s density, or by studying blood
KEY TERMS
Aneurysm —A symptomless bulging of a weak arterial wall that can rupture, leading to stroke.
Cerebral ateriography —A procedure that allows a wire to be inserted in blood vessels in the brain which generates an image of diseases in these arteries.
Contrast (agent, medium) —A substance injected into the body that illuminates certain structures that would otherwise be hard to see on the radiograph (film).
Gantry —A name for the couch or table used in a CT scan. The patient lies on the gantry while it slides into the x-ray scanner.
Hematoma —An accumulation of blood, often clotted, in a body tissue or organ, usually caused by a break or tear in a blood vessel.
Metastasis —Secondary cancer, or cancer that has spread from one body organ or tissue to another.
Osteoporosis —A loss of bone minerals.
Radiologist —A medical doctor specially trained in radiology (x ray) interpretation and its use in the diagnosis of disease and injury.
Spiral CT —Also referred to as helical CT, this method allows for continuous 360-degree x-ray image capture.
Thoracic —Refers to the chest area. The thorax runs between the abdomen and neck and is encased in the ribs.
vessel location and activity. The speed and convenience of CT often allows for detection of hemorrhage (bleeding) before symptoms even occur.
Body scans
The body CT scan can identify abnormal body structures and organs. A CT scan may indicate tumors or cysts, enlarged lymph nodes, abnormal collections of fluids, blood, fat, or cancer metastasis. Tumors resulting from metastasis (movement of the cancer from the primary site of cancer growth to a distant site) are different in makeup than primary (original) tumors.
Chest scans
In addition to those findings that may indicate aortic aneurysms (rupture of the largest artery in the body), chest CT studies can show other problems in the heart and lungs, and distinguish between an aortic aneurysm and a tumor adjacent to the aorta. CT will not only show differences between air, water, tissues and bone, but will also assign numerical values to the various densities. Coin-sized lesions in the lungs may be indicative of tuberculosis or tumors. CT will help distinguish among the two. Enlarged lymph nodes in the chest area may indicate Hodgkin’s disease (a blood disorder).
Resources
BOOKS
Abeloff, M. Clinical Oncology, 2nd Ed. Orlando, Florida: Churchill Livingstone, Inc., 2000.
Springhouse Corporation. Illustrated Guide to Diagnostic Tests. Springhouse, PA: Springhouse Corporation, 1998.
PERIODICALS
Holbert, J. M. “Role of Spiral Computed Tomography in the Diagnosis of Pulmonary Embolism in the Emergency Department.” Annals of Emergency Medicine (May 1999): 520–28.
ORGANIZATIONS
American College of Radiology. 1891 Preston White Drive, Reston, VA 22091. Telephone: (800) ACR-LINE. <http://www.acr.org>.
Laith Farid Gulli, M.D.
Teresa G. Norris, R.N.
Computed Tomography
Computed tomography
Definition
Computed tomography (CT) scanning is a valuable diagnostic tool that provides physicians with views of internal body structures. During a CT scan, multiple x rays are passed through the body, producing cross-sectional images, or "slices, " on a cathode-ray tube (CRT), a device resembling a television screen. These images can then be preserved on film for examination.
Purpose
CT scans are used to image bone, soft tissues, and air. Since the 1990s, CT equipment has become more affordable and available. CT scans have become the imaging exam of choice for the diagnoses of most solid tumors. Because the computerized image is sharp, focused, and three-dimensional, many structures can be better differentiated than on standard x rays.
Common indications for CT scans include:
- Sinus studies. The CT scan can show details of sinusitis, bone fractures, and the presence of bony tumor involvement. Physicians may order a CT scan of the sinuses to provide an accurate map for surgery.
- Brain studies. Brain CT scans can detect hematomas, tumors, strokes, aneurysms, and degenerative or infected brain tissue. The introduction of CT scanning, especially spiral CT, has helped reduce the need for more invasive procedures such as cerebral angiography.
- Body scans. CT scans of the chest, abdomen, spine, and extremities can detect the presence of tumors, enlarged lymph nodes, abnormal collection of fluid, and vertebral disc disease. These scans can also be helpful in evaluating the extent of bone breakdown in osteoporosis.
- Heart and aorta scans. CT scans can focus on the thoracic or abdominal aorta to locate aneurysms and other possible aortic diseases. A newer type of CT scan, called electron beam CT, can be used to detect calcium buildup in arteries. Because it is a new technology, it is not yet widely used and its indications are not yet well-defined.
- Chest scans. CT scans of the chest are useful in distinguishing tumors and in detailing accumulation of fluid in chest infections.
Precautions
Pregnant women or those who could possibly be pregnant should not have a CT scan, particularly a full body or abdominal scan, unless the diagnostic benefits outweigh the risks. If the exam is necessary for obstetric purposes, technologists are instructed not to repeat films if there are errors. Pregnant patients receiving a CT scan or any x ray exam away from the abdominal area may be protected by a lead apron; most radiation, known as scatter, travels through the body, however, and is not blocked by the apron.
Contrast agents are often used in CT exams, though some types of tumors are better seen without it. Patients should discuss the use of contrast agents with their doctor, and should be asked to sign a consent form prior to the administration of contrast. One of the common contrast agents, iodine, can cause allergic reactions. Patients who are known to be allergic to iodine or shell-fish should inform the physician prior to the CT scan; a combination of medications can be given to such patients before the scan to prevent or minimize the reaction. Contrast agents may also put patients with diabetes at risk of kidney failure, particularly those taking the medication glucophage.
Description
Computed tomography, also called CT scan, CAT scan, or computerized axial tomography, is a combination of focused x-ray beams and the computerized production of an image. Introduced in the early 1970s, this radiologic procedure has advanced rapidly and is now widely used, sometimes in the place of standard x rays.
CT equipment
A CT scan may be performed in a hospital or outpatient imaging center. Although the equipment looks large and intimidating, it is very sophisticated and fairly comfortable. The patient is asked to lie on a gantry, or narrow table, that slides into the center of the scanner. The scanner looks like a doughnut and is round in the middle, which allows the x-ray beam to rotate around the patient. The scanner section may also be tilted slightly to allow for certain cross-sectional angles.
CT procedure
The gantry moves very slightly as the precise adjustments for each sectional image are made. A technologist watches the procedure from a window and views the images on a computer screen. Generally, patients are alone during the procedure, though exceptions are sometimes made for pediatric patients. Communication is possible via an intercom system.
It is essential that the patient lie very still during the procedure to prevent motion blurring. In some studies, such as chest CTs, the patient will be asked to hold his or her breath during image capture.
Following the procedure, films of the images are usually printed for the radiologist and referring physician to review. A radiologist can also interpret CT exams on the computer screen. The procedure time will vary in length depending on the area being imaged. Average study times are from 30 to 60 minutes. Some patients may be concerned about claustrophobia but the width of the "doughnut" portion of the scanner is such that many patients can be reassured of openness. Doctors may consider giving sedatives to patients who have severe claustrophobia or difficulty lying still.
The CT image
While traditional x-ray machines image organs in two dimensions, often resulting in organs in the front of the body being superimposed over those in the back, CT scans allow for a more three-dimensional effect. CT images can be likened to slices in a loaf of bread. Precise sections of the body can be located and imaged as cross-sectional views. The screen before the technologist shows a computer's analysis of each section detected by the x-ray beam. Thus, various densities of tissue can be easily distinguished.
Contrast agents
Contrast agents are often used in CT exams and in other radiology procedures to illuminate certain details of anatomy more clearly. Some contrasts are natural, such as air or water. A water-based contrast agent is sometimes administered for specific diagnostic purposes. Barium sulfate is commonly used in gastroenterology procedures. The patient may drink this contrast or receive it in an enema. Oral or rectal contrast is usually given when examining the abdomen or cells, but not when scanning the brain or chest. Iodine is the most widely used intravenous contrast agent and is given through an intravenous needle.
If contrast agents are used in the CT exam, these will be administered several minutes before the study begins. Patients undergoing abdominal CT may be asked to drink a contrast medium. Some patients may experience a salty taste, flushing of the face, warmth or slight nausea, or hives from an intravenous contrast injection. Technologists and radiologists have the equipment and training to help patients through these minor reactions and to handle more severe reactions. Severe reactions to contrast are rare, but do occur.
Newer types of CT scans
The spiral CT scan, also called a helical CT, is a newer version of CT. This type of scan is continuous in motion and allows for the continuous re-creation of images. For example, traditional CT allows the technologist to take slices at very small and precise intervals one after the other. Spiral CT allows for a continuous flow of images, without stopping the scanner to move to the next image slice. A major advantage of spiral CT is the ability to reconstruct images anywhere along the length of the study area. Because the procedure is faster, patients are required to lie still for shorter periods of time. The ability to image contrast more rapidly after it is injected, when it is at its highest level, is another advantage of spiral CT's high speed.
Electron beam CT scans are another newer type of CT technology that can be used to detect calcium buildup in arteries. These calcium deposits are potential risk factors for coronary artery disease. Electron beam CT scans take pictures much more quickly than conventional CTs, and are therefore better able to produce clear images of the heart as it pumps blood. Because it is a newer and expensive test, electron beam CT scanning is not widely used.
Some facilities will have spiral, electron, and conventional CT available. Although spiral is more advantageous for many applications, conventional CT is still a superior and precise method for imaging many tissues and structures. The physician will evaluate which type of CT works best for the specific exam purpose.
Preparation
If a contrast medium is administered, the patient may be asked to fast for about four to six hours prior to the procedure. Patients will usually be given a gown (like a typical hospital gown) to be worn during the procedure. All metal and jewelry should be removed to avoid artifacts on the film. Depending on the type of study, patients may also be required to remove dentures.
Aftercare
Generally, no aftercare is required following a CT scan. Immediately following the exam, the technologist will continue to watch the patient for possible adverse contrast reactions. Patients are instructed to advise the technologist of any symptoms, particularly respiratory difficulty. The site of contrast injection will be bandaged and may feel tender following the exam.
Risks
Radiation exposure from a CT scan is similar to, though higher than, that of a conventional x ray. Although this is a risk to pregnant women, the risk for other adults is minimal and should produce no effects. Severe contrast reactions are rare, but they are a risk of many CT procedures.
Normal results
Normal findings on a CT exam show bone, the most dense tissue, as white areas. Tissues and fat will show as various shades of gray, and fluids will be gray or black. Air will also look black. Intravenous, oral, and rectal contrast appear as white areas. The radiologist can determine if tissues and organs appear normal by the sensitivity of the gray shadows.
Abnormal results
Abnormal results may show different characteristics of tissues within organs. Accumulations of blood or other fluids where they do not belong may be detected. Radiologists can differentiate among types of tumors throughout the body by viewing details of their makeup.
Sinus studies
The increasing availability and lowered cost of CT scanning has lead to its increased use in sinus studies, either as a replacement for a sinus x ray or as a follow-up to an abnormal sinus radiograph. The sensitivity of CT allows for the location of areas of sinus infection, particularly chronic infection. Sinus tumors will show as shades of gray indicating the difference in their density from that of normal tissues in the area.
Brain studies
The precise differences in density allowed by CT scan can clearly show tumors, strokes, or lesions in the brain area as altered densities. These lighter or darker areas on the image may indicate a tumor or hematoma within the brain and skull area. Different types of tumors can be identified by the presence of edema, by the tissue's density, or by studying blood vessel location and activity. The speed and convenience of CT often allows for detection of hemorrhage before symptoms even occur.
Body scans
The body CT scan can identify abnormal body structures and organs. A CT scan may indicate tumors or cysts, enlarged lymph nodes, abnormal collections of fluids, blood or fat, or cancer metastasis . Tumors resulting from metastasis are different in makeup than primary (original) tumors.
Chest scans
In addition to those findings which may indicate aortic aneurysms, chest CT studies can show other problems in the heart and lungs, and distinguish between an aortic aneurysm and a tumor adjacent to the aorta. CT will not only show differences between air, water, tissues and bone, but will also assign numerical values to the various densities. Coin-sized lesions in the lungs may be indicative of tuberculosis or tumors. CT will help distinguish among the two. Enlarged lymph nodes in the chest area may indicate Hodgkin's disease .
Teresa G. Norris
Resources
BOOKS
Abeloff, M. Clinical Oncology, 2nd Ed. Orlando, Florida:Churchill Livingstone, Inc., 2000.
Springhouse Corporation. Illustrated Guide to Diagnostic Tests. Springhouse, PA: Springhouse Corporation, 1998.
PERIODICALS
Holbert, J. M. "Role of Spiral Computed Tomography in the Diagnosis of Pulmonary Embolism in the Emergency Department." Annals of Emergency Medicine (May 1999): 520-28.
ORGANIZATION
American College of Radiology. 1891 Preston White Drive, Reston, VA 22091. (800) ACR-LINE. <http://www.acr.org>.
KEY TERMS
Aneurysm
—The bulging of the blood vessel wall. Aortic aneurysms are the most dangerous. Aneurysms can break and cause bleeding.
Contrast (agent, medium)
—A substance injected into the body that illuminates certain structures that would otherwise be hard to see on the radiograph (film).
Gantry
—A name for the couch or table used in a CT scan. The patient lies on the gantry while it slides into the x-ray scanner.
Hematoma
—A collection of blood that has escaped from the vessels. It may clot and harden, causing pain to the patient.
Metastasis
—Secondary cancer, or cancer that has spread from one body organ or tissue to another.
Radiologist
—A medical doctor specially trained in radiology (x ray) interpretation and its use in the diagnosis of disease and injury.
Spiral CT
—Also referred to as helical CT, this method allows for continuous 360-degree x-ray image capture.
Thoracic
—Refers to the chest area. The thorax runs between the abdomen and neck and is encased in the ribs.
QUESTIONS TO ASK THE DOCTOR
- Why is a CT scan recommended in my case?
- What are the benefits associated with this procedure?
- What are the risks associated with this procedure?
- How do I prepare for the CT scan?
- When will I know the results?
Computed tomography
Computed tomography
Definition
Computed tomography scanning, also called CT scan, CAT scan, or computerized axial tomography, is a diagnostic tool that provides views of internal body structures using x rays. In the field of mental health, a CT scan may be used when a patient seeks medical help for symptoms that could possibly be caused by a brain tumor. These symptoms may include headaches, emotional abnormalities, or intellectual or memory problems. In these cases, a CT scan may be performed to "rule out" a tumor, so that other tests can be performed in order to establish an accurate diagnosis .
Purpose
CT scans are used to image bone, soft tissues, and air. Since the 1990s, CT equipment has become more affordable and available. CT scans have become the imaging exam of choice for the diagnoses of most solid tumors. Because the computerized image is sharp, focused, and three-dimensional, many structures can be better differentiated (visualized) when compared with standard x rays.
Common indications for CT scans include:
- • Sinus studies. The CT scan can show details of sinusitis, bone fractures, and the presence of bony tumor involvement. Physicians may order a CT scan of the sinuses to provide an accurate map for surgery.
- • Brain studies. Brain CT scans can detect hematomas (blood clotted mass), tumors, strokes, aneurysms (a blood vessel that ruptures), and degenerative or infected brain tissue. The introduction of CT scanning, especially spiral CT, has helped reduce the need for more invasive procedures such as cerebral angiography (inserting a wire through an artery to where it will reach brain vessels for visualization in real time).
- • Body scans. CT scans of the chest, abdomen, spine, and extremities can detect the presence of tumors, enlarged lymph nodes, abnormal collection of fluid, and vertebral disc disease. These scans can also be helpful in evaluating the extent of bone breakdown in osteoporosis.
- • Heart and aorta scans. CT scans can focus on the thoracic (chest) or abdominal aorta to locate aneurysms and other possible aortic diseases. A newer type of CT scan, called electron beam CT, can be used to detect calcium buildup in arteries. Because it is a new technology, it is not yet widely used and its indications are not yet well-defined.
- • Chest scans. CT scans of the chest are useful in distinguishing tumors and in detailing accumulation of fluid in chest infections.
Precautions
Pregnant women or those who could possibly be pregnant should not have a CT scan, particularly a full body or abdominal scan, unless the diagnostic benefits outweigh the risks. If the exam is necessary for obstetric purposes, technologists are instructed not to repeat films if there are errors. Pregnant patients receiving a CT scan or any x ray exam away from the abdominal area may be protected by a lead apron; most radiation, known as scatter, travels through the body, however, and is not totally blocked by the apron.
Contrast agents are often used in CT exams, though some types of tumors are better seen without it. Patients should discuss the use of contrast agents with their doctor, and should be asked to sign a consent form prior to the administration of contrast. One of the common contrast agents, iodine, can cause allergic reactions. Patients who are known to be allergic to iodine or shellfish should inform the physician prior to the CT scan; a combination of medications can be given to such patients before the scan to prevent or minimize the reaction. Contrast agents may also put patients with diabetes at risk of kidney failure, particularly those taking the medication glucophage.
Description
Computed tomography, is a combination of focused x-ray beams and the computerized production of an image. Introduced in the early 1970s, this radiologic procedure has advanced rapidly and is now widely used, sometimes in the place of standard x rays.
CT equipment
A CT scan may be performed in a hospital or outpatient imaging center. Although the equipment looks large and intimidating, it is very sophisticated and fairly comfortable. The patient is asked to lie on a gantry, or narrow table, that slides into the center of the scanner. The scanner looks like a doughnut and is round in the middle, which allows the x-ray beam to rotate around the patient. The scanner section may also be tilted slightly to allow for certain cross-sectional angles.
CT procedure
The gantry moves very slightly as the precise adjustments for each sectional image are made. A technologist watches the procedure from a window and views the images on a computer screen. Generally, patients are alone during the procedure, though exceptions are sometimes made for pediatric patients. Communication is possible via an intercom system.
It is essential that the patient lie very still during the procedure to prevent motion blurring. In some studies, such as chest CTs, the patient will be asked to hold his or her breath during image capture.
Following the procedure, films of the images are usually printed for the radiologist and referring physician to review. A radiologist can also interpret CT exams on the computer screen. The procedure time will vary in length depending on the area being imaged. Average study times are from 30 to 60 minutes. Some patients may be concerned about claustrophobia (a feeling of being "closed in") but the width of the "doughnut" portion of the scanner is such that many patients can be reassured of openness. Doctors may consider giving sedatives to patients who have severe claustrophobia or difficulty lying still (such as small children).
The CT image
While traditional x-ray machines image organs in two dimensions, often resulting in organs in the front of the body being superimposed over those in the back, CT scans allow for a more three-dimensional effect. CT images can be likened to slices in a loaf of bread. Precise sections of the body can be located and imaged as cross-sectional views. The screen before the technologist shows a computer's analysis of each section detected by the x-ray beam. Thus, various densities of tissue can be easily distinguished.
Contrast agents
Contrast agents are often used in CT exams and in other radiology procedures to illuminate certain details of anatomy more clearly. Some contrasts are natural, such as air or water. A water-based contrast agent is sometimes administered for specific diagnostic purposes. Barium sulfate is commonly used in gastroenterology procedures. The patient may drink this contrast or receive it in an enema. Oral or rectal contrast is usually given when examining the abdomen or cells, but not when scanning the brain or chest. Iodine is the most widely used intravenous contrast agent and is given through an intravenous needle.
If contrast agents are used in the CT exam, these will be administered several minutes before the study begins. Patients undergoing abdominal CT may be asked to drink a contrast medium. Some patients may experience a salty taste, flushing of the face, warmth or slight nausea, or hives from an intravenous contrast injection. Technologists and radiologists have the equipment and training to help patients through these minor reactions and to handle more severe reactions. Severe reactions to contrast are rare, but do occur.
Newer types of CT scans
The spiral CT scan, also called a helical CT, is a newer version of CT. This type of scan is continuous in motion and allows for the continuous re-creation of images. For example, traditional CT allows the technologist to take slices at very small and precise intervals one after the other. Spiral CT allows for a continuous flow of images, without stopping the scanner to move to the next image slice. A major advantage of spiral CT is the ability to reconstruct images anywhere along the length of the study area. Because the procedure is faster, patients are required to lie still for shorter periods of time. The ability to image contrast more rapidly after it is injected, when it is at its highest level, is another advantage of spiral CT's high speed.
Electron beam CT scans are another newer type of CT technology that can be used to detect calcium buildup in arteries. These calcium deposits are potential risk factors for coronary artery disease. Electron beam CT scans take pictures much more quickly than conventional CTs, and are therefore better able to produce clear images of the heart as it pumps blood. Because it is a newer and expensive test, electron beam CT scanning is not widely used.
Some facilities will have spiral, electron, and conventional CT available. Although spiral is more advantageous for many applications, conventional CT is still a superior and precise method for imaging many tissues and structures. The physician will evaluate which type of CT works best for the specific exam purpose.
Preparation
If a contrast medium is administered, the patient may be asked to fast for about four to six hours prior to the procedure. Patients will usually be given a gown (like a typical hospital gown) to be worn during the procedure. All metal and jewelry should be removed to avoid artifacts on the film. Depending on the type of study, patients may also be required to remove dentures.
Aftercare
Generally, no aftercare is required following a CT scan. Immediately following the exam, the technologist will continue to watch the patient for possible adverse contrast reactions. Patients are instructed to advise the technologist of any symptoms, particularly respiratory difficulty. The site of contrast injection will be bandaged and may feel tender following the exam.
Risks
Radiation exposure from a CT scan is similar to, though higher than, that of a conventional x ray. Although this is a risk to pregnant women, the risk for other adults is minimal and should produce no effects. Severe contrast reactions are rare, but they are a risk of many CT procedures.
Normal results
Normal findings on a CT exam show bone, the most dense tissue, as white areas. Tissues and fat will show as various shades of gray, and fluids will be gray or black. Air will also look black. Intravenous, oral, and rectal contrast appear as white areas. The radiologist can determine if tissues and organs appear normal by the sensitivity of the gray shadows.
Abnormal results
Abnormal results may show different characteristics of tissues within organs. Accumulations of blood or other fluids where they do not belong may be detected. Radiologists can differentiate among types of tumors throughout the body by viewing details of their makeup.
Sinus studies
The increasing availability and lowered cost of CT scanning has led to its increased use in sinus studies, either as a replacement for a sinus x ray or as a follow-up to an abnormal sinus radiograph. The sensitivity of CT allows for the location of areas of sinus infection, particularly chronic infection. Sinus tumors will show as shades of gray indicating the difference in their density from that of normal tissues in the area.
Brain studies
The precise differences in density allowed by CT scan can clearly show tumors, strokes, or lesions in the brain area as altered densities. These lighter or darker areas on the image may indicate a tumor or hematoma within the brain and skull area. Different types of tumors can be identified by the presence of edema (fluid), by the tissue's density, or by studying blood vessel location and activity. The speed and convenience of CT often allows for detection of hemorrhage (bleeding) before symptoms even occur.
Body scans
The body CT scan can identify abnormal body structures and organs. A CT scan may indicate tumors or cysts, enlarged lymph nodes, abnormal collections of fluids, blood, fat, or cancer metastasis. Tumors resulting from metastasis (movement of the cancer from the primary site of cancer growth to a distant site) are different in makeup than primary (original) tumors.
Chest scans
In addition to those findings which may indicate aortic aneurysms (rupture of the largest artery in the body), chest CT studies can show other problems in the heart and lungs, and distinguish between an aortic aneurysm and a tumor adjacent to the aorta. CT will not only show differences between air, water, tissues and bone, but will also assign numerical values to the various densities. Coin-sized lesions in the lungs may be indicative of tuberculosis or tumors. CT will help distinguish among the two. Enlarged lymph nodes in the chest area may indicate Hodgkin's disease (a blood disorder).
Resources
BOOKS
Abeloff, M. Clinical Oncology, 2nd Ed. Orlando, Florida: Churchill Livingstone, Inc., 2000.
Springhouse Corporation. Illustrated Guide to Diagnostic Tests. Springhouse, PA: Springhouse Corporation, 1998.
PERIODICALS
Holbert, J. M. "Role of Spiral Computed Tomography in the Diagnosis of Pulmonary Embolism in the Emergency Department." Annals of Emergency Medicine (May 1999): 520-28.
ORGANIZATIONS
American College of Radiology. 1891 Preston White Drive, Reston, VA 22091. (800) ACR-LINE. <http://www.acr.org>.
Laith Farid Gulli, M.D.
Teresa G. Norris R.N.
Computed Tomography
Computed tomography
Definition
Computed tomography (CT), formerly referred to as computerized axial tomography (CAT), is a common diagnostic imaging procedure that uses x rays to generate images (slices) of the anatomy.
Purpose
Computed tomography (CT) is an x-ray imaging procedure used for a variety of clinical applications. CT is used for spine and head imaging, gastrointestinal imaging, vascular imaging (e.g., detection of blood clots), cancer staging and radiotherapy treatment planning, screening for cancers and heart disease, rapid imaging of trauma, imaging of musculoskeletal disorders, detection of signs of infectious disease, and guidance of certain interventional procedures (e.g., biopsies). CT is the preferred imaging exam for diagnosing several types of cancers, and along with the chest x ray, CT is the most commonly performed procedure for imaging the chest. CT is also used to perform noninvasive angiographic imaging to assess the large blood vessels.
CT may be performed on newborns, infants, children, and adolescents. In children, CT is most frequently used in the hospital emergency department to evaluate the effects of trauma, especially to the head, face, brain, and spine, and to diagnose or rule out appendicitis and other abdominal disorders because a scan can be completed in less than 20 seconds. Chest CT examinations are used to assess complications from infectious diseases, such as pneumonia and tuberculosis , inflammation of the airways, and birth defects. CT scans of the pelvic area are used to image ovarian cysts and tumors, bladder abnormalities, urinary tract stones, kidney disease, and bone disease. Head CT scans are used to examine the brain and sinuses. For children with cancer, CT is used to assist in treatment planning and to monitor cancer progression and response to treatment. For children requiring complex surgeries (e.g., brain, spine), CT is often used to produce images of the anatomy that help surgeons plan the surgery. Newer CT scanners, called multislice or multidetector CT, are used to rapidly image newborns to assess congenital heart defects.
Description
CT is performed using a specialized scanner, an x-ray system, a patient table, and a computer workstation. The CT scanner is shaped like a large square with a hole in the center or round like a doughnut. X rays are produced in the form of a beam that rotates around the patient. During a CT scan, the patient table is moved through the center hole as x-ray beams pass through the patient's body. The x rays are converted into a series of black-and-white images, each of which represents a "slice" of the anatomy.
CT scans are conducted by a technologist with specialized training in x rays and CT imaging. During scanning, the technologist operates the CT scanner using a computer located in an adjacent room. Because movement during the scan can cause inaccurate images, the technologist instructs the patient via an intercom system to hold their breath and not move during the x rays. The scan itself may only take five to 15 minutes, but total examination time may be up to 30 minutes, since the patient must be prepped and positioned. Abdominal CT scans usually require that the child drink a solution that contains a dye, called oral contrast, that shows up on the CT images to help better define internal organs. For pelvic scans, contrast material may be delivered via the rectum. Some CT scans also require the injection of contrast material into the vein to help define the blood vessels and surrounding tissue.
The images from CT examination are called slices because they are acquired in very small (millimeter-size) sections of the body. The image slices are displayed on a computer monitor for viewing or printed as a film. A radiologist interprets the x-ray images produced during the CT examination. For emergency CT scans, images are interpreted immediately so that the child can be treated as soon as possible. For non-urgent outpatient CT scans, the radiologist interprets the images and sends a report to the referring physician within a few days.
For emergency situations, CT scans are performed in a hospital radiology department in conjunction with the emergency department. For non-urgent conditions, CT scans can be performed on an outpatient basis in a hospital radiology department or outpatient imaging center. In small hospitals or hospitals in rural areas, a CT scanner may not be permanently located in the hospital; rather, a mobile imaging service will be contracted to bring a specially designed trailer with CT equipment to the hospital on prescheduled days.
Precautions
CT scans expose the child to radiation, and overuse of CT scanning has received attention from organizations that regulate medical radiation exposure. Although no side effects have been linked to radiation exposure from CT imaging, the Food and Drug Administration has issued guidance to physicians regarding levels of radiation during pediatric CT examinations. New CT scanners have preset imaging features that allow scanning at the lowest radiation dose for the child's weight and age.
Oral contrast may be unpleasant tasting, although chocolate, vanilla, and fruit flavors may be available. Injected contrast can cause sensations of heat or cold through the body. Some children may have allergic reactions to the contrast material, although severe reactions are rare. Parents should inform CT staff if their child has ever had a reaction to any medication, contrast material, or anesthesia. Because contrast material may contain iodine, sensitivity to contrast material may occur if the child has other allergies to iodine or seafood, and CT staff should be informed if the child has such allergies. Also, because CT contrast material can affect kidney function, parents should notify CT staff if their child has kidney disease.
Preparation
Abdominal CT examinations usually require fasting for at least 12 hours before the scan. If the intestines will also be imaged, a laxative before the scan is required. Parents should alert CT staff if children are diabetic and taking insulin, since hypoglycemia can occur with missed meals.
Before the CT scan, the patient has to change into a hospital gown. When oral contrast is necessary, patients need to arrive at least one hour before the scan to drink the contrast solution. During the scan, the child is asked to lie on the CT table. Positioning devices, such as head cradles or knee rests, may be used. For very young or very active children, foam or Velcro restraints may be used to minimize movement during imaging. Or sedation may be used if children cannot remain still. After positioning the child, the technologist inserts an intravenous catheter to inject contrast material.
CT scanners may frighten young children, so prior to the imaging examination, the basic procedure should be explained to help reduce fear . Some radiology departments offer special patient education booklets for children that help explain imaging procedures.
Aftercare
No special aftercare is required following CT scans, unless sedation or general anesthesia was used during the scan. In these cases, children are required to remain in a supervised recovery area for an hour or more following the procedure to be monitored for reactions to anesthesia. If injected contrast material is used, some minor first aid (small bandage, pain relief) for the injection site may be necessary.
Risks
Radiation exposure is a risk during CT examinations. However, the radiation from a CT scan is usually less than that from regular x rays, and the benefits of the examination far outweigh the minor radiation dose received during the scan.
Some children may have reactions to anesthesia or sedation, including headaches, shivering, or vomiting . Rarely, severe anaphylactic reactions can occur that require emergency treatment.
Parental concerns
Younger children may be frightened of the CT scanner, and a parent or other family member may be required to be present in the scanning room. To help alleviate fear, taking the child into the CT room to see the equipment prior to the procedure may be helpful. To reduce risk of radiation exposure, anyone remaining in the scanning room during x-ray delivery will have to wear a lead apron on shield.
KEY TERMS
Anaphylaxis —Also called anaphylactic shock; a severe allergic reaction characterized by airway constriction, tissue swelling, and lowered blood pressure.
Radiologist —A medical doctor specially trained in radiology, the branch of medicine concerned with radioactive substances and their use for the diagnosis and treatment of disease.
Resources
BOOKS
Margolis, Simeon, et al. The Johns Hopkins Consumer Guide to Medical Tests: What You Can Expect, How You Should Prepare, What Your Results Mean. New York: Rebus Inc., 2002.
Medical Tests: A Practical Guide to Common Tests. Boston, MA: Harvard Health Publications, 2004.
Segen, J. C., et al. The Patient's Guide to Medical Tests: Everything You Need to Know about the Tests Your Doctor Prescribes. New York: Facts on File, 2002.
Shannon, Joyce Brennfleck. Medical Tests Sourcebook: Basic Consumer Health Information about Medical Tests. Detroit, MI: Omnigraphics, 2004.
PERIODICALS
Harvey, D. "Evaluating Pediatric Trauma: Imaging vs. Lab Tests." Radiology Today 5 (August 2, 2004): 14–16.
ORGANIZATIONS
American College of Emergency Physicians. 2121 K St., NW, Suite 325, Washington, DC 20037. Web site: <www.acep.org>.
American College of Radiology. 1891 Preston White Dr., Reston, VA 20191. Web site: <www.acr.org>.
Radiological Society of North America. 820 Jorie Blvd., Oak Brook, IL 60523–2251. Web site: <www.rsna.org/>.
WEB SITES
"CT Scan." Emedicine, November 1, 2004. Available online at <www.emedicinehealth.com/Articles/11618-1.asp> (accessed December 21, 2004).
"Pediatric CT (Computerized Tomography)." Radiology Info: The Radiology Information Source for Patients. Available online at <http://www.radiologyinfo.com/content/pediact.htm> (accessed December 21, 2004).
Jennifer Sisk, MA