Dictionary:
com·put·er·ized axial tomography (kəm-pyū'tə-rīzd')  |
Tomography in which computer analysis of a series of cross-sectional scans made along a single axis of a bodily structure or tissue is used to construct a three-dimensional image of that structure. The technique is used in diagnostic studies of internal bodily structures, as in the detection of tumors or brain aneurysms.
| 5min Related Video: computerized axial tomography |
| Oncology Encyclopedia: Computed Tomography |
Key Terms: Aneurysm, Contrast, Gantry, Hematoma, Metastasis, Radiologist, Spiral CT.
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:
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 xray 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 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, 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.
Questions to Ask the Doctor
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.
Resources
Books
Abeloff, M. Clinical Oncology. 2nd ed. Orlando, Florida: Churchill Livingstone, Inc., 2000.
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.
—Teresa G. Odle
| Dental Dictionary: computed tomography |
A radiographic body scanning technique in which thin or narrow layer sections of the body can be imaged for diagnostic purposes. The technique uses a computer-linked x-ray machine to focus the x-rays on a particular section of the body to be viewed.
| Children's Health Encyclopedia: 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.
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
[Article by: Jennifer Sisk, MA]
| Veterinary Dictionary: computed tomography |
A radiological imaging technique that produces images of ‘slices’ through a patient's body. See also computed tomography.
| Wikipedia: Computed tomography |
Computed tomography (CT) is a medical imaging method employing tomography. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation. The word "tomography" is derived from the Greek tomos (slice) and graphein (to write). Computed tomography was originally known as the "EMI scan" as it was developed at a research branch of EMI, a company best known today for its music and recording business. It was later known as computed axial tomography (CAT or CT scan) and body section röntgenography.
CT produces a volume of data which can be manipulated, through a process known as "windowing", in order to demonstrate various bodily structures based on their ability to block the X-ray/Röntgen beam. Although historically the images generated were in the axial or transverse plane, orthogonal to the long axis of the body, modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3D) representations of structures. Although most common in medicine, CT is also used in other fields, such as nondestructive materials testing. Another example is the DigiMorph project at the University of Texas at Austin which uses a CT scanner to study biological and paleontological specimens.
Contents |
In the early 1900s, the Italian radiologist Alessandro Vallebona proposed a method to represent a single slice of the body on the radiographic film. This method was known as tomography. The idea is based on simple principles of projective geometry: moving synchronously and in opposite directions the X-ray tube and the film, which are connected together by a rod whose pivot point is the focus; the image created by the points on the focal plane appears sharper, while the images of the other points annihilate as noise. This is only marginally effective, as blurring occurs only in the "x" plane. There are also more complex devices which can move in more than one plane and perform more effective blurring.
Tomography had been one of the pillars of radiologic diagnostics until the late 1970s, when the availability of minicomputers and of the transverse axial scanning method, this last due to the work of Godfrey Hounsfield and South African born Allan McLeod Cormack, gradually supplanted it as the modality of CT.
The first commercially viable CT scanner was invented by Sir Godfrey Hounsfield in Hayes, United Kingdom at EMI Central Research Laboratories using X-rays. Hounsfield conceived his idea in 1967,[1] and it was publicly announced in 1972. Allan McLeod Cormack of Tufts University in Massachusetts independently invented a similar process, and both Hounsfield and Cormack shared the 1979 Nobel Prize in Medicine.[2]
The original 1971 prototype took 160 parallel readings through 180 angles, each 1° apart, with each scan taking a little over five minutes. The images from these scans took 2.5 hours to be processed by algebraic reconstruction techniques on a large computer. The scanner had a single photomultiplier detector, and operated on the Translate/Rotate principle.
It has been claimed that thanks to the success of The Beatles, EMI could fund research and build early models for medical use.[3] The first production X-ray CT machine (in fact called the "EMI-Scanner") was limited to making tomographic sections of the brain, but acquired the image data in about 4 minutes (scanning two adjacent slices), and the computation time (using a Data General Nova minicomputer) was about 7 minutes per picture. This scanner required the use of a water-filled Perspex tank with a pre-shaped rubber "head-cap" at the front, which enclosed the patient's head. The water-tank was used to reduce the dynamic range of the radiation reaching the detectors (between scanning outside the head compared with scanning through the bone of the skull). The images were relatively low resolution, being composed of a matrix of only 80 x 80 pixels. The first EMI-Scanner was installed in Atkinson Morley Hospital in Wimbledon, England, and the first patient brain-scan was made with it in 1972.
In the U.S., the first installation was at the Mayo Clinic. As a tribute to the impact of this system on medical imaging the Mayo Clinic has an EMI scanner on display in the Radiology Department.
The first CT system that could make images of any part of the body and did not require the "water tank" was the ACTA (Automatic Computerized Transverse Axial) scanner designed by Robert S. Ledley, DDS at Georgetown University. This machine had 30 photomultiplier tubes as detectors and completed a scan in only 9 translate/rotate cycles, much faster than the EMI-scanner. It used a DEC PDP11/34 minicomputer both to operate the servo-mechanisms and to acquire and process the images. The Pfizer drug company acquired the prototype from the university, along with rights to manufacture it. Pfizer then began making copies of the prototype, calling it the "200FS" (FS meaning Fast Scan), which were selling as fast as they could make them. This unit produced images in a 256x256 matrix, with much better definition than the EMI-Scanner's 80x80
A form of tomography can be performed by moving the X-ray source and detector during an exposure. Anatomy at the target level remains sharp, while structures at different levels are blurred. By varying the extent and path of motion, a variety of effects can be obtained, with variable depth of field and different degrees of blurring of 'out of plane' structures.[4]:25
Although largely obsolete, conventional tomography is still used in specific situations such as dental imaging (orthopantomography) or in intravenous urography.
Digital tomosynthesis combines digital image capture and processing with simple tube/detector motion as used in conventional radiographic tomography. Although there are some similarities to CT, it is a separate technique. In CT, the source/detector makes a complete 360-degree rotation about the subject obtaining a complete set of data from which images may be reconstructed. In digital tomosynthesis, only a small rotation angle (e.g., 40 degrees) with a small number of discrete exposures (e.g., 10) are used. This incomplete set of data can be digitally processed to yield images similar to conventional tomography with a limited depth of field. However, because the image processing is digital, a series of slices at different depths and with different thicknesses can be reconstructed from the same acquisition, saving both time and radiation exposure.
Because the data acquired is incomplete, tomosynthesis is unable to offer the extremely narrow slice widths that CT offers. However, higher resolution detectors can be used, allowing very-high in-plane resolution, even if the Z-axis resolution is poor. The primary interest in tomosynthesis is in breast imaging, as an extension to mammography, where it may offer better detection rates with little extra increase in radiation exposure.
Reconstruction algorithms for tomosynthesis are significantly different from conventional CT, because the conventional filtered back projection algorithm requires a complete set of data. Iterative algorithms based upon expectation maximization are most commonly used, but are extremely computationally intensive. Some manufacturers have produced practical systems using off-the-shelf GPUs to perform the reconstruction.
Since its introduction in the 1970s, CT has become an important tool in medical imaging to supplement X-rays and medical ultrasonography. Although it is still quite expensive, it is the gold standard in the diagnosis of a large number of different disease entities. It has more recently begun to also be used for preventive medicine or screening for disease, for example CT colonography for patients with a high risk of colon cancer. Although a number of institutions offer full-body scans for the general population, this practice remains controversial due to its lack of proven benefit, cost, radiation exposure, and the risk of finding 'incidental' abnormalities that may trigger additional investigations.
CT scanning of the head is typically used to detect:
CT can be used for detecting both acute and chronic changes in the lung parenchyma, that is, the internals of the lungs. It is particularly relevant here because normal two dimensional x-rays do not show such defects. A variety of different techniques are used depending on the suspected abnormality. For evaluation of chronic interstitial processes (emphysema, fibrosis, and so forth), thin sections with high spatial frequency reconstructions are used - often scans are performed both in inspiration and expiration. This special technique is called High Resolution CT (HRCT). HRCT is normally done with thin section with skipped areas between the thin sections. Therefore it produces a sampling of the lung and not continuous images. Continuous images are provided in a standard CT of the chest.
For detection of airspace disease (such as pneumonia) or cancer, relatively thick sections and general purpose image reconstruction techniques may be adequate. IV contrast may also be used as it clarifies the anatomy and boundaries of the great vessels and improves assessment of the mediastinum and hilar regions for lymphadenopathy; this is particularly important for accurate assessment of cancer.
CT angiography of the chest is also becoming the primary method for detecting pulmonary embolism (PE) and aortic dissection, and requires accurately timed rapid injections of contrast (Bolus Tracking) and high-speed helical scanners. CT is the standard method of evaluating abnormalities seen on chest X-ray and of following findings of uncertain acute significance.
More than 62 million scans are ordered each year, according to the 2007 New England Journal of Medicine study. 31% of 62 million (19,2 million) is used for lung CT's.
CT pulmonary angiogram (CTPA) is a medical diagnostic test used to diagnose pulmonary embolism (PE). It employs computed tomography to obtain an image of the pulmonary arteries.
It is a preferred choice of imaging in the diagnosis of PE due to its minimally invasive nature for the patient, whose only requirement for the scan is a cannula (usually a 20G).
MDCT (multi detector CT) scanners give the optimum resolution and image quality for this test. Images are usually taken on a 0.625 mm slice thickness, although 2 mm is sufficient. 50 - 100 mls of contrast is given to the patient at a rate of 4 ml/s. The tracker/locator is placed at the level of the Pulmonary Arteries, which sit roughly at the level of the carina. Images are acquired with the maximum intensity of radio-opaque contrast in the Pulmonary Arteries. This is done using bolus tracking.
CT machines are now so sophisticated that the test can be done with a patient visit of 5 minutes with an approximate scan time of only 5 seconds or less.
A normal CTPA scan will show the contrast filling the pulmonary vessels, looking bright white. Ideally the aorta should be empty of contrast, to reduce any partial volume artifact which may result in a false positive. Any mass filling defects, such as an embolus, will appear dark in place of the contrast, filling / blocking the space where blood should be flowing into the lungs.
With the advent of subsecond rotation combined with multi-slice CT (up to 64-slice), high resolution and high speed can be obtained at the same time, allowing excellent imaging of the coronary arteries (cardiac CT angiography). Images with an even higher temporal resolution can be formed using retrospective ECG gating. In this technique, each portion of the heart is imaged more than once while an ECG trace is recorded. The ECG is then used to correlate the CT data with their corresponding phases of cardiac contraction. Once this correlation is complete, all data that were recorded while the heart was in motion (systole) can be ignored and images can be made from the remaining data that happened to be acquired while the heart was at rest (diastole). In this way, individual frames in a cardiac CT investigation have a better temporal resolution than the shortest tube rotation time.
Because the heart is effectively imaged more than once (as described above), cardiac CT angiography results in a relatively high radiation exposure around 12 mSv. For the sake of comparison, a chest X-ray carries a dose of approximately 0.02[5] to 0.2 mSv and natural background radiation exposure is around 0.01 mSv/day. Thus, cardiac CTA is equivalent to approximately 100-600 chest X-rays or over 3 years worth of natural background radiation. Methods are available to decrease this exposure, however, such as prospectively decreasing radiation output based on the concurrently acquired ECG (aka tube current modulation.) This can result in a significant decrease in radiation exposure, at the risk of compromising image quality if there is any arrhythmia during the acquisition. The significance of radiation doses in the diagnostic imaging range has not been proven, although the possibility of inducing an increased cancer risk across a population is a source of significant concern. This potential risk must be weighed against the competing risk of not performing a test and potentially not diagnosing a significant health problem such as coronary artery disease.
It is uncertain whether this modality will replace invasive coronary catheterization. Currently, it appears that the greatest utility of cardiac CT lies in ruling out coronary artery disease rather than ruling it in. This is because the test has a high sensitivity (greater than 90%) and thus a negative test result means that a patient is very unlikely to have coronary artery disease and can be worked up for other causes of their chest symptoms. This is termed a high negative predictive value. A positive result is less conclusive and often will be confirmed (and possibly treated) with subsequent invasive angiography. The positive predictive value of cardiac CTA is estimated at approximately 82% and the negative predictive value is around 93%.
Dual Source CT scanners, introduced in 2005, allow higher temporal resolution by acquiring a full CT slice in only half a rotation, thus reducing motion blurring at high heart rates and potentially allowing for shorter breath-hold time. This is particularly useful for ill patients who have difficulty holding their breath or who are unable to take heart-rate lowering medication.
The speed advantages of 64-slice MSCT have rapidly established it as the minimum standard for newly installed CT scanners intended for cardiac scanning. Manufacturers are now actively developing 256-slice and true 'volumetric' scanners, primarily for their improved cardiac scanning performance.
The latest MSCT scanners acquire images only at 70-80% of the R-R interval (late diastole). This prospective gating can reduce effective dose from 10-15mSv to as little as 1.2mSv in follow-up patients acquiring at 75% of the R-R interval. Effective doses at a centre with well trained staff doing coronary imaging can average less than the doses for conventional coronary angiography.
CT is a sensitive method for diagnosis of abdominal diseases. It is used frequently to determine stage of cancer and to follow progress. It is also a useful test to investigate acute abdominal pain (especially of the lower quadrants, whereas ultrasound is the preferred first line investigation for right upper quadrant pain). Renal stones, appendicitis, pancreatitis, diverticulitis, abdominal aortic aneurysm, and bowel obstruction are conditions that are readily diagnosed and assessed with CT. CT is also the first line for detecting solid organ injury after trauma.
Oral and/or rectal contrast may be used depending on the indications for the scan. A dilute (2% w/v) suspension of barium sulfate is most commonly used. The concentrated barium sulfate preparations used for fluoroscopy e.g. barium enema are too dense and cause severe artifacts on CT. Iodinated contrast agents may be used if barium is contraindicated (for example, suspicion of bowel injury). Other agents may be required to optimize the imaging of specific organs, such as rectally administered gas (air or carbon dioxide) or fluid (water) for a colon study, or oral water for a stomach study.
CT has limited application in the evaluation of the pelvis. For the female pelvis in particular, ultrasound and MRI are the imaging modalities of choice. Nevertheless, it may be part of abdominal scanning (e.g. for tumors), and has uses in assessing fractures.
CT is also used in osteoporosis studies and research alongside dual energy X-ray absorptiometry (DXA). Both CT and DXA can be used to assess bone mineral density (BMD) which is used to indicate bone strength, however CT results do not correlate exactly with DXA (the gold standard of BMD measurement). CT is far more expensive, and subjects patients to much higher levels of ionizing radiation, so it is used infrequently.
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 recognised with a 0.2 mm resolution.
There are several advantages that CT has over traditional 2D medical radiography. First, CT completely eliminates the superimposition of images of structures outside the area of interest. Second, because of the inherent high-contrast resolution of CT, differences between tissues that differ in physical density by less than 1% can be distinguished. Finally, data from a single CT imaging procedure consisting of either multiple contiguous or one helical scan can be viewed as images in the axial, coronal, or sagittal planes, depending on the diagnostic task. This is referred to as multiplanar reformatted imaging.
CT is regarded as a moderate to high radiation diagnostic technique. While technical advances have improved radiation efficiency, there has been simultaneous pressure to obtain higher-resolution imaging and use more complex scan techniques, both of which require higher doses of radiation. The improved resolution of CT has permitted the development of new investigations, which may have advantages; compared to conventional angiography for example, CT angiography avoids the invasive insertion of an arterial catheter and guidewire; CT colonography (also known as virtual colonoscopy or VC for short) may be as useful as a barium enema for detection of tumors, but may use a lower radiation dose. CT VC is increasingly being used in the UK as a diagnostic test for bowel cancer and can negate the need for a colonoscopy.
The greatly increased availability of CT, together with its value for an increasing number of conditions, has been responsible for a large rise in popularity. So large has been this rise that, in the most recent comprehensive survey in the United Kingdom, CT scans constituted 7% of all radiologic examinations, but contributed 47% of the total collective dose from medical X-ray examinations in 2000/2001.[6] Increased CT usage has led to an overall rise in the total amount of medical radiation used, despite reductions in other areas. In the United States and Japan for example, there were 26 and 64 CT scanners per 1 million population in 1996. In the U.S., there were about 3 million CT scans performed in 1980, compared to an estimated 62 million scans in 2006.[7]
The radiation dose for a particular study depends on multiple factors: volume scanned, patient build, number and type of scan sequences, and desired resolution and image quality. Additionally, two helical CT scanning parameters that can be adjusted easily and that have a profound effect on radiation dose are tube current and pitch.[8]
The increased use of CT scans has been the greatest in two fields: screening of adults (screening CT of the lung in smokers, virtual colonoscopy, CT cardiac screening and whole-body CT in asymptomatic patients) and CT imaging of children. Shortening of the scanning time to around one second, eliminating the strict need for subject to remain still or be sedated, is one of the main reasons for large increase in the pediatric population (especially for the diagnosis of appendicitis).[7] CT scans of children have been estimated to produce non-negligible increases in the probability of lifetime cancer mortality leading to calls for the use of reduced current settings for CT scans of children.[9] These calculations are based on the assumption of a linear relationship between radiation dose and cancer risk; this claim is controversial, as some but not all evidence shows that smaller radiation doses are less harmful.[7] Estimated lifetime cancer mortality risks attributable to the radiation exposure from a CT in a 1-year-old are 0.18% (abdominal) and 0.07% (head)—an order of magnitude higher than for adults—although those figures still represent a small increase in cancer mortality over the background rate. In the United States, of approximately 600,000 abdominal and head CT examinations annually performed in children under the age of 15 years, a rough estimate is that 500 of these individuals might ultimately die from cancer attributable to the CT radiation .[10] The additional risk is still very low (0.35%) compared to the background risk of dying from cancer (23%).[10] However, if these statistics are extrapolated to the current number of CT scans, the additional rise in cancer mortality could be 1.5 to 2%. Furthermore, certain conditions can require children to be exposed to multiple CT scans. Again, these calculations can be problematic because the assumptions underlying them could overestimate the risk.[7]
CT scans can be performed with different settings for lower exposure in children, although these techniques are often not employed. Surveys have suggested that currently, many CT scans are performed unnecessarily. Ultrasound scanning or magnetic resonance imaging are alternatives (for example, in appendicitis or brain imaging) without the risk of radiation exposure. Although CT scans come with an additional risk of cancer, especially in children, the benefits that stem from their use outweighs the risk in many cases.[10] Studies support informing parents of the risks of pediatric CT scanning.[11]
| Examination | Typical effective dose (mSv) | (milli rem) |
|---|---|---|
| Chest X-ray | 0.1 | 10 |
| Head CT | 1.5[12] | 150 |
| Screening mammography | 3[7] | 300 |
| Abdomen CT | 5.3[12] | 530 |
| Chest CT | 5.8[12] | 580 |
| Chest, Abdomen and Pelvis CT | 9.9[12] | 990 |
| CT colonography (virtual colonoscopy) | 3.6 - 8.8 | 360 - 880 |
| Cardiac CT angiogram | 6.7-13[13] | 670 - 1300 |
| Barium enema | 15[7] | 1500 |
| Neonatal abdominal CT | 20[7] | 2000 |
For purposes of comparison the average background exposure in the UK is 1-3 mSv per annum.
Because contrast CT scans rely on intravenously administered contrast agents in order to provide superior image quality, there is a low but non-negligible level of risk associated with the contrast agents themselves. Many patients report nausea and discomfort, including warmth in the crotch which mimics the sensation of wetting oneself. Certain patients may experience severe and potentially life-threatening allergic reactions to the contrast dye.
The contrast agent may also induce kidney damage. The risk of this is increased with patients who have preexisting renal insufficiency, preexisting diabetes, or reduced intravascular volume. In general, if a patient has normal kidney function, then the risks of contrast nephropathy are negligible. Patients with mild kidney impairment are usually advised to ensure full hydration for several hours before and after the injection. For moderate kidney failure, the use of iodinated contrast should be avoided; this may mean using an alternative technique instead of CT e.g. MRI. Perhaps paradoxically, patients with severe renal failure requiring dialysis do not require special precautions, as their kidneys have so little function remaining that any further damage would not be noticeable and the dialysis will remove the contrast agent.
The main issue within radiology today is how to reduce the radiation dose during CT examinations without compromising the image quality. Generally, a high radiation dose results in high-quality images. A lower dose leads to increased image noise and results in unsharp images. Unfortunately, as the radiation dose increases, so does the associated risk of radiation induced cancer - even though this is extremely small. A radiation exposure of around 1200 mrem (similar to a 4-view mammogram) carried a radiation-induced cancer risk of about a million to one. However, there are several methods that can be used in order to lower the exposure to ionizing radiation during a CT scan.
See the entries or paragraphs of the same name in the MRI and 2D-FT NMRI and Spectroscopy articles. The basic mathematics of the 2D-Fourier transform in CT reconstruction is very similar to the 2D-FT NMRI, but the computer data processing in CT does differ in detail, as for example in the case of the volume rendering or the artifacts elimination algorithms that are specific to CT.
X-ray slice data is generated using an X-ray source that rotates around the object; X-ray sensors are positioned on the opposite side of the circle from the X-ray source. The earliest sensors were scintillation detectors, with photomultiplier tubes excited by (typically) cesium iodide crystals. Cesium iodide was replaced during the eighties by ion chambers containing high pressure Xenon gas. These systems were in turn replaced by scintillation systems based on photo diodes instead of photomultipliers and modern scintillation materials with more desirable characteristics. Many data scans are progressively taken as the object is gradually passed through the gantry. They are combined together by the mathematical procedures known as tomographic reconstruction. The data are arranged in a matrix in memory, and each data point is convolved with its neighbours according with a seed algorithm using Fast Fourier Transform techniques. This dramatically increases the resolution of each Voxel (volume element). Then a process known as Back Projection essentially reverses the acquisition geometry and stores the result in another memory array. This data can then be displayed, photographed, or used as input for further processing, such as multi-planar reconstruction.
Newer machines with faster computer systems and newer software strategies can process not only individual cross sections but continuously changing cross sections as the gantry, with the object to be imaged, is slowly and smoothly slid through the X-ray circle. These are called helical or spiral CT machines. Their computer systems integrate the data of the moving individual slices to generate three dimensional volumetric information (3D-CT scan), in turn viewable from multiple different perspectives on attached CT workstation monitors. This type of data acquisition requires enormous processing power, as the data are arriving in a continuous stream and must be processed in real-time.
In conventional CT machines, an X-ray tube and detector are physically rotated behind a circular shroud (see the image above right); in the electron beam tomography (EBT) the tube is far larger and higher power to support the high temporal resolution. The electron beam is deflected in a hollow funnel shaped vacuum chamber. X-rays are generated when the beam hits the stationary target. The detector is also stationary. This arrangement can result in very fast scans, but is extremely expensive.
The data stream representing the varying radiographic intensity sensed at the detectors on the opposite side of the circle during each sweep is then computer processed to calculate cross-sectional estimations of the radiographic density, expressed in Hounsfield units. Sweeps cover 360 or just over 180 degrees in conventional machines, 220 degrees in EBT.
CT is used in medicine as a diagnostic tool and as a guide for interventional procedures. Sometimes contrast materials such as intravenous iodinated contrast are used. This is useful to highlight structures such as blood vessels that otherwise would be difficult to delineate from their surroundings. Using contrast material can also help to obtain functional information about tissues.
Pixels in an image obtained by CT scanning are displayed in terms of relative radiodensity. The pixel itself is displayed according to the mean attenuation of the tissue(s) that it corresponds to on a scale from +3071 (most attenuating) to -1024 (least attenuating) on the Hounsfield scale. Pixel is a two dimensional unit based on the matrix size and the field of view. When the CT slice thickness is also factored in, the unit is known as a Voxel, which is a three dimensional unit. The phenomenon that one part of the detector cannot differentiate between different tissues is called the "Partial Volume Effect". That means that a big amount of cartilage and a thin layer of compact bone can cause the same attenuation in a voxel as hyperdense cartilage alone. Water has an attenuation of 0 Hounsfield units (HU) while air is -1000 HU, cancellous bone is typically +400 HU, cranial bone can reach 2000 HU or more (os temporale) and can cause artifacts. The attenuation of metallic implants depends on atomic number of the element used: Titanium usually has an amount of +1000 HU, iron steel can completely extinguish the X-ray and is therefore responsible for well-known line-artifacts in computed tomograms. Artifacts are caused by abrupt transitions between low- and high-density materials, which results in data values that exceed the dynamic range of the processing electronics.
Windowing is the process of using the calculated Hounsfield units to make an image. A typical display device can only resolve 256 shades of gray, some specialty medical displays can resolve up to 1024 shades of gray. These shades of gray can be distributed over a wide range of HU values to get an overview of structures that attenuate the beam to widely varying degrees. Alternatively, these shades of gray can be distributed over a narrow range of HU values (called a "narrow window") centered over the average HU value of a particular structure to be evaluated. In this way, subtle variations in the internal makeup of the structure can be discerned. This is a commonly used image processing technique known as contrast compression. For example, to evaluate the abdomen in order to find subtle masses in the liver, one might use liver windows. Choosing 70 HU as an average HU value for liver, the shades of gray can be distributed over a narrow window or range. One could use 170 HU as the narrow window, with 85 HU above the 70 HU average value; 85 HU below it. Therefore the liver window would extend from -15 HU to +155 HU. All the shades of gray for the image would be distributed in this range of Hounsfield values. Any HU value below -15 would be pure black, and any HU value above 155 HU would be pure white in this example. Using this same logic, bone windows would use a "wide window" (to evaluate everything from fat-containing medullary bone that contains the marrow, to the dense cortical bone), and the center or level would be a value in the hundreds of Hounsfield units. To an untrained person, these window controls would correspond to the more familiar "Brightness" (Window Level) and "Contrast" (Window Width).
Although CT is a relatively accurate test, it is liable to produce artifacts, such as the following.
These appear as dark lines which radiate away from sharp corners. It occurs because it is impossible for the scanner to 'sample' or take enough projections of the object, which is usually metallic. It can also occur when an insufficient X-ray tube current is selected, and insufficient penetration of the x-ray occurs. These artifacts are also closely tied to motion during a scan. This type of artifact commonly occurs in head images around the pituitary fossa area.
This appears as 'blurring' over sharp edges. It is due to the scanner being unable to differentiate between a small amount of high-density material (e.g. bone) and a larger amount of lower density (e.g. cartilage). The processor tries to average out the two densities or structures, and information is lost. This can be partially overcome by scanning using thinner slices.
Probably the most common mechanical artifact, the image of one or many 'rings' appears within an image. This is usually due to a detector fault.
This appears as graining on the image and is caused by a low signal to noise ratio. This occurs more commonly when a thin slice thickness is used. It can also occur when the power supplied to the X-ray tube is insufficient to penetrate the anatomy.
This is seen as blurring and/or streaking which is caused by movement of the object being imaged.
Streaking appearances can occur when the detectors intersect the reconstruction plane. This can be reduced with filters or a reduction in pitch.
This can give a 'cupped appearance'. It occurs when there is more attenuation in the center of the object than around the edge. This is easily corrected by filtration and software.
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Because contemporary CT scanners offer isotropic, or near isotropic, resolution, display of images does not need to be restricted to the conventional axial images. Instead, it is possible for a software program to build a volume by 'stacking' the individual slices one on top of the other. The program may then display the volume in an alternative manner.
Multiplanar reconstruction (MPR) is the simplest method of reconstruction. A volume is built by stacking the axial slices. The software then cuts slices through the volume in a different plane (usually orthogonal). Optionally, a special projection method, such as maximum-intensity projection (MIP) or minimum-intensity projection (mIP), can be used to build the reconstructed slices.
MPR is frequently used for examining the spine. Axial images through the spine will only show one vertebral body at a time and cannot reliably show the intervertebral discs. By reformatting the volume, it becomes much easier to visualise the position of one vertebral body in relation to the others.
Modern software allows reconstruction in non-orthogonal (oblique) planes so that the optimal plane can be chosen to display an anatomical structure. This may be particularly useful for visualising the structure of the bronchi as these do not lie orthogonal to the direction of the scan.
For vascular imaging, curved-plane reconstruction can be performed. This allows bends in a vessel to be 'straightened' so that the entire length can be visualised on one image, or a short series of images. Once a vessel has been 'straightened' in this way, quantitative measurements of length and cross sectional area can be made, so that surgery or interventional treatment can be planned.
MIP reconstructions enhance areas of high radiodensity, and so are useful for angiographic studies. mIP reconstructions tend to enhance air spaces so are useful for assessing lung structure.
Where different structures have similar radiodensity, it can become impossible to separate them simply by adjusting volume rendering parameters. The solution is called segmentation, a manual or automatic procedure that can remove the unwanted structures from the image.
Some slices of a cranial CT scan are shown below. The bones are whiter than the surrounding area. (Whiter means higher attenuation.) Note the blood vessels (arrowed) showing brightly due to the injection of an iodine-based contrast agent.
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A volume rendering of this volume clearly shows the high density bones.
After using a segmentation tool to remove the bone, the previously concealed vessels can now be demonstrated.
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