COMPUTED TOMOGRAPHY (CT SCAN)

Computed tomography (CT) Scan also called computerized axial tomography (CAT) scan or body section röntgenography is the process of the creating a cross-sectional tomography plane (slice) of any part of the body. The word "tomography" is derived from the Greek tomos (slice) and graphein (image). A patient is scanned by an x-ray tube rotating about the body part being examined. A detector assembly detects the radiation The image which is reconstructed by a computer using x-ray absorption measurement collected al multiple points about the periphery of the part being scanned.
Today, CT is a well-accepted imaging modality for many body applications, since CT imaging often provide a great deal of unique diagnostic information. CT is used for a wide variety of neurologic and somatic procedures. CT provides diagnostic information that cannot be achieved with any other method. The most common procedures involve the head (e.g., brain, skull, sinuses, facial bones, orbits, IACs and sella tursica),chest, abdomen and pelvic (e.g., liver, gallbladder, pancreas, spleen,kidney,adrenal glands, intestines, reproductive organs). Computed tomography is used to detect abnormalities such as blood clots, cysts, fractures, infections, and tumors in internal structures (e.g., bones, muscles, organs, soft tissue). It also can be used to detect abnormalities in the neck and spine (e.g., vertebrae, intervertebral discs, spinal cord) and in nerves, blood vessels upper and lower extremities.


The procedure also may be used to guide the placement of instruments within the body (e.g., to perform a biopsy) and drainage of fluid collections offer an alternative to surgery for some patients. Although the procedures are considered invasive, they offer shorter recovery periods, no exposure to anesthesia, and less risk of infection. CT is also used in radiaio oncology for radiation therapy treatment planning. CT Scan taken through the treatment field, with the patient in treatment position, have drastically improved the accuracy and quality of therapy provided.

The amount of radiation used in a CT scan is low, and the procedure is considered to be safe. However, CT scans should be used with caution in women who are pregnant, especially during the first trimester. Other diagnostic tests (e.g., ultrasound) may be used during pregnancy.

Comparison with conventional radiography

Reviewing conventional radiography helps explain the uniquensess of CT diagnostic information. When a conventional x-ray exposure is made, the transmitted radiation passes through the patien and is detected by x-ray film or an image-intensifer phosphor. First, for each exposure to radiation, one diagnostic image with a fixed density and contrast is produced. Second, all body structures are superimposed on one sheet of x-ray film. Thus, the highlighting of certain anatomy requires exact positioning of the patient. Often the use of contrast agents, and frequently more than one exposure.

Low tissue density that would normally be abscured by higher-density anatomy on a conventional radiograph can be clearly visualized with CT. for this reason CT is valuable in neurologic work in which the brain is surrounded by the skull. Like wise, in many body examinations. Low tissue density that would otherwise be hidden or blend with surrounding anatomy can be clearly visualized.

Although it seems obvious, it should also be noted that the CT image displays the entire cross section of the slice of anatomy that was scanned. Thus the size and location of any pathologic condition can be determined with extreme accuracy within a given CT slice. With conventional radiography, multiple exposures and contrast media are often required to estimate the size and location of the diseased area.

Contrast of Image : CT measures and can reveal significantly more minute differences in x-ray attenuation than can be recorded by conventional radiography. For example, conventional radiography requires a minimum difference in tissue of a 2% to 5% to radiographyically separate the structures. CT can resolve differences in tissue density as low as 0.5%. in Figure below the gray and the white matter in the brain can be distinguished easily.

Image manipulation : In conventional radiography, only a single radiography with a fixed contrast and density is obtained for each patient exposure to radiation. Once the film has been processed, the patient must be exposed to radiation again to produce another image. The CT image, on the other hand, is the result of complex mathematical calculation that the computer performs to reconstruct an image which is stored in the computer’s memory. The CT image is displayed on the monitor and can be altered in many ways.

HISTORICAL DEVELOPMENT

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.

The first successful clinical demonstration of CT was conducted in 1970 by Godfrey Newbold Hounsfield from the Central Research Laboratory of EMI, Ltd and Dr.James Ambrose, a physician at Atkinson Morley’s Hospital in London , England are generally given credit for development of CT. In 1971 the first full-scale unit for head scanning was installed at Atkinson Morley’s Hospital, Wimbledon, England. Its value for providing neurologic information enabled it to again rapid acceptance.

The first CT units in the United States were installed in 1973 at the Mayo Clinic and Massachusets General Hospital. In 1974, Dr. Robert Ledley at Georgetown University Medical Center developed the first scanner capable of visualizing any section of the body (whole body scanner) which greatly expanded the diagnostic capabilities of CT.

TECHNICAL ASPECTS

To obtain one axial image, a series of steps is performed by the computer. The tube rotates about the patient, radiating the area of interest. The detector measure the remnant radiation, translate it into an attenuation coefficient, and relay it to the computer. When the computer receives the data from detector, it creates a CT number based on the average intensity of the remnant radiation

CT numbers are also termed Hounsfield units (HU). CT numbers or Hounsfield units ( HU in honor of the inventor Godfrey Newbold Hounsfield) are defined as relative comparison of x-ray attenuation of each voxel of tissue with an equal volume of water. CT numbers or HU varies proportionately with tissue density ( high CT number indicate dense tissue, low CT number indicate less dense tissue).

In general, they are related to the attenuation coefficient of water (µw) as follow :
HU = (µ- µw) x 1000 x 1/ µw
Table 1. Sample CT numbers for various tissues.
Tissue CT number (HU)

Metal +2000 to +4000
Bone +1000
Liver +40 to +60
Aorta +35 to +50
White matter ~+20 to +30 HU
Grey matter ~+37 to +45 HU
Tumor +25 to +100
Blood ( Fluid) +25 to +50
Blood (clotted) +50 to +75
Blood (old) +10 to +15
Muscle +10 to +40
Kidney +30
Cerebrospinal fluid +15
Gall Bladder +5 to +30
Cyst -5 to +10
Water 0
Orbits -25
Fat -50 to -100
Air -1000

In accordance with this system, lesions whose attenuation values are close to that of water are consistent with, but not specific for, cysts. Lesions composed solely or predominantly of fat produce negative CT numbers; however, some types of liposarcoma contain great amounts of fat, and some forms of lipoma reveal abundant nonfatty tissue. haematomas characteristically demonstrate inhomogeneous areas with regions of both high attenuation (approximately 50 HU) and low attenuation (approximately 10 HU) in the subacute stage and homogeneous areas of low attenuation (120 HU) in the chronic stage. The measurement of attenuation values of bone lesions may be more difficult, especially in narrow bones in which the contribution of the cortex may prohibit accurate assessment.

The identification of gas in soft tissue or bone by CT is possible owing to its very low attenuation value. Gas within a vertebral body documented by CT, for example, is an important sign of ischaemic necrosis of bone. Intraosseous gas is also identified in some cases of osteomyelitis and in subchondral cysts (pneumatocysts), particularly in the ilium and vertebral body.

SCANNER COMPONENTS

The major components of a CT scanner are the computer and operator console, the gantry, and the table. Scanner will have slight variations in design and appearance according to manufactures.


The gantry houses the x-ray tube, data acquisition system (DAS; part of the detector assembly that converts analog signals to digital signals ttaht can be used by the CT computer), and detector for radiation production and detection. Every gantry has an opening, or aperture, to accommodate most patients. The Gantry can be tilted in either direction.

The table is an automated device linked to the computer and gantry. CT tables are made of either wood or low-density carbon composite, both of which will support the patients without causing image artifacts

The operator console is the point from which the operator controls the scanner. In this area operator (radiographer) can adjust the examination protocols, adjust the image by changing the width or center (level) of the window.

The Procedures of CT Scan Examination

Before undergoing a CT scan, patients must remove all metallic materials (e.g., jewelry, clothing with snaps, zippers) and may be required to change into a hospital gown that will not interfere with the x-ray images. Patients lie on a movable table, which is slipped into a doughnut-shaped computed tomography scanner.
To provide clear images, patients must remain as still as possible during CT scan. At certain points during a CT scan of the chest or abdomen, the radiographers may ask the patient not to breathe for a few seconds. CT scans can be performed on an outpatient basis, unless they are part of a patient's inpatient care. Although each facility may have specific protocols in place, generally, CT scans follow this process:
1. When the patient arrives for the CT scan, he/she will be asked to remove any clothing, jewelry, or other objects that may interfere with the scan.
2. If the patient will be having a procedure done with contrast, an intravenous (IV) line will be started in the hand or arm for injection of the contrast medication. For oral contrast, the patient will be given medication to swallow.
3. The patient will lie on a scan table that slides into the gantry
4. As the scanner begins to rotate around the patient, x-rays will pass through the body for short amounts of time.
5. A detector assembly detect the x-rays exiting the patient and feeds back the information, referred to as raw data to the host computer.
6. The computer will transform the information into an image to be interpreted by the radiologist.

CONTRAST AGENT

A contrast agent (e.g., iodine-based dye, barium solution) may be administered prior to CT scan to allow organs and structures to be seen more easily. Contrast agents can be administered through a vein (IV), by injection, or taken orally. Patients usually are instructed not to eat or drink for a few hours prior to contrast injection or IV because the dye may cause stomach upset. Patients may be required to drink an oral contrast solution 1–2 hours before CT scan of the abdomen or pelvis.
Contrast dye may cause a rash, itching, or a feeling of warmth throughout the body. Usually, these side effects are brief and resolve without treatment. Antihistamines may be administered to help relieve symptoms.
A severe anaphylactic reaction (e.g., hives, difficulty breathing) to the contrast dye may occur. This reaction, which is rare, is life threatening and requires immediate treatment. Patients with a prior allergic reaction to contrast dye or medication and patients who have asthma, emphysema, or heart disease are at increased risk for anaphylactic reaction. Epinephrine, corticosteroids, and antihistamines are used to treat this condition. Rarely, contrast dye may cause kidney failure. Patients with diabetes, impaired kidney function, and patients who are dehydrated are at higher risk for kidney failure.
Advances in computed tomography technology
Advances in computed tomography technology include the following:
• high-resolution computed tomography
This type of CT scan uses very thin slices (less than one-tenth of an inch), which are effective in providing greater detail in certain conditions such as lung disease.
• helical or spiral computed tomography
During this type of CT scan, both the patient and the x-ray beam move continuously, with the x-ray beam circling the patient. The images are obtained much more quickly than with standard CT scans. The resulting images have greater resolution and contrast, thus providing more detailed information.
• ultrafast computed tomography (also called electron beam computed tomography)
This type of CT scan produces images very rapidly, thus creating a type of "movie" of moving parts of the body, such as the chambers and valves of the heart. This scan may be used to obtain information about calcium build-up inside the coronary arteries of the heart.
• Multidetector computed tomography: Multidetector computed tomography(MDCT) is also known by a confusing array of other terms such as multidetector CT, multidetector-row computed tomography, multidetector-row CT, multisection CT, multislice computed tomography, and multislice CT MSCT).
In MDCT or MSCT, a two-dimensional array of detector elements replaces the linear array of detector elements used in typical conventional and helical CT scanners. The two-dimensional detector array permits CT scanners to acquire multiple slices or sections simultaneously and greatly increase the speed of CT image acquisition. Image reconstruction in MDCT or MSCT is more complicated than that in single section CT. Nonetheless, the development of MDCT has resulted in the development of high resolution CT applications such as CT angiography and CT colonoscopy. .
• Combined computed tomography and positron emission tomography (PET/CT)
The combination of computed tomography and positron emission tomography technologies into a single machine is referred to as PET/CT. PET/CT combines the ability of CT to provide detailed anatomy with the ability of PET to show cell function and metabolism to offer greater accuracy in the diagnosis and treatment of certain types of diseases, particularly cancer. PET/CT may also be used to evaluate epilepsy, Alzheimer's disease, and coronary artery disease.

Patient Radiation Doses

The various factor affecting patient dose are;patient thickness, generator and tube factors (kilovoltage, filtration, tube current, scan on time and focal-spot size), gantry factors (beam collimation, slice width and overlap, scan orientation, and detector efficiency), and image quality desired.
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.
1. New software technology can significantly reduce the radiation dose. The software works as a filter that reduces random noise and enhances structures. In this way, it is possible to get high-quality images and at the same time lower the dose by as much as 30 to 70 percent.
2. Individualize the examination and adjust the radiation dose to the body type and body organ examined. Different body types and organs require different amounts of radiation.
3. Prior to every CT examination, evaluate the appropriateness of the exam whether it is motivated or if another type of examination is more suitable.