introduction

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introduction

مشاركةبواسطة دكتور كمال سيد » الجمعة مايو 24, 2013 3:43 pm

X-ray computed tomography

X-ray computed tomography, also computed tomography (CT scan) or computed axial tomography (CAT scan), is a medical imaging procedure that utilizes computer-processed X-rays to produce tomographic images or 'slices' of specific areas of the body. These cross-sectional images are used for diagnostic and therapeutic purposes in various medical disciplines.
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.

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Schematic representation of CT scanner.

CT produces a volume of data that 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 beam. Although historically the images generated were in the axial or transverse plane, perpendicular 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 archaeological uses such as imaging the contents of sarcophagi. Individuals responsible for performing CT exams are called Radiologic Technologists or Radiographers and are required to be licensed in most states.

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A patient is receiving a CT scan for cancer. Outside of the scanning room is an imaging computer that reveals a 2D image of the body's interior.

Usage of CT has increased dramatically over the last two decades in many countries. An estimated 72 million scans were performed in the United States in 2007. One study estimated that as many as 0.4% of current cancers in the United States are due to CTs performed in the past and that this may increase to as high as 1.5-2% with 2007 rates of CT usage; however, this estimate is disputed. Kidney problems following intravenous contrast agents may also be a concern in some types of studies.

Diagnostic use

Since its introduction in the 1970s, CT has become an important tool in medical imaging to supplement X-rays and medical ultrasonography. It has more recently been used for preventive medicine or screening for disease, for example CT colonography for patients with a high risk of colon cancer, or full-motion heart scans for patients with high risk of heart disease.

A number of institutions offer full-body scans for the general population although this practice goes against the advice and official position of many professional organizations in the field.

CT head

CT scanning of the head is typically used to detect infarction, tumors, calcifications, hemorrhage and bone trauma. Of the above, hypodense (dark) structures can indicate infarction and edema, hyperdense (bright) structures indicate calcifications and haemorrhage and bone trauma can be seen as disjunction in bone windows.
Tumors can be detected by the swelling and anatomical distortion they cause, or by surrounding edema. Ambulances equipped with small bore multi-sliced CT scanners respond to cases involving stroke or head trauma.

Lungs

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 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. Therefore, it produces a sampling of the lung and not continuous images.

Pulmonary angiogram

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(Example of a CTPA, demonstrating a saddle embolus (dark horizontal line) occluding the pulmonary arteries (bright white triangle

CT pulmonary angiogram (CTPA) is a medical diagnostic test used to diagnose pulmonary embolism (PE). It employs computed tomography and an iodine based contrast agent to obtain an image of the pulmonary arteries.

Cardiac CT

With the advent of subsecond rotation combined with multi-slice CT (up to 320-slices), high resolution and high speed can be
(obtained at the same time, allowing excellent imaging of the coronary arteries (cardiac CT angiography

Abdominal and pelvic CT

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.

Extremities CT scan

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.

Advantages

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. The improved resolution of CT has permitted the development of new investigations, which may have advantages; compared to conventional radiography, for example, CT angiography avoids the invasive insertion of a catheter.
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 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. In addition, two helical CT scanning parameters that can be adjusted easily and that have a profound effect on radiation dose are tube current and pitch. Computed tomography (CT) scan has been shown to be more accurate than radiographs in evaluating anterior interbody fusion but may still over-read the extent of fusion.

Adverse effects

1)
Cancer

The ionizing radiation in the form of x-rays used in CT scans are energetic enough to directly or indirectly damage DNA.
This and other types of DNA damage are occasionally not corrected properly by cellular repair mechanisms.
Such damage to the DNA occasionally leads to cancer. The estimates of harm from CT are partly based on similar radiation exposures experienced by those present during the atomic bomb explosions in Japan during the second world war and those of nuclear industry works.
There is a small increased risk of cancer with CT scans. It is estimated that 0.4% of current cancers in the United States are due to CTs performed in the past and that this may increase to as high as 1.5–2% with 2007 rates of CT usage;
however, this estimate is disputed.
This would be equivalent to one in 1000 to one in 2000 increased risk of developing a fatal cancer per 10mSv CT scan, or 29,000 new cancer cases in the United States due to the number of scans done in 2007 and 2100 new cancers in the United Kingdom. This additional risk is still low compared to the background risk of dying from cancer of ~20%. The most common cancers caused by radiation exposure are thought to be lung cancer, breast cancer, thyroid cancer, stomach cancer and leukemia.

A person's age plays a significant role in the subsequent risk of cancer. Estimated lifetime cancer mortality risks from an abdominal CT of a 1-year-old is 0.1% or 1:1000 scans. The risk for someone who is 40 years old is half that of someone who is 20 years old with substantially less risk in the elderly.
The International Commission on Radiological Protection estimates that the risk to a fetus being exposed to 10 mGy (a unit of radiation exposure, see Gray (unit)) increases the rate of cancer before 20 years of age from 0.03% to 0.04% (for reference a CT pulmonary angiogram exposes a fetus to 4 mGy).
A 2012 review did not find an association between medical radiation and cancer risk in children noting however the existence of limitations in the evidences over which the review is based.

CT scans can be performed with different settings for lower exposure in children with most manufacturers of CT scans as of 2007 having this function built in. Furthermore, certain conditions can require children to be exposed to multiple CT scans. Studies support informing parents of the risks of pediatric CT scanning.

2)
Contrast

In the United States half of CT scans involve intravenously injected radiocontrast agents. The most common reactions from these agents are mild, including nausea, vomiting and an itching rash; however, more severe reactions may occur. Overall reactions occur in 1 to 3% with nonionic contrast and 4 to 12% of people with ionic contrast. Skin rashes may appear in 1 to 3% within a week.

The old radiocontrast agents caused anaphylaxis in 1% of cases while the newer, lower-osmolar agents cause reactions in 0.01–0.04% of cases.[ Death occurs in about two to 30 people per 1,000,000 administrations with newer agents being safer.
When deaths do occur it is more typically in those who are female, elderly or in poor health and is secondary to either anaphylaxis or acute renal failure.
The contrast agent may induce contrast-induced nephropathy. This occurs in 2 to 7% of people who receive these agents, with greater risk in those who have preexisting renal insufficiency, preexisting diabetes, or reduced intravascular volume. People 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. Those 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.

In addition to the use of intravenous contrast, orally administered contrast agents are frequently used when examining the abdomen. These are frequently the same as the intravenous contrast agents, merely diluted to approximately 10% of the concentration. However, oral alternatives to iodinated contrast exist, such as very dilute (0.5–1% w/v) barium sulfate suspensions.

Dilute barium sulfate has the advantage that it does not cause allergic-type reactions or kidney failure, but cannot be used in patients with suspected bowel perforation or suspected bowel injury, as leakage of barium sulfate from damaged bowel can cause fatal peritonitis.
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