A CT scanner, the machine used to proceed with computed tomography, is composed of a rotating frame which has an x-ray tube on one side and a detector on the opposite side. As the rotating frame spins around the patient, a fan beam of x-ray continuously passes through the patient. At each rotation of the frame, the data stream representing the varying radiographic intensity sensed reaching the detectors is computer processed to calculate cross-sectional estimations of the radiographic density. A process called windowing calculates the radiographic density, expressed in Hounsfield units, to make very detailed images in 256 shades of gray. The shades vary in function of the density, becoming more white when more dense.

Often, to accentuate the difference of intensity on the pictures, contrast material such as intravenous iodinated contrast or a dilute suspension of barium sulfate will be administered to the patient. It helps getting clearer images and it highlights structures difficult to see otherwise, like blood vessels.  It also helps to obtain functional information about tissues.

Three dimensional reconstruction

Another method called volume rendering is also commonly used to create three dimensional representations of the body parts. The image below shows several cross-sectional slices of head.

Using the same cross-sectional slices of head, it is possible to create a 3D representation of the internal structure of the head by a procedure called segmentation. It removes all the unwanted structures from the image and enables us to model, for example, the brain vessels.

Newer machines with more advanced computer systems and software strategies can process what is called helical or spiral CT. Instead of creating individual cross sections, the CT scanner processes continuously changing cross sections as the gantry with the patient is slowly slid through the X-ray circle. This procedure is very useful because it makes three dimensional structures easier to create. Also, it may detect small abnormal areas better than do conventional CT. Moreover, it is faster, so the test lasts less long than with a conventional CT.

History

The first CT system was invented by British engineer Godfrey Newbold Hounsfield and by South Africa-born physicist Allan Cormack in 1972. The original prototype took several hours to acquire the data for a single semi-rotation, which took 160 parallel readings through 180 angles of 1° apart. The images were low resolution: about 80 x 80 pixels. Since that time, the speed and the resolution have considerably increased: today’s CT scanner reconstructs high resolution images from millions of data points in less than a second.

Uses

CT is used in the diagnosis of a large number of different disease entities and to determinate stage of cancer and to follow progress. But it is not yet well-performing in detection of tumors, coming behind magnetic resonance imaging.

 Cranial CT

Cranial CT are most frequently operated for the diagnosis of cerebrovascular accidents and intracranial hemorrhage, but it is also useful to detect intracranial pressure and to evaluate facial and skull fractures. It is helpful for planning craniofacial and dentofacial deformities and to diagnose the causes of chronic sinusitis.

 Chest CT

Extremities

 “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.”

Advantages

-A diagnosis effectuated with CT may eliminate the need for exploratory surgery.

-CT is completely painless.

-In comparison to conventional x-rays, CT scanning provides very detailed pictures of a large quantity of organs and tissues.

-CT can help planning radiotherapy

-The difference between tissues that differ in physical density by less than 1% can be distinguished because of the high contrast resolution.

-CT can be used for all anatomical regions, including those susceptible of patient motion or breathing because of the very short scan times varying from 500 milliseconds to a few seconds.

To obtain higher-resolution imaging and because of the complex scan techniques, the level of radiation varies from moderate to high. For example, in the most recent survey in the UK, CT scans constituted 7% of all radiologic examinations, but contributed to 47% of the total dose of x-rays emitted in medical examinations.

     In very rare cases, contrast agents may cause allergic symptoms such as mild itching or hives. Still in very rare cases, it can cause severe allergic reactions which include shortness of breath and swelling of the throat or other parts of the body. They can also induce kidney damage, principally to patients who have preexisting renal insufficiency, diabetes or reduced intravascular volume. For people who have a normal kidney function, the risks are negligible

Computed tomography is very useful to detect, diagnose or evaluate physical troubles. Even if there are some risks related to the use of moderate to high x-rays doses, the considerable benefits of CT make it a method of medical imaging frequently used throughout the world

Sources

http://en.wikipedia.org/wiki/Computed_axial_tomography

http://www.chl.lu/html/glossaire_biomedical/computed_tomography.html

http://www.imaginis.com/ct-scan/

http://www.cancer.gov/cancertopics/factsheet/Detection/CT