CT Scan | Summary

Purpose:
CT scan(Computed Tomography) is a diagnostic machine that provides us a detailed cross sectional view of all type of tissues; the doctor can see slices of the organ or even three dimensional views. Usually it is used to diagnosing of cancer; we can see the precise location and size of the tumor. It is also used for helping to plan radiation treatments or to guide biopsies or other invasive procedures.

Description of instrument:
The most used CT is the one known us the third generation.
Fan shaped x-ray beam is made to pass through body, it rotate by the gantry.
After its passes the body it detected by ionic gas chamber like xenon or solid state like cesium iodide, the detectors are collimated. Multiple projections are taken to reconstruct a single image.
Each unit of tissue called voxel is represented on a computer display matrix called pixel. Each pixel is assigned a CT number. And then an image is accepted.The gantry uses a strong engine 60kw.
X-ray tube- require high anode hear capacity, that why it is rotating.
The beam is collimated as it exits the tube and again before it strikes the detector.
Collimation defines the beam thickness and reduces scattering.

Scientific principle:
CT number: Number used to represent the mean x-ray attenuation associated with each vocal. Numbers are normally expressed in terms of Hounsfield units (HU). Measured values of attenuation are transformed into CT numbers using the international Hounsfield scale:

CT Number= (µ(Material)-µ(Water))/(µ(water)) *1000

Where µ is the effective linear attenuation coefficient for the x-ray beam.The CT number scale is defined so that water has a value of 0 HU and air a value of -1000 HU.
Image quality: it is characterized in terms of contrast, noise, and spatial resolution.
Usually the quality of the image is a tradeoff between these 3 factors and radiation dose.
Contrast: is the difference it the HU values between tissues. Its increases as kVp decrease and not affected by mA or scan time.
It can be increased artificially by adding a contrast medium such as iodine.
Noise: is determined by the number of photons used to make the image, because the STDV is the root of the number of the photons.
It is increased us increasing the kPv, mA, scan time, or voxel size by decreasing matrix size.
Resolution: is the ability to discriminate between adjacent cells and is a function of pixel size.


Operation:
The patient lies down on a platform that moves slowly through the hall, while the x-ray tube is rotating. Each round scans a narrow slice. In this spiral motion the computer varies the intensity of the beam to the optimum of the tissue that is examined.
Later, the computer processes all the information to give us an image of the organ.


Safety features:

The exposure is about 10mSv.
The dose is increased with number of slices, slice thickness, scan time, kVp, mA.
The patient is protected by shielding in the areas that is not examined to decrease the exposure.
Pregnant women most inform the doctor.
Nursing mothers should wait for 24 hours after contrast injection before resuming breast feeding.
Some people get allergic reaction to iodine that is used sometimes as a contrast material. But it is rare.



Internet sites:
http://www.vrundavan.com/ctscan.htm
http://www.medicinenet.com/CAT_Scan/article.htm
http://www.radiologyinfo.org/content/ct_of_the_body.htm
http://www.drs.dk/guidelines/ct/quality/Page043.htm
http://science.howstuffworks.com/cat-scan2.htm