BASIC PRINCIPLE OF CT

BY:BISHAL KHANAL

Principle The internal structure of an object can be reconstructed from multiple

projection. A large number of data along a thin line of 2D section is acquired to

reconstruct the structure within that slice. This involves five main stages:-1) Scanning phase and data acquisition system (DAS).2) Pre-processing of raw data.3) Image reconstruction.4) Linear attenuation coefficient of each pixel is converted into

Hounsfield unit (HU).5) Display and Recording.

Scanning phase After the patient lies in the gantry the following

steps are done such asa. Field of view is selected.b. FOV cuts into thinner and smaller slices.c. Placing a matrix grid on thin slices.d. Scanning of selected slice from multiple

projection and acquire data.e. Select next slice and repeat scanning.

Data Acquisition System

It revolves tube along with detector. It consist of detector which converts intensity

information into digital signal.

Pre-processing of data  

IMAGE RECONSTRUCTION The computer hardware and software convert the raw data of CT into CT

image through the process called image reconstruction. It is done by filter back projection. In this pre-processed data are sending into array process by the help of

filter back projection. ARRAY PROCESS:-it is small computer in the host processer which does the

mathematical calculation. Function:- The main function is the mathematical manuplation of data very quickely. It help in reconstruction of linear attuation coefficient. Design of array process:- High speed memory. High speed bus.

LAC of each pixel is converted into Hounsfield unit

 

Hounsfield Unit

TISSUES CT NUMBER RANGE IN HU Air Lungs Fat Water Muscle Soft tissue Cortical bone Trabecular bone

-1000 -900 to -300 -120 to –80 0 10 to 30 10 to 30 50 to 100 500 to 1000

Display and Recording

Digital imaging is based on followingMatrix size

Pixel bit depthIt is the ability of pixel to show different shades of grey.We can differentiate the shades of grey by window width(W) and window level(L).

Window width and Window level Window width (W) It decides the shades. It depends on contrast. Narrow window width having high

contrast wider window width having low contrast.

Decreasing the window width, increasing the contrast of the image so good for looking at differences in soft tissues.

Increasing the window width allows structures with a large pixel range to be viewed.

Window level(L) It is the mid-point of the

window width. It identifies the type of

tissues. It controls the CT

number which determine the position of window.

Recording Recording of the image produce by CT scanner is usually

done with the multi-format camera. A more recently introduced recording system uses a

laser beam to directly record the image on the system.

Multiform camera When exposed signal is send to multiform camera the

image form internal monitor is focused onto a small sectional film.

CT image are usually imaged onto a sheet of film that are 35x43 cm.

Informate ranging form 6 to 15 image.

Laser techonology Instead of exposing the film to an image displayed on an

internal television monitor, a laser camera uses a digital. Image data directly and scan the film link by line ,

bulding of the image gradually. A laser beam is scanned over the film and intensity of

lesser beam is determined by the pixel value at that point in the image data.

The narrow beam gemeratry of leaser beam create less scattered light in the system and therefore laser camera produces film image with more contrast than do multiformate camera.