Thursday, 19 March 2015

COMPONENTS OF A COMPUTED TOMOGRAPHY SCANNER

COMPONENTS OF A COMPUTER TOMOGRAPHY SCANNER




In continuation of our discussion on how a CT scanner works, we will look at the main components of a CT scanner which are
A.      The gantry
B.      The table top
C.      The computed processing unit (CPU)
D.      The display monitors
THE GANTRY
The gantry is the most conspicuous part of the CT machine. It is oval in shape and has a central wide opening through which the patient on the table can pass in and out. The gantry is also able to tilt in 2 directions (either up or down).
The gantry houses the following
·         The X-ray tube(s)
·         The collimator system
·         Array of detectors
·         The data acquisition system.



X-RAY TUDE
 The X-ray tube is the where the X-ray beam is generated. We need not go into the technicalities of the X-ray tube and how the X-rays are generated. But note that because of the high kVp requirement of a CT scan, the X-ray tube is usually a rotating anode tube and is modified for high capacity generation. It also has an elaborate cooling mechanism






THE COLLIMATOR SYSTEM
The collimator system is used to shape the X-ray beam into a narrow fan beam. The collimator system is simply made up of thick lead metals that have an opening that is adjustable. The X-ray is made to pass through the opening. The opening is adjusted to determine the size of the beam; so this means that the narrowness of the collimator determines the narrowness of the X-ray beam. Please note that slice thickness is determined by adjusting the collimators. So a narrower fan beam means a smaller slice thickness. *when we get to the concept of scanning protocols, we will explain this better.


DETECTOR ARRAY
A detector array is an arrangement of radiosensitive materials that are used to record the amount of x-ray coming out from the patient. We earlier noted that the amount of x-ray passing through the patient is absorbed differently by the different tissues. So what this detector array does is to capture these different energy x-ray photons.
A detector can either be a
1.       solid state scintillators (e.g. Sodium iodide)
2.       Xenon gas detectors
Solid state scintillators are able to give out visible light photons when they interact with X-ray photons. These light photons are amplified by photomultipliers and incorporated to a photodiode in the data acquisition system. For xenon gas detectors, the x-ray photons cause and ionization in the chambers proportional to their energy. This ionization leads to electrical charges that are coupled into the data acquisition system. *We shall time to discuss the detector system separately and also talk about the different detector arrangements (single row and multiple row detectors) and how it affects scanning protocols.




DATA ACQUSITION SYSTEM
This is an elaborate electrical and electronic collection. Its sole purpose is to convert the X-ray photon information gathered by the detectors into an electric current that can be easily processed. It has photodiode system that converts the photo-energy or ionization charges from the solid state scintillators or xenon gas detectors respectively. This electric current is now fed into the computer processing unit for integration and mathematical processing and reconstruction.


THE TABLE TOP:
The table top has a radiolucent top that the patient is placed. It has a mechanism for moving the patient in and out through the gantry.
*we will discuss the concept of pitch in relations to table movement subsequently.




COMPUTED PROCESSING UNIT AND DISPLAY MONITOR
The computed processing unit is a where the image reconstruction takes place. The data collected from the Data acquisition system is fed into the CPU and the reconstruction phase (calculating the differential attenuation coefficients) of making a CT image happens here.
The display monitor is where the final CT image is displayed. The differential attenuation coefficients are assigned different CT numbers and the different CT numbers are displayed on the screen as different shades of grey. This is the final image that we appreciate as a CT image.




in our next discussion, we will talk about the different generations of CT scanners and the different adjustments and advancement in CT technology.
i am hoping that after, we will talk about protocol managements in CT and then move into the CT scanning procedures.  

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