1. Conventional and Computed Tomography

2. Introduction
Is a radiographic technique that employs motion to show anatomical structures lying in the a plane tissue while blurring or eliminating the detail in images of structures above and below the plane of the interest

4. Principle
The principle is based on synchronous movement of 2 or 3 elements in a tomographic system
Tomographic units synchronize the movements of the x-ray tube and the image receptor in opposite directions around stationary fulcrum (pivot point) during the exposure
The fulcrum area is sharp

5. Principle
The longer the blurring the less opportunity to create a sharp image
The greater distance to the fulcrum the greater the blurring
The further the object is from the fulcrum the greater difference between its projected motion of the image receptor and the motion of the image receptor
This cause image more sharp

6. Tomographic Quality
The tomographic amplitude is the total distance the tube travels
The tomographic amplitude is equal or greater to the exposure amplitude
Exposure amplitude is the total distance the tube travels during the exposure
Blur is the smearing that results in the loss of nearly all recorded detail of objects outside the focal plane

7. Tomographic Quality Increased blurring causes decreased density
It is effected by tomographic amplitude, distance from the fulcrum, distance from the image receptor, and orientation of tube motion
Distance from the fulcrum has a direct relationship to blur width
Distance from the image receptor has a direct relationship to blur width

8. Tomographic Quality
Orientation of tube motion has a direct relationship to blur width
Fulcrum controls the section level, and it may be fixed and patient can move up and down to change section level
Focal plane is the section
Section thickness is the width of the focal and its control by exposure angle

10. Tomographic Quality
Exposure angle inversely proportional to section thickness
The tomographic images exhibit less contrast than static image
Section Interval is the distance between fulcrum levels

11. Types of Motion Linear it moving along a straight line
The major quality problem is SID and OID
Curvilinear reduce magnification and maintain SID and OID
Circular
Elliptical
Figure eight
Trispiral
Hypocyclodial

13. Exposure Factors
Time it must match the time of the x-ray tube to complete
Complex tomographic motion often require 3-6 seconds
mA usually used below 100 to set long time
30% more mAs is needed for wider angle tomography
Zonography tomographs usually require the same mAs as static radiograph

14. Exposure Factors
kV 15% rule is a critical tool to determine the adjustment
5% change in kVp is required to produce visible density
Zonography is a narrow angle tomography exposure amplitudes less than 10o

16. Computed Tomography

17. Scanner Generations
1- First generation 2- Second generation 3- Third generation 4- Fourth generation

18. Gantry Is the movable frame of the CT unit
It contains the x-ray tube and the detectors
It maintain the alignment of the tube and the detectors and contains the equipment necessary to perform the scanning movement
The aperture for the patient is cm
Do not force the obese patient into the aperture

19. Gantry Most gantrys can be angled up to 30o
Table angulation can be sometimes be used in place of gantry angulation
Positioning lights are usually mounted on the gantry intense white halogen lights and low power laser lights
There are often 3 positioning lights for accurate sagittal, coronal, and transverse centering

20. X-ray Tubes
CT images produce massive amounts of heat in the x-ray tube
Focal spots size use 0.6 mm and 1.2 mm
0.6 mm use a pulsed beam to reduce the heat load
Modern CT pulsed scanner tubes operate at 120 kVp, 1-5 msec pulses
X-ray tube produce million heat unit
Liquid and air-cooled tube housing design have been developed

21. X-ray Tubes
The radiation beam is double collimated to assist in eliminating scatter information
Slight misalignment can create ring artifact image
The anode is aligned with their long axis perpendicular to the scanner plane, this to prevent heel effect
Collimation controls voxel length
Can be vary between 1mm to 13 mm usually control by the software program
The dimension width determine by section thickness or voxel length

22. Detectors CT detectors should have:
(a) high capture efficiency i.e. how well the detectors receive the photon from the patient and that can be control by detectors size and the distance between detectors
(b) high absorption efficiency i.e. how well the detectors convert incoming photons, it determine by the material used as well the size and thickness of the detector

23. Detectors
(c) high conversion efficiency i.e. how well the detector convert the absorb photon to analog or digital signal
CT detectors should have also high stability, fast response time, and wide dynamic range which is the ratio between the largest signal to the smallest that can be measured
Typical modern scanners are capable of dynamic range of 1,000,000:1

24. Computer
Is design to control the data acquisition, process and display, and storage
The computer should be in enclosed room with controlled temperature and humidity
CT console provide the radiographer access to the software program that data acquisition, controls data processing and display and storage functions

25. Computer
A system program is used to start up the CT unit, this program turns on and perform quality assurance, and record various problems
The CT console operate from the menu simply uses a keyboard, light pen

26. Data Acquisition
Controls the tube and detector collimation (pixel size), matrix size, gantry angle, table top entrance, section increment movement, kVp, mA, scan speed

27. Display Console
Controls the digital image production process, that compile the image and display parameters, such as window width and level

28. Exposure Factor Most CT performed at 120 kVp
Time is not a factor as it must be controlled by scanning program
mA should be setup
Dual energy scanning units required usually 80 kVp and 140 kVp

29. Artifacts Motion Metal or Star Beam Hardening Partial Volume Effect
Ring artifacts