conventional and computed tomography l.
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Conventional and Computed Tomography. 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. Principle.

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Conventional and Computed Tomography

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  • 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
  • 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
  • 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
tomographic quality
Tomographic Quality
  • The tomographic amplitude is the total distance the tube travels
  • The tomographic amplitude is equal or greater to the exposure amplitude
  • Exposure amplitudeis the total distance the tube travels during the exposure
  • Bluris the smearing that results in the loss of nearly all recorded detail of objects outside the focal plane
tomographic quality7
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 receptorhas a direct relationship to blur width
tomographic quality8
Tomographic Quality
  • Orientation of tube motionhas a direct relationship to blur width
  • Fulcrumcontrols the section level, and it may be fixed and patient can move up and down to change section level
  • Focal planeis the section
  • Section thicknessis the width of the focal and its control by exposure angle
tomographic quality10
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
types of motion
Types of Motion
  • Linearit moving along a straight line
  • The major quality problem is SID and OID
  • Curvilinearreduce magnification and maintain SID and OID
  • Circular
  • Elliptical
  • Figure eight
  • Trispiral
  • Hypocyclodial
exposure factors
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
exposure factors14
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
scanner generations
Scanner Generations
  • 1- First generation2- Second generation3- Third generation4- Fourth generation
  • 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 50-58 cm
  • Do not force the obese patient into the aperture
  • 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
x ray tubes
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 0.5-5 million heat unit
  • Liquid and air-cooled tube housing design have been developed
x ray tubes21
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
  • 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
  • (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
  • 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
  • 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
data acquisition
Data Acquisition
  • Controls the tube and detector collimation (pixel size), matrix size, gantry angle, table top entrance, section increment movement, kVp, mA, scan speed
display console
Display Console
  • Controls the digital image production process, that compile the image and display parameters, such as window width and level
exposure factor
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
  • Motion
  • Metal or Star
  • Beam Hardening
  • Partial Volume Effect
  • Ring artifacts