Conventional and Conventional and Computed TomographyComputed Tomography

IntroductionIntroduction

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

PrinciplePrinciple

The principle is based on synchronous The principle is based on synchronous movement of 2 or 3 elements in a movement of 2 or 3 elements in a tomographic system tomographic system

Tomographic units synchronize the Tomographic units synchronize the movements of the x-ray tube and the movements of the x-ray tube and the image receptor in opposite directions image receptor in opposite directions around stationary fulcrum (pivot point) around stationary fulcrum (pivot point) during the exposureduring the exposure

The fulcrum area is sharpThe fulcrum area is sharp

PrinciplePrinciple

The longer the blurring the less The longer the blurring the less opportunity to create a sharp imageopportunity to create a sharp image

The greater distance to the fulcrum the The greater distance to the fulcrum the greater the blurringgreater the blurring

The further the object is from the fulcrum The further the object is from the fulcrum the greater difference between its the greater difference between its projected motion of the image receptor projected motion of the image receptor and the motion of the image receptorand the motion of the image receptor

This cause image more sharpThis cause image more sharp

Tomographic QualityTomographic Quality

The tomographic amplitudeThe tomographic amplitude is the total is the total distance the tube travelsdistance the tube travels

The tomographic amplitude is equal or The tomographic amplitude is equal or greater to the exposure amplitudegreater to the exposure amplitude

Exposure amplitudeExposure amplitude is the total distance is the total distance the tube travels during the exposurethe tube travels during the exposure

BlurBlur is the smearing that results in the is the smearing that results in the loss of nearly all recorded detail of objects loss of nearly all recorded detail of objects outside the focal planeoutside the focal plane

Tomographic QualityTomographic Quality

Increased blurring causes decreased densityIncreased blurring causes decreased density It is effected by tomographic amplitude, It is effected by tomographic amplitude,

distance from the fulcrum, distance from the distance from the fulcrum, distance from the image receptor, and orientation of tube motionimage receptor, and orientation of tube motion

Distance from the fulcrumDistance from the fulcrum has a direct has a direct relationship to blur widthrelationship to blur width

Distance from the image receptorDistance from the image receptor has a has a direct relationship to blur widthdirect relationship to blur width

Tomographic QualityTomographic Quality

Orientation of tube motionOrientation of tube motion has a direct has a direct relationship to blur widthrelationship to blur width

FulcrumFulcrum controls the section level, and it controls the section level, and it may be fixed and patient can move up and may be fixed and patient can move up and down to change section leveldown to change section level

Focal planeFocal plane is the section is the section Section thicknessSection thickness is the width of the focal is the width of the focal

and its control by exposure angleand its control by exposure angle

Tomographic QualityTomographic Quality

Exposure angle inversely proportional to Exposure angle inversely proportional to section thicknesssection thickness

The tomographic images exhibit less The tomographic images exhibit less contrast than static imagecontrast than static image

Section Interval Section Interval is the distance between is the distance between fulcrum levelsfulcrum levels

Types of MotionTypes of Motion

LinearLinear it moving along a straight lineit moving along a straight line The major quality problem is SID and OIDThe major quality problem is SID and OID CurvilinearCurvilinear reduce magnification and reduce magnification and

maintain SID and OIDmaintain SID and OID CircularCircular EllipticalElliptical Figure eightFigure eight TrispiralTrispiral HypocyclodialHypocyclodial

Exposure FactorsExposure Factors

Time it must match the time of the x-ray Time it must match the time of the x-ray tube to complete tube to complete

Complex tomographic motion often require Complex tomographic motion often require 3-6 seconds3-6 seconds

mA usually used below 100 to set long timemA usually used below 100 to set long time 30% more mAs is needed for wider angle 30% more mAs is needed for wider angle

tomographytomography Zonography tomographs usually require Zonography tomographs usually require

the same mAs as static radiographthe same mAs as static radiograph

Exposure FactorsExposure Factors

kV 15% rule is a critical tool to determine kV 15% rule is a critical tool to determine the adjustmentthe adjustment

5% change in kVp is required to produce 5% change in kVp is required to produce visible densityvisible density

Zonography is a narrow angle tomography Zonography is a narrow angle tomography exposure amplitudes less than 10exposure amplitudes less than 10oo

Computed TomographyComputed Tomography

Scanner GenerationsScanner Generations

1- First generation1- First generation2- Second generation2- Second generation3- Third generation3- Third generation4- Fourth generation4- Fourth generation

GantryGantry

Is the movable frame of the CT unitIs the movable frame of the CT unit It contains the x-ray tube and the detectorsIt contains the x-ray tube and the detectors It maintain the alignment of the tube and the It maintain the alignment of the tube and the

detectors and contains the equipment detectors and contains the equipment necessary to perform the scanning movementnecessary to perform the scanning movement

The aperture for the patient is 50-58 cmThe aperture for the patient is 50-58 cm Do not force the obese patient into the Do not force the obese patient into the

aperture aperture

GantryGantry

Most gantrys can be angled up to 30Most gantrys can be angled up to 30oo

Table angulation can be sometimes be Table angulation can be sometimes be used in place of gantry angulationused in place of gantry angulation

Positioning lights are usually mounted on Positioning lights are usually mounted on the gantry intense white halogen lights and the gantry intense white halogen lights and low power laser lights low power laser lights

There are often 3 positioning lights for There are often 3 positioning lights for accurate sagittal, coronal, and transverse accurate sagittal, coronal, and transverse centering centering

X-ray TubesX-ray Tubes

CT images produce massive amounts of heat CT images produce massive amounts of heat in the x-ray tubein the x-ray tube

Focal spots size use 0.6 mm and 1.2 mmFocal spots size use 0.6 mm and 1.2 mm 0.6 mm use a pulsed beam to reduce the heat 0.6 mm use a pulsed beam to reduce the heat

loadload Modern CT pulsed scanner tubes operate at Modern CT pulsed scanner tubes operate at

120 kVp, 1-5 msec pulses120 kVp, 1-5 msec pulses X-ray tube produce 0.5-5 million heat unitX-ray tube produce 0.5-5 million heat unit Liquid and air-cooled tube housing design Liquid and air-cooled tube housing design

have been developed have been developed

X-ray TubesX-ray Tubes The radiation beam is double collimated to The radiation beam is double collimated to

assist in eliminating scatter informationassist in eliminating scatter information Slight misalignment can create ring artifact Slight misalignment can create ring artifact

imageimage The anode is aligned with their long axis The anode is aligned with their long axis

perpendicular to the scanner plane, this to perpendicular to the scanner plane, this to prevent heel effectprevent heel effect

Collimation controls voxel lengthCollimation controls voxel length Can be vary between 1mm to 13 mm usually Can be vary between 1mm to 13 mm usually

control by the software programcontrol by the software program The dimension width determine by section The dimension width determine by section

thickness or voxel lengththickness or voxel length

DetectorsDetectors

CT detectors should have:CT detectors should have: (a) high capture efficiency i.e. how well the (a) high capture efficiency i.e. how well the

detectors receive the photon from the detectors receive the photon from the patient and that can be control by patient and that can be control by detectors size and the distance between detectors size and the distance between detectorsdetectors

(b) high absorption efficiency i.e. how well (b) high absorption efficiency i.e. how well the detectors convert incoming photons, it the detectors convert incoming photons, it determine by the material used as well the determine by the material used as well the size and thickness of the detectorsize and thickness of the detector

DetectorsDetectors

(c) high conversion efficiency i.e. how well (c) high conversion efficiency i.e. how well the detector convert the absorb photon to the detector convert the absorb photon to analog or digital signalanalog or digital signal

CT detectors should have also high stability, CT detectors should have also high stability, fast response time, and wide dynamic range fast response time, and wide dynamic range which is the ratio between the largest signal which is the ratio between the largest signal to the smallest that can be measuredto the smallest that can be measured

Typical modern scanners are capable of Typical modern scanners are capable of dynamic range of 1,000,000:1 dynamic range of 1,000,000:1

ComputerComputer

Is design to control the data acquisition, Is design to control the data acquisition, process and display, and storageprocess and display, and storage

The computer should be in enclosed room The computer should be in enclosed room with controlled temperature and humiditywith controlled temperature and humidity

CT console provide the radiographer CT console provide the radiographer access to the software program that data access to the software program that data acquisition, controls data processing and acquisition, controls data processing and display and storage functions display and storage functions

ComputerComputer

A system program is used to start up the A system program is used to start up the CT unit, this program turns on and perform CT unit, this program turns on and perform quality assurance, and record various quality assurance, and record various problemsproblems

The CT console operate from the menu The CT console operate from the menu simply uses a keyboard, light pensimply uses a keyboard, light pen

Data AcquisitionData Acquisition

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

Display ConsoleDisplay Console

Controls the digital image production Controls the digital image production process, that compile the image and process, that compile the image and display parameters, such as window width display parameters, such as window width and leveland level

Exposure FactorExposure Factor

Most CT performed at 120 kVpMost CT performed at 120 kVp Time is not a factor as it must be Time is not a factor as it must be

controlled by scanning programcontrolled by scanning program mA should be setupmA should be setup Dual energy scanning units required Dual energy scanning units required

usually 80 kVp and 140 kVpusually 80 kVp and 140 kVp

ArtifactsArtifacts

MotionMotion Metal or StarMetal or Star Beam HardeningBeam Hardening Partial Volume EffectPartial Volume Effect Ring artifactsRing artifacts