Limitations of Radiography 3D body rendered in 2D Structures superimposed on image Structure of interest viewed through underlying overlying structures Multiple views used to adequately visualize structure ID: 911406
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Slide1
Computed Tomography Basics
Slide2Limitations of Radiography
3D body rendered in 2D
Structures superimposed on image
Structure of interest viewed through underlying / overlying structures
Multiple views used to adequately visualize structure.
Patient
X-ray
Beam
Film
Slide3Limitations of Radiography
Density (gray shade) dictated by total attenuation encountered by beam
Thin highly-attenuating objects have same density as thicker low-attenuating object.
Patient
X-ray
Beam
Image
Thin dense
object
Thick less
dense object
Slide4Radiography vs. CTBoth based on differential attenuation of x-rays passing through bodyRadiography
“Shadowgraph” using x-ray light source
CTCross-sectional imageImage computed from pencil beam intensity measurements through only slice of interest
Slide5X-Ray vs CT
View anatomy without looking through underlying / overlying structures
improves contrastTightly collimated beamminimizes scattered radiationimproves contrastVery small contrast differences visible
CT X-ray
Beam
Conventional
X-ray Beam
Slide6Conventional (not Computed) TomographyPredated
CT by decades
Application ExamplesIVPInner ear
Philips
Polytome
Slide7Body Section TomographyNot CT
Body Section Imaging
Goalkeep plane of interest in focusblur all other plansenhances contrastPopularity decreased because ofCTMRI
Slide8Conventional Tomography BlurringImage produced on filmObjects above or below fulcrum plane change position on film & thus blur
Slide9Conventional vs Axial Tomography
Conventional Cut
CT Axial Cut
Slide10Limitations of Conventional TomographyOverlying / underlying structures blurred, not removed
5-10% subject contrast difference required for objects to appear different
many anatomic systems don’t have this much subject contrast
Slide11CT AdvantagesExcellent low-contrast resolutionsensitive detectorssmall beam size produces little scatter
Much better than film
Slide12CT DisadvantagesPoorer spatial resolution than radiographyHigher doseArtifacts at abrupt transitions
bone / soft tissue interfaces
metallic objects
Slide13CT Early Units
4 minute scans
1 slice5 minute reconstruction80 X 80 matrixhead onlywater bag fit tightly around head
Slide14Data Acquisitioncross sectional image reconstructed from many line transmission measurements made in different directions
Tube
Detector
Slide15Projection MeasurementsRadiation detector generates a voltage proportional to radiation intensity
Slide16Image ReconstructionMinicomputer does its thing
Analog to Digital
(A to D) conversion
Slide17CT - Improvements
All CT generations measure same transmission intensities in many directions
ImprovementsProtocol for obtaining many line transmissions# of line transmissions obtained simultaneouslyDetector locationOverall acquisition speed
Slide181st GenerationTranslate / Rotate
Slide193rd Generation CTWide angle fan beam
Rotational motion only / no translation
detectors rotate with tube30o beamMany more detectors
Slide203rd Generation CT
Patient
Z-axis orientation perpendicular to page
Slide213rd Generation (Non-spiral) CT
Tube rotates once around patient
Table stationarydata for one slice collectedTable increments one slice thicknessRepeat
Tube rotates opposite direction
Slide22Spiral CT (late 1980’s)
Continuous rotation of gantry
Slip ring technology
Patient moves slowly but continuously through gantry
No dead time as gantry
reverses
Much faster
Slide23Spiral CT
Patient
Z-axis orientation perpendicular to page
Slide24Data AquisitionSlice by sliceOne slice at a time
Volume acquisition
data for an entire volume collectedpatient moves in axial direction during scantube traces spiral-helical path through patient
Before ~ 1990
Since ~ 1990
Slide25Spiral CT Advantagesvolume data acquisition in single breath holdno delay between slicesimproved 3D imaging
improved multi-planar image reformatting
Special applicationsbone mineral contentradiation treatment planningCT angiography
Slide26Multi-slice CT (2000’s)
Multiple rows of fan beam detectors
Wider fan beam in axial directionTable moves much fasterSubstantially greater throughput
Slide27Multi-slice CT AdvantagesScans at much greater speed OR
Allows scanning of same volume with thin slices
Makes possible additional clinical applications