basics_of_ct scan(lovish vij; eic 2 ; 101285007)

7/28/2019 Basics_of_CT Scan(Lovish Vij; Eic 2 ; 101285007)

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Basic Principles of

CT Scanning


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The basics of CT

CT imaging chain

System components

Acquisition methods Image quality

Applications


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X-ray: The beginning

X-Rays founded in 1895 by

Wilhelm Conrad Roentgen


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CT: The beginning

CT founded in 1970 by Sir Godfrey Hounsfield

Engineer with EMI, LTD.

first applications were in neuroradiology


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CT Scanner

X-Ray modality used to the body

in cross section

Used to determine extent of trauma

location and type of tumors

status of blood vessels

pre surgical planning


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CT System


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Basic CT scanner components

Gantry

X-Ray Tube

Detector Control Console


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Gantry

CT X-ray tube

High voltage generator

Detector array Data acquistion system

Slip ring


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The CT X-ray Tube

Anode heat capacity

3.5 MHU up to 28 MHU

Determines maximum mAs

Determines volume length

Dictates generator size


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Detector Elements

Capture energy that has not been

attenuated by the patient


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Control console

Set scan parameters kVp, mA, scan time, reconstruction

filter, etc.

Set scan mode Surview, Axial or Spiral

IRS (Image reconstructionSystem)

Review and archive images

Post-processing


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CT

CT - Computed Tomography

CAT Scan - Computerized Axial

Tomography


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Scanning methods

Surview AP,Lat

Surview, Scanogram , Topogram.

Conventional CT

Axial Start/stop

Volumetric CT Helical or spiral CT

Continuous acquisition


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Digital Projection

X-ray tube and detector remain stationary

Patient table moves continuously

With X-rays on

Produces an image covering a range of

anatomy Similar to a conventional X-ray image, e.g. flat

plate of the abdomen

Image used to determine scan location


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Axial CT

X-ray tube and detector rotate360

Patient table is stationary

With X-rays on

Produces one cross-sectionalimage

Once this is complete patient ismoved to next position

Process starts again at thebeginning


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Volume CT

X-ray tube and detector rotate 360

Patient table moves continuously

With X-rays on

Produces a helix of image information This is reconstructed into 30 to 1000 images


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Single Slice = One 10mm slice per rotationDual Slice = Two 5mm slices per rotation

Quad Slice = Four 2.5mm slices per rotation

Multislice Effectiveness

Everything is better

Resolution 2x 4x-8x

Speed same same

Volume same same

Power same same

Dual Quad


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. .

single detector arc dual detector arc

pre-patient collimation

post-patient collimation

x-ray tube focal spot

_ __

Mx8000

Dual Slice

Dual Slice Detector Optimized for2 Slice Acquisition


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Approximately 10%

more efficient than

matrix detectors

Variable Wide Area Detector

AsymmetrixVariable detector length Fixed detector length

Quad Detector Technology

variable wide area detector

Variable slice thickness

4 x 1mm

4 x 5mm

4 x 2.5mm

2 x 0.5mm

2 x 8mm

2 x 10mm


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8 Element

2-D array

4 Slices

Quad TechnologyHow it works


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CT

CT attenuation information

CT image quality


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Attenuation

X-ray beam passes through patient

Each structure attenuates X-ray beam

differentlyAccording to individual densities

Radiation received by detector varies

according to these densities


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Density information

Transferred from detector to CT computer

(A to D converter)

Reconstructed by computer into a cross-sectional image

Displayed on screen

Each pixel displayed on monitor has varying brightness

The greater the attenuation, the brighter the pixel

The less attenuation, the darker the pixel


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Density information

Density values

correspond to a

range of numbers

Hounsfield scale


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Window settings

Window width

Determines range of CT numbers displayed on an image

Values above this range = white

Values below this range = black

Window level

Sets the center CT number displayed on the monitor

Determines the location on the Hounsfield scale about which the

window width will be centered


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CT image quality

Spatial resolution

Ability to resolve small

objects in an image

Measured in lp/cm


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CT image quality

Spatial resolution

Ability to resolve small

objects in an image

Measured in lp/cm


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Isotropic Imaging

True 0.5mm Isotropic imaging


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CT image quality

Contrast resolution

Ability to differentiate small density differences in an image


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Post Processing Options

Visualization of

vasculature in

relation to pathology

Show course of vessels Show stent placement

Define vascular stricture


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Neuro-Angiography

Circle of Willis


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Thin-SliceSpiralLungs


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MIP4D-Angio

MasterCutPanoramic View

RenalArteries


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Ext. Spiral Acq.Spiral Acquisition

UltraFast 0.5 sec

Coverage 1400mm

Pitch 1.75

Acq. Time 41.5 secFOV 420 mm

ST2.5 mm

Rec. Incr. 1.6 mm

Std Res. 8 lp/cm

120 kV, 96 mAs

CTDI100w 7.3 mGy

CTDIFDAw 4.9 mGy

Extended Spiral Acquisition

basics_of_ct scan(lovish vij; eic 2 ; 101285007)

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