Basics of CT All

7/21/2019 Basics of CT All

1/39

Basic Principles of

CT Scanning

Ilan Shelef MD , Head of Neuroradiology Unit,

Soroka Medical Center, Beer Sheva, Israel


2/39

The basics of CT

CT imaging chain

System components

Acquisition methods Image quality

Applications


3/39

X-ray: The beginning

X-Rays founded in 1895 by

Wilhelm Conrad Roentgen


4/39

CT: The beginning

CT founded in 1970 by Sir Godfrey Hounsfield

Engineer with EMI, LTD.

first applications were in neuroradiology


5/39

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


6/39

CT System


7/39

Basic CT scanner components

Gantry

X-Ray Tube

Detector Control Console


8/39

Gantry

CT X-ray tube

High voltage generator

Detector array Data acquistion system

Slip ring


9/39

The CT X-ray Tube

Anode heat capacity

3.5 MHU up to 28 MHU

Determines maximum mAs

Determines volume length

Dictates generator size


10/39

Detector Elements

Capture energy that has not been

attenuated by the patient


11/39

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


12/39

CT

CT - Computed Tomography

CAT Scan - Computerized Axial

Tomography


13/39

Scanning methods

Surview AP,Lat

Surview, Scanogram , Topogram.

Conventional CT

Axial Start/stop

Volumetric CT Helical or spiral CT

Continuous acquisition


14/39

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


15/39

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


16/39

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


17/39

Volume Scanning: Pitch

1:1 2:1


18/39

Advantages of Volume CT

More coverage in a breath-hold Chest, Vascular studies, trauma

Reduced misregistration of slices

Improved MPR, 3D and MIP images Potentially less IV contrast required

Gapless coverage

Arbitrary slice positioning


19/39

Fundamentals of Multislice CT


20/39

Multislice Fundamentals

Everything is better

(R)esolution

Z-axis, spatial, low contrast

(S)peed

Temporal -

bolus capture, stopped motion (V)olume

Thin slice -

organ-specific coverage

(P)ower

Enough photons -

uncompromising image quality


21/39

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


22/39

. .

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


23/39

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


24/39

8 Element

2-D array

4 Slices

Quad TechnologyHow it works


25/39

CT

CT attenuation information

CT image quality


26/39

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


27/39

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


28/39

Density information

Density values

correspond to a

range of numbers

Hounsfield scale


29/39

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


30/39

CT image quality

Spatial resolution

Ability to resolve small

objects in an image

Measured in lp/cm


31/39

CT image quality

Spatial resolution

Ability to resolve small

objects in an image

Measured in lp/cm


32/39

Isotropic Imaging

True 0.5mm Isotropic imaging


33/39

CT image quality

Contrast resolution

Ability to differentiate small density differences in an image


34/39

Post Processing Options

Visualization of

vasculature in

relation to pathology

Show course of vessels Show stent placement

Define vascular stricture


35/39

Cervical SpineSpiral Acquisition

Rotation0.75 sec

Coverage160mm

Pitch0.875

Acq.Time36 secFOV250 mm

ST1.0mm

Rec.Incr.0.6 mm

Std Res.8 lp/cm

120 kV, 200 mAsCTDI100w39mGy

CTDIFDAw17mGy

Thin-Slice Spiral Neck


36/39

Neuro-Angiography

Circle of Willis


37/39

Thin-SliceSpiralLungs


38/39

MIP4D-Angio

MasterCutPanoramic View

RenalArteries


39/39

Ext. Spiral Acq.Spiral Acquisition

UltraFast0.5 sec

Coverage1400mm

Pitch1.75

Acq. Time41.5 secFOV420 mm

ST2.5 mm

Rec. Incr.1.6 mm

Std Res.8 lp/cm

120 kV, 96 mAs

CTDI100w7.3 mGy

CTDIFDAw4.9 mGy

Extended Spiral Acquisition

Basics of CT All

Documents

Transcript of Basics of CT All