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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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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 6.5 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 – Digital radiograph, axial or volume

• Houses reconstructor

• 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

• Digital projection – AP, PA, Lat or Oblique projection

– Surview, Scanogram

• 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 rotate 360°

• Patient table is stationary – With X-ray’s “on”

• Produces one cross-sectional image

• Once this is complete patient is moved to next position – Process starts again at the

beginning

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Volume CT

• X-ray tube and detector rotate 360°

• Patient table moves continuously – With X-ray’s “on”

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

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Volume Scanning: Pitch

1:1 2:1

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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

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Fundamentals of Multislice CT

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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

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Single Slice = One 10mm slice per rotation

Dual 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 for 2 Slice Acquisition

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Approximately 10%

more efficient than

matrix detectors

Variable Wide Area Detector

Asymmetrix™

Variable detector length Fixed detector length

Quad Detector Technology

• Philips patented 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 Technology How it works

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2x0.5mm

4x1mm

4x2.5mm

4x5mm

2x10mm

p-plane

fused fused

to FEE

Asymmetrix™ Technology Variable slice thickness

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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 differently

– According 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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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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Cervical Spine Spiral Acquisition

Rotation – 0.75 sec

Coverage – 160 mm

Pitch – 0.875

Acq. Time – 36 sec

FOV – 250 mm

ST – 1.0 mm

Rec. Incr. – 0.6 mm

Std Res. – 8 lp/cm

120 kV, 200 mAs

CTDI100w – 39 mGy

CTDIFDAw – 17 mGy

Thin-Slice Spiral Neck

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Neuro-Angiography

Circle of Willis

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Thin-Slice Spiral Lungs

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MIP 4D-Angio

MasterCut Panoramic View

Renal Arteries

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Ext. Spiral Acq. Spiral Acquisition

UltraFast – 0.5 sec

Coverage – 1400mm

Pitch – 1.75

Acq. Time – 41.5 sec

FOV – 420 mm

ST –2.5 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

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CT Scanners

• Provide a window into the body

• Customer considerations – How many patients

– Referring physicians

– Budget

– Upgrade expectations

• Philips has ALL the answers

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