Transcript Basic Principles of CT Scanning

Basic Principles of
CT Scanning
CT
• CT - Computed Tomography
• CAT Scan - Computerized Axial
Tomography
The basics of CT
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CT imaging chain
System components
Acquisition methods
Image quality
Applications
X-ray: The beginning
• X-Rays founded in 1895 by
Wilhelm Conrad Roentgen
CT: The beginning
• CT founded in 1970 by Sir Godfrey Hounsfield
– Engineer with EMI, LTD.
– first applications were in neuroradiology
CT Scanner
• X-Ray modality used to view the
body in cross section
• Used to determine
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extent of trauma
location and type of tumors
status of blood vessels
pre surgical planning
CT System
Basic CT scanner components
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Gantry
X-Ray Tube
Detector
Control Console
Gantry
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CT X-ray tube
High voltage generator
Detector array
Data acquistion system
Slip ring
The CT X-ray Tube
• Anode heat capacity
– 3.5 MHU up to 28 MHU
• Determines maximum mAs
• Determines volume length
• Dictates generator size
Detector Elements
• Capture energy that has not been
attenuated by the patient
Control console
• Set scan parameters
– kVp, mA, scan time, reconstruction
filter, etc.
• Set scan mode
– Surview, Axial or Spiral
• IRS (Image reconstruction
System)
• Review and archive images
• Post-processing
Scanning methods
• Surview
– AP,Lat
– Surview, Scanogram , Topogram….
• Conventional CT
– Axial
• Start/stop
• Volumetric CT
– Helical or spiral CT
• Continuous acquisition
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
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
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
Volume Scanning: Pitch
1:1
2:1
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
Fundamentals of Multislice CT
Multislice Fundamentals
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Everything is better
(R)esolution
– Z-axis, spatial, low contrast
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(S)peed
– Temporal bolus capture, stopped motion
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(V)olume
– Thin slice –
organ-specific coverage
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(P)ower
– Enough photons uncompromising image quality
Multislice Effectiveness
• Everything is better
– Resolution
– Speed
– Volume
– Power
Single Slice =
Dual Slice =
Quad Slice =
Dual
2x
same
same
same
One 10mm slice per rotation
Two 5mm slices per rotation
Four 2.5mm slices per rotation
Quad
4x-8x
same
same
same
Dual Slice Detector Optimized for
2 Slice Acquisition
Mx8000
Dual Slice
x-ray tube focal spot
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pre-patient collimation
post-patient collimation
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single detector arc
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dual detector arc
Quad Detector Technology
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variable wide area detector
Variable slice thickness
Asymmetrix™
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4 x 1mm
4 x 5mm
4 x 2.5mm
2 x 0.5mm
2 x 8mm
2 x 10mm
Variable detector length
Fixed detector length
Approximately 10%
more efficient than
matrix detectors
Variable Wide Area Detector
Quad Technology
How it works
8 Element
2-D array
4 Slices
CT
• CT attenuation information
• CT image quality
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
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
Density information
• Density values
correspond to a
range of numbers
– Hounsfield scale
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
CT image quality
• Spatial resolution
– Ability to resolve small
objects in an image
– Measured in lp/cm
CT image quality
• Spatial resolution
– Ability to resolve small
objects in an image
– Measured in lp/cm
Isotropic Imaging
• True 0.5mm Isotropic imaging
CT image quality
• Contrast resolution
– Ability to differentiate small density differences in an image
Post Processing Options
• Visualization of
vasculature in
relation to pathology
– Show course of vessels
– Show stent placement
– Define vascular stricture
Thin-Slice Spiral Neck
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
Neuro-Angiography
Circle of Willis
Thin-Slice Spiral Lungs
Renal Arteries
MasterCut
Panoramic View
MIP
4D-Angio
Extended Spiral Acquisition
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