CT

1
CT
2
Computed Tomography

• CT Basics
• Principle of Spiral CT
• Scan Parameter Image Quality
• Optimizing Injection Protocols
• Clinical Applications

3
X-Ray Discovery

• X-ray was discovered by a German scientist
  Roentgen 100 years ago.
• This made people for the first time be able to
• view the anatomy structure of
• human body without operation

• But it's superimposed
• And we couldn't view soft tissue

4
History of Computed Tomography
1963 - Alan Cormack developed a mathematical
method of reconstructing images from x-ray
projections

• My name is Godfrey Hounsfield
• I work for the Central Research
• Labs. of EMI, Ltd in England
• I developed the the first clinically
• useful CT scanner in 1971

• Early 1970s

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CT Broke the Barrier
For the first time we could view
- Tomographic or Slice anatomy- Density
difference

• But it's time consuming
• And resolution needs to be improved

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Concept of X-ray Attenuation
SCATTERED X-RAYS

• An X-ray beam passing through
• the body is attenuated (loses its
• energy) by
• Absorption
• Scattering

Incident X-ray
Transmitted ray
BODY TISSUE
Absorption by the tissue is proportional to the
density
More dense tissue
Less dense tissue
MORE ATTENUATION
LESS ATTENUATION
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How does CT Work?
8
How does CT Work?
X-ray goes through collimator therefore penetrate
only an axial layer of the object, called
"slice"
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How does CT Work?

• Patient is placed in the center of the
  measurement field
• X-ray is passed through the patients slice from
  many direction along a 360 path
• The transmitted beams are captured by the
  detectors which digitizes these signals
• These digitized signals called raw data are sent
  to a computer which create the CT image

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How is CT Image generated?
The object slice is divided into small volume
elements called voxels. Each voxel is assigned a
value which is dependent on the average amount of
attenuation
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How is CT Image generated?
The attenuation values are transferred to the
computer where they are coded used to create a
slice image
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CT Generations Design

• Generation is used to label CT tube-detector
  designs

3rd Generation Design Rotating tube detector
4th Generation Design Fixed ring detector
13
Slip-ring Technology

• Power is transmitted through parallel sets of
  conductive rings
• instead of electrical cables
• ? Continuous Gantry Rotation
• ? Prerequisite for Spiral CT

Non Slip-ring Scanner
Slip-ring Scanner
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Computed Tomography

• CT Basics
• Principle of Spiral CT
• Scan Parameter Image Quality
• Optimizing Injection Protocols
• Clinical Applications

15
What is Spiral Scan? -- just 4C

• Continuously rotating tube/detector system
• Continuously generating X-ray
• Continuously table feed
• Continuously data acquisition

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Continuous data acquisition
Volume Data
A
B
Reconstruction of arbitrary slices (either
contiguous or overlapping) within the scanned
volume Distance between the slices is called
Increment
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Contiguous Image Reconstruction
?
Slice Thickness

• Increment Slice Thickness
• No Overlap
• No Gaps

?
Increment
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Overlapping Image Reconstruction
?
?
Slice
Thickness
Overlap
?

• Increment lt Slice Thickness
• Overlap of slices
• Closer image interval
• More images created

?
Increment
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Image Reconstruction with Gaps
?
Slice Thickness

• Increment gt Slice Thickness
• Gaps between slices
• Images are further apart
• Less images created

?
Increment
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Shallow Inspiration
Deep Inspiration
Standard CT / Slice Imaging

• Misregistration due to different
• respiratory levels between slices

• Unable to resconstruct images at
• arbitrary position

Partial Volume Effect

• Slice imaging is slow

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Spiral CT / Volume Imaging

• Scan the whole region of
• interest in one breath hold

• No gaps since radiation always
• transmits the whole volume

• Reconstruction of overlapping
• images without additional dose

• Retrospective reconstruction
• of slices in arbitrary position
• within the scanned volume

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Computed Tomography

• CT Basics
• Principle of Spiral CT
• Scan Parameter Image Quality
• Optimizing Injection Protocols
• Clinical Applications

23
Scan Parameters

• X-ray Tube Voltage (kVp)
• X-ray Tube Current (mA)
• Scan Time (s)
• Slice thickness or Collimation (mm)

• Table Speed (mm/rot) or Feed per 360 rotation
• Pitch
• Interpolation Process
• Increment (mm)

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Table Speed Pitch

• Table Speed is defined as distance traveled in mm
  
• per 360º rotation

Pitch gt
Table Feed per rotation
Collimation
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30s
Pitch 2 covers 2x distance as Pitch 1
10mm P1
30s
More Coverage in the same time with extended
Pitch!!
10mm P2
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Scan Range 300mm
30s
15s
10mm P2 20 mm/s
10mm P1 10 mm/s
Cover the same volume in shorter time with
extended Pitch
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Interpolation Algorithm

• Converts volume data into slice images

Interpolation
To reduce artifacts due to table motion during
spiral scanning, we use a special reconstruction
process called INTERPOLATION
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Slim Algorithm
Wide Algorithm
?
?
2 (18052) 464 raw data
2 x 360 720 raw data
Wide algorithm produces a broader image
thickness Wide algorithm uses more raw data gt
less image noise
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Pitch 2 scanning produces a broader image
thickness Pitch 2 scanning does not increase
image noise
PITCH 2
PITCH 1
30 increase in image thickness with Pitch 2
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Slice Sensitivity Profile ( SSP )
SSP describes the effective slice thickness of
an image and to what extent anatomy within that
slice contribute to the signal
Image signal
RESOLUTION
Ideal SSP
Collimation width of x-ray beam slice profile
SSP
All points within the slice contribute equally
points outside of the slice do not contribute to
the image at all .
Z-axis (mm)
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Slice Profile (SP)

• Effective slice thickness of an image

Slice Profile Resolution

• Factors influencing Slice Profile
• Collimation
• Pitch
• Interpolation algorithm (360 or 180)

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Factors influencing SSP

• Collimator width
• collimation gt SSP
• Spiral CT
• Table speed or Pitch
• Interpolation Algorithm
• gt mathematical process required to reconstruct
  axial images from the spiral volume data set

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Pitch Slice Profile
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Slim vs Wide SSP Comparison
Slice Profile Slim Broaden Wide
Braden Pitch One 5.0 mm 0
6.3 mm 26 Pitch Two 6.5 mm 30
10.8 mm 116
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SLIM 464 degree Less photons
WIDE 720 degree More photons
SSP Spatial resolution
SSP Spatial resolution
Smoother image
Noisier image
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Slim - Advantages

• Improved Z Resolution
• Reduced partial volume artifacts
• Slim extended Pitch
• Longer coverage
• Same coverage with shorter scan time or thinner
  slices
• Less radiation dose

37
Wide - Advantages

• Noise Reduction
• Smoother image
• Useful for scanning huge patient
• Only for scanning at
• Pitch One

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Slice Profile Comparison
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Optimizing the Scanning Parameters
Lesion smaller than 1cm
SCAN RANGE 150mm
10/10/10 (15s)
5/10/5 (15s)
Slice Profile 10mm
Slice Profile 6.5mm
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Smallest Possible Effective Slice Thickness
Scan Length (mm)
Smallest Collimation (mm)
Table Speed (mm/s)
Scan Duration (s)
Scan Duration
Depends on the scan length patients
breath-hold compliance
Table Speed Pitch Factor
1 lt Pitch lt 2 ? to cover the whole volume in one
breath-hold
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Computed Tomography

• CT Basics
• Principle of Spiral CT
• Scan Parameter Image Quality
• Optimizing Injection Protocols
• Clinical Applications

42
Injection Protocols
Site

• Peripheral vein eg. antecubital vein
• 19-20 gauge needle or IV catheter

Volume
80 - 150 ml ? patients weight region of
interest
Flow Rate
2 - 5 ml/s ? cardiac output
Scan Delay
Delay between injection initiation the start
of the scan sequence
Concentration
300 mg I/ml non-ionic contrast
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Tailoring Scan Injection Protocols
Injection Duration must be equal to or greater
than Scan Time
Enhancement Curve of the Target Region
HU
HU
250
250
200
200
150
150
100
100
50
50
CONTRAST
CONTRAST
NaCl
Times
Times
Bolus Duration lt scan time Insufficient,
inhomogeneous opacification
Bolus Duration scan time Uniform, maximum
opacification
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Contrast Bolus Timing
Determines optimal scan delay for spiral CTA
sequence
HU
Time-density curve of the target region
250
200
150
Early
Late
Optimal Window
100
50
Times
CONTRAST
NaCl
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Test Bolus Procedure

• 10-20 ml of contrast is injected
• at the chosen rate for spiral

• After a delay of 8-10s, low-dose,
• single-level axial images are
• acquired every 2s at the starting
• point of the imaging volume

• Dynamic Evaluation to generate
• a Time-density curve

Imaging Volume for spiral CTA
Dynamic scans at this position
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Dynamic Evaluation

• Dynamic Scans

• ROI placed in the Aorta

• Time-density curve
• Scan Delay ? Peak Enhancement Time

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Computed Tomography

• CT Basics
• Principle of Spiral CT
• Scan Parameter Image Quality
• Optimizing Injection Protocols
• Clinical Applications

48
Dual Phase Liver Exam Liver Metastases

• Arterial Phase

• Venous Phase

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Single Plane Imaging with Multiplanar Results
Oblique recon. of Aorta
2D reconstruction based on a serial of axial
images along a certain axis
Sagittal
Coronal
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CT Angiography
Max. Intensity Projection
Surface Shaded Display (3D)
Spine 3D image AVM
Femoral Arteries CT Angiogram
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3D Post-processing
3D Bronchoscopy Lesion in the right upper lobe
branch
Colour Segmentation 3D
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Volume Rendering Technique
Transparent color image
Transparent image
Solid Image
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Virtual Endoscopy
Bronchoscopy

• Real Time Fly through
• Reverse Perspective
• Axial Image reference
• High Resolution

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The END