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Advanced CT systems
and
Their Performance
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SourceSource
Detector
Generations source
detector
Advantages Disadvantages
collimation
collimation Detector
movement
1st Gen.
2nd Gen.
single
single
Pencil
beam
Fanbeamlet
Fanbeam
single
no
multiple
yes
3rd Gen.
single
4th Gen.
single
Fanbeam
Stationary
ring
no
multiple
Fanbeam
Stationary
ring
no
5th Gen.
6th Gen.
single
7th Gen. single
8th Gen.
single
Fanbeam
Narrow
many
many
Multiple
cone- beam arrays
wide
FPD
cone- beam
no
yes
yes
Trans.+
Rotates
Trans.+
Rotates
Faster
than 1G
Faster
Rotates
than 2G
together
Source
Higher
Rotates
efficiency
only
than 3G
No
Ultrafast
movement for cardiac
3rdGen.+
bed trans.
3rdGen.+
bed trans.
no
No
scatter
3rd Gen.
faster 3D
imaging
faster 3D
imaging
Large 3D
slow
Low
efficiency
High cost
and Low
efficiency
high
scatter
high
cost
higher
cost
higher
cost
Relatively
slow
4th Generation CT Scanners
Rotate/Stationary
• Fan beam geometry
• More than 4800 detectors
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5th generation: Electron Beam CT (EBCT)
- x-ray source is not x-ray tube but a focused, steered,
microwave-accelerated EB incident on a tungsten target.
- It has no moving parts .
- Target covers one-half of the imaging circle; detector array
covers the other half.
- Images in
- less than
- 50ms.
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Electron Beam CT (EBCT)
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EBCT(CONT’D)
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Spiral (Helical) CT:
Reciprocating rotation (A) versus fast continuous rotation using
slip-ring technology (B)
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12
(A) Pitch =1
(B) Pitch = 2
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MULTISLICE SPIRAL CT
• Introduced at the 1998.
• They are based multiple detector.
rows ranging between 8, 16, 24, 32 and
64 depending on the manufacturer.
• The overall goal is to improve the
volume coverage speed
performance.
• Complete x-ray tube/detector array
rotation in less than 1s.
• Partial scan images can be
obtained in approximately 100ms.
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256-slice cone-beam CT detector
REAL-TIME CT FLUOLOROSCOPY
• CT fluoroscopy acquire
dynamic images in real time.
• Fast continuous imaging, fast
image reconstruction &
continuous image display.
• Patient movement is low
during Tube rotation.
• Fast image Reconstruction
algorithm is required.
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CT ANGIOGRAPHY (CTA)
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CT VIRTUAL REALITY
IAMAGING
• The use of virtual reality is the
creation the inner views of
tubular structures.
• Offers both endoluminal and
extra luminal information.
• It reduces complication (eg.
infection and perforation).
• Four requirements:
–
–
–
–
data acquisition
image processing
3D rendering
image display and analysis.
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What is displayed in CT images?
T  water
CT# 
 1000HU
water
Water: 0HU
Air: -1000HU
Typical medical scanner display:
[-1024HU,+3071HU],
Range:
4096  2
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12 bit per pixel is required in display.
Hounsfield scales for typical tissues
+3071
W
L
For most of the display device, we can
only display 8 bit gray scale.
This can only cover a range of 2^8=256
CT number range. Therefore,
for a target organ, we need to map the CT
numbers into [0,255] gray
scale range for observation purpose. A
0
255 window level and window
width are utilized to specify a display.
-1024

0


 CT #( L  W / 2)
Displayed Gray scale  
I max
W



I max

CT #  L  W / 2
L  W / 2  CT #  L  W / 2
CT #  L  W / 2
Windowing in CT image display
Scintillator Properties of CT
•
•
•
•
Transparency
X-ray stopping power
Light output and efficiency for lower dose
Primary speed (Fast decay time or quicker
response)
• Luminescent afterglow for quicker speed
response
• Radiation damage
PRIMARY SPEED
• Primary speed is the rise time of the output signal in response to
a constant x-ray input (~10-3-10-6seconds supporting 0.5 second scanning
time).
• It is also the time constant for the first component of exponential
decay of that output after the input is turned off.
Clinical Significance:
• Primary speed is critical to maintaining high resolution during
sub-second scanning. It must be fast enough to prevent blurring
especially at the perimeter of the scan FOV.
• A slower primary speed can be seen as shading across high
contrast edges, such as the skin-air interface.
AFTERGLOW
• Afterglow is the second time component of
the exponential decay of the output after the
x-ray source is turned off.
• Clinical Significance:
Afterglow will result in arcing artifacts
extending from low attenuation anatomy into
areas of higher attenuation. It also decreases
in plane spatial resolution.
RADIATION DAMAGE
• Radiation damage is the darkening of the material with radiation
exposure over time.
•
It results in a gain or a shift in output for a given x-ray exposure.
• It can also cause changes in Z- Axis uniformity. This is especially
true for translucent materials.
Clinical Significance:
• Radiation damage causes changes in gain that require frequent recalibration.
• It can result in changes in Z-axis uniformity which are more
severe. These can cause rings or spots especially when scanning
anatomy that changes rapidly along Z.
TRANSPARENCY
• A transparent material allows light to be transmitted with
very little scatter. Most light rays can pass through with a
shorter more direct path for light.
• In a translucent material light rays scatter many times as
they travel from the creation site to the photodiode. This
longer path length can result in more self absorption, lower
net light output, and greater susceptibility to radiation
damage.
Clinical Significance:
• Transparency results in higher light output, better signal to
noise, better Z-axis uniformity and reduced radiation
damage.
X-RAY STOPPING POWER
• Stopping power is defined as the thickness of
material needed to stop 98% of incident x-rays in
the typical 140 kVp CT beam (~2-3 mm).
Clinical Significance:
• The thinner the material needed to stop 98% of the
x-rays, the greater the light output at the diode.
LIGHT OUTPUT
• Relative light signal at the diode for a given x-ray
input (~70% at 610nm).
Clinical Significance:
• Low light output can result in performance due to
less electronic noise vs. quantum noise for thin
slice, large patient, low dose application.
• It can also result in low signal artifacts such as
streaking at the shoulders and hips.
EMMISSION SPECTRUM
• The emission spectrum of a scintillator is the relative
intensity of light output at a given wavelength.
Clinical Significance:
• In addition to diode matching for optimal electronic signal
output, the emission spectrum of a scintillator can impact
the design flexibility of detector systems and its long term
stability.
– In addition to x-ray radiation, scintillator emission in the photo
active range can impact detector aging.
DIODE MATCHING and
RELATIVE OUTPUT
• Total signal output is a function of how well the emission
spectrum of the detector material and sensitivity spectrum
of photo diode match.
• The closer the output spectrum of the detector matches the
sensitivity profile of the photo diode, the higher the
resultant electrical signal.
Clinical Significance:
• Can reduce effective light output with the expected low
signal impacts when scanning large patients with thin
slices.
Relative Diode Sensitivity
• HiLight: 60% @ 610nm
• Gadolinium Oxysulfide: 40% @510nm
• Cadmium Tungstate: 42% @530nm
• Relative Output
• HiLight: 60% x 70% = 42%
• Gadolinium Oxysulfide: 40% x 80% = 32%
• Cadmium Tungstate: 42% x 30% = 13%
Quality criteria for CT images
The slice sensitivity profile (SSP)
1) For conventional CT and spiral/helical CT.
2) SSP is wider for 360-degree linear interpolation algorithms.
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Scanner performance: technical
parameters (I)
• CT Number Accuracy
– CT number depends on tube voltage, filtration, object thickness
– CT number of water is by definition equal to 0
– Measured CT number should be < ± 4 HU in the central ROI
• CT Number Linearity
– It concerns the linear relationship between the calculated CT number and the
linear attenuation coefficient of each element of the object
– Deviations from linearity should be < ± 5 HU
• CT Number Uniformity
– It relates to the fact that a CT number of each pixel in the image of an
homogeneous object should be the same over various regions
– The difference in the CT number between a peripheral and a central region of an
homogeneous test object should be < 8HU
– Differences are largely due to beam hardening phenomenon
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Scanner performance: technical
parameters (II)
• Spatial Resolution
– The high contrast resolution determines the
minimum size of detail visualized in the plane of the
slice with a contrast >10%.
It is affected by:
• the reconstruction algorithm
• the detector width
• the effective slice thickness
• the object to detector distance
• the X-ray tube focal spot size
• the matrix size.
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Scanner performance: technical
parameters (III)
• Spatial Resolution
– The low contrast resolution determines the size of
detail that can be visibly reproduced when there is
only a small difference in density relative to the
surrounding area
•
Low contrast resolution is considerably limited by
noise.
• The perception threshold in relation to contrast and
detail size can be determined, for example, by means
of a contrast-detail curve.
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Scanner performance: technical
parameters (IV)
• Slice Thickness
– The slice thickness is determined in the
center of the field of view.
– The use of post-patient collimation to
reduce the width of the image slice
leads to very significant increases in
the patient dose
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CT number uniformity
Axial image of
an
homogenous
phantom
can be assessed at the same time as measuring noise, by placing
four additional ROI (N, E, S and W) at positions near the edge of
the image of a uniform phantom 40
CT number linearity
CT number linearity is assessed using a phantom containing
inserts of a number of different materials (materials should
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cover a wide range of CT numbers
Low contrast resolution
Typical image
of the Catphan
LCR insert
Low contrast resolution (or low contrast detectability) is
often quoted as the smallest visible object at a given
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contrast for a given dose
Spatial resolution (high contrast)
 The number of line pairs per
cm just visible in the image is
approximately equivalent to the
2% value of the MTF
 This result can then be
compared with the 2% MTF
The resolution is quoted as the spatial frequency (in line
pairs / cm) at which the modulation falls to 50%, 10% or
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2% MTF.
Z-Sensitivity (Imaged slice width)
Plan view of a test object used to measure imaged slice widths for axial
scans, to assess the accuracy of post patient collimation, and to
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calculate the geometric efficiency
for the scanner
Dosimetry - CTDI in air (helical)
Axial slice positions
Helical scan (pitch 1)
The Computed Tomography Dose Index (CTDI) in air can be
measured using a 10cm pencil
ionization chamber, bisected
45
by the scan plane at the isocentre.
Dosimetry - CTDI in Perspex
Phantoms
Insert to plug holes
Body phantom (or annulus
Head phantom
to fit over haed phantom)
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Dosimetry - CTDI in Perspex
Phantoms
 Central and peripheral CTDI’s are used to calculate
weighted CTDI, CTDIw:
1
n CTDI w =
C
(
1
2
CTDI 100, c + CTDI 100, p
3
3
)
 CTDIws can be compared against diagnostic reference
levels for standard scan examinations
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CT Noise Characteristics
• For low mAs values
– standard deviation decreases with increasing
mAs
• For higher mAs values
– standard deviation stays fairly constant
Transition point mAs
should not increase
throughout scanner life
Standard
Deviation
mAs
CT Noise Characteristics
• Excessive noise can be caused by
– detector sensitivity
– electronic noise in detector amplifier circuits
– reduced output per mAs
Imaging performance (Noise)
 Noise is generally assessed using
cylindrical phantoms, which are either
filled with water or made of a tissue
equivalent material
 Once an axial image of the phantom has
been acquired, noise is obtained from the
standard deviation in CT number in a
region of interest (ROI) placed centrally
within the image
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Imaging performance (Noise)
Region of
interest
(ROI)
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SNR is dependent on dose, as in X-ray.
Notice how images become grainier and our
ability to see small objects decreases as dose
decreases.
There are some similarities with X-ray. But
we also see some important differences.
Actions that can influence image quality
 Avoid bad viewing conditions (e.g. lack of monitor
brightness or contrast, poor spatial resolution, etc)
 Improve insufficient skill to use the workstation
capabilities to visualize images (window level, inversion,
magnification, etc).
 Reduce artifacts due to incorrect digital post-processing
(creation of false lesions or pathologies)
 Compromise between image quality and compression
level in the images
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