ct basic principles modified
TRANSCRIPT
ImPACT Day: 23rd July 2002
Basic Principles of CT Scanning
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 2
Basic principles of CT scanning
• Basic theory of CT
– What’s inside the gantry?
– Data acquisition
– Image reconstruction
• Variations in scanner design
– Generation
– Detector type
– Number of projections and detectors
– Slip-ring scanning
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 3
Why CT?
• Conventional radiography suffers from the
collapsing of 3D structures onto a 2D image
• Although resolution is lower in CT, it has
extremely good low contrast resolution,
enabling the detection of very small changes in
tissue type
• CT gives accurate diagnostic information about
the distribution of structures inside the body
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 4
Basic principles of CT scanning
• Basic theory of CT
– What’s inside the gantry?
– Data acquisition
– Image reconstruction
• Variations in scanner design
– Generation
– Detector type
– Number of projections and detectors
– Slip-ring scanning
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 5
Construction of a CT scanner
Whizzo CT Company
x-ray tube
x-ray detectors
slip rings
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 6
TUBE
DETECTORS
APERTURE
In practice
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 7
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 8
Basic principles of CT scanning
• Basic theory of CT
– What’s inside the gantry?
– Data acquisition
– Image reconstruction
• Variations in scanner design
– Generation
– Detector type
– Number of projections and detectors
– Slip-ring scanning
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 9
Data acquisition
X-ray tube
Slice width Fan beam
Detectors
Collimators
X-ray emission in all directions
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 10
What are we measuring?
• The average linear attenuation
coefficient, µ, between tube and detectors
• Attenuation coefficient
reflects the degree to
which the x-ray
intensity is reduced
by a material
x-ray tube
detector
attenuation
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 11
Projections
• 2D views - ‘projections’ at angles all the
way round the patient
– sample µ at each
detector to generate
a projection
– rotate tube and
detectors a small
amount and repeat
the measurements atte
nu
ation
detector
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 12
Basic principles of CT scanning
• Basic theory of CT
– What’s inside the gantry?
– Data acquisition
– Image reconstruction
• Variations in scanner design
– Generation
– Detector type
– Number of projections and detectors
– Slip-ring scanning
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 13
Back projection
• Reverse the process of measurement of
projection data to reconstruct an image
• Each projection is ‘smeared’ back across
the reconstructed image
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 14
Back projection
2 projections 4 projections
8 projections 16 projections 32 projections
Original object
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 15
Filtered back projection
• Back projection produces blurred trans-axial images
• Projection data needs to be filtered before reconstruction
• Different filters can be applied for different diagnostic purposes
– Smoother filters for viewing soft tissue
– Sharp filters for high resolution imaging
• Back projection process same as before
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 16
Filtered back projection
• Filter applied to projection data
-15
-10
-5
0
5
10
15
20
25
30
-15
-10
-5
0
5
10
15
20
25
30
0 2 4 6 8 10
Frequency (lp/cm)
Am
pli
ficati
on
smooth
sharp
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 17
Filtered back projection
16 projections
4 projections 2 projections 8 projections
64 projections 64 projections
(not filtered)
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 18
Filtered back projection
original
back
projected
image
filtered back
projected
image
Profile
Filtered profile
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 19
m to CT number
• Originally measured was the distribution of m
• m values are scaled to that of water to give the
CT number
mtissue - mwater
• CT number = x 1000
mwater
• Water = 0; Air = -1000; Bone = ~1000
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 20
CT number flexibility
• We can change the appearance of the image
by varying the window width and level
• This spreads a small range of CT numbers over
a large range of grayscale values
• This makes it easy to detect very small
changes in CT number
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 21
CT number window
• Same image data at different WL and WW
WL -593, WW 529 WL -12, WW 400 -1000 HU
4000+ HU
WW WL
0 HU
-1000 HU
4000+ HU
WW WL
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 22
CT image
Slice
Width
Picture
Element
(PIXEL)
512
PIXELS
Volume
Element
(VOXEL)
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 23
Basic principles of CT scanning
• Basic theory of CT
– What’s inside the gantry?
– Data acquisition
– Image reconstruction
• Variations in scanner design
– Generation
– Detector type
– Number of projections and detectors
– Slip-ring scanning
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 24
Scanner generation
3rd generation
4th generation
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 25
Basic principles of CT scanning
• Basic theory of CT
– What’s inside the gantry?
– Data acquisition
– Image reconstruction
• Variations in scanner design
– Generation
– Detector type
– Number of projections and detectors
– Slip-ring scanning
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 26
Detector type
Pressurised xenon gas
Ionisation
Signal
Scintillation
Photon capture
Light
Photo-diode
Signal
XENON SOLID STATE
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 27
Xenon detectors
_
+
Xenon Atom
X-ray
Electrical signal
Negative electron Positive xenon ion + -
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 28
Electrical signal
Ceramic scintillators
Scintillator
Photo-diode
Visible photon
X-ray
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 29
• Xenon - Single detector chamber
sub-divided by electrodes
• Solid state - Detector array made up of
individual elements
Xenon vs. solid state
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 30
Basic principles of CT scanning
• Basic theory of CT
– What’s inside the gantry?
– Data acquisition
– Image reconstruction
• Variations in scanner design
– Generation
– Detector type
– Number of projections and detectors
– Slip-ring scanning
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 31
• Lower range scanners:
400 - 600 detectors
• Top range scanners:
650 - 900 detectors (per row)
Typically, for 3rd generation scanners:
Number of detectors
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 32
• Lower range scanners:
600 - 1000 per image
• Top range scanners:
1100 - 1800 per image
Number of projections
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 33
Basic principles of CT scanning
• Basic theory of CT
– What’s inside the gantry?
– Data acquisition
– Image reconstruction
• Variations in scanner design
– Generation
– Detector type
– Number of projections and detectors
– Slip-ring scanning
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 34
Conventional CT Systems
• • Power to X-Ray Tube via Cord
• • Scan CW and CCW to Wind/Unwind
Cord
• • Tube Rotates Around Stationary Patient
• (Table Position is Incremented Between
Acquisitions)
• • Interscan Delays:
3.5 Seconds Between Slices
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 35
Power
supply
Projection
data
Slip ring scanning
Slip Rings Slip-rings
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 37
Low voltage
AC
mains
Low
voltage
slip ring
High
voltage
generator X-ray
tube
High voltage
AC
mains
High
voltage
generator
High
voltage
slip ring X-ray
tube
High voltage v low voltage slip rings
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 38
• Faster conventional axial scanning
(stepwise table feed)
– Interscan delay governed only by time
taken for table to move to new position
(~ 1 sec.)
Advantages of slip ring scanners
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 39
• Cine scanning
(no table feed)
– continuous series of images at one position
• “CT fluoroscopy”
– new image reconstructed several times
during one rotation
Advantages of slip ring scanners
• Spiral scanning
(continuous table feed)
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 40
Spiral (Helical) CT Systems
• Power to X-ray Tube via Slip Ring -
Allows Continuous Rotation, No InterScan Delays
• Table Moves as Tube Rotates
• Synthesizing Projection Data via Interpolation
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 41
0
z, mm
t, sec
Direction of Continuous Patient Transport
Center of Spiral Path
Width of Spiral Path (From Collimation)
Pitch=1; Contiguous Spiral
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 42
0
z, mm
t, sec
Center of Image
Width of Image (From Collimation)
Contiguous Reconstruction - No Overlap
Image Number 1 2 3
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 43
0
z, mm
t, sec
Direction of Continuous Patient Transport
Center of Spiral Path
Width of Spiral Path (From Collimation)
Pitch=2; Extended Spiral
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 44
0
z, mm
t, sec
Overlapping Reconstruction - 50% Overlap
Image Number 1 2 3
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 45
Image Formation
u Collect Projection Data
u These Data are NOT all at same table
position, so:
u Synthesize a set of Planar Projection Data
via Interpolation (interpolate between
views taken at same projection angle, but
different table positions), then
u Use Filtered Back Projection on
Synthesized Planar Data
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 46
Selected Image Plane (Arbitrary)
Path of Continuously Rotating X-ray Tube (and Projection Data)
0
z, mm
t, sec
Direction of Continuous Patient Transport
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 47
Selected Image Plane (Arbitrary)
Path of Continuously Rotating X-ray Tube (and Projection Data)
0
z, mm
t, sec
Direction of Continuous Patient Transport
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 48
Image Formation
u Because a VOLUME of Data is Acquired and
Interpolation is used, Images can be formed
ANYWHERE -->
• Slice Location is Arbitrary
u Therefore, Overlapping Images can be created – (e.g. 10 mm thick, 5 mm apart).
u Slice Thickness Is Determined by Collimation
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 49
Index
• Index = Interval at which images are
reconstructed
• (e.g. image collimation of 10 mm with
index of 5 mm means that images are
reconstructed every 5 mm).
• % Overlap = Percent of image that
overlaps with adjacent image. (e.g. 10
mm collimation with 5 mm reconstruction
interval is a 50% overlap).
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 50
Data Acquisition
Pitch = Table Movement per Rotation
X-ray Beam Collimation
• Contiguous Spiral
Pitch = 1 (10 mm / 10 mm)
• Extended (Non-Contiguous) Spiral
Pitch = 2 (20 mm/ 10 mm)
• Overlapping Spiral
Pitch = 1/2 ( 5 mm / 10 mm)
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 51
Spiral CT - Differences From Axial
Image Quality (Assuming 180 reconstruction
algorithm):
• Effective Slice Thickness Increases with Pitch
(~10% increase for pitch 1; ~ 30% for pitch 2;
~ 70% for pitch 3)
• Hence volume averaging increases with pitch
• Noise is greater than Conventional Axial under
same conditions, but does not change with pitch
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 52
Spiral CT - Differences From Axial
Radiation Dose:
For Pitch of 1 ~ Same as Comparable Contiguous
Conventional Scans
For Pitch 1.5 - Approximately 2/3 that of
Contiguous Scans
For Pitch 2 - Approximately 1/2 of Contiguous
Scans
• Radiation Dose Proportional to 1/pitch
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 53
Spiral CT Systems
System Requirements:
• High Heat Unit Capacity Tube to Sustain
Continuous Scanning- Now 5 and 6 Million
Heat Units
• Faster Rotation Times to Get Through a
Volume Even Quicker; < 0.5 second rotation
times available
• Faster Data Transfer Systems to Get Data
From DAS - Fiber Optics
• Faster Computers to Reconstruct
- 0.1 to 0.5 seconds per image
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 54
Spiral CT Systems
Advantages over Conventional:
• Faster Acquisition
• Follow Contrast Quicker
• Begin to Image Physiology/Angiography
• Reduce Breathing Artifacts/Misregistration
ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 55
Spiral CT Systems
Limitations:
• Motion Blurring (Effective Slice is Thicker)
• Data/Image Overload
• Memory and Data Transfer May Be
Limiting Factors