CT

Data Collection Basics
 X-ray source & detector must be in & stay in alignment
 Beam moves (scans) around patient
 many transmission measurements
Patient
X-Ray beams

Data Collection Basics
 Pre-patient beam
 collimated to pass only through slice of interest
 shaped by special bow tie filter for uniformity
Filter Patient

Data Collection Basics (cont)
 Beam attenuated by patient
 Transmitted photons detected by scanner
 Detected photon intensity converted to electrical signal (analog)
 Electrical signal converted to digital value
 A to D converter
 Digital value sent to reconstruction computer

CT “Ray”
 That part of beam falling onto a single detector
Ray

Each CT Ray
 attenuated by patient
 projected onto one detector
 detector produces electrical signal
 produces single data sample

CT View
 # of simultaneously collected rays

Scan Requires Many Data Samples
 # Data Samples = [# data samples per view] X
[# views]
 # Data Samples = [# detectors] X
[# data samples per detector]

Acquisition Geometries
 Pencil Beam
 Fan Beam
 Spiral
 Multislice

Pencil Beam Geometry
 Tube-detector assembly translates left to right
 Entire assembly rotates 1 o
1st Generation CT
Tube
1 o
Detector

Tube
Fan Beam Geometry
3nd Generation
Detectors
2nd Generation
4th Generation

Comparing Long vs. Short Geometry
Long Geometry
•
Smaller fan angle
•
Longer source-detector distance
•
Lower beam intensity
•
Lower patient dose
•
More image noise
•
Less image blur
•
Requires larger gantry
Scan
FOV
Scan
FOV

Spiral Geometry




X-ray tube rotates continuously around patient
Patient continuously transported through gantry
No physical wiring between gantry & x-ray tube
Requires “Slip Ring” technology
Slip
Rings
Interconnect
Wiring
Tube
Detector

What’s a Slip Ring?

Slip Rings
 Electrical connections made by stationary brushes pressing against rotating circular conductor
 Similar to electric motor / generator design

X-Ray Generator Configurations with Slip Ring Technology
 Problem:
 Supply high voltage to a continually rotating x-ray tube?
 Options
 #1
 Stationary Generator & Transformer
 #2


Stationary Generator
Transformer & x-ray tube rotate in gantry
 #3
 Transformer, generator & tube rotate in gantry

Option #1: Stationary High Voltage
Transformer
Incoming
AC Power
X-Ray
Generator
Primary
Voltage
High Voltage
Transformer
Secondary
Voltage
X-Ray
Tube

Option #1: Stationary High Voltage
Transformer
Secondary
Voltage
Line Voltage
Generator
Primary Voltage
HV
Transformer
 high voltage must pass through slip rings
Tube
Slip
Rings
Detector

Option #2: Rotating High Voltage
Transformer
Incoming
AC Power
X-Ray
Generator
Primary
Voltage
High Voltage
Transformer
Secondary
Voltage
X-Ray
Tube

Option #2: Rotating High Voltage
Transformer
Line Voltage Generator
Primary
Voltage
HV Transformer
 low voltage must pass through slip rings
Slip
Rings
Tube
Detector

Rotating Generator
Incoming
AC Power
X-Ray
Generator
Primary
Voltage
High Voltage
Transformer
Secondary
Voltage
X-Ray
Tube

Rotating Generator
 low line voltage must pass through slip rings
Line
Voltage
Generator
Slip
Rings HV Transformer
Tube

Spiral CT Advantages
 Faster scan times
 minimal interscan delays
 no need to stop / reverse direction of rotation
 Slip rings solve problem of cabling to rotating equipment
 Continuous acquisition protocols possible

X-Ray System Components
 X-Ray Generator
 X-Ray Tube
 Beam Filter
 Collimators

X-Ray Generator
 3 phase originally used
 Most vendors now use high frequency generators
 relatively small

 small enough to rotate with x-ray tube can fit inside gantry

X-Ray Tube

X-Ray Tube
 Must provide sufficient intensity of transmitted radiation to detectors
 Radiation incident on detector depends upon
 beam intensity from tube
 patient attenuation

 beam’s energy spectrum patient

 thickness atomic #
 density

Maximizing X-Ray Tube Heat
Capacity



 rotating anode
 high rotational speed small target angle large anode diameter focal spot size appropriate to geometry
 distances
 detector size

Special Considerations for Slip Ring
Scanners

 continuous scanning means


Heat added to tube faster
No cooling between slices
Need
 more heat capacity
 faster cooling

Why not use a Radioactive Source instead of an X-Ray Tube?



High intensity required
 X-ray tubes produce higher intensities than sources
Single energy spectrum desired


Produced by radioactive source
X-ray tubes produce spectrum of energies
Coping with x-ray tube energy spectrum
 heavy beam filtering (see next slide)
 reconstruction algorithm corrects for beam hardening

CT Beam Filtration
 Hardens beam
 preferentially removes low-energy radiation
 Removes greater fraction of lowenergy photons than high energy photons

 reduces patient exposure
Attempts to produce uniform intensity & beam hardening across beam cross section
Filter Patient

CT Beam Collimation
 Pre-collimators
 between tube & patient
Tube
 Post-collimators
• between patient & detector
Detector

Pre-Collimation
 Constrains size of beam
 Reduces production of scatter
 May have several stages or sets of jaws
Tube
Pre-collimator
Detector

Post-Collimation
 Reduces scatter radiation reaching detector
 Helped define slice (beam) thickness for some scanners
Tube
Post-collimator
Detector

CT Detector Technology:
Desirable Characteristics
 High efficiency
 Quick response time
 High dynamic range
 Stability

CT Detector Efficiency
 Ability to absorb & convert x-ray photons to electrical signals

Efficiency Components
 Capture efficiency
 fraction of beam incident on active detector
 Absorption efficiency
 fraction of photons incident on the detector which are absorbed
 Conversion efficiency
 fraction of absorbed energy which produce signal

Overall Detector Efficiency
Overall detector efficiency = capture efficiency
X absorption efficiency
X conversion efficiency

Capture Efficiency
 Fraction of beam incident on active detector

Absorption Efficiency
Fraction of photons incident on the detector which are absorbed
 Depends upon detector’s

 atomic # density

 size thickness
 Depends on beam spectrum capture efficiency
X absorption efficiency
X conversion efficiency

Conversion Efficiency
 Ability to convert x-ray energy to light
GE “Gemstone
Detector” made of garnet

Conversion Efficiency
 Ability to convert x-ray energy to light
Siemens
UltraFastCeramic (UFC)
CT Detector
•
Proprietary
•
Fast afterglow decay
UFC Material
UFC Plate

Response Time
 Minimum time after detection of
1st event until detector can detect
2nd event
 If time between events < response time, 2 nd event may not be detected
 Shorter response time better

Stability
 Consistency of detector signal over time
 Short term
 Long term
 The less stable, the more frequently calibration required

Dynamic Range
 Ratio of largest to smallest signal which can be faithfully detected
 Ability to faithfully detect large range of intensities
 Typical dynamic range:
1,000,000:1
 much better than film

Detector Types: Gas Ionization
 X-rays converted directly to electrical signal
Filled with
Air
X-Rays
+
Ionization
Chamber
-
- + Electrical
Signal

CT Ionization Detectors
 Many detectors (chambers) used
 adjacent walls shared between chambers
 Techniques to increase efficiency
 Increase chamber thickness
 x-rays encounter longer path length
 Pressurize air (xenon)
 more gas molecules encountered per unit path length
X-Rays thickness

Older Style Scintillation Detectors
 X-rays fall on crystal material
 Crystal glows
 Light flash directed toward photomultiplier (PM) tube
 Light directed through light pipe or conduit
 PM tube converts light to electrical signal
 signal proportional to light intensity
PM
Electrical
Signal

Detector Types: Scintillation
 X-ray energy converted to light
 Light converted to electrical signal
X-Rays Light
Scintillation
Crystal
Photomultiplier
Tube
Electrical
Signal

Photomultiplier Tubes
 Light incident on Photocathode of PM tube
 Photocathode releases electrons
+
-
X-Rays
Scintillation
Crystal
Light
Photocathode
Dynodes
PM
Tube

Photomultiplier Tubes
 Electrons attracted to series of dynodes
 each dynode slightly more positive than last one
+ + +
-
+
X-Rays
Scintillation
Crystal
Light
Photocathode
+
Dynodes
PM
Tube

Solid State Detectors
 Crystal converts incident x-rays to light
 Photodiode semiconductor current proportional to light
X-Rays Light
Photodiode
Semiconductor
Electrical
Signal

Photodiode
 Made of two types of materials
 p-type
 n-type
 Lens focuses light from crystal onto junction of p & n type materials
X-Rays Light
Lens p n
Junction

Photodiode
 Light controls resistance of junction
 Semiconductor current proportional to light falling on junction
X-Rays Light
Lens p n
Junction

Solid State Detectors
 Output electrical signal amplified
 Fast response time
 Large dynamic range
 Almost 100% conversion & photon capture efficiency
 Scintillation materials

 cadmium tungstate high-purity ceramic material

Detector Electronics
From
Detector
Pre-Amplifier
Increases signal strength for later processing
Logarithmic Amplifier
Analog to Digital
Converter
To
Computer
Compresses dynamic range;
Converts transmission intensity into attenuation data

Logarithms
Log
10 x = ? means 10 ?
= x?
 logarithms are exponents
 log
10 x is exponent to which 10 is raised to get x
 log
10
100 =2 because 10 2 =100

Logarithms
Input Logarithm
100,000
10,000
1,000
100
10
1
5
4
3
2
1
0
Using logarithms the difference between
10,000 and 100,000 is the same as the difference between 10 and 100

Compression
1,000
Hard to distinguish between 1 & 10 here
3 = log 1000
2 =log 100
1 = log 10
0 = log 10
1 10 100 1000
Input Logarithm
100,000
10,000
1,000
100
10
1
5
4
3
2
1
0
1 10 100 1000
Difference between
1 & 10 the same as between 100 & 1000
Logarithms stretch low end of scale; compress high end

Logarithmic Amplifier
 accepts widely varying input
 takes logarithm of input
 amplifies logarithm
 logarithm output dynamic range now appropriate for A/D conversion
Input Logarithm
100,000
10,000
1,000
100
10
1
5
4
3
2
1
0

Improving Quality & Detection
 Geometry


Smaller detectors
Smaller focal spot



Larger focus-detector distance
Smaller patient-detector distance
 Thinner slices less patient variation over slice thickness distance