1. Computed Tomography Data Acquisition

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

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

4. 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

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

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

7. CT View
# of simultaneously collected rays

8. Acquisition Geometries
Pencil Beam
Fan Beam
Spiral
Multislice

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

10. What’s a Slip Ring?

11. High Voltage Transformer
Rotating Generator
Primary Voltage
Secondary Voltage
Incoming AC Power
X-Ray Generator
High Voltage Transformer
X-Ray Tube
Stationary
Rotating
Slip Rings

13. Spiral CT Advantages Faster scan times
minimal interscan delays
no need to stop / reverse direction of rotation
Slip rings allow continuous rotation of tube & detector
Continuous acquisition protocols possible

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

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

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

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

18. Efficiency Components
Capture efficiency (0-1)
fraction of beam incident on active detector
Absorption efficiency (0-1)
fraction of photons incident on the detector which are absorbed
Conversion efficiency (0-1)
fraction of absorbed energy which produce signal

19. Overall Detector Efficiency (Also 0-1)
capture efficiency X absorption efficiency X conversion efficiency

20. Capture Efficiency
Fraction of beam incident on active detector

21. 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

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

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

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

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

26. 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

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

28. 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

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

30. Logarithms Log10x = ? means 10? = x? logarithms are exponents
log10x is exponent to which 10 is raised to get x
log10100 =2 because 102=100

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

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

33. 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 5
10,000 4
1,000 3
100 2 10 1 1

34. 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