1. Computed Tomography RAD309
Data Acquisition

2. Data Acquisition
Data acquisition represents the first step in process of image production
X-ray tube & detectors collect information systematically
Collect large number of x ray transmissions around the patient

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

4. Data Collection Basics
Pre-patient beam
collimated to pass only through slice of interest
shaped by special filter for uniformity

5. 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

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

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

8. CT Projection -or- View
# of simultaneously collected rays

9. Acquisition Geometries
Pencil Beam
Fan Beam
Spiral

10. DA Geometries Parallel beam, translate rotate motion
Fan beam, translate rotate motion
Fan beam, complete rotation tube/detector
Fan beam, complete rotation of tube around stationary ring of detectors
Special: high speed CT, stationary/stationary, multiple targets tube
Spiral, rotate/translate
Multiple detector rows

11. 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

12. X Ray System Initially used low energy gamma rays
Problem: low radiation intensity rate, large source size, low source strength, high cost
Use of X ray tubes
Benefit: high radiation intensity, high contrast ct scanning
Problem: heterogeneous beam , does not obay Lamber-Beer Exponential Law

13. 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

14. CT Beam Filtration Shapes beam to appear monochromatic and satisfy
reconstruction process
Hardens beam
Removes greater fraction of low-energy photons than high energy photons
reduces patient exposure
Shapes energy distribution to produce uniform intensity & beam cross section
Patient
Filter

15. Patient Protection Post-collimators Pre-collimators
between tube & patient
Tube
Post-collimators
between patient & detector
Detector

16. Pre-Collimation Constrains size of beam
Reduces amount of scatter produced
Designed to minimize beam divergence
Often consists of several stages or sets of jaws
Tube
Detector
Pre-collimator

17. Post-Collimation Helps define slice (beam) thickness
Reduces scatter radiation reaching detector
Improves image quality
Tube
Detector
Post-collimator

18. Detectors Capture radiation from patient Converts to electrical signal
Then they are converted to binary coded information

19. CT Detector Characteristics
Efficiency
Response time
Dynamic range
Reproducibility and Stability

20. 1. Efficiency
Ability to capture, absorb & convert x-ray photons to electrical signals

21. Efficiency Components
a. Capture efficiency
Efficiency of detector to obtain transmitted photons from patient
Size of detector area, distance between 2 detectors
b. Absorption efficiency
no. of photons absorbed
Z , density, size, thickness of detector
c. Conversion efficiency
fraction of absorbed energy which produce signal

22. Overall Detector Efficiency
capture efficiency X absorption efficiency X conversion efficiency

23. Absorption Efficiency
Depends upon detector’s
atomic #
density
size
thickness
Depends on beam spectrum

24. 2. Response Time
“Speed with which detector can detect an x ray event and recover to detect the next one”
Minimum time after detection of 1st event when detector can detect 2nd event
If time between events shorter than response time, second event may not be detected
Shorter response time better

25. 3. Dynamic Range
Ability to faithfully detect large range of intensities
“Ratio of largest signal to be measured to the precision of the smallest signal to be discriminated”
Typical dynamic range: 1,000,000:1
much better than film

26. 4. Stability “Steadiness” of detector system
Consistency of detector signal over time
The less stable, the more frequently calibration required

27. Detector Types 2 principles:
Convert x-ray into light ---electrical signal
Scintillation detector
Convert x-ray directly into electrical signal
Gas ionization detector

28. Scintillation Detectors
Crystal couple to photomultiplier tube
X ray falls on crystal ---light flashes (glow)
Light directed to PM
Light hits Photocathode in PM and releases electrons

29. Scintillation X-ray energy converted to light
Light converted to electrical signal
Photomultiplier Tube
X-Rays
Light
Electrical
Signal
Scintillation
Crystal

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

31. Gas Ionization Detector
Series of individual chambers separated by tungsten plates
X ray falls on each chamber– (+/- ions)
+ ions move to – plate, - ions to + plate
The migration produces electrical signal

32. Gas Ionization + - Ionization Chamber
X-rays converted directly to electrical signal
Filled with Air
Ionization
Chamber
X-Rays
Electrical
Signal + - - +

33. 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
thickness
X-Rays

34. Detector Array Slice by Slice – one arc of detector array
Volume – one arc of detector array, acquires volume of tissue then separated by computed to slice by slice

35. DAS Detector electronics Location: between detector and computer
Role of translator
Measure transmitted radiation beam
Encodes measurement to binary data
Transmits binary data to computer

36. Components of DAS Amplifier Log Amplifier
Analog to Digital Converter (digital data)
Digital Transmission to computer

37. Log Amplification
Transmission measurement data must be changed into attenuation and thickness data
Attenuation = log of transmission x thickness

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

39. DA and Sampling Radiation falling on detector
Each samples the beam intensity on it
Not enough samples = artifacts appear
To increase number of measurement/samples available for reconstruction and improve image quality

40. Improving Quality & Detection
Geometry
Smaller detectors
Closer packed detectors
Smaller patient-detector distance
Thinner slices
less patient variation over slice thickness distance