1. CT Physics
Lecture 3

2. Data Acquisition: Major Components
Gantry
Generator
Xray Tube
Collimator + Filter
Detector Array
DAS – Data Acquisition System
Patient table

3. 3rd Generation CT
Discussed generations of CT scanners – modern day scanner is 3rd generation with rotating x-ray tube and detectors.

4. 3rd Generation CT System

6. What do we mean by cross section?

7. Gantry Ring-shaped part of the CT scanner Houses Slip rings Generator
Cooling system
X-ray source
Filtration
Collimation
Detectors

8. Gantry Aperture (The Hole) Tilt capabilities Laser Commonly 70 cm
Large Bore – 80 – 90 cm (Toshiba Acquilon LB)
Tilt capabilities
Varies from system to system but usually between +/- 12 to +/- 30 degrees in 0.5 degree increments
Laser
Positioning ISOCENTER

9. Gantry X Y Z

10. Gantry
Important to recognize that especially in the imaging of a bariatric patient that despite a listed aperture size that it does not account for the table height for which has to advance into the bore

11. Couch Carbon Fiber Top Weight limit Strong & rigid Low absorption
Floats and Rests on pedestal
Vertical and Horizontal movement
Weight limit
Generally 450 lbs.
Philips Brilliance Large Bore = 650 lbs.

12. Couch Typical Maximum Scan Range
Not comparable to the length of the couch
Generally can cover approximately 162 cm of scan length
Most modern scanners do have a general increase in scan length

13. What is a Slip Ring?
Its what made helical scanning possible by providing continuous rotation without conventional cables
Enables the transmission of power and electrical signals from a stationary to a rotating structure.
Consists of conductive rings and brushes which facilitates the transfers
Allows for faster scan times and continuous acquisitions without cable worry

14. What is a Slip Ring?
Brushes enable transmission of power by sliding in and out of grooves on the stationary ring
2 Brush designs
Wire Brush – Conductive wire as contact
Composite Brush – Conductive material as contact

15. What is a Slip Ring?

16. What is a Slip Ring? High Voltage Slip Ring
AC power delivered to high voltage generator
Supplies Slip Ring which powers tube
GENERATOR DOES NOT ROTATE WITH TUBE
Low Voltage Slip Ring
More Common today
AC Power is transferred to slip rings by brushes
Provides power to high voltage (step up) transformer then to tube
POSITIONED TO ROTATE WITH TUBE

17. Generator Produce high voltage for the creation of x-ray photons
Modern scanners use High Frequency Generator
Parameters we can control:
kVp (80, 100, 120, 140)
mA ( )
Time (0.5 – 2 sec)
mA * time = mAs

18. Heat Capacity Modern CT units can accommodate 3 – 5 million HU
Heat Unit
Product of kVp, mA and seconds – the heat generated.
Eg 75kVp X 100 mA x 2 s = heat units
When heated to capacity machinery will automatically compensate
Decrease kV
Decrease mA
Decrease time
End result will result in imaging that is sub-optimal due to the increased presence of noise on the images

19. X-Ray Source Tube: Getter: Lead lined cast steel:
NOT PYREX GLASS
Lead lined to further contain potential leakage or off focus radiation
Greater cooling properties
More efficient isolation of high voltages
Getter:
Barium
Helps ensure vacuum by absorbing any air molecules potentially released by the target during operation

20. CT Unique Components Bow Tie Filter
Shaped in order to compensate for the attenuations within both the head and the body
Effectively hardens the beam and equalizes the amount of radiation reaching the image receptor

21. CT Unique Components Collimator 2 kinds Pre-Patient
Beam Collimation - Single Detector
Detector Collimation – Muti- Detector
Pre-Patient
Adjustable; influenced by focal spot size (Penumbra)
Ensures constant beam width at the detectors
Detector (Pre-detector)
Assist in shaping the beam
Remove any scattered radiation from reaching the detectors

22. Collimation Restricts the x-ray beam to a specific area
Reduces scatter radiation
Improves contrast resolution
Decreases patient dose

23. Collimation Specified in number of detector rows x detector width.
Eg. 16 x (20 mm)

24. Detectors
Purpose – capture attenuated radiation from the patient and convert it into an electrical signal
Signal then converted into digital data

25. Detector Characteristics
Efficiency
How well the detector receive attenuated photons
Capture Efficiency
Efficiency in which detectors can obtain attenuated photons
Influenced by detector size and distance between detectors
Absorption Efficiency
Number of photons absorbed by detectors
Atomic #; Density of material; Size and Thickness of the face

26. Detector Characteristics
Stability
Detector response
How often is calibration needed?
Response Time
How quickly the detector can detect a photon and how quick can it recover in order to record the next event
Dynamic Range
Ratio of the largest signal to measured to the precision of the smallest that is discriminated; …Accuracy…
Product of Capture Efficiency, Absorption Efficiency, and Conversion Efficiency
Afterglow
continuous luminescence after event

27. Types of Detectors 1.) Scintillation Detectors
Scintillation crystal coupled to a photodiode tube
X-Ray falls onto crystal which upon interaction creates photons of light
Light gets directed to the photomultiplier increasing the light
Light strikes photocathode which emits electrons
Electrons pass through dynodes which are arranged and maintained resulting in a signal

28. Scintillation Detector
Solid State Photodiode Scintillation Crystals (Solid State)
Photodiode – calcium tungstate and ceramic to which the crystals are bonded by fiber optics
Allows current flow when exposed to light
Current is proportional to the amount of light
Extremely fast response time
Conversion Efficiency = 99 %
Capture Efficiency = 99 %
Dynamic Range = 1,000,000 to 1

29. Types of Detectors 2.) Gas Ionization Chamber Think AEC…
Series of individual highly pressurized gas (Xenon) filled chambers which are separated by metal plates (Tungsten)
X-Ray strikes gas filled chambers
Gas becomes ionized
Plates are charged + and -, e- migrate to the positive plate, positive ions to the negative plate
As ions move produces a small signal depending on the number of photons

30. Gas Ionization Detectors
Pressure 30 Atm to increase probability of event
Excellent stability
Zero Afterglow
Detection efficiency less than scintillation detector
50 – 60 % compared to 95 – 100 %
No longer utilized…

31. Multi-slice Detectors
Dual Row
Introduced in 1992
Allowed for faster volume coverage than single slice
Dynamic Focal Spot – position of focal spot switched by a computer system during each scan in order to double sampling and measurements
Produced two contiguous slices with great resolution
con·tig·u·ous
adjective \kən-ˈti-gyə-wəs, -gyü-əs\ —used to describe things that touch each other or are immediately next to each other

32. Multi-slice Detectors
Multi-row(slice) Detectors
Purpose = increase volume coverage speed of both single and dual slice scanners
IE. 16 slice scanner will be 16 times faster than single slice
Commonly used for machines which acquire from 4 to 320 slices per rotation
Consists of multiple separate detector rows which can image simultaneously up to the maximum amount of slices per 360 degree rotation
16 slice scanner = 16 slices per rotation…Duh

33. ARRAY TYPES

34. Multi-slice Detectors Two Different Types
Matrix Array Detectors
AKA Fixed Array
All detector cells are equal in dimension along an array
Perfect Cubes one right after the other
Adaptive Array Detectors
Detector cells have different sizes along an array

36. Multi-slice Detectors
Number of slices is dependent on configuration used
Data collection “channels”
Configuration can use each individual detector to produce that size slice
Ie mm detector can produce mm slices
Or….can use combination to produce larger slices
Ie mm detector can produce mm slices or 4-5 mm slices

37. Multi-slice Detectors

38. DAS Data Acquisition System
Refers to all the electronic components which lie after the detectors but before the computer
Performs 3 functions
Measures attenuated radiation
Converts measurements to digital
Transmits digital data set to computer

39. Attenuation = log transmission * thickness…or
DAS
DAS Components:
Pre-amplifier – takes the weak electrical signal from the detectors and boosts it so that it can be more easily converted
Logarithmic amplifier – performs the conversion of attenuated transmission data to logarithmic data which is sent across to ADC
Conversion of transmission to attenuation and thickness
Attenuation = log transmission * thickness…or
µ1 + µ2 + µ3 = (ln I0 / 1) (I / x)
µ = linear attenuation coefficient
I0 = Original intensity
I = Transmitted intensity
x = Thickness of Object

41. DAS ADC Optoelectronics
Analog to Digital Converter – divides the electrical signals into multiple portions
The more portions the better the digital signal…now how do we get more portions again? Hmmmmmm…
Help determine the bit depth (grey scale) of image
Optoelectronics
Series of lens and light diodes which moves data transmissions from the ADC to the computer at tremendously high speed
50 million bits / sec.

42. Data Acquisition
In order to ensure that an adequate amount of measurements (samples) are obtained for optimal quality…
Utilize thin slices to reduce sample artifact…
Closely Packed Detector Array – closer they are, the more are available to receive radiation, the more samples obtained…
Quarter Shifted Detector Arc – provides 2 sets of data which can be individually reconstructed
Siemens developed

43. References Image from of Sprawls.com
Stewart Bushong “Radiologic Science for Technologists”
Bushberg et al., “The Essential Physics of Medical Imaging”
Wikipedia
Impact.org