1. Ch 7. Quality of X-ray Beams
The physics of Radiation Therapy, pp
Ch 7. Quality of X-ray Beams
Leung Chung Man

2. Introduction Ideal way to describe radiation quality
Specify the spectral distribution of x-ray beam
e.g. energy fluence in each energy interval
Difficult to measure
Not necessary in most clinical situations
HVL is crude but simpler way
More interest in penetration of the beam into the patient
The penetrating ability of radiation is often described as the quality of the radiation

3. Half-value Layer Filters Measurement of Beam Quality Parameters
Measurement of Megavoltage Beam Energy
Measurement of Energy Spectrum

4. Half-value Layer Definition
Thickness of an absorber of specified composition required to attenuate the intensity of the beam to half its original value
Although HVL can described the quality of all beams, it usually used in x-ray beams produced by radiation generators
γray beam is usually stated in terms of the energy
A known emission spectrum
e.g. 60Co → 1.17 and 1.33 MeV (average 1.25 MeV)
Cobalt-60 beam
X-ray beams are usually describe by HVL
Heterogeneous in energy

5. Half-value Layer Low-energy x-ray beams (< MV)
HVL + kVp
Megavoltage x-ray range
The quality is specified by the peak energy and rarely by HVL
The beam is so heavily filtered through the transmission type target and flattening filter
Additional filtration do not significantly alter the beam quality or its HVL
The average energy = 1/3 of the peak energy

6. Filters Half-value Layer Measurement of Beam Quality Parameters
Measurement of Megavoltage Beam Energy
Measurement of Energy Spectrum

7. Energy of x-ray beam
200 kVp
Filters
Curve A → Al
Curve B → Sn + Al
Curve C → Sn + Cu + Al
(58 – 69 keV)
(29.2 keV)
K edge of Cu → 9 keV
(30 – 70 keV)
Fig 7.1. Schematic graph showing changes in spectral distribution of 200 kVp x-ray beam with various filters.

8. Filters The character of the energy spectrum of x-ray beam
The effect of x-ray beam filtered by 1-mm-thick aluminum filter
The effect of so-called inherent filtration
The effects of attenuation
The glass envelope of the x-ray tube
The surrounding oil
The exit window of the tube housing
Equal to about 1-mm Al
K characteristic x-rays produced in the tungsten target
58 and 69 keV
K characteristic x-rays produced in the tin target
29.2 keV

9. Filters Combination filters Containing plates of Sn, Cu, Al
Increase HVL of the orthovoltage beams
Without reducing the beam intensity
Thoraeus filters
Arrange in proper order
High Z material nearest the x-ray target
Table7.1. Thoraeus Filter Used with Orthovoltage X-Rays
Filter
Composition
Thoraeus I
0.2 mm Sn mm Cu + 1 mm Al
Thoraeus II
0.4 mm Sn mm Cu + 1 mm Al
Thoraeus III
0.6 mm Sn mm Cu + 1 mm Al

10. Filters Application of filter in clinical radiation machine Machine
Diagnostic & superficial x-ray energy range
Primary aluminum filter (mmAl)
Orthovoltage range
Combination filter (range: 1-4 mm Cu)
Cesium & Cobalt teletherapy machines
No filter (Monoenergetic)
Megavoltage x-ray beam
Inherent filtration of the transmission target
Flattening filter
(Primary purpose: make beam intensity uniform in cross-section)

11. Measurement of Beam Quality Parameters
Half-value Layer
Filters
Measurement of Beam Quality Parameters
Measurement of Megavoltage Beam Energy
Measurement of Energy Spectrum

12. Measurement of Half-Value Layer
Correlation to the linear attenuation coefficient (μ)
HVL = μ
Attenuation data are obtained by measuring transmitted exposure through absorbers of varying thickness but constant composition

13. HVL ↑
Filter thickness ↑
Beam “harder” ↑
Choice of the filter
Suitable HVL
Acceptable beam output

14. Measurement of Peak Voltage
Two parameters give an appropriate specification of radiation quality in superficial and orthovoltage range
HVL
Tube potential (kVp)

15. Direct measurement of Peak Voltage
Voltage Divider
Several high resistances are connected in series to form a resistance tower
Placed across the high tension leads
V = I x R
Sphere-Gap Method
High voltage lead of the x-ray tube is connected to a polished metallic sphere by a cable adaptor
The distance between the two sphere is reduced until an electric spark passes between them
Calculation of peak voltage
Critical distance
Correction for air density and humidity

16. Indirect Measurement of Peak Voltage
Fluorescence Method
Two principles
Peak photon energy is given by peak potential (hvmax in keV = kVp)
K edge absorption occurs when the photon energy ≧ binding energy of the K shell electron
Using materials of several different K absorption edges , one can calibrated the kVp dial on the machine

17. Absorbs most of scattered radiation from attenuator
measures characteristics as well as scattered x-rays.
Absorbs most of scattered radiation from attenuator
measures radiation transmitted through the attenuator
Fig 7.2. Experimental arrangement for measuring tube voltage by K fluorescence method.

18. Tube voltage (kVp) = K edge threshold of the attenuator (keV)
Fig 7.3. Plot of the transmitted (chamber 1 reading) to scattered radiation (chamber 2 reading) as a function of tube kilovoltage

19. Indirect Measurement of Peak Voltage
Attenuation Method
Principle
Observation of X-ray beam at high filtration
Slope of the transmission curve depends on the peak kilovoltage
Apparatus (detector)
Ion chamber with 2 caps of Cu / Al of different thickness
Calibration : Determining the ratio of the detector response as a function of kVp of x-ray beams (accurately known peak potentials)
An unknown kVp of an x-ray beam can be estimated from the calibration curve
Limitation
Accuracy
Depends strongly on the wave form of the x-ray tube potential

20. Indirect Measurement of Peak Voltage
Penetrameter (typical)
Principle
Comparing transmission through 2 materials with x-ray absorptions that change differently with photon energy
Structure
Polyethylene rectangular central reference block
Identical metal step wedges on both sides
Aluminum wedge → low kV range
Brass wedge → high kV range
Measuring Process
Radiographed in beam with heavy filtration and scattering shielding
Obtain the “matching step position” by using optical density ratios of adjacent wedge and reference areas
Calibrate and measuring unknown kV

21. Indirect Measurement of Peak Voltage
Penetrameter (Ardran-Crooks cassette)
Commercial version → Wisconsin Test Cassette
Consist of film which is partly cover with
Slow intensifying screen
Uncover for reference
Fast intensifying screen
Superimposed with copper step system
Matching step density with the reference depends on the kV
Calibration for measuring unknown peak voltage

22. Measurement of Effective Energy
Heterogenous x-rays beam is convenient to express the quality in terms of the effective energy
Definition of effective energy of an x-ray beam
The energy of photons in a monoenergetic beam which is attenuated at the same rate (having the same HVL) as the radiation in question
HVL = μ
Effective Energy

23. Measurement of Effective Energy
Effective energy (keV)
Fig 7.4. Plot of effective energy as a function of half-value layer. Data calculated from the attenuation coefficients of monoenergetic photon beams

24. Measurement of Megavoltage Beam Energy
Half-value Layer
Filters
Measurement of Beam Quality Parameters
Measurement of Megavoltage Beam Energy
Measurement of Energy Spectrum

25. Measurement of Megavoltage Beam Energy
Measure the complete spectrum of a MV x-rays beam
Calculation of thin target bremsstrahlung spectra
Scintillation spectrometry
Photoactivation
Practical method of determining the MV beam energy by measuring
Percent depth dose distribution
Tissue-air ratios
Tissue-maximum ratios
Comparing the data with the published data (e.g. Hospital Physictist’s Association)

26. Measurement of Megavoltage Beam Energy
Photoactivation ratio (PAR) method
Sensitive method of monitoring x-ray beam spectral quality
Process
Irradiating a pair of foils
Activated by the photodisintegration process
Use scintillation counter to measure the induced radioactivity in the foil
Ratio of induced actitvities → PAR → peak photon energy
More sensitive method than the conventional method measuring HVL in water

27. Measurement of Energy Spectrum
Half-value Layer
Filters
Measurement of Beam Quality Parameters
Measurement of Megavoltage Beam Energy
Measurement of Energy Spectrum

28. Measurement of Energy Spectrum
Scintillation spectrometer
Fig 7.6. Energy spectrum of an x-ray beam determined by scintillation spectrometer

29. Measurement of Energy Spectrum
e- ejected in the crystal
Produce ionization and excitation of crystal atoms
Produce light photon
Striking the photosensitive surface of photomultiplier tube
Eject low-energy photoelectrons
Collected and multiplied by photomultiplier dynodes
Sort out electronically different-size pulses by multichannel pulse height analyzer