Ch 7. Quality of X-ray Beams The physics of Radiation Therapy, pp. 45 - 70 Ch 7. Quality of X-ray Beams Leung Chung Man

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

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

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

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

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

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.

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

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 + 0.25 mm Cu + 1 mm Al Thoraeus II 0.4 mm Sn + 0.25 mm Cu + 1 mm Al Thoraeus III 0.6 mm Sn + 0.25 mm Cu + 1 mm Al

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)

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

Measurement of Half-Value Layer Correlation to the linear attenuation coefficient (μ) HVL = 0.693 μ Attenuation data are obtained by measuring transmitted exposure through absorbers of varying thickness but constant composition

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

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

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

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

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.

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

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

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

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

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 = 0.693 μ Effective Energy

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

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

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)

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

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

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

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