CHAPTER-7 - QUALITY OF X-RAY BEAM

Ch 7. Quality of X-ray Beams
A Radiation Oncologist’s perspective
Dr Kanhu Charan Patro
Mr A C Prabu
Mr A Srinu 1

Faiz M. Khan, PhD
• Faiz M. Khan, PhD
• Professor Emeritus Department
of Radiation Oncology
• University of Minnesota
Medical School
• Minneapolis, Minnesota
2
John P. Gibbons,
PhD
Chief of Clinical Physics
Mary Bird Perkins Cancer
Center
Baton Rouge, Louisiana

Slow my flow
• Basic wordings
• Parameters of X-Ray beam quality
• Half value layer
• Peak voltage
• Mean energy
• Effective energy
• Energy spectrum
• Depth dose distribution
• Measurement of energy spectrum
• Specification of clinical beam quality
3

Define the term quality of X-Ray?
The penetrating ability of the radiation is
often described as the quality of the radiation
4

Specification of beam quality
The quality of clinical photon beam can be
usually described by penetrability of x ray
beam
1. HVL
2. kVp
3. MV
4. PDD
5. Ratio of depth doses under reference
conditions etc.
5

• Superficial orthovoltage X Ray
• HVL and kVp
• Gamma Rays
• The energy of nuclide origin
• Like Cobalt
• 1.33 Mev and 1.17Mev
• Avg. 1.25 Mev
• Megavoltage beam
• The kinetic energy of electron on target
• As X Ray beam is heterogeneous
• Maximum energy designated by MV[megavolts]
• As if the beam were generated by applying that voltage on X Ray tube
6
Specification of beam quality

Voltage
• Determines the speed of electrons that travel from cathode to
anode.
• When voltage is increased, the speed of the electrons is increased.
• When the speed of the electrons is increased, the electrons strike the
target with greater force and energy.
• Resulting in a penetrating x-ray beam with a short wavelength.
• Measured in volts or kilovolts (kV’s) This is adjusted according to
individual diagnostic needs of the patient.
15

Bremsstrahlung X-ray generation
• The X-rays produced by the
primary beam electrons
comprise mostly a large (almost
infinite) number of low-energy
X-rays.
• Beam hardening is the
phenomenon that occurs when
an x-ray beam comprised of
polychromatic energies passes
through an object, resulting in
selective attenuation of lower
energy photons.
16

X-ray Beam Quality
• The beam of radiation we use in dentistry may be described in two
ways:
• 1. Quality = penetrating ability - energy of the photon beam
• 2. Quantity = amount - # of photons in beam
20

Kilovoltage
• Kilovoltage is controlled by the kilovoltage peak dial, or kVp. (Only on
some machines)
• Determines the wavelength (quality) or speed and energy of the
wave.
• Density= overall darkness or blackness of an image.
• Adjustment in the kVp = a change in the density.
21

Kilovoltage
• Quality = penetrating ability
• kVp is the electrical force needed to move the electrons from the –
cathode to the +anode
• kVp setting is the only factor influencing the penetrating ability
(quality, power) of beam
22

- shorter wavelength, more penetrating- also more X Rays produced
Increased kVp
23

-longer wavelength, less penetrating, fewer X-Rays produced
Decreased kVp
24

Types of radiation
• Gamma Ray
• Cobalt
• X Ray
• KV
• 4,6,10,15 MV
26

Which is important?
• Quality
• Spectral distribution
• Quantity
• Fluence
27

Define it?
• An ideal way to describe the quality of an x-ray beam is to specify its
spectral distribution, that is, energy fluence in each energy interval
• However, spectral distributions are difficult to measure and,
furthermore, such a complete specification of the beam quality is not
necessary in most clinical situations.
• Since the biologic effects of x-rays are not very sensitive to the
quality of the beam, in radiotherapy one is interested primarily in the
penetration of the beam into the patient rather than its detailed
energy spectrum.
• Thus, a crude but simpler specification of the beam quality is often
used, namely the half-value layer
29

Describing radiation beam
• Quality
• Spectral distribution
• Quantity
• Fluence
• Measurements
• HVL
30

Quantity
• Fluence
• Fluence can be defined as the total number of particles (typically Gamma
Ray Photons) crossing over a sphere of unit cross section which surrounds a
Point Source of Ionizing Radiation. .
31

Penetration and spectral distribution?
32

• Because all x-ray beams produced by radiation generators are
heterogeneous in energy (i.e., possess continuous energy spectra
that depend on the
1. Peak voltage,
2. Target material
3. Beam filtration
Factors effecting energy spectra?
33

• Inherent filtration is the filtration or reduction in radiation energy
due to absorption by the material necessary to provide the vacuum,
the electrical insulation, and mechanical rigidity of the X-ray tube.
• This so-called inherent filtration is equivalent to approximately 1-mm
Al in most x-ray tubes
Inherent filtration?
34

• Crude but simpler specification of the beam quality is often
used, namely the half-value layer-HVL
• Single parameter specifying the overall penetrating ability of
the beam
Specification of beam quality?
35

• In the case of low-energy x-ray beams (below megavoltage
range), it is customary to describe the quality in terms of
HVL together with kVp, although HVL alone is adequate for
most clinical applications.
• On the other hand, in the megavoltage x-ray range, the
quality is specified by the peak energy and rarely by the
HVL.
• The reason for this convention is that in the megavoltage
range the beam is so heavily filtered through the
transmission-type target and the flattening filter that any
additional filtration does not significantly alter the beam
quality or its HVL
Describing beam quality?
36

Before Marriage - you are choosing[filtering]
• Social status
• Education
• Profession
• In laws
• Etc.
38

After Marriage - you are blocking
• Social status
• Education
• Profession
• In laws
• Etc.
39

40
1. Half-value Layer
2. Filters
3. Measurement of Beam Quality Parameters
4. Measurement of Megavoltage Beam Energy
5. Measurement of Energy Spectrum

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
41

HVL-Half value layer
• 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
• HVL of x-ray beam depends upon energy spectrum which is a function of
primarily peak voltage and the filtration [inherent and added]
• The HVL is related to the linear attenuation coefficient by following
equation
• HVL= 0.693/μ
42

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
43

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
44

HVL
45
•HVL ↑
•Filter thickness ↑
•Beam “harder” ↑
•Choice of the filter
•Suitable HVL
•Acceptable beam output

HVL
• HVL- l/l0=1/2
• HVL must be measured under narrow beam and good geometry
• Subsequent HVL are more than before
• It explains about overall penetrating power of beam
• Given by 0.693/μ
• As filter thickness increases the average energy of the transmitted beam
increases or the beam becomes increasingly harder
• By increasing the filtration in such an X-Ray beam one increases the
penetrating power or the HVL of beam
47

48
1. Half-value Layer
2. Filters

Defining the filters
• An X-ray filter is a material placed in front of an X-ray source in order to reduce the
intensity of particular wavelengths from its spectrum and selectively alter the distribution
of X-ray wavelengths within a given beam.
• For medical purposes, X-ray filters are used to selectively attenuate, or block out, low-
energy rays during x-ray imaging (radiography). Low energy x-rays (less than 30 keV)
contribute little to the resultant image as they are heavily absorbed by the patient's soft
tissues (particularly the skin).
• Thus, it is favorable to remove these low energy X-rays from the incident light beam. X-ray
filtration may be inherent due to the X-ray tube and housing material itself or added from
additional sheets of filter material.
• The minimum filtration used is usually 2.5 mm aluminium (Al) equivalent, although there is
an increasing trend to use greater filtration. Manufacturers of
modern fluoroscopy equipment utilize a system of adding a variable thickness of copper
(Cu) filtration according to patient thickness. This typically ranges from 0.1 to 0.9 mm Cu.
49

Various filters in practice
1. Molybdenum - Used in Mammography
2. Rhodium - Used in Mammography with Rhodium anodes
3. Aluminium - Used in general radiography x-ray tubes
4. Copper - Used in general radiography - especially
in pediatric applications.
5. Silver - Used in Mammography with tungsten anode
6. Tantalum - Used in fluoroscopy applications with tungsten anodes
7. Niobium - Used in radiography and dental radiography with
tungsten anodes
8. Erbium - Used in radiography with tungsten anodes
50

Schematic graph showing changes in spectral distribution of
200 kVp x-ray beam with various filters
51
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)
(30 – 70 keV)
K edge of Cu → 9 keV
1. Bremsstrahlung pinching is due to the atomic mass.
2. The denser the atom, the higher the X-Ray Absorption.
3. Only the higher energy X-Rays pass through the filter,
appearing as if the Bremsstrahlung continuum had been
pinched

52
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
53
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

Application of filter in clinical radiation machine
54
Machine Filter
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 1. Inherent filtration of the
transmission target
2. Flattening filter
(Primary purpose: make beam intensity
uniform in cross-section)

The flattening flatter
• The bremsstrahlung generated
when high energy electron
strikes the target is forwardly
peaked.
• To make the beam intensity
uniform across the field, a
flattening filter is inserted in the
beam.
• The filter is usually made of
lead, although tungsten,
uranium, steel, aluminum or a
combination.
55

Bow-tie filter in CBCT
• Varian uses two custom-designed aluminium filters called “Bow-tie”
that equalize x-ray intensity laterally across the detector for two
different modes of acquisition.
• In Elekta, the kV panel can be positioned at three different field of
view (FOV) positions, namely S (small FOV), M (medium FOV) and
L (Large FOV).
• The bow-tie filter (F1) is inserted between the source and the patient
to reduce intensity variations across the detector
56

59
1. Half-value Layer
2. Filters
3. Measurement of Beam Quality
Parameters

The effective energy
• Because x-ray beams used in radiology are heterogeneous in energy,
it is convenient sometimes to express the quality of an x-ray beam in
terms of the effective energy.
• The heavily filtered X-Ray beam effective or average energy
approximately one third of peak energy
• A 6 MV of linear accelerator of an average energy is 2Mev
60

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
61

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
• Attenuation Method.
• Penetrometer
62

64
1. Half-value Layer
2. Filters
4. Measurement of Megavoltage Beam
Energy

65
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)

66
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 activities → PAR → peak photon energy
• More sensitive method than the conventional method
measuring HVL in water

67
1. Half-value Layer
2. Filters

1. An apparatus used for recording and measuring spectra,
especially as a method of analysis.
2. A spectrometer is any instrument used to probe a property of light as a
function of its portion of the electromagnetic spectrum, typically its
wavelength, frequency, or energy.
3. The property being measured is usually intensity of light, but other
variables like polarization can also be measured.
Spectrometer and spectrometry
68

Scintillation
• Scintillation is the process in which the energy from a certain
radiation interacting with a volume of sensitive material (called
a scintillator) is converted into electromagnetic waves.
• The frequency of the emitted electromagnetic waves is within or
near the visible spectrum
69

71
Measurement of Energy Spectrum
• Scintillation spectrometer
Fig 7.6. Energy spectrum of an x-ray beam determined by scintillation spectrometer

73
Sort out electronically different-size pulses by multichannel pulse
height analyzer
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
Measurement of Energy Spectrum

Summary
• Quality of x-ray beams is specified by kVp, filtration, and HVL (for diagnostic,
superficial, and orthovoltage beams); and MV and percent depth dose in water
(for megavoltage x-rays).
• Quality of cobalt-60 beams is designated simply as cobalt-60 because it is known
that they have the same energy, namely g rays of 1.17 and 1.33 MeV.
• Peak voltage (kVp) applied to an x-ray generator can be measured directly (e.g.,
voltage divider, sphere-gap method) or indirectly (e.g., fluorescence,
attenuation, or a penetrameter device such as an Adrian-Crooks cassette).
• Peak energy (MV) of a megavoltage x-ray beam can be measured directly by
scintillation spectrometry or by photoactivation of appropriate foils (e.g., PAR
method).
• Effective or equivalent energy of an x-ray beam is the energy of a monoenergetic
photon beam that has the same HVL as the given beam.
• Energy spectrum of an x-ray beam can be measured by scintillation
spectrometry.
74

The following table used in problem 1 to 3
describes the measured beam intensity as a
function of aluminum filter thickness:

1. What is the first HVL for this beam ?
a) 1.7mm Cu
b) 2.0mm Al
c) 2.7mm Al
d) 0.5mm Al
e) 0.5mm Cu

2. What is the attenuation coefficient m for this beam in aluminum?
a) 0.26mm-1
b) 0.26mm-1
c) 0.41mm-1
d)1.4mm-1
e) 2.0mm-1

3. The homogenecity coefficient is defined as the first HVL divided by the
second . What is the homogenecity coefficient for this beam?
a) 0.15
b) 0.18
c) 1
d) 5.6
e) 6.5

a) Unfiltered X ray beam
b) Filtered X ray beam
c) Monoenergetic beam
d) All three will have the same HVL
e) Not enough information is given
4.Suppose there were three photon beam with the same peak
energy ; one from an X ray machine without filtration, one from
an Xray machine with 2.0mm Al filtration, and one monoenergetic.
Which beam would have the largest HVL?

a) 0.029cm-1
b) 0.039cm-1
c) 0.049cm-1
d) 0.059cm-1
e) 0.069cm-1
• 5.The beam quality for megavoltage photon beams is sometimes specified in
terms of their ionization ratio, or the ratio of the doses at depth of 20 to 10 cm
for a fixed source detector distance and meter setting . If a nominal 6MV
photon beam has an ionization ratio of 0.68, what is the approximate
attenuation coefficient of this beam , neglecting scatter radiation?

6.HVL should be measured under good
geometry conditions that require:
a) A narrow beam with negligible scatter
b) Abroad beam with full scatter
c) Detector positioned far away from the absorber to avoid scatter
d) Chamber imbedded in a phantom at a sufficient depth to provide
full build-up matter

7.A Thoreaus filter for orthovoltage beams
must be inserted with :
a) Aluminium filter facing the patient
b)Tin filter facing the patient
c) Copper filter facing the patient
d)Lead filter facing the X-ray tube

8.The following instrument(s) can be used to
measure kVp directly if the high tension leads of
the x-ray tube are accessible:
a) Penetrameter
b) Voltage divider
c) Sphere-gap apparatus
d) Wisconsin test cassette

9.HLV measured for a 6 MV beam turned out
to be13.3 mm Pb .For this beam:
a) The mass attenuation coefficient is 4.59 x 10-3 m2 kg-1 in lead
b) The mass energy absorption coefficient in lead is the same as in
water
c) The mass attenuation coefficient is greater than the mass energy
absorption coefficient.
d) The effective energy is approx. 2 Mev.

10.The peak photon energy in a megavoltage
beam can be determined by:
a) Measuring HLV layer in lead
b) Comparing depth dose distribution with published data
c) Measuring MVp using a voltage divider
d) Measuring photoactivation ratio

CHAPTER-7 - QUALITY OF X-RAY BEAM

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