7.1-7.3: Half-Value Layer, Filters, & Measurement of Beam Quality Parameters
- The quality of a radiation beam refers to its penetrating ability and is ideally specified by its spectral distribution.
- A spectral distribution displays the frequency of photon energies along a cross section (energy fluence), e.g., the number of photons that have an energy ‘x’, ‘y’, and ‘z’.
- A simpler method of specifying the beam quality is by the half-value layer (HVL), and it is the thickness of an absorber required to attenuate the initial intensity of the incident beam by half.
- As x-ray beams possess a continuous energy spectra (dependent on the kVp, target material, and beam filtration), it is more practical to specify the beam by a single parameter that describes the overall penetrating ability of the beam.
- Attenuation by the components that make up the x-ray tube is referred to as inherent filtration (~ 1 mm Al) and involves the surrounding the glass envelope, the surrounding oil, and the exit window of the tube housing.
- K-edge absorption spike seen on a spectral distribution for a tungsten target can be reduced by using a combination filter.
- By combining materials of differing K-edge absorption (tin = 29 keV; copper = 9 keV), the K-edge absorption spike for tungsten which occurs at 69 MeV can be mitigated.
- The beam will also become more penetrative, but the overall output of the beam will decrease.
- A Thoraeus filter is an example of a combination filter which is made up of tin, copper, and aluminum.
- A y-ray beam, such as a Co-60, can be specified directly in terms of its energy because the energy emitted by the source is, for the most part, homogeneous (Co-60: 1.17 MeV & 1.33 MeV, or 1.25 MeV).
- As x-ray beams possess a continuous energy spectra (dependent on the kVp, target material, and beam filtration), it is more practical to specify the beam by a single parameter that describes the overall penetrating ability of the beam.
- Usage of the HVL is primarily seen in specifying the quality of kilovoltage (kV) beams, but not for megavoltage (MV) beams.
- MV beams are hardened due to filtration by the transmission target and flattening filter, and thus, using further filtration will not significantly alter the beam intensity.
- MV beams are instead specified by average energy which is approximately one-third of the peak energy.
- The HVL of a beam is dependent on the linear attenuation coefficient (μ) which will vary from material to material, and can be determined by the equation: HVL = 0.693 / μ.
- The HVL of a beam is measured using an absorber of varying thickness and a detector. The absorber is placed at a distance far enough away from the source to obtain a narrow beam. The detector is placed far enough behind the absorber to measure only the transmitted photons, and not the scattered photons and leakage radiation from the x-ray generator.
- The detector measurements at varying absorber thicknesses are plotted onto a semilogarithmic graph to determine the HVL.
- For a polyenergetic beams, the subsequent HVLs will increase in thickness because the low-energy component of the energy spectrum are preferentially filtered resulting in a more penetrating beam.
- Thus, the slope of the attenuation curve decreases with increasing the number of HVLs.
- If the absorber thickness becomes too thick, a beam may become softened by Compton scattering taking place within the absorber.
- Increasing the filtration will reduce the beam output, and so a balance must be had between suitable filtration and an acceptable beam output.
- For a polyenergetic beams, the subsequent HVLs will increase in thickness because the low-energy component of the energy spectrum are preferentially filtered resulting in a more penetrating beam.
- The kilovoltage peak (kVp) is commonly used alongside the HVL to specify kilvoltage beams. The x-ray tube potential can be determined by direct or indirect methods.
- Direct methods involve making measurements via accessing the high-tension circuits of the x-ray equipment which are normally sealed.
- Using voltage dividers is a direct method in which a resistance tower, i.e., a circuit in which several high resistances are connected in series, is placed across the high-tension leads.
- The total voltage is then divided among the separate resistances that form the resistance tower.
- By the equation V = I * R, the effective voltage across any two resistances can be determined by multiplying the effective current through the resistance tower and the resistance between the two points.
- The sphere-gap is another direct method where each high-voltage lead of the x-ray tube is connected to a metallic sphere by a cable adapter.
- These spheres are held via stands, and the distance between the spheres is reduced until an electric spark passes between them.
- This critical distance allows for the calculation of the peak voltage across the x-ray tube.
- Using voltage dividers is a direct method in which a resistance tower, i.e., a circuit in which several high resistances are connected in series, is placed across the high-tension leads.
- The fluorescence method is an indirect method that is based upon two principles:
- First, the maximum photon energy (hv_max) is numerically to the peak potential (kVp), and second, that the K-edge absorption in characteristic x-ray production for a given shell occurs when the photon energy is just equal to or greater than the K-shell electron binding energy.
- Two ionization chambers are used with one placed directly behind the attenuator, and the other is placed lateral (90 degrees) to the attenuator.
- The attenuator is also angled by 45 degrees such that the attenuator surface faces lateral ion chamber.
- The chamber placed behind the attenuator measures the transmitted x-rays while the lateral chamber measures the scattered and fluoresecent (characteristic) radiation.
- The transmitted and scattered radiation inherently increases with an increase in tube voltage, but when the tube voltage is raised above the K-edge, there is a sudden spike in absorption such that the transmitted radiation will decrease and the secondary radiation will increase due to characteristic x-rays.
- When the lateral ion chamber records an increased measurement due to the K-edge absorption, the kVp can be concluded to equal the K-shell binding energy.
- Using K-absorption edges of varying attenuators will allow for the calibration of the kVp dial on the machine.
- Direct methods involve making measurements via accessing the high-tension circuits of the x-ray equipment which are normally sealed.
- A polyenergetic beam can be expressed as an effective energy, i.e., as a monoenergetic beam that possesses the same attenuation rate as the polyenergetic beam.
- The effective energy can be found by finding a monoenergetic beam with the same linear attenuation coefficient (μ), or, as μ is related to the HVL, the HVL as that of the polyenergetic beam.
7.4: Measurement of Megavoltage Beam Energy
- A megavoltage x-ray beam can be specified by a single energy parameter if the electron beam energy prior to its incidence on the target is determined.
- A more practical method to specifying a beam is to measuring beam data (PDD, TMR, and TAR) and to compare them with published data, but this method serves as an approximation as depth dose distributions are relatively insensitive to small changes in the peak energy.
- The x-ray beam spectral quality can be sensitively monitored by the photoactivation ratio (PAR) which involves a thin target bremsstrahlung spectra, scintillation spectrometery, and photoactivation.
- A pair of foils (one foil being more sensitive to high energies than the other) are irradiated and become activated by the photodisintegration process.
- Photodisintegration is a process by which an atomic nucleus absorbs a high-energy photon and will resultantly enter into an excited state to then immediately decay by emitting subatomic particles.
- The induced radioactivity in the foils is measured using a scintillation counter, and the ratio of induced activities gives the PAR, which can be related to the peak photon energy.
7.5: Measurement of Energy Spectrum
- The HVL specifies a therapeutic beam as an approximation and cannot be used with radiation detectors (e.g., film, diodes, and types of ion chambers) that will show a large variation to different photon energies.
- The spectral distribution of photons is what actually characterizes beam, and can be determined experimentally by scintillation spectrometry.
- The scintillation spectrometer is comprised of a crystal and a photomultiplier tube that is attached to the crystal.
- A photon beam that is incident on the crystal will produce ionization and excitations.
- Along the ejected electron tracks, photons in the optical and ultraviolet will arise.
- When these light photons strike the photocathode of the photonmultiplier tube, low-energy photoelectrons are ejected.
- A photon beam that is incident on the crystal will produce ionization and excitations.
- These photoelectrons are then collected and multiplied by about a million times by the photomultiplier dynodes, resulting in an output pulse proportional to the energy of the original x-ray photon entering the crystal.
- The number of photons with a particular energy can be determined by a multichannel pulse height analyzer.
- Each channel corresponds to a particular input photon energy, and the different-size pulses can be sorted to their respective channels.
- The spectrum is displayed in photons per unit energy as a function of photon energy.
- The number of photons with a particular energy can be determined by a multichannel pulse height analyzer.
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