Фото: Теории и практики
Автор: Академик, профессор, д.т.н Юрий Дехтярь — директор института биомедицинской инженерии и нанотехнологии АН Латвии.
Соавторы: Mohamad Zakaria, Vladimir Noskov
Riga Technical University | RTU
Institute of Biomedical Engineering and Nanotechnologies
The purpose of this study is to describe the spectrometer and the technology that is used to investigate the influence of X-ray radiation on exo-electron emission (EEE) on bone specimens. This paper addresses an approach to detect radiation-induced modifications in bone structure using method of exo-electron emission (EEE). The bone specimens were irradiated with high energy X-rays having different doses, dose rates and photon energies for that purpose. Prior to that, factors such as multiple heating of bone specimens, bone specimen storage time were studied as these factors may influence the accuracy and validity of the EEE measurement results from the bone specimens.
Использование экзоэлектронного эмиссионного спектрометра для изучения структурных изменений в кости из-за воздействия ионизирующего излучения.
Use of the exo-electron emission spectrometer to explore structural alterations in bone due to ionising radiation exposure. Available from: https://www.researchgate.net/publication/282010712_Use_of_the_exo-electron_emission_spectrometer_to_explore_structural_alterations_in_bone_due_to_ionising_radiation_exposure [accessed Jan 25 2018].
Key words: biomedical; radiation; dosimetry; X-ray; bones; exo-electron emission; technology; nano.
Technologies used in radiation therapy are among the very developing areas of biomedical instrumentation. The new fully automated machines deliver radiation to the right target volume and at the right time depending on organ motion. However, to reach the best radiation therapy treatment, an accurate dosimetry of the irradiated target should be provided.
A signal characterising an absorbed dose to bone could be measured from both the entire volume of the dosimeter stuff and from its surface layer. The surface plays the specific role because the cells in a human organism are adhered by the bone surface to supply biochemical reactions for the bone’s life. As the result of radiation therapy, the cancer cells are expected to be killed. However, when the bone is positioned close to an irradiated organ the radiation therapy aims to provide as low as possible dose for the bones. Ionizing radiation which is usually used during radiation therapy damages molecular/atomic bonds of the tissues.
According to Dekhtyar (2000) and Zakaria (2012), exoelectron emission (EEE) is among the sensitive methods for detecting structural imperfections on the surfaces of materials. The EEE technique has a high sensitivity to radiation induced structural imperfections and gives a possibility to characterize a thin (~100Å) surface layer of the solid (Sakurai et al, 1999; Manesh, 1985; Dekhtyar, 1987). Exoelectron emission can be used to analyse radiation damage in silicon semiconductors, carbon reinforced plastics and natural bone composites (Sakurai et al, 1999). In all these cases, the amount of emitted charge is directly proportional to the number of radiation induced imperfections and has a wave function dependence on the radiation exposure (Manesh, 1985).
Exoelectron dosimetry shows the influence of ionic crystals after mechanical treatment and exposure to heat or light. Electron emission from the surface may be spontaneous or require thermal or optical stimulation. Using this method, emission maxima were observed at different temperatures (Katashev, 1998; Zakaria, 2007). As the sample is heated, these electrons are emitted from the surface with a wide spectrum of low energies, overcoming the barrier potential of the surface and any positive charges created there by ionizing radiation (Katashev, 1995; Zakaria, 2007). The emission as a function of temperature is the ‘glow curve’. Alternatively, electron emission from a surface can be stimulated by light, known as Photo Stimulated Exo-electron Emission (PSEE). If both light and heat are used to stimulate electron emission it is called Photo Thermo-stimulated Exo-electron emission (PTSE) (Katashev, 1998).
The characteristics of this process are:
1) The instability of the radiation induced defects in the surface with time even while the external conditions are maintained constant;
2) The small size of the registered currents (approximately 10-15 to 10-19 Ao);
3) The insignificant energy of the electrons (0.01 up to 6.0 eV);
4) The thinness of the emitting layer (100 nm to 104 nm)
Despite the dependence of EEE on the above factors, it represents a useful tool for investigating a wide range of relaxation processes in the surface layers of solids, including biological materials, such as strain relaxation, oxidation, annealing of point defects, re-coupling of broken bonds, and phase transformations. The mechanisms of PTSE have been presented in more detail (Dekhtyar, 1987; Katashev, 1998).
Free charges created by ionizing radiation are trapped in defects or impurities and become liberated by thermal or photo stimulation, whereupon they can return to their ground state or if located near the surface of the material may be emitted from it (Dekhtyar, 1987; Bolotov et al, 1980; Zakaria et al, 2008; Bogucharska, 2002). The exo-electron emission current (I) depends on the stimulating photon energy in accordance with the equation for the photoelectric effect near the threshold energy:
I α ( hν — φ )m…………….(1)
hν is the photon energy;
φ is the photoelectric work function;
m is a power index;
For thermo stimulated emission, the exo-electron current (I) depends on the rate of decrease of the concentration of imperfections (C) with temperature (T):
I α — dC/dT…………………………………(2)
dC/dT = -A[C(T)]n.exp(-E/kT)…………….(3)
A is a pre-exponential multiplier;
n is the order of the relaxation reaction;
E is the activation energy;
k is the Boltzman constant;
This model predicts that the PTSE current is directly proportional to the rate of annealing of structural imperfections and the total emitted charge (Q) is directly proportional to the defects concentration at T0, the initial temperature at the start of annealing (Fig. 1). The information received at its registration is represented usually as the exoemission intensity I dependence of temperature of the sample Т linearly varied in time.
The properties of the studied surface are usually expressed in terms of Tmax, the temperature at which the maximum emission occurs, E, the energy of activation of PTSE, Imax, the intensity of the maximum and I0, the emission current registered in the absence of heating of the sample. Q is the area under the glow curve (Katashev, 1998; Bogucharska, 2002; Zakaria et al., 2008). Since the temperature is varied linearly with time, the intensity (I) of the Exo-emission current is expressed as a function of temperature, I= f(T). In other words, the intensity of Exo-emission reflects the rate of decrease of the concentration of emission centres as the temperature is increased. If the initial and final temperatures are T0 and Tf,
The exo-electron emission current from a sample was measured before and after irradiation and the corresponding glow curves obtained, denoted by Qo and Qirr respectively. The difference between them is directly proportion to the concentration of defects induced by the radiation.
ΔQ = Qirr – Qo ………….(5)
Megavoltage radiation delivered during radiotherapy causes structural imperfections in bone. Since bone has similar properties to semiconductor (Bonch-Bruevich & Klashnikov, 1977), these imperfections are often electrically active and the resulting electric fields can affect the migration of osteoclasts and osteoblasts near the bone surface. Therefore it is important to investigate the radiation — induced imperfections in the surface of bones.
Radiation used to treat carcinogenic diseases has the energy of photons varying from 1-20 MeV. These photons do not kill only the cancer cells but also have effects on the structures of bones. Such structure changes also alter electron density of states that may influence, with time, biomedical reactions affecting bone life condition. As a result, biochemical reactions between the bone and the surrounding cells targeted to remodel tissue could be influenced (Eriksen & Kassem, 1992). The bone surface, where the cells adhere to supply biochemical has the greatest impact (Schakenrad, 1996; Demster, 2000).
The purpose of this study is to describe the development of EEE measurement technique and its use in experiments to investigate the influence of X-ray irradiation on bones. It also examines the possibility to measure EEE from the bone material which is irradiated in the real radiation therapy conditions and modes.
2. Materials and Method
2.1 The preparation of bone specimens
Bovine bone has a similar structure to human bone (Katz, 1995). Samples of bovine tibia from a freshly slaughtered 18 month old cow were cleaned of adjacent soft tissue and marrow and cut perpendicular to the long axis to provide specimens of thickness 0.2 cm, surface area of 0.36 cm¬¬2 and volume 0.72 cm3, with a density of 1.5-1.7 g/ cm3 (Fig.2). The bone samples were dried to avoid any influence of the humidity of the bones on exoelectron emission.
2.2 The exo-electron spectrometer
The equipment was developed “in-house” at the laboratory of Institute of Biomedical Engineering and Nanotechnology of Riga Technical University in Latvia. The Exo-electron spectrometer comprises an evacuated chamber enclosing a mounting table for the bone specimens with a window to admit light from the photo-stimulator, vacuum tight connections for the thermo-stimulator and a registration system for emitted electrons (Fig. 3).
A mechanical rotary pump and a diffusion pump reduced the air pressure in the chamber to a minimum of 10-4 Pa, which was monitored by an ionization gauge and meter, taking less than 15 minutes to reach the minimum pressure. The bone samples were pinned to the surface of an annealed copper block fixed on a hollow steel tube, inside which the thermo-stimulating lamp was located. The table was orientated at an angle of 45° relative to the incident photo stimulation light beam.
Photo-stimulation of the bone specimen was provided by a 30W deuterium lamp, emitting a continuous spectrum of light with photon energies in the range of 3.1 eV to 6.2 eV (±2%). The light was focused by a quartz lens onto a diffraction monochromator to select the wavelength and then by a quartz lens to a 1mm focal spot on the surface of the bone specimen. A 600W halogen lamp or 450W tungsten filament lamp provided thermal stimulation of the bone specimen, with a thermocouple maintaining linear temperature control as a function of time. The heating rate could be varied from 0.1 to 0.5 °C per second over a temperature range from 293 °K to 673 °K (20 °C to 400°C). The thermocouple provided the measurement of temperature with an uncertainty of 0.3 °C.
The electron emission registration system ( EERS ) comprised a sensitive electron multiplier with a gain of 7 x 108 when biased with a voltage up to 4000V, with 150 V applied across the input stage. The dark current from the electron detector was 0.1 impulses per second and capable of recording up to a maximum of 105 impulses per second. The aperture of the electron detector was 8 mm diameter, sufficient to collect a significant number of electrons from the bone sample but exclude extraneous electrons. The current of the emitted particles was measured by the channel secondary electron multiplier operated in single electron counting mode (Fig. 4a). A set of four grids across the aperture could be biased with retarding voltages (with a minimum of 0.5 V) to select electrons from the bone specimen according to their energy and eliminate noise (Fig.4b). The output was connected through an impedance matching amplifier to a histogramming memory, which recorded the number of pulses as a function of temperature and a range of voltages on the grids (Fig.4).
The electron aperture and grids were aligned parallel to the surface of the bone specimen and coaxial to the photo-stimulated spot on its surface. A supply voltage of the amplifier shows the dependence of the emission current on the applied voltage in which the biasing potentials were optimized to minimize the slope of the plateau and maximize the output current (Fig.5).
2.3 The irradiation procedure
The irradiation of bone samples were performed at the Latvian National Oncology Centre using a Philips SL75-10 ( 8 MV photons ) accelerator and a Varian Clinac 2100CD (6 MV and 18 MV photons). Radiation pulses of 2 μsec duration and at repetition rates varying from 100 to 600 per second delivered a dose of approximately 0.3 mGy per pulse. The dose rate was approximately 10 Gy/minute measured at 1m from the target in a 10cm square field in a water phantom at a depth of 10cm.
Build-up bolus and backscatter material was used during the specimen set up under the Linac. One beam was used during irradiation. The doses were ‘prescribed’ to the surface of the bone specimens due to the specimen small thickness. A reference dosimeter of Linac with ion chamber was used to verify the dose to bone specimen prior to bone irradiation with a range of error of ± 1 %.
Table 1 shows the planned series of experiments for irradiation of bone samples, the number of specimens, the photon energy, the dose delivered to each specimen and the dose rate employed for both fractionated and non-fractionated deliveries.
2.4 Optimization of the Exo-emission measurements
The EEE current was measured using bone specimens before and after irradiation, with only photo-stimulation for the purpose of calibrating the system. Photo-stimulation and thermo-stimulation were applied simultaneously to determine the magnitude of radiation-induced defects in the bone specimen. The area under the emission graph (glow curve) is directly proportional to the total number of emitted electrons (Q):
— X……….. (6)
T0 and T f are the initial and final temperatures of the EEE spectrum.
X- is the background
The uncertainty of the measurement was 4% (two standard deviations).
3. Results and discussion
This section investigates the factors that may affect bone specimens’ exoemission measurements and are done before irradiation with ionising radiation to reduce the uncertainties of experimental results after irradiation. Such factors are: Storage — time induced decay of photoemission, Temperature dependence of the emission current at different storage time, and effects of multiple heating on I(T) regularities. After taking into consideration all the results of these factors, ionising radiation is applied on the bone specimens.
To evaluate the correct specimens’ measurements, it is necessary to investigate the time effect on different steps of the experiment. To analyse the current dependence on the storage time at room temperature, the photothermostimulated emission was recorded during subsequent of days (Figure 6). The level of uncertainty is 10% (two standard deviations).
Figure 6 clearly suggests a decrease of the dependence of EEE current on the storage time over a period of 30 days. The time t1 between samples cutting and irradiation must be more than 10 days as well as the time t2 for performance of exoemission test after irradiation should not be more than 5 days. The above procedure of the specimen preparation provided measurements of (I) with a maximal error ± 10 % at level of significance 5%.
Figure 7 shows the dependence of the exoelectron emission current on storage time and thermo-stimulation temperature, normalized to T = 200 °C.
From Figure 7, we can see a decrease in the emission two days after preparation and that indicates a relaxation of the bone surface. However, no further decrease was observed on the fourth day. This suggests that three days storage were sufficient to achieve full relaxation of the bone surface. In subsequent experiments, specimens were stored for 10 days before analysis.
To analyse the influence of heating at the explored temperature range on the EEE the I(T) spectra were measured as a result of multiple heating. The difference between EEE spectra for the first and subsequent heating demonstrates the bone features was changed irreversible with the temperature growth till 200 oC (Figure 8). Besides, the spectra for the second and following heating were alike, that implies absence of material changes due to the subsequent heating cycles. Heating of the specimens till +200oC during PTSE measurement changes the bone emission properties in the irreversible manner. Variations of the heating rate affect the shape of PTSE spectra.
Figure 8 indicates that almost all emission occurs in the first heating cycle and no significant emission was observed in subsequent heating cycles. Heating the specimens up to 200 °C changes the bone surface irreversibly. Therefore, same bone specimen cannot be used twice in the experiment.
To find an optimal mode of photostimulation, PTSE from bone was measured using different photon energies (h). The value of I(Tmax)/I(Tf) reflected in PTSE signal-to-noise ratio, was calculated depending on h (Figure 9). The dependence I = f (T) is a differential line of the burn off, reflecting the speed of change of properties of the researched object connected to concentration of the emission centres C.
From Figure 9 we can conclude that PTSE depends on the energy of photo-stimulation. The ratio of I(Tmax ) / I(Tf) shows a maximum for stimulating photons of around 5.6 eV for a thermo-stimulated heating rate of 10 °C per minute. This photon energy was employed subsequently to maximize the signal to noise ratio. From the later, we conclude that optimal EEE registration mode is reached when the rate of heating is chosen equal to 10oC/min and photostimulation is provided by photons having the energy equal to 5.6 eV.
After taking the above factors into consideration and choosing bone specimens with similar properties for optimally accurate results, ionising radiation (X-ray) was applied and measurements were done using exoelectron emission spectrometer and the results below have been found.
Figures 10, 11 and 12 show the total emitted charge ( Q ) (concentration of radiation induced imperfections) as a function of dose for 8 MV, 6 MV and 18 MV photon irradiation respectively for total delivered doses of 2, 4, 6, 8 and 10 Gy.
Figure 10 shows the Q behavior for fractional and non-fractional modes for E= 8 MeV. The deviation of Q for different dose rates at fractional mode was close to the uncertainty of ΔQ (16%). It may be concluded from this Figure that is no significant difference in the value of Q as a result of delivering the radiation dose in 1Gy or 2 Gy fractions compared with non-fractionated delivery and thus all subsequent irradiations were non-fractionated ones. There seems to be no linear dependence on dose at 8 MV, either fractionated or non-fractionated. The irradiation of bone specimens by 6MV and 18MV photons were carried out at dose rates of 2 Gy/min and 6 Gy/min.
Figure 11 shows the Q behaviour for a bone material after radiation by gamma photons with Energy E= 6MeV. Radiation had been performed with different dose rate (2 Gy/min and 6 Gy/min respectively). From Figure 11 it is concluded that EEE has a response when the dose rate is 6 Gy/min but does not have the response at 2 Gy/min.
Figure 12 shows the Q behaviour of bone material after irradiation by radiation photons with energy E= 18 MeV. Irradiation of bones had been performed with different dose rate (2 Gy/min and 6 Gy/min respectively).
Thus Figure 11 shows no dependence of the emitted charge on dose delivered at 2 Gy/min but a linear dependence at 6Gy/min, whereas Figure 12 shows a linear dependence on dose for irradiation by 18 MV photons at both dose rates. We can not have a proper explanation on the reason behind such graphs behaviour of Figures 10, 11, and 12. Further research should be done to investigate the rational behind such results. However, what we can say from the last three Figures (Figures 10, 11, 12) that bone material has the potential to become as EEE dosimeter at least for the delivered doses until 10 Gy. However the ranges of the radiating particles energy and dose rates are limited. For electron irradiation of 6 MeV the dose rate should be greater than 6 Gy/min while for 18 MeV the dose rate could be greater than 2 Gy/min.
The paper investigated the use of exoelectron emission to study changes in bone due to exposure to ionizing radiation. The ultimate goal is to use this type of measurement for radiation dosimetry. Moreover, this paper illustrated a number of difficulties to study exoelectron emission for superficial bony structures after radiation therapy. Very high doses are needed to get a signal, a signal that varies with absorbed dose.
The paper addressed an approach to detect radiation-induced modifications in bone structure using method of exo-electron emission (EEE). This method was very popular in the 1970s as method of non-destructive testing and control of surfaces, as well as a dosimetry technique for skin surface dose. The authors of this paper developed and upgraded their instrument for application in biological research and bioengineering.
Selection of bones as test samples is very promising because in radiobiology the focus so far mostly on soft tissue cancers whereas effects of radiation on bones got much less attention.
Technologies for EEE measurements have been done to get the highest exoemission maxima and reproducible results. Exoemission measurements should be conducted after the 10th day of specimen preparation. Moreover, the energy of photons should be equal to 5.6 eV and measurements should be done at the 1st heating cycle. The Exoelectron Emission Spectrometer could detect changes in the EEE of the same bone specimens before and after irradiation.
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