Comparison of dose measurements in CT using a photodiode and a small ion chamber

Owing to the advance of multislice computed tomography (CT), the dosimetric protocol currently used in CT has become inadequate. Instead of dosimetry based on the measurement of the Computed Tomography Dosimetry Index (CTDI) using a pencil ion chamber (IC) 100 mm in length, the use of a short IC and the calculation of the dose equilibrium (D_eq) at the location of the chamber are proposed. The objective of this work was to compare the performance of a short IC and a commercial photodiode (BPW34FS) to measure the accumulated dose at the center of the scan length L, D_L(0), and to obtain the equilibrium dose D_eq using the two detectors. The result for L = 100 mm was compared with the result of the pencil chamber. The results indicate that the commercial photodiode is suitable to measure the accumulated dose at the center of the scan length L as compared with the ICs. This methodology allows measurements of the accumulated dose for any desired scan length, allowing measurement of the equilibrium dose D_eq if the phantom is long enough to allow it.     

Absorbed photon dose measurement and calculation for some patient organs examined by computed tomography

Patient doses from computed tomography (CT) examinations are usually expressed in terms of dose index, organ doses, and effective dose. The CT dose index (CTDI) can be measured free-in-air or in a CT dosimetry phantom. Organ doses can be measured directly in anthropomorphic Rando phantoms using thermoluminescent detectors. Organ doses can also be calculated by the Monte Carlo method utilizing measured CTDI values. In this work, organ doses were assessed for three main CT examinations: head, chest, and abdomen, using the different mentioned methods. Results of directly measured doses were compared with calculated doses for different organs in the study, and also compared with published international studies.     

Studies on the application of a low-voltage peak to the postsurgical follow-up CT scan in abdominal cancer patients in order to reduce the exposure of patients to radiation

This study examined the radiation dose, computed tomography (CT) number, contrast and image quality of patients requiring periodic follow-up abdominal CT examinations at various tube voltages. The subjects were divided into two groups. One group consisted of patients who underwent a clinical analysis and the other group was a phantom one. Somatom Sensation 16 (Siemens, Erlangen, Germany) was used. Twenty patients who underwent a periodic follow-up examination by CT were selected randomly. The tube current was fixed to 150 mA, and the tube voltage was adjusted according to the appropriate value of each examination. The computed tomography dose index (CTDI) values were measured. The CT number of each organ was measured by setting up a 1 cm diameter return on investment (ROI) in the abdominal organs at the same height of the first lumbar vertebra using images of the arterial phase. Two radiologists in consensus graded the quality of the abdominal images into three groups. An abdomen-shaped acrylic phantom was used in the phantom study. An ion chamber was inserted into the holes located at the center and periphery of the phantom, where the radiation dose was automatically displayed on the reader. Tube voltages of 80, 100, 120 and 140 kVp were applied to the phantom (diluted contrast medium with water at 1:10 ratio) and the phantom was scanned. The CT number was measured from a 1 cm diameter ROI at the center of the image. The CTDI value decreased by 36% at 100 kVp (7.50 mGy) compared with that at 120 kVp (11.70 mGy). According to the radiologists’ evaluation, there were 17 equivalent, 3 acceptable and 0 unacceptable levels in the group of 20 subjects. The radiation dose in the phantom study decreased with increasing tube voltages from 80 to 140 kVp. The peripheral and central doses decreased by 38% and 41%, respectively. The CT numbers at 80, 100, 120 and 140 kVp were 1365.9±4.4, 1046.1±3.7, 862.8±3.2 and 737.5±3.0 HU, respectively. In conclusion, in a follow-up observation for the detection of a recurrence or metastasis after surgery or patients with chronic abdominal diseases, the exposure doses can be reduced using a low-voltage peak CT examination without greatly changing the image quality.     

A novel method for observation by unaided eyes of nitrogen ion tracks and angular distribution in a plasma focus device using 50 Hz–HV electrochemically-etched polycarbonate detectors

A novel ion detection method has been developed and studied in this paper for the first time to detect and observe tracks of nitrogen ions and their angular distribution by unaided eyes in the Amirkabir 4 kJ plasma focus device (PFD). The method is based on electrochemical etching (ECE) of nitrogen ion tracks in 1 mm thick large area polycarbonate (PC) detectors. The ECE method employed a specially designed and constructed large area ECE chamber by applying a 50 Hz–high voltage (HV) generator under optimized ECE conditions. The nitrogen ion tracks and angular distribution were efficiently (constructed for this study) amplified to a point observable by the unaided eyes. The beam profile and angular distribution of nitrogen ion tracks in the central axes of the beam and two- and three-dimensional iso-ion track density distributions showing micro-beam spots were determined. The distribution of ion track density along the central axes versus angular position shows double humps around a dip at the 0° angular positions. The method introduced in this paper proved to be quite efficient for ion beam profile and characteristic studies in PFDs with potential for ion detection studies and other relevant dosimetry applications.     

Studying the potential of point detectors in time-resolved dose verification of dynamic radiotherapy

Modern megavoltage x-ray radiotherapy with high spatial and temporal dose gradients puts high demands on the entire delivery system, including not just the linear accelerator and the multi-leaf collimator, but also algorithms used for optimization and dose calculations, and detectors used for quality assurance and dose verification. In this context, traceable in-phantom dosimetry using a well-characterized point detector is often an important supplement to 2D-based quality assurance methods based on radiochromic film or detector arrays. In this study, an in-house developed dosimetry system based on fiber-coupled plastic scintillator detectors was evaluated and compared with a Farmer-type ionization chamber and a small-volume ionization chamber. An important feature of scintillator detectors is that the sensitive volume of the detector can easily be scaled, and five scintillator detectors of different scintillator length were thus employed to quantify volume averaging effects by direct measurement. The dosimetric evaluation comprised several complex-shape static fields as well as simplified dynamic deliveries using RapidArc, a volumetric-modulated arc therapy modality often used at the participating clinic. The static field experiments showed that the smallest scintillator detectors were in the best agreement with dose calculations, while needing the smallest volume averaging corrections. Concerning total dose measured during RapidArc, all detectors agreed with dose calculations within 1.1 ± 0.7% when positioned in regions of high homogenous dose. Larger differences were observed for high dose gradient and organ at risk locations, were differences between measured and calculated dose were as large as 8.0 ± 5.5%. The smallest differences were generally seen for the small-volume ionization chamber and the smallest scintillators. The time-resolved RapidArc dose profiles revealed volume-dependent discrepancies between scintillator and ionization chamber response, which confirmed that correction factors for ionization chambers in high temporal and spatial dose gradients are dominated by the volume averaging effect. The unique scaling of the scintillator volumes indicated how such time-dependent volume averaging corrections could be quantified. The time-resolved measurements further supported the claim that small-volume water equivalent detectors are most likely to accurately detect changes in dose delivery, although exact positioning of detectors remains critical.     

The role of dose distribution gradient in the observed ferric ion diffusion time scale in MRI-Fricke-infused gel dosimetry

Ferric ion diffusion is a detrimental factor in MRI-Fricke-infused gel dosimetry. In this study, a novel approach involving MR image subtraction and a fast image-based dosimetry technique to study ferric ion diffusion effects is presented. The fast image-based approach allows studying dose profile degradation within minutes post-irradiation. The relationship between the rate of dose profile deterioration and dose distribution gradients can be elucidated with the improved imaging temporal resolution also. Our results showed that for a dose distribution with gradient 4 Gy/mm or higher, ferric ion diffusion causes apparent dose profile degradation in 0.5-1 h post-irradiation. For a gradual dose gradient change of 2.1 Gy/mm or smaller, dose profile degradation appears insignificant for a two-hour elapsed diffusion time. These observations agree well with the theoretical analysis of a square dependence between dose profile degradation and dose distribution gradient. Because all stereotactic radiosurgery procedures produce steep dose distributions and because the ideal "snapshot" of MR scanning cannot be achieved, knowledge of the ferric ion diffusion time scale is important in experimental designs in order to avoid potential measurement errors in MRI-Fricke-agarose gel dosimetry.     

Angular response characteristic of pyramid shape fast neutron dosimeter using CR-39 films

CR-39 is a highly sensitive etched track detector for neutron monitoring and dosimetry applications but its dose equivalent response is strongly direction dependent with respect to the incident neutrons. This is considered to be a major drawback for their use. In the present study, an attempt has been made to develop a pyramid shaped dosimeter, which consists of polyethylene material of thickness 1 mm with the provision to hold three CR-39 films at an angle of 35° to each other. The response of CR-39 in this configuration under optimum electrochemical etching at elevated temperature have been found nearly angular independent and therefore the dosimeter can be used for neutron monitoring, i.e. personnel as well as area monitoring.     

Temperature compensation techniques for resonantly enhanced sensors and devices based on optical microcoil resonators

It is well known that environmental effects have a major influence on the optical stability of resonantly enhanced sensors and devices based on optical microfiber, namely in the configuration of a microcoil resonator. We propose a geometric design to reduce such effects by chirping the refractive index of successive paired turns in the microcoil resonator. The resistance to external effects such as temperature drifts can be considerably improved by optimizing the coupling coefficients and chirping profile, such that the wavelength span of the resonant condition is maximized without compensating its sensitivity to the desired measurand. We also demonstrate another technique based on resonant wavelength tuning using a compact piezoelectric ceramic disk measuring 3 mm in diameter and 1 mm in thickness, attaining tunability as high as 6.5 pm/100 V.     

Large phase coherence effects in GaMnAs-based nanostructures: Towards a quantum spintronics

Quantum coherent transport of spin-polarized carriers is observed on a very unusual large scale within epitaxial nanowires of GaMnAs, a diluted ferromagnetic semiconductor. From the analysis of the amplitude of strong universal conductance fluctuations, an effective phase coherence length of about 100 nm is inferred at T=100 mK, which is one order of magnitude larger than in a granular 3d-metal ferromagnets. Together with the temperature and bias dependence of these reproducible fluctuations, their wire-length dependence is studied in single-domain sub-micron long nanowires with a perprendicular anisotropy. In particular, variations for two equivalent probe configurations are shown when the length becomes comparable to the actual phase coherence length. This result forecasts the possible observation of non-local voltage drops in GaMnAs nanostructures smaller than about 200 nm. Generally speaking, this research contributes to pave the way towards the realization of quantum spintronics devices.     

Evaluation of epitaxial silicon diodes as dosimeters in X-ray mammography

In this work we report on results obtained with 2 rad-hard n-type epitaxial silicon diodes in mammography X-ray dosimetry. One sample was not irradiated before using as a dosimeter, while the other received a ⁶⁰Co gamma-ray pre-dose of 200 kGy. Both unbiased devices operated in a short-circuit mode as on-line radiation dosimeters for quality assurance in medical imaging dosimetry. The irradiation was performed using 28 kV and 35 kV X-ray beams from a Pantak/Seifert generator, previously calibrated by standardized ionization chamber. The dosimetric response of these devices was investigated with respect to the repeatability, long term stability, sensitivity dependence on energy and dose-rate, charge-dose linearity and directional response. The calibration coefficients of each diode, in terms of air kerma, were also determined. These dosimetric parameters of both diodes fully meet the requirements of IEC 61674 norm, confirming their use as a reliable alternative choice for mammography photon dosimetry within the dose range of 60 μGy-10 Gy (unirradiated EPI diode); for the pre-irradiated EPI diode upper limit of dose was not reached up to now. Nevertheless, it still remains to be investigated whether or not the pre-irradiation procedure influences on the response long-term stability of EPI devices. These studies are under way.     

高纯区熔单晶硅高增益光晶体管的研究

区熔单晶硅与直拉单晶硅以及其他半导体材料相比杂质含量少,少子寿命长,所以以区熔单晶硅为衬底制作的光晶体管在弱的光信号下仍然有高的增益,适宜于弱光探测.报道了以区熔单晶硅为衬底的光晶体管的实验结果.为了保持区熔高纯单晶硅内的少子寿命,背面淀积了一层掺磷多晶硅作为外吸杂层.已经测量得到对于实验中发射极直径为2mm的光晶体管在波长为0.83 μm的入射光照射下,光功率低至0.16 nW时,光晶体管的增益仍然高达4400.     

First positron experiments at NEPOMUC

The in-pile positron source NEutron induced POsitron source MUniCh (NEPOMUC) of the new Munich research reactor FRM-II is now operated at the nominal reactor power of 20 MW. Recently, intensity and positron beam profile measurements were performed at 30 eV and 1 keV, respectively. For this purpose, NaI-scintillators detect the 511 keV γ-radiation of positrons that annihilate at a removable target in the beam line. The beam profile is determined with a micro-channel plate detector and a CCD-camera. In the present arrangement of NEPOMUC's instrumentation the positron beam is connected to a coincident Doppler broadening (CDB) facility and to a positron induced Auger electron spectroscopy (PAES) analysis chamber. First experiments were carried out in order to show the performance of these new spectrometers. An overview of the positron beam facility is given and first experimental results of PAES are presented.     

Reduction in radiation dose with reconstruction technique in the brain perfusion CT

The principal objective of this study was to verify the utility of the reconstruction imaging technique in the brain perfusion computed tomography (PCT) scan by assessing reductions in the radiation dose and analyzing the generated images. The setting used for image acquisition had a detector coverage of 40 mm, a helical thickness of 0.625 mm, a helical shuttle mode scan type and a rotation time of 0.5 s as the image parameters used for the brain PCT scan. Additionally, a phantom experiment and an animal experiment were carried out. In the phantom and animal experiments, noise was measured in the scanning with the tube voltage fixed at 80 kVp (kilovolt peak) and the level of the adaptive statistical iterative reconstruction (ASIR) was changed from 0% to 100% at 10% intervals. The standard deviation of the CT coefficient was measured three times to calculate the mean value. In the phantom and animal experiments, the absorbed dose was measured 10 times under the same conditions as the ones for noise measurement before the mean value was calculated. In the animal experiment, pencil-type and CT-dedicated ionization chambers were inserted into the central portion of pig heads for measurement. In the phantom study, as the level of the ASIR changed from 0% to 100% under identical scanning conditions, the noise value and dose were proportionally reduced. In our animal experiment, the noise value was lowest when the ASIR level was 50%, unlike in the phantom study. The dose was reduced as in the phantom study.     

Construction of an electron analyzer with a wide acceptance solid angle

We have developed a new electron energy analyzer with a large solid angle of 0.14π, which is comparable to that of cylindrical mirror analyzer. Typical energy resolution was ΔE/E₀ ∼ 0.016 for the aperture of 1 mm and central radius of 100 mm, and typical angular resolution was less than 0.5°.     

Investigation of a fibre-coupled beryllium oxide (BeO) ceramic luminescence dosimetry system

The purpose of this study is to investigate the potential use of a beryllium oxide (BeO) ceramic as a radioluminescence (RL) and optically stimulated luminescence (OSL) probe material for fibre-coupled luminescence dosimetry. A portable dosimetry system, named RL/OSL BeO FOD was developed, consisting of a 1 mm diameter, 1 mm long BeO ceramic cylinder coupled to a silica/silica optical fibre. The reader measures the RL signal and also uses a 450 nm laser diode to stimulate the BeO ceramic. A second background optical fibre is used to remove the stem effect. The RL/OSL BeO FOD was characterised in a solid water phantom, using a 6 MV x-ray beam. The RL was found to be reproducible and have a linear response to doses ranging from 30 cGy–15 Gy and dose rates from 100 cGy/min – 600 cGy/min. The OSL response was linear to doses of 10 Gy, becoming supralinear at higher doses. Measured percentage depth curves using the RL/OSL BeO FOD agreed with those measured using an IC15 ion chamber to within 5%, beyond the build up region. It was also found that the RL from BeO ceramic is unaffected by the delivered dose to the probe and hence, it remains constant for a given dose-rate. The insensitivity of the RL to accumulated dose makes BeO ceramic potentially capable of accurate dose-rate measurements without any corrections for the accumulated dose. This study demonstrates the feasibility of BeO ceramic as a versatile fibre-coupled luminescence dosimeter probe.     

Characterization of a Ce3+ doped SiO2 optical dosimeter for dose measurements in HDR brachytherapy

Aim of this work was to study the application of a new miniaturized Ce³⁺ doped SiO₂ scintillation detector to in vivo dosimetry in high dose rate brachytherapy. Energy, dose-rate, temperature and angular dependences of the detector response to ¹⁹²Ir HDR brachytherapy fields were investigated, as well as sensitivity reproducibility and linearity. To this aim, two ad hoc phantoms were designed and developed to perform measurements in water. Intra-session reproducibility resulted to be very high, however inter-session reproducibility showed too high statistical variation. Detector response resulted to increase linearly with dose (R² = 0.997), with no evidence of energy and dose-rate dependence. Sensitivity resulted to increase linearly with temperature (R² = 0.995), with a 0.2% increase each °C. Finally, no significant angular dependence for the source moving around a circle in the azimuthal plane centered at the scintillator was observed. The obtained results show that the proposed detector is suitable for in vivo real-time dosimetry in high dose rate brachytherapy.     

A compact TOF-SANS using focusing lens and very cold neutrons

We are developing a high-resolution small angle neutron scattering instrument for very cold neutrons (VCN). Our concept includes a magnetic lens for focusing of the beam at the detector plane. The lens consists of one permanent-magnet sextupole array rotating outside another stationary sextupole array, to focus a pulsed white beam of neutrons. Thus the instrument operates in time of flight mode. The prototype magnetic lens has a bore of 15 mm diameter and length of 66 mm, producing a magnetic field gradient oscillating from 1.5×10⁴ to 5.9×10⁴ T/m², with frequency ≤25 Hz. A torque-canceling magnet around the lens suppresses the torque of rotation from the outer array to 1/3.We have demonstrated the performance of the lens, over wavelength range from 30 to 48 Å, on the PF2-VCN beam line at the Institut Laue-Langevin, France. The focused beam image was the same size as the source, without chromatic aberration, with focal length of 1.14 m. We also studied the performance of this configuration for high-resolution SANS, in a compact geometry (just 5 m long). The measurable q range of this system was 0.009 Å⁻¹≤q≤0.3 Å⁻¹ or 0.004 Å⁻¹≤q≤0.08 Å⁻¹ for sample to detector distances of 100 and 465 mm, respectively. Here, we present the results of our lens characterization study along with the SANS results on a tri-block copolymer (F127 Pluronic) and on a stretched polymer blend (with the Shish-Kebab structure).     

Multi-wavelength generation of an extremely narrow pulse using a ring resonator system for bio-cells microscopy

We propose a new system of an extremely narrow light pulse generation for optical microscopy applications using a nonlinear ring resonator system. The system consists of one nano and three micro-optical ring resonators, which can be used to generate the 50 fm (10⁻¹⁵ m) optical spectral width at the broad wavelength spectrum. By using a soliton pulse with a pulse width of 50 ps, peak power of 1 W, center wavelength at 550 nm, and after the soliton pulse is launched into the first ring device, the chaotic pulses are generated within the first ring. The chaotic filtering behaviors are performed by using the second and the third ring devices, whereas the extremely short pulse, i.e. narrow spectral width, can be generated by using the extended nano-ring device. The broad spectrum of the harmonic waves is generated and filtered, which is of use in optical tomography. Results obtained have shown that the generation of the broad spectrum of short pulse with width 100 fm and peak power 60 mW is achieved. The possibility of using such a system for nondestructive bio-cells microscopy, for visualizing bio-cells and for bio-cells tomography is also discussed in detail.     

Simulations of grazing-incidence pumped Ni-like Sn X-ray laser at 11.9 nm

Grazing-incidence pumped Ni-like Sn X-ray laser media at 11.9 nm (4d-4p, J = 0-1) is modelled using code EHYBRID and a post-processor code. The required atomic data are obtained using the Cowan code. In this study the pre-formed plasma is pumped on longitudinal direction with a grazing angle. Detailed simulations were performed to optimize the driving laser configurations. Relatively high gain is produced for the Ni-like Sn X-ray laser at 11.9 nm with long pre-pulse and short main pulse drive energy of only 100 mJ on 4 mm slab targets. Using low intensity pre-pulse prior to long pulse decreases the electron density gradient. X-ray resonance lines between 13 and 25 Å emitted from tin plasma have been simulated using post-processor coupled with EHYBRID. The ratio of these resonance lines can be used to measure electron temperature of the laser produced Sn plasma.     

Effects on skin dose from unwanted air gaps under bolus in photon beam radiotherapy

The dose under small air gaps located under bolus material of up to 10 mm have been measured with an Attix parallel plate ionization chamber and radiochromic film. For a 6 MV x-ray beam with 10 mm bolus, an air gap of 2 mm produced no reduction in skin dose when measured with the Attix chamber. An air gap of 4 mm will introduce a reduction of dose to the basal layer of approximately 0–4% depending on field size, angle of incidence and other patient specific parameters and a reduction of up to 10% could be seen at the basal cell layer for a 10 mm air gap. The 10% reduction was for a small x-ray field at 60 degrees angle of incidence and was a reduction in dose from 100% of D_max down to 90% of D_max. Results at oblique angles of incidence show that larger reductions in dose are seen with increasing angle of incidence. Radiochromic film results agree with the Attix chamber results measuring 2%±2% decrease for a 4 mm gap and 4%±2% for a 10 mm gap at normal incidence. Clinically, results show that small air gaps can reduce skin dose, however, at least 90% of maximum dose is still delivered for air gaps up to 10 mm.