Progress in quality control of alanine dosimeters

The ESR/alanine system has been used for different applications, over several years, as a classical routine dosimetry service. This service is based on alanine dosimeters constituted by pellets manufactured by our laboratory, which are presented with some specifications of use and different parameters bound to the intrinsic quality of the dosimeter itself. These parameters were obtained through the strict application of a production protocol and a validation protocol created, tested and validated under a quality assurance system. Every year, one batch of photon dosimeters is produced and controlled using these protocols and the specifications associated with it are checked. Two categories of data are considered separately: physical parameters and experimental data obtained by measurement on our ESR system are given with their associated standard deviation.     

An automated measurement system for characterization of RF and gradient coil parameters

A fully automated laboratory-based measurement system for characterization of coil system parameters is presented. This method uses an inexpensive personal computer (PC)-controlled stepper motor positioning system in conjunction with a network/spectrum analyzer and an analog-to-digital converter (A/D) board that allows high resolution data acquisition in an unattended manner. A graphical interface was created for complete control of stepper motor movement, measurement, and data acquisition. The system is capable of performing a wide range of measurements that can, either individually or combined, characterize radiofre-quency (RF) and gradient coils used in MRI. Measurement methods, theory, and results for conductor and shield current distributions, mutual impedance, and magnetic fields are given. Comparisons with theoretical calculations are included to validate the accuracy and utility of the system.     

Behaviour of alanine dosimeters in an ozone environment

Polymer-alanine dosimeters (PADs) have been irradiated, in air with a concentration of 5 ppm of ozone, with doses up to 10 kGy, by means of a ⁹⁰Sr source. The electron spin resonance (ESR) signals were compared with the signals obtained after irradiations in normal air, and the amplitude reduction was found to be negligible. The evolution of the ESR signal of PADs irradiated in normal air and thereafter stored in air with 5 ppm ozone concentration was also investigated. No change was observed over a period of 2 months.     

Electron spin relaxation times for the alanine radical in two dosimeters.

The spin-lattice relaxation times for the alanine radical in dosimeters were measured by long-pulse saturation recovery and by inversion recovery. The strong dependence of the recovery time constants on experimental conditions show that spectral diffusion processes contribute to enhanced relaxation rates, analogous to what was observed previously for 60Co gamma-irradiated polycrystalline l-alanine. The spectral diffusion processes make it possible to run quantitative CW spectra of the alanine radical at substantially higher microwave powers than could be used in the absence of spectral diffusion processes.     

Temperature stabilization of alanine dosimeters used for food processing and sterilization

The International Atomic Energy Agency has established a dose quality audit service for radiation processing facilities. The objective of the service is to provide an independent check on the routine dosimetry system in use at the facility. The audit service is based on the use of alanine EPR dosimetry. Generally, alanine dosimeters are irradiated at the facility together with a product, and the response is then analyzed at the IAEA laboratory. Practice of the audit service has shown that the main uncertainty in alanine dosimetry is due to absence of temperature control at the irradiation facilities. Here, a method for stabilizing the temperature of the dosimeter during irradiation is proposed.     

Responses of alanine dosimeters to irradiations at cryogenic temperatures

Present and future high-energy accelerators and particle detectors use and will use super-conducting technologies where components are submitted to radiation in a cryogenic environment. Radiation tests and dosimetry calibration have to be made under these conditions. Alanine-ESR dosimetry is world-wide recognised as a standard method for high-dose measurements at room-temperature irradiation. From earlier work, the system also seems suitable for low-temperature applications, despite a decrease of sensitivity. The aim of the present work was to estimate the temperature dependence of five different alanine based dosimeters. The dose-response functions at 290, 77 and 5 K were determined between 10 Gy and 80 kGy. All the results were consistent among the five laboratories: at a given dose, the amplitude of the ESR signal decreases with the irradiation temperature, moreover, this effect is found to be dose-dependent.     

A unique calibration curve for alanine EPR dosimetry systems

The alanine EPR dose-response curve was proposed to follow a saturating exponential function. In this study the characteristic parameter D₃₇3of the exponential function was estimated for two types of dosimeters and different EPR spectrometer configurations. The results suggest that only below about 0.3 mW the alanine EPR signal is not saturated with microwave power. A unique alanine calibration curve is suggested for measurements with non-saturating microwave power.     

Measurements with alanine and film dosimeters for industrial 10 MeV electron reference dosimetry

Alanine and radiochromic film dosimeters were irradiated with absorbed doses from 3 to 10 kGy at a 10 MeV electron accelerator under typical radiation processing conditions. The nominal doses were measured by graphite calorimeters and the dosimeters were irradiated in graphite phantoms. The dosimeters were calibrated by irradiation with ⁶⁰Co photons.The ratios of calorimeter to film and alanine dosimeter readings were generally within 0.99 ± 0.01 for 10 mm phantoms, but larger differences were observed for 20 mm phantoms. The discrepancies may be due to uncertainties in irradiation geometry, or in the stopping power ratios applied.     

Development of a secondary standard for the absorbed dose to water based on the alanine EPR dosimetry system.

The primary measurand in the dosimetry for radiation therapy is the absorbed dose to water, D(W). An alanine/EPR based secondary standard for D(W) for high-energy photon and electron radiation is presently under construction at the Physikalisch-Technische Bundesanstalt. A technique of transfer dosimetry that makes it possible to reduce the reproducibility to a level below 0.5% in the 5-50 Gy range is presented. It takes advantage of a reference sample. Results of investigations of some effects, such as environmental humidity, are presented.     

Electron arc therapy treatment planning verification with alanine/EPR dosimetry.

Alanine/EPR dosimetry was applied to measure dose distributions during electron arc therapy treatments generated by electron beams moving isocentrically over a stated arc. Alanine-polyethylene pellets, prepared at ISS according to the NIST recipe, inserted in a homogeneous cylindrical and anthropomorphic phantoms, were used. Preliminary, alanine response to static electron beams in the (6-20) MeV nominal energy range was studied. Then alanine dosimetry was applied to determine the dose versus the gantry angle at a reference point in a cylindrical homogeneous phantom. Finally, arc therapy treatment planning verification was performed in anthropomorphic phantom.     

An automated portable night vision testing system

This report describes a new testing system designed to assess night vision parameters of the military population in the field, and to quantify the effects of various environmental stress factors, such as extended exposure to high altitude, on the dark adaptation process. The instrument is based on an established procedure for dark adaptation measurement in which the subject continuously adjusts the threshold luminance of a recurrently flashing stimulus. The device described here represents a modernized version of the original technique, which features an automated testing procedure and provides for computerized data translation of the dark adaptation function. It also offers the advantages of rugged construction and field portability not available in clinical style instruments.     

An automated intelligent diagnostic system for the interpretation of umbilical artery Doppler velocimetry

The objective is to develop an automated intelligent diagnostic system for the interpretation of umbilical artery velocity waveforms. An ultrasound instrument with pulsed-wave Doppler is connected to a microcomputer by means of a frame grabber. After data acquisition, umbilical Doppler velocimetry is handled as a pattern recognition (feature extraction and classification) and decision-making problem. Automated image processing (enhancement, smoothing/thresholding and edge detection) and analysis are used for feature extraction. Six waveform indices obtained by feature extraction are used as input layer to vector quantization which classifies waveforms into six groups. A clinical decision is assigned to each group by the medical expert. Our system is trained by 278 and 380 waveform images of 94 normal and 157 high risk pregnancies, respectively. The system was tested with 193 and 61 images of normal and risky pregnancies; it was demonstrated that sensitivity and specificity of the system are 54.1% and 80.3%, respectively.