Film Calibration Research Articles

Development and initial evaluation of a spectral microdensitometer for analysing radiochromic films

Radiation dose deposited on a radiochromic film is considered as a dose image. A precise image extraction system with commensurate capabilities is required to measure the transmittance of the image and translate it to radiation dose. This paper describes the development of a spectral microdensitometer which has been designed to achieve this goal under the conditions of (a) the linearity and sensitivity of the dose response curve of the radiochromic film being highly dependent on the wavelength of the analysing light, and (b) the inherent high spatial resolution of the film. The microdensitometer consists of a monochromator which provides an analysing light of variable wavelength, a film tray on a high-precision scanning stage, a transmission microscope coupled to a thermoelectrically cooled CCD camera, a microcomputer and corresponding interfaces. The measurement of the transmittance of the radiochromic film is made at the two absorption peaks with maximum sensitivities. The high spatial resolution of the instrument, of the order of micrometres, is achieved through the use of the microscope combined with a measure-and-step technique to cover the whole film. The performance of the instrument in regard to the positional accuracy, system reproducibility and dual-peak film calibration was evaluated. The results show that the instrument fulfils the design objective of providing a precise image extraction system for radiochromic films with micrometre spatial resolution and sensitive dose response.

1000 poster Film calibration for IMRT patient QA verification

1000 poster Film calibration for IMRT patient QA verification

Effect of processing time delay on the dose response of Kodak EDR2 film.

Kodak EDR2 film is a widely used two-dimensional dosimeter for intensity modulated radiotherapy (IMRT) measurements. Our clinical use of EDR2 film for IMRT verifications revealed variations and uncertainties in dose response that were larger than expected, given that we perform film calibrations for every experimental measurement. We found that the length of time between film exposure and processing can affect the absolute dose response of EDR2 film by as much as 4%-6%. EDR2 films were exposed to 300 cGy using 6 and 18 MV 10 x 10 cm2 fields and then processed after time delays ranging from 2 min to 24 h. An ion chamber measured the relative dose for these film exposures. The ratio of optical density (OD) to dose stabilized after 3 h. Compared to its stable value, the film response was 4%-6% lower at 2 min and 1% lower at 1 h. The results of the 4 min and 1 h processing time delays were verified with a total of four different EDR2 film batches. The OD/dose response for XV2 films was consistent for time periods of 4 min and 1 h between exposure and processing. To investigate possible interactions of the processing time delay effect with dose, single EDR2 films were irradiated to eight different dose levels between 45 and 330 cGy using smaller 3 x 3 cm2 areas. These films were processed after time delays of 1, 3, and 6 h, using 6 and 18 MV photon qualities. The results at all dose levels were consistent, indicating that there is no change in the processing time delay effect for different doses. The difference in the time delay effect between the 6 and 18 MV measurements was negligible for all experiments. To rule out bias in selecting film regions for OD measurement, we compared the use of a specialized algorithm that systematically determines regions of interest inside the 10 x 10 cm2 exposure areas to manually selected regions of interest. There was a maximum difference of only 0.07% between the manually and automatically selected regions, indicating that the use of a systematic algorithm to determine regions of interest in large and fairly uniform areas is not necessary. Based on these results, we recommend a minimum time of 1 h between exposure and processing for all EDR2 film measurements.

Surface dose extrapolation measurements with radiographic film

Assessment of surface dose delivered from radiotherapy x-ray beams for optimal results should be performed both inside and outside the prescribed treatment fields. An extrapolation technique can be used with radiographic film to perform surface dose assessment for open field high energy x-ray beams. This can produce an accurate two-dimensional map of surface dose if required. Results have shown that the surface percentage dose can be estimated within ±3% of parallel plate ionization chamber results with radiographic film using a series of film layers to produce an extrapolated result. Extrapolated percentage dose assessment for 10 cm, 20 cm and 30 cm square fields was estimated to be 15% ± 2%, 29% ± 3% and 38% ± 3% at the central axis and relatively uniform across the treatment field. The corresponding parallel plate ionization chamber measurements are 16%, 27% and 37%, respectively. Surface doses are also measured outside the treatment field which are mainly due to scattered electron contamination. To achieve this result, film calibration curves must be irradiated to similar x-ray field sizes as the experimental film to minimize quantitative variations in film optical density caused by varying x-ray spectrum with field size.

Thermoluminescent response of TLD-100 irradiated with 20 keV electrons and the use of radiochromic dye films for the fluence determination

We have measured the LiF:Mg,Ti (TLD-100) fluence response and supralinearity function to 20 keV electrons in the fluence interval between 5 × 10 9 and 4 × 10 12 cm −2. TLD-100 shows linear response up to 2 × 10 10 cm −2, followed by supralinearity and saturation after 10 12 cm −2. Peak 5 is slightly supralinear, f( n) max=1.1±0.1, while high temperature peaks reach up to f( n) max≈8. Peak 5 saturates at n≈1×10 11 cm −2, fluence smaller than any of the saturating fluences of the high temperature peaks. We have also measured the glow curve shape of TLD-100 irradiated with 40 keV electrons, beta particles from a 90Sr/ 90Y source and 1.3 and 6.0 MeV electrons from accelerators. Results are interesting and unexpected in that, for a given macroscopic dose, electrons show a smaller relative contribution of high-temperature peaks with respect to peak 5 than heavy ions or X- and γ-rays. The 20 and 40 keV electron irradiations were performed with a scanning electron microscope using radiochromic dye film to measure fluence. Since film calibrations were performed using 60Co γ-rays which expose the totality of the film volume, the use of this method with low energy electrons required to develop a formalism that takes into account the sensitive thickness of the film in relation to the range of the incident particles.

Uranium and thorium thin film calibrations by particle track techniques

Particle track techniques, which enable estimation of the uranium and thorium contents in films where energy self-absorption is negligible, are presented. These techniques allow calibration of uranium and thorium thin films with adequate precision to be used in neutron fluence determinations for fission-track dating. Calibration via a particles is relatively simple for the case of uranium films, whereas for thorium films it is necessary to use the spectrometric characteristics of the employed track detector. Besides those based on a particles, calibration procedures via fission fragments are presented both for uranium as well as for thorium films.

Rapid radiographic film calibration for IMRT verification using automated MLC fields.

A method for measuring a film sensitometric curve using a single sheet of film exposed with a two field step-and-shoot MLC treatment was developed and tested with Kodak XV2 and EDR2 films. With this technique a film sensitometric curve can be completed in only 10 minutes, making it practical to generate new film calibrations daily. This method is applicable to film calibrations for all purposes, but is particularly useful in IMRT treatment verification due to the method's use of small fields. This method agrees with the traditional large-field multifilm calibration within 0.5% and will produce sensitometric curves with errors less than 1% throughout the dose range, including uncertainties in dose delivery, film response, and optical density measurements. OD values for XV2 and EDR2 films were consistent in the middle of exposure areas at high depths, but the XV2 film penumbra regions showed large amounts of over-response as the calibration depth increased. If XV2 film is used for IMRT treatment verification, it is necessary to reduce the fluence of low energy photons in areas around the film by using thin lead shields. EDR2 film was shown to have minimal energy dependence, as it accurately represented penumbra areas and yielded identical sensitometric curves generated with 6 and 18 MV photons. However, its darker tint may make it more sensitive to scanning laser film digitizers' horizontal nonuniformities. This single film method proved to be superior to the traditional calibration method and allows fast daily calibrations of films for highly accurate IMRT delivery verifications.

On the application of magneto-optic films in NDE

Some applications of magneto-optic (MO) films in the field of nondestructive evaluation (NDE) are presented. The method for imaging and measuring of magnetic fields on micrometer scale is based on the analysis of changes of the magnetic domain structure caused by the detected field. A process of MO film evaluation and calibration is described. The existence of the leakage field around a fatigue crack in nonmagnetic austenitic material is detected by a highly sensitive MO film. An experimentally determined distribution of the excitation field of an eddy current probe is compared with results obtained by the finite element method (FEM) numerical procedure.

Dosimetry of rotational partial-skin electron irradiation

Background and purpose: Often, the most appropriate treatment for superficially and extensively spreading tumors of the skin is to use electron irradiation at enlarged distances. Rotational skin electron irradiation is a proven method for the treatment of the entire skin surface. We here report modifications of this technique in the set-up of partial-skin electron irradiation and the results of dosimetric examinations with regard to optimal shielding, dose profiles and depth dose curves under various irradiation conditions.Materials and methods: Irradiation was performed using electron beams with nominal energies of 6 MeV from a linear accelerator. The phantom was located on a rotating platform at a source-surface distance SSD=300 cm. A horizontal slit aperture (height: 32 cm) within a 2 cm thick polymethylmethacrylate (PMMA) shielding plate near the phantom was used to define the size of the irradiated region. Influences on dose distributions due to scattering processes on the PMMA edges were investigated using a flat ionization chamber and films. Absolute dose measurements and film calibration were made with the flat chamber. The quality of bremsstrahlung radiation behind the shielding was determined with a thimble ionization chamber in the phantom.Results and conclusions: The results of rotational partial-skin electron irradiation reveal some of the investigated shielding geometries to be optimal. Depth dose distributions and dose rates correspond to the results obtained in total skin electron rotational irradiation. It is possible to apply the dose superficially in the first millimeters of the skin; the dose maximum is located at a depth of 0–2 mm, the 80% isodose at 9 mm. The amount of bremsstrahlung contamination is 2.5%. The local amount of absorbed dose per monitor unit depends strongly on patient/phantom cross-section geometry. At our institute, rotational partial-skin electron irradiation was implemented into clinical routine in 1997.

Potentialities and practical limitations of absolute neutron dosimetry using thin films of uranium and thorium applied to the fission track dating

Neutron dosimetry using natural uranium and thorium thin films makes possible that mineral dating by the fission-track method can be accomplished, even when poor thermalized neutron facilities are employed. In this case, the contributions of the fissions of 235U, 238U and 232Th induced by thermal, epithermal and fast neutrons to the population of tracks produced during irradiation are quantified through the combined use of natural uranium and thorium films. If the Th/U ratio of the sample is known, only one irradiation (where the sample and the films of uranium and thorium are present) is necessary to perform the dating. However, if that ratio is unknown, it can be determined through another irradiation where the mineral to be dated and both films are placed inside a cadmium box. Problems related with film manufacturing and calibration are discussed. Special attention is given to the utilization of thin films having very low uranium content. The problems faced suggest that it may be better to substitute these films by uranium doped standard glasses calibrated with “thicker” uranium films (thickness greater than 1.5 × 10 −3 μm).

Development of a unique deposition process for thin film calibration sensors

Abstract A new approach for coating thin metal films on pure aluminum for use in sensors has been developed. The process consists of three steps: (1) spontaneous electrolytic copper cementation onto aluminum directly from an organic solvent, (2) electroplating cadmium on the copper-modified aluminum substrate, and (3) cold rolling to enhance the properties and size of the final deposit. The unique part of the technology is the use of the galvanic coating process to plate copper from an organic solvent. The adherence approaches levels which are not attainable using conventional cementation procedures from aqueous solutions. The results of some preliminary tests on the effect of parameters such as the chemical composition of the phases, loading parameters, temperature and oxygen content on the rate of reaction and the quality of the copper deposit are presented. The copper nuclei deposited on the aluminum serve as initiation sites for the deposition of a uniform and adherent cadmium deposit which serves as the thin film sensor. The special Cd 108 isotope layer necessary for the particular application is approximately 600 nm thick after rolling.

Autoradiographic film evaluated with desktop scanner

A procedure is described herein for the densitometric calibration of autoradiographic films, utilizing a desktop scanner. The method yields a calibration accuracy of ±0.03 reflection density units over the range from 0 to 1.7. The calibration procedure was applied to Amersham “Hyperfilm-βmax” autoradiographic film against 14C, 36Cl, 60Co, 133Ba, 137Cs and 241Am. An exponential function was used to describe the useful range of the characteristic curves of this film, which is between 1×10 4 and 1×10 5 (discharges mm −2) for 14C and 36Cl and between 10 5–10 6 (discharge mm −2) for the other gamma-emitting isotopes.

Calibration of single-surface hot films and in-line hot-film arrays in laminar or turbulent flows

Experimental and numerical investigations were performed concerning the heat balance and calibration of single-surface hot films and surface hot-film arrays in laminar or turbulent flows. Emphasis is placed on the influence of the thermal interaction among sensor, fluid and the wall and among different sensors from an in-line hot-film array on the calibration of the sensors. Experimental and numerical investigations of the coupled flow and structure temperature field in the vicinity of an in-line surface hot-film array are presented. The mechanism of thermal interactions is demonstrated, namely the strong effect of an increasing thermal boundary layer thickness on the convective and conductive heat losses of each single sensor of the array due to an increasing number of upstram sensors being `operated'. The numerical simulations were performed both for laminar and the turbulent flow as well as for a different number of sensors. The results are compared with experiments demonstrating that, due to the thermal interaction, the downstream decrease in the total heat losses of a sensor from an in-line array has a great effect on the calibration.

Appropriateness of plane pressure-sensitive film calibration for contact stress measurements in articular joints

Objective. To examine how accurately calibration stains from pressure-sensitive film samples compressed between perfectly flat surfaces represent contact pressures measured with the film inserted between curved joint surfaces. Design. Experimental study. Background. Contact pressure distributions and areas in joints are often quantified using pressure-sensitive film inserted between the articular surfaces. Absolute pressure values are recovered from the film through a calibration procedure. Since the calibration is typically performed in a different environment from the experimental measurements, a systematic error in the pressure magnitudes may be introduced. Methods. Calibration stains were produced with an Instron machine using a flat and a slightly curved indenter. Calibration curves were fitted to the raw data of both indenters using a model representing the functional characteristics of the pressure-sensitive film. Results. No systematic differences between the calibration curves from the flat and the curved indenter were observed. Conclusions. It appears that calibration with the film in a flat configuration is adequate for contact pressure measurements between moderately curved articular surfaces.

Implementation of the Varian EDW into a commercial RTP system

An enhanced dynamic wedge system (EDW) has been installed onto our Clinac 600C (6 MV) linear accelerator. This paper addresses and describes the measurements taken and subsequent analysis required to enable the planning of EDW using our commercial radiotherapy treatment planning (RTP) system. This implementation into a `closed' commercial system is performed without the use of specialized measurement equipment or the necessity of introducing new calculation algorithms and is therefore independent of the RTP manufacturer. We consider that, for incorporation of techniques such as EDW into routine clinical use, a simple verifiable method of inclusion into the RTP system must be achieved. This paper also presents a methodology for quality control of dynamic fields chosen for clinical use which is quick and easy to use by virtue of its use of film dosimetry. We present the method of film calibration used in this work.

Absolute measurements in the VUV spectral range with a luminescent detector

A description is given of a simple luminescent detector of radiation in the VUV range (10–100 nm) and of a method for absolute calibration of the sensitivity (quantum efficiency) of this detector. The detector and multilayer normal-incidence mirrors were used in determination of the absolute yield of the radiation from a laser plasma and in absolute calibration of the UF-4 photographic film at the wavelength of 18 nm.

The use of radiochromic film in treatment verification of dynamic stereotactic radiosurgery

Standard silver-based films are usually too sensitive to be used as direct indicators of dose in dynamic radiosurgery because of optical saturation. This paper describes the use of a new radiochromic film to measure 6-MV radiosurgery doses and dose distributions in a head phantom. Dose calibration of the radiochromic film was performed in the range of 2.3-50.2 Gy using light of 632- and 530-nm wavelengths. Radiosurgery dose distributions were measured using the radiochromic film in a head phantom undergoing the same treatment as a patient, and were compared with the planned distributions. For an example case (nominal 2.0-cm-diam cone), film measurement verified the calculated dose distribution in one plane. The simple measurement technique described led to experimental uncertainties of +/- 0.1 cm for the 90% and 50% isodose lines, +/- 0.3 cm for the 20% line, and +/- 0.5 cm for the 10% line. Isocenter dose was measured with an uncertainty of +/- 3%. Refinements to the technique should allow more precise measurements. It is concluded that the radiochromic film, with some limitations, is a convenient and useful tool for dynamic radiosurgery quality assurance.

Calibration of Film for Accurate Megavoltage Photon Dosimetry

An accurate method of converting film density to dose is presented. For films oriented parallel to the beam’s central axis, calibration curves were produced at several depths using Kodak XV-2 film for cobalt-60, 4MV, and 18MV beams. Then the appropriate curve was employed to convert the film density to dose at a specific depth. It is hypothesized that the change in film response with depth is due to changes in the photon spectra at depth in a phantom.

Quantitative determination of the salinity of seawater droplet in 10−6 to 10−10 gram

The rate of evaporation of seawater droplets in the air-sea boundary layer can be estimated by its salinity change compared to the sea surface salinity. A micro-chemical method based on Farlow (1954) is developed for quantitative determination of the salinity of an individual droplet without the error due to further evaporation after sampling. A halide ion-sensitive sampling surface is prepared by colloidally dispersing brown silver dichromate into the gelatin layer of a commercially available film. The reaction of soluble chlorides with the silver salt leaves a clear white halo with diametera on the brown film to reveal the volume of the droplet. After the film is developed in a water vapor saturated atmosphere, the halo grows to diameterd with the original as the embryo to indicate the quantity of chloride ion involved in the action. The ratio ofd/(a1.5) is a function of the salinity of the droplet. The method can be used to determine the salinity of seawater droplet of 10−6 to 10−10 gram. Details of the principle and preparation, and calibration of the reagent film, are presented.

Calibrated soft x-ray spectroscopy using transmission grating, pinhole and film

The combination of transmission diffraction grating (free-standing gold bars), pinhole and film can be used for quantitative measurements of the x-ray spectra in the 0.1–1.5-keV region by adopting the procedures described. The steps consist of film calibration and deconvolution of overlapping contributions from different spectral orders. In addition, spatial resolution is available in the direction perpendicular to the dispersion axis for each x-ray energy. The working example presented is the characterization of an x-ray source created by a laser-target interaction, for possible use in x-ray microscopy or x-ray lithography.