Performance of a new star-bar phantom designed for MTF calculations in x-ray imaging systems.

PURPOSE: A new phantom, designed and manufactured for modulation transfer function (MTF) calculations is presented in this work. The phantom has a star-bar pattern and is manufactured in stainless steel. Modulation transfer function determinations are carried out with the new phantom and with an edge phantom to compare their performance and to compare them with previous theoretical predictions. METHODS: The phantoms are imaged in an x-ray imaging system using different beam qualities and different entrance air KERMA. Methods, previously developed for synthetic images and simulations, are adapted to real measurements, solving practical implementation issues. RESULTS: In the case of the star-bar, in order to obtain optimal MTF determinations it is necessary to accurately determine the center of the pattern. Also, to avoid underestimates in MTF calculations, the length in pixels of each of the scanning circumferences must be an integer multiple of the number of cycles in the pattern. Both methods, star-bar and edge, give similar mean values of the MTF in all cases analyzed. Also, the dependence with frequency of the experimental MTF standard deviation (SD) agrees with the theoretical expressions presented in previous works. In this regard, the precision is better for the star-bar method than for the edge and differences in precision between both methods are higher for the lowest beam quality. CONCLUSIONS: The star-bar phantom can be used for MTF determinations with the advantage of having an improved precision. However, precision is reduced when the radiation quality increases. This fact suggests that, for the highest beam qualities, materials with an attenuation coefficient greater than that of steel should be used to manufacture the phantom.

Optimizing the recalibration process in radiochromic film dosimetry.

Intra-lot, inter-scan and other variabilities in radiochromic film dosimetry may have a severe impact on absolute dosimetry with this dosimeter. In the literature, several dosimetry protocols may be found characterized by different calibration functions and different film response variables. Also, the re-calibration methods found in the literature correct and minimize the impact of the variabilities in the absolute dose estimates. In this work, several recalibration methods and dosimetry protocols are evaluated. In order to find optimal configurations, their accuracy is compared, and the accuracy level that can be reached in each case is discussed. The efficient protocol and the parameter escalation are used to recalibrate EBT3 films from two different film batches. The mean absolute deviations between known doses and estimated doses for eight dose levels are obtained and compared with the self calibration of each reading, named intrinsic film calibration. Eight film sheets from two different lots and two digitizers are used. The parameter escalation method with a four-level recalibration using net optical density (NOD) and a power law as dosimetry protocol obtains the highest accuracy. Regarding the number of control strips, increasing the number from two to three makes the parameter escalation protocol to come close to intrinsic film calibration in all cases, but has a less important effect on the efficient protocol. Regardless the choice of the sensitometric variables, using the appropriate recalibration method results in accuracy levels typical of self calibration of the film. In addition, the parameter escalation method provides better results than the efficient protocol with three calibration strips.

Monte Carlo uncertainty analysis of dose estimates in radiochromic film dosimetry with single-channel and multichannel algorithms.

PURPOSE: To provide a multi-stage model to calculate uncertainty in radiochromic film dosimetry with Monte-Carlo techniques. This new approach is applied to single-channel and multichannel algorithms. MATERIAL AND METHODS: Two lots of Gafchromic EBT3 are exposed in two different Varian linacs. They are read with an EPSON V800 flatbed scanner. The Monte-Carlo techniques in uncertainty analysis provide a numerical representation of the probability density functions of the output magnitudes. From this numerical representation, traditional parameters of uncertainty analysis as the standard deviations and bias are calculated. Moreover, these numerical representations are used to investigate the shape of the probability density functions of the output magnitudes. Also, another calibration film is read in four EPSON scanners (two V800 and two 10000XL) and the uncertainty analysis is carried out with the four images. RESULTS: The dose estimates of single-channel and multichannel algorithms show a Gaussian behavior and low bias. The multichannel algorithms lead to less uncertainty in the final dose estimates when the EPSON V800 is employed as reading device. In the case of the EPSON 10000XL, the single-channel algorithms provide less uncertainty in the dose estimates for doses higher than four Gy. CONCLUSION: A multi-stage model has been presented. With the aid of this model and the use of the Monte-Carlo techniques, the uncertainty of dose estimates for single-channel and multichannel algorithms are estimated. The application of the model together with Monte-Carlo techniques leads to a complete characterization of the uncertainties in radiochromic film dosimetry.

The incidence of the different sources of noise on the uncertainty in radiochromic film dosimetry using single channel and multichannel methods.

The influence of the various sources of noise on the uncertainty in radiochromic film (RCF) dosimetry using single channel and multichannel methods is investigated in this work. These sources of noise are extracted from pixel value (PV) readings and dose maps. Pieces of an RCF were each irradiated to different uniform doses, ranging from 0 to 1092 cGy. Then, the pieces were read at two resolutions (72 and 150 ppp) with two flatbed scanners: Epson 10000XL and Epson V800, representing two states of technology. Noise was extracted as described in ISO 15739 (2013), separating its distinct constituents: random noise and fixed pattern (FP) noise. Regarding the PV maps, FP noise is the main source of noise for both models of digitizer. Also, the standard deviation of the random noise in the 10000XL model is almost twice that of the V800 model. In the dose maps, the FP noise is smaller in the multichannel method than in the single channel ones. However, random noise is higher in this method, throughout the dose range. In the multichannel method, FP noise is reduced, as a consequence of this method's ability to eliminate channel independent perturbations. However, the random noise increases, because the dose is calculated as a linear combination of the doses obtained by the single channel methods. The values of the coefficients of this linear combination are obtained in the present study, and the root of the sum of their squares is shown to range between [Formula: see text] and [Formula: see text] over the dose range studied. These results indicate the random noise to play a fundamental role in the uncertainty of RCF dosimetry: low levels of random noise are required in the digitizer to fully exploit the advantages of the multichannel dosimetry method. This is particularly important for measuring high doses at high spatial resolutions.

On the re-calibration process in radiochromic film dosimetry.

PURPOSE: The accuracy and precision of the dose estimates obtained with radiochromic film dosimetry are investigated in a clinical environment. The improvement in the accuracy of dose estimates reached with corrective methods is analyzed. Two novel re-calibration algorithms for radiochromic film dosimetry are presented. METHODS: Two different EBT3 lots are evaluated in two different centres. They are calibrated in Varian linacs and read in two different EPSON scaners. Once the lots are calibrated, three films per lot are considered and divided into stripes that are exposed to known doses. Several dosimetry protocols usually employed in radiochromic film dosimetry are used to convert film responses to absorbed doses. These protocols are characterized by different choices of the film responses or different sensitometric curves. Finally, the accuracy and reproducibility of the dose estimates is investigated with and without the corrective methods. RESULTS AND CONCLUSIONS: The variabilities that affect radiochromic film dosimetry, such as intra-lot variability, inter-scan variability, post-exposure time and film autodevelopment may give rise to inaccuracies in the dose estimates. However, the implementation of re-calibration methods leads to more accurate dose estimates. All the investigated protocols showed more accurate and reproducible results when the re-calibrated methods were employed. So, the novel re-calibration methods may be applied in order to improve the accuracy and reproducibility of radiochromic film dosimetry.

Characterization of noise and digitizer response variability in radiochromic film dosimetry. Impact on treatment verification.

PURPOSE: To study how noise and scanner response variability affect radiochromic film dosimetry. METHODS: Five treatment plans were analyzed in this work with two different multichannel protocols: the multichannel algorithm of Mayer et al. and the efficient protocol of Lewis et al. RESULTS AND CONCLUSION: The multichannel protocol of Mayer et al. is not able to compensate variability in scanner response, which is an important issue for radiochromic film dosimetry. The efficient protocol compensates variations of scanner response, so dose values and gamma scores become more accurate and reproducible. The compensation of digitizer scan variability of the efficient protocol, together with time averaging improve radiochromic film dosimetry. Noise is related to selected resolution in the scanner, our results show that if high resolution measurements are required, de-noising should be considered.

Technical Note: Statistical dependences between channels in radiochromic film readings. Implications in multichannel dosimetry.

PURPOSE: This note studies the statistical relationships between color channels in radiochromic film readings with flatbed scanners. The same relationships are studied for noise. Finally, their implications for multichannel film dosimetry are discussed. METHODS: Radiochromic films exposed to wedged fields of 6 MV energy were read in a flatbed scanner. The joint histograms of pairs of color channels were used to obtain the joint and conditional probability density functions between channels. Then, the conditional expectations and variances of one channel given another channel were obtained. Noise was extracted from film readings by means of a multiresolution analysis. Two different dose ranges were analyzed, the first one ranging from 112 to 473 cGy and the second one from 52 to 1290 cGy. RESULTS: For the smallest dose range, the conditional expectations of one channel given another channel can be approximated by linear functions, while the conditional variances are fairly constant. The slopes of the linear relationships between channels can be used to simplify the expression that estimates the dose by means of the multichannel method. The slopes of the linear relationships between each channel and the red one can also be interpreted as weights in the final contribution to dose estimation. However, for the largest dose range, the conditional expectations of one channel given another channel are no longer linear functions. Finally, noises in different channels were found to correlate weakly. CONCLUSIONS: Signals present in different channels of radiochromic film readings show a strong statistical dependence. By contrast, noise correlates weakly between channels. For the smallest dose range analyzed, the linear behavior between the conditional expectation of one channel given another channel can be used to simplify calculations in multichannel film dosimetry.

Technical Note: MTF determination from a star bar pattern image.

PURPOSE: A procedure to obtain the MTF of an image system from a star bar pattern image is presented. The results obtained are compared to the standard analysis of an edge image (as described in the norm IEC 62220-1). METHODS: The images used are artificially generated following a procedure that simulates sampling, blurring, and noise present in a real phantom image. The MTF from a star bar pattern is obtained from the analysis of circular scans over the star pattern image and concentric to it. The radius of the scanning circumference is proportional to the spatial frequency of the curve arising from the scan, and the amplitude of this curve is directly related to the system MTF for that frequency. An oversampling procedure is also followed to minimize the effect of noise in the image. In order to investigate the MTF in a particular direction, the image analysis is restricted to an angular sector centered around that direction. RESULTS: The MTFs calculated following this procedure show a very good agreement with those obtained following the standard IEC 62220-1 for low noise levels. As the noise increases, however, the star bar method is more accurate than the edge method. CONCLUSIONS: The procedure presented provides a good method to investigate the MTF in any spatial direction. Finally, as the whole star bar pattern image is used for MTF computation, the high signal to noise ratio guarantees a good immunity against noise.