M.E. Brandan

TL response of LiF: Mg,Ti exposed to intermediate energy 1H, 3He, 12C, 16O and 20Ne ions

The dependence of the thermoluminescent (TL) LiF : Mg,Ti response as a function of linear energy transfer (LET) has been investigated, through a careful analysis of the induced glow curve. We have irradiated TLD-100 chips in a stack with low fluences (5 × 105–5 × 107 cm−2) of a variety of ions: 1, 3, 25 and 40 MeV µ−1 hydrogen, 25 and 40 MeV µ−1 helium, 15, 25 and 40 MeV µ−1 carbon, 25 MeV µ−1 oxygen and 40 MeV µ−1 neon, spanning a LET interval from 1.55 to 517 keV μ m−1. For all peaks, except peak 8, the measured TL response is a linear function of deposited energy. We have observed differences between the LET dependence of the low- (4 and 5) and the high- (6a to 9) temperature peaks: the low-temperature peaks decrease their presence in the glow curve as LET increases, while the high-temperature peaks do the opposite. The peak 8 response exhibits linear–supralinear behaviour as a function of dose, the strongest LET dependence and the highest efficiency relative to gamma rays (about 20 near 200 keV μ m−1). The observed evidence of the glow curve shape evolution with LET and the response to gamma rays suggests that the entities responsible for the LiF : Mg,Ti high- and low-temperature peaks have widely different properties.

Study of the TL response of LiF:Mg,Ti to 3 and 7.5 MeV helium ions: Measurements and interpretation in terms of the track interaction model

Abstract We have measured the LiF:Mg,Ti (TLD-100) thermoluminescent response, efficiency and supralinearity, to 3 and 7.5 MeV helium ions in the fluence interval between 6×107 to 1.5×1011 cm−2. Ion irradiations were performed using a flux of elastically scattered ions from a thin gold target. The measured efficiencies with respect to 60Co γ-rays are equal to 0.074 and 0.174 at 3 and 7.5 MeV, respectively, for the response of peak 5. The supralinearity functions for peaks 4, 5, 6a, 6b, 7, 8, 9 and the total TL signal indicate that supralinearity is an increasing function of energy and that it increases with peak temperature. The data for peaks 5, 8 and 9 have been successfully described by Monte Carlo simulations of the track interaction model. The fitted parameters show that peak 5 displays a special behaviour which requires the use of a much larger effective track radius than the higher temperature peaks.

Response of radiochromic dye films to low energy heavy charged particles

Abstract We have studied the possible use of radiochromic dye films (RCF) as heavy charged particle dosemeters. We present the results of irradiating two commercial RCF (GafChromic HD-810 and MD-55-1) with 1.5, 2.9 and 4.4 MeV protons, 1.4, 2.8, 4.7, 5.9, 6.8 MeV 4He ions and 8.5 and 12.4 MeV 12C ions, at proton doses from about 1 Gy up to 3 kGy, helium ions doses from 3 Gy to 5 kGy and carbon ion doses from 30 Gy to 20 kGy. The films were scanned and digitized using commercial equipment. For a given particle, the response per unit dose at different energies indicates an energy dependence of the sensitivity, which is discussed. Comparison was made for the use of a standard spectrophotometer to obtain optical density readings versus a white light scanner.

Supralinear response and efficiency of LiF:Mg,Ti to 0.7, 1.5 and 3 MeV protons

Abstract The thermoluminescent (TL) response, supralinearity function, and relative efficiency with respect to 60 Co γ-rays, of TLD-100 exposed to 0.7, 1.5 and 3 MeV protons have been measured. The TL response of the glow curve peaks using two different deconvolutions, separating in either 7 (3–9) or 9 peaks (3–10 and peaks 6a and 6b), was investigated. Results for the supralinearity function of peaks 5 and 7–9 and the relative response of peaks 5 and 7 are insensitive to the chosen deconvolution scheme. At all energies we observe that peak 5 is slightly supralinear and that supralinearity increases as a function of the peak temperature. The relative efficiencies, measured for the total TL signal (area under the glow curve) and peaks 5 and 7, decrease with the proton energy having values of 0.33, 0.46 and 0.70 (total TL signal), 0.22, 0.32 and 0.53 (peak 5) and 1.4, 2.6 and 3.6 (peak 7) at 0.7, 1.5 and 3 MeV, respectively. These data agree with the systematics reported in the literature. The measurements of the supralinearity function are compared with Monte Carlo track interaction model calculations. The analysis suggests that the model ion track effective radius and the saturation radius are independent of the incident ion energy.

SU‐FF‐I‐34: PSF and S/P in Mammography: A Validation of Simulations Using the GEANT4 Code

Purpose: To study GEANT4 predictions for low-energy (10–40 keV) photon transmission and scattering through tissue-like thick matter, specifically to compare the predicted scatter to primary ratio (S/P) and line and point spread functions (LSF and PSF) against published and new measurements. Method and Materials: Using the GEANT4 code we have performed Monte Carlo simulations of the 1978 Barnes and Brezovich beam stop measurements of S/P, and the 2000 Cooper et al. edge detection experiments to evaluate S/P and LSF. Also, we have measured the PSF with a tungsten 1 mm diameter collimator on 3 – 6 cm thick lucite phantoms, using a Senographe 2000D unit operated at 26–28 kVp. In the simulations we have reproduced the exact experimental geometrical conditions, as well as the rest of parameters, according to the information contained in the original reports. Results: The S/P simulation and data agree within 3–10% accuracy, depending on the data set. The largest difference is with respect to Cooper et al., probably partially related to an inaccurate simulation of the original experimental conditions. For the LSF, the agreement between simulation and data is better than 15%, and for our PSF, better than 12 % at distances smaller than 5 mm. This last value should be taken as an upper limit, because of the lack of complete information on the features of the clinical unit. Conclusion: The analysis of these comparisons confirms that the GEANT4 predictions for low energy photon scattering are described with an accuracy better than 12%, at least up to 8 cm tissue thickness.

Validation of a Custom‐made Software for DQE Assessment in Mammography Digital Detectors

This works presents the validation of a custom‐made software, designed and developed in Matlab, intended for routine evaluation of detective quantum efficiency DQE, according to algorithms described in the IEC 62220‐1‐2 standard. DQE, normalized noise power spectrum NNPS and pre‐sampling modulation transfer function MTF were calculated from RAW images from a GE Senographe DS (FineView disabled) and a Siemens Novation system. Calculated MTF is in close agreement with results obtained with alternative codes: MTF_tool (Maidment), ImageJ plug‐in (Perez‐Ponce) and MIQuaELa (Ayala). Overall agreement better than ≈90% was found in MTF; the largest differences were observed at frequencies close to the Nyquist limit. For the measurement of NNPS and DQE, agreement is similar to that obtained in the MTF. These results suggest that the developed software can be used with confidence for image quality assessment.

TU-C-I-609-01: The Differences Method for the Tissue Structured Noise Determination in Digital Mammography

Purpose: To propose a new method for the estimation of the tissue structured noise, based on the differences distributions among neighboring pixels. Method and Materials: Breast tissue structured noise (σT) is determined as the difference between the total and system noise. The system noise (σSYS), which includes quantum and apparatus noise, is determined using digital mammography images without an object. The total noise is obtained from the differences distributions among neighboring pixels in the region where the compressed breast thickness is constant. The range of raw pixel values is split into intervals to suppress the quantum noise variation contribution and σT is determined for each interval. 280 Senographe 2000D clinical images from 100 unselected patients were collected and analyzed. Also based on the differences distributions for a flat phantom, we propose a method to estimate spatial resolution for pixelized detectors. Results: The average measured value of σT /σSYS is 0.45±0.15 for the 10 cycles/mm frequency and for all the pixel value intervals. The absolute value of σT for the same frequency is estimated to be 1.7 ± 1.0 microns in microcalcification thickness equivalent. The σT is also determined down to 1 cycle/mm frequency. The estimated Senographe 2000D spatial resolution is approximately 60 microns. Conclusion: The proposed method for σT and resolution estimation is very simple and fast enough to use in online image processing, not requiring a complex signal/background analysis. The main limitation of the method for low frequencies (less than 1 cycle/mm) is statistical.

A novel method to use radiochromic dye films to determine dose under proton irradiation

The response of commercial radiochromic dye films exposed to 0.7, 1.5 and 3.0 MeV protons, in the dose interval between 1−103 Gy, has been studied. The response of the plastics (MD-55 and HD-810 from Gafchromic) to the radiation was evaluated by digitalizing and analyzing the films with commercial computer systems. Curves of response as a function of dose show an approximately linear behaviour followed by saturation. The response in the lower dose interval suggests a possible use of these methods in heavy charged particle medical applications. To verify consistency with the traditional spectrophotometer reading procedure, the films were also irradiated and evaluated with 60Co gamma rays.

TU-CD-207-03: Time Evolution of Texture Parameters of Subtracted Images Obtained by Contrast-Enhanced Digital Mammography (CEDM)

Purpose: To evaluate the time evolution of texture parameters, based on the gray level co-occurrence matrix (GLCM), in subtracted images of 17 patients (10 malignant and 7 benign) subjected to contrast-enhanced digital mammography (CEDM). The goal is to determine the sensitivity of texture to iodine uptake at the lesion, and its correlation (or lack of) with mean-pixel-value (MPV). Methods: Acquisition of clinical images followed a single-energy CEDM protocol using Rh/Rh/48 kV plus external 0.5 cm Al from a Senographe DS unit. Prior to the iodine-based contrast medium (CM) administration a mask image was acquired; four CM images were obtained 1, 2, 3, and 5 minutes after CM injection. Temporal series were obtained by logarithmic subtraction of registered CM minus mask images.Regions of interest (ROI) for the lesion were drawn by a radiologist and the texture was analyzed. GLCM was evaluated at a 3 pixel distance, 0° angle, and 64 gray-levels. Pixels identified as registration errors were excluded from the computation. 17 texture parameters were chosen, classified according to similarity into 7 groups, and analyzed. Results: In all cases the texture parameters within a group have similar dynamic behavior. Two texture groups (associated to cluster and sum mean) show a strong correlation with MPV; their average correlation coefficient (ACC) is r^2=0.90. Other two groups (contrast, homogeneity) remain constant with time, that is, a low-sensitivity to CM uptake. Three groups (regularity, lacunarity and diagonal moment) are sensitive to CM uptake but less correlated with MPV; their ACC is r^2=0.78. Conclusion: This analysis has shown that, at least groups associated to regularity, lacunarity and diagonal moment offer dynamical information additional to the mean pixel value due to the presence of CM at the lesion. The next step will be the analysis in terms of the lesion pathology. Authors thank PAPIIT-IN105813 for support. Consejo Nacional de Ciencia Y Tecnologia, PAPIIT-IN105813

WE‐G‐103‐01: Dual‐Energy Temporal Study of Contrast‐Enhanced Digital Mammography in 20 Patients: The Use of a Pixel‐By‐Pixel Weighting Formalism for the Subtraction

PURPOSE To study the possible correlation between iodine uptake in contrast-enhanced digital mammography images and microvessel density (MVD) in breast lesions. METHODS 20 BIRADS 4-5 patients were included. Mask images were acquired with low-and high-energy spectra from a SenographeDS. Iodine-based contrast medium (CM) was mechanically injected and high-energy CM images acquired between 1 and 5 minutes after injection. A biopsy was obtained after the images and specific biomarkers for newly-formed blood and lymphatic microvessels were applied; MVD was later evaluated in microscope fields. The subtraction combined dual-energy and temporal modalities. LE masks were subtracted from weighted CM images, and weight factor was a matrix obtained from the masks, containing pixel-by-pixel anatomical and radiological information. Iodine uptake in subtracted temporal series was quantified by contrast between lesion and normal glandular tissue. Alternative metrics for contrast quantification were evaluated. Contrast was transformed into iodine mass-thickness using calibrated samples. RESULTS 12 cases were malignant and 8, benign. The matrix-based subtraction formalism severely reduced anatomic noise in resulting images, compared with alternative techniques based on mean pixel values within regions-of-interest. Five types of time-intensity curves were identified, qualitatively similar to MRI. If iodine uptake in the lesion was evaluated, most frequent curve was plateau type, for cancer and malignant cases. The normalized Weber contrast indicated a frequency distribution with zero-contrast cases for benign lesions and most-frequent curve type washout for cancer. No correlation was found between contrast indicators and MVD. Blood and lymphatic MVD were correlated (r=0.94, p<0.05) and mean blood MVD values in cancer were about twice those in benign cases. CONCLUSION This subtraction formalism, previously validated in non-homogeneous phantom images, adds quantitative features to the resulting clinical images. The statistical distribution of contrast curves for cancer and benign cases seems to be the most relevant parameter with a possible diagnostic value. FUNDING DGAPA-UNAM Grants 102610 and 105813; Conacyt Grant Salud 2009-01-112374. No conflict of interest.