Elena Salvagnini

Quantification of scattered radiation in projection mammography: Four practical methods compared

PURPOSE Four different practical methodologies of quantifying scattered radiation for two different digital mammographic systems are compared. The study considered both grid in and grid out geometries for two different antiscatter grid types, a typical linear grid and a cellular grid design. The aim was to find quick and reproducible methods that could be used in place of the beam stop technique. METHODS The scatter to primary ratio (SPR) and the scatter fraction (SF) were used to quantify scattered radiation as a function of poly(methyl methacrylate) (PMMA) thickness, grid position, and beam quality. The four scatter estimation methods applied were (1) the beam stop method, (2) a hybrid method that combined measured detector (scatter-free) modulation transfer function (MTF) data and a Monte Carlo simulation of the scatter point spread function, (3) from the low frequency drop data taken from the system MTF, and (4) from the edge spread function (ESF) measured in the presence of PMMA. Repeatability error was assessed for all methods. RESULTS SPR results acquired with the beam stop method ranged from 0.052 to 0.187 for the system with linear grid and from 0.012 to 0.064 for the cellular grid system, as PMMA thickness was increased from 20 to 80 mm. With the grid removed, beam stop SPR was similar for both systems, ranging between 0.268 and 1.124, for corresponding MTF thicknesses. The direct MTF method had a maximum difference of 24% from the beam stop SPR and SF data for all conditions except the cellular grid in geometry, where maximum difference in SPR was 0.044 (164%). The ESF technique gave large differences from the beam stops for both grid geometries but agreement was within 21% for the grid out geometry. Repeatability error with beam stops was between 1% and 5% for the grid out geometries, while for the grid in cases it was 13% and 87% for the linear and cellular grids, respectively. Repeatability error for the direct MTF method applied to both systems and grid geometries ranged between 3% and 12%. CONCLUSIONS All three alternative methods to the beam stop technique gave reasonable estimates of SPR without grid, with a maximum difference of 24% (mean difference 8%). For the grid in geometry, the direct MTF method gave a maximum difference of 24% for the linear grid system, while maximum percentage difference was 119% (absolute difference of 0.042) for the system with the cellular grid, where SPR values were low. Except for cases where the SPR is very low, the direct MTF method offers a quick and reproducible alternative to the beam stop technique.

Design and application of a structured phantom for detection performance comparison between breast tomosynthesis and digital mammography.

This paper introduces and applies a structured phantom with inserted target objects for the comparison of detection performance of digital breast tomosynthesis (DBT) against 2D full field digital mammography (FFDM). The phantom consists of a 48 mm thick breast-shaped polymethyl methacrylate (PMMA) container filled with water and PMMA spheres of different diameters. Three-dimensionally (3D) printed spiculated masses (diameter range: 3.8-9.7 mm) and non-spiculated masses (1.6-6.2 mm) along with microcalcifications (90-250 µm) were inserted as targets. Reproducibility of the phantom application was studied on a single system using 30 acquisitions. Next, the phantom was evaluated on five different combined FFDM & DBT systems and target detection was compared for FFDM and DBT modes. Ten phantom images in both FFDM and DBT modes were acquired on these 5 systems using automatic exposure control. Five readers evaluated target detectability. Images were read with the four-alternative forced-choice (4-AFC) paradigm, with always one segment including a target and 3 normal background segments. The percentage of correct responses (PC) was assessed based on 10 trials of each reader for each object type, size and imaging modality. Additionally, detection threshold diameters at 62.5 PC were assessed via non-linear regression fitting of the psychometric curve. The reproducibility study showed no significant differences in PC values. Evaluation of target detection in FFDM showed that microcalcification detection thresholds ranged between 110 and 118 µm and were similar compared to the detection in DBT (range of 106-158 µm). In DBT, detection of both mass types increased significantly (p  =  0.0001 and p  =  0.0002 for non-spiculated and spiculated masses respectively) compared to FFDM, achieving almost 100% detection for all spiculated mass diameters. In conclusion, a structured phantom with inserted targets was able to show evidence for detectability differences between FFDM and DBT modes for five commercial systems. This phantom has potential for application in task-based assessment at acceptance and commissioning testing of DBT systems.This paper introduces and applies a structured phantom with target objects for the comparison of detection performance of digital breast tomosynthesis (DBT) against full field digital mammography (FFDM). The phantom consists of a 48 mm thick breast-shaped polymethyl methacrylate (PMMA) container filled with water and PMMA spheres of different diameters. Three-dimensionally (3D) printed spiculated masses (diameter range: 3.8-9.7 mm) and non-spiculated masses (1.6-6.2 mm) along with microcalcifications (90-250 µm) were inserted as targets. Reproducibility of the phantom application was studied on a single system using 30 acquisitions. Next, the phantom was evaluated on five different combined FFDM & DBT systems and target detection was compared for FFDM and DBT modes. Ten phantom images in both FFDM and DBT modes were acquired on these 5 systems using automatic exposure control (AEC). Five readers evaluated target detectability. Images were read with the four-alternative forced-choice (4-AFC) paradigm, with always one segment including a target and 3 normal background segments. The percentage of correct responses (PC) was assessed based on 10 trials of each reader for each object type, size and modality. Additionally, detection threshold diameters at 62.5 PC were assessed via non-linear regression fitting of the psychometric curve. The reproducibility study showed no significant differences in PC values. Evaluation of target detection in FFDM showed that microcalcification detection thresholds ranged between 110 and 118 µm and were similar compared to the detection in DBT (range of 106-158 µm). In DBT, detection of both mass types increased significantly (p=0.0001 and p=0.0002 for non-spiculated and spiculated masses respectively) compared to FFDM, achieving almost 100% detection for all spiculated mass diameters. In conclusion, a structured phantom with inserted targets was able to show evidence for detectability differences between FFDM and DBT modes for five commercial systems. This phantom has potential for application in task-based assessment at acceptance and commissioning testing of DBT systems.

Model observer detectability as a substitute for contrast detail analysis in routine digital mammography quality control

This work investigated the substitution of CDMAM contrast detail (c-d) analysis with detectability (d’) from a nonprewhitening eye filter model observer (NPWE) for routine quality control (QC). Routinely acquired QC data for 53 systems were analyzed: 13 computed radiography (CR) and 40 integrated detector (DR) systems. For a given system, threshold gold thickness from the c-d analysis (T) was calculated from 16 images and compared against d’ calculated for 0.1 and 0.25 mm diameter discs. The d’ data were calculated from the routine 50 mm PMMA AEC image and the measured pre-sampling detector modulation transfer function (MTF). Threshold gold thickness T and d’ were plotted as function of MGD and compared. The Fuji CR and the Agfa CR systems had the highest T values compared to the other systems. The Hologic systems were found to have a low value of T, compared to the other systems, for both disc diameters. The NPWE results reflected the performance seen for T data for the majority of the systems with the exception of the Fuji CR and Konica CR systems. The Hologic systems gave unexpectedly low d’ results or unexpectedly low T values. The correspondence between the two quality indices was examined with the Pearson correlation statistical test. This test was not applicable to the GE Essential systems because all systems are grouped together at the same working point so the result of r is about 0. For all other groups of systems the test gave good results (larger than -0.65).

Software framework for simulating clusters of microcalcifications in digital mammography

Observer performance experiments for lesion detection are an accepted means of assessing the imaging performance of radiological imaging systems Simulation methods for clusters of microcalcifications have been proposed for creating images with abnormal pathology for its use in such experiments We report on a software tool that can generate simulated clusters of microcalcifications for different exposure parameters and different digital mammography systems The effect of the simulation steps on microcalcification templates, (namely exposure settings, breast thickness, modulation transfer function (MTF) and pixel size) is demonstrated and validated Results were evaluated in terms of the clusters peak contrast (PC) for three cases: for different exposure conditions within a given system, for different systems and for different system MTF calculation methods As expected, with higher tube voltage and for insertion into thicker breast simulating material, the lesion contrast decreases while the position of the peak remains unchanged When different systems are considered with the same exposure settings, the observed difference in the PCs is related to the blurring due to the different MTF and the pixel size of the systems; a shift in the peak position is also observed, due to resampling This functional and user-friendly system could be used by other researchers for performing comparative studies of mammographic imaging systems.

Grid-less imaging with antiscatter correction software in 2D mammography: the effects on image quality and MGD under a partial virtual clinical validation study

This work investigated the effect of the grid-less acquisition mode with scatter correction software developed by Siemens Healthcare (PRIME mode) on image quality and mean glandular dose (MGD) in a comparative study against a standard mammography system with grid. Image quality was technically quantified with contrast-detail (c-d) analysis and by calculating detectability indices (d’) using a non-prewhitening with eye filter model observer (NPWE). MGD was estimated technically using slabs of PMMA and clinically on a set of 11439 patient images. The c-d analysis gave similar results for all mammographic systems examined, although the d’ values were slightly lower for the system with PRIME mode when compared to the same system in standard mode (-2.8% to -5.7%, depending on the PMMA thickness). The MGD values corresponding to the PMMA measurements with automatic exposure control indicated a dose reduction from 11.0% to 20.8% for the system with PRIME mode compared to the same system without PRIME mode. The largest dose reductions corresponded to the thinnest PMMA thicknesses. The results from the clinical dosimetry study showed an overall population-averaged dose reduction of 11.6% (up to 27.7% for thinner breasts) for PRIME mode compared to standard mode for breast thicknesses from 20 to 69 mm. These technical image quality measures were then supported using a clinically oriented study whereby simulated clusters of microcalcifications and masses were inserted into patient images and read by radiologists in an AFROC study to quantify their detectability. In line with the technical investigation, no significant difference was found between the two imaging modes (p-value 0.95).

Grid-Less Imaging with Anti-scatter Correction Software in 2D Mammography: A JAFROC Study Using Simulated Lesions

Purpose: To perform a virtual clinical trial study to assess the justification of the grid-less mammography acquisition mode with scatter correction software, as developed by Siemens Healthcare PRIME mode. Materials and methods: The study was performed on a Siemens mammography unit using the conventional acquisition mode system 1 and a second system used PRIME. Mean glandular doses MGD were compared from data of 5981 images. A paired t-test for all thickness groups 69i¾?mm separately and combined had shown a significantly higher average MGD for system 1 NON-PRIME when compared to system 2 PRIME, with an overall decrease of 11.7i¾?%. The next phase in justification focused on detectability performance, in particular for screening applications. A dataset mimicking an enriched screened population was created by simulating previously developed anthropomorphic mass models and microcalcification clusters in 60 out of 100 normal mammograms of system 1 NON-PRIME. The same physical lesions were then simulated into 60 out of 100 PRIME, normal mammograms. Care was taken to simulate each lesion model in matched mammograms PRIME-NON PRIME in terms of BI-RADS score, in a region with the same background glandularity obtained after analysis with Volpara and in a breast of the same thickness group. All images were visualized with ViewDEX software and four radiologists performed the free search detectability study. A JAFROC analysis was executed and detectability was quantified by means of the AUC. Results: Present approach allowed the realization of paired virtual clinical data sets starting from 200 normal mammograms. The results of all readers separately as well as combined showed approximately the same AUC for PRIME and NON-PRIME 0.57 vs 0.60, and the ANOVA analysis showed no statistical significant difference in detectability of the lesions between PRIME and NON-PRIME p-value 0.36. The same result was found if the dataset was subdivided for both types of lesions: masses p-value 0.88 and microcalcification clusters p-value 0.33. Conclusion: Results state that the MGD is significantly lower in PRIME mode than with the conventional acquisition while lesion detectability remained constant for all four radiologists.

A comparison of mammographic systems for different breast thicknesses using model observer detectability

This work investigated image quality as a function of PMMA thickness on a variety of mammography systems. Image quality was quantified by calculating detectability (d’) using a non-prewhitening with eye filter model observer (NPWE) from routinely acquired quality control (QC) data of twelve digital radiography (DR) systems. The sample of systems included two mammography devices equipped with the Siemens PRIME upgrade and one system with the Claymount SmartBucky detector. The d’ data were calculated for a 0.1 and 0.25 mm diameter gold discs using images of homogeneous PMMA (thickness from 2 to 7 cm), all from the routinely performed AEC test. The GE Essential systems had the highest d’ values for low thicknesses and the lowest d’ values for high thicknesses. The Hologic Selenia Dimension systems had the most constant detectability curve, ensuring high d’ values at high thicknesses. This was achieved by increasing the mean glandular dose (MGD) at higher thicknesses compared to the other systems. The Siemens PRIME and the Claymount system detectability results were comparable to the standard FFDM systems. Mean glandular dose at 5, 6 and 7 cm PMMA and gold threshold thickness at 5 cm PMMA were also evaluated. The Claymount system had a high (but acceptable) threshold gold thickness (T) compared to the other systems. This was probably caused by the low dose at which this DR detector operates. Results of NPWE d’ and CDMAM analysis showed the same trends.

Effective Detective Quantum Efficiency (eDQE) Measured for a Digital Breast Tomosynthesis System

This paper presents effective detective quantum efficiency (eDQE) results for a digital breast tomosynthesis (DBT) system. Poly(methyl methacrylate) (PMMA) blocks of thickness 2, 4, 6 and 7 cm were imaged under automatic exposure control (AEC) in standard 2D digital (planar) mammography (with anti-scatter grid) and DBT mode (without anti-scatter grid). Modulation transfer function (MTF) for the projection images was measured in the front-back and left-right (i.e. tube-travel for DBT) directions using a 0.8 mm thick steel edge at positions 2, 4, 6 and 7 cm above the breast table. NNPS data required for eDQE calculation were calculated from the AEC projection images (the ~0° projection AEC image for DBT). The eDQE at 0.5 mm− 1 in planar mammography mode was relatively stable at ~0.25 as PMMA thickness changed from 2 to 7 cm. For DBT, blurring from the focus motion and scattered radiation reduced eDQE at 6 and 7 cm PMMA.

Impact of compressed breast thickness and dose on lesion detectability in digital mammography: FROC study with simulated lesions in real mammograms

PURPOSE The aim of this work was twofold: (1) to examine whether, with standard automatic exposure control (AEC) settings that maintain pixel values in the detector constant, lesion detectability in clinical images decreases as a function of breast thickness and (2) to verify whether a new AEC setup can increase lesion detectability at larger breast thicknesses. METHODS Screening patient images, acquired on two identical digital mammography systems, were collected over a period of 2 yr. Mammograms were acquired under standard AEC conditions (part 1) and subsequently with a new AEC setup (part 2), programmed to use the standard AEC settings for compressed breast thicknesses ≤49 mm, while a relative dose increase was applied above this thickness. The images were divided into four thickness groups: T1 ≤ 29 mm, T2 = 30-49 mm, T3 = 50-69 mm, and T4 ≥ 70 mm, with each thickness group containing 130 randomly selected craniocaudal lesion-free images. Two measures of density were obtained for every image: a BI-RADS score and a map of volumetric breast density created with a software application (VolparaDensity, Matakina, NZ). This information was used to select subsets of four images, containing one image from each thickness group, matched to a (global) BI-RADS score and containing a region with the same (local) volpara volumetric density value. One selected lesion (a microcalcification cluster or a mass) was simulated into each of the four images. This process was repeated so that, for a given thickness group, half the images contained a single lesion and half were lesion-free. The lesion templates created and inserted in groups T3 and T4 for the first part of the study were then inserted into the images of thickness groups T3 and T4 acquired with higher dose settings. Finally, all images were visualized using the ViewDEX software and scored by four radiologists performing a free search study. A statistical jackknife-alternative free-response receiver operating characteristic analysis was applied. RESULTS For part 1, the alternative free-response receiver operating characteristic curves for the four readers were 0.80, 0.65, 0.55 and 0.56 in going from T1 to T4, indicating a decrease in detectability with increasing breast thickness. P-values and the 95% confidence interval showed no significant difference for the T3-T4 comparison (p = 0.78) while all the other differences were significant (p < 0.05). Separate analysis of microcalcification clusters presented the same results while for mass detection, the only significant difference came when comparing T1 to the other thickness groups. Comparing the scores of part 1 and part 2, results for the T3 group acquired with the new AEC setup and T3 group at standard AEC doses were significantly different (p = 0.0004), indicating improved detection. For this group a subanalysis for microcalcification detection gave the same results while no significant difference was found for mass detection. CONCLUSIONS These data using clinical images confirm results found in simple QA tests for many mammography systems that detectability falls as breast thickness increases. Results obtained with the AEC setup for constant detectability above 49 mm showed an increase in lesion detection with compressed breast thickness, bringing detectability of lesions to the same level.