Publications by authors named "Nicholas W Marshall"

15 Publications

  • Page 1 of 1

Investigation of single-shot beam quality measurements using state of the art solid-state dosimeters for routine quality assurance applications in mammography.

Phys Med 2021 Jul 23;88:242-249. Epub 2021 Jul 23.

Medical Imaging Research Center, KU Leuven, Herestraat 49, 3000 Leuven, Belgium; Department of Radiology, UZ Leuven, Herestraat 49, 3000 Leuven, Belgium. Electronic address:

Purpose: To assess if single shot acquisitions with solid-state dosimeters as well as Robson's method could replace ionization chambers for tube output and HVL measurements, saving medical physicists time.

Material And Methods: The energy responses of 4 solid-state dosimeters with automatic calculation of HVL were compared to ionization chamber measurements. Five anode/filter combinations were tested: Mo/Mo, Mo/Rh, Rh/Rh, W/Rh and W/Ag, from 24kVp to 35kVp. Tube output was measured free in air. HVL was measured using the solid-state dosimeters (single-shot acquisition), then manually with aluminum sheets and finally using the parametrization method of Robson.

Results: Deviations in tube output and HVL related to energy response in SSD were small in the 25-32 kVp range, and for tube output typically within 3%. Extrapolation using the Robson parametrization was within 5%, except for one device and for all W/Rh. Deviations of the HVL using the single shot approach were within 10% of the gold standard data. Larger deviations were found at the extreme tube voltages of 24kVp and 35kVp (maximum of 24%).

Conclusion: With the assumption that deviations in tube output of 5% and for HVL of 10% are acceptable, all tested solid state dosimeters met this criterion in the tube voltage range of 26kVp to 32kVp. Robson's method worked well for the spectra for which the method was developed, making both alternative approaches trustworthy for routine quality assurance purposes.
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http://dx.doi.org/10.1016/j.ejmp.2021.07.006DOI Listing
July 2021

On the relevance of modulation transfer function measurements in digital mammography quality control.

J Med Imaging (Bellingham) 2021 Mar 27;8(2):023505. Epub 2021 Apr 27.

KU Leuven, Department of Imaging and Pathology, Leuven, Belgium.

The relevance of presampling modulation transfer function (MTF) measurements in digital mammography (DM) quality control (QC) is examined. Two studies are presented: a case study on the impact of a reduction in MTF on the technical image quality score and analysis of the robustness of routine QC MTF measurements. In the first study, two needle computed radiography (CR) plates with identical sensitivities were used with differences in the 50% point of the MTF ( ) larger than the limiting value in the European guidelines ( change between successive measurements). Technical image quality was assessed via threshold gold thickness of the CDMAM phantom and threshold microcalcification diameter of the L1 structured phantom. For the second study, presampling MTF results from 595 half-yearly QC tests of 55 DM systems (16 types, six manufacturers) were analyzed for changes from the baseline value and changes in between successive tests. A reduction of 20% in of the two CR plates was observed. There was a tendency to a lower score for task-based metrics, but none were significant. Averaging over 55 systems, the absolute relative change in between consecutive tests (with 95% confidence interval) was 3% (2.5% to 3.4%). Analysis of the maximum relative change from baseline revealed changes of up to for one a-Se based system and for a group of CsI-based systems. A limit of 10% is a relevant action level for investigation. If exceeded, then the impact on performance has to be verified with extra metrics.
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http://dx.doi.org/10.1117/1.JMI.8.2.023505DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8076849PMC
March 2021

Methodology to create 3D models of COVID-19 pathologies for virtual clinical trials.

J Med Imaging (Bellingham) 2021 Jan 4;8(Suppl 1):013501. Epub 2021 Jan 4.

KU Leuven, Medical Physics and Quality Assessment, Leuven, Belgium.

We describe the creation of computational models of lung pathologies indicative of COVID-19 disease. The models are intended for use in virtual clinical trials (VCT) for task-specific optimization of chest x-ray (CXR) imaging. Images of COVID-19 patients confirmed by computed tomography were used to segment areas of increased attenuation in the lungs, all compatible with ground glass opacities and consolidations. Using a modeling methodology, the segmented pathologies were converted to polygonal meshes and adapted to fit the lungs of a previously developed polygonal mesh thorax phantom. The models were then voxelized with a resolution of and used as input in a simulation framework to generate radiographic images. Primary projections were generated via ray tracing while the Monte Carlo transport code was used for the scattered radiation. Realistic sharpness and noise characteristics were also simulated, followed by clinical image processing. Example images generated at 120 kVp were used for the validation of the models in a reader study. Additionally, images were uploaded to an Artificial Intelligence (AI) software for the detection of COVID-19. Nine models of COVID-19 associated pathologies were created, covering a range of disease severity. The realism of the models was confirmed by experienced radiologists and by dedicated AI software. A methodology has been developed for the rapid generation of realistic 3D models of a large range of COVID-19 pathologies. The modeling framework can be used as the basis for VCTs for testing detection and triaging of COVID-19 suspected cases.
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http://dx.doi.org/10.1117/1.JMI.8.S1.013501DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7791575PMC
January 2021

Survey of chest radiography systems: Any link between contrast detail measurements and visual grading analysis?

Phys Med 2020 Aug 28;76:62-71. Epub 2020 Jun 28.

KU Leuven, Medical Physics & Quality Assessment Group, Herestraat 49, B-3000 Leuven, Belgium; UZ Gasthuisberg, Department of Radiology, Herestraat 49, B-3000 Leuven, Belgium.

Purpose: To evaluate image quality of chest radiography for a number of systems in Belgium, using a contrast-detail (c-d) test object and Visual Grading Analysis (VGA) of an anthropomorphic phantom.

Methods: The study comprised 22 chest imaging systems in Belgium. C-d data were measured using Leeds TO20 test object, imaged using poly(methyl methacrylate) (PMMA) thicknesses of 9, 13 and 16 cm. Images of the Lungman phantom, with additional tissue-equivalent chest plates to represent different patient sizes, were then acquired. Perceived image quality was evaluated using VGA by three radiologists. Images were acquired at a patient equivalent position with system-specific exposure settings for Posterior-Anterior chest protocol. Incident air kerma (IAK) was measured using a solid-state dosemeter.

Results: C-d results showed large differences between the systems. Total number of visible discs ranged from 38 to 83 (for 9 cm PMMA) with a consistent average drop of 10% as PMMA thickness was systematically increased. However, no correlation was found between number of visible discs and IAK. Perceived image quality scored by the readers from the Lungman images decreased with increasing phantom thickness, however no correlation of VGA score with IAK was seen. Moderate correlation was found between the VGA score of one of the readers and the TO20 results, and no correlation for the rest.

Conclusions: The spread in dose and image quality measures was high and no correlation was seen between either image quality measure and IAK, suggesting the need for optimization. A more powerful tool is required for task-based optimization in chest radiography.
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http://dx.doi.org/10.1016/j.ejmp.2020.06.014DOI Listing
August 2020

Systematic approach to a channelized Hotelling model observer implementation for a physical phantom containing mass-like lesions: Application to digital breast tomosynthesis.

Phys Med 2019 Feb 17;58:8-20. Epub 2019 Jan 17.

Dept. of Medical Physics and Quality Assessment, KU Leuven, Leuven, Belgium; Dept. of Radiology, UZ Leuven, Belgium.

Purpose: to develop a channelized model observer (CHO) that matches human reader (HR) scoring of a physical phantom containing breast simulating structure and mass lesion-like targets for use in quality control of digital breast tomosynthesis (DBT) imaging systems.

Methods: A total of 108 DBT scans of the phantom was acquired using a Siemens Inspiration DBT system. The detectability of mass-like targets was evaluated by human readers using a 4-alternative forced choice (4-AFC) method. The percentage correct (PC) values were then used as the benchmark for CHO tuning, again using a 4-AFC method. Three different channel functions were considered: Gabor, Laguerre-Gauss and Difference of Gaussian. With regard to the observer template, various methods for generating the expected signal were studied along with the influence of the number of training images used to form the covariance matrix for the observer template. Impact of bias in the training process on the observer template was evaluated next, as well as HR and CHO reproducibility.

Results: HR performance was most closely matched by 8 Gabor channels with tuned phase, orientation and frequency, using an observer template generated from training image data. Just 24 DBT image stacks were required to give robust CHO performance with 0% bias, although a bias of up to 33% in the training images also gave acceptable performance. CHO and HR reproducibility were similar (on average 3.2 PC versus 3.4 PC).

Conclusions: The CHO algorithm developed matches human reader performance and is therefore a promising candidate for automated readout of phantom studies.
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http://dx.doi.org/10.1016/j.ejmp.2018.12.033DOI Listing
February 2019

Translation from murine to human lung imaging using x-ray dark field radiography: A simulation study.

PLoS One 2018 29;13(10):e0206302. Epub 2018 Oct 29.

Department of Imaging and Pathology, KU Leuven, Leuven, Belgium.

Recent studies on murine models have demonstrated the potential of dark field (DF) x-ray imaging for lung diseases. The alveolar microstructure causes small angle scattering, which is visualised in DF images. Whether DF imaging works for human lungs is not a priori guaranteed as human alveoli are larger and system settings for murine imaging will probably have to be adapted. This work examines the potential of translating DF imaging to human lungs. The DF contrast due to murine and human lung models was studied using numerical wave propagation simulations, where the lungs were modelled as a volume filled with spheres. Three sphere diameters were used: 39, 60 and 80 μm for the murine model and 200, 300 and 400 μm spheres for the human model. System settings applied for murine lung response modelling were taken from a prototype grating interferometry scanner used in murine lung experiments. The settings simulated for human lung imaging simulations combine the requirements for grating interferometry and conventional chest RX in terms of x-ray energy and pixel size. The DF signal in the simulated murine model was consistent with results from experimental DF data. The simulated linear diffusion coefficient for medium alveoli diameters was found to be (1.31±0.01)⋅10-11 mm-1, 120 times larger than those of human lung tissue ((1.09±0.01)⋅10-13 mm-1). However, as the human thorax is typically a factor 15 times larger than that of murine animals, the overall DF effect in human lungs remains substantial. At the largest lung thickness and for the DF setup simulated, human lungs have an estimated DF response of around 0.31 and murine lungs of 0.23. Dark field imaging can therefore be considered a promising modality for use in human lung imaging.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0206302PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6205805PMC
April 2019

Characterisation of noise and sharpness of images from four digital breast tomosynthesis systems for simulation of images for virtual clinical trials.

Phys Med Biol 2017 03 2;62(6):2376-2397. Epub 2017 Feb 2.

National Coordinating Centre for the Physics of Mammography, Royal Surrey County Hospital, Guildford, Surrey GU2 7XX, United Kingdom.

In-depth evaluation of the noise and sharpness characteristics of FujiFilm Innovality, GE SenoClaire, Hologic Selenia Dimensions and Siemens Inspiration digital breast tomosynthesis (DBT) systems was performed with the intention of improving image simulation for virtual clinical trials. Noise power spectra (NPS) and modulation transfer function curves (MTF) were measured for planar modes and for the first and central projections for DBT modes. In DBT mode, the x-ray beam was blocked for the projections before the central projection in order to remove the influence of lag and ghosting from the previous images. A quadratic fit between the NPS and linearised pixel value gave the noise coefficients for planar and DBT imaging modes. The spatial frequencies corresponding to an MTF of 0.5 (MTF) were calculated from the MTF measurements made on the breast support and at 40 mm above the breast support. This was done for the first and the central projections. The percentage of signal carried over from the first projection to subsequent images (lag) was measured using a slit. The noise associated with lag was also evaluated. The DBT modes typically had lower electronic noise coefficients but higher structural noise coefficients compared to the respective planar mode MTF measured 40 mm above the table was between 6% and 47% lower for continuous scanning systems compared to 1% lower for step and shoot systems. For wide angle DBT, the MTF of the first projection was 18% (FujiFilm) and 28% (Siemens) lower than for the central projection. Lag in the second projection was 2.2%, 0.3%, 0.8% for the FujiFilm, GE and Hologic systems respectively. In all cases, the noise associated with lag was negligible. Current modelling frameworks for virtual clinical trials of breast DBT systems need to be adapted to account for signals from lag and variations in the MTF at wide angles.
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http://dx.doi.org/10.1088/1361-6560/aa5dd9DOI Listing
March 2017

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

Med Phys 2016 Sep;43(9):5104

Department of Imaging and Pathology, Radiology, KUL, Herestraat 49, Leuven B-3000, Belgium and Department of Radiology, Radiology, UZ Gasthuisberg, Herestraat 49, Leuven B-3000, Belgium.

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.
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http://dx.doi.org/10.1118/1.4960630DOI Listing
September 2016

Performance evaluation of a retrofit digital detector-based mammography system.

Phys Med 2016 Feb 20;32(2):312-22. Epub 2016 Jan 20.

Department of Radiology, UZ Gasthuisberg, Herestraat 49, B-3000 Leuven, Belgium.

A retrofit flat panel detector was integrated with a GE DMR+ analog mammography system and characterized using detective quantum efficiency (DQE). Technical system performance was evaluated using the European Guidelines protocol, followed by a limited evaluation of clinical image quality for 20 cases using image quality criteria in the European Guidelines. Optimal anode/filter selections were established using signal difference-to-noise ratio measurements. Only small differences in peak DQE were seen between the three anode/filter settings, with an average value of 0.53. For poly(methyl methacrylate) (PMMA) thicknesses above 60 mm, the Rh/Rh setting was the optimal anode/filter setting. The system required a mean glandular dose of 0.54 mGy at 30 kV Rh/Rh to reach the Acceptable gold thickness limit for 0.1 mm details. Imaging performance of the retrofit unit with the GE DMR+ is notably better than of powder based computed radiography systems and is comparable to current flat panel FFDM systems.
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http://dx.doi.org/10.1016/j.ejmp.2016.01.002DOI Listing
February 2016

Tailoring automatic exposure control toward constant detectability in digital mammography.

Med Phys 2015 Jul;42(7):3834-47

Department of Radiology, UZ Gasthuisberg, Herestraat 49, Leuven B-3000, Belgium.

Purpose: The automatic exposure control (AEC) modes of most full field digital mammography (FFDM) systems are set up to hold pixel value (PV) constant as breast thickness changes. This paper proposes an alternative AEC mode, set up to maintain some minimum detectability level, with the ultimate goal of improving object detectability at larger breast thicknesses.

Methods: The default "opdose" AEC mode of a Siemens MAMMOMAT Inspiration FFDM system was assessed using poly(methyl methacrylate) (PMMA) of thickness 20, 30, 40, 50, 60, and 70 mm to find the tube voltage and anode/filter combination programmed for each thickness; these beam quality settings were used for the modified AEC mode. Detectability index (d'), in terms of a non-prewhitened model observer with eye filter, was then calculated as a function of tube current-time product (mAs) for each thickness. A modified AEC could then be designed in which detectability never fell below some minimum setting for any thickness in the operating range. In this study, the value was chosen such that the system met the achievable threshold gold thickness (Tt) in the European guidelines for the 0.1 mm diameter disc (i.e., Tt ≤ 1.10 μm gold). The default and modified AEC modes were compared in terms of contrast-detail performance (Tt), calculated detectability (d'), signal-difference-to-noise ratio (SDNR), and mean glandular dose (MGD). The influence of a structured background on object detectability for both AEC modes was examined using a CIRS BR3D phantom. Computer-based CDMAM reading was used for the homogeneous case, while the images with the BR3D background were scored by human observers.

Results: The default opdose AEC mode maintained PV constant as PMMA thickness increased, leading to a reduction in SDNR for the homogeneous background 39% and d' 37% in going from 20 to 70 mm; introduction of the structured BR3D plate changed these figures to 22% (SDNR) and 6% (d'), respectively. Threshold gold thickness (0.1 mm diameter disc) for the default AEC mode in the homogeneous background increased by 62% in going from 20 to 70 mm PMMA thickness; in the structured background, the increase was 39%. Implementation of the modified mode entailed an increase in mAs at PMMA thicknesses >40 mm; the modified AEC held threshold gold thickness constant above 40 mm PMMA with a maximum deviation of 5% in the homogeneous background and 3% in structured background. SDNR was also held constant with a maximum deviation of 4% and 2% for the homogeneous and the structured background, respectively. These results were obtained with an increase of MGD between 15% and 73% going from 40 to 70 mm PMMA thickness.

Conclusions: This work has proposed and implemented a modified AEC mode, tailored toward constant detectability at larger breast thickness, i.e., above 40 mm PMMA equivalent. The desired improvement in object detectability could be obtained while maintaining MGD within the European guidelines achievable dose limit. (A study designed to verify the performance of the modified mode using more clinically realistic data is currently underway.).
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http://dx.doi.org/10.1118/1.4921417DOI Listing
July 2015

Comparison of digital breast tomosynthesis and 2D digital mammography using a hybrid performance test.

Phys Med Biol 2015 May 24;60(10):3939-58. Epub 2015 Apr 24.

Department of Imaging and Pathology, Division of Medical Physics & Quality Assessment, KU Leuven, Herestraat 49 Box 7003, 3000 Leuven, Belgium. Department of Radiology, University Hospitals Leuven, Herestraat 49, 3000 Leuven, Belgium.

This paper introduces a hybrid method for performing detection studies in projection image based modalities, based on image acquisitions of target objects and patients. The method was used to compare 2D mammography and digital breast tomosynthesis (DBT) in terms of the detection performance of spherical densities and microcalcifications. The method starts with the acquisition of spheres of different glandular equivalent densities and microcalcifications of different sizes immersed in a homogeneous breast tissue simulating medium. These target objects are then segmented and the subsequent templates are fused in projection images of patients and processed or reconstructed. This results in hybrid images with true mammographic anatomy and clinically relevant target objects, ready for use in observer studies. The detection study of spherical densities used 108 normal and 178 hybrid 2D and DBT images; 156 normal and 321 hybrid images were used for the microcalcifications. Seven observers scored the presence/absence of the spheres/microcalcifications in a square region via a 5-point confidence rating scale. Detection performance in 2D and DBT was compared via ROC analysis with sub-analyses for the density of the spheres, microcalcification size, breast thickness and z-position. The study was performed on a Siemens Inspiration tomosynthesis system using patient acquisitions with an average age of 58 years and an average breast thickness of 53 mm providing mean glandular doses of 1.06 mGy (2D) and 2.39 mGy (DBT). Study results showed that breast tomosynthesis (AUC = 0.973) outperformed 2D (AUC = 0.831) for the detection of spheres (p  <  0.0001) and this applied for all spherical densities and breast thicknesses. By way of contrast, DBT was worse than 2D for microcalcification detection (AUC2D = 0.974, AUCDBT = 0.838, p  <  0.0001), with significant differences found for all sizes (150-354 µm), for breast thicknesses above 40 mm and for heights above the detector of 20 mm and above. In conclusion, the hybrid method was successfully used to produce images for a detection study; results showed breast tomosynthesis outperformed 2D for spherical densities while further optimization of DBT for microcalcifications is suggested.
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http://dx.doi.org/10.1088/0031-9155/60/10/3939DOI Listing
May 2015

Effective detective quantum efficiency for two mammography systems: measurement and comparison against established metrics.

Med Phys 2013 Oct;40(10):101916

UZ Gasthuisberg, Medical Imaging Research Center and Department of Radiology, Herestraat 49, B-3000 Leuven, Belgium and SCK●CEN, Boeretang 200, B-2400 Mol, Belgium.

Purpose: The aim of this paper was to illustrate the value of the new metric effective detective quantum efficiency (eDQE) in relation to more established measures in the optimization process of two digital mammography systems. The following metrics were included for comparison against eDQE: detective quantum efficiency (DQE) of the detector, signal difference to noise ratio (SdNR), and detectability index (d') calculated using a standard nonprewhitened observer with eye filter.

Methods: The two systems investigated were the Siemens MAMMOMAT Inspiration and the Hologic Selenia Dimensions. The presampling modulation transfer function (MTF) required for the eDQE was measured using two geometries: a geometry containing scattered radiation and a low scatter geometry. The eDQE, SdNR, and d' were measured for poly(methyl methacrylate) (PMMA) thicknesses of 20, 40, 60, and 70 mm, with and without the antiscatter grid and for a selection of clinically relevant target/filter (T/F) combinations. Figures of merit (FOMs) were then formed from SdNR and d' using the mean glandular dose as the factor to express detriment. Detector DQE was measured at energies covering the range of typical clinically used spectra.

Results: The MTF measured in the presence of scattered radiation showed a large drop at low spatial frequency compared to the low scatter method and led to a corresponding reduction in eDQE. The eDQE for the Siemens system at 1 mm(-1) ranged between 0.15 and 0.27, depending on T/F and grid setting. For the Hologic system, eDQE at 1 mm(-1) varied from 0.15 to 0.32, again depending on T/F and grid setting. The eDQE results for both systems showed that the grid increased the system efficiency for PMMA thicknesses of 40 mm and above but showed only small sensitivity to T/F setting. While results of the SdNR and d' based FOMs confirmed the eDQE grid position results, they were also more specific in terms of T/F selection. For the Siemens system at 20 mm PMMA, the FOMs indicated Mo/Mo (grid out) as optimal while W/Rh (grid in) was the optimal configuration at 40, 60, and 70 mm PMMA. For the Hologic, the FOMs pointed to W/Rh (grid in) at 20 and 40 mm of PMMA while W/Ag (grid in) gave the highest FOM at 60 and 70 mm PMMA. Finally, DQE at 1 mm(-1) averaged for the four beam qualities studied was 0.44 ± 0.02 and 0.55 ± 0.03 for the Siemens and Hologic detectors, respectively, indicating only a small influence of energy on detector DQE.

Conclusions: Both the DQE and eDQE data showed only a small sensitivity to T/F setting for these two systems. The eDQE showed clear preferences in terms of scatter reduction, being highest for the grid-in geometry for PMMA thicknesses of 40 mm and above. The SdNR and d' based figures of merit, which contain additional weighting for contrast and dose, pointed to specific T/F settings for both systems.
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http://dx.doi.org/10.1118/1.4820362DOI Listing
October 2013

Comparison of signal to noise ratios from spatial and frequency domain formulations of nonprewhitening model observers in digital mammography.

Med Phys 2012 Sep;39(9):5652-63

Dipartimento di Fisica dell' , Università di Ferrara, Ferrara, Italy.

Purpose: Image quality indices based upon model observers are promising alternatives to laborious human readings of contrast-detail images. This is especially appealing in digital mammography as limiting values for contrast thresholds determine, according to some international protocols, the acceptability of these systems in the radiological practice. The objective of the present study was to compare the signal to noise ratios (SNR) obtained with two nonprewhitening matched filter model observer approaches, one in the spatial domain and the other in the frequency domain, and with both of them worked out for disks as present in the CDMAM phantom.

Methods: The analysis was performed using images acquired with the Siemens Novation and Inspiration digital mammography systems. The spatial domain formulation uses a series of high dose CDMAM images as the signal and a routine exposure of two flood images to calculate the covariance matrix. The frequency domain approach uses the mathematical description of a disk and modulation transfer function (MTF) and noise power spectrum (NPS) calculated from images.

Results: For both systems most of the SNR values calculated in the frequency domain were in very good agreement with the SNR values calculated in the spatial domain. Both the formulations in the frequency domain and in the spatial domain show a linear relationship between SNR and the diameter of the CDMAM discs.

Conclusions: The results suggest that both formulations of the model observer lead to very similar figures of merit. This is a step forward in the adoption of figures of merit based on NPS and MTF for the acceptance testing of mammography systems.
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http://dx.doi.org/10.1118/1.4747267DOI Listing
September 2012

Quantification of scattered radiation in projection mammography: four practical methods compared.

Med Phys 2012 Jun;39(6):3167-80

Department of Radiology, UZ Gasthuisberg, Herestraat 49, B-3000 Leuven, Belgium.

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.
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http://dx.doi.org/10.1118/1.4711754DOI Listing
June 2012

Physical evaluation of a needle photostimulable phosphor based CR mammography system.

Med Phys 2012 Feb;39(2):811-24

Department of Radiology, UZ Gasthuisberg, Herestraat 49, Leuven, Belgium.

Purpose: Needle phosphor based computed radiography (CR) systems promise improved image quality compared to powder phosphor based CR units for x-ray screening mammography. This paper compares the imaging performance of needle CR cassettes, powder based CR cassettes and a well established amorphous selenium (a-Se) based flat panel based mammography system, using consistent beam qualities.

Methods: Detector performance was assessed using modulation transfer function (MTF), normalized noise power spectrum (NNPS), and detective quantum efficiency (DQE). Mammography system performance was assessed against levels from the European Guidelines, including threshold gold thickness (c-d), relative signal difference to noise (SdNR) and mean glandular dose, for automatic exposure control settings suggested by the manufacturers. The needle based Agfa HM5.0 CR detector was compared against the single sided readout Agfa MM3.0R and dual sided readout Fuji Profect CS powder CR plates using a 28 kV Mo/Rh spectrum, while a 28 kV W/Rh spectrum was used to compare the Agfa HM5.0 against the Siemens MAMMOMAT Inspiration a-Se based system.

Results: MTF at 5 mm(-1) was 0.16 and 0.24 for the needle CR detector in the fast and slow scan directions, respectively, indicating a slight improvement (∼20%) over the two powder CR systems but remained 50% lower than the result at 5 mm(-1) for the a-Se detector (∼0.55). Structured screen noise was lower for the needle phosphor compared to the powder plates. CR system gain, estimated from the measured absorption fraction and NNPS results, was 6.3 for the (single sided) needle phosphor and 5.1 and 7.2 for the single sided and dual sided powder phosphor systems. Peak DQE at ∼100 μGy was 0.47 for the needle system compared to peak DQE figures of 0.33 and 0.46 for the single sided readout powder plates and dual sided readout plates. The high frequency DQE (at 5 mm(-1)) was 0.19 for the needle CR plates, a factor of approximately 3 greater than for the powder CR plates. At 28 kV W/Rh, 2 mm Al, peak DQE for the needle CR system was 0.45 against a value of 0.50 for the a-Se detector. The needle CR detector reached the Acceptable limit for 0.1 mm details in the European Guidelines at a mean glandular dose (MGD) of approximately 1.31 mGy imaged at 28 kV Mo/Rh, compared to figures of 2.19 and 1.43 mGy for the single sided and dual sided readout powder CR systems. The a-Se detector could reach the limit at 0.65 mGy using a 28 kV W/Rh spectrum, while the needle CR system required 1.09 mGy for the same spectrum.

Conclusions: Imaging performance for the needle CR phosphor technology, characterized using MTF and DQE and threshold gold thickness demonstrated a clear improvement compared to both single and dual sided reading powder phosphor based CR systems.
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http://dx.doi.org/10.1118/1.3675403DOI Listing
February 2012
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