Francesca Di Lillo

A Monte Carlo study of monoenergetic and polyenergetic normalized glandular dose (DgN) coefficients in mammography

We investigated the influence of model assumptions in GEANT4 Monte Carlo (MC) simulations for the calculation of monoenergetic and polyenergetic normalized glandular dose coefficients (DgN) in mammography, focussing on the effect of the skin thickness and composition, of the role of compression paddles and of the bremsstrahlung processes. We showed that selecting a skin thickness of 4 mm instead of 1.45 mm produced DgN values with deviations from 9% to 32% for x-ray spectra routinely adopted in mammography. Consideration of the bremsstrahlung radiation had a weak influence on monoenergetic DgN. Simulations (in the range 8-40 kVp) which included consideration of bremsstrahlung radiation, a skin thickness of 1.45 mm and a 2 mm thick compression paddles produced polyenergetic DgN coefficients up to 19% higher than corresponding literature data. Adding a 2 mm thick adipose layer between the skin layer and the radiosensitive portion of the breast produces polyenergetic DgN values up to 15% higher than those routinely adopted. These findings provide a quantitative estimate of the influence of model parameters on the calculation of the mean glandular dose in mammography.

Cone-beam micro computed tomography dedicated to the breast.

We developed a scanner for micro computed tomography dedicated to the breast (BµCT) with a high resolution flat-panel detector and a microfocus X-ray tube. We evaluated the system spatial resolution via the 3D modulation transfer function (MTF). In addition to conventional absorption-based X-ray imaging, such a prototype showed capabilities for propagation-based phase-contrast and related edge enhancement effects in 3D imaging. The system limiting spatial resolution is 6.2mm-1 (MTF at 10%) in the vertical direction and 3.8mm-1 in the radial direction, values which compare favorably with the spatial resolution reached by mini focus breast CT scanners of other groups. The BµCT scanner was able to detect both microcalcification clusters and masses in an anthropomorphic breast phantom at a dose comparable to that of two-view mammography. The use of a breast holder is proposed in order to have 1-2min long scan times without breast motion artifacts.

A Monte Carlo model for mean glandular dose evaluation in spot compression mammography

Purpose To characterize the dependence of normalized glandular dose (DgN) on various breast model and image acquisition parameters during spot compression mammography and other partial breast irradiation conditions, and evaluate alternative previously proposed dose‐related metrics for this breast imaging modality. Methods Using Monte Carlo simulations with both simple homogeneous breast models and patient‐specific breasts, three different dose‐related metrics for spot compression mammography were compared: the standard DgN, the normalized glandular dose to only the directly irradiated portion of the breast (DgNv), and the DgN obtained by the product of the DgN for full field irradiation and the ratio of the mid‐height area of the irradiated breast to the entire breast area (DgNM). How these metrics vary with field‐of‐view size, spot area thickness, x‐ray energy, spot area and position, breast shape and size, and system geometry was characterized for the simple breast model and a comparison of the simple model results to those with patient‐specific breasts was also performed. Results The DgN in spot compression mammography can vary considerably with breast area. However, the difference in breast thickness between the spot compressed area and the uncompressed area does not introduce a variation in DgN. As long as the spot compressed area is completely within the breast area and only the compressed breast portion is directly irradiated, its position and size does not introduce a variation in DgN for the homogeneous breast model. As expected, DgN is lower than DgNv for all partial breast irradiation areas, especially when considering spot compression areas within the clinically used range. DgNM underestimates DgN by 6.7% for a W/Rh spectrum at 28 kVp and for a 9 × 9 cm2 compression paddle. Conclusion As part of the development of a new breast dosimetry model, a task undertaken by the American Association of Physicists in Medicine and the European Federation of Organizations of Medical Physics, these results provide insight on how DgN and two alternative dose metrics behave with various image acquisition and model parameters.

Dose Volume Distribution in Digital Breast Tomosynthesis: A Phantom Study

Monte Carlo (MC) calculations for breast dosimetry in digital breast tomosynthesis (DBT) require experimental validations. We measured the 3-D dose distribution in breast phantoms, using XR-QA2 radiochromic films, compared to dose maps obtained with a previously validated MC code. Film sheets were positioned at the entrance surface, at the bottom surface as well as at four depths between adjacent slabs in the five-slabs 50-mm-thick phantoms simulating a compressed breast. We employed a homogeneous PMMA phantom, for the method validation, and a heterogeneous (BR 50/50) phantom for a preliminary study in a complex breast phantom. Irradiations were made at 40 kV at ±25° and 0° in craniocaudal view. A continuous scan over 15° was carried out for the homogeneous phantom. In the direction of the beam axis the dose decreases down to 12% of the entrance value. In the transverse planes, the dose varies up to 17%; in the heterogeneous phantom, it decreases to 25% in the direction of the beam axis. In transverse planes the maximum dose variations are up to 18% at

Monte Carlo Evaluation of Normalized Glandular Dose Coefficients in Mammography

{\theta = 0^\circ }

Suitability of low density materials for 3D printing of physical breast phantoms

, whereas the dose varies up to 22% in angular views. The simulations agreed with the measured values within the measurement uncertainties.

Monte Carlo evaluation of glandular dose in cone-beam X-ray computed tomography dedicated to the breast: Homogeneous and heterogeneous breast models

The mean glandular dose in mammography is evaluated via the normalized glandular dose coefficients DgN, calculated via Monte Carlo simulations. The conversion from dose to the homogenous mixture to dose in the glandular tissue is made by considering an energy-dependent correction factor, G, which is the weighted mean of the energy absorption coefficients of adipose and glandular tissues. The authors implemented a GEANT4 code and evaluated, in the range 8-80i¾?keV, the influence on the calculation of DgN values by 1 the method of G-weighting the dose, 2 the inclusion of bremsstrahlung radiation and 3 the energy threshold under which electrons are not tracked. The results for monochromatic DgN show that evaluating G retrospectively causes an underestimation up to 5i¾?%, and that not considering bremsstrahlung or setting high electron energy cutoff may cause a bias up to 1i¾?%, in the calculation of monochromatic DgN. These deviations may be negligible for polychromatic mammographic spectra.

Performance of the mediPROBE compact gamma camera for coded aperture imaging

Breast physical phantoms are a basic tool for the assessment and verification of performance standards in daily clinical practice of x-ray breast imaging modalities. They are also invaluable in testing and evaluation of new x-ray breast modalities to be potentially established, e.g. breast computed tomography, dual-energy breast CT and phase-contrast mammography and tomography. Nowadays, there is a lack or there are only a limited number of breast physical phantoms available for this purpose. The aim of this study is to explore a range of 3D printing materials such as resins, PLA, ABS, Nylon etc, to determine their attenuation and refractive properties, and to finally compare them to the properties of the breast tissues: adipose, glandular and skin. To achieve this goal, step-wedge phantoms were computationally modeled and then manufactured using stereolithographic and fused-deposition modeling technologies. X-ray images of the phantoms were acquired, using monochromatic beam at ID17, ESRF, Grenoble for three energies-30 keV, 45 keV and 60 keV. Experimental data were further processed to obtain the linear attenuation coefficients of these materials. Comparison with theoretical data for the linear attenuation coefficients and the refractive indexes for breast tissues was performed. From the studied materials, most of the resins, Nylon, Hybrid, PET-G show absorption properties close to the glandular tissue, while ABS shows absorption characteristics close to these of the adipose tissue. For phase-contrast imaging, it turns out that the ABS combined with resin-based materials to represent the adipose and glandular tissues, respectively may be a good combination for manufacturing of a phantom suitable for these studies. These results can be used for the design and the construction of a new physical anthropomorphic phantom of the breast with improved anatomical and radiological characteristics dedicated for advanced mammography imaging techniques implemented at higher photon energies.

Volume dose distribution in digital breast tomosynthesis: A phantom study

PURPOSE In cone-beam computed tomography dedicated to the breast (BCT), the mean glandular dose (MGD) is the dose metric of reference, evaluated from the measured air kerma by means of normalized glandular dose coefficients (DgNCT). This work aimed at computing, for a simple breast model, a set of DgNCT values for monoenergetic and polyenergetic X-ray beams, and at validating the results vs. those for patient specific digital phantoms from BCT scans. METHODS We developed a Monte Carlo code for calculation of monoenergetic DgNCT coefficients (energy range 4.25-82.25 keV). The pendant breast was modelled as a cylinder of a homogeneous mixture of adipose and glandular tissue with glandular fractions by mass of 0.1%, 14.3%, 25%, 50% or 100%, enveloped by a 1.45 mm-thick skin layer. The breast diameter ranged between 8 cm and 18 cm. Then, polyenergetic DgNCT coefficients were analytically derived for 49-kVp W-anode spectra (half value layer 1.25-1.50 mm Al), as in a commercial BCT scanner. We compared the homogeneous models to 20 digital phantoms produced from classified 3D breast images. RESULTS Polyenergetic DgNCT resulted 13% lower than most recent published data. The comparison vs. patient specific breast phantoms showed that the homogeneous cylindrical model leads to a DgNCT percentage difference between -15% and +27%, with an average overestimation of 8%. CONCLUSIONS A dataset of monoenergetic and polyenergetic DgNCT coefficients for BCT was provided. Patient specific breast models showed a different volume distribution of glandular dose and determined a DgNCT 8% lower, on average, than homogeneous breast model.

Evaluation of the BreastSimulator Software Platform for Breast Tomography: Preliminary Results

The aim of this work is to assess the performance of a prototype compact gamma camera (MediPROBE) based on a CdTe semiconductor hybrid pixel detector for coded aperture imaging. This probe can be adopted for various tasks in nuclear medicine such as preoperative sentinel lymph node localization, breast imaging with 99mTc-sestamibi and thyroid imaging with iodinated radioisotopes. The hybrid detector is an assembly of a 1mm thick CdTe semiconductor detector (sensitive area 14.08×14.08 mm2) bump-bonded to a photon-counting CMOS readout circuit of the Medipix2 series (256×256 square pixels, 55-μm pitch) or with a photon counting, energy-sensitive Timepix detector (128×128 square pixels, 110-μm pixel pitch). MediPROBE is equipped with a set of three interchangeable knife-edge pinhole collimators (hole aperture of 0.78 mm, 1 mm, or 1.9 mm) and a set of two coded aperture masks with 0.070-mm or 0.080-mm round holes. We performed measurements of field of view (FOV), system spatial resolution, and signal-difference-to-noise ratio (SDNR) by using a point-like gamma-emitting 241Am source (60 keV). The detector is operated at a single low-energy threshold of about 5 keV. At 60 keV the sensitivity is 0.0085 cps/kBq (w/0.78 mm pinhole). The system spatial resolution was 2.57 mm FWHM (w/ 0.78 mm pinhole) at a source-collimator distance of 45 mm, and 0.560 mm FWHM (w/ coded aperture 0.080 mm) at a source-collimator distance of 50 mm. We observed greater values of SDNR with the use of the coded aperture and the Timepix detector. A clinical trial is ongoing to evaluate the performance of MediPROBE for gamma-ray nuclear imaging.