Paola Baldelli

Evaluation of the minimum iodine concentration for contrast-enhanced subtraction mammography

Early manifestation of breast cancer is often very subtle and is displayed in a complex and variable pattern of normal anatomy that may obscure the disease. The use of dual-energy techniques, that can remove the structural noise, and contrast media, that enhance the region surrounding the tumour, could help us to improve the detectability of the lesions. The aim of this work is to investigate the use of an iodine-based contrast medium in mammography with two different double exposure techniques: K-edge subtraction mammography and temporal subtraction mammography. Both techniques have been investigated by using an ideal source, like monochromatic beams produced at a synchrotron radiation facility and a clinical digital mammography system. A dedicated three-component phantom containing cavities filled with different iodine concentrations has been developed and used for measurements. For each technique, information about the minimum iodine concentration, which provides a significant enhancement of the detectability of the pathology by minimizing the risk due to high dose and high concentration of contrast medium, has been obtained. In particular, for cavities of 5 and 8 mm in diameter filled with iodine solutions, the minimum concentration needed to obtain a contrast-to-noise ratio of 5 with a mean glandular dose of 2 mGy has been calculated. The minimum concentrations estimated with monochromatic beams and K-edge subtraction mammography are 0.9 mg ml(-1) and 1.34 mg ml(-1) for the biggest and smallest details, respectively, while for temporal subtraction mammography they are 0.84 mg ml(-1) and 1.31 mg ml(-1). With the conventional clinical system the minimum concentrations for the K-edge subtraction mammography are 4.13 mg ml(-1) (8 mm diameter) and 5.75 mg ml(-1) (5 mm diameter), while for the temporal subtraction mammography they are 1.01 mg ml(-1) (8 mm diameter) and 1.57 mg ml(-1) (5 mm diameter).

A prototype of a quasi-monochromatic system for mammography applications

Improvement in image contrast and dose reduction, in mammographic x-ray imaging, can be achieved using narrow energy band x-ray beams in the 16-24 keV range. As part of an Italian Government funded project, a quasi-monochromatic system for mammography applications has been developed. The system is based on a tunable narrow energy band x-ray source operating in the 16-24 keV energy range. The bremsstrahlung beam is monochromatized via Bragg diffraction by a highly oriented pyrolytic graphite mosaic crystal (HOPG). The scanning system provides a large field (18 x 24 cm2) of quasi-monochromatic x-rays with energy resolution ranging from 10% at 18 keV to 17.2% at 24 keV. The system has been characterized in terms of fluence rate and energy resolution. An x-ray tube developed ad hoc allows us to acquire images in a reasonable time to minimize the motion blur. A qualitative analysis has been performed in order to know if the prototype system performances are far from a clinical application, by evaluating the spatial resolution, the field uniformity and the image quality as a function of the quasi-monochromatic beam energy. Dose evaluation has been performed as a function of the energy and compared to a conventional system for mammography. The quasi-monochromatic prototype system can produce comparable image quality at half the dose.

Quasi-monochromatic x-rays for diagnostic radiology

Monochromatic x-ray beams are desirable in various fields of diagnostic radiology; in fact a reduction of the dose and an enhancement of the contrast could be achieved. In this work two different methods to monochromatize x-ray beams produced by conventional tubes have been compared. In the first one the beam is obtained via Bragg diffraction on mosaic crystal and in the second one by attenuating the polychromatic beam with aluminium filters. We have simulated quasi-monochromatic x-ray spectra by setting suitable values of Braggs angle to obtain beams tuned to 20, 30, 40 and 50 keV with the SHADOW code, an x-ray tracing program designed to study the propagation and the interaction of a photon beam through an optical system. We have validated such a program by comparing some calculated data with measurements carried out on an experimental apparatus. Attenuated polychromatic x-ray spectra have been simulated by setting appropriate values of aluminium filters and potential with the SPECTRUM PROCESSOR, the software version of the Catalogue of Spectral Data for Diagnostic X-Rays, which provides radiographic x-ray spectra that can be attenuated with several material filters. The relation between the energy resolution and the flux as a function of the mean energy has been investigated and results have been compared. Results show that quasi-monochromatic x-ray beams produced via Bragg diffraction exhibit, for a given mean energy and energy resolution, a higher total flux compared to attenuated spectra.

Dose comparison between conventional and quasi-monochromatic systems for diagnostic radiology

Several techniques have been introduced in the last year to reduce the dose to the patient by minimizing the risk of tumour induced by radiation. In this work the radiological potential of dose reduction in quasi-monochromatic spectra produced via mosaic crystal Bragg diffraction has been evaluated, and a comparison with conventional spectra has been performed for four standard examinations: head, chest, abdomen and lumbar sacral spine. We have simulated quasi-monochromatic x-rays with the Shadow code, and conventional spectra with the Spectrum Processor. By means of the PCXMC software, we have simulated four examinations according to parameters established by the European Guidelines, and calculated absorbed dose for principal organs and the effective dose. Simulations of quasi-monochromatic laminar beams have been performed without anti-scatter grid, because of their inherent scatter geometry, and compared with simulations with conventional beams with anti-scatter grids. Results have shown that the dose reduction due to the introduction of quasi-monochromatic x-rays depends on different parameters related to the quality of the beam, the organ composition and the anti-scatter grid. With parameters chosen in this study a significant dose reduction can be achieved for two out of four kinds of examination.

Quantitative analysis of the effect of energy separation in k-edge digital subtraction imaging.

The aim of the work is to quantitatively compare the effect of the energy separation in the k-edge digital subtraction imaging technique. Images of a custom-made, iodine filled (k-edge = 33.17 keV) test object have been acquired with monochromatic x-ray beams (energy spread <0.1 keV) at the ID17 biomedical beamline of the ESRF. Image acquisition has been performed using two energy separations, namely 0.65 keV (32.85 keV and 33.5 keV, respectively) and 4.4 keV (31.2 keV and 35.6 keV, respectively), using beams of energies on either side of the iodine k-edge. Signal and signal-to-noise ratio (SNR) analysis has been performed as a function of DeltaE and the contrast medium concentrations. The results show that the SNR values measured for DeltaE < 1 keV are only slightly higher than those measured for DeltaE = 4.4 keV. This preliminary study shows that monochromaticity and the energy separation obtained with quasi monochromatic beams from conventional x-ray sources might be suitable for this imaging technique.

Comment on ''Perspectives of medical X-ray imaging''

Abstract In the paper “Perspectives of medical X-ray imaging” (Nucl. Instr. and Meth. A 466 (2001) 99) the authors infer, from simple approximations, that the use of HOPG monochromator has no advantage in mammography compared to existing systems. We show that in order to compare imaging properties of different X-ray sources it is necessary to evaluate the spectra after the attenuation of the tissue to be imaged. Indeed, quasi-monochromatic X-ray sources have the potential to enhance image contrast and to reduce patient dose.

A comparative study of physical image quality in digital and synthetic mammography from commercially available mammography systems

We present a comparison between full field digital mammography and synthetic mammography, performed on several mammography systems from four different manufacturers. The analysis is carried out on both the digital and synthetic images of two commercially available mammography phantoms, and focuses on a set of objective metrics that encode the geometrical appearance of imaging features of diagnostic interest. In particular, we measured sizes and contrasts of several clusters of microcalcification specks, shapes and contrasts of circular masses, and the power spectrum of background regions mimicking the heterogeneous texture of the breast parenchyma. Despite the potential issues of tomosynthesis in terms of image blurring, the synthetic images do not highlight any globally significant differences in the rendering of the details of interest, when compared to the original digital mammograms: relative contrasts are generally preserved, as well as the geometry of broad structures. We conclude that, as far as the considered objective metrics are concerned, the image quality of synthetic mammography does not exhibit significant differences with respect to the one of full field digital mammography, for all the considered systems.

A first experience of quality control in full field digital mammography

A simple phantom for periodic quality control measurement on full field digital mammography systems has been developed. It was employed with three different digital mammography units, by collecting data on detector linearity and uniformity, image contrast and spatial resolution, reproducibility of detector and automatic exposure control device. Results show the phantom is able to give information about some detector characteristics and can detect problems related to the stability of the automatic exposure control system. It is also fast and easy to use, as required in a quality assurance program.

Application of dual-energy techniques to digital mammography

A dual-energy technique which employs the basis decomposition method is being investigated for application to digital mammography. A three-component phantom was doubly exposed with the digital mammography system manufactured by General Electric. The “low” and “high” energy images were recorded with a Mo/Mo anode-filter combination at 25 kV and a Rh/Rh combination at 40 kV, respectively. The total dose was kept within the acceptable levels of conventional mammography. The first hybrid image obtained with the dual-energy algorithm is presented in comparison with a conventional radiograph of the phantom.