M. Marziani

Dual-energy tissue cancellation in mammography with quasi-monochromatic x-rays

Dual-energy radiography has not evolved into a routine clinical examination yet due to intrinsic limitations of both dual-kVp imaging and single-exposure imaging with conventional x-ray sources. The recent introduction of novel quasi-monochromatic x-ray sources and detectors could lead to interesting improvements, especially in mammography where the complex structure of healthy tissues often masks the detectability of lesions. A dual-energy radiography technique based on a tissue cancellation algorithm has been developed for mammography, with the aim of maximizing the low intrinsic contrast of pathologic tissues while being able to minimize or cancel the contrast between glandular and fat tissues. Several images of a plastic test object containing various tissue equivalent inserts were acquired in the energy range 17-36 keV using a quasi-monochromatic x-ray source and a scintillator-coated CCD detector. Images acquired at high and low energies were nonlinearly combined to generate two energy-independent basis images. Suitable linear combinations of these two basis images result in the elimination of the contrast of a given material with respect to another. This makes it possible to selectively cancel certain details in the processed image.

Narrow energy band X-rays via mosaic crystal for mammography application

Abstract Monochromatic X-ray beams are desirable in various fields of diagnostic radiology. We are investigating an imaging system based on a narrow energy band X-ray source for mammography application. The narrow energy band is obtained by making use of a mosaic crystal that reflects X-rays from a conventional X-ray tube via Bragg diffraction. In this paper the characteristics of the system in terms of energy resolution and fluence rate are reported. Small field (1.1 × 3.0 cm 2 ) radiographs of Plexiglas phantoms with a thickness up to 3.6 cm have been obtained with 18, 20 and 22 keV quasi-monochromatic beams. The field size is limited by the crystal size along one dimension and by the desired energy spread along the other one. A digital imaging system has been used to correct for non-uniformity of the X-ray beam.

MTF evaluation of a phosphor-coated CCD for x-ray imaging

A novel digital x-ray detector has been assembled. The imaging system is based on a phosphor-coated charge-coupled device (CCD) obtained by direct deposition of a gadolinium oxysulphide scintillator onto the detector surface. The modulation transfer function has been measured along the two directions of the digital coordinates with the narrow slit technique. A resolution limit of about 20 line pairs per mm has been obtained for both directions. The high spatial resolution currently demanded in mammography can be achieved with this imaging system.

AUTORADIOGRAPHY WITH SILICON STRIP DETECTORS

A digital autoradiography system based on double sided silicon strip detectors (1.6 × 1.6 mm2 active surface with 100 μm pitch) has been developed and successfully tested with beta-emitting tracers. It is shown here that the system is able to perform imaging of organic material with specific sensitivity as small as 0.002 nCi/mm2, and to record activity measurements with good linearity in the range 0.002–20 nCi/mm2. Autoradiographic images of clusters of mammary cells marked with ortho-(32P)phosphate, obtained with an exposure time of about 10 min are presented.

A Monte Carlo simulation of the possible use of Positron Emission Tomography in proton radiotherapy

Abstract We have used the Monte Carlo technique to evaluate the applicability of Positron Emission Tomography to in vivo dosimetry for proton radiotherapy. A fair agreement has been found between Monte Carlo results and experimental data. The simulation shows that PET can be useful especially for in vivo Braggs peak localization.

Evaluation of a digital X-ray detector based on a phosphor-coated CCD for mammography

Abstract The imaging performance of a phosphor-coated CCD system for mammography application is evaluated. The coated CCD is obtained by direct deposition of a Gd2O2S:Eu powder onto a detector surface. Modulation transfer function, noise power spectrum, and detective quantum efficiency are measured. Their dependence on the input beam X-ray energy is discussed. Images of a test phantom for mammography are compared with those obtained with a screen-film combination.

Digital imaging in radiology: preliminary results obtained with a high spatial resolution 2D silicon detector

Double-sided microstrip silicon crystals are being tested as detectors for X-rays in the diagnostic energy range (10-100 keV) for digital radiology. An analog-to-digital-converter and CAMAC based acquisition system has been developed to study the imaging capabilities of a silicon microstrip detector with 100 and 200 mu m read-out pitch. The authors present the first images of submillimeter high-contrast phantoms obtained with an X-ray mammography tube operating at high flux density. A preliminary contrast transfer function study was performed; a low frequency contrast of about 0.97 for a high-contrast phantom and a decrease of contrast at a frequency of 5 1p/mm corresponding to the detector intrinsic spatial resolution (100 mu m) were measured. >

Energy spectra measurements of X-ray emission from electron interaction in a dense plasma focus device

Electrons generated during a pinch implosion in a hollow anode Mather-like plasma focus device (PF) are considered as a possible X-ray source via the impinging of those particles on medium and high-Z targets. A usual PF device has been slightly modified to optimise the X-ray production and their measurements by means of a suitable and non-invasive spectrometer. This ensemble allows measurements of X-rays generated booth by electrons turning back to the anode and by target collision of the so-called relativistic electron beam. The spectrum of the emitted photons is evaluated by using a differential absorption based technique. The X-ray spectrometer consists of a stack of LiF dosimeters which act both as detectors and filters to give curves of attenuated intensities. Finally, the energy distribution is calculated from such attenuation curves using an iterative procedure based on spectral algebra formalism.

Optimization of radiography applications using x-ray beams emitted by compact accelerators. Part I. Monte Carlo study of the hard x-ray spectrum

PURPOSE A 3-year project called LABSYNC has been recently funded by the European Commission, with the aim of designing a radiation facility based on a compact light source, i.e., a laboratory-sized commercial synchrotron, capable of accelerating electrons up to 6 or 20 MeV. An accurate spectral description of hard x rays emitted from thin targets, irradiated by electron beams circulating in the storage ring, is of primary interest for the design and the characterization of a beamline. This article, Part I, aims at optimizing some of the parameters which are critical for the design of medical applications based on the above compact light source. The goal was to evaluate the dependence of photon fluence and beam monochromaticity on electron-beam energy, target material, and thickness. METHODS The transport of 6 and 20 MeV electrons in a thin molybdenum, rhodium, and tungsten target is studied by means of Monte Carlo simulations using MCNPX. Configurations of the x-ray output port, different from the default forward-directed emission of the beam, are also investigated. A comparison with reference spectra for general diagnostic radiology and mammography is carried out. RESULTS It is shown that the emitted x-ray beams can be far more intense than those generated by conventional x-ray tubes for radiography applications. The profiles of the calculated polychromatic spectra resemble those generated by conventional x-ray tubes, with x-ray energies up to the energy of the incident-electron beam. An appreciable improvement in the monochromaticity of the beams can be obtained by viewing the x-ray emission from an output port antiparallel to the direction of the incident-electron beam. CONCLUSIONS The optimum target thickness for tungsten target spectra is practically constrained by a trade-off between bremsstrahlung efficiency and focal-spot size requirements. A larger margin for optimization of target thickness is probably available for mammographic spectra. The constraint of a backward-directed (or, to a lesser extent, orthogonal) output port is to be considered mandatory for minimizing the high-energy tail of the spectral distribution and keeping the radiation dose to a reasonable level. It is also fundamental to evaluate the impact of the high-energy tail of the emitted spectra in x-ray imaging applications, since the energy range involved is significantly beyond the diagnostic range. This topic will be dealt with in Part II of the article.

Digital system based on a bichromatic x-ray source and a single-photon counting device: a single-exposure dual-energy mammography approach

In this work we exploit the advantages of using a bi-chromatic X-rays source coupled with a single photon counting pixel detector to perform a feasibility study for dual energy mammography. This technique allows enhancing the contrast between different breast tissues by composing two images acquired at two different energies. The high and low energy images have been acquired by a single X-ray shot. The bi-chromatic beam has been produced per diffraction of polychromatic photons by a monochromator crystal. The imaging system is based on a single photon counting silicon pixel detector. The data read-out is performed by a VLSI Integrated Circuit bump-bonded to the sensor. The energy threshold of each electronics channel can be individually trimmed. We set the threshold of one pixel below 16 keV while the threshold of the neighboring pixel between 16 and 32 keV. With a single exposure the information from both energies is recorded. After separation between low and high threshold pixels, we obtained two independent images. We acquired radiographs of phantoms made of three different materials. Appling a dual energy algorithm, we obtained synthesized images where any of the three materials is removed from the radiograph, enhancing the contrast between the two remaining.