Sergio Zucca

Experimental study of Compton scattering reduction in digital mammographic imaging

In mammography, the first cause of image contrast reduction arises from the photons scattered inside the examined organ. The amount of Compton scattering strongly depends on the irradiation area and on the distance between the organ and the X-ray detector. We have experimentally evaluated how these geometrical conditions affect the scattering fraction. Our experimental setup includes a single photon counting device based on a silicon pixel detector as X-ray sensor; a lucite cylinder to simulate the breast tissue, and a lead collimator to define the irradiation area. We have evaluated the contrast and the signal-to-noise ratio for images acquired in different conditions.

Semiconductor pixel detectors for digital mammography

Abstract We present some results obtained with silicon and gallium arsenide pixel detectors to be applied in the field of digital mammography. Even though GaAs is suitable for medical imaging applications thanks to its atomic number, which allows a very good detection efficiency, it often contains an high concentrations of traps which decrease the charge collection efficiency (CCE). So we have analysed both electrical and spectroscopic performance of different SI GaAs diodes as a function of concentrations of dopants in the substrate, in order to find a material by which we can obtain a CCE allowing the detection of all the photons that interact in the detector. Nevertheless to be able to detect low contrast details, efficiency and CCE are not the only parameters to be optimized; also the stability of the detection system is fundamental. In the past we have worked with Si pixel detectors; even if its atomic number does not allow a good detection efficiency at standard thickness, it has a very high stability. So keeping in mind the need to increase the Silicon detection efficiency we performed simulations to study the behaviour of the electrical potential in order to find a geometry to avoid the risk of electrical breakdown.

Spectroscopic and imaging capabilities of a pixellated photon counting system

We are studying the performance of various thickness GaAs pixel detectors bump-bonded to a dedicated photon counting chip (PCC) for medical imaging applications in different energy ranges. In this work we present the experimental results obtained with a 600 μm thick pixel matrix (64×64 square pixels, 170 μm side) in the 60–140 keV energy range to evaluate the possible use of such a system in the nuclear medicine field. In particular, we have measured the spectroscopic properties of the detector (charge collection efficiency, energy resolution and detection efficiency) and evaluated the discrimination capability of the electronics. Then we have measured the imaging properties of the whole system in terms of Point Spread Function and using a home made thyroid phantom. We present also a comparison with a traditional gamma camera and an evaluation, made by both experimental measurements and software simulations, of the imaging characteristics related to the use of a collimation system.

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.

Imaging spectroscopic performances for a Si based detection system

We present the imaging and spectroscopic capabilities of a system based on a single photon counting chip (PCC) bump-bonded on a Si pixel detector. The system measures the energy spectrum and the flux, produced by a standard mammographic tube. We have also made some images of low contrast details, achieving good results.

Experimental test of a new technique of background suppression in digital mammography

A multiple-exposure technique in digital mammography has been developed to suppress the physical background in the image due to Compton scattering in the body. A pair of X-ray masks, shaped in a projective geometry and positioned upstream and downstream the patient, are coupled mechanically and moved in four steps along a square pattern in order to irradiate the full area in four consecutive short exposures. A proof-of-principle apparatus is under test with a breast phantom and a standard mammographic X-ray unit. Results are reported.

Study of GaAs detectors characteristics for medical imaging

In this work we present the results of a systematic study about SI GaAs detectors as a function of substrate and contact type, geometry and thickness. This study has been stimulated from the interest in using GaAs as a detector for medical imaging applications. GaAs detectors have been produced using crystals grown with different techniques and changing both the thickness (in the range 200 /spl mu/m-1 mm) and the contacts type and geometry. We have measured the current-voltage characteristics and, using radioactive sources (/sup 109/Cd, 20 keV photons, /sup 241/Am, 60 keV photons, /sup 99m/Tc, 140 keV photons), we have studied the performance of our detectors in terms of charge collection efficiency and energy resolution as a function of the bias voltage. Besides we have also studied the electrical and spectroscopic properties of GaAs detectors with different types and concentrations of the dopants in the substrate. So we have found the optimal doping type and concentration to have the best spectroscopic performances and the higher breakdown voltage. Simulation programs made with Monte Carlo methods have been developed to describe the electric field distribution and the transport of charge carriers toward the electrodes in GaAs detectors. In these simulations we have considered the presence of deep energy levels in the bandgap, the thickness, the bias voltage and the charge deposition in the crystal after photon interaction.

Full-field images of mammographic phantoms obtained with a single photon counting system

As the use of digital radiographic equipment in the morphological imaging field is becoming largely diffuse, the research of new and more performing devices from public institutions and industrial companies is in constant progress. Many of these devices are based on solid-state detectors as X-ray sensors. Semiconductor pixel detectors, originally developed in the high energy physics environment, have been then proposed as digital detector for medical imaging applications. In this paper a digital single photon counting device, based on silicon and GaAs pixel detectors, is presented. The detector is a thin slab of semiconductor crystal equipped with an array of 64 by 64 square contacts, 170-μm side. The data read-out is performed by a VLSI integrated circuit named Photon Counting Chip (PCC), developed within the MEDIPIX collaboration. Each chip cell geometrically matches the sensor pixel. It contains a charge preamplifier, a threshold comparator and a 15 bits pseudo-random counter and it is coupled to the detector by means of bump-bonding. Most important advantages of such a system, with respect to a traditional X-rays film/screen device, are the wider linear dynamic range (3x104) and the higher performance in terms of MTF and DQE. Electronics read-out performance as well as imaging capabilities of the digital device will be presented. Images of mammographic phantoms acquired with a standard mammographic tube will be compared with radiographs obtained with traditional film/screen systems.

Use of radiation dose index monitoring software in a multicenter environment for CT dose optimization

Radiation dose index monitoring (RDIM) systems are useful software tools that allow radiological data collection and patient dose monitoring. Based on DICOM standard, the dose information of digital radiological equipment can be collected in various ways, for example, by analysis of DICOM headers, by optical character recognition of the patient record, by DICOM-modality performed procedure steps (MPPS) or by DICOM-radiation dose structured report (RDSR). The RDSR in particular is a hierarchically structured document encoding all relevant radiation exposure information, both for accumulated and per single irradiation event; it enables easy and universal comprehensive dose monitoring for DICOM modalities [1, 2]. RDIM software can extract exposure data for freestanding archiving and analysis. Analogous to PACS, such a dose database could be defined as “Dose Archive and Communication System” (DACS). Italian law requires the recording of all imaging and treatment procedures performed with ionising radiation [3]. The latest EU Council Directive 2013/59/EURATOM (BSS) [4] strengthens requirements for recording and reporting doses from radiological procedures; it states that Member States shall ensure that information relating to patient exposure forms part of the report of the medical radiological procedures. Likewise, the ALARA (“as low as reasonably achievable”) principle for dose optimization must be applied in all cases and the diagnostic reference levels (DRLs) [5, 6] have to be regularly reviewed. In this scenario, the RDIM systems become valuable tools for radiological departments. RDIM software, in fact, can provide exposures of all imaged patients, and whole distributions of dose indices for any specific equipment and protocol. With RDIM, statistical analysis of dosimetric indices can be performed

Digital mammographic application of a single photon counting pixel detector

We present the imaging capabilities of a system based on a single photon counting chip, bump-bounded to a Si pixel detector. The detector is 300 µm thick a matrix of 64 x 64 square pixels with a dimension side of 170 µm. The active area is about 1.2 cm2. The photon counting chip matches the geometry of the detector so it has 4096 asynchronous read-out cells, each containing a charge preamplifier, a leading edge comparator and a pseudorandom counter. We have tested the image quality and the stability of our acquisition system imaging some phantoms used for mammografic quality checks. Our system allows to see low contrast details with a patient dose comparable with that given in mammographic examinations.