MICROSTRIPS CRYSTALLINE SILICON DETECTOR FOR DIGITAL MAMMOGRAPHY ABSTRACT

Transcription

1 MICROSTRIPS CRYSTALLINE SILICON DETECTOR FOR DIGITAL MAMMOGRAPHY A. Leyva 1*, L. M. Montaño 2, A. E. Cabal 1, M. Fontaine 2, L. Bolaños 1, A. Díaz 3, F. Padilla 4, C. C. Díaz 2, I. Piñera 1, Y. Abreu 1, C. Cruz 1, A. Cerna 2, C. M. Ortiz 5, H. Mercado 6 1. Centro de Estudios Avanzados y Desarrollo Nuclear, C. Habana, Cuba 2. Centro de Investigaciones y Estudios Avanzados del IPN, D.F., México 3. Agencia de Energía Nuclear y Tecnologías de Avanzada, C. Habana, Cuba 4. Instituto Superior de Tecnologías y Ciencias Aplicadas, C. Habana, Cuba 5. Clínica de Mama y Gineco-oncología, Hospital General Tacuba, ISSSTE, D.F., México 6. Universidad de Guadalajara, Guadalajara, México *- Corresponding author: aleyva@ceaden.edu.cu ABSTRACT The present paper summarizes the results obtained in the evaluation of microstrips crystalline silicon detectors designed for high-energy physics experiments, as a useful X-ray detector in advanced medical radiography, specifically in digital mammography. Research includes the evaluation of the electrical and spectrometric parameters of these devices, as well as the acquisition of twodimensional radiography of some mammography phantoms using the scanning method. The paper also shows the digital images of biological samples taken from breast biopsies, where it is possible to identify the presence of possible pathological tissues. The obtained results support the real possibility that these advanced detectors can be satisfactorily introduced in the digital medical radiography. INTRODUCTION The introduction of semiconducting radiation detectors in medical imaging in substitution of photographic classic films reports considerable advantages for diagnosis techniques and patients. The instantaneously obtaining of radiographies in digital form with a better resolution and contrast, their easy storage, their possible transmission by phone line, the duplication without quality loss, and the reduction of the patient time exposition to the rays X time are some of these advantages [1]. ISBN

2 Due to these reasons numerous laboratories around the world, included the CEADEN Detectors Laboratory, dedicate efforts to investigate in this field with the objective of introducing the detectors that were initially conceived for high energies physic in the medical radiology [2-5]. At the present time, X-ray systems based on semiconducting detectors are already available in the market, and the scientists and technologists concentrate on the development and manufacture of more efficient detectors and detection systems, searching for the optimum detectors integration with its associated low-noise multichannel electronics. Novel acquisition and imaging processing methods are being developed and tested, as for example, the acquisition using dual energy, technique which significantly improves the image quality. The paper presents the results obtained in the experimental evaluation of crystalline silicon microstrips detector (CSMD) in digital mammography. This detector was developed in the frame of collaboration between several European and Latin American institutions. MATERIALS AND METHODS The tested CSMD was originally designed for researches in the field of high energy physic at CERN. Figure 1 shows the detector with 128 microstrips and two ASICs RX64, which simultaneously process signals and store data from each detector strip. Details about the detector operation and its characteristics can be found in [6, 7]. Figure microstrips crystalline silicon semiconducting detector. Figure 2. Self-made micro-calcifications and fibers mammographic phantoms. The radiographies were obtained using a BEDE generator with X-rays tubes of Cu (8.04 kev) and Mo (18 kev), with 20 kv acceleration voltage in all the cases. The scanning method was used to obtain two-dimensional radiographies with different geometric configurations sourceobject-detector. The acquired data mathematical processing, as well as its presentation in 2D images was carried out using the commercial program Origin v.7.0 [8]. A schematic representation of the used phantoms is shown in figure 2. These two phantoms were made taking into account the data reported in the literature for accreditation phantoms Gammex 156, designed to evaluate image quality in digital mammography systems [9]. Three aluminum ISBN

3 oxide cylinders and four different size nylon fibers were employed in phantoms to simulate micro-calcifications and fibrous structures respectively. Dimensions of mammographic details are shown in figures. All biological samples were obtained from mammary biopsies. RESULTS The main results obtained during the electric and spectrometric detector characterization processes are presented in figure 3(a-c). (a) (b) (c) Figure 3. Main analogue parameters distributions of the RX64: a) gain, b) equivalent noise charge and c) offset at the discriminator output. The detector fulfills all basic requirements concerning gain, noise and discriminator offset spread. It has a mean gain equal to 62.4 µv/el., and a mean value of el. rms and 3.3 mv for respectively the equivalent noise charge and the offset at the discriminator output respectively. Obtained value for noise guarantees that the detector signal/noise relationship stays above 10 for photons energies higher than 5.5 kev. Similar results were obtained for the rest of detector microstrips. The system energy calibration was carried out for each detector strip using four radioactive sources. In figure 4 the resulting spectrum is shown. The mathematical fitting of this calibration (figure 5) shows a good linearity (R = ) that guarantee the reliable detector use in spectrometric evaluations. The characterization results indicate that this detector can be used in medical imaging for the realization of radiographic studies by classic methods, or using advanced techniques as dual energy subtraction for the improvement of obtained image quality. Two-dimensional radiographies were carried out using an X-rays generator with Cu and Mo tubes. Images were taken without any filtration, neither special technique for radiographies optimization. They are only the result of X-rays transmission and absorption in tissues and its registration in the detector. ISBN

4 Figure 4. Detector energy calibration. Figure 5. Mathematical fitting of energy calibration. Obtained radiography of microcalcifications is shown in figure 6. In this image the three details are observed with a very good definition. Each incrustation is observed with different gray tonalities in correspondence with its different dimensions. Figure 6 Microcalcifications phantom radiography obtained with Cu X-ray tube, t adq = 2 s and collimation 100 μm. (a) (b) (c) Figure 7. Fibrous structures phantom radiographies obtained with Mo X-ray tube, collimation 100 μm: (a) t adq = 10 s and 60 steps; (b) t adq = 10 s and 100 steps; (c) t adq = 30 s and 120 steps. The image contrasts are good as result fundamentally of the existing wide difference among the densities of phantom materials Another effect should be expected from Al 2O3 acrylic ISBN

5 experiments with the fibrous structures phantom because in that case the difference between the densities is very small Nylon acrylic. Nevertheless, figure 7 shows the fibrous structures phantom radiography obtained with the Mo tube under different experimental conditions. The image shown in figure 7 (c) allows to identify adequately all the phantom details. Finally, figures 8 present the digital radiographies of different biological samples (mamma biopsies) obtained following the same scanning procedure used with the phantoms and employing the Cu tube. Figure 8. Digital radiographic images of five different biological samples obtained from breast tissue biopsies. In these images the clearest regions correspond to those areas with higher mass density and therefore, higher radiations absorption, while the darkest belong to slight tissues where the photons interaction probability with the material is smaller. There is possible to observe in radiographies the presence of some structures where the photons are strongly absorbed. These are in fact the microcalcifications, pretumoral injure with high mass density and therefore, high X-rays absorption. CONCLUSIONS The main electric and spectrometric parameters of the evaluated 128 microstrips crystalline silicon detector are suitable for it possible application in the medical digital imaging, specifically in mammography, where the used X-rays are in the energy interval where de detector should work satisfactorily, with a good gain and low noise. The obtained digital radiographies of micro-calcifications and fibrous structures phantoms, as well as of biological samples, support the potentiality of these detectors in digital mammography, with possibilities to be introduced in other fields like angiography. ISBN

6 Acknowledgments The authors express its gratefulness for the financial support provided by the European Alpha program, CONACYT of Mexico and PRN-AENTA of Cuba. REFERENCES [1] Moy J. P., Recent developments in X-ray imaging detectors, Nucl. Instr. and Meth., A, 442, (2001), [2] Midgley S., Berry A., Benci N., Morton S., Phillips D., Smith P., Troja S. and Lewis R., Hybrid pixel detector development for medical radiography, Nucl. Instr. and Meth., A, 573 (1), (2007), [3] Heijne E. H. M., Microelectronics technologies for new detectors in medical imaging, Nucl. Instr. and Meth., A, 571 (1), (2007), 7-9. [4] Olsena U. L., Badelb X., Linnrosb J., Di Michiel M., Martin T., Schmidt S. and Poulsen H. F., Development of a high-efficiency high-resolution imaging detector for kev X- rays, Nucl. Instr. and Meth., A, 576 (1), (2007), [5] Condeles J.F., Ghilardi Netto T. and Mulato M., Lead iodide films as X-ray sensors tested in the mammography energy región, Nucl. Instr. and Meth., A, 577 (3), (2007), [6] Bollini D., Cabal A. E., Dabrowski W., Diaz A., Gambaccini M., Giubellino P., Grybos P., Idzik M., Marzari-Chiesa A., Montano L. M., Prino F., Ramello L., Sitta M., Swientek K., Wheadon R. and Wiacek P., Energy resolution of a silicon strip detector with the RX64 ASIC designed for X-ray imaging, Nucl. Instr. and Meth., A, 515 (3), (2003), [7] Leyva A., Montaño L. M., Fontaine M., de la Mora R., Padilla F. and Cabal A. E., Radiography imaging by 64/128 microstrip crystalline detector at different X-ray energies, Nucleus, 39, (2006), [8] Code system Origin v. 7.0, OriginLab Corporation, [9] Computerized Imaging Reference Systems, Inc., CIRS, ISBN