Marcos Fernandez

Performance tests of two portable mini gamma cameras for medical applications.

We have developed two prototypes of portable gamma cameras for medical applications based on a previous prototype designed and tested by our group. These cameras use a CsI(Na) continuous scintillation crystal coupled to the new flat-panel-type multianode position-sensitive photomultiplier tube, H8500 from Hamamatsu Photonics. One of the prototypes, mainly intended for intrasurgical use, has a field of view of 44×44mm2, and weighs 1.2kg. Its intrinsic resolution is better than 1.5mm and its energy resolution is about 13% at 140keV. The second prototype, mainly intended for osteological, renal, mammary, and endocrine (thyroid, parathyroid, and suprarenal) scintigraphies, weighs a total of 2kg. Its average spatial resolution is 2mm; it has a field of view of 95×95mm2, with an energy resolution of about 15% at 140keV. The main advantages of these gamma camera prototypes with respect to those previously reported in the literature are high portability and low weight, with no significant loss of sensitivity and spatial resolution. All the electronic components are packed inside the mini gamma cameras, and no external electronic devices are required. The cameras are only connected through the universal serial bus port to a portable PC. In this paper, we present the design of the cameras and describe the procedures that have led us to choose their configuration together with the most important performance features of the cameras. For one of the prototypes, clinical tests on melanoma patients are presented and images are compared with those obtained with a conventional camera.

On the Use of Front-End Cascode Rectifiers Based on Normally On SiC JFET and Si MOSFET

The new wide band-gap semiconductor devices provide new properties to be explored. Normally on silicon carbide (SiC) JFET power devices have several advantages, in particular low switching losses and the potential capabilities of high temperature and high reverse blocking voltage. Looking for improving the overall efficiency in power converters, new structures based on these power devices might be studied. In this paper, a cascode rectifier based on normally on SiC JFET is presented and analyzed. This rectification structure can be applied as front-end rectifier stage for ac-dc power converters, increasing the overall efficiency of these topologies. A second cascode rectifier based on silicon (Si) MOSFET is also studied, as a low-cost alternative. A simple static forward characterization and a brief dynamic behavior analysis of the proposed cascode rectifier structure are made. Both cascode structures are compared with traditional Si rectifier diodes and front-end rectifiers, using an active power factor corrector (PFC) interleaved boost converter. As a result of this comparison, an efficiency improvement as high as two points is obtained. An additional OR gate based on a diode is also used as second test circuit to compare the proposed structures to the traditional Si rectifiers. A reduction between 75% and 55% of the total loss are obtained in this second experimental test.

Detector optimization of a small animal PET camera based on continuous LSO crystals and flat panel PS-PMTs

We have built a PET camera for small animals based on continuous block LSO crystals coupled to a PS-PMT flat panel. When working with continuous crystals, surface treatment is an important factor that strongly determines the main characteristics of the detector module. As a part of the work done for the development of our small animals PET camera, we have investigated the effect of the scintillator crystal surface treatment on the PET detector module performances, in order to optimize crystal configuration. We present the results for spatial resolution, image compression and energy resolution obtained when using different surface treatments in continuous LSO crystals. These results are compared with those obtained from simulations that have been carried out using DETECT2000 package.

Tapped-inductor buck HB-LEDs AC-DC driver operating in Boundary Conduction Mode for replacement incandescent bulb lamps

It is known that High-Brightness Light-Emitting Diodes (HB-LEDs) are potential successors of incandescent bulb lamps due to its high luminous efficiency and high lifetime. To realize these advantages the HB-LED ballast must be durable and efficient. Also, for this specific application the ac-dc HB-LEDs ballast needs high-step-down ratio, high Power Factor (PF) and low-cost. This paper presents a tapped-inductor buck Power Factor Corrector (PFC) operating in Boundary Conduction Mode (BCM) design for replacement incandescent bulb lamps. This low-cost solution presents the adequate high-step-down ratio without galvanic isolation in order to produce an output voltage of about 20 V from line voltage. Also, the tapped-inductor buck PFC maintains high efficiency in comparison to other PFCs based on one stage (i.e. flyback PFC). A static analysis, distortion analysis of the input current, design procedure and an average small signal model of the ac-dc HB-LEDs driver have been carried out in this paper in order to provide an adequate design procedure. Finally, a 12 W experimental prototype was developed in order to validate the theoretical results presented.

Depth of interaction measurement in gamma ray imaging detectors with continuous scintillation crystals

A design for an inexpensive depth of interaction (DOI) detector for gamma rays, suitable for nuclear medical applications, especially positron emission tomography (PET), has been developed, studied by simulations and tested experimentally. The detector consists of a continuous LSO-scintillator of dimensions 42/spl times/42/spl times/10 mm/sup 3/ and a new compact large-area (49/spl times/49 mm/sup 2/) position sensitive photo-multiplier (PSPMT) H8500 from Hamamatsu. Since a continuous crystal is used, the scintillation light distribution is not destroyed and its first 3 moments can be used to determine the energy (0th moment), the centroids along the x- and y-direction (1st moments) and the depth of interaction (DOI), which is strongly correlated to the distributions width and thus its standard deviation (2nd moment). The simultaneous computation of these moments allows a three-dimensional reconstruction of the position of interaction of the /spl gamma/-rays within the scintillating crystal and will be realized by a modified position sensitive proportional (PSP) resistor network. No additional photo detectors or scintillating crystals are needed. According to previous Monte Carlo simulations which estimated the influence of Compton scattering for 511 keV /spl gamma/-rays, the transport of the scintillation light within the detector assembly and also the behavior of the modified PSP resistor network, we expect a spatial resolution of /spl lsim/ 2 mm and a DOI resolution of /spl ap/ 5 mm. The first experimental results presented here yield an intrinsic spatial resolution of /spl lsim/ 1.8 mm and 2.6 mm for the x- and y-direction respectively and a DOI resolution /spl lsim/ 1 cm. Further we measured an energy resolution of 12%-18%.

Digital implementation of the feedforward loop of the asymmetrical half-bridge converter for LED lighting applications

An asymmetrical half-bridge converter (AHBC) has proved to be a promising candidate for LED lighting applications. It provides high efficiency and galvanic isolation, and it can be easily built without electrolytic capacitor. On the other hand, its main drawback is its poor attainable bandwidth. In any two-stage ac-dc LED driver based on the AHBC, the first stage is a power factor corrector converter, which has to be also implemented without electrolytic capacitor. As a consequence, its output voltage (input voltage of the AHBC) presents a low-frequency ripple. Due to the poor bandwidth of the AHBC, this voltage ripple will be transferred to the converter output voltage, leading to flickering. Due to the complex and nonlinear transfer function of the AHBC, any analog feedforward loop has to be tuned for a given operating point, leading to a poor performance when the AHBC moves away from that point. In this paper, a digital feedforward loop is proposed in order to solve this problem. The digital implementation allows the feedforward loop to perfectly cancel the ripple under any condition (e.g., output voltage variation due to dimming). Besides, this digital feedforward loop has been designed and simplified considering the specific purpose of canceling flickering in the emitted light. In this way, it can be easily implemented in small-size microcontrollers. Experimental results with a 40-W prototype prove the usefulness of the proposed feedforward loop.

Self-driven synchronous-rectification technique for the Asymmetrical Half-Bridge converter in led lighting applications

Due to their high reliability and luminous efficacy, HB-LEDs are being widely used in lighting applications and, therefore, their power supplies are required to have also high reliability and efficiency. A very common approach for achieving this in ac-dc applications is using a two-stage topology. The Power Factor Corrector Boost converter operating in Boundary Conduction Mode is a very common converter used as first stage. It is normally designed without electrolytic capacitors, improving reliability but also increasing the low-frequency ripple of the output voltage. The Asymmetrical Half-Bridge (AHB) is a perfect option for the second stage as it has very high efficiency, it operates at constant switching frequency and its output filter is small (i.e., it can be easily implemented without electrolytic capacitors). Moreover, the AHB is an excellent candidate for Self-Driven Synchronous Rectification (SD-SR) as its transformer does not have dead times. However, the standard configuration of the SD-SR must be modified in this case in order to deal with the transformer voltage variations due to the input voltage ripple and, more important, due to the LED dimming state. This modification is presented in this paper. The experimental results obtained with a 60-W topology show that efficiency of the AHB may be very high (94.5%).

The Master's Thesis: An Opportunity for Fostering Presentation Skills

Presentation skills, such as oral expression and public speaking, have normally been relegated to the background in engineering degree programs. In recent years, however, the labor market has specifically demanded these kinds of skills in engineers. Accordingly, new engineering degrees, adapted to the goals of the Bologna Declaration or ABET criteria, consider presentation skills as being fundamental transferable skills. In practice, however, many engineering degree programs do not specifically foster these skills even though they are included in the syllabus. This paper proposes a presentation-skills training that uses the Masters thesis as an opportunity for fostering presentation-related skills. This activity has students deliver a scheduled series of rehearsals, in front of their classmates and tutors, for their officially assessed presentation of their Masters thesis work. The paper also presents a Web tool specifically designed for uploading recordings of the rehearsal presentations for feedback online as a complementary method for fostering presentation-related skills. Finally, the results of carrying out the proposed resource over a 4-year period from 2009 to 2013 are discussed; they show that students following the proposed methodology had higher than average marks, all receiving an A+, and 82% of them receiving an A+ with distinction.

On the use of front-end cascode rectifiers based on normally-on SiC JFET and Si MOSFET

The new wide band-gap semiconductor devices provide new properties to be explored. Normally-on Silicon Carbide (SiC) JFET power devices have several advantages, in particular low switching losses, high temperature operation and high reverse voltage capability. Looking for improve the overall efficiency in power converters, new structures based on these power devices might be studied. In this paper, a cascode rectifier based on normally-on SiC JFET is presented and analyzed. This new rectification structure can be applied as front-end rectifier stage for AC-DC power converters, increasing the overall efficiency of these topologies. A second cascode rectifier based on Silicon (Si) MOSFET is also studied, as a low cost alternative. Both cascode structures are compared with traditional Si rectifier diodes and front-end rectifiers, using three different test circuits: a full bridge rectifier, a passive Power Factor Corrector (PFC) voltage doubler and an active PFC interleaved boost converter. As a result of this comparison, an efficiency improvement as high as two points is obtained on each tested circuit.

Fully digital trigger and pre-processing electronics for planar positron emission mammography (PEM) scanners

Digital trigger algorithms performed over large sequences of data can be an efficient way to shift processing time from baseline samples or singles to coincident photo multiplier tube pulses. This paper describes a coincidence processing system intended to implement fully digital trigger and pre-processing algorithms for a positron emission tomography scanner dedicated to breast imaging. In order to efficiently address all the emerging issues of high resolution PET detectors, a high performance DSP processor has been embedded into the backbone of the system. Signals from 12 channels of a dual-head PET camera are acquired in free-running sampling while a first stage of FIFO memories, implemented using a Virtex-II Pro FPGA, translate data from a sequential sample-by-sample processing basis to a more efficient block-by-block one. This approach enables us to carry out trigger and pre-processing tasks in parallel. Moreover, the scheme offers additional benefits: intrinsic temporal coherence, zero data acquisition dead time, and a more flexible software approach to pre- and post-processing issues (from pile-ups and scatter correction to pre-reconstruction processing). It also heavily reduces the bandwidth required for the link to the host computer, enabling the use of a high speed USB port. The DAQ system is capable of handling count rates up to 10 Mevents/s, pre-processing the samples on the fly and lastly delivering data to the host computer for image reconstruction.