Sergio Santos Muhlen

The resonance frequencies on mechanically pre-stressed ultrasonic piezotransducers

The piezotransducers employed in high power ultrasound are composed of piezoelectric ceramics and metallic pieces. These transducers are mechanically pre-stressed in order to avoid the ceramic fractures when high voltage is applied under resonance. The resonance and anti-resonance frequencies are shifted depending on the level of applied mechanical pre-stressing. This paper discusses some causes of this shifting on a experimental study. The discussion takes into account the variations on characteristic parameters of the ceramics and the acoustic coupling between parts of the transducer.

The mechanical pre-stressing in ultrasonic piezotransducers.

Composed piezotransducers submitted to mechanical pre-stressing present shifts on resonance and anti-resonance frequencies. Changes on characteristic parameters of the ceramic and in the coupling between the parts of the transducer can be the causes for this behavior. In applications where the level of pre-stressing is low (up to 50 MPa) the parameters of the ceramic are not altered, therefore, the shifting on frequencies are attributed to coupling between parts. This paper describes a mathematical model to explain this effect based on difference of effective cross-section between transducers parts under pre-stressing. The results show a proportional relation between pre-stressing and effective coupling of the parts.

The influence of the thickness of non-piezoelectric pieces on pre-stressed piezotransducers.

The mechanical pre-stress applied in piezotransducers used to generate high power ultrasound is needed to avoid ceramics fracture on traction cycle. Pre-stress levels inferior to 50 MPa can yield resonance shifting due to effectiveness of acoustic coupling between transducer pieces. Symmetrical transducers with different thickness of passive parts were submitted to axial mechanical pre-stress up to 50 MPa and their resonances were measured. The experimental results show the increasing of the resonances frequencies with the level of applied pre-stress. Similar effect is verified in simulations by using a model based on Masons equivalent electric circuit. Due to the similarity of these effects, a relation between applied pre-stress and pieces coupling was proposed for the transducer assembled. In addition, the dependence of the thickness of non-piezoelectric pieces on the coupling effectiveness between them is discussed. The results show that transducers with small thickness present more expressive shifting resonance ratio.

Design of an optimized high-power ultrasonic transducer

The design of an optimized ultrasonic transducer for renal lithotripsy is discussed. The device consists of a concentrator (horn) associated with piezoelectric (PZT) ceramic rings to obtain maximum power transfer to the kidney stones. The transducer works at 31 kHz, and the vibratory amplitude measured on the application tip is 0.12 mm. The theoretical modeling of the horn is discussed, considering the propagation of mechanical waves in solids, with specific boundary conditions. The choice of materials and the technological aspects concerning assembly, insulation, and cooling are considered. The equivalent electrical circuit is studied and the impedance curve plotted in order to design the electronic driver.<<ETX>>

A low-cost transmittance transducer for measurement of blood oxygen saturation in extracorporeal circuits

Designs for a for low-cost dual wavelength transducer based on light transmittance and a disposable cuvette for monitoring oxygen saturation (SO/sub 2/) in extracorporeal arterial and venous blood are presented. The transducer utilizes red and infrared light-emitting diodes and a large photodiode; it is designed to attach to a flow-through curvette modified from 3/8-in/spl times/3/8-in bypass-circuit connectors. A mock extracorporeal circulation system was assembled to evaluate operation of the transducer at a controlled blood SO/sub 2/ and the relationship between light transmittance and hemodilution. SO/sub 2/ was calculated based on multiple linear regression analyses. The results show a high correlation between the SO/sub 2/ obtained with the equipment designed and values measured with commercial gasometric equipment in the range of 50% to 100% (r/sup 2/=0.976, error <2%). The method presented allows continuous and real time measurement of whole blood SO/sub 2/ with a low-cost transmittance transducer.

Virtual instrument for experimental studies in biological systems

Biological tissues, especially excitable cells, may have their biophysical properties altered under adequate stimulation. To better understand the interactions between biological tissues and stimuli with different energy modalities, it is necessary to precisely control the generation of the stimulus and the temporal parameters of its application, and to synchronize its delivery with the acquisition of the biological signal of interest. The objective of the present work is to present a virtual instrument (VI) developed to control the shape and timing of the delivered stimulus to tissues in vitro and simultaneously acquire the biological signal. For stimuli delivered by electronic devices, the system controls the programmable waveform generator and the data acquisition board (DAQ) via the universal serial bus (USB) communication interface used to capture the biological signal. The LabVIEW® development platform was used to build the graphical interfaces for controlling all stimulation and acquisition parameters. The graphical interface has three modules: 1) the configuration module of the stimulus generation and application includes a table where each sequence of stimulation parameters can be inserted individually or in batch; 2) the acquisition module, consisting of a screen that displays in real time the acquired signals during the experiment, and a field to enter addresses to save the acquired data to files; 3) the post-processing module, which includes a set of statistical analysis tools, frequency analysis, and digital filtering. Both the biological signal of interest and the synchronization signal for stimulation are recorded in the files. The developed VI can be applied in the study of stimulation of virtually any kind of biological preparation. This system was applied to the study on the effects of high power ultrasound on the heart.

Radiation from mammography: Diagnostic or cancer induction?

Establishing the biological effects and the safe level of exposure to ionizing radiation remains, so far, an objective to be reached. Mammography is an important exam in the early detection of breast tumors, but since the genes BRCA1 and BRCA2 were identified as a main cause of hereditary breast cancer, there is growing concern of exposing patients carrying those mutations to ionizing radiation. It could increase mutations in these genes and other suppressors, and promote the development of tumor in those patients. In this study we propose an in vitro initial experiment aiming to determine the radiation trigger level for detectable mutations in genes BRCA, and thus a safer level of radiation due to mammography for patients carrying a mutated allele of the genes BRCA1 or BRCA2. Breast cells from one patient were cultivated and irradiated with different doses of X-rays generated by commercial mammography equipment. The results have shown that exposition to ionizing radiation promotes measurable alterations in breast cell grown.

New Challenges in Controlling EMI in Hospitals

Over the last twenty years, hospitals have been attempting to incorporate wireless technology into their dayto-day operations. Clinical engineers who are responsible to facilitate the integration of wireless technology into hospitals are faced with many challenges. Wireless technology has been evolving from networks with a small number of relatively high power sources to networks with a large number of relatively low power sources. This has made characterizing the hospital electromagnetic environment a complicated process and the associated potential for electromagnetic interference (EMI) difficult to determine. There is no globally-accepted comprehensive protocol to evaluate the function/malfunction of medical devices exposed to radio-frequency (RF) fields. In addition, there is often uncertainty in determining the degree of clinical impact that a malfunction has. The evaluation of malfunctions and their severity is not only a technical matter related to expected behavior of the medical devices, but must be evaluated considering: the clinical status of the patient, the medical staff opinion, institutional mission and the intended application of the device. There is a need for the development of RF management policies to deal with the changing reality of wireless in hospitals and to develop more effective clinical impact of EMI testing. Risk mitigation policy is a part of the evidence-based management programs.

Methodology for identification of susceptible areas to electromagnetic interference in the hospital

Electromagnetic interference (EMI) is the functional alteration in equipment due its exposition to electromagnetic fields. In Hospitals many patient care units have a high concentration of electrical and electronic medical devices, most of them are susceptible to EMI. The study of interference and electromagnetic compatibility (EMC) in medical devices involves the use of measurement instruments and controlled environments, which most of Brazilian hospitals cannot afford. However, the occurrence of functional alterations in a medical device may cause serious damages to the patient therefore this problem cannot be ignored. The aim of this work is to propose a simple methodology to identify patient care units in hospital that are susceptible to radiated EMI. When these environments are identified and characterized, preventive actions can be performed.

Advantages in the Application of Conductive Shielding for AC Magnetic Field in MRI Exam Rooms

Due to the increasing demand for installation of Magnetic Resonance Imaging (MRI) equipment in clinics and hospitals, and the difficulty to choose the location for installation of these equipment because of their high sensitivity to 60 Hz magnetic field sources, this study was carried out to present a practical solution for shielding 60 Hz magnetic field in the hospital environment. MRI is a technique that produces 3-D (volumetric) tomographic images of high resolution without using ionizing radiations. The quality of images is greatly influenced by magnetic fields of the environment, especially of 50/60 Hz, which results in the need of shielding the space where the equipment are installed. This study proposes the use of aluminum in the construction of the MRI room shield for the many advantages presented by this material when compared to ferromagnetic materials: it is lighter, easier to handle, bend, rivet or weld, it does not rust and dispenses sturdy supports for fixing on walls, resulting in the best cost-benefits ratio in short and long terms. We performed computational simulation and experiments with shields of rectangular geometries assembled with aluminum and ferromagnetic materials. The aluminum shield has proved to be advantageous, since it presents shielding effectiveness to 60 Hz magnetic field similar to those of Fe–Si GNO under certain conditions, and radio frequencies shielding also, with lower cost in the installation and maintenance of shielding in MRI rooms.