RF Signal Frequency Research Articles

Non-Gaussian statistics and temporal variations of the ultrasound signal backscattered by blood at frequencies between 10 and 58 MHz.

Very little is known about the blood backscattering behavior and signal statistics following flow stoppage at frequencies higher than 10 MHz. Measurements of the radio frequency (rf) signals backscattered by normal human blood (hematocrit = 40%, temperature = 37 degrees C) were performed in a tube flow model at mean frequencies varying between 10 and 58 MHz. The range of increase of the backscattered power during red blood cell (RBC) rouleau formation was close to 15 dB at 10 and 36 MHz, and dropped, for the same blood samples, below 8 dB at 58 MHz. Increasing the frequency from 10 to 58 MHz raised the slope of the power changes at the beginning of the kinetics of aggregation, and could emphasize the non-Gaussian behavior of the rf signals interpreted in terms of the K and Nakagami statistical models. At 36 and 58 MHz, significant increases of the kurtosis coefficient, and significant reductions of the Nakagami parameter were noted during the first 30 s of flow stoppage. In conclusion, increasing the transducer frequency reduced the magnitude of the backscattered power changes attributed to the phenomenon of RBC aggregation, but improved the detection of rapid growth in aggregate sizes and non-Gaussian statistical behavior.

In Vitro Ultrasound Measurement at the Human Femur

Measurements of broadband ultrasonic attenuation (BUA) represent an established means of skeletal status assessment in osteoporosis. Today, the skeletal site that is the most widely used clinically is the calcaneus (heel bone). However, we hypothesized that fracture risk predictions could be improved if ultrasound measurements were performed directly at the main site of fracture, e.g., the proximal femur. The goal of this paper is to demonstrate in vitro the feasibility of quantitative ultra sound (QUS) imaging at the upper part of fee femur, and to investigate the relationships of BUA to bone mineral density (BMD). Forty-four excise human femurs were measured in transmission with a pair of focused 0.5-MHz central frequency transducers. Two-dimensional scans were performed, and the radio frequency (rf) signals were recorded at each measurement point. A data acceptance criterion for region of interest (ROI) selection was established based on the linearity of the frequency-dependent attenuation. Five measurements with repositioning were performed on each sample to determine the reproducibility. Dual energy x-ray absorptiometry (DXA) scans have been performed on the samples for BMD measurements: Three ROIs were selected in the specimens: greater, trochanter intertrochanteric, and femoral neck regions. Coefficient of variations were in the range 1.6% to 2.5%. The determination coefficients (r(2)) between BUA and BMD in site-matched ROIs were 0.81, 0.78, and 0.73, respectively, for the greater trochanteric, intertrochanteric, and femoral neck regions. Our results are consistent with data previously shown at the calcaneus and demonstrate the feasibility of QUS measurements at the femur in vitro, with reasonable reproducibility.

Bias Dependence of Intermodulation Distortion Asymmetry in Heterojunction Bipolar Transistor Using Nonlinear Large-Signal Model

The asymmetry between lower and upper intermodulation distortions (IMD) in a heterojunction bipolar transistor (HBT), which can be observed under certain baseband termination and bias conditions, is analyzed using a large-signal model. This approach enables harmonic balance analysis of IMD asymmetry in nonlinear circuits using HBTs and, therefore, overcomes the limitations of the conventional Volterra-series analysis. Closed-form expression derived from Volterra analysis is also employed to present comparisons with the large-signal model approach. Measurements indicate that the asymmetry is very sensitive to baseband termination, which varies with the envelope frequency of the input RF signal. Specifically, zero crossings of asymmetry are observed at each resonance frequency where the reactive part of baseband termination changes its sign rapidly. It is also found that the asymmetry is enhanced at a specific small bias current where the dominance of the 3rd order nonlinearity is greatly reduced by cancellation with the 2nd order nonlinearity. The bias condition for asymmetry occurrence is calculated, using a simple analytical equation, to be related to measurement data. All the measurements are verified successfully by nonlinear simulation using large-signal model. Finally, the advantage of the large-signal model approach compared to Volterra analysis for asymmetry prediction is discussed.

Intravascular ultrasound elastography: an overview

The composition and morphology of the atherosclerotic lesion are currently considered more important determinants of acute coronary ischemic syndromes than the degree of stenosis. When a lesion is unstable, it may rupture and cause an acute thrombotic reaction. A rupture prone plaque contains a large lipid pool covered by a thin fibrous cap. The stress in the cap increased with decreasing thickness. Additionally, it may be weakened by macrophage infiltration. Intravascular ultrasound elastography might be an ideal technique to assess the presence of lipid pools and identify high stress regions. Elastography is a technique to assess local mechanical properties of tissue. The underlying principle is that the deformation of tissue by a mechanical excitation is a function of its mechanical properties. The deformation of the tissue is determined using ultrasound. For intravascular purposes, the intraluminal pressure is used as the excitation force. The radial strain in the tissue is obtained by cross-correlation techniques on the radio frequency (rf) signal. The strain is colour-coded and plotted as a complimentary image to the IVUS echogram. Elastography was validated in vitro using diseased human coronary and femoral arteries. After the ultrasound experiments, the specimens were processed for routine histology to counterstain collagen, smooth-muscle cells, and macrophage activity. Regions were segmented in the elastograms based on their strain values. Next, the dominant plaque type (fibrous, fibro-fatty or fatty) was defined by observers blinded to the elastographic result. These experiments demonstrate that the strain in the three plaque types is different (Kruskall–Wallis p<0.001). Especially between fibrous and fatty tissue, a highly significant difference (Wilcoxon p<0.001) was found. In vivo, the technique is validated in an atherosclerotic Yucatan mini-pig animal model. High-resolution echo frames (30 frames per second) were acquired near end-diastole. In this phase of the pressure cycle, catheter motion was minimal. Frames with a pressure difference of approx. 5 mm Hg were taken to determine the elastograms. This in vivo validation study in Yucatan mini-pigs revealed higher strain values in fatty material (ANOVA p<0.001). All cross-sections with a fatty plaque were identified with the elastogram by the presence of high strain values. Additionally, data are acquired in patients referred for Percutaneous Transluminal Coronary Angioplasty with the same set-up as tested in the animal study. Ultrasound data of soft, fibrous, calcified and stented plaques are acquired near end-diastole. The elastogram of soft plaques, as identified from the deformation during the pressure cycle, reveals strain values of 1% with increased strain up to 2% at the shoulders of the plaque. Calcified material, as identified from the echogram, shows low strain values of 0–0.2%. The elastogram of stented plaques reveals very low strain values, except for two regions: these are between the stent struts and at the shoulders of the plaque. In conclusion, intravascular elastography appears to be a unique tool to determine local mechanical properties in atherosclerotic lesions to identify fibrous and fatty tissue. Experiments have demonstrated the feasibility of this technique to be applied in vivo.

New Structures of Quantized Current Plateaus in Semiconductor Turnstile Devices

Quantized current plateaus appear in the I - V characteristics of turnstile devices realized by modulating the tunnel barriers with a radio frequency (rf) signal of frequency f and phase difference π. We report two new results. (1) New fine structures resulting from the quantized level spacing Δ E appear. Then, the quantized current flows through the barriers via several excited states of the system with a fixed number of electrons, as the source drain voltage increases. These structures are smeared out when the temperature k B T becomes comparable to Δ E . However the quantized plateaus remain. The quantized plateaus are washed out when k B T becomes comparable to the charging energy e 2 / C , where C is the capacitance of the dot. (2) When the rf frequency f becomes larger than the leakage current J ( f =0)/ e , which is the time-averaged current at the frequency f =0, the time-averaged current J dc is quantized in the adiabatic regime. On the other hand, when f is smaller than J ( f =0)/ e , J dc is mu...

Multiturn flux transformers for rf SQUID magnetometers

This work showed experimentally how the planar multiturn input coil of a flux transformer could be integrated with a coplanar resonator while still permitting one to couple radio frequency (rf) signals to an rf SQUID. A new layout of a multiturn flux transformer with one pickup loop and two input coils, one single turn and one multiturn, has been fabricated on (001) substrates and evaluated. A planar double-hole washer SQUID was inductively coupled to the two separate input coils via a flip chip configuration. With this layout the best magnetometer showed magnetic field white noise of at 77 K in a magnetically shielded surrounding.

Preliminary study in differential contrast echography

Microbubble-based contrast agents have backscattering properties that differ greatly from those of soft tissues. These agents exhibit nonlinear scattering properties in response to incident acoustic energy, causing harmonic components in reflected energy. Even in linear scattering conditions, an important difference is observed in the frequency dependence of their backscatter coefficient, when compared to that of tissues. In this situation, any such differential behaviour can be exploited by imaging instrumentation to enhance the detection of agent-containing vessels against background tissue. The resulting B-mode image brightness (or pixel level) can thus be the representation of a local parameter computed from radio frequency (rf) signal processing of the raw echoes. The object of this article is to report on parametric imaging studies performed with Sonovue ™ (formerly code named BR1, Bracco Research SA), aimed at assessing the contrast-enhancing potential of spectral rf signal processing. The technique used, termed here “differential contrast echography” (DCE), is applicable in real-time B-mode imaging. It is based on an on-line subtractive approach, using dual-channel signal processing to implement differential filtering and demodulation. In this preliminary work, DCE was implemented off-line, on rf echo signals digitised from commercial B-mode scanners. Data acquisition, DCE processing and complete B-mode image reconstruction were programmed on a personal computer. The images presented were produced from test phantoms as well as animal phased array scanning. The phantom included a flow channel, background scattering material, fixed echogenic targets and echo-free regions. Animal scanning was performed on rabbit liver. The results obtained from DCE show promising contrast-enhancing properties. The regions containing no contrast agent are significantly suppressed from the image, preferentially leaving the regions perfused by the contrast agent. This property was favourable to experimenting with image-overlay presentations, superimposing colour-coded DCE imaging with standard log-compressed grey-scale B-mode, in a way analogous to duplex imaging combining colour Doppler and B-mode.

RF coupling to single crystal BSCCO under c-axis bias

We have explored rf coupling to BSCCO single crystals with c-axis bias currents over a wide range of frequencies. A structure was developed that afforded the necessary impedance transformations at several frequencies from dc to 35 GHz. At low frequencies, connections were made to a coaxial cable. At X-band, the sample was placed in front of a short section of waveguide, forming a resonator. Above 12 GHz, the sample mount formed a coplanar resonator. At 35 GHz it was used as the end plate of a cylindrical cavity with degenerate mode spoilers and a loaded Q-factor of 3800 at 38.3 GHz. Fabricated BSCCO samples showed evidence of hysteretic Josephson junction stack behavior at 77 K. Application of low frequency rf signals caused steps to appear in the sample I-V. At frequencies above 6.4 GHz only a reduction of critical current was observed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Radio frequency phase shifting at the source simplifies NMR spectrometer design

Two different approaches to the digital control of the phase of the radio frequency (rf) signal are reviewed. Given a fast reproducible digital phase control, a method is presented for performing digitally some of the traditionally analog spectrometer functions. As a result, the spectrometer design is simplified considerably. Selection of different phase settings during the transmission of the rf pulse and during signal detection and other digital operations are discussed as examples.

Frequency conversion in surface acoustic wave devices on n-GaAs

Abstract A study of RF signal frequency multiplication and mixing with simultaneous surface acoustic wave (SAWrpar; excitation by a reverse-biased Al interdigital transducer on epitaxial GaAs has been performed. The dependence of SAW excitation efficiency and excited wave phase on the bias voltage applied to the transducer have been measured. The frequency responses of tunable SAW devices on n-GaAs wafers are presented.

5217010 ECG amplifier and cardiac pacemaker for use during magnetic resonance imaging

A device for monitoring a patient or pacing a patient is disclosed which can safely operate in a MRI system. The device uses unique RF filtering and shielding to attenuate voltages on the leads resulting from the high frequency RF signals produced in the MRI. The device is uniquely shielded to prevent induced currents from disrupting the amplifying and processing electronics. The device uses an optional secondary low pass or band reject filter to eliminate interference from the MRI's gradient magnetic field. The device uses optional inductors placed close to electrodes to limit RF currents through the electrodes. Several embodiments of the RF filter are taught which depend on the number of sensing leads, whether the leads are shielded, whether the RF filter is contained in the electronic shielded housing or whether single or multistage filtering is employed. The device may operate as an extended ECG monitor or may be an implantable MRI safe pacemaker.

406 MHz ELT signal spectra for SARSAT

The signal format and spectral properties of the 406-MHz emergency locator transmitter (ELT) used in the search and rescue satellite aided tracking (SARSAT) system are examined. The ELT improves location estimate accuracies and can relay information about the particular aircraft and its problem, by means of the digitally modulated message fields. It is shown that due to the RF signal frequency characteristics and the Doppler shift, processing must be performed over a frequency band of approximately 25 kHz. Through the use of the fast Fourier transformation (FFT), the frequency spectrum of the ELT is analyzed, taking account of effects due to noise, multiple simultaneously received signals, and Doppler shift. It is demonstrated that the FFT provides an effective means for detecting and recognizing the presence of one or more ELT signals over this 25-kHz frequency band. Some recommendations are made to improve the spectral characteristics and the performance of the ELT. >

Development and Characterization of a Radio Frequency-Transparent Ablator

Abstract Earth and planetary entry vehicles require heat shield materials whose transparency to radio frequency (rf) signals is unaffected by the ablation process. Teflon or fused silica are currently used for this purpose, but these materials are heavy and difficult to attach. For applications such as Mars entry vehicles or lifting bodies where low heating rates prevail, low-density, rf-transparent ablators are desired. To fill this need, Martin Marietta Corporation has developed a rf-transparent composition designated SLA-220. This ablator, a silica-filled elastomeric silicone, has a density of 0.25 g/cm3 and forms a siliceous rather than a carbonaceous char. Pyrolysis of SLA-220 at slow heating yields a pure silica residue. As heating rate increases, the volatile pyrolysis products tend to undergo secondary decomposition and to deposit carbon while passing through the hot char. Therefore, rf signal attenuation increases with heating rate. Electrical properties of SLA-220 (dielectric constant, signal attenuation) have been measured before and after ablation. Ablation testing has been conducted at heating rates ranging from 6 to 60 Btu/ft2-sec. Thermal properties (conductivity, specific heat, expansion) and mechanical properties (strength, elastic modulus, elongation) have been determined. The properties of SLA-220 are compared with those of 0.50 and 0.70 g/cm3 porous Teflon ablators to demonstrate the advantages of SLA-220 over other low-density rf-transparent ablative systems.