Raw Radiofrequency Signals Research Articles

Ultrasound Detection of Scatterer Concentration by Weighted Entropy

Ultrasound backscattering signals depend on the microstructures of tissues. Some studies have applied Shannon entropy to analyze the uncertainty of raw radiofrequency (RF) data. However, we found that the sensitivity of entropy in detecting various scatterer concentrations is limited; thus, we propose a weighted entropy as a new information entropy-based approach to enhance the performance of scatterer characterization. A standard simulation model of ultrasound backscattering was used to generate backscattered RF signals with different number densities of scatterers. The RF signals were used to estimate the weighted entropy according to the proposed algorithmic scheme. The weighted entropy increased from 0.08 to 0.23 (representing a dynamic range of 0.15) when the number density of scatterers increased from 2 to 32 scatterers/mm2. In the same range of scatterer concentration, the conventional entropy increased from 0.16 to 0.19 (a dynamic range of 0.03). The results indicated that the weighted entropy enables achieving a more sensitive detection of the variation of scatterer concentrations by ultrasound.

SU‐D‐210‐05: The Accuracy of Raw and B‐Mode Image Data for Ultrasound Speckle Tracking in Radiation Therapy

Purpose:For ultrasound speckle tracking there is some evidence that the envelope‐detected signal (the main step in B‐mode image formation) may be more accurate than raw ultrasound data for tracking larger inter‐frame tissue motion. This study investigates the accuracy of raw radio‐frequency (RF) versus non‐logarithmic compressed envelope‐detected (B‐mode) data for ultrasound speckle tracking in the context of image‐guided radiation therapy.Methods:Transperineal ultrasound RF data was acquired (with a 7.5 MHz linear transducer operating at a 12 Hz frame rate) from a speckle phantom moving with realistic intra‐fraction prostate motion derived from a commercial tracking system. A normalised cross‐correlation template matching algorithm was used to track speckle motion at the focus using (i) the RF signal and (ii) the B‐mode signal. A range of imaging rates (0.5 to 12 Hz) were simulated by decimating the imaging sequences, therefore simulating larger to smaller inter‐frame displacements. Motion estimation accuracy was quantified by comparison with known phantom motion.Results:The differences between RF and B‐mode motion estimation accuracy (2D mean and 95% errors relative to ground truth displacements) were less than 0.01 mm for stable and persistent motion types and 0.2 mm for transient motion for imaging rates of 0.5 to 12 Hz. The mean correlation for all motion types and imaging rates was 0.851 and 0.845 for RF and B‐mode data, respectively. Data type is expected to have most impact on axial (Superior‐Inferior) motion estimation. Axial differences were <0.004 mm for stable and persistent motion and <0.3 mm for transient motion (axial mean errors were lowest for B‐mode in all cases).Conclusions:Using the RF or B‐mode signal for speckle motion estimation is comparable for translational prostate motion. B‐mode image formation may involve other signal‐processing steps which also influence motion estimation accuracy. A similar study for respiratory‐induced motion would also be prudent.This work is support by Cancer Research UK Programme Grant C33589/A19727.

Measurement of Internal Diameter Changes and Pulse Wave Velocity in Fetal Descending Aorta Using the Ultrasonic Phased-Tracking Method in Normal and Growth-Restricted Fetuses

Phased tracking (PT) is an ultrasound-based technique that enables precise measurement of a target velocity. The aims of this study were to use PT to evaluate arterial pulse waveform, pulse wave velocity and fetal pulse pressure in normal and growth-restricted fetuses. One hundred fetuses with normal development and 15 fetuses with growth restriction were analyzed. Ultrasonic raw radiofrequency signals were captured from a direction perpendicular to the vascular axis at the fetal diaphragmatic level for the difference in internal dimensions (DID), or simultaneously from different directions for the pulse wave velocity. Pulsatile movement of the proximal and distal intima of the vessels was analyzed using PT. The fetal DID exhibited no significant changes in growth-restricted fetuses. Pulse wave velocity (3.8 ± 0.32 m/s vs. 2.2 ± 0.069 m/s, p < 0.001) and estimated pulse pressure (6.9 ± 0.90 kPa vs. 2.5 ± 0.18 kPa, p < 0.001) were significantly elevated in growth-restricted fetuses. Assessment of DID and pulse wave velocity of the descending aorta using PT is a feasible, non-invasive approach to evaluation of fetal hemodynamics.

Magnetically-coated silica nanospheres for dual-mode imaging at low ultrasound frequency

To experimentally investigate the acoustical behavior of different dual-mode nanosized contrast agents (NPCAs) for echographic medical imaging at low ultrasound (US) frequency. We synthesized three different nanosized structures: (1) Pure silica nanospheres (SiNSs); (2) FePt-iron oxide (FePt-IO)-coated SiNSs; and (3) IO-coated SiNSs, employing three different diameter of SiNS-core (160, 330 and 660 nm). Tissue mimicking phantoms made of agarose gel solution containing 5 mg of different NPCAs in 2 mL-Eppendorf tubes, were insonified by a commercial echographic system at three different low US pulse values (2.5, 3.5 and 4.5 MHz). The raw radiofrequency signal, backscattered from each considered NPCA containing sample, has been processed in order to calculate the US average backscatter intensity and compare the acoustic behavior of the different NPCA types. The highest US contrast was exhibited by pure SiNSs; FePt-IO-coated SiNSs acoustical behavior followed a similar trend of pure SiNSs with a slight difference in terms of brightness values. The acoustic response of the examined NPCAs resulted function of both SiNS diameter and US frequency. Specifically, higher US frequencies determined higher value of the backscatter for a given SiNS diameter. Frequency-dependent enhancement was marked for pure SiNSs and became less remarkable for FePt-IO-coated SiNSs, whereas IO-coated SiNSs resulted almost unaffected by such frequency variations. Pure and FePt-IO-coated SiNSs evidenced an image backscatter increasing with the diameter up to 330 nm. Conversely, among the types of NPCA tested, IO-coated SiNSs showed the lowest acoustical response for each synthesized diameter and employed US frequency, although a diameter-dependent raising trend was evidenced. The US characterization of magnetically covered SiNS shows that FePt-IO, rather than IO, was the best magnetic coating for realizing NPCAs suitable for dual mode imaging of deep organs, combining US and magnetic resonance imaging.

Intravascular ultrasound tissue characterization. I like the rainbow but... what's behind the colours?

Intravascular ultrasound (IVUS) remains the gold standard for, precise, high-quality visualization of atherosclerotic plaque on the vessel wall.1 Conventional grey-scale IVUS provides unique insights into the underlying substrate of atherosclerotic disease, and classical studies have demonstrated that plaque echogenicity correlated with its histological composition. However, despite its ability to provide direct insight on plaque composition, grey-scale IVUS has been unable to identify vulnerable plaques.2 The reason for this appears to be multifaceted. First, the current resolution of the technique is unable to define some subtle characteristics of these plaques, including the thickness of the fibrous cap. Secondly, ‘thrombotic-prone’ plaques have several pathological substrates apart from the classical ‘vulnerable plaque’ (i.e. thin cap fibroateroma) and, phenotypically, constitute a moving target. Thirdly, further accuracy to identify better the distinct histological components of atherosclerotic plaques with IVUS is clearly required. Some technological developments have attempted to overcome the limitations of conventional IVUS to elucidate plaque components.2–6 All of them rely on sophisticated mathematical analyses of the raw radiofrequency signal, aim to obtain real-time clinically meaningful tissue characterization, and provide a “ rainbow ” of appealing colours encoding different plaque components.2–6 Direct use of radiofrequency data avoids signal alterations induced by automated processing and gain adjustments. Integrated backscatter IVUS (IB-IVUS) uses the fast Fourier transformation to analyse frequencies within the signal and provides colour-coded tissue maps of plaque components (lipid, fibrous, and calcified) enabling tissue characterization with a good correlation with histological and angioscopic findings.3 Some provocative preliminary studies have even suggested that IB-IVUS may allow the detection of changes in plaque characteristics induced by lipid-lowering drugs and could also help to predict future coronary events.2,4 Alternative modalities of tissue characterization, namely wavelet analysis of radiofrequency signals, have also been validated to … *Corresponding author. Email: falf{at}hotmail.com