Geoffrey Vince

This document suggests standards for the acquisition, measurement, and reporting of radiofrequency data analysis (virtual histology - VH) intravascular ultrasound (IVUS) studies. Readers should view this document as the authors' best attempt in an area of rapidly evolving investigation, an area where rigorous evidence is not yet available or widely accepted. Nevertheless, this document is based on known pathologic data as well as previously reported imaging data; where practical, this data is summarised in the current document, a document which will also include recommendations for future evolution of the technology.

Focused broadband miniature polyvinylidene fluoride-trifluoroethylene (PVDF TrFE) ultrasonic transducers were investigated for intravascular (IVUS) second-harmonic imaging. Modeling and experimental studies demonstrated that focused transducers, unlike conventional flat transducers, build up second harmonic peak pressures faster and stronger, leading to an increased SNR of second harmonic content within the coronary geometry. Experimental results demonstrated that focused second harmonic pressures could be controlled to occur at specific depths by controlling the f-number of the transducer. The experimental results were in good agreement with the modeled results. Experiments were conducted using three imaging modalities: fundamental 20 MHz (F20), second harmonic 40 MHz (H40), and fundamental 40 MHz (F40). The lateral resolutions for a 1-mm transducer (f-number 3.2) at F20, F40, and H40 were experimentally measured to be 162, 123, and 124 microm, respectively, which agreed well with the theoretical calculations with <<8% error. Lateral resolution was further characterized in the three modes, using a micromachined phantom consisting of fixed bars and spaces with widths ranging from 20 to 160 microm. H40 exhibited better lateral resolution, clearly displaying 40- and 60-microm bars with about 4 dB and 7 dB greater signal strength compared with F20. Ex vivo human aorta images were obtained in the second-harmonic imaging mode to show the feasibility of high resolution second-harmonic IVUS using focused transducers.

Improvement of resolution in ultrasound based devices for tissue characterization has been of real interest in the field of minimally invasive imaging such as intravascular ultrasound. Harmonics obtained from the nonlinear properties of the tissue improve resolution and reduce artifacts. Unfortunately, the harmonics produced from the conventional unfocused transducers lie outside the arteries and hence the advantages with harmonic imaging cannot be utilized. A promising way where harmonics can be made to occur inside the arteries is by developing Micro-Electro-Mechanical Systems (MEMS) based focused transducers. Broadband (fractional bandwidths of 75%), high frequency (35-45 MHz) focused transducers were developed using polyvinylidene fluoride-trifluoroethylene (PVDF TrFE). Low pass (33 MHz cut off) and High pass (35 MHz cut off) filters were designed and developed for second harmonic imaging applications in pulse-echo mode. Second harmonic axial radiation patterns have been modeled for focused and unfocused sources and the focused source showed higher harmonic peak pressures. Second harmonic axial radiation patterns for various f-number transducers showed that the harmonic peaks occurred at corresponding focal lengths proving the possibility of obtaining harmonic images from desired locations by adjusting the focal length of the transducer. The second harmonic beam at 40 MHz for a 1 mm transducer was measured in pulse-echo mode and compared with the fundamental beam. Along the focal plane, the transmitted harmonic beam had a beam width of 65 mum whereas the fundamental had a beam width of 90 mum. The experimental values agree very closely with the theoretical calculations that the harmonic beam width should be 1/radic2 times the fundamental beam width. In order to demonstrate contrast enhancement using harmonics, silicon MEMS phantom consisting of fixed bars and spaces alternately with widths ranging from 20 mum to 160 mum was scanned using 20 MHz fundamental beam. It was shown that the harmonic beam exhibited better contrast, especially for 40 mum and 60 mum bars with about 4 dB and 7 dB greater signal strength compared to fundamental. Tissue harmonic imaging capabilities of the transducers were also displayed via successful in vitro imaging of a section of human aorta. The harmonic images provided additional information in comparison to fundamental. This work illustrates that focused transducers produce higher harmonic peak pressures and that the harmonics can be caused to occur as a function of focus. These results coupled with the demonstration of high-resolution, high-contrast harmonic images leads us to believe that this could be an ideal candidate for next generation second harmonic imaging applications

In a pilot study, radiofrequency backscatter data was collected in the paravertebral (PV) spaces of 4 healthy individuals. Using the associated gray scale ultrasound and Doppler data as guidance, regions-of-interest (ROIs) were chosen to represent five tissue types found in and around the PV space – rib shadow, pleura, superior costotransverse ligament, intercostal vessel (artery or vein), and the PV space away from the vessel. ROI sizes of 1.0 mm, 1.5 mm, and 2.0 mm square were examined for auto-regressive (AR) orders of 10, 20, 30, and 40 and bandwidths of 3dB, 6dB, 20dB. Spectral estimations were performed for each ROI size, AR order, and bandwidth over the A-lines of the ultrasound radiofrequency data. The spectra were averaged and normalized using data collected from a tissue phantom. Eight spectral parameters &ndash; Y-intercept, slope, and mid-band fit of the regression line, maximum dB of the spectra, frequency at maximum dB, minimum dB of the spectra, frequency at minimum dB, and integrated backscatter were calculated for each spectral estimate and used to create ensembles of bagged tree classifiers. An ROI size of 2.0 mm, bandwidth of 20 dB, and AR order 10 had the lowest out-of-bag error at 0.315, and averaged across all tissue types, an accuracy of 89.15&percnt;, sensitivity of 0.70, specificity of 0.93, and Youden’s Index (YI) of 0.62. These results show that the identification of the five tissues types in radiofrequency backscatter from intercostal ultrasound is feasible.