Frequency-Shifted Narrowband Transmit Pulse Sequencing and Data Compounding Strategy for Improved H-scan Ultrasound Imaging and Tissue Characterization

Abstract

Tissue characterization using ultrasound (US) methods is a valuable tool for assessing tissue health and function. The newer H-scan US methodology is a pixel-level tissue characterization technique where the received backscattered US signals are linked to the size of the scatterer source including that from cell aggregates. Here we detail implementation of a new frequency-shifted narrowband transmit pulse sequencing and data compounding strategy for performing H-scan US imaging. Termed spectrum US imaging (SUI), the primary motivation was to improve relative scatterer size (RSS) estimation and quality of the resultant H-scan US images. The arbitrary waveform generator capability of a research scanner (Vantage 256, Verasonics Inc) was used to implement a frequency-shifted narrowband US transmit pulsing strategy. These narrowband pulses were equally spaced in the frequency domain to utilize the entire bandwidth of the linear array transducer. A total of 27 different frequency-shifted pulse sequences were implemented in the 3.3 to 11.3 MHz range. Next, matched to the center frequency of the different transmit pulses used to collect the US images during this frequency sweep, a filter bank was applied to the backscattered US data to measure the relative strength of the received signals. The signal envelope for each of these filtered and compounded data sequences was then calculated using a Hilbert transformation. This data was then color coded whereby the lower frequency backscattered US signal components are assigned to a redder channel and the higher frequency signals to bluer channel. Preliminary in vitro studies involved use of gelatin-based phantom that were embedded with either 30 or 40 µm spherical US scatterers. Images were collected using both SUI and traditional H-scan US imaging as comparison. Overall, SUI locally estimates the RSS and spatial distribution of aggregate US scatterers.