Chirp-Coded Ultraharmonic Imaging with a Modified Clinical Intravascular Ultrasound System

Abstract

Imaging plaque microvasculature with contrast-enhanced intravascular ultrasound (IVUS) could help clinicians evaluate atherosclerosis and guide therapeutic interventions. In this study, we evaluated the performance of chirp-coded ultraharmonic imaging using a modified IVUS system (iLab™, Boston Scientific/Scimed) equipped with clinically available peripheral and coronary imaging catheters. Flow phantoms perfused with a phospholipid-encapsulated contrast agent were visualized using ultraharmonic imaging at 12 MHz and 30 MHz transmit frequencies. Flow channels with diameters as small as 0.8 mm and 0.5 mm were visualized using the peripheral and coronary imaging catheters. Radio-frequency signals were acquired at standard IVUS rotation speed, which resulted in a frame rate of 30 frames/s. Contrast-to-tissue ratios up to 17.9 ± 1.11 dB and 10.7 ± 2.85 dB were attained by chirp-coded ultraharmonic imaging at 12 MHz and 30 MHz transmit frequencies, respectively. These results demonstrate the feasibility of performing ultraharmonic imaging at standard frame rates with clinically available IVUS catheters using chirp-coded excitation.

Combining Subharmonic and Ultraharmonic Modes for Intravascular Ultrasound Imaging: A Preliminary Evaluation

“…1(a)) based on the iLab ™ IVUS scanner (Boston Scientific/Scimed, Natick, MA) and equipped with single-element imaging catheters — either Atlantis PV (8.5 F, 15 MHz center frequency, clinical use: peripheral imaging) or Atlantis SR Pro (3.2 F, 40 MHz center frequency, clinical use: coronary imaging) was employed. The −3 dB (full-width-half-maximum) fractional bandwidth of these transducers were 25 %, and 49 % (Shekhar et al, 2016). The imaging catheter was connected to the motor drive via a breakout box (provided by Boston Scientific) to circumvent the transmit and receive circuitry of the iLab ™ IVUS system (Yu et al, 2014).…”

“…Accordingly, the transducers with 15 MHz and 40 MHz center frequencies were used to transmit pulses at 12 MHz and 30 MHz, respectively. Specifically, the excitation pulse for 12 MHz and 30 MHz transmit consisted of linear frequency modulated chirps (10 % fractional bandwidth), with 1.67 μ s and 0.67 μ s pulse duration, respectively (Shekhar and Doyley, 2012; Shekhar et al, 2016). Excitation pulses were tapered using a 70 % Tukey window to reduce pulse distortion.…”

“…For imaging at 30 MHz transmit frequency, a custom lowpass filter (E&I, Rochester, NY, USA) with −3 dB cutoff frequency of 34 MHz was used to filter the amplifier output to eliminate nonlinearity in the transmit signal. The IVUS transducers were calibrated as reported earlier (Shekhar et al, 2016). Peak pressures measured on axis with the peripheral and coronary imaging catheters at distances of 5 mm and 2.5 mm were 2.3 and 1.4 MPa, respectively.…”

“…However, the CTR of subharmonic images produced in these studies was comparable to fundamental imaging, likely due to the limited bandwidth of the transducer, and the lack of contrast agent-specific pulsing sequences. Recently, we demonstrated the feasibility of ultraharmonic IVUS imaging using commercially available catheters (Shekhar et al, 2016; Huntzicker et al, 2016) by implementing long duration chirp pulses to attain CTRs ranging from 11 – 18 dB. Although these results are encouraging, combining nonlinear imaging modes could improve the performance of CE-IVUS imaging substantially.…”

“…In this technical note, we investigated the feasibility of performing combined subharmonic and ultraharmonic CE-IVUS imaging using a single transmission. Commercially available peripheral and coronary imaging catheters were used along with a prototype IVUS system (Shekhar et al, 2016), and vessel flow phantoms perfused with a phospholipid-encapsulated contrast agent were visualized. The performance of standalone subharmonic and ultraharmonic imaging was compared with combined-mode CE-IVUS imaging.…”