Ultrasound‐gated computed tomography coronary angiography: Development of ultrasound transducers with improved computed tomography compatibility
Authors: Strassle Rojas Stephan , Collins Graham C. , Tridandapani Srini , Lindsey Brooks D. 
Published: June 2021

Purpose

Cardiovascular disease (CVD) is a leading cause of death worldwide, with coronary artery disease (CAD) accounting for nearly half of all CVD deaths. The current gold standard for CAD diagnosis is catheter coronary angiography (CCA), an invasive, expensive procedure. Computed tomography coronary angiography (CTCA) represents an attractive non‐invasive alternative to CCA, however, CTCA requires gated acquisition of CT data during periods of minimal cardiac motion (quiescent periods) to avoid non‐diagnostic scans. Current gating methods either expose patients to high levels of radiation (retrospective gating) or lead to high rates of non‐diagnostic scans (prospective gating) due to the challenge of predicting cardiac quiescence based on ECG alone. Alternatively, ultrasound (US) imaging has been demonstrated as an effective indicator of cardiac quiescence, however, ultrasound transducers produce prominent streak artifacts that disrupt CTCA scans. In this study, a proof‐of‐concept array transducer with improved CT‐compatibility was developed for utilization in an integrated US‐CTCA system.

Methods

Alternative materials were tested radiographically and acoustically to replace the radiopaque acoustic backings utilized in low frequency (1–4 MHz) cardiac US transducers. The results of this testing were used to develop alternative acoustic backings consisting of varying concentrations of aluminum oxide in an epoxy matrix via simulations. On the basis of these simulations, single element test transducers designed to operate at 2.5 MHz were fabricated, and the performance of these devices was characterized via acoustic and radiographic testing with micro‐computed tomography (micro‐CT). Finally, a first proof‐of‐concept cardiac phased array transducer was developed and its US imaging performance was evaluated. Micro‐CT images of the developed US array with improved CT‐compatibility were compared with those of a conventional array.

Results

Materials testing with micro‐CT identified an acoustic backing with a measured radiopacity of 1008 HU, more than an order of magnitude lower than that of the acoustic backing (24,000 HU) typically used in cardiac transducers operating in the 1–4 MHz range. When utilized in a simulated transducer design, this acoustic backing yielded a −6‐dB fractional bandwidth of 57%, similar to the 54% bandwidth of the transducer with the radiopaque acoustic backing. The developed 2.5 MHz, single element transducer based on these simulations exhibited a fractional bandwidth of 51% and signal‐to‐noise ratio (SNR) of 14.7 dB. Finally, the array transducer developed with the acoustic backing having decreased radiopacity exhibited a 56% fractional bandwidth and 10.4 dB single channel SNR, with penetration depth >10 cm in phantom and in vivo imaging using the full array.

Conclusions

The first attempt at developing a CT‐compatible ultrasound transducer is described. The developed CT‐compatible transducer exhibits improved radiographic compatibility relative to conventional cardiac array transducers with similar SNR, bandwidth, and penetration depth for US imaging, according to phantom and in vivo cardiac imaging. A CT‐compatible US transducer might be used to identify cardiac quiescence and prospectively gate CTCA acquisition, reducing challenges associated with current gating approaches, specifically relatively high rates of non‐diagnostic scans for prospective ECG gating and high radiation dose for retrospective gating.