Department of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA, USA.
Wallace H Coulter Department of Biomedical Engineering, Georgia Institute of Technology, Atlanta, GA, USA.
Med Phys. 2021 Aug;48(8):4191-4204. doi: 10.1002/mp.15023. Epub 2021 Jul 11.
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.
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.
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.
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.
心血管疾病（CVD）是全球主要的死亡原因，其中冠状动脉疾病（CAD）占所有 CVD 死亡人数的近一半。CAD 诊断的金标准是导管冠状动脉造影（CCA），这是一种侵入性、昂贵的程序。计算机断层冠状动脉造影（CTCA）代表了一种有吸引力的非侵入性替代 CCA 的方法，然而，CTCA 需要在心脏运动最小（静止期）期间采集 CT 数据的门控，以避免非诊断性扫描。目前的门控方法要么使患者暴露在高剂量的辐射下（回顾性门控），要么由于仅基于心电图预测心脏静止状态的挑战性而导致高比例的非诊断性扫描（前瞻性门控）。另一方面，超声（US）成像已被证明是心脏静止状态的有效指标，但是，超声换能器产生的条纹伪影会干扰 CTCA 扫描。在这项研究中，开发了一种具有改进的 CT 兼容性的概念验证相控阵换能器，用于集成的 US-CTCA 系统。
对替代材料进行了射线照相和声学测试，以替代低频（1-4MHz）心脏 US 换能器中使用的不透射线声背衬。对这些测试的结果进行了研究，以开发出由不同浓度的氧化铝组成的替代声学背衬，该背衬通过模拟制作而成。在此基础上，设计了工作频率为 2.5MHz 的单元件测试换能器，并通过微计算机断层扫描（micro-CT）进行了声学和射线照相测试，对这些设备的性能进行了表征。最后，开发了第一个概念验证的心脏相控阵换能器，并对其 US 成像性能进行了评估。利用 micro-CT 对具有改进 CT 兼容性的开发型 US 阵列进行成像，并将其与传统阵列进行比较。
微 CT 材料测试确定了一种声背衬，其测量的射线不透明度为 1008 HU，比通常用于 1-4MHz 范围内工作的心脏换能器的声背衬（24000 HU）低一个数量级。当用于模拟换能器设计时，这种声背衬产生的-6dB 分数带宽为 57%，与具有射线不透明度的声背衬的 54%带宽相似。基于这些模拟开发的 2.5MHz 单元件换能器的分数带宽为 51%，信噪比（SNR）为 14.7dB。最后，开发的具有较低射线不透明度的阵列换能器具有 56%的分数带宽和 10.4dB 的单个通道 SNR，在体模和体内成像中，穿透深度>10cm，可使用整个阵列。
描述了开发 CT 兼容超声换能器的首次尝试。根据体模和体内心脏成像，开发的 CT 兼容换能器在具有相似 SNR、带宽和穿透深度的情况下，与传统的心脏阵列换能器相比，具有更好的射线照相兼容性。CT 兼容的 US 换能器可用于识别心脏静止状态并前瞻性地门控 CTCA 采集，从而减少当前门控方法的相关挑战，特别是前瞻性 ECG 门控的相对较高比例的非诊断性扫描和回顾性门控的高辐射剂量。
