University of Rochester, Rochester, NY, USA.
Med Phys. 2012 Apr;39(4):2049-60. doi: 10.1118/1.3694101.
Subharmonic intravascular ultrasound imaging (S-IVUS) could visualize the adventitial vasa vasorum, but the high pressure threshold required to incite subharmonic behavior in an ultrasound contrast agent will compromise sensitivity-a trait that has hampered the clinical use of S-IVUS. The purpose of this study was to assess the feasibility of using coded-chirp excitations to improve the sensitivity and axial resolution of S-IVUS.
The subharmonic response of Targestar-p(TM), a commercial microbubble ultrasound contrast agent (UCA), to coded-chirp (5%-20% fractional bandwidth) pulses and narrowband sine-burst (4% fractional bandwidth) pulses was assessed, first using computer simulations and then experimentally. Rectangular windowed excitation pulses with pulse durations ranging from 0.25 to 3 μs were used in all studies. All experimental studies were performed with a pair of transducers (20 MHz/10 MHz), both with diameter of 6.35 mm and focal length of 50 mm. The size distribution of the UCA was measured with a Casy(TM) Cell counter.
The simulation predicted a pressure threshold that was an order of magnitude higher than that determined experimentally. However, all other predictions were consistent with the experimental observations. It was predicted that: (1) exciting the agent with chirps would produce stronger subharmonic response relative to those produced by sine-bursts; (2) increasing the fractional bandwidth of coded-chirp excitation would increase the sensitivity of subharmonic imaging; and (3) coded-chirp would increase axial resolution. The experimental results revealed that subharmonic-to-fundamental ratios obtained with chirps were 5.7 dB higher than those produced with sine-bursts of similar duration. The axial resolution achieved with 20% fractional bandwidth chirps was approximately twice that achieved with 4% fractional bandwidth sine-bursts.
The coded-chirp method is a suitable excitation strategy for subharmonic IVUS imaging. At the 20 MHz transmission frequency and 20% fractional bandwidth, coded-chirp excitation appears to represent the ideal tradeoff between subharmonic strength and axial resolution.
次谐波血管内超声成像（S-IVUS）可使血管外膜血管可视化，但在超声造影剂中激发次谐波行为所需的高压阈值会降低灵敏度-这一特性阻碍了 S-IVUS 的临床应用。本研究旨在评估使用编码啁啾激发提高 S-IVUS 灵敏度和轴向分辨率的可行性。
首先通过计算机模拟，然后通过实验评估 Targetar-p（TM），一种商业微泡超声造影剂（UCA）对编码啁啾（5％-20％分数带宽）脉冲和窄带正弦脉冲（4％分数带宽）的次谐波响应。所有研究均使用脉冲持续时间为 0.25 至 3 μs 的矩形窗激励脉冲。所有实验研究均使用一对换能器（20 MHz/10 MHz）进行，换能器直径均为 6.35mm，焦距均为 50mm。UCA 的粒径分布用 Casy（TM）细胞计数器测量。
模拟预测的压力阈值比实验确定的压力阈值高一个数量级。但是，所有其他预测均与实验观察结果一致。预测结果表明：（1）用啁啾激发造影剂会产生比正弦脉冲更强的次谐波响应；（2）增加编码啁啾激励的分数带宽会提高次谐波成像的灵敏度；（3）啁啾会提高轴向分辨率。实验结果表明，啁啾产生的次谐波与基波比高于类似持续时间的正弦脉冲产生的次谐波与基波比。20%分数带宽啁啾获得的轴向分辨率约为 4%分数带宽正弦脉冲的两倍。
编码啁啾方法是次谐波 IVUS 成像的合适激励策略。在 20 MHz 发射频率和 20%分数带宽下，编码啁啾激励似乎在次谐波强度和轴向分辨率之间达到了理想的折衷。
