Lashkari Bahman, Mandelis Andreas
Center for Advanced Diffusion-Wave Technologies (CADIFT), Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, M5S 3G8, Canada.
Rev Sci Instrum. 2011 Sep;82(9):094903. doi: 10.1063/1.3632117.
In this work, a detailed theoretical and experimental comparison between various key parameters of the pulsed and frequency-domain (FD) photoacoustic (PA) imaging modalities is developed. The signal-to-noise ratios (SNRs) of these methods are theoretically calculated in terms of transducer bandwidth, PA signal generation physics, and laser pulse or chirp parameters. Large differences between maximum (peak) SNRs were predicted. However, it is shown that in practice the SNR differences are much smaller. Typical experimental SNRs were 23.2 dB and 26.1 dB for FD-PA and time-domain (TD)-PA peak responses, respectively, from a subsurface black absorber. The SNR of the pulsed PA can be significantly improved with proper high-pass filtering of the signal, which minimizes but does not eliminate baseline oscillations. On the other hand, the SNR of the FD method can be enhanced substantially by increasing laser power and decreasing chirp duration (exposure) correspondingly, so as to remain within the maximum permissible exposure guidelines. The SNR crossover chirp duration is calculated as a function of transducer bandwidth and the conditions yielding higher SNR for the FD mode are established. Furthermore, it was demonstrated that the FD axial resolution is affected by both signal amplitude and limited chirp bandwidth. The axial resolution of the pulse is, in principle, superior due to its larger bandwidth; however, the bipolar shape of the signal is a drawback in this regard. Along with the absence of baseline oscillation in cross-correlation FD-PA, the FD phase signal can be combined with the amplitude signal to yield better axial resolution than pulsed PA, and without artifacts. The contrast of both methods is compared both in depth-wise (delay-time) and fixed delay time images. It was shown that the FD method possesses higher contrast, even after contrast enhancement of the pulsed response through filtering.
在这项工作中，对脉冲光声（PA）成像模式和频域（FD）光声成像模式的各种关键参数进行了详细的理论和实验比较。根据换能器带宽、光声信号产生原理以及激光脉冲或线性调频脉冲参数，从理论上计算了这些方法的信噪比（SNR）。预测了最大（峰值）信噪比之间存在很大差异。然而，结果表明，在实际中，信噪比差异要小得多。对于来自地下黑色吸收体的FD-PA和时域（TD）-PA峰值响应，典型的实验信噪比分别为23.2 dB和26.1 dB。通过对信号进行适当的高通滤波，可以显著提高脉冲光声的信噪比，这能使基线振荡最小化但无法消除。另一方面，通过相应地增加激光功率和减小线性调频脉冲持续时间（曝光），可以大幅提高FD方法的信噪比，以便保持在最大允许曝光准则范围内。计算了信噪比交叉线性调频脉冲持续时间作为换能器带宽的函数，并确定了使FD模式具有更高信噪比的条件。此外，结果表明，FD轴向分辨率受信号幅度和有限的线性调频脉冲带宽的影响。脉冲的轴向分辨率原则上由于其较大的带宽而更优；然而，信号的双极性形状在这方面是一个缺点。由于互相关FD-PA中不存在基线振荡，FD相位信号可以与幅度信号相结合，以产生比脉冲光声更好的轴向分辨率，且无伪影。在深度方向（延迟时间）图像和固定延迟时间图像中对两种方法的对比度进行了比较。结果表明，即使通过滤波对脉冲响应进行对比度增强后，FD方法仍具有更高的对比度。
