Christensen Alexandra, Hall Timothy J, Feltovich Helen, Rosado-Mendez Ivan
Department of Medical Physics, University of the Wisconsin, Madison, Madison, WI, USA.
Department of Medical Physics, University of the Wisconsin, Madison, Madison, WI, USA.
Ultrasound Med Biol. 2025 Sep;51(9):1589-1603. doi: 10.1016/j.ultrasmedbio.2025.06.006. Epub 2025 Jul 1.
Speckle statistics are fundamentally related to the resolution of an ultrasound image. Existing models for speckle statistics do not account for changes in resolution with imaging depth, a reality of clinical pulse-echo ultrasound, thereby posing challenges to interpretation. The purpose of this work is to evaluate and address this shortfall in first-order speckle statistics analysis.
Simulated ultrasonic speckle from a low scatterer density medium was created with known acquisition parameters, and speckle statistics of the Nakagami and homodyned K distributions were estimated with and without compensating for the expected change in the resolution cell size, defined by the ultrasound pulse volume, over the field of view. Compensation methods using resolution estimates from the predicted acoustic field, image autocorrelation, or spectral analysis were compared. The absolute number of scatterers per cubic millimeter was also calculated from corrected speckle statistics estimates. The results were validated with experiments in a low scatterer density phantom using a clinical scanner, and in in vivo rhesus macaque cervix and human Achilles tendon images.
It was shown in both simulations and phantoms that, when no compensation was applied, the expansion of the resolution cell size caused a saturation of Nakagami m and homodyned K parameter estimates at depths beyond the focal distance, even when scatterer concentration was constant throughout the depth of the phantoms. This confounding factor was reduced by compensating for the changing resolution cell size, resulting in a decrease in the normalized root mean squared errors between estimates at focus and estimates at all depths (7.8 ± 0.3% to 5.1 ± 0.3% in m, 68 ± 2% to 9.1 ± 0.3% in α, and 40 ± 1% to 6.8 ± 0.3% in k in simulated acquisitions; 21.5 ± 0.4% to 15.4 ± 0.4% in m, 291 ± 15% to 76 ± 12% in α, and 39 ± 1% to 21 ± 1% in k in phantom acquisitions). Images from in vivo analysis before and after compensation resulted in an increase in median contrast-to-noise ratio between the cervix and background and the Achilles tendon and background.
Compensation for changes in ultrasonic resolution with depth prevents saturation in speckle statistics estimates, thus providing more relevant information about tissue properties. The methods described in this work reduce the system dependence of speckle statistics analysis, thus addressing a barrier to the clinical application and adoption of speckle statistics.
散斑统计与超声图像的分辨率密切相关。现有的散斑统计模型未考虑成像深度对分辨率的影响，而这在临床脉冲回波超声中是实际存在的情况，从而给解读带来了挑战。本研究的目的是评估并解决一阶散斑统计分析中的这一不足。
利用已知的采集参数生成来自低散射体密度介质的模拟超声散斑，并在不补偿和补偿预期的分辨率单元大小变化（由超声脉冲体积定义）的情况下，估计在视野范围内的Nakagami分布和同相K分布的散斑统计量。比较了使用预测声场分辨率估计、图像自相关或频谱分析的补偿方法。还从校正后的散斑统计估计中计算了每立方毫米散射体的绝对数量。结果在使用临床扫描仪的低散射体密度体模实验以及恒河猴子宫颈和人体跟腱的体内图像实验中得到了验证。
模拟和体模实验均表明，在不进行补偿时，即使体模整个深度范围内散射体浓度恒定，分辨率单元大小的扩大也会导致在焦距以外深度处Nakagami参数m和同相K参数估计值出现饱和。通过补偿分辨率单元大小的变化，这一混淆因素得以减少，使得焦点处估计值与所有深度处估计值之间的归一化均方根误差降低（模拟采集中，m从7.8±0.3%降至5.1±0.3%，α从68±2%降至9.1±0.3%，k从40±1%降至6.8±0.3%；体模采集中，m从21.5±0.4%降至15.4±0.4%，α从291±15%降至76±12%，k从39±1%降至21±1%）。补偿前后的体内分析图像显示，子宫颈与背景以及跟腱与背景之间的对比度噪声比中位数有所增加。
补偿超声分辨率随深度的变化可防止散斑统计估计值饱和，从而提供更多关于组织特性的相关信息。本研究中描述的方法降低了散斑统计分析对系统的依赖性，从而解决了散斑统计临床应用和推广的一个障碍。
