Abstract
Introduction
Over the last years different elastography techniques have been implemented into nearly all High-End ultrasound systems from nearly all distributors and have already been established as a non-invasive method to assess the stiffness of different abdominal organs and different elastography techniques have been compared to test their reliability [3, 14]. Routine imaging methods for the work-up of unclear scrotal massing like the B-mode sonography are still inevitable and play a crucial role in clinical patient management but more sophisticated ultrasound techniques like shear-wave elastography (SWE) are gaining a more important role by preoperatively determining tissue elasticity on a non-invasive basis [4, 13]. Acoustic Radiation Force Impulse (ARFI) as well as Supersonic Shear Imaging (SSI) are both non-invasive methods to evaluate tissue stiffness. Both techniques rely on elastographies that are based upon shear-wave speed, though the exact methods for generating and detecting shear-wave propagation in tissue do vary in detail [6, 9]. A big benefit of both methods is that they can be performed routinely during classical B-mode ultrasound examinations because of their implementation into the ultrasound system. Both methods have been tested in several studies, mainly for the assessment of fibrosis in patients with liver fibrosis [3, 14]. The key benefit of these methods in comparison to classic strain imaging is the independency from operator reproducibility that results in a decreased number of artefacts [12]. While ARFI has been already tested for the evaluation of tissue stiffness in the male testes, SSI as a relatively new method has not been evaluated yet and until now there has been no comparison made of both methods in evaluating tissue stiffness of the testes [4, 13]. The purpose of this study was to evaluate and compare the results of SSI and ARFI in the measurement of the stiffness of healthy male testes.
Materials and methods
Between February 2015 and April 2105 a total of 58 male healthy volunteers were examined using ARFI and SSI. The local ethics committee approved this study. The study data were collected in compliance with the principles of the Helsinki/Edinburgh Declaration of 2002. Informed consent of all patients was obtained prior to each ultrasound and elastography examination. The authors followed the ethical guidelines for publication in Clinical Hemorheology and Microcirculation [1].
For the evaluation of tissue stiffness using the ARFI technique we used a 9-L4 linear ultrasound probe together with a Siemens Acuson S2000® High-end ultrasound system (Siemens HealthCare, Erlangen, Germany) (Fig. 1).
For the evaluation of tissue stiffness using the SSI technique we used a SL15-4 linear ultrasound probe together with an Aixplorer (SuperSonic Imagine, Aix-en-Provence, France) ultrasound system (Fig. 2).
A single investigator with more than 2 years of experience assessed all ultrasound examinations and elastography measurements in order to rule out interobserver variability. The numbers of measurements, the location of the ultrasound transducer, the procedures for the positioning of the region of interest (ROI) were identical for the ARFI and for SSI examinations. All volunteers were positioned in a supine position and for imaging optimization purposes they were instructed to hold their penises up towards their abdomen in order to preserve optimal conditions. B-mode ultrasound was initially performed for screening purposes and to exclude major testicular pathologies. After initial B-mode ultrasound, SWE was assessed using the ARFI setting of the ultrasound device. In general, the examiner was mindful to avoid any manual pressure on the testes to diminish artefacts. SWE was measured using ROIs in three different parts of each testis, using ROIs in the upper pole, the central portion and the lower pole of each testis using the standard elastography software settings of each ultrasound device. The examinations were afterwards repeated with the other ultrasound device using the SSI method. Both methods generated numerical results measured in m/s. All evaluated results were statistically processed using the paired sample t test and IBM SPSS version 21 program for Mac (IBM, Armonk, NY). P-values lower <0.05 were considered statistically significant.
Results
We measured a mean shear-wave velocity value of 0.81 m/s for the ARFI examination and a mean velocity of 1.1 m/s for the SSI examination.
Shear-wave velocities determined by SSI were all significantly higher in all areas than values gained in the ARFI mode. (p < 0.001 to p = 0.015).
For the upper pole of the left and right testes shear-wave velocity values were 0.43 m/s (95% CI: 0.33– 0.54) (p < 0.001) and 0.26 m/s (95% CI: 0.17– 0.34) (p < 0.001) for the SSI technique, respectively.
For the testicular central portion values were 0.09 m/s (95% CI: 0.02– 0.15) higher (p = 0.015) in the left testes and 0.18 m/s (95% CI: 0.10– 0.25) (p < 0.001) higher in the right testes using SSI.
The measurements in the lower testicular poles showed a higher shear-wave velocity for SSI as well. In the left testes mean values were 0.39 m/s (95% CI: 0.28– 0.49) (p < 0.001) higher and in the right testes 0.34 m/s (95% CI: 0.25– 0.43) higher (p < 0.001) compared to the ARFI technique.
Discussion
The shear wave velocity measurements with SSI were all significantly higher than the velocities measured with the ARFI method. These results are in line with several other studies about the same topic but with different target organs [2, 14]. An assumption is made that the different velocities measured with both methods are caused by systemic differences in velocity measurements, making it difficult to directly compare both methods [14]. Phantom model studies about the same topic reported similar results with significantly different shear wave velocities measured using ARFI and SSI methods [5]. This results in a recommendation to not directly interchange SSI and ARFI velocity values. To our knowledge, this study is the first to describe this systemic difference in velocities for the human male testes. Since elastography of the testes is becoming more popular among radiologists and urologists, a better understanding of these systemic differences is important to compare and interpret results in daily clinical routine as well as in a scientific context. Without the knowledge of this difference reports can be misinterpreted by different physicians leading to unnecessary examinations and possibly changing patient management. The awareness for this problem is already appreciated by the scientific ultrasound community and resulted in a consensus statement of the major scientific researchers regarding the different used and clinically implemented elastography techniques [10].
This study was limited by some factors. Firstly, this study was a mono-center study with only one examiner evaluating the testes with both ultrasound systems. This might result in a subjective choice of ROI placement by the examiner possibly leading to an interobserver bias. The fact that only one examiner performed all measurements leads to a lack in assessing interobserver-variability. Additionally, this study was conducted with a relatively small number of volunteers (n = 58). The conduction of further studies with bigger cohorts will be of great importance in the future for evaluation the reliability of both methods.
Conclusion
Both, ARFI and SSI techniques enabled to assess quantitative and qualitative testicular stiffness and proved to be feasible for clinical application. For the definition of tissue stiffness of testicular tissue, it is mandatory to distinguish between the different elastography techniques. This will be important for the examination of e.g. scrotal masses and other testicular pathologies and in follow-up examinations.