Abstract
PURPOSE:
Real-time shear-wave elastography (SWE) is a newly developed technique for the sonographic quantification of tissue elasticity, which already is used in the assessment of breast and thyroid lesions. Due to limited overlying tissue, the testes are ideally suited for assessment using shear wave elastography. To our knowledge, no published data exist on real-time SWE of the testes.
MATERIALS AND METHODS:
Sixty six male volunteers (mean age 51.86±18.82, range 20–86) with no known testicular pathology underwent normal B-mode sonography and multi-frame shear-wave elastography of both testes using the Aixplorer ® ultrasound system (SuperSonic Imagine, Aix en Provence, France). Three measurements were performed for each testis; one in the upper pole, in the middle portion and in the lower pole respectively. The results were statistically evaluated using multivariate analysis.
RESULTS:
Mean shear-wave velocity values were similar in the inferior and superior part of the testicle (1.15 m/s) and were significantly lower in the centre (0.90 m/s). These values were age-independent. Testicular stiffness was significantly lower in the upper pole than in the rest of the testis with increasing volume (p = 0.007).
CONCLUSION:
Real-time shear-wave elastography proved to be feasible in the assessment of testicular stiffness. It is important to consider the measurement region as standard values differ between the centre and the testicular periphery. Further studies with more subjects may be required to define the normal range of values for each age group. Useful clinical applications could include the diagnostic work-up of patients with scrotal masses or male infertility.
Introduction
Two main types of elastography are implemented on diagnostic ultrasound systems to assess tissue stiffness, differing in how the tissue is deformed for measurement. The first approach to achieve wide clinical use is referred to as strain imaging or quasi-static elastography [17, 26]. An external compression is applied while tracking axial displacements. Differences in displacements provide an image of relative estimated strains with respect to surrounding tissue [23]. However, providing a quantitative map of tissue stiffness is hindered by the unknown stresses resulting from the compression. Additionally, displacements can be affected by tissue structures outside of the imaging plane complicating interpretation [25]. The second approach creates a remote deformation inside the tissue of interest by pushing with a focused ultrasound beam. A transverse shear-wave is generated from the acoustic radiation force of the focused beam. The speed of this shear wave can be tracked and estimated providing a quantitative image of tissue stiffness, as the elasticity or stiffness of tissue are related to the shear wave speed [8, 28]. Higher shear wave velocities correspond to harder tissue. A main advantage over strain based techniques in addition to an absolute measure of tissue stiffness provided by shear wave techniques is improved ease of use and reproducibility, due in part to the user influence in the tissue deformation [3]. Over the last few years, the shear wave based approaches have found growing clinical use (eg. liver fibrosis, breast and thyroid) and are now provided by a number of diagnostic ultrasound manufacturers [2, 15].
Real-time shear-wave elastography (SWE) was used in this study [5]. Multiple acoustic pulses are emitted at increasing focal depths into the tissue to create a quasi-plane shear wave. The coherent interference of the multiple shear-waves increases tissue displacements and the extent of shear wave propagation, reducing the number of pushes to provide a large field of view [5, 26]. This technology has been used in a number of applications, for example with breast and thyroid lesions [2].
The knowledge of elastography in testis imaging is limited. Real-time tissue Elastosonography (RTE) and Virtual touch tissue imaging quantification (VTIQ, Siemens Healthcare, Erlangen, Germany) were described to be helpful for the differentiation of scrotal masses [11]. D’Anastasi et al. described ARFI elastography to be feasible in the assessment of testicular stiffness [7]. The goal of this study is to determine baseline values of normal testicular stiffness obtained by real-time shear-wave elastography, using the new Aixplorer ultrasound device by Supersonic Imagine.
Materials and methods
Study population
The study was performed in accordance with the Declaration of Helsinki and local ethics committee standards. Informed consent was obtained from each volunteer prior to the examination. The authors complied with the ethical guidelines for publication in Clinical Hemorheology and Microcirculation (Ethical guidelines for publication in Clinical Hemorheology and Microcirculation. Clin Hemorheol Microcirc, 2010). The testes of 67 male volunteers (n = 132) were included in this study. The study subjects had no known history of testicular cancer, male infertility or testicular torsion. Two patients lost the right testicle because of prior injuries.
The mean age was 51.86±18.82 years, ranging from 20–86 years. The study population was divided into three different age groups.
The first group (A) included males up to the age of 40 (n = 20).
The second group (B) contained males from 41–60 (n = 21).
The third group (C) consisted of males above the age of 61 (n = 22).
Obtained testicular volumes were put into three different size groups.
Group 1 contained all testes with a size of ≤10 ml.
Group 2 included males with a testicular volume of 10.1–15 ml.
Group 3 consisted of testes with a volume >15 ml.
Ultrasound and elastographic imaging
We used the ‘Aixplorer’ ultrasound system manufactured by Supersonic Imagine (Aix-en-Provence, France) in combination with the SL15-4 linear ultrasound transducer for our study. To avoid inter-observer variability all examinations were performed by the same investigator. B-mode ultrasound was performed on all subjects, using the SL15-4 linear ultrasound transducer having a bandwidth spanning 4–15 MHz. The testicular parenchyma was first scanned for irregularities, then testicular volume was measured in three dimensions and calculated according to the formula: length×width×height×0.523. Subsequently, shear-wave elastography was performed on both testes, using the linear transducer. During the examination, the pressure of the ultrasound transducer on the testes was kept to a minimum in order to avoid compression and artificially increasing shear-wave velocity measurements. A colour-coded two-dimensional elastographic map was obtained and saved on the ultrasound system. On the stored images, three regions of interest (ROI or Q-Box ®), each ROI providing a mean shear-wave velocity value in meters/second, were placed one in the upper pole of the testes, one in the centre and one in the lower pole. We tried to avoid the mediastinum testis when selecting our ROI. In the color-coded maps blue coloured areas represent low shear-wave velocity, thus lower tissue stiffness. Yellow- or red-coloured areas showed spots of higher tissue stiffness.
Data and statistical analysis
The mean shear-wave velocity of each ROI was used for statistical analysis. For comparison between two groups the Mann-Whitney test was used, and for comparison between three groups the Kruskal-Wallis test with subsequent post-hoc tests was used. P values below 0.05 were regarded as significant. All calculations were performed using the software STATISTICA 10 (StatSoft, Tulsa, OK).
Results
No significant difference in mean shear-wave velocities were observed between the left (n = 67; median 1.10 m/s) or right testicles (n = 65; median 1.07 m/s) in our group of patients for equivalent regions (Mann-Whitney-Test; p = 0.148). As a result, no further analyses was pursued with respect to between testicle stiffness.
Testicular stiffness according to the region of measurement
Tissue stiffness was measured at the centre, upper and lower pole of each testis (Fig. 1). No significant difference in mean shear wave velocity (1.15 m/s) was observed between the inferior and superior poles of the testes. However, the centre of the testicle was significantly softer (0.90 m/s; Kruskal-Wallis-Test; p < 0.001) than the two exterior regions (Fig. 2).
Dependence of the testicular tissue stiffness according to the men’s age
As described above, the center was softer than the exterior regions of the testicle, but this investigation was statistically age-independent in our current study. For the inferior part of the testicle, the tissue stiffness was measured for group A at 1.10 m/s and for group B and C at 1.20 m/s each (p = 0.065) (Fig. 3a). Measurements of the centre of the testicle showed in all groups a similar mean shear-wave velocity of 0.90 m/s (p = 0.558) (Fig. 3b). The mean shear-wave velocity at the superior part of the testes for group A was 1.00 m/s and was not significantly increased for group B and C (1.20 m/s each) (p = 0.119) (Fig. 3c).
The influence of the testicular volume to the testicular tissue stiffness
There were no significant differences in tissue stiffness between the three size groups, in the inferior and central regions of the testicle. For the different testicle size groups, measurement of the mean shear-wave velocity in the inferior region decreased in a non-significant way from 1.20 m/s (size ≤10 ml) to 1.15 m/s (size 10.1 ml–15 ml) and 1.10 m/s (size >15 ml) (Fig. 4a) (p = 0.158). There were similar values measured in the central region (for all sizes 0.90 m/s; p = 0.557) (Fig. 4b). In the superior region, there was a significant decrease (p = 0.007) of the mean velocity from 1.20 m/s (size ≤10 ml and 10.1–15 ml), to 0.95 m/s (size >15 ml) (Fig. 4c).
Discussion
Sonography is an essential component of the diagnostic work-up of unclear scrotal masses and male infertility [1, 27]. Unclear masses include testicular cancer, which makes up about 5% of all urologic tumours, mostly occurring in younger men in their third or fourth decade [14]. A definite differentiation between benign and malignant lesions is often not possible on imaging, which may lead to unnecessary operative explorations and even orchiectomies [1]. Sonography is also the first imaging modality used in the diagnostic evaluation of male infertility. The main causes of male infertility, which affects about 7.5% of all couples of reproductive age [16], are a varicocele, undescended testes, hypogonadism, genetic disorders, prior genital surgery or urogenital infections [13, 21].
Elastographic techniques might offer additional information about the composition of testicular tissue in the investigation of testicular masses, as well as aiding in the diagnostic assessment of male infertility.
The variation in testicular tissue stiffness could be explained by the anatomical structure of the testis. The testicle is segmented by multiple small septa, dividing the testicle in up to 400 lobules and interrupted by the Rete testis, an anastomosing network of delicate tubules in the hilum of the testicle. The size of these lobules are larger and longer in the middle of the testis. The mediastinum testis consists of supporting connective tissue and larger vessels drained by centripetal veins and lymphatics from the lobules [4, 24].
The testes undergo age-related changes. With advancing age in men, testicular size, sperm quality, and numbers of all germ cell types, including Sertoli cells and Leydig cells will decrease. Histomorphological examinations showed that the volume occupied by the seminiferous tubules decreases, whereas tissue occupied by the testicular interstitium remains constant [12]. Several studies on age-related testicular involution on the microscopic level showed germ cell degeneration and replacement with a collagenized tunica propria with myoid cells in the sense of a tubular sclerosis and atrophy. Furthermore an accumulation of lipid droplets in the Sertoli cell cytoplasm can be described. As this development is similar to alterations observed after experimental ischemia, it is suggested that vascular lesions may play an important role in age-related testicular atrophy [10, 19].
Absolute values of shear-wave velocities for the testes differ from one ultrasound device to another. In a study performed by our group using a Siemens Acuson S2000™ system to examine the viscoelastic properties of testicular tissue of healthy volunteers, a significant association between older age, lower testicular volumes and higher tissue stiffness has been found [7]. The age- and volume-related elevated stiffness in our patient collective, although non-significant in our current study, is consistent with these findings. In our current study, we observed a significant difference in shear-wave velocity between the peripheral areas and the centre of the testis, which was not observed in our group’s previous study, but no significant difference between the superior and lower portions, which was consistent with our previous findings. One possible explanation for the observed differences is inter-observer variability in the choice of ROI for SWV measurements between studies. Another potential reason which might have contributed to these differences could be the smaller ROI sampling size used for multi-frame elastography. These differences might present a challenge in the comparison of the viscoelastic properties of testicular tissue using different sonoelastrographic devices.
Another study conducted by our group examined uncertain scrotal lesions using the VTIQ (Virtual Touch Imaging Quantification, Siemens Healthcare, Erlangen, Germany) technology, permitting the storage of generated data as a two-dimensional color-coded map, from which retrospective generation of numerical and graphical values is possible. The study found significant differences between the stiffness of normal testicular tissue and malignant lesions. The assessment of different scrotal lesions is certainly a topic of interest for future studies with three-dimensional shear-wave elastography. Analysis of the orthogonal planes generated by the volumetric transducer (transverse, axial and coronal planes) may allow an accurate distinction between tumorous and normal tissue, which would be consistent with the observations of Athanasiou et al. in the assessment of breast lesions [2].
Conclusion
Multi-frame shear-wave elastography appears to be a feasible tool in the assessment of viscoelastic properties of testicular tissue. Operator-independence, reproducibility, quantitative estimate of tissue stiffness, and ease of use are the key advantages of real-time shear-wave elastography [10]. Simplified operability and the possibility to store entire three-dimensional datasets for later analysis were advantages of the Aixplorer ultrasound device used in our current study. After having defined values for normal testicular tissue and compared these data with devices from other venders, the next steps could include gathering information about various scrotal pathologies, for example the evaluation of scrotal masses or of patients with male infertility. Our data suggest that in the assessment of such pathologies, factors like age, area of measurement and testicular volume are an important consideration.