Factors affecting the shear wave elastic quantitative measurement of penile tissue in rats

According to the data shown in Table 1, we found that SWQ followed a normal distribution (P = 0.282), but PAP_S and PAP_C did not (P < 0.001). Scatter plots (Figure 3A) showed that SWQ was linearly correlated with PAP_S and PAP_C, respectively. There was no multicollinearity between PAP_S and PAP_C (tolerance = 0.385, VIF = 2.595). The Durbin–Watson statistic (= 1.418) and residual plots (Figures 3B–D) showed that residuals possessed normality, independence, and equal variance. Multiple linear regression results (Table 2) showed that the regression equation of the PAP_S, PAP_C, and SWQ (SWQ = 10.376 – 0.05 PAP_S – 0.07 PAP_C) was of statistical significance (F = 21.153, P < 0.001). The content of SMCs and CFs was negatively correlated with SWQ, affecting 42.6% of the total variation in SWQ (R² = 0.426).

Figure 3.

2D-SWE is a new noninvasive ultrasound technology that has been successfully applied in clinical practice. The brief workflow of 2D-SWE (Figure 4) is as follows [1]: the exciting acoustic impulse emitted by the probe is continuously focused on the tissue to generate acoustic radiation force, which in turn leads to tissue vibration and shear wave formation. The shear wave propagating transversely in the tissue will be captured rapidly by the detected pulse. The SWQ value reflecting the tissue elastic characteristics can be calculated accordingly by measuring the shear wave velocity.

Figure 4.

2D-SWE could quantitatively evaluate the tissue elastic characteristics and detect pathological changes in the tissue components by measuring the SWQ value. For example, 2D-SWE is available for follow-up and close monitoring in patients with kidney allograft fibrosis by assessing the degree of fibrosis, providing superiority over serum creatinine in the detection of early tubulointerstitial fibrosis [8]. 2D-SWE is also applied to distinguish benign from malignant breast lesions as the elastic characteristics of breast lesions correlate significantly with the content of CFs [9].

Besides, 2D-SWE is a reliable technique with high specificity and sensitivity for quantitative assessment of the elastic characteristics in the penile tissue [10, 11]. The SWQ value is correlated with the International Index of Erectile Function (IIEF-5) and Erection Hardness Scale (EHS) scores [11]. By measuring the penile SWQ value, 2D-SWE is able to detect and localize nonpalpable lesions in PD patients' curved penis undetected through conventional ultrasound [12], evaluate the degree of fibrosis in corpus cavernosum [13], differentiate the malignant from benign penile mass [14], and quantitatively evaluate the rigidity changes of corpus cavernosum in vascular ED patients [15]. Accurate assessment of the penile tissue composition seems imperative for the precise diagnosis of penile diseases. For example, pathological changes in penile tissue components will eventually result in organic ED. Timely and accurate assessment of the penile tissue components is the key to improving the accuracy of the diagnosis of ED.

Currently, penis biopsy and surgery are the only clinical methods for the assessment of penile tissue components. However, these methods are invasive and require high technical skills for the operators. Complications like penile pain, hematoma, fibrosis, scarring, and induration are prone to occur after invasive penile examinations, which seriously affect patients' physical and mental health. Thus, penis biopsy and surgery are not available as routine clinical examinations for patients with penile diseases. As the penis is a private external reproductive organ of the human body, patients always have difficulty accepting invasive examinations on it compared with other organs. For example, psychological pressure and potential complications will occur and aggravate the condition in ED patients after an invasive penile examination. Consequently, noninvasive penile examinations play an essential role in the diagnosis, treatment, and follow-up of patients with penile diseases. There is a critical need to explore a novel method for the noninvasive assessment of penile tissue components.

Studies have demonstrated that pathological changes in the component and content of penile tissue would affect the SWQ value measured by 2D-SWE [16]. Identifying the main factors affecting the penile SWQ value is the prerequisite for accurate diagnosis of the penile disease using 2D-SWE. Given that no relevant studies have been reported, this study was performed to analyze the main factors affecting the penile SWQ value.

SMCs (40%–52% in humans) and CFs (40%–65% in humans) are the major components of the penile tissue and the main components involved in erectile function [4, 5]. SMCs relaxation is the primary process of penile erection. CFs are essential components involved in penile stretching. During the process of penile erection, CFs prevent overstretching of the basement membrane and SMCs and protect the endothelial and underlying cell layers from disruption [17]. Furthermore, as components of the intracavernosal fibrous framework, CFs ensure normal erection, prevent the penis from deforming and bending during erection, and form a fibrous sheath to prevent arterial collapse and nerve compression [18]. Type III CFs are involved in the process of contraction during wound healing. Increased CFs content would result in plaque contracture and penile deformity. In theory, the composition and content of the tissue through which shear wave passes could directly affect the SWQ value. Therefore, the hypothesis of this study was that the content of SMCs and CFs may be the main factors affecting changes in penile SWQ.

In this study, our results showed that the regression equation of the SWQ with the PAP_S and PAP_C was statistically significant. The content of SMCs and CFs was one of the main factors affecting the penile SWQ value, which verified the hypothesis we proposed.

Although our data demonstrated that the content of SMCs and CFs was negatively correlated with SWQ, there were several limitations in this study. First, PAP_S and PAP_C did not follow a normal distribution. The small sample size may attribute to it. Second, the regression degree of the model (R² < 0.5) was low, which indicated that there were other factors affecting the penile SWQ that need to be explored. An exploratory study in a new field might contribute to the reason why the P value of PAPs (P = 0.086) in the regression equation was more than 0.05 but smaller than 0.1. Additional research on a large amount of clinical data is needed before 2D-SWE can be put into application confidently.

SMCs and CFs are the main factors affecting the SWQ value and are the primary tissue components determining the SWQ when quantitatively evaluating the elastic characteristics of penile tissue with 2D-SWE.