A new method to quantify penile erection hardness: real-time ultrasonic shear wave elastography.

Background: An important indicator of penile erectile function is erection hardness (EH), which is currently evaluated by the semi-quantitative erectile hardness score (EHS). EH increases continuously during the course of an erection, so although it is statistically a continuous variable, the EHS is a grade variable. We propose a new method for real-time quantitative measurement of penile EH using ultrasonic shear wave elastography (SWE). Methods: The study group comprised 40 patients with erectile dysfunction (ED) and 20 normal controls who all underwent real-time SWE to measure tissue stiffness (Young's modulus, YM) of the penile corpus cavernosum and tunica albuginea during erection, at rest and at different EH grades induced by intracavernosal injection (ICI) of prostaglandin. The examiner gently placed the high frequency probe on the ventral penis and got the two-dimensional longitudinal US image of penis, then switched to SWE mode, the appropriate region of interest (ROI) was selected, then a 3-5 mm circle ("Q-box") was automatically set to by machine to measure the SWE of the corpus cavernosum and tunica albuginea on the left side. Results: In both the ED patients and normal controls, YM slightly decreased in the corpus cavernosum during erection but the stiffness of tunica albuginea increased significantly with increasing EH (resting: 21.66 +/- 4.21 kPa, grades 1-4: 32.61 +/- 4.27, 54.86 +/- 8.69, 128.02 +/- 20.66, and 223.23 +/- 23.61 kPa, respectively). Because EH grades 1 and 2 clinically predicted failure to complete sexual intercourse, and EH grades 3 and 4 predicted ability, the cutoff point for the YM of the tunica albuginea to evaluate whether sexual intercourse could be completed was 81.60 kPa. Conclusions: Our data demonstrated that tunica albuginea stiffness, not corpus cavernosum stiffness, provided a good clinical imaging index indicator for evaluating penile EH. The tunica albuginea stiffness changed continuously during penile erection, which could be measured quantitatively by SWE. Compared with the EHS, measuring the YM of the penile tunica albuginea using SWE is a new and objective technique for quantitatively assessing EH. Because of its objective and quantifiable characteristics, measuring YM enables more accurate evaluation of the effect on EH of various treatments for ED.