This study showed that the MRE of the prostate gland using an external air driver with a frequency of 60 Hz is successful in evaluating tissue stiffness. All ten patients could well tolerate the mechanical vibration of the driver without any complications. This implies that the MRE of the prostate gland is a safe noninvasive method that could be added to conventional MRI. It was also possible to obtain the wave images which passed through the prostate gland and measured the tissue stiffness from the MRE examinations of all ten patients.
The mean tissue stiffness of the whole prostate gland in this study was 4.40 ± 0.71 kPa when compared with the previous MRE studies of J. Kemper et al.  He performed MRE using an external driver attached to the pubic bone by a 1.5 T scanner in seven healthy volunteers at a vibration frequency of 85 Hz and reported the mean values of elasticity inside the peripheral zone and central zone that were 3.3 ± 0.5 kPa and 2.2 ± 0.3 kPa, which were lower than the current study stiffness mean value. After that Sahebjavaher et al. studied MRE using a 3.0 T scanner transperineal electromechanical transducer at a frequency of 70 Hz in six healthy volunteers and found mean shear stiffnesses of 11.5 ± 2.9, 13.8 ± 4.5 and 13.2 ± 5.0 kPa for the peripheral, central and transition zones . That study showed a much higher value of stiffness than in this present study, although the study was performed in healthy volunteers. This might be caused by the different electromechanical transducer in this study and an air driver as in the current study, a different wave frequency and different setting of volunteers. The recent study of prostate MRE was performed by Dittmann F et al. to assess the elasticity of prostate gland in 12 healthy volunteers using a 1.5 T scanner with MRE and three externally placed pressurized-air drivers at vibration frequencies of 60, 70, and 80 Hz. They found the shear wave speed of the entire prostate gland at a frequency of 60 Hz was 2.21 ± 0.22 m/s and there was no significant differences of values of shear wave speed between the peripheral zone (2.23 ± 0.20 m/s) and the central gland (2.18 ± 0.26 m/s) . The shear wave speed (Vs) to shear modulus (µ, reported in kPa) conversion using the equation µ = ρVs2 (ρ = soft tissue density, assumed to be 1,000 kg/m3) was used. So the mean stiffness values of the entire prostate gland, peripheral zone, and central gland from Dittmann’s study were 5.02, 4.97, and 4.75 kPa, which were slightly higher than in the present study. There were limitations to compare the results of the current study to other previous studies due to differences in MR scanner, MRE hardware, protocol, technique, population, and post-processing software. From the present results, it can be recommended that further studies to evaluate the reliability of MRE in different settings should be done.
For evaluation of test–retest reliability without repositioning of patients and driver, this study achieved good reproducibility (ICC 0.82) for mean stiffness and moderate reproducibility for maximum and SD of stiffness (ICC 0.74 and 0.65). Minimum stiffness had poor reproducibility (ICC 0.42). The different values between two tests might be caused by inhomogeneous waves traversing the prostate gland which was is a small organ located deeply in the pelvis and the movement of adjacent organs such as bowel peristalsis which might affect imaging acquisition. Excellent reproducibility was found for several acquired MRE slices (ICC 0.93) which mainly depended on the size of prostate gland, especially the prostate height. For analysis of the number of areas of increased stiffness on stiffness images, this study also found excellent reliability between two examinations (ICC 0.99). The study, however, did not reposition the subjects and actuators which might induce small changes in wave patterns and wave amplitudes. As the result of reproducibility analysis in the study of Dittmann F et al., they found good test–retest reproducibility despite repositioning of subjects and actuators between measurements (ICC = 0.88 and 0.78 in the central gland and peripheral zone) .
For the analysis of MRI findings, there was a nearly equal proportion of BPH components in this study; four patients with predominated glandular component, three patients with predominated stromal component, and three patients with equal glandular and stromal components. Most patients (90%) had a bilateral TZ and retrourethral enlargement pattern (type 3) and only one patient (10%) had a pedunculated with bilateral TZ and retrourethral enlargement pattern (type 5). These results were potentially the same as the Randall et al. classification . There were no patients with another pattern of BPH in this study which might be caused by too small a population in the study.
For the analysis of focal areas of increased stiffness seen on stiffness images, about half of them (50.6%) corresponded to a stromal component on T2W images and more than one-third (37.9%) correspond to mixed glandular and stromal components. Additionally, it was found that the glandular predominant group tended to have a lower maximal stiffness than when stromal predominant or equal in its glandular-stromal component. These results supported the review of Wasserman et al.  that the stromal component in BPH leads to an increased resistance of prostatic parenchyma causing an increase in tissue stiffness.
Most other findings found in MRI were minimal ascites in the pelvic cavity, a small area of hemorrhage in the prostate gland and small prostatic utricle cysts which were not corresponding to areas of increased stiffness on stiffness images. Therefore, it can be assumed that these findings might not cause an increase in tissue stiffness. The study found, however, that some features which were seen on patients with high mean stiffness values including prostatic calcification, type-5 BPH pattern and the large prostate volume, it was possible to make an hypothesis that these features might be associated with an increase in tissue stiffness. Further study with more subjects would give more information about this hypothesis.
To the best of our knowledge, this study is the first study to exclusively describe MRE of the prostate gland in Thailand. There are several limitations to the study. Firstly, the study was a prospective study which had too small a number of subjects. Patients recruited into the study were less than was expected because of the Covid-19 situation in the country. Secondly, there might be a selection biases especially in the population-based selection from BPH patients with high PSA levels who were requested to have an MRI to screen for prostate cancer. Other patients with BPH may have more findings. Thirdly, the study had no healthy volunteers to compare the results with which would make a more reliable study. Fourthly, it did not demonstrate the pathological diagnosis which is the gold standard for diagnosis of BPH and its correlation to MRI and MRE findings. Lastly, it tried to perform prostate MRE using the drivers and post-processing software which were developed for measuring stiffness in the liver for the staging of liver fibrosis. The protocol might not be suitable for measuring stiffness in prostate gland so the results in the study might not represent valid tissue stiffness values. It is believed, however, that the results of the study would be beneficial for the development of an MRE protocol as an additional diagnostic tool for evaluating prostate disease and a guide to study more about prostate MRE in the future.