Tomoelastography of the prostate using multifrequency MR elastography and externally placed pressurized‐air drivers

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Through advances in technology, the clinical applications of multiparametric MRI in the noninvasive assessment of the prostate have expanded from locoregional staging to tumor detection, localization, characterization, risk stratification, surveillance, and image guidance for targeted biopsy 1. Standard MRI protocols include T2‐weighted imaging for assessment of morphology, diffusion‐weighted imaging for assessment of cell density, and dynamic contrast‐enhanced imaging for assessment of vascularization 3. In addition to these MRI markers, preliminary studies based on ultrasound shear wave elastography (SWE) indicate that the stiffness of prostatic lesions correlates with the Gleason score 4. However, SWE is limited in penetration depth of the mechanical stimulus and requires the use of endorectal transducers. Because of their invasiveness, these transducers are not suited for screening examinations or for acquisition of reference values in healthy volunteers. For this reason, consistent stiffness values are still lacking in the literature 5. This motivates the development of externally placed elastography for integration into multiparametric MRI examinations tailored for screening or follow‐up imaging of the prostate.
Several challenges exist for the advancement of prostate magnetic resonance elastography (MRE) 6. The most relevant are shear wave damping due to the central location of the prostate in the pelvis, and resolution of stiffness details due to the high heterogeneity in small tissue volumes. To overcome these challenges, different techniques have been proposed including transurethral 7, endorectal 8, and transperineal 10 drivers. The transurethral approach has not been demonstrated in humans so far, whereas the endorectal approach allows application of mechanical vibrations at higher frequencies but is more invasive than approaches with externally placed drivers. The transperineal approach uses an externally placed driver and is thus better suited for screening examinations but requires specialized driver hardware dedicated to prostate examinations with installation times of 8 to 10 min before each MRE scan 11.
Our study has three main objectives: (i) Demonstrate the technical feasibility of prostate MRE using robust externally placed pressurized‐air drivers, which have been used in previous investigations of other organs including the liver, spleen, pancreas, and kidneys 12; (ii) propose a multifrequency MRE protocol with a short scan time of less than 10 min, including full 3D wave field acquisition and tomoelastography wave‐speed reconstruction 14: and (iii) use the new method to analyze regional prostatic stiffness in healthy volunteers and patients.
Although reproducibility and reference values are measured in healthy volunteers, a case study of five subjects with benign and malignant alterations of the prostate is presented for demonstrating the clinical feasibility of the protocol.

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