· To assess the feasibility of 3T Magnetic Resonance Elastography (MRE) for evaluating renal tumors with surgical pathology as the gold standard

· To correlate the distribution of MRI shear modulus elastography maps to same-level MRI anatomical sequences including high-resolution HASTE and post contrast VIBE.

· To correlate the distribution of MRI shear modulus elastography maps to gross pathological morphology: cellular distribution, vascular architecture and areas of necrosis to determine how the underlying architecture affects biomechanical properties.

With the growing use of cross-sectional imaging, the incidence of incidental asymptomatic small renal tumors (≤ 4 cm) has risen steadily [1, 2]. Although most of these lesions represent renal cell carcinoma (RCC), up to 20% will be benign [3]. A recent estimate suggests that more than 5600 benign renal tumors are resected yearly in the United States [4]. Renal oncocytoma is the most common benign lesion, accounting for up to 15% of all renal tumors, and cannot be differentiated from RCC reliably by conventional imaging [5, 6]. On gross pathology oncocytomas have a characteristic macroscopic appearance of homogenous dark brown lesions with a frequent central scar and absence of necrosis. Microscopically they consist of tight cellular nests surrounded by myxoid or hyalinized stroma. Conversely, clear cell RCCs, the most common subtype, consist of soft yellow material alternating with fibrous or mucoid areas, and are frequently hemorrhagic and necrotic. Microscopically they are composed of clear cells, rich in lipids and glycogen, surrounded by an extensive capillary network.We hypothesize that renal tumor MRE biomechanical properties are determined by the underlying tumor architecture, including their cellular distribution, vascular architecture and nature of the extracellular-extravascular space.Following IRB approval, informed consent, and initial MRE optimization in 5 healthy volunteers, 5 patients with renal tumors (≤ 4 cm) scheduled for partial or total nephrectomy were recruited prospectively. In addition to standard MRI (T2 HASTE, DWI and post contrast T1 VIBE), MRE was performed on a 3T MRI system (Biograph mMR, Siemens Healthcare, Erlangen, Germany) based upon a prototype 2D multi-slice interleaved gradient echo sequence synchronized with the transducer's vibrations, TE = 7.38 ms, motion encoding gradient amplitude = 30 mT/m and GRAPPA acceleration factor = 2 [7]. Mechanical vibrations at 30Hz were transmitted from the mechanical transducer sited over the kidney of interest. Four consecutive breath-holds of 17 sec each (3 motion-encoding directions and one reference scan) provided MRE data within 6 consecutive slices of 128 x 88 pixels at 3 mm isotropic resolution and 4 wave phase offsets. Reconstruction of viscoelastic parameters used firstly the application of the curl operator for removal of the compressional component, secondly an iterative method comparing the measured with the simulated curl-field within a subzone of 11 x 11 x 5 pixels using the data as boundary condition on the surface of the subzone (similar to [8]). Viscoelastic parametric maps, including storage modulus (elasticity, kPa) and viscosity/elasticity ratio [y=2/π*atan(Gl/Gd)] were generated and regions of interest (ROI) drawn around tumors.One patient was unable to tolerate the MRI. Four MRI datasets, including MRE, were successfully acquired. Mechanical vibrations showed good unilateral wave penetration (image 1). Histological assessment revealed 2 renal oncocytomas (images 2 and 3) and 2 clear cell RCC (images 4 and 5). The two oncocytomas showed homogeneous intermediate T2 HASTE signal with central hyperintensity and homogeneous contrast enhancement on T1 VIBE. RCCs showed spatially heterogeneous T2 HASTE signal and T1 VIBE contrast enhancement. Mean ADC and Minimum ADC values were not obviously discriminatory, ranging between 1258.16 - 1823.29 x 10^-6 mm²/s and 381.00 - 861.00 x 10^-6 mm²/s respectively across tumors. Areas of necrosis and the central scar were excluded from MRE analysis. Mean elasticities were 1.14 and 2.50 kPa; y ratios were 0.52 and 0.40 for each oncocytoma respectively. Mean elasticities for each RCCs were higher at 3.50 and 3.60 kPa; corresponding y ratios were lower at 0.3 and 0.15 kPa respectively.Our initial exploratory data suggests that the biomechanical properties differ between clear RCC and oncocytoma, with higher mean elasticity and lower viscosity/elasticity ratios in RCCs, raising its potential for clinical application. Ongoing prospective recruitment with pathological correlation will provide confirmatory evidence.