Physicians and imaging scientists interested quantitative biomarkers for renal allograft function.To apply multifrequency magnetic resonance elastography (MMRE) to patients with transplant kidneys for the stiffness-based assessment of renal allograft function.22 patients (age range 23–73 years, 7 females) with transplant kidneys underwent MRE (1) examinations. MMRE (2) was performed on a 1.5-T scanner with four vibration frequencies (40 to 70 Hz with 10 Hz increments) using a non-magnetic piezoelectric driver mounted to a transducer mat. For each drive frequency, full wave fields were recorded at eight time points of the vibration period in 7 contiguous coronal image slices of 2.5*2.5*2.5 mm³ resolution. The total acquisition time was approximately 4 minutes. As the transplant kidneys locate in the iliac fossa, respiratory motion was negligible and the patients maintained a shallow and regular breathing throughout the MMRE session. Data processing is detailed in (3): multifrequency dual elasto visco (MDEV) was carried out yielding elastograms of the magnitude of the complex shear modulus |G^*| which mainly reflect the stiffness of the kidney. Clinical data such as the glomerular filtration rate (GFR), resistive index (RI, a sonographic index for altered renal blood flow) were obtained, biopsy was performed in patients with dysfunctional transplants and the last Banff-Score representing the grade of kidney fibrosis was collected.In Fig.1 MRE magnitude images (T2-weighted) and the corresponding elastogram are shown. The patient has two transplant kidneys visible in the coronal slice, the right one (blue outline) is functional (FN) while the left one is highly atrophic and dysfunctional (Dys, red outline). Example shear wave images are shown in Fig.2. To ensure that comparison of stiffness values between FN and Dys are made in regions of same sizes and tissue type (bias from boundary effect related to MRE reconstruction), a refined region-of-interest (ROI) was automatically delineated which contained 100 pixels of the highest |G^*| values (center image of Fig.1). In the reconstructed elastograms (|G^*|-map) shown in Fig.1, FN has marked higher intensity as compared to Dys indicating higher renal stiffness. Group-mean |G^*|-values were significantly higher in FN than Dys (9.50 ± 1.77 kPa [FN], 6.96 ± 1.78 kPa [Dys], p < 0.001 [U-test], Fig.3a). A cutoff value of 7.24 kPa provided sensitivity (83.33%) and specificity (86.67%) for detecting renal allograft dysfunction in kidney transplants with an AUROC-value of 0.9083 (95% CI 0.80-1.02; p < 0.001). We also tested the correlation between |G^*| and clinical data. |G^*| positively correlated with GFR (r = 0.54, p = 0.012), while a negative correlation was found between |G^*| and RI (r = -0.50, p = 0.021), as shown in Fig.3b and Fig.3c.In this study, MR elastography was applied to kidney transplant patients with stable and impaired renal allograft function. Interestingly, we found that the FN are stiffer than Dys with interstitial fibrosis (BANFF 6) and tubular atrophy. This suggests that renal stiffness is determined by multiple contributions from fibrosis, arterial blood flow and perfusion pressure. We hypothesize that in FN, the normal filtering capacity leads to elevated hydrostatic pressure which results in higher stiffness values. On the other hand, stiffness of Dys might be influenced by two competing factors, advanced fibrosis and diminished perfusion/blood flow, a larger influence of the hydrostatic pressure would result in a decreased renal stiffness. Our observations agree to reports based of an animal study (4) and 11 patients (5). Moderate positive correlation between MRE and GFR suggests that diminished renal functionality is accompanied by a reduction in renal stiffness, very likely due to the decrease in perfusion pressure. Additionally, a negative correlation between |G^*| and RI indicates that increased vascular compliance which associates with reduced renal blood flow leads to reduced renal stiffness.MMRE has good diagnostic accuracy in detecting renal allograft dysfunction. Renal stiffness is significantly lower in patients with dysfunctional transplant kidney and moderately correlates with glomerular filtration rate and resistive index. MMRE may serve as a non-invasive imaging maker to detect renal allograft dysfunction in an early stage and to monitor renal allograft function longitudinally.