Intravoxel incoherent motion analysis of renal allograft diffusion with clinical and histopathological correlation in pediatric kidney transplant patients

Clare B Poynton^{1}, Marsha Lee^{2}, Yi Li^{1}, Zoltan Laszik^{3}, John D Mackenzie^{1}, Pauline W Worters^{4}, and Jesse Courtier^{1}

This study was conducted after obtaining IRB approval and written informed consent from each subject. Between 10/2014 and 9/2015, 14 pediatric renal transplant recipients (mean age 15.7 $$$\pm$$$ 2.9) were prospectively scanned on a clinical 3T magnetic resonance (MR) scanner, prior to obtaining an ultrasound-guided renal transplant biopsy. An echo-planar DTI sequence was performed in the coronal plane using four b values (200, 400, 600, 800 $$$\textrm{s/mm}^{2}$$$), 20 diffusion directions, FOV=36cm, TE=90ms, TR=2500ms, matrix size= 192 x 192, and scan-time=90s . For each b-value, an accompanying T2-weighted (b=0) image was acquired. Data from 11 subjects was analyzed following inspection of the dicom data for image quality. The magnetization, M, at each voxel was modeled according to Eq 1:

$$M = M_{0} ( f_{p}\, \textrm{exp}(-b D_{p}) + (1 - f_{p})\, \textrm{exp}(-b D_{t}))$$

where $$$M_{0}$$$ is the total magnetization, $$$f_{p}$$$ is the perfusion fraction, $$$D_{p}$$$ is the pseudo-diffusivity, and $$$D_{t}$$$ is the tissue-diffusivity [4]. The IVIM parameters, $$$\mathbf{x} =\{ f_{p}, D_{p}, D_{t}\}$$$, were estimated for each voxel and each diffusion direction independently according to Eq 2:

$$\mathbf{x}^{*} = \underset{\mathbf{x}}{\arg \min} \displaystyle\sum_{b_{j}}\Big(M - M_{0}( f_{p} \, \textrm{exp}(-b_{j} D_{p}) + (1 - f_{p})\, \textrm{exp}(-b_{j} D_{t}))\Big)^{2} \qquad \textrm{s.t.} \qquad f_{p} \in [0,1], \; D_{p} > 0, \; \textrm{and} \; D_{t} > 0, $$

where $$$b_{j} \in \{200, 400, 600, 800 \}$$$ . Eq 2 was solved in Matlab using a standard constrained nonlinear optimization technique. For each subject, final IVIM maps of each parameter estimate were obtained by computing the mean across all diffusion directions. Regions of interest (ROIs) were defined for each subject on the cortex (n=1) and medulla (n=3), using the coronal plane of a single b0 image. Medullary ROIs of uniform size (5x5 voxels) were defined in the lower-polar, inter-polar, and upper-polar regions. For each subject, mean values of $$$D_{p}$$$, $$$D_{t}$$$, and $$$f_{p}$$$ were computed from the cortical ROI and the medually ROI that corresponded to the biopsy site. Subjects were grouped according to whether the biopsy resulted in a change in clinical management (change vs. no change). Group differences in cortical and medullary IVIM estimates and intra-group cortico-medullary differences in tissue-diffusivity were assessed using two-tailed t-tests.

[1] Eisenberger U, Thoeny HC, Binser T, Gugger M, Frey FJ, Boesch C, Vermathen P. Evaluation of renal allograft function early after transplantation with diffusion-weighted MR imaging. Eur Radiol 2010;20: 1374–1383.

[2] Hueper K, Gutberlet M, Rodt T, Gwinner W, Lehner F, Wacker F, Galanski M, Hartung D. Diffusion tensor imaging and tractography for assessment of renal allograft dysfunction-initial results. Eur Radiol 2011;21:2427–2433.

[3] Lanzman RS, Ljimani A, Pentang G, Zgoura P, Zenginli H, Kropil P, Heusch P, Schek J, Miese FR, Blondin D, Antoch G, Wittsack HJ. Kidney transplant: functional assessment with diffusion-tensor MR imaging at 3T. Radiology 2013;266:218–225.

[4] Le Bihan D, Breton E, Lallemand D, Grenier P, Cabanis E, Laval-Jeantet M. MR imaging of intravoxel incoherent motions: application to diffusion and perfusion in neurologic disorders. Radiology 1986; 161:401–407.

[5] Notohamiprodjo M, Chandarana H, Mikheev A, Rusinek H, Grinstead J, Feiweier T, Raya JG, Lee VS, Sigmund EE. Combined intravoxel incoherent motion and diffusion tensor imaging of renal diffusion and flow anisotropy. Magn Reson Med. 2015 Apr;73(4):1526-32.

Fig 1. b0 images (A,C) and mean *D*_{t} estimates
(B,D) from a representative patient from the no change group (A,B) and the change group (C,D) are shown above. The yellow
boxes show enlarged views of the inter-polar (A,B) and lower-polar (C,D) areas
surrounding the medullary ROIs (red boxes).

Fig 2. Box plots
showing the min, max, first and third quartile, and median of each IVIM
parameter in medullary ROIS of both groups are shown above. The change group shows lower estimates in
all three parameters and this difference is statistically significant for *D*_{t} (p=0.017).

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)

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