Despite increasing interest in functional magnetic resonance imaging techniques, compartment-specific comparisons between donor kidneys and the same kidneys shortly after implant have remained limited. Therefore, in this preliminary case series, we aimed to explore early postoperative cortex- and medulla-specific multiparametric magnetic resonance imaging patterns in living-donor kidney transplant by directly comparing donor and recipient measurements within matched grafts. We assessed multiparametric magnetic resonance imaging parameters by comparing donor and early postoperative measurements acquired between postoperative days 19 and 23 in 3 transplant recipients. We categorized percent differences in parameters as stable, mild, moderate, or marked according to their magnitude; these classifications were descriptive. In analyses, donor-to-recipient variations were more pronounced in medullary parameters than in cortical measurements across all 3 grafts, and whole kidney values generally paralleled medullary behavior. Perfusion-related metrics, particularly pseudodiffusion and perfusion fraction, demonstrated the most prominent postoperative changes. In radar plots, donor kidneys displayed compact and overlapping medulla-to-cortex profiles, whereas transplanted kidneys showed expansion or distortion of medullary-dominant parameters, particularly in perfusion-related indices. These ratio-based visualizations emphasized redistribution toward medulla-driven physiological alterations in the early postoperative phase. The reproducible compartment-specific patterns observed in this series support the feasibility of multiparametric magnetic resonance imaging as a tool for early physiological monitoring after kidney transplant. Further validation in larger, longitudinal cohorts will be required to determine the clinical utility of corticomedullary metrics as potential biomarkers of early graft status.

Key words : Graft microstructure, Perfusion dynamics, Tissue oxygenation

Introduction
Kidney transplantation requires reliable, noninvasive tools to evaluate graft physiology during the early postoperative period. Although laboratory parameters such as serum creatinine and estimated glomerular filtration rate provide global functional assessment, they do not offer compartment-specific information regarding renal microcirculation, oxygenation, or tissue integrity. Early physiological alterations, such as edema, microvascular adaptation, and changes in oxygen demand, may precede overt functional decline. Conventional imaging modalities, including ultrasonography and computed tomography, are primarily used to detect structural, vascular, or urologic complications; however, they cannot quantify graft microstructure, perfusion dynamics, or tissue oxygenation.¹ Multiparametric magnetic resonance imaging (mpMRI) offers a noninvasive approach to simultaneously characterize renal oxygenation, microvascular perfusion, and tissue microstructure within a single examination.^1,2 By integrating complementary functional techniques, mpMRI provides compartment-specific physiological information that may complement routine clinical and laboratory assessments. One key component of mpMRI is blood-oxygen-level–dependent (BOLD) imaging, which provides indirect information about renal tissue oxygen bioavailability through the paramagnetic properties of deoxyhemoglobin.³ Increased effective transverse relaxation rate (R2*) values are generally associated with reduced oxygen availability within the graft parenchyma; however, R2* cannot distinguish the underlying cause of oxygenation changes and may be influenced by perfusion, blood volume fraction, and other physiological factors.⁴ Renal oxygenation is physiologically heterogeneous: the medulla normally operates at lower oxygen tension than the cortex and is therefore more vulnerable to hypoxic stress. Accordingly, medullary R2* values are typically higher than cortical values in functioning kidneys, reflecting the corticomedullary oxygen gradient.^5,6 This physiological gradient underscores the importance of compartment-specific analysis in transplant evaluation, as medullary alterations may precede global whole-kidney changes (Figure 1a). In addition to oxygenation-sensitive imaging, diffusion-weighted MRI evaluates the movement of water molecules within renal tissue and provides the apparent diffusion coefficient (ADC) as a measure of overall water mobility. Changes in ADC may reflect alterations in tissue structure, including edema, cellular injury, or fibrosis.⁷ Intravoxel incoherent motion (IVIM) analysis further separates true molecular diffusion (D) from perfusion-related motion, generating parameters such as pseudodiffusion (D*) and perfusion fraction (pF).⁸ In kidney transplantation, diffusion-based parameters have been associated with early graft dysfunction and microstructural changes before overt clinical deterioration becomes apparent⁹ (Figure 1b). Despite increasing interest in functional MRI techniques, compartment-specific comparisons between donor kidneys and the same kidneys shortly after implantation remain limited. Therefore, in this preliminary case series, we aimed to explore early postoperative, cortex- and medulla-specific mpMRI patterns in living-donor kidney transplant by directly comparing donor and recipient measurements within matched grafts. All protocols involving human subjects were approved by the Başkent University ethics committee before the study began, and the protocols conformed to the ethical guidelines of the 1975 Helsinki Declaration. Written informed consent was obtained from the patients or their guardians.

Case Report

Methods
All MRI examinations were performed on a 1.5T MR scanner (Magnetom Avanto, Siemens Healthcare) at Başkent University using a standardized renal multiparametric imaging protocol.^10,11 To minimize physiological variability, participants underwent imaging after a minimum 4-hour fasting period with controlled oral hydration. The complete examination required approximately 25 minutes. Imaging was acquired in a coronal-oblique plane aligned with the long axis of the kidney. Three to five slices centered at the renal hilum were obtained to ensure consistent corticomedullary coverage in both donors and recipients, in accordance with established technical recommendations. Recipient imaging was performed during the early postoperative period (postoperative days 19-23) in hemodynamically stable patients without early surgical complications. The mpMRI protocol included BOLD imaging, diffusion-weighted imaging, IVIM, diffusion tensor imaging (DTI), and dynamic contrast-enhanced (DCE) sequences. Postprocessing was performed with Diffusion Calculator software(developed by the authors; https://github.com/Atakanisik/Diffusion_Calculator) for extraction of diffusion and IVIM parameters, whereas R2* maps were generated from BOLD acquisitions using dedicated pMRI software (ParametricMRI; https://www.parametricmri.com/). Although DTI and DCE sequences were acquired as part of the institutional renal imaging protocol, the present report focused on noncontrast functional parameters (BOLD, diffusion-weighted imaging, and IVIM), and DTI- and contrast-based DCE-derived metrics were not included in our present analysis. Quantitative measurements were obtained separately from the cortex, medulla, and whole kidney parenchyma. Cortical and medullary values were derived from manually placed circular regions of interest measuring approximately 0.20 to 0.25 cm² per slice. Whole kidney regions of interest were manually drawn on coronal images, excluding the renal hilum and collecting system while encompassing both cortical and medullary tissue. Medulla-to-cortex ratios were calculated to facilitate compartment-level comparisons between donor kidneys and the same grafts after transplant. Region of interest placement followed a predefined and consistent approach across all examinations.

Case presentations
We assessed multiparametric MRI findings by comparing donor and early postoperative measurements acquired between postoperative days 19 and 23 in 3 transplant recipients. Quantitative BOLD (R2*) and IVIM-derived parameters (D, D*, and pF) are summarized in Table 1, whereas representative R2* maps and medulla-to-cortex (M/C) radar plots are shown in Figure 2 and Figure 3, respectively. We categorized percentage differences as stable, mild, moderate, or marked according to their magnitude; these classifications are descriptive and do not indicate statistical significance. Kidney volume increased postoperatively in all cases, with mild enlargement in case 1 and case 2 and a more pronounced increase in case 3 (Table 1). Color-coded R2* maps (Figure 2) demonstrated preservation of the normal corticomedullary gradient in donor kidneys, characterized by relatively higher medullary R2* values compared with the cortex. In transplanted kidneys, compartment-specific alterations were observed. In case 1, attenuation of the corticomedullary gradient was evident, with relatively increased R2* values particularly in the upper and lower poles. In case 2, the transplanted kidney located in the left iliac fossa was evaluated in the presence of a chronically rejected graft in the contralateral iliac region. In case 3, increased medullary and whole-kidney R2* values were observed compared with the corresponding donor kidney. These visual findings corresponded to the quantitative R2* differences summarized in Table 1 and reflect early postoperative variability in corticomedullary oxygenation patterns. In diffusion and IVIM analyses, donor-to-recipient variations were more pronounced in medullary parameters than in cortical measurements across all 3 grafts, and whole-kidney values generally paralleled medullary behavior. Perfusion-related metrics, particularly D* and pF, demonstrated the most prominent postoperative changes. In cases 1 and 2, marked reductions in D* were observed, whereas case 3 showed substantial increases in whole-kidney D*. In contrast, true diffusion (D) and ADC showed more variable and less consistently compartment-specific shifts (Table 1). Radar plots based on M/C ratios (Figure 3) further illustrated these compartment-specific differences. Donor kidneys displayed compact and overlapping M/C profiles, whereas transplanted kidneys showed expansion or distortion of medullary-dominant parameters, particularly in perfusion-related indices. These ratio-based visualizations emphasized redistribution toward medulla-driven physiological alterations in the early postoperative phase. During follow-up, case 1 demonstrated biopsy-confirmed graft injury at 3 months attributed to polyomavirus nephropathy. Case 2 maintained clinically stable graft function without biopsy-proven rejection. Case 3 had no biopsy for suspected rejection but experienced infectious complications involving the urinary tract. At 3 months posttransplant, estimated glomerular filtration rate values improved to 48 mL/min (case 1), 57 mL/min (case 2), and 86 mL/min (case 3).

Discussion
Across all 3 cases, mpMRI findings showed sensitivity to early postoperative physiological alterations in transplanted kidneys. Assessment of BOLD-derived R2* values showed compartment-specific variations, underscoring early shifts in renal oxygenation. Renal oxygenation is physiologically heterogeneous, with the medulla operating at lower baseline oxygen tension than the cortex and therefore exhibiting greater susceptibility to hypoxic stress.^3,5,6 This corticomedullary oxygen gradient has also been demonstrated in transplanted kidneys using BOLD MRI.^12-14 The medulla-dominant pattern shown in our series was consistent with prior literature highlighting the intrinsic physiological vulnerability of the renal medulla. Importantly, consensus recommendations have emphasized that R2* reflects not only oxygen bioavailability but also vascular and hemodynamic influences, necessitating cautious interpretation in the transplant setting.¹⁰ Beyond oxygenation-sensitive imaging, diffusion- and perfusion-related diffusion parameters demonstrated early postoperative alterations. The most prominent changes were observed in perfusion-related metrics, particularly D* and pF, which reflect microvascular flow components within renal tissue.^8,9 In kidney transplantation, diffusion-based parameters have been associated with early graft dysfunction and histologic alterations, supporting the interpretation that early shifts in D* and pF may represent vascular adaptation or transient perfusion imbalance after implantation.^9,13,14 True diffusion (D) and ADC exhibited comparatively smaller or more variable changes. Prior studies have shown that diffusion metrics primarily reflect tissue microstructure, including interstitial edema or fibrosis, rather than isolated perfusion changes.^7,9 The relative stability of cortical diffusion parameters in our series may therefore suggest preserved cortical microstructure during the early postoperative phase. The use of medulla-to-cortex ratios provided an interpretable framework for tracking redistribution of functional load between compartments. Ratio-based analysis may reduce interindividual variability and enhance sensitivity to regional physiological shifts, particularly in the context of the inherent corticomedullary gradient.^3,5 In our cases, whole kidney measurements largely mirrored medullary behavior, suggesting that early graft physiology may be predominantly driven by medullary adaptation. Overall, these findings are consistent with prior multiparametric transplant studies demonstrating that diffusion- and oxygenation-sensitive MRI parameters can detect early graft changes before overt clinical deterioration.^2,9,11,15 By directly comparing donor and early recipient measurements within the same grafts, this case series highlighted a reproducible medulla-dominant postoperative physiological pattern.

Conclusions
Multiparametric MRI provides a noninvasive approach to assessment of graft physiology at the corticomedullary level. In this 3-case series, early postoperative changes were primarily medullary and most evident in perfusion-related diffusion parameters (D and pF), reflecting the susceptibility of the renal medulla to hemodynamic and oxygenation fluctuations after transplantation. Whole kidney measurements paralleled medullary trends, suggesting that early graft adaptation is largely medulla-driven. Although limited by small sample size and incomplete histologic correlation, the reproducible compartment-specific patterns observed in this series support the feasibility of mpMRI as a tool for early physiological monitoring following kidney transplantation. Further validation in larger, longitudinal cohorts will be required to determine the clinical utility of corticomedullary metrics as potential biomarkers of early graft status.