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ARTICLE
Evaluation of Transplanted Kidneys and Comparison with Healthy Volunteers and Kidney Donors with Diffusion-Weighted Magnetic Resonance Imaging: Initial Experience

Objectives: The aim of this study was to evaluate the feasibility of diffusion-weighted magnetic resonance, by comparing imaging in renal allograft recipients for functional assessment of kidney transplants versus imaging of these features in healthy volunteers and kidney donors with native kidneys.

Materials and Methods: Seventy renal transplant recipients (group A) with stable graft function at postoperative month 1, 40 healthy volunteers (group B), and 40 kidney donors (group C) underwent diffusion-weighted magnetic resonance imaging. An echo-planar diffusion-weighted imaging sequence was performed in coronal orientation by using 6 b values (0, 200, 400, 600, 800, 1000 s/mm2). The apparent diffusion coefficients were determined for the upper and lower poles of the kidney cortex and medulla. Relations between apparent diffusion coefficients and allograft function, determined by the estimated glomerular filtration rate (comparing rates > 60 mL/min/1.73 m2 [group A1] versus < 60 mL/min/1.73 m2 [group A2]), were investigated in renal transplant recipients, and apparent diffusion coefficients in groups A, B, and C were compared.

Results: Apparent diffusion coefficients were sta­tistically higher in group A1 than in group A2 (P < .05) and statistically higher in group A than in groups B and C (P < .001). There were no significant differences between groups B and C (P > .05).

Conclusions: We observed that apparent diffusion coefficients of transplanted kidneys at postoperative month 1 were higher than values in native kidneys of healthy volunteers and kidney donors. In addition, apparent diffusion coefficients of transplanted kidneys with estimated glomerular filtration rates > 60 mL/min/1.73 m2 were higher than transplanted kidneys with rates < 60 mL/min/1.73 m2.


Key words : Apparent diffusion coefficient, End-stage renal disease, Estimated glomerular filtration rate, Kidney transplantation

Introduction

Kidney transplant is the best treatment modality for patients with end-stage renal disease.1 Acute allograft dysfunction and early, accurate, and safe detection of this situation remains a major diagnostic and therapeutic challenge.2 Dysfunction could be due to allograft rejection, infection, urinary or vascular obstruction, drug nephrotoxicity, delayed graft function, acute tubular necrosis, or dehydration, with each requiring different treatment modalities.1,3 Accurate differentiation between causes of allograft dysfunction is based on the combination of clinical findings and histopathologic examination of an allograft biopsy.1 In practice, biopsy is the only method that allows identification of parenchymal abnormalities of the allograft.3 However, a needle biopsy from an allograft may be associated with serious morbidity, such as hematuria requiring transfusion, obstruction of the allograft by clots, hypovolemic shock, and massive bleeding that may lead to graft nephrectomy.1 An additional important problem associated with allograft biopsy is the substantial number of nondiagnostic and false results.3 Long-term monitoring using biopsy is controversial.3 Unfortunately, there is no efficient and noninvasive method to assess graft function and to diagnose the cause of allograft dysfunction.1,3 Dop­pler ultrasonography is a noninvasive diagnostic method for hemodynamic assessment of renal allograft, but it is not specific for renal graft dysfunction because there is no reliable difference between acute rejection and other parenchymal pathologies.1

Diffusion-weighted magnetic resonance imaging (DW-MRI) is an established technique for the diag­nosis of acute stroke and has lately gained increased importance in imaging outside the brain for functional imaging.4 Diffusion-weighted MRI allows us to obtain a functional parenchymal assessment, based on its property, to provide information on the Brownian motion of water molecules in tissues.5 The mean apparent diffusion coefficient (ADC) is calculated as a quantitative parameter from DW-MRIs and can provide information on diffusion and perfusion.5,6 Because the main kidney functions are related to transport of water (glomerular filtration, active and passive tubular reabsorption, and secretion), diffusion characteristics may provide a useful insight into the functional consequences of different renal diseases.2

Diffusion-weighted MRI has been used to examine transplanted kidneys in both animal and human studies.1-7 Renal DW-MRI may be used to compare healthy versus pathologic kidney function with good levels of feasibility and reproducibility.5 The diffusion characteristics of the kidney may provide information on renal function and may help in the characterization of various renal abnormalities.8

In this study, the aim was to evaluate the feasibility of DW-MRI for functional assessment of transplanted kidneys in renal allograft recipients, by comparing these assessments with those shown in native kidneys of healthy volunteers and kidney donors.

Materials and Methods

This study was approved by the local ethics committee, and patients provided written informed consent. Group A comprised 70 renal transplant recipients, with mean age of 42 ± 12 years (range,18-67 y), composed of 39 men (mean age, 41 ± 12 y; range, 18-66 y) and 31 women (mean age, 43 ± 11 y; range, 23-67 y) who had undergone kidney trans­plant between September 2013 and July 2015 in our hospital and who had stable graft function. Group B comprised 40 healthy volunteers, with mean age of 43 ± 10 years (range, 27-68 y), composed of 20 men (mean age, 42 ± 11 y; range, 27-68 y) and 20 women (mean age, 44 ± 10 y; range, 30-68 y). Group C comprised 40 kidney donors, with mean age of 44 ± 10 years (range, 32-69 y), composed of 20 men (mean age, 43 ± 11 y; range, 33-68 y) and 20 women (mean age, 45 ± 10 y; range, 32-69 y). All of the renal recipients had undergone DW-MRI at postoperative month 1. Healthy volunteers and kidney donors had undergone DW-MRI for both right and left kidneys. Kidney donors, a physician-selected population, are required to pass strict tests; therefore, we agreed that their results may not resemble a healthy volunteer group at the same age group in the population. Because of this, we decided to include a healthy volunteer and a kidney donor group to compare these groups for DW-MRI parameters.

Antithymocyte globulin induction therapy was administered at the time of transplant and at 3 days thereafter; tacrolimus (target level of 8-10 ng/mL), mycophenolate mofetil (1 g twice per day), and prednisolone were started as immunosuppressive therapy. Tacrolimus was maintained with a target level of 6 to 8 ng/mL at 3 months after transplant. None of the patients experienced acute rejection episodes during DW-MRI. Kidney transplant procedures were performed by the same surgical team, which was also responsible for the follow-up care. Renal transplant recipients (group A) were divided into 2 groups, on the basis of estimated glomerular filtration rates (eGFR): group A1 had eGFR > 60 mL/min/1.73 m2 and group A2 had eGFR < 60 mL/min/1.73 m2.

All of the DW-MRIs were performed using a 1.5-T magnetic resonance apparatus (Siemens Avonto 1.5 T, Munich, Germany), with an 8-channel phased-array coil. Diffusion-weighted MRI was acquired using a free-breathing, single-shot echo-planar sequence and was performed in coronal orientation by using six b values (0, 200, 400, 600, 800, 1000 s/mm2). The apparent diffusion coefficients (ADC) were determined for the upper and lower poles of kidney cortex and medulla with placement of ellipsoid regions of interest (1 cm in diameter). No special patient preparation was needed, such as fasting or drinking, before the examination. Two radiologists who were blinded to the results analyzed all images and measured the ADC values. The mean of both readings was used for the final statistical analyses.

In all participants, blood samples were obtained on the day of MRI examination, and eGFR was calculated by using the Modification of Diet in Renal Disease equation.9 Relations between ADC values and allograft function, which was determined by eGFR, were investigated in renal transplant recipients. Apparent diffusion coefficients of renal transplant recipients (group A), healthy volunteers (group B), and kidney donors (group C) were compared. Apparent diffusion coefficients of right and left kidneys of healthy volunteers and kidney donors were also compared.

Sample sizes for all of the groups were estimated with a power analysis. Statistical analyses were performed with SPSS software (SPSS: An IBM Company, version 15.0, IBM Corporation, Armonk, NY, USA). Comparisons of 2 independent groups were done with t test if the variables provided condition of normal distribution and Mann-Whitney U test if not. Dependent group analysis was done with paired sample t test. P < .05 indicated a statistically significant difference.

Results

In group A, mean eGFR was 74.2 ± 23.6 mL/min/1.73 m2 (range, 39-134 mL/min/1.73 m2) and mean creatinine level was 1.14 ± 0.34 mg/dL. There were no statistically significant differences between men and women regarding age and eGFR (mean eGFR of 73.7 vs 73 mL/min/1.73 m2) (P > .05). Mean ADC values for upper pole cortex (UPC), upper pole medulla (UPM), lower pole cortex (LPC), lower pole medulla (LPM), and total ADC (ADCtot) were 1658 ± 169 × 10-6 s/mm2 (range, 1266-2045 × 10-6 s/mm2), 1606 ± 148 × 10-6 s/mm2 (range, 1150-2083 × 10-6 s/mm2), 1650 ± 127 × 10-6 s/mm2 (range, 1353-1989 × 10-6 s/mm2), 1586 ± 118 × 10-6 s/mm2 (range, 1112-1795 × 10-6 s/mm2), and 1585 ± 110 × 10-6 s/mm2 (range, 1389-1879 × 10-6s/mm2). Apparent diffusion coefficients of the cortex were statistically higher than that of the medulla (P < .001), as shown in Table 1. There were no statistically significant differences between men and women for UPC, UPM, LPC, LPM, and ADCtot (P > .05) as shown in Table 2. In addition, no significant differences were detected between men and women for UPC-UPM and LPC-LPM differences (P > .05).

Group A1 included 47 recipients (25 men, 22 women, mean age of 41 ± 12 y; range, 18-67 y) whose mean eGFR was 86.6 ± 18.4 mL/min/1.73 m2 (range, 60-134 mL/min/1.73 m2). Mean ADC values for UPC, UPM, LPC, LPM, and ADCtot were 1675 ± 184 × 10-6 s/mm2 (range, 1266-2045× 10-6 s/mm2), 1629 ± 157 × 10-6 s/mm2 (range, 1150-2083× 10-6 s/mm2), 1664 ± 126 × 10-6 s/mm2 (range, 1353-1989 × 10-6 s/mm2), 1602 ± 112 × 10-6 s/mm2 (range, 1112-1795 × 10-6 s/mm2), and 1642 ± 112 × 10-6 s/mm2 (range, 1389-1879 × 10-6 s/mm2).

Group A2 included 23 patients (14 men, 9 women, mean age of 45 ± 11 y; range, 25-63 y) whose mean eGFR was 48.6 ± 5.5 mL/min/1.73 m2 (range, 37-59 mL/min/1.73 m2). Mean ADC values for UPC, UPM, LPC, LPM, and ADCtot were 1616 ± 128 × 10-6 s/mm2 (range, 1299-1868 × 10-6 s/mm2), 1559 ± 114 × 10-6 s/mm2 (range, 1385-1882 × 10-6 s/mm2), 1616 ± 123 × 10-6 s/mm2 (range, 1364-1875 × 10-6 s/mm2), 1546 ± 122 × 10-6 s/mm2 (range, 1233-1788× 10-6 s/mm2), and 1584 ± 98 × 10-6 s/mm2 (range, 1396-1841× 10-6 s/mm2).

There were no statistically significant difference between group A1 and group A2 for age and sex (P > .05) (Table 3). Mean creatinine level was statistically higher in group A2 than in group A1 (1.49 ± 0.28 vs 0.95 ± 0.18 mg/dL; P < .001), and eGFR was statistically lower in group A2 than in group A1 (48.6 ± 5.5 vs 86.6 ± 18.4 mL/min/1.73 m2; P < .001) as shown in Table 3. Mean ADC values for UPC, UPM, LPC, LPM, and ADCtot were statistically higher in group A1 than in group A2 (P < .05) (Table 3). Apparent diffusion coefficients for the cortex (UPC, LPC) were statistically higher than in the medulla (UPM, LPM) for both group A1 and group A2 (P < .001). Mean UPC-UPM difference of ADC values was 46 ± 116 for group A1, and mean UPC-UPM difference of ADC values was 65.3 ± 77.4 for group A2, with no significant differences between groups A1 and A2 (P = .473). Mean LPC-LPM difference of ADC values was 61.4 ± 92.9 for group A1, and mean LPC-LPM difference of ADC values was 70.2 ± 73.6 for group A2, with no significant differences between groups A1 and A2 (P = .693).

For the 40 healthy volunteers in group B, mean creatinine level was 0.74 ± 0.12 mg/dL and mean eGFR was 100.7 ± 10.1 mL/min/1.73 m2. In group B, 80 kidneys were evaluated and their mean ADC values for UPC, UPM, LPC, LPM, and ADCtot were 1465 ± 124 × 10-6 s/mm2 (range, 1229-1723 × 10-6s/mm2), 1401 ± 120 × 10-6 s/mm2 (range, 1128-1731 × 10-6 s/mm2), 1485 ± 156 × 10-6 s/mm2 (range, 1102-1781 × 10-6 s/mm2), 1438 ± 131 × 10-6 s/mm2 (range, 1169-1709 × 10-6 s/mm2), and 1448 ± 120 × 10-6 s/mm2 (range, 1203-1684 × 10-6 s/mm2). Apparent diffusion coefficients of the cortex (UPC, LPC) were statistically higher than those of the medulla (UPM, LPM) (P < .001). There were no statistically significant difference between men and women (P > .05) (Table 4) and between right and left kidneys (P > .05) (Table 5). Also no significant dif­ference was detected between right and left kidneys for UPC-UPM and LPC-LPM differences (P > .05).

For the 40 kidney donors in group C, mean creatinine level was 0.73 ± 0.11 mg/dL and mean eGFR was 104.7 ± 13.5 mL/min/1.73 m2. In total, 80 kidneys were evaluated and their mean ADC values for UPC, UPM, LPC, LPM, and ADCtot were 1481 ± 112 × 10-6 s/mm2 (range, 1260-1719 × 10-6 s/mm2), 1396 ± 108 × 10-6 s/mm2 (range, 1176-1701 × 10-6 s/mm2), 1471 ± 114 × 10-6 s/mm2 (range, 1205-1692 × 10-6 s/mm2), 1408 ± 111 × 10-6 s/mm2 (range, 1150-1628 × 10-6 s/mm2), and 1438 ± 102 × 10-6 s/mm2 (range, 1221-1673 × 10-6 s/mm2). Apparent diffusion coefficients of cortex (UPC, LPC) were statistically higher than those shown in the medulla (UPM, LPM) (P < .001). There were no statistically significant differences between men and women (P > .05) (Table 6) and between right and left kidneys (P > .05) (Table 7). Also, no significant difference was detected between right and left kidneys for UPC-UPM and LPC-LPM differences (P > .05).

There were no statistically significant differences among groups A, B, and C for age and sex (P > .05) (Tables 8 and 9). Mean creatinine level was statistically higher in group A than in groups B and C (1.14 ± 0.34 vs 0.74 ± 0.12 mg/dL and 0.73 ± 0.11 mg/dL; P < .001), and eGFR was statistically lower in group A than in groups B and C (73.4 ± 23.7 vs 100.7 ± 10.1 mL/min/1.73 m2 and 104.7 ± 13.5 mL/min/1.73 m2; P < .001), as shown in Table 8. Apparent diffusion coefficients were statistically higher in group A than in groups B and C for UPC, UPM, LPC, LPM, and ADCtot (P < .001) (Table 8). However, there were no significant differences detected among groups A, B, and C for UPC-UPM and LPC-LPM differences (P > .05).

There were no statistically significant differences between groups B and C for age, sex, creatinine level, eGFR, and ADC values (P > .05) (Tables 8 and 9).

Discussion

Kidney transplant is the best treatment option, as it results in improved quality of life and better prognosis in patients with end-stage renal disease.4 Long-term graft survival has improved and is reported to be about 90% during the 1st year after transplant.4 However, early graft dysfunction is encountered in up to 30% of renal allografts.4 Medical complications, including acute rejection, acute tubular necrosis, drug-related toxicity, and delayed graft function, are difficult to diagnose.4 Biopsy is still the only reliable technique to differentiate these entities.4 However, biopsy has its limitations due to sampling errors or concomitant presence of acute tubular necrosis and acute rejection.4 In addition, biopsy is invasive and carries risks of complications, such as bleeding, infection, and graft loss.4 Therefore, DW-MRI can be a promising nonin­vasive technique to provide structural and functional information on diffusion.4

Diffusion-weighted MRI is a technique used to show molecular diffusion, which is the Brownian motion of the spin in biologic tissues.1 Diffusion-weighted MRI is a standard method of central nervous system imaging.3 It is already an established method used routinely in the diagnosis of acute stroke.1 The kidney is well suited for diffusion studies because of its high blood flow and its fluid transport function.1 Diffusion-weighted MRI has emerged as a promising noninvasive functional technique in renal imaging.6 As a quantitative parameter calculated from DW-MRI, the ADC combines the effects of capillary perfusion and water diffusion in the extracellular-extravascular space.1

There are limited data on the use of DW-MRI in the evaluation of renal allografts. Yang and associates reported that ADC values in the cortex and medulla decreased significantly in an acute rejection model of transplanted kidneys in rats.7 This significant dif­ference perhaps was explained by the relatively low b values applied in this study. When comparing renal allografts and isografts early after transplant to normal native kidneys, ADCs in allografts were decreased relative to those in native kidneys, whereas ADCs in isografts were similar to those in native kidney.7

In a study of renal recipients reported by Thoeny and associates in 2006, 15 patients with renal allo­grafts and stable kidney function and 15 age- and sex-matched healthy volunteers underwent DW-MRI.2 They reported that ADCs were significantly higher in the cortex than in the medulla of native kidneys; however, in transplanted kidneys, ADCs were almost identical in the medulla and the cortex.2 Furthermore, medullary diffusion parameters were almost identical in transplanted and native kidneys, but cortical ADCs were substantially higher in native kidneys than in transplanted kidneys.2 Their findings were corroborated with previous studies for native kidneys.7,10,11 In contrast, some investigators pre­viously reported higher ADC values in the medulla than in the cortex but they used only low b values.12,13 Thoeny and associates explained the lack of corticomedullary differentiation in allografts as a result of renal denervation or activity of immuno­suppressive agents in their study.2 Yang and associates reported previously that ADC values in rats with renal allografts were lower than the ADC values in rats with native kidneys.2,7 Renal allografts in rats were investigated during the first 4 days after transplant, whereas Thoeny and associates reported their measurements obtained more than 100 days after transplant.2,7

Xu and associates reported that the ADCs were significantly lower in impaired kidneys than in normal kidneys, and there was a positive correlation between the ADCs and glomerular filtration rates.8 In addition, in earlier studies, it was shown that hydration increases ADC and hydronephrosis could influence ADC values.14,15 Moreover, patients with acute and chronic renal failure showed decreased ADC values compared with healthy volunteers.10,12 In patients with high serum creatinine values, Fukuda and associates observed a decrease in ADC compared with patients with normal values.16

Blondin and associates assessed the clinical value of DW-MRI in the functional evaluation of trans­planted kidneys.17 Their study included 32 patients who were divided into 4 groups: (1) patients with stable renal allograft function of at least 6 months, (2) patients with acute deterioration of allograft function and patients who recently underwent transplant (< 14 days) with good (3) and decreased (4) renal function. They reported that the differences in ADC between groups 1 and 2 and between group 3 and 4 were statistically significant.

In another recent study, researchers found that ADC values in patients with stable kidney function were significantly higher than in patients with altered kidney function.1 In that study, all patients were evaluated on day 14 after transplant. However, regions of interest were placed at the middle of the kidney, including the entire renal parenchyma; therefore, ADCs for cortex and medulla were not evaluated separately.

Palmucci and associates performed assessments of DW-MRI in 35 transplant patients for the functional evaluation of transplanted kidneys.5 They divided patients into 3 groups according to their creatinine clearance level, with group A having clearance > 60 mL/min, group B having clearance > 30 and ≤ 60 mL/min, and group C having clearance ≤ 30 mL/min. When they compared mean values of ADC between groups A and C, the investigators observed a significant difference, with higher ADC values among patients with normal creatinine clearance (> 60 mL/min). A comparison of group B versus C did not show a significant difference, nor did group A and group B reveal a significant difference. Measurement of ADC was achieved by placing the region of interest in the cortex of the transplanted kidney; for each patient, 3 regions of interest were calculated (upper, middle, and lower zones) to obtain a mean score. The patients were analyzed at a mean posttransplant time of 12.3 months (range, 1-42 months).

Wypych-Klunder and associates evaluated 15 patients (11 men and 4 women) with a mean age of 52 years (range, 24-71 y).3 Mean time from transplant was 15 days. Measurements were performed in the axial plane using b values of 600 and 1000 s/mm2 using 4 random regions of interest within the cortex and in 3 random regions of interest within the medulla of transplanted kidneys. Apparent diffusion coefficients measured at b value 1000 s/mm2 in the cortex were higher in patients with eGFR ≤ 30 mL/min/1.73 m2 than in patients with eGFR > 30 mL/min/1.73 m2. They suggested that the best quality parameters offer ADC measurements in the renal cortex using b value 1000 s/mm2, and ADC values in the renal cortex measured at b value 1000 s/mm2 showed a relation with eGFR.

Lanzman and associates prospectively evaluated 40 renal transplant recipients (25 men and 15 women) with a mean age of 49.6 ± 14.9 years.2 The time interval between renal transplant and the MRI examination ranged from 3 days to 11 years, with a median of 16 days. An echo-planar diffusion-tensor MRI sequence was performed in coronal orientation by using 5 b values (0, 200, 400, 600, 800 s/mm2) and 20 diffusion directions. Three ellipsoid regions of interest of approximately 10 to 15 pixels were placed in the medulla, and a region of interest of 60 to 100 pixels was drawn to cover the renal cortex on 0 b-value images. Apparent diffusion coefficients were compared between patients with good or moderate allograft function (eGFR > 30 mL/min/1.73 m2) and patients with impaired function (eGFR ≤ 30 mL/min/1.73 m2). They reported that mean ADCs of renal cortex and medulla were significantly higher in those with good or moderate function than in patients with impaired function.

Eisenberger and associates prospectively evaluated 13 healthy kidney donors (9 women and 4 men; mean age of 55 ± 12 y; range, 38-72 y) and their cor­responding recipients (4 women and 9 men; mean age of 50 ± 10 y; range, 33-68 y) in donors before donation and in donors and recipients at a mean time of day 8 and months 3 and 12 after donation.18 Diffusion-weighted MRI examinations were per­formed in donors within 3 weeks (mean of 8 ± 7 d) before explantation and in donors and recipients at approximately 8 days (donors at mean of 8 ± 3 d and recipients at mean of 9 ± 3 d), 3 months (donors at mean of 103 ± 24 d; recipients at mean of 99 ± 12 d), and 12 months (donors at mean of 358 ± 18 d; recipients at mean of 356 ± 14 d) after living-donor transplant. They used 10 b values (10, 20, 40, 70, 100, 150, 250, 400, 550, and 700 s/mm2) and up to 3 ellipsoid regions of interest, with one in the upper, one in the middle, and one in the lower pole positioned in both the cortex and medulla on a maximum of 5 sections covering large parts of the kidneys. They reported that ADCtot values in the nontransplanted kidneys of donors increased in the medulla and cortex 1 week after donation. Medullary but not cortical ADCtot values stayed increased for up to 1 year. Total ADC values in allografts of recipients were stable. Compared with values obtained before transplant in donors, the corticomedullary difference was reduced in allografts. Cortical ADCtot values correlated with eGFR rates in recipients but not in donors. Cortical ADCtot values in the same kidney before transplant in donors correlated with those in recipients on day 8 after transplant.
Kaul and associates compared DW-MRI of renal allografts on postoperative day 7 and corresponding kidney biopsy results.19 The ADC values were found to be slightly lower in the medulla than in the cortex in transplanted kidneys with normal function. They also reported a significant reduction of ADC values in both the cortex and medulla in allografts displaying dysfunction. They suggested that such a reduction was correlated with the degree of rejection on the biopsies. Furthermore, the increase in ADC values was observed during the recovery from rejection, suggesting the usefulness of DW-MRI for therapy monitoring after rejection episodes.

In our study, we revealed that ADC values of transplanted kidneys at postoperative month 1 were statistically higher than shown in healthy volunteers and kidney donors. It may be due to compensatory hypertrophy mechanism. Eisenberger and associates showed a rapid increase of ADCs in the remaining nontransplanted kidney in living donors due to glomerular and also tubular enlargement.18 Hyper­trophy is also a well-known adaptation process in transplanted kidneys and in the remaining non­transplanted kidneys.18,20,21 In addition, we did not find a lack in corticomedullary difference in trans­planted kidneys using DW-MRI. There was no statistical difference between UPC-UPM and LPC-LPM differences in all groups. To our knowledge, our study has the highest study population number of transplant patients in all studies that used DW-MRI (70 renal transplant recipients and 80 healthy counterparts). Also, our prospective study perfor­med measurements of transplanted kidneys at postoperative month 1. In the other recent studies, the interval between DW-MRI examination and transplant time varied widely. In the first report by Thoeny and associates in 2006, it was suggested that cortical ADC values in healthy volunteers were higher than in transplant patients, but measurements were performed at a mean time of 8.8 months after transplant.2 We think that the high cortical ADC values in healthy volunteers compared with trans­plant recipients could be due to the regression of the compensatory adaptation mechanisms, develo­pment of interstitial fibrosis, and glomerular sclerosis in transplant recipients. Eisenberger and associates also suggested in their study that medullary, but not cortical, ADCtot values stayed increased for up to 1 year in donors but were stable in allografts of recipients.18 Moreover, they suggested that cortical ADCtot values in the same kidney before transplant in donors correlated with those in recipients on day 8 after transplant.18 According to these results, it is conflicting that ADC values of healthy volunteers were higher than values shown in transplant recipients because at least there will be no statistically significant difference if the ADC values were stable after trans­plant compared with the ADC values before donation.

There are conflicting published data as to whether ADCs are higher in the renal cortex or in the medulla. Although several earlier studies reported higher ADCs in the medulla than in the cortex, more recent studies have revealed higher ADCs in the cortex than in the medulla.4 The difference seems to be due to the use of different b values in different studies. In our study, we found statistically higher ADCs in the cortex in all groups.

Correlation of eGFR and ADC values in trans­planted kidneys were reported in many recent studies.1,3,5,6,8,18 It was shown that ADC values are significantly decreased in recipients with eGFR ≤ 30 mL/min/1.73 m2 compared with patients having eGFR > 60 mL/min/1.73 m2.1,3,5,8 As a limitation of our study, we did not have a patient group with eGFR ≤ 30 mL/min/1.73 m2. In our study ADC values were statistically higher in those with eGFR > 60 mL/min/1.73 m2 than in those with eGFR < 60 mL/min/1.73 m2, similar to that shown recently.5

Magnetic resonance imaging has limitations in clinical practice. Unlike ultrasonography, it is less frequently available because the equipment is not portable, the cost is relatively high, and it requires specialized personnel who may not be available 24 hours/day.22 Also, there is not a consensus in the literature about MRI technique, b values, and the indications and time of the imaging.

Conclusions

Apparent diffusion coefficients of kidneys with DW-MRI are strongly correlated with creatinine and eGFR levels in transplant patients. Renal allografts with lower eGFR have lower ADC than normal grafts. Therefore, a significant decrease in ADC can be a sign of acute renal allograft dysfunction. Cortical ADC values are higher than medulla in all groups, similar to that shown in previous reports. Apparent diffusion coefficients are statistically higher in renal transplant recipients than in healthy volunteers and kidney donors at postoperative month 1 due to compensatory hypertrophy mechanisms.

Diffusion-weighted MRI is a promising nonin­vasive technique in diagnosis of acute renal allograft impairment. Also, it may be helpful for renal recipients to have DW-MRI at regular intervals for early detection of microstructural changes and to indicate the time point of performing allograft biopsy. There are limitations and conflicting data in the literature about MRI for evaluation of trans­planted kidneys and its value in clinical practice; therefore, further prospective studies are needed to investigate this issue and to generate a consensus.


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DOI : 10.6002/ect.2016.0341


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From the Departments of 1General Surgery and 2Radiology, Baskent University School of Medicine, Ankara, Turkey
Acknowledgements: The authors declare that they have no sources of funding for this study, and they have no conflicts of interest to declare.
Corresponding author: Ümit Özçelik, Baskent University School of Medicine, Department of General Surgery, Altunizade mahallesi, Oymaci sokak no: 7, Uskudar, Postal code 34662, Istanbul, Turkey
Phone: +90 216 554 1500
E-mail: dr_umit_ozcelik@yahoo.com