Role of diffusion-weighted MRI in diagnosis of acute renal allograft dysfunction: a prospective preliminary study

Objective

The aim was to evaluate the effects of diagnostic performance of diffusion-weighted (DW) MRI in the assessment of acute impairment of transplanted kidneys.

Methods

From January 2009 to January 2010, 49 patients with stable renal allograft function (Group 1) and 21 patients with acute graft impairment (Group 2) were included in the study. All patients were evaluated with coronal T₂ weighted (T₂W) and DW MRI of the kidney. Patients in Group 2 underwent graft biopsy to determine the underlying histopathological aetiology. Apparent diffusion coefficient (ADC) was calculated and the kidneys were studied for any areas of diffusion restriction. Two radiologists, who were blinded to the results of histopathology, independently interpreted the T₂W and DW images.

Results

The histopathological diagnosis ofGroup 2 (21 patients) was acute cellular rejection (ACR) in 10, acute tubular necrosis (ATN) in 7 and immunosuppressive toxicity in 4 patients. ADC values in Group 1 were significantly higher compared with Group 2 (p<0.001), patients with ACR (p<0.001), patients with ATN (p<0.001) and patients with drug toxicity (p<0.001). Using 2×10⁻³ mm² s⁻¹ as a cut-off, there was no overlap between the ADC values of patients with normal graft function and those with ATN. Both ACR and ATN had a low ADC value, but on the ADC map the kidney in cases of ATN appears heterogeneous with a characteristic mosaic pattern resembling the Tiger skin. There was no significant T₂W morphological difference between the two groups.

Conclusion

These results show how DW MRI is a promising new technique for the diagnosis of acute renal transplant dysfunction.Renal transplantation is the preferred method of renal replacement therapy in end-stage renal disease [1]. Acute deterioration in function, in a transplanted kidney, is a diagnostic and a therapeutic challenge. It could be due to infection, renal allograft rejection, urinary or vascular obstruction, ciclosporin or tacrolimus nephrotoxicity, dehydration or acute tubular necrosis, and each requires distinctly different management [2]. Accurate differentiation between previous causes relies on combination of clinical findings and histopathological examination of a biopsy from the transplanted kidney. However, a needle biopsy from a transplanted kidney may be associated with serious morbidity, such as haematuria requiring transfusion, obstruction of the graft by clots, hypovolaemic shock and intraperitoneal haemorrhage that may lead to graft nephrectomy [3]. Unfortunately, there is no non-invasive tool that can diagnose the aetiology of acute graft dysfunction. Ultrasonography, including colour Doppler imaging, is a non-invasive diagnostic method that provides flowmetric quantitative parameters for the haemodynamic assessment of the renal transplant. These values present certain sensitivity but are not specific of renal graft dysfunction because there is no reliable differentiation between acute rejection and other parenchymal pathology [4].Diffusion-weighted (DW) MRI is an established method used in the diagnosis of acute stroke [5]. Diffusion-weighted imaging (DWI) provides quantification of Brownian motion of water protons by calculating the apparent diffusion coefficient (ADC), and can be used for in vivo quantification of the combined effects of capillary perfusion and diffusion [6]. Since the main kidney functions are related to transportation 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.DW MRI has been used to examine transplanted kidneys in both animal and human studies [7-9]. However, none of these studies examined the value of DW MRI in identification of the underlying aetiology of acute graft dysfunction. Therefore, the aim of our study was to assess the clinical value of DW MRI in the diagnosis of the underlying aetiology of acute renal allograft dysfunction. In the future, this will allow us to reduce the need for invasive ultrasound-guided biopsies. which have a high-risk of complication.