Acute and Chronic Kidney disease are still important causes of morbi-mortality.
Due to important limitations of laboratorial markers like microalbuminuria and serum creatinine, other techniques have been studied over the last decades.
DWI assesses restriction to water molecules diffusion, as well as blood microcirculation and tubular flow, being a promising biomarker of the renal microstructure.
Changes on renal perfusion, glomerular and tubular flow can thus modify both renal DWI signal and Apparent Diffusion Coefficient (ADC).
APN, CKD, and obstruction typically exhibit restriction to water diffusion, with corresponding low cortical ADC values.
Hematomas, abscesses and sediment also exhibit restriction and tend to appear particularly bright at high b values (1200 s/mm^2).
In the setting of acute infection, the lack of ionizing radiation makes DWI very attractive during pregnancy and childhood.
Also, Diffusion Tensor Imaging (DTI), by measuring fractional anisotropy, provides an even better characterization of the renal microstructure, particularly in CKD, being able to depict alterations earlier than conventional blood biomarkers.
After adequate standardization of these sequences, still under investigation, the need for biopsy and contrast administration can be obviated, highlighting DWI not only as a diagnostic tool but also as a way to follow-up and guide treatment decisions.
APN DWI was shown to be more sensitive than ultrasound to detect the infected segments during APN.
Combining clinical data, DWI could have a significant impact on confirming treatment effectiveness during follow-up, especially in children and pregnant women.
DWI may even replace contrast-enhanced MRI for detection of APN, thus avoiding the potential side effects of contrast agents.
In DWI, hyperintense (on b-1200 images) areas with corresponding low ADC values correspond to the infected segment (Figure 1, 2, 3).
OBSTRUCTION AND SEDIMENT Acute unilateral renal obstruction due to kidney stones leads to a decrease in renal blood flow and GFR, ultimately resulting in interstitial fibrosis and irreversible renal damage if left untreated.
Conflicting results have been observed in several studies.
However, a prospective study by Thoeny HC et al found a significant reduction in the ADC value of obstructed vs unobstructed kidneys [1].
In transplanted kidneys, children with ureteropelvic junction obstruction and pregnant women DWI may also serve as a non-invasive diagnostic modality (Figure 2,3).
ABSCESSES Hyperintense on T2 weighted images and with a T1 hypointense periphery, they are distinguished from cysts by their restriction to water diffusion (Figure 4).
HEMATOMAS Hematomas and blood generally have a variable signal intensity on T2 weighted imaging, depending on the age of blood products.
As a rule, moderate to high T1 signal intensity and a low T2 signal intensity are to be expected in the acute and early subacute setting.
However, in Polycystic Kidney Disease, hemorrhagic cysts can pose a challenge when the background is filled with other cysts and suspected foci of malignization.
In this context, DWI can provide invaluable information, appearing as a rim of high DWI signal, sometimes in crescentic forms (Figure 5).
CKD One of the most important features of CKD is cortical interstitial fibrosis, its evaluation determining renal prognosis and an early diagnosis and prompt treatment, since elevation of serum creatinine is a late occurrence.
Numerous studies[2,3,4] demonstrate DWI discriminates between a healthy and a sick kidney and DTI came as a more comprehensive method of evaluating the renal microstructure, since besides water mobility, it shows water´s directionality, expressed as fractional anisotropy(FA).
A low FA means there is homogeneous water movement, with no directionality (e.g. cyst), whereas a high FA is displayed in tubules and vessels with water movement in one direction.
Lu L et al[5] showed that the renal medulla displays a low FA and a low ADC in diabetic nephropathy, highlighting the promise of MRI as a biomarker, particularly when microalbuminuria has its limitations.
ADC correlates also with renal function[6,7] and displayed a good correlation with cortical fibrosis and cortical lesions in histology[8] (Figure 6).