Diffusion-weighted imaging of the urinary tract – beyond tumors
Diffusion-weighted imaging of the urinary tract – beyond tumors
Afonso Fonseca, João Lopes Dias
To provide a pictorial review, with MRI images from our institution, depicting the Diffusion-weighted imaging (DWI) features of acute kidney disease (including Acute Pyelonephritis (APN), hydronephrosis, abscesses, hematomas and sediment) and Chronic Kidney Disease(CKD).
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 . 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 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 (Figure 6).
Figure 1. Sixty-two year-old man with fever, chills, raised inflammatory markers and positive urocultures for Klebisella pneumoniae. In the left iliac fossa, the renal transplant exhibits cortical triangular areas of restricted diffusion, compatible with pyelonephritis.
Figure 2. Eighty-four year-old man with left hydronephrosis, diffuse hyperenhancing urothelial thickening and endoluminal sediment/debris. There are also multiple focus of restricted diffusion in the renal cortex (ADC value of 0.972x10^-3 mm^2/s), compatible with pyelonephritis.
Figure 3. Right kidney with hydronephrosis and pyonephrosis, in a 62 year-old patient with a pyeloureteral junction syndrome (a right JJ stent is noted). The right renal parenchyma has multiple peripheral areas with restricted diffusion, associated with the obstructive condition.
Figure 4. Eighty-three year-old woman with cervical cancer with fever of unknown focus. Abdominal MRI shows a T2 hyperintense lesion in the upper pole of the right kidney, exhibiting restricted diffusion (ADC value of 0.855x10^-3 mm^2/s), compatible with an asbcess.
Figure 5. ADPKD in a 45 year-old man, with a ruptured hemorrhagic cyst and an extensive hematoma that exhibits restricted diffusion with low ADC values (0.676x10^-3 mm^2/s).
Figure 6. MRI of the abdomen revealing a left kidney of a 55 year-old woman with renal cortical defects, exhibiting areas of lower enhancement, appearing in DWI sequences as geographic areas of restricted diffusion (sequelae of previous pyelonephritis, with fibrosis).
CONCLUSION: The importance of the DWI sequence in MRI goes well beyond tumor detection, particularly in the kidney. It is an increasingly recognized biomarker of renal function and fibrosis, namely in CKD. The role of DWI in APN, abscesses, hematomas and obstruction has also been explored and its imaging findings should figure in the checklist of the urogenital radiologist. References:  Thoeny HC, Binser T, Roth B, Kessler TM, Vermathen P. Noninvasive assessment of acute ureteral obstruction with diffusion-weighted MR imaging: a prospective study. Radiology. 2009 Sep;252(3):721-8  Li Q, Li J, Zhang L, Chen Y, Zhang M, Yan F. Diffusion-weighted imaging in assessing renal pathology of chronic kidney disease: A preliminary clinical study. Eur J Radiol. 2014 May;83(5):756-62.  Liu Z, Xu Y, Zhang J, Zhen J, Wang R, Cai S, Yuan X, Liu Q. Chronic kidney disease: pathological and functional assessment with diffusion tensor imaging at 3T MR. Eur Radiol. 2015 Mar;25(3):652-60.  Xu X, Fang W, Ling H, Chai W, Chen K. Diffusion-weighted MR imaging of kidneys in patients with chronic kidney disease: initial study. Eur Radiol. 2010 Apr;20(4):978-83.  Lu L, Sedor JR, Gulani V, et al. Use of diffusion tensor MRI to identify early changes in diabetic nephropathy. Am J Nephrol. 2011;34(5):476-482  Friedli I, Crowe LA, de Perrot T, Berchtold L, Martin PY, de Seigneux S, Vallée JP. Comparison of readout-segmented and conventional single-shot for echo-planar diffusion-weighted imaging in the assessment of kidney interstitial fibrosis. J Magn Reson Imaging. 2017 Dec;46(6):1631-1640. Friedli I, Crowe LA, Viallon M, Porter DA, Martin PY, de Seigneux S, Vallée JP. Improvement of renal diffusion-weighted magnetic resonance imaging with readout-segmented echo-planar imaging at 3T. Magn Reson Imaging. 2015 Jul;33(6):701-8.  Friedli I, Crowe LA, de Perrot T, Berchtold L, Martin PY, de Seigneux S, Vallée JP. Comparison of readout- segmented and conventional single-shot for echo-planar diffusion-weighted imaging in the assessment of kidney interstitial fibrosis. J Magn Reson Imaging. 2017 Dec;46(6):1631-1640. doi: 10.1002/jmri.25687.