| Abstract|| |
Over the last decade, state-of-the-art magnetic resonance MRI has become a valuable partner in the clinical imaging arena to tackle a wide variety of kidney diseases in a non-invasive manner, mainly due to its high tissue contrast and multiplanar imaging capabilities. Paramagnetic contrast agents have further improved the performance of MRI of the kidney since their administration seems to be applicable in virtually all patients, irrespective of their age, renal function and their ability to cooperate. Moreover, MRI is particularly helpful for further differentiation of lesions that are equivocal on CT and/or ultrasound. Further technical developments of applied MR-techniques and further improvements in spatial resolution will expand the imaging possibilities and create new tracts and challenges in the MRI evaluation of kidney disease. An overview of the current status of MRI in the diagnosis of renal abnormalities, which includes description of technique and normal anatomy and congenital variants, is given. The benefits of MRI in diagnosing diseases of renal parenchyma mass lesions such as cysts, renal cystic diseases, and benign and malignant tumors is reviewed. Furthermore, the MRI diagnosis of vascular diseases, diffuse renal parenchymal diseases, renal infectious diseases and the pathology of the renal collecting system will be discussed.
Keywords: MRI, Renal, Vascular, Technique, Tumors.
|How to cite this article:|
Verswijvel G, Oyen R. Magnetic Resonance Imaging in the Detection and Characterization of Renal Diseases. Saudi J Kidney Dis Transpl 2004;15:283-99
|How to cite this URL:|
Verswijvel G, Oyen R. Magnetic Resonance Imaging in the Detection and Characterization of Renal Diseases. Saudi J Kidney Dis Transpl [serial online] 2004 [cited 2019 Nov 13];15:283-99. Available from: http://www.sjkdt.org/text.asp?2004/15/3/283/32978
| Introduction|| |
Detection of renal pathology by ultrasonography (US), computed tomography (CT) and magnetic resonance imaging (MRI) relies on high-quality examinations. Continuous refinement of these techniques and optimization of their use is an essential prerequisite to expand the range of 'detectable' kidney diseases. Over the last decade state-of-theart magnetic resonance MRI has become a valuable partner in the clinical imaging arena to tackle a wide variety of kidney diseases in a non-invasive manner, mainly due to its high tissue contrast and multiplanar imaging capabilities. Recent technical advances have largely overcome the problem of respiration induced motion artifacts. Paramagnetic contrast agents have further improved the performance of MRI of the kidney since their administration seems to be applicable in virtually all patients, irrespective of their age, renal function and their ability to cooperate. Moreover, MRI is particularly helpful for further differentiation of lesions that are equivocal on CT and/or ultrasound. Different MR-techniques can now be combined in the same imaging session to establish a 'one-stop-shopping' or 'all-in-one' imaging modality in the assessment of diseases that affect the kidneys and the urinary tract. 1 This can be achieved without the risks of iodinated contrast material or exposure to radiation. It is a cost-effective diagnostically relevant method, especially for patients who are likely to be prone to a cascade of examinations spread over multiple (hospital-stay) days. Once pathology has been detected, the radiologist is faced with additional tasks such as the determination of malignancy, ascertain its stage and/or operability, assess if further examinations are necessary or not, etc. All this must be considered when performing an MR- study of a patient with a potential renal disease. Further technical developments of applied MR-techniques and further improvements in spatial resolution will expand the imaging possibilities and create new tracts and challenges in the MRI evaluation of kidney disease.
An overview of the current status of MRI in the diagnosis of renal abnormalities is given.
| Technique|| |
A basic MRI examination of the kidneys involves adequate imaging of the renal parenchyma, renal vessels, renal parenchymal (and lesional) perfusion and of the collecting system. Therefore, standard single shot T2-weighted sequences with and without fat saturation are combined with T1-weighted images precontrast and postgadolinium-chelate administration. The latter can be obtained in a dynamic sequence (breath hold spoiled gradient echo) obtaining images in the corticomedullary, parenchymal and excretory phases. Precontrast and postcontrast image acquisition in additional planes (coronal and sagittal) can be helpful in the evaluation of different disorders and display better the individual anatomical (or surgical) relationships. Furthermore, three dimensional (3D) MR-angiography can be combined in the same imaging session, allowing evaluation of the main renal artery with diagnostically sufficient image quality to tackle an underlying stenosis. Finally, contrast enhanced MR-urography images can be obtained seven to ten minutes after contrast injection, with or without supplementary administration of furosemide , [Figure - 1]. Post procedural processing of the images can improve the diagnostic quality, using subtraction/addition methods or volume of interest editing techniques. However, in case of a dilated collecting system it is possible to image the excretory tract with T2 weighted sequences ('MR-hydrography') without administration of contrast  [Figure - 2].
Usually the examination is performed with a 1.5 (or higher) Tesla system. A phased array body coil is preferable to optimize signal-tonoise ratio. Patients are usually examined in the supine position. With suboptimal filling of the collecting system i.e. in case of severe dilatation, an additional sequence is obtained in the prone position to attempt to further improve contrast enhanced MR-urography images.
In the majority of patients, the examination will be performed with intravenous injection of gadolinium-chelates (0.1 or 0.2 mmol/kg body weight).Their elimination is predominantly via renal excretion (glomerular filtration) and without reabsorption. Hence, these contrast agents are excellent for evaluation of the morphology of the kidneys, renal and lesional perfusion, and renal function. Within the collecting system the contrast is diluted by the urine and thus increases its T1-relaxation giving the urine high signal intensity on T1weighted images. In high concentrations, however, these contrast agents provoke magnetic susceptibility and signal intensity loss on T1-weighted images. Then, the urine has low signal intensity on T1-weighted images. This explains the typical layering effect/ artifact in a dilated collecting system or in the urinary bladder: the low signal intensity posterior part has a high contrast concentration; the upper part has high signal intensity due to contrast dilution. Turning the patient and repeating the sequence easily solve this problem. An advantage of gadolinium chelates is the possibility of performing contrast enhanced studies in patients with moderate renal insufficiency or in patients allergic to iodinated contrast materials. The clinically applied dosages are 0.1 to 0.2 mmol/kg body weight.  Since some agents are eliminated almost exclusively by the kidneys, administration in patients with strongly decreased renal function should be evaluated against the diagnostic benefit and the examination in this group of patients is probably best followed by dialysis.  During pregnancy it is not recommended to use intravenous gadolinium chelates since their ultimate safety has not yet been proven in humans. The incidence of general allergic reactions to gadolinium containing contrast media is only 1-2 % and severe systemic anaphylactic reactions are very rare. 
| Image Findings|| |
Normal Anatomy and Congenital Variants
In general, MRI provides adequate visualization of the anatomy of the kidneys. Since they are embedded in the retroperitoneal fat, they are easily displayed on T1- and T2-weighted images. The perirenal area is bordered by renal fascia (Gerota's fascia), recognizable as a thin hypointense line on T1- and T2-weighted images. The renal cortex that arches over the pyramids (cortex base) and extends towards the renal sinus (septal or junctional cortex) is lower in signal intensity on T2-weighted images due to the high water content of the tubuli that colligate in the pyramids. By using respiratory triggered or diaphragmatic pacing methods, it is now possible to distinguish corticomedullary differentiation in the kidney in normal subjects. This allows identification of hypertrophic septal cortex, which might present as a pseudotumor on ultrasound. The fibroelastic capsule enveloping the kidney is not visible on MRI images. Persisting fetal lobation causes indentations on the renal contour centered on the septal cortex in between the pyramids and with normal cortical thickness. It is best appreciated on coronal or axial images. The collecting system and the large vessels are adequately studied with appropriate imaging techniques (MR-urography, MR-angiography). Anatomical variants such as collecting system duplication or multiple renal arteries can be demonstrated with exquisite detail. Variants in the position of the kidneys and/or fusion anomalies (ectopic kidney, horseshoe kidney, crossed fused ectopy…) can be evaluated in different spatial planes and should be differentiated from pathological conditions. Especially in young children, MRI is the method of choice due to the lack of irradiation.
Diseases of the Renal Parenchyma
Ultrasonography and CT increased the earlier detection of renal tumors. This improves the cure rate of renal malignancy. On the other hand this has increased the detection of all types of renal masses in asymptomatic patients, including masses not requiring surgery. Imaging focuses on distinguishing mass lesions requiring surgery from those where surgery has to be avoided (not necessarily malignant from benign lesions). Renal cysts (simple and complicated), abscesses, hematomas, infarcts, inflammatory pseudotumors, angiomyolipomas should be identified and differentiated from primary malignant renal tumors, lymphoma and renal metastases. With growing experience, it even becomes possible to differentiate subgroups of renal parenchymal tumors. Lesional biopsy is indicated in selected patients only.
Simple renal cysts present as a rounded homogeneous hypointensity mass on T1-weighted images, and as rounded homogeneous hyperintense lesion on T2-weighted images, reflecting the water content. The cyst wall is thin and almost imperceptible and there is no enhancement of any portion of the cyst after intravenous administration of gadolinium.  [Figure - 3] Cysts are solitary or multiple; the number of cysts increases with increasing age. The volume of the cysts varies from a few milliliters to several liters.
Cysts are considered to be complex when a) there is focal or diffuse thickening of the wall, b) the content is 'not clear fluid', c) there are internal septations, d) there are calcifications at septations or in the wall, e) there is any combination of these abnormalities. Hemorrhagic cysts are frequently seen on MRI. The signal intensity depends on the time of imaging. The majority has a high signal intensity on T1- and T2- weighted images since most of the imaging studies are performed in the subacute phase, i.e. 1 to 26 weeks after the initial intracystic bleeding  [Figure - 4]. Acute or early subacute hemorrhage contains intracellular deoxyhemoglobin or intracellular methemoglobin, which turns the cystic content hypointense on T2-weighted sequences.  Such cysts resemble neoplastic pathology and differentiation can only be achieved after intravenous contrast administration. Region of interest (ROI) measurements on comparable pre- and serial postcontrast T1-weighted images are required to prove the lack of enhancement in a complicated cyst. It is essential to evaluate it on delayed scans so weak enhancement of some subtypes of malignant renal tumors can be verified. , Septations reflect fibrin strands from previous bleeding or infection or are due to close juxtaposition of two cysts. Septations should be thin (2 mm or less) and should not contain enhancing nodules.  Focally or diffusely thickened septations and/or cystic wall enhances almost always to some degree after intravenous contrast administration. Accurate differentiation between a benign and a malignant lesion is often not feasible and careful follow-up or surgery is often required , [Figure - 5] a, b. Since calcium does not contain any protons it will signal void on MRI and its detection is therefore less easy (and sometimes impossible) compared to CT. Nevertheless, since the adjacent tissue is clearly identified, MRI is indicated in the evaluation of the calcified cysts. The underlying tumor can readily be identified. 
The benign multilocular cystic nephroma consists of multiple loci separated by rather thick septations. Its peak incidence is in the middle-aged females and in young boys. , The origin of the lesion is usually near the poles of the kidney that has a tendency to protrude into the renal pelvis. , Whatever the imaging modality used, it remains impossible to distinguish this lesion from a cystic renal cell carcinoma based on imaging criteria and, therefore, it should be verified by surgery , [Figure - 6] a, b.
Renal Cystic Diseases
MRI is particularly useful in patients with congenital cystic diseases and syndromes involving the kidneys. Autosomal dominant polycystic kidney disease (ADPKD) is characterized by bilaterally enlarged kidneys with multiple cysts of varying sizes on MRI examinations.  Coronal and sagittal images usually demonstrate the extent of the disease. The cysts involve all portions of the renal parenchyma and almost consistently display varying signal intensities due to the presence of intracystic blood degeneration products.  Postgadolinium images are useful in the determination of associated carcinoma in this setting. Extrarenal cysts in the liver, pancreas, and seminal vesicles can be evaluated in the same image session. Autosomal recessive polycystic kidney disease (ARPKD) is an inherited disorder characterized by unobstructive collecting duct ectasia, hepatic biliary duct ectasia and fibrosis of the kidneys and the liver.  MRI usually demonstrates enlarged kidneys with a varying number of small (usually <1cm) cysts throughout the parenchyma. The enlargement of the kidneys is usually not as pronounced as in ADPKD. , In the medullary cystic disease ('nephronophtisis-uremic medullary cystic disease complex) small (1-2 cm) cysts are limited to the confines of the renal medulla, and usually in both kidneys [Figure - 7].  It is a salt wasting nephropathy leading to renal failure around the fourth decade of life. In the end-stage renal disease, cortical atrophy occurs with preservation of smooth contours. 
Medullary sponge kidney (MSK) is characterized by the presence of cystic or tubular dilatations of the collecting ducts, frequently complicated with calcifications. Contrast enhanced studies in the excretory phase show contrast filled tubular structures radiating from the calyx in the papilla. , The abnormality may be generalized and bilateral, but sometimes a single or few pyramids may be involved.
Acquired cystic disease of dialysis occurs in approximately 50% of patients with long standing hemodialysis. At the time of development, the kidneys are usually atrophic. On MRI, small cysts are found superficial in the renal cortex, often without significant contour expansion.  Coronal and sagittal images are very useful in evaluating these patients. Hemorrhagic cysts are frequently present and MRI is the method of choice in the evaluation of these patients, especially to rule out associated renal carcinoma; both of parenchymal and urothelial origin. A recent observation in patients with renal insufficiency secondary to lithium salts induced nephropathy was the presence of numerous renal microcysts ranging from 1 to 2 mm, and best demonstrated on MRI. 
Angiomyolipoma (AML) is the most frequent benign lesion of the renal parenchyma. It is composed of a variable amount of three elements: vascular structures ('angio'), smooth muscle structures ('myo') and mature fat ('lipoma').  In its sporadic form, the AML is found incidentally in asymptomatic patients during ultrasound, CT or MRI studies. However, the AML may be symptomatic manifesting as spontaneous bleeding or hemorrhage after minor abdominal trauma. The likelihood of hemorrhage increases with tumor size and the presence of intralesional pseuodaneurysms.  In syndromes as in tuberous sclerosis (Booneville's disease), the AML tends to be multiple and bilateral.  Lymphangiomyomatosis is another condition that is associated with the presence of angiomyolipomas in the kidneys.  Because of the high signal of fat on T1weighted images, these lesions are easily recognized on MRI. Fat suppressed or out-ofphase T1-weighted images lower the signal of fat, and thus confirm the presence of fat, unlike in hemorrhagic lesions where the high signal on T1 is not affected by such change [Figure - 8]. In a small number of cases, smooth muscle or vascular components predominate the picture that renders the differentiation from renal cell carcinoma difficult or impossible. , MRI can be helpful since pure angiomyomas tend to have low signal intensity on both T1- and T2- weighted images. Small amounts of fat have been described in rare cases of renal carcinoma. , However, malignant fat containing renal cell carcinoma tends to be heterogeneous, hypovascular and often contains coarse calcifications that are unlikely to be present in angiomyolipoma.
Benign tumors originating from the renal tubular epithelial cells include adenoma, oncocytoma and metanephric adenoma. At present, most pathologists agree that there are no reliable criteria to distinguish renal adenomas from renal cell carcinoma based on tumor size.
Adenoma is a benign tumor incidentally found on resection specimens, smaller than 0.5 cm in diameter, and with a typically papillary pattern. Its chromosomal abnormalities resemble that of a papillary carcinoma. Based on this observation, it has to be considered as a precursor of this malignancy. It should be clear that adenoma is not a radiological diagnosis and the term should be avoided in radiological reports. 
Renal oncocytoma is an uncommon (1-5%) benign neoplasm derived from the tubular epithelium. Based on imaging studies, it is correctly identified preoperatively in about 10-20 % of cases. , Malignancy with distant metastasis has been described,  but it is now believed that these malignant oncocytomas are probably chromophobe carcinomas rather than oncocytomas. , The features of oncocytomas on pre-contrast MRI include a low signal intensity mass on T1-weighted images, increased signal intensity on T2-weighted images, the presence of a capsule, a central scar or central stellate pattern and the absence of hemorrhage or necrosis.  Early contrast enhanced images will display a predominant centripetal arterial perfusion. A central stellate scar is only rarely identified and not exclusive for oncocytoma since clear cell carcinoma may have a similar presentation.  Oncocytoma may become very bulky lesions (10-20 cm) and in these cases the term 'giant oncocytoma' is used. 
Renal cell carcinoma (RCC) was believed to originate from adrenal residues ('hypernephroma'). In the 1960's, Oberling and coworkers described its true origin from the proximal renal tubule and the tumor was renamed as RCC.  For many years, it was considered as a single entity until 1986 when Thoenes and coworkers proposed a new classification of RCC known as the Mainz classification. Since then, RCC is no longer considered a single pathological entity but a group of different renal cancers, all derived from the renal tubular epithelium with different clinical, pathological, phenotypic and genotypic features, and with a different prognosis.  Malignant renal parenchymal epithelial neoplasms include the classical clear cell carcinoma, papillary (or chromophilic) carcinoma, chromophobic carcinoma, and collecting duct carcinoma.
Clear cell carcinoma is the most common renal neoplasm in adults that represents about 70% to 80% of tumors derived from tubular epithelium.  This tumor can be as small as 1 cm or less and discovered incidentally, or very bulky leading to clinical symptoms like pain, hematuria, and a palpable mass besides a variety of paraneoplastic symptoms. Even small tumors can have an aggressive biological behavior and have the potential to metastasize. The classical tumor is solid, well circumscribed or invasive in the perirenal fat, the renal sinus or the vascular structures. Cystic degeneration can occur in 10-15% of cases. Ten percent of the cases are multifocal and 1% are bilateral.  These tumors are slightly hypointense on T1-weighted MRI and slightly hyperintense on the T2-weighted images. One of the key features is the presence of tumor necrosis, which is frequently observed even in small lesions.  Typically, this intralesional necrosis has an eccentric location: it extends to the periphery of the lesion usually at the opposite site of the renal parenchyma.  This feature is an important morphological discriminator in the differentiation from other solid masses.  On MRI, necrosis displays low signal intensities on T1- and high signal intensities on T2-weighted images. Calcifications are difficult to detect on MRI. A pseudocapsule presents as a peripheral hypointense rim on T1- and T2-weighted images  [Figure - 9]a. In general, encapsulated RCC has a favorable pathological stage. At times, small tumors may be difficult to be detected on T1- and T2-weighted images alone, hence postcontrast evaluation is always mandatory. After injection of gadolinium, at least 80% of clear cell carcinomas are hypervascular in the arterial phase (corticomedullary phase) of contrast enhancement, while 20% are isoor hypovascular compared to the renal cortex [Figure - 9]b.  In the venous phase (parenchymal phase) or excretory phase the lesions become hypointense compared to the renal cortex due to wash-out [Figure - 9]c. Extension into the major renal veins and further into inferior vena cava may occur and is evaluated in the same MR-study. , Cystic clear cell (RCC) may be difficult to be differentiated from benign pathology as the multilocular cystic nephroma. 
Papillary carcinoma is the second most common malignant renal parenchymal tumor that represents 10-15%. Two subtypes are usually seen: 1) the tumor is large with solid or predominately cystic multinodules 2) the lesions are multifocal (ipsilateral or bilateral), usually with one or two large lesions "mother" lesions and several small ,less than 1 cm, 'daughter' lesions. , These lesions are usually hypointense on the T1-weighted MRI , while the signal intensities on T2- weighted images are variable (iso- to hypo- and slightly hyperintense)  [Figure - 10]a,b. This probably reflects different cytological variants (basophilic, eosinophilic, sarcomotoid) with different nuclear grades. Papillary carcinomas are hypovascular compared to the renal cortex after intravenous administration of gadolinium.
However, isovascularity may occur as well, usually with a wash-out of contrast on delayed images.
The chromophobic carcinoma is rare and accounts for less than 5% of the renal carcinomas.  There is evidence that this tumor nomas.  There is evidence that this tumor arises from the intercalated cells of the collecting ducts. With growing experience, it seems that there are two peak incidences: one in the middle aged women, and another in the older men. At the time of discovery, the lesions tend to be very large and to arise at the pole of the kidney. The chromophobic carcinomas are well circumscribed and typically solid without intralesional necrosis or hemorrhage. These lesions are usually hypointense on the T1-weighted MRI and iso- to slightly low in signal intensity on the T2weighted images.  After administration of gadolinium chelates, these lesions are hypovascular compared to the renal parenchyma.
The collecting duct carcinoma (or Bellini duct carcinoma) is a tumor, which is derived from the collecting ducts and constitutes only 1% of renal carcinomas.  Typically, it infiltrates the kidney with preservation of the renal shape, in contrast to the other parenchymal carcinomas, which lead to a 'ball' type of lesion. , The infiltrative pattern of this tumor mimics the parenchymal involvement by an infiltrative undifferentiated transitional cell carcinoma. , Frequently lymph node metastases are present at the time of the diagnosis and the prognosis is very poor. 
Many primary malignant diseases may metastasize to the kidney. Usually the renal metastases appear as multiple bilateral renal masses and frequently have the same imaging characteristics as the primary tumor on T1and T2-weighted MRI and the same perfusion characteristics.  They may resemble renal lymphoma and papillary carcinoma, obviating lesional biopsy. 
Renal lymphoma usually occurs in the setting of widespread systemic disease. However, isolated renal lymphoma can occur. Three patterns have been described: direct renal or perirenal infiltration from adjacent retroperitoneal disease, solitary or multiple focal masses and diffuse infiltration. , Lymphomatous tissue is usually slightly hypointense on T1-weighted MRI and hypointense to isointense on T2-weighted images. After administration of gadolinium, the lymphoma appears hypovascular compared to the renal parenchyma. , Contrary to the renal cell carcinoma, intratumoral necrosis is only very rarely observed, even in large masses.  Vascular encasement is frequently present in lymphoma, rather than vascular invasion  [Figure - 11]. These features can be recognized on MRI imaging.
Differentiation of these different lesions is a challenge to the radiologist. An effort should be made for in-depth preoperative radiological differentiation since this can further orientate therapy. Currently, MRI has the same potential as CT scan for detection, characterization and staging of renal parenchymal masses. In selected cases, MRI can provide additional information after a state-of-the-art performed CT. ,
| Vascular Diseases|| |
Renal vein thrombosis can be isolated or associated with a tumor.  The latter occurs almost exclusively in clear cell renal carcinoma. Tumor thrombus has the same signal intensity as the primary renal tumor on T1and T2- weighted MRI; MRI shows a patchy flow pattern reflecting neoangeogenesis. , In acute isolated thrombosis the kidney is usually swollen with persistent nephrogram on MRI that can be observed after injection of gadolinium. 
The aorta and its major branches including the renal artery supply can be studied adequately by MR-angiography (MRA) [Figure - 12]a,b. The best results are currently obtained by 3D contrast enhanced MRA methods. , Technical improvements, such as sensitivity encoding (SENSE), have decreased the acquisition time and the requirement for long breath holding, which improved the image quality as well as the reliability and technical success of 3D-MRA. The diagnostic accuracy is equal to the CT-angiography (CTA). Both techniques have sensitivities and specificities ranging from 90-100%.  The major advantage of MRA above CTA is the lack of ionizing radiation and the use of contrast agents not affecting the renal function. Furthermore, it has been shown that MRA is sufficient to evaluate the renal transplant donors. The flow quantification techniques may provide additional information on renal perfusion in case of renal artery stenosis. However, further advances in MRA are still necessary to improve the visualization of the intrarenal vessels, which still form an important advantage of CTA.
| Diffuse Renal Parenchymal Diseases|| |
In MRI, findings of glomerular disease are usually non-specific. Symmetrical swelling of the kidney, prolonged nephrogram and increased enhancement of the kidney usually result in high signal intensity of the medulla. The loss of corticomedullary differentiation is another non-specific finding on T1-weighted MRI.
In chronic diseases, cortical atrophy occurs with preservation of a smooth contour.  Medullary atrophy may be present as well and partially replaced by hilar fat (renal sinus replacement lipomatosis).
In case of tubulointerstitial disease processes the findings are also non-specific and MRI is not able to differentiate among its different causes. 
MRI can be very helpful in the diagnosis of iron deposition in the renal cortex in cases of renal cortical hemosiderosis. , In these cases, the renal cortex has a very low signal intensity on T2-weighted images, due to the T2 effect of the iron atom. 
| Infectious Diseases|| |
In acute pyelonephritis (APN), MRI can be used to evaluate the extension and severity of the disease. The findings on MRI include renal swelling, cone shaped regions of high signal intensity in the renal medulla, on T1 images due to the presence of protaceous images due to the presence of protaceous material in the tubules. Furthermore, there may be triangular areas in the parenchyma of slightly lower signal intensity on T2 images due to the infiltration by inflammatory cells and perinephric fluid with mild dilatation of the collecting system.  These areas are hypovascular in the corticomedullary and parenchymal phases after administration of gadolinium; a striated nephrogram is observed on delayed scans. Recently, areas of acute pyelonephritis are found to be hyperintense on diffusion weighted images.  The most sensitive sequences seem to be TIR-sequences after gadolinium.
The renal abscess due to APN or hematogenous infection can be detected on MRI as a mass lesion with a necrotic center (due to the presence of pus) and a contrast enhancing thick peripheral wall.  Secondary signs of inflammation (such as swelling, perinephric fluid) can also be found on the images. The center of the abscess is usually hyperintense on diffusion-weighted images [Figure - 13] a,b,c. 
Pyonephrosis is demonstrated on MRI as a fluid-debris level in an obstructed collecting system with thickening and contrast enhancing of the wall of the renal pelvis, calyces and ureter. 
Xanthogranulomatous pyelonephritis (XGP) is characterized by the presence of collections of foamy macrophages ('xanthoma cells') and other inflammatory cells. MRI can be helpful in its diagnosis, since these foamy macrophages often result in high signal intensity on T1-weighted images. , Other image features correspond to those found on CT.
Malakoplakia is a rare chronic granulomatous inflammatory disease, usually seen in the setting of a compromised immune system. MRI can demonstrate nodules that coalesce to become diffuse or undergo suppuration with abscess formation. 
Diseases of the Urinary Tract
Calculi are the most common lesions in the collecting system. Calculi present as areas of signal void on MRI, regardless of their composition. MRI has a low sensitivity in the detection of calculi, compared to CT. When found, they are best displayed on T2- weighted images where the urine is high and the stone is low in signal intensity. Other filling defects due to blood clots, sloughed papillae or fungus balls can also be demonstrated as non-enhancing mass lesions in the collecting system. Air in the collecting system may result in artifacts in the non-dependent parts of the system.
Dilatation of the collecting system can be evaluated with great accuracy on MRI.  Dilatation of the collecting system is not necessarily obstructive. Possible causes include high flow status, congenital anomaly, ureteropelvic junction stenosis and atony after obstruction. All these causes can be evaluated with MRI, especially when combing the 'classical' cross sectional sequences with MR-urography and MR-angiography ('tailored approach'). ,,
Primary transitional cell carcinoma, squamous cell carcinoma, secondary lymphoma, metastatic tumors affecting the collecting system can be evaluated and staged accurately with MRI. ,
| Future Directions|| |
Functional MR-imaging of the kidneys is currently under experimental investigation and is expected to enter the clinical imaging arena in the near future.  Glomerular filtration, tubular concentration, blood perfusion, macrophage infiltration, diffusion  and oxygenation are contributive elements that can be evaluated by non-invasive imaging methods.  The latter require new imaging approaches such as the use of exogenous contrast agents such as iron oxide particles and endogenous contrast agents like deoxyhemoglobin. Further studies are required to evaluate the reproducibility and understand the physiological background of the experimental findings.
| Conclusion|| |
MR-imaging has become a valuable tool in the clinical non-invasive imaging evaluation of a patient with kidney disease. Many kidney diseases can be evaluated with great accuracy and with a combination of different MR-techniques. An 'all-in-one' (or 'one-stop shopping') imaging modality can be established for the evaluation of disease affecting the kidneys and the urinary tracts, providing all the necessary diagnostic information for management. Future techniques will hopefully broaden the clinical application of MRI in the evaluation of renal disease. Since MRI does not involve radiation or iodinated contrast material, it is easy to understand why this technique has already become the standard in the evaluation of a normal potential kidney donor. 
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Ziekenhuis Oost Limburg, Department of Radiology, Schiepse Bos 6, 3600 Genk, Belgium
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8], [Figure - 9], [Figure - 10], [Figure - 11], [Figure - 12], [Figure - 13]