MRI Atlas of Renal Pathology Mary Kitazono Patrick M Colletti MD University of Southern California Keck School of Medicine Start firstname.lastname@example.org, email@example.com
Table of Contents • Renal MRI Technical Considerations • Indications for Renal MRI • MRI Atlas of Normal Renal Anatomy • MRI Atlas of Renal Pathology • Vascular Disorders • Diseases of the Renal Parenchyma • Obstructive Uropathy • References • MRI Tutorial • Proton MR Tutorial • Pulse Sequences • MR Angiography • MRI Imaging Strategies
Renal MRI Technical Considerations • Select a Pulse Sequence and Protocol • Set Parameters: • TR (repetition time): determines T1 and T2-weighting • TE (echo time): determines T2-weighting • Flip angle: determines T1-weighting in gradient echo sequences • Maximize image quality • Voxel size: smaller = improved resolution • Acquisition time: faster = better temporal resolution • Reduce Motion Artifacts • Paramagnetic Contrast Enhancement • Renal MR Angiography • MR Urography
Basic Renal ImagingProtocol • Localizer: HASTE/ssFSE/ssTSE • Coronal plane, 8 mm slices, breath held • In-phase and opposed-phase T1: Dual echo spoiled gradient echo: • To characterize fat-containing lesions; axial plane, 6-8 mm slices, breath held • Include a precontrast fat-suppressed T1 to differentiate fat from blood • Fat-suppressed T2: Fast/TurboSE or GE • To characterize lesion, lymphadenopathy, and fluid; axial plane, 6-8 mm slices, respiratory triggering • Dynamic Gd-enhanced T1: 2D or 3D spoiled GE • Contrast enhanced MRA: to characterize vascularity; axial plane, 6-8 mm slices, breath held • Non-contrast MRA • To evaluate renal vein and IVC for patients with renal cell carcinoma • MR Urography • To evaluate the collecting system
Motion Artifact Reduction • Causes of physiologic motion artifact: • respiration • cardiovascular pulsation • bowel peristalsis • Due to the posterior location of the kidneys, there is much less artifact as compared to liver imaging, however various methods for reducing motion artifact can be employed.
Motion Artifact Reduction • Ways to reduce motion artifact: • antero-posterior phase encoding for axial imaging (since artifacts develop in the direction of the phase encoding) • cranio-caudal phase encoding combined with foldover suppression for coronal imaging • breath-holding, with or without retrospective averaging of individual breath-holds • single-shot and fast imaging pulse sequences • fat-suppression techniques • intermittent sampling of data using respiratory ordered-phase encoding, respiratory gating, or respiratory triggering (though not usually required) • wrapping an elastic garment around the patient’s abdomen to minimize anatomic movement during respiration
post-contrast Gadolinium • Chelates of Gadolinium (e.g. Gd-DTPA) • commonly used paramagnetic contrast agents • completely filtered in the kidney, making it useful for examining excretory kinetics • provides excellent corticomedullary delineation in the early perfusion phase (10-20 seconds after injection) and in the early excretory phase (50-90 seconds after injection) • safe (approved by the FDA) • MR imaging with Gd-DTPA is the appropriate choice for all children and women of childbearing age, patients with iodinated contrast allergy, renal insufficiency and solitary kidney, and for following the progression of disease over time.
pre-contrast post-contrast Paramagnetic Contrast Enhancement • How they work: Paramagnetic contrast agents have unpaired electrons, which create small local magnetic fields in the surrounding tissue • Main effect: “proton relaxation enhancement,” or shortening of the T1 relaxation time in the surrounding tissues, increasing the signal intensity and tissue contrast • Requirements:blood flow and a compromised capillary basement membrane • Use: to assess the function of renal vessels and to enhance lesions for diagnosis, especially vascularized tumors
Renal MR Angiography • Advantages of MRA over conventional angiography: • Gadolinium chelates are safe, even for patients with renal insufficiency, whereas iodinated contrast agent can cause allergic reactions and nephrotoxicity. • MRA is a non-invasive procedure, not requiring local anesthesia or recovery time. • 3D MRA allows a more accurate assessment of lesions, particularly atheromatous plaques. • MRA can usually demonstrate accessory renal vessels. • MRA can elucidate the significance of stenosis by depicting the presence of poststenotic dilatation, delayed renal enhancement, and/or reduced renal mass. • Incidental renal pathology can be detected. • Phase contrast flow MRA can provide a quantitative measurement of renal perfusion.
Renal MR Angiography • Disadvantages of MRA: • MRA requires precise timing and calculation of the rate of contrast flow. Inaccuracies can result in signal loss if image acquisition begins too late, or venous enhancement, instead of arterial enhancement, if acquisition is begun too early. • MRA requires the patient to hold his or her breath and remain motionless for a considerable amount of time. • As with all forms of MR imaging, MRA can not be performed on patients with pace-makers or implanted ferromagnetic devices.
MR Urography Clinical Application: to evaluate ureteral obstruction and congenital urinary tract abnormalities
Contents MR Urography • Static MR Urography: • Technique: Heavily T2-weighted sequence similar to MRCP • Works best for dilated collecting systems • Can even be used in patients with severe renal insufficiency • No imaging delay or IV contrast necessary, but overlapping fluid-filled structures can interfere • Tip: imaging nondilated ureters can be enhanced by hydrating the patient and administed a low dose of furosemide before the exam • Excretory MR Urography: • Technique: T1-weighted 3D gradient echo sequence after IV contrast administration • Can demonstrate nondilated as well as dilated ureters • No interference by overlapping fluid-filled structures • Cannot be used in patients with severe renal insufficiency
Indication for Renal MRI • Detection, localization, and characterization of renal masses(due to its multiplanar capability) • Staging renal cell carcinoma • Detection and differentiation between renal cysts, hematomas and angiomyolipomas • Assessment of kidney functionusing dynamic imaging • Visualization of the renal arteriesusing MR Angiography • Preoperative planning and postoperative evaluation
Renal Cysts • The Bosniak classification system, which is based on imaging findings,can be used to differentiate benign renal cysts from angiomyolipomas and renal cell carcinomas
Staging Renal Cell Carcinoma • MRI is the imaging modality of choice for staging renal cell carcinoma: • multiplanar capability • superior soft tissue contrast • ability to depict tumor extension and invasion • ability to detect enlarged lymph nodes
Renal Transplantation • MRI is useful for both preoperative planning and post-transplant evaluation of the caliber of the collecting system and vasculature • Peritransplant fluid collections, which may cause urinary tract obstruction, or regions of infarction (best seen with contrast) can be detected in the postoperative period
Contents Utility of Dynamic Contrast-Enhanced MRI • In addition to providing exceptional anatomical detail, dynamic contrast-enhanced MR can provide a wealth of information about renal function including perfusion and glomerular filtration rates, giving MR imaging a clear advantage over real time ultrasonography and radionuclide renography. Excretory Function Spatial Resolution Blood Flow Real time ultrasonography with color doppler Radionuclide renography Dynamic contrast-enhanced functional MRI
Adrenals Renal sinus fat Retroperitoneal fat Normal Renal Anatomy T1-Weighted Image Kidneys are surrounded byhigh-signalretroperitoneal fat, which is contained by the low-signalperirenal fascia Liver Spleen T2-Weighted Image
c m c m Normal Renal Anatomy • T1- weighted images allow corticomedullary differentiation between the intermediate-signal cortex and the low-signal medullary pyramids • The absence of a distinct corticomedullary differentiation on T1-weighted images is a sensitive, though nonspecific, indication of renal parenchymal disease
Normal Renal Anatomy T1-Weighted Image T2-Weighted Image • The renal pelvis,calyces, and infundibuli can normally be seen because of their water content, appearing as hypointense on T1-weighted images and hyperintense on T2-weighted images • Low-signal ureters extend obliquely, inferiorly, and medially
* * * * Contents Normal Renal Anatomy a a • This MR angiogram demonstrates the renal arteries (*) extending from the abdominal aorta (a) in the axial oblique (top) and coronal views (bottom). a • Renal vessels are linear or tortuous areas of signal void • Renal arteries extend horizontally from the aorta to the kidneys; renal veins extend obliquely towards the IVC
Vascular Disorders that cause Renal Pathology • Pre-renal vascular disorders: • Aortic dissection • Arteritis (ie Takayasu Arteritis) • Renal artery stenosis • Renal artery aneurysm • Renal vascular disorders: • Renal AVM • Polyarteritis nodosa • Hematomas • Post-renal vascular disorders: • Renal vein or IVC thrombosis
Aortic Dissection First Pass Second Pass Delayed
Takayasu Arteritis • Takayasu arteritis is a granulomatous vasculitis of medium and large-sized arteries • Classic clinical syndrome = ocular disturbances and marked weakening of the pulse in the upper extremities (“pulseless disease”) due to fibrous thickening of the aortic arch and narrowing of the great vessels arising in the arch • One third of cases also have involvement of the remainder of the aorta and its branches, including the renal arteries
Renal Artery Stenosis • Best visualized with 3D contrast enhancedMRA • Proximal lesions are the easiest to visualize • Post-stenotic dilatation • Small, smooth kidney • Arterial collaterals occasionally noted
Renal Artery Aneurysm • Classified morphologically as saccular or fusiform • Annular calcification is common, especially in atherosclerotic aneurysms • A large aneurysm may produce a concave deformity on the adjacent pelvocalyceal system • Occasionally an aneurysm appears as a soft tissue mass accentuated by surrounding sinus fat • Rapid contrast enhancement, with delayed washout of the aneurysm • Look for pulsation artifact Calcified Renal Artery Aneurysm
GRE Renal AVM • AVM usually occurs in the medulla and may cause a mass effecton the collecting system • Peripheral, curvilinear calcification may be present in a cavernous AVM • An enlarged feeding artery and draining vein may be seen • The entire mass enhances with contrast
Renal AVM 46 y/o man with hematuria. There is a large left upper pole AVM with massive IVC dilatation
Polyarteritis Nodosa • Polyarteritis nodosa is a systemic vasculitis resulting from non-infectious transmural necrotizing inflammation of small or medium-sized muscular arteries • Only the “classic” form, in which intermediate-sized arteries (the arcuate arteries) are affected, is seen in the kidneys • It often results in irregular arterial dilation, nodularity, obstruction, and less commonly, infarction
Polyarteritis Nodosa • Multipleaneurysms in the arcuate arteries • The aneurysms are 2 mm- 3mm andsharply defined • Arterial involvement is focal, random, and episodic • Infarcts are also common • Because small vessels are spared, glomerulonephritis is not seen infarcts
Polyarteritis Nodosa • Although the kidney is the organ that is most commonly involved, polyarteritis nodosa (PAN) is a systemic disease that can infect any organ or system, including the central nervous system as shown below PAN in the brain (a) a PAN in the spinal cord (b, c, d) b c d
a b c d Chronic Subcapsular Hematoma • Homogenous collection of hypodense fluid (a) that is bright on T1 (b, c) and T2 images (d)
Contents Renal Vein Tumor Thrombosis • T1-weighted MR images or MRA are best for depicting renal vein thrombosis
Diseases of the Renal Parenchyma • Introduction • Glomerular Diseases • Glomerulonephritis • Glomerular lesions associated with systemic disease • Tubular and Interstitial Diseases • Acute Tubular Necrosis • Tubulointerstitial Nephritis • Acute and Chronic Pyelonephritis • Cystic Diseases • Neoplasms • Angiomyolipoma • Renal cell carcinoma • Wilm’s Tumor
Imaging the Renal Parenchyma • The renal parenchyma is often evaluated using a breath-hold fat suppressed T1-weighted spoiled GRE sequence in the axial plane, performed before and approximately 3-5 minutes after contrast administration. • The coronal plane is particularly useful in evaluating lesions at the poles of the kidneys as well as lesions that extend outside the renal parenchyma. • To characterize cystic lesions, a breath-hold T2-weighted half-Fourier single-shot turbo spin echo sequence in the coronal plane is often performed. Gadolinium enhancement and fat suppression methods can be used to further characterize the cystic components.
1. Infarct 2. Chronic pyelonephritis Irregular Renal Contour • The renal capsule is normally smooth. There are many causes of an irregular renal contour: 3. Papillary necrosis
Renal Scar Differential Diagnosis: Unilateral: • Reflux nephropathy • Previous renal surgery Bilateral: • With Normal Calyces: • Renal Infarcts • With Abnormal Calyces: • Analgesic nephropathy • Reflux nephropathy Chronic pyelonephritis
Renal Scar Differential Diagnosis: Unilateral: • Reflux nephropathy • Previous renal surgery Bilateral: • With Normal Calyces: • Renal Infarcts • With Abnormal Calyces: • Analgesic nephropathy • Reflux nephropathy +c +c Cortical infarcts
Glomerulonephritis • Acute glomerulonephritis: nonspecific, dense collection of inflammatory edema • Chronic glomerulonephritis: • marked, symmetric reduction in the size of both kidneys • smooth contours with normal calyces and papillae • abundant peripelvic fat • calcification of the renal cortex may be seen Chronic Glomerulonephritis with iron loading
Siderotic Nodules • Iron overload caused by genetic hemochromatosis or secondary hemochromatosis can result in the deposition of iron deposits in the kidneys and, if severe, the formation of siderotic nodules.
Myoglobinuria • Coronal T1-weighted images in a patient with iron overload. Note the low signal renal parenchyma and loss of the corticomedullary distinction due to the very short longitudinal relaxation times of the iron in myoglobin. (b) postcontrast administration: note the medullary enhancement (a) precontrast
Acute Tubular Necrosis • Acute, reversible renal failure due to severe ischemia or toxin exposure • Clinical symptoms include oliguria (or anuria) and proteinuria • Both kidneys are smooth and globally enlarged due to interstitial edema
Acute Tubular Necrosis • Immediate contrast enhancement following an intravenous bolus injection of Gd-DTPA • Contrast enhancement persists (often longer than 24 hours), with little opacification of the pelvocalyceal system due to tubular blockage by debris A T1-weighted GE sequence depicts persistent contrast enhancement characteristic of ATN
Pyelonephritis • Acute Pyelonephritis • Characteristic features: renal enlargement,increased water content, fascial thickening, and striated wedge-shaped enhancement • T1: loss of corticomedullary distinction and dark striations • T2: high signal intensity perinephric fluid • Emphysematous Pyelonephritis:air within the renal parenchyma appears as tiny focal areas of markedly decreased signal on both T1- and T2- weighted images • Chronic Pyelonephritis: calyceal dilatation, focal loss, and cortical attenuation Xanthogranulomatous pyelonephritis- note low-signal intensity striations
Xanthogranulomatous Pyelonephritis • Chronic suppurative bacterial infection in the collecting system due to a partial obstruction • Calculi, especially staghorn calculi composed of struvite, are seen in the pelvis in 75% • Characteristic MR findings: • low-signal intensity striations • renal fascia thickening • scarred pelvis • dilated calyces • chronic inflammation • Extra-renal extension of the inflammatory process is common
Chronic inflammation of the renal parenchyma may form multiple yellow-colored masses due to the accumulation of lipid-laden macrophages (b, c). • The kidney is massively enlarged by multiple uniform masses of slightly lower attenuation than the surrounding parenchyma (a). c b Xanthogranulomatous Pyelonephritis a
post-contrast Xanthogranulomatous Pyelonephritis pre-contrast • Marked enhancement is seen surrounding the dilated calyces following IV contrast administration (caused by the inflammatory infiltrate). • (a)pre-contrast;(b, c, d)post-contrast a b c d