Kidneys

CHAPTER 18 Kidneys




CLINICAL CONSIDERATIONS


The kidneys play a central role in homeostasis. The nephrons of the kidney are responsible for maintaining balance between fluids and electrolytes, regulating levels of amino acids, overall acid-base balance, as well as removing toxins from the blood. The kidney also has endocrine functions, helping to control blood pressure, bone mineralization, and erythrocyte production. Although each kidney is about the size of a fist, the approximately one million nephrons per kidney require nearly 20% of the total cardiac output to perform this multitude of functions.



Laboratory Assessment of Renal Function



Serum Creatinine


Despite the complexity of renal physiology, many attempt to assess renal function with a simple quantitative measure, the serum creatinine. Creatinine is a breakdown product of creatine, found within muscle. Most serum creatinine is excreted in the urine; therefore, if renal function is compromised, levels of creatinine in the serum increase.


Serum creatinine, however, is dependent not only on its disposal but also its production. Because production of creatinine is affected by sex, age, muscle mass, protein intake, and liver function, the serum creatinine can be an inaccurate predictor of renal function, particularly in those at the extremes of age and body weight.


Perhaps of even greater importance, serum creatinine is not a sensitive test for minor insults to the kidney in otherwise healthy individuals. A healthy individual who donates a kidney is likely to maintain a normal serum creatinine despite the loss of 50% of parenchymal tissue. However, this individual is more likely to show a decline in renal function from an additional insult. This illustrates the amount of reserve function that must be compromised before renal injury can be detected with a serum creatinine level. Thus, any upward trend in the serum creatinine value should be viewed with concern when considering the administration of potentially nephrotoxic or renally excreted intravenous contrast media.




Renal function is better evaluated by measured creatinine clearance, which takes into account not only the amount of creatinine in the blood but also the amount of creatinine within a specified volume of urine over a given period.




RENAL ANATOMY



Renal Parenchyma


The kidneys can be divided into three main regions from cranial to caudal. Each end of the kidney is commonly called a pole. The portion of the kidney between the poles is called the interpolar region and contains the renal hilum (Fig. 18-1).



On axial sections, the polar regions of the kidney typically form a closed circle or “donut” shape, with the “hole” formed by renal sinus fat. The anteromedial aspect of the interpolar region is interrupted by the renal hilum to make a C shape. In this region, the anterior and posterior hilar lip is identified (Fig. 18-2). In most kidneys, the renal hilum faces more anteromedial in the upper half of the kidney and more directly medial in the lower half.



The solid renal parenchyma consists of the peripheral renal cortex and more central renal medulla. The nephrons within the cortex comprise some of the most highly perfused parenchymal tissue in the body. More tenuous vascular supply to the renal medulla makes it more susceptible to ischemia. Each segmental branch of the renal artery divides into multiple interlobar arteries that course along the periphery of the medullary pyramids and causes small interlobular branches (Fig. 18-3). Because the interlobular arteries form an arch overlying the pyramid, they are called the arcuate arteries. These terminal branches have no collateral circulation.




Renal Collecting System


Urine that is concentrated in the renal papilla is subsequently excreted into a lumen lined with transitional epithelium. The small portion of the lumen surrounding the papilla is called the calyx. The shape of the calyx is formed by the impression of the renal papilla. Increasing pressure within the lumen initially distends the fornices (acutely angled portions of the calyx along the sides of the papillae), whereas the central portion of the papillary impression is preserved. Chronic obstruction, however, results in damage to the papilla, evident in the “clubbed” calyx of papillary necrosis (Fig. 18-4).



A simple calyx receives urine from a single papilla; a compound calyx receives urine from multiple papillae (Fig. 18-5). Urine from the calyces flows to the renal sinus via tributaries called infundibula. Occasionally, a papilla will communicate directly with an infundibulum or the renal pelvis and is considered to be an aberrant papilla. Unlike other filling defects within the renal collecting system (e.g., tumor, stone, clot), an aberrant papilla usually has a small fornix around it, seen as a halo on conventional urography (Fig. 18-6). However, sometimes ureteroscopy is required to confirm the diagnosis in patients with hematuria.




Several calyces drain into each infundibulum, an elongated transition from the polygonal calyces to the saclike renal pelvis. The renal pelvis then tapers like a funnel to join the ureter. The region where the renal pelvis joins the ureter is called the ureteropelvic junction (UPJ).


If the renal pelvis is entirely within the confines of the renal sinus, it is considered intrarenal. If the renal pelvis extends out of the renal sinus, it is considered to be an extrarenal pelvis (Fig. 18-7). Because an extrarenal pelvis is not confined by the renal parenchyma, there is a tendency for it to expand. Although this dilatation of the renal pelvis may occasionally mimic hydronephrosis, delicate and sharply defined calyces and thin infundibula can be used to differentiate an extrarenal pelvis from obstruction.






Congenital Variants of Renal Anatomy




Abnormalities of Lobation


As the lobules of metanephric blastema coalesce to form each kidney, they do not always result in a smooth, uniform band of cortex. A junctional cortical line is a common septum of capsule typically seen on ultrasound as an echogenic line at the site of fusion between the superior pole and middle third of the kidney (Fig. 18-8). When multiple clefts in the renal cortex are present throughout the kidney, it is described as fetal lobulation. Fetal lobulation is best differentiated from renal scars during the corticomedullary phase of enhancement on computed tomography (CT) or magnetic resonance imaging (MRI) because cortex can be followed into the indentation that occurs between calyces (Fig. 18-9). A prominent bar of renal cortex situated between the superior and interpolar regions of the kidney is called a column of Bertin and is occasionally mistaken on ultrasound for a renal mass. Normal parenchymal enhancement on CT or MRI allows definitive characterization.








Preprocedure Anatomic Considerations



Evaluation of the Living Renal Donor


Living renal donor allografts account for more than half of the transplanted kidneys in the United States. Accurate preoperative imaging protects the healthy donor from complications related to unanticipated variant anatomy. Literature supports the use of either multidetector computed tomography (MDCT) or MRI in donor evaluation. A potential benefit of MRI is the lack of exposure to ionizing radiation, although unenhanced CT would still be required to detect stones (the presence of stones increases the donor’s risk for renal insufficiency later in life and could disqualify them as a donor candidate).


Imaging must provide detailed images of the renal parenchyma and a survey of arterial, venous, and ureteral anatomy. Table 18-1 provides a quick guide itemizing key imaging findings in the potential renal donor.


Table 18-1 Imaging the Living Renal Donor





















Inspected Areas Comments
Stones Always include unenhanced computed tomographic images to look for renal stones.
Renal arteries



Leftrenal vein




Right renal vein Note number of veins by inspecting inferior vena cava along entire length of kidney.
Ureters Look for duplication, large extrarenal pelvis.







Crossing Vessels in Ureteropelvic Junction Obstruction


Conventional surgery for congenital UPJ obstruction involves an open pyeloplasty, in which some tissue is removed from the wall of the saclike renal pelvis to form a more tapered, efficient, funnel-shaped renal pelvis. Some forms of congenital UPJ obstruction are now treated with transureteroscopic endopyelotomy in which an incision is made from within the ureter using a ureteroscope. Because the fascia of the retroperitoneum prevents significant extravasation, the incision usually heals to form a larger lumen. If, however, a vessel crosses the UPJ at the level of obstruction, a blind incision made from the inside of the ureteral lumen can result in severe hemorrhage. Made aware of such a vessel, the urologist may choose to perform an alternate procedure to avoid hemorrhagic complications.


Recent advances in MDCT and MRI permit cross-sectional vascular studies to replace conventional angiography before UPJ repair (Fig. 18-18). Box 18-2 provides some tips regarding crossing vessels in UPJ obstruction.





NORMAL IMAGING APPEARANCE OF THE KIDNEYS



Ultrasound Appearance of the Kidneys


Ultrasound permits real-time optimization of imaging relative to the axis of each kidney. In most cases, the kidneys are situated with the inferior poles slightly more lateral and anterior than the superior poles. Each kidney should always be evaluated in long axis (coronal, sagittal, or both, depending on sonographic window) and axial to the kidney.


The cortex of a normal kidney is usually less echogenic than the adjacent normal liver. When the renal cortex is more echogenic than the adjacent liver, there is a high correlation with renal disease, although sensitivity is relatively low, according to Platt and colleagues (Fig. 18-19). When echogenicity of the renal cortex equals that of the liver, renal function is abnormal in approximately 38% of cases. In clinical practice, it is probably best to categorize the renal cortex as hypoechoic, isoechoic, or hyperechoic compared with normal liver, and then state a correlative risk for associated renal parenchymal disease (Table 18-2).



Table 18-2 Association between Renal Cortical Echogenicity and Renal Parenchymal Disease















Echogenicity of Renal Cortex * Prevalence of Renal Disease
Hypoechoic Low
Isoechoic Moderate
Hyperechoic High

* Echogenicity as compared with liver.


Renal size can be measured in several ways. Calculation of the estimated renal volume is considered by some to be the most accurate assessment of renal size available with ultrasound, although renal length alone is more commonly reported. Most radiologists consider 10 to 12 cm to be an approximate reference range for renal length in adults, allowing for an additional 1 cm in either direction for patients at the extremes of height. Size disparity greater than 1.5 cm between kidneys should raise suspicion that one kidney is abnormal.



Computed Tomographic Appearance of the Kidneys


The presence or absence of intravenous contrast media, as well as the phase of contrast enhancement, are key factors that determine the appearance of the renal parenchyma on CT (Table 18-3).



The corticomedullary phase is prolonged in the presence of ureteral or venous obstruction and can persist for days in cases of acute tubular necrosis (ATN; Fig. 18-20). The vascularity of some tumors may be most apparent during this phase (Fig. 18-21). However, small, low-attenuation lesions in the medulla are often obscured during this phase. The uniform high attenuation of the nephrographic phase provides an optimal background for detecting small, low-attenuation lesions in the renal parenchyma (Fig. 18-22).





The visible contrast seen in the excretory phase has been concentrated many-fold. In fact, evaluation of the renal collecting system during the excretory phase often requires window and level settings approaching those used for evaluating the osseous structures (Fig. 18-23). Some centers use diuretics or fluid bolus, or both, during CT urography to dilute the excreted contrast to improve assessment of the urothelium.



Some divide the excretory phase into the early excretory phase (contrast mainly confined to the kidney) and late excretory phase (contrast in the ureters). The early excretory phase begins as early as 120 seconds after injection. Although ureteral contrast media is typically present before 3 minutes, longer delays provide more predictable opacification.




IMAGING EVALUATION FOR RENAL FAILURE


The goal of imaging patients with renal failure is to identify a correctable cause in an effort to recover or preserve renal function.




Ultrasound Evaluation for Renal Failure


Ultrasound is usually used in the initial evaluation of the patient with newly diagnosed renal failure. Even when there is another plausible explanation for decreased renal function (e.g., known prerenal causes), ultrasound offers the opportunity to rapidly and noninvasively identify a potentially correctible cause of renal failure. Table 18-5 summarizes a checklist approach to the ultrasound examination.


Table 18-5 Checklist Approach to Ultrasound for Renal Failure





















Ultrasound Finding Comments
Hydronephrosis

Kidney size

Cortical echogenicity Increased echogenicity has high association with parenchymal disease
Resistive index (RI) Acute tubular necrosis usually results in an increased RI, whereas prerenal causes usually do not have an increased RI; postrenal causes often increase the RI, but hydronephrosis should be present in those cases
Bladder distension If present, suspect neurogenic bladder or outlet obstruction






Magnetic Resonance Evaluation for Renal Failure


Unenhanced MRI can also be used to diagnose obstruction and identify the source (Fig. 18-27). MR angiography can be useful for the diagnosis of renal vascular abnormalities. Use of MR contrast agents in renal failure poses a lower risk than iodinated contrast material for exacerbating renal failure, but there is evidence that gadolinium-based MR contrast media pose some risk for systemic complications (nephrogenic systemic fibrosis) and should be used with caution in patients with severe or acute renal insufficiency. Static-fluid (T2-weighted) MR urography and phase-contrast MR angiography are useful techniques that do not require intravenous contrast material.




Nuclear Scintigraphy for Renal Failure


Renal scintigraphy can be performed with a variety of agents to provide assessment of either function or structure of the kidneys. Advantages of scintigraphy include accurate quantitative measurement of function and parenchymal mass without the risks for nephrotoxicity associated with iodinated contrast media or nephrogenic systemic fibrosis associated with gadolinium contrast agents.


Technetium 99m-mercaptoacetyltriglycin (MAG3) is excreted by the kidneys (mainly through secretion by proximal tubules) and provides evaluation of renal function, particularly in cases of suspected obstruction. Because repeat imaging does not expose the patient to additional radiation, multiple phases including delayed images may be obtained and allow the creation of quantitative curves that define the initial filling and then clearing of dilated collecting system structures. This method is the standard in evaluation of UPJ obstruction and often is used for other types of chronic obstruction. The dynamics of obstruction and quantification of relative renal function between the two kidneys may be important considerations in two general circumstances: (1) it is unclear whether obstruction is severe enough to warrant surgical intervention; or (2) significant parenchymal atrophy exists, and the relative merits of repair and nephrectomy are being compared. A furosemide challenge is often administered after initial excretion is observed to measure the impact of diuresis on the clearance of radiotracer from the renal pelvis.


Technetium 99m dimercaptosuccinic acid (DMSA) and glucoheptonate (GHA) are both used for evaluation of renal parenchyma. Despite different methods of accumulation, each is sequestered by the renal cortex, providing an opportunity to quantify the volume of renal parenchymal tissue in each kidney. The most common indication for cortical scintigraphy is to evaluate kidneys that have been injured by vesicoureteral reflux, chronic obstruction, or severe or repeated urinary infections. Poorly functioning kidneys with little residual parenchymal volume may be removed because preservation offers opportunities for future complications (infection, hypertension) without contributing significantly to renal function. The presence of significant renal parenchyma may justify surgical repair to maximize the functional contribution of that kidney.



DIFFUSE ABNORMALITIES OF THE KIDNEY



Size and Contour of Diffuse Renal Disease




Unilateral Small Smooth Kidney


Global insult to one kidney may result in unilateral atrophy that is uniform and smooth. Table 18-7 lists causes of unilateral smooth renal atrophy. The most common cause is renal artery stenosis (see Fig. 18-25). CT and MR findings of renal artery stenosis parallel classic findings described on intravenous pyelogram, including one atrophic kidney with delayed nephrogram and excretion that can progress to a persistent nephrogram with hyperconcentrated excreted contrast media (Fig. 18-28). When the fine, weblike complex of ureteral arteries is recruited to contribute to collateral circulation, enlarged vessels are seen surrounding the proximal ureter, causing the classic “ureteral notching” seen on intravenous urogram (IVU).


Table 18-7 Causes of Unilateral Small Smooth Kidney









Cause Comments
Renal artery stenosis

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Mar 6, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Kidneys

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