Laboratory Assessment of Renal Function

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).

Figure 18-1 Annotated three-dimensional volume rendering of the left kidney acquired using a combined nephrographic phase and excretory phase during computed tomographic urography demonstrates regional anatomy of the kidney.

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.

Figure 18-2 Annotated axial image of the right kidney from a contrast-enhanced computed tomographic scan demonstrates hilar anatomy of the kidney.

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.

Figure 18-3 Illustration demonstrating basic intrarenal arterial anatomy.

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).

Figure 18-4 Illustration demonstrating the relation between the renal papilla and calyx. The normal appearance of the calyx is created by the impression of the renal papilla. A, Tips of the fornices are sharply defined. B, Mild hydronephrosis results in rounding of the fornices with mild shortening of the papillary impression. C, More severe hydronephrosis results in more pronounced shortening of the papilla. D, If pressure on the papilla persists, the ischemic papilla undergoes necrosis, allowing the calyx to protrude outward toward the cortex.

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.

Figure 18-5 Illustration demonstrating the anatomy of the renal collecting system.

Figure 18-6 Frontal image of the left kidney from an intravenous urogram demonstrating an aberrant renal papilla in the lower pole infundibulum.

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.

Figure 18-7 Axial image of the left kidney from a contrast-enhanced computed tomographic scan demonstrates an extrarenal pelvis. The ureter and calyces were not dilated (not shown), helping to differentiate this anatomic variant 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.

Figure 18-8 Junctional cortical line seen on a long-axis ultrasound image of the right kidney.

Figure 18-9 Coronal computed tomographic image in the corticomedullary phase shows normal corticomedullary differentiation along the lobulated contour, consistent with fetal lobulation.

Renal Ectopia

The upper pelvis is the most common ectopic location for the kidney; most cases are also associated with abnormalities of rotation. Extraaortic origin of the renal arteries and accessory renal arteries are common. When both kidneys are on the same side crossed ectopia is present, because the ureter from one kidney must cross the midline to insert into the bladder (Fig. 18-10). Crossed ectopia can be either fused or unfused. When fused, the condition is described as crossed fused ectopia. The fused kidneys can have a variety of orientations, including side by side, in-line, or perpendicular. Thoracic kidneys are the least common form of renal ectopia.

Figure 18-10 Crossed ectopia on intravenous pyelogram. The patient had right flank pain but had a solitary calcification in the left pelvis on plain radiograph (not shown). Retrograde urogram shows a calculus in the left ureter. Right-sided pain was related to crossed renal ectopia.

Abnormalities of Fusion

Horseshoe kidneys result from midline fusion of the kidneys, typically at the level of the origin of the inferior mesenteric artery. The isthmus connecting the kidneys is variable, ranging from normal renal cortex to a thin fibrous band. Blood supply is variable and often includes extraaortic and multiple vessel origins. The axes of the renal moeities are abnormal with the inferior “poles” angled medially. Prominent extrarenal pelves are typically positioned anteriorly (Fig. 18-11). Pancake kidney describes a more severe fusion anomaly with a single, flat kidney positioned low in the pelvis with an anterior collecting system drained by either one or two ureters.

Figure 18-11 Three-dimensional volume rendering from contrast-enhanced multidetector computed tomography examination of the kidneys demonstrates typical orientation of a horseshoe kidney.

Ureteral Duplication

Duplication of the urinary tract is discussed in detail in Chapter 19. Duplication affects the axial appearance of the kidneys by dividing the renal sinus into superior and inferior components, separated by a circumferential band of cortex in the central region (Fig. 18-12).

Figure 18-12 Axial sections of the right kidney from contrast-enhanced computed tomography demonstrate a bar of renal parenchyma separating renal hila in the superior and inferior poles, consistent with duplication.

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	Look carefully for accessory arteries at upper and lower poles (Fig. 18-13). Note diameter of arteries because small accessory arteries may be sacrificed in many cases. Note distance from origin to the first arterial division (Fig. 18-14). Identify abnormal course of main or accessory right renal artery anterior rather than posterior to inferior vena cava (Fig. 18-15).
Leftrenal vein	Look for retroaortic or circumaortic left renal vein. Note that retroaortic components are usually near the inferior poles (Fig. 18-16). Anterior components of circumaortic vein can be small. Note large lumbar veins (Fig. 18-17). Note multiple or large gonadal veins.
Right renal vein	Note number of veins by inspecting inferior vena cava along entire length of kidney.
Ureters	Look for duplication, large extrarenal pelvis.

Figure 18-13 Volume rendering from a computed tomographic scan of the kidneys shows bilateral supernumerary renal arteries (three on right, two on left). Note origin of inferior accessories near inferior poles on each side.

Figure 18-14 Axial maximum intensity projection image from the arterial phase of a contrast-enhanced computed tomographic scan from a prospective renal donor demonstrates early prehilar branching of the left renal artery. In this case, the right kidney had more favorable anatomy for laparoscopic donor nephrectomy.

Figure 18-15 Relation between the right renal artery and the inferior vena cava (IVC). A, Axial image from contrast-enhanced computed tomography (CT) demonstrates an accessory right renal artery coursing anterior to the IVC. B, Axial CT image from a different patient demonstrates the more common location of the right renal artery posterior to the IVC.

Figure 18-16 Coronal reformation from contrast-enhanced computed tomography performed for renal donation demonstrates a retroaortic left renal vein crossing the aorta well inferior to the level of the renal hila. IVC, Inferior vena cava.

Figure 18-17 Coronal maximum intensity projection image from a contrast-enhanced computed tomographic scan demonstrates a dilated and tortuous lumbar veins joining the left renal vein.

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.

Figure 18-18 Single-detector computed tomographic images from ureteropelvic junction deformity in the right side of a horseshoe kidney. A, Axial image demonstrates the dilated renal pelvis and crossing vessel. B, A curved planar reformation of the crossing vein demonstrates its course. Surgery was successful and the surgeon confirmed the anatomic survey was correct. IVC, Inferior vena cava.

BOX 18-2 Crossing Vessels in Ureteropelvic Junction Obstruction

1. Crossing vessels may be either arterial or venous, so remember to take a close look at both vascular phases.

2. The term crossing vessel does not necessarily imply a causative relation. In fact, some crossing vessels are near but not precisely at the level of obstruction. Any vessels adjacent to a site of potential endopyelotomy pose a potential hazard and should be reported.

3. Crossing vessels may be either anterior or posterior to the ureter, although anterior is more common.

4. The relation between the UPJ and vascular anatomy can be quite complex. Best results are achieved if the radiologist can review the examination directly on a three-dimensional workstation.

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).

Figure 18-19 Sagittal ultrasound image of the right kidney demonstrates increased size and echogenicity of the kidney, findings typical of human immunodeficiency virus nephropathy. This kidney measured 14 cm in length. The left kidney (not shown) had a similar appearance.

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

* 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).

Table 18-3 Utility of Different Phases of Renal Contrast Enhancement

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).

Figure 18-20 Axial image from unenhanced computed tomography of the kidneys performed 2 days after an angiographic procedure demonstrates stasis of contrast in the renal cortex, resulting in a persistent corticomedullary phase of enhancement. Note that there is no contrast in the aorta.

Figure 18-21 Axial images from contrast-enhanced computed tomography demonstrate transient enhancement of a small renal cell carcinoma. A, Enhancement of the mass is conspicuous in the corticomedullary phase. B, Low-attenuation lesion in the late nephrographic/early excretory phase is less suspicious in appearance.

Figure 18-22 Axial images of the left kidney from a three-phase renal computed tomographic scan demonstrate improved conspicuity of low-attenuation lesions of the renal medulla during the nephrographic phase. A, A low-attenuation lesion is difficult to identify during the corticomedullary phase. B, The lesion becomes more conspicuous during the nephrographic phase.

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.

Figure 18-23 Axial image of the left kidney obtained in the excretory phase of a computed tomographic urogram demonstrates the effects of window settings on visualizing structures near excreted contrast. A, Soft-tissue windows demonstrate no filling defect. B, A small calyceal defect is seen when the same image is viewed using bone windows. The defect proved to be blood clot from papillary necrosis.

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.

Magnetic Resonance Imaging

The renal cortex and medulla both have high signal intensity on T2-weighted images resulting in poor corticomedullary differentiation. However, T1-weighted images provide good corticomedullary differentiation. Calcifications and renal calculi are notoriously poorly demonstrated with MRI. The phases of nephrogram development and contrast excretion parallel those seen on contrast-enhanced CT with one notable exception (Fig. 18-24). Unlike the excretory phase of enhanced CT, signal intensity within the renal collecting systems is reduced on T1- and T2-weighted MR images once excreted gadolinium-based contrast media becomes sufficiently concentrated. This phenomenon is due to T2-shortening and susceptibility (T2*) effects caused by concentrated gadolinium, and can potentially obscure filling defects and urothelial lesions.

Figure 18-24 Normal magnetic resonance imaging appearance of the kidneys. A, Steady-state free precession, (B) T2-weighted with fat saturation, (C) T1-weighted, (D) T1-weighted with fat saturation, (E) postcontrast corticomedullary phase, and (F) postcontrast nephrographic phase.

Types and Causes of Renal Failure

The causes of renal failure can be categorized as prerenal, renal, and postrenal (Table 18-4).

Table 18-4 Causes of Renal Failure

* Entities for which sonography is most useful.

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

Increased Echogenicity

Increased cortical echogenicity is associated with many forms of chronic renal parenchymal disease and indicates a renal cause for renal failure. When abnormal echogenicity is detected, it is important to note whether it is unilateral or bilateral. Chronic glomerulonephritis usually causes bilateral increased renal echogenicity with smooth atrophy, whereas renal artery stenosis usually causes a similar but unilateral appearance (Fig. 18-25). Bilateral echogenic kidneys with renal hypertrophy can be seen associated with human immunodeficiency virus disease (see Fig. 18-19). The presence of contour irregularity usually indicates scarring, suggesting prior infection, reflux, or infarction. An acute change in renal cortical echogenicity is occasionally seen with pyelonehritis.

Figure 18-25 Sagittal ultrasound image of the left kidney demonstrates a unilateral small, smooth, echogenic kidney in a patient with renal artery stenosis. The right kidney has a normal appearance (not shown). The left kidney measured 7.8 cm, and the right kidney measured 10.9 cm. Arterial stenosis was confirmed by magnetic resonance angiography. If this appearance were present bilaterally, chronic renal disease such as chronic glomerulonephritis would be a more likely explanation.

Hydronephrosis

Hydronephrosis is important to detect, because obstructive uropathy is often reversible if identified early. It is important to note, however, that the appearance of hydronephrosis does not necessarily indicate urinary obstruction (see Hydronephrosis and Its Mimics section later in this chapter).

For most people, obstruction of a single ureter does not induce renal failure. Obstruction can cause renal failure if it is bilateral (Box 18-3) or if there is preexisting disease in the unobstructed kidney. Ultrasound can often identify the cause in cases of bilateral obstruction (Fig. 18-26). In cases of unilateral obstruction with acute renal failure, sonographic evaluation may show evidence of chronic renal parenchymal disease in the unobstructed kidney.

BOX 18-3 Causes of Bilateral Hydronephrosis

Neurogenic bladder

Urethral stricture/valves

Bladder mass at trigone

Benign prostatic hyperplasia

Pelvic mass (cervical, rectal, uterine, prostate cancer)

Cystocele or uterine prolapse

Retroperitoneal fibrosis

Bilateral ureteral stones

Figure 18-26 Ultrasound performed for acute renal failure demonstrates bilateral hydronephrosis caused by a bladder tumor. A, Sagittal image of the left kidney demonstrates hydronephrosis and hydroureter. The right kidney had a similar appearance (not shown). B, Transverse image of the bladder demonstrates a large bladder tumor in the region of the trigone.

Computed Tomographic Evaluation for Renal Failure

CT is occasionally used to evaluate patients with renal failure. In most cases, unenhanced CT is performed when the duration and cause of renal failure are unknown because exposure to iodinated contrast media could impair recovery of renal function. Unenhanced CT can identify hydronephrosis and hydroureter, urinary stones, and some masses. Renal size and cortical thickness can be assessed in a manner similar to ultrasound. CT angiography is occasionally performed when a vascular causative factor is suspected (renal artery stenosis or renal vein thrombosis) and MRI is contraindicated.

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.

Figure 18-27 T2-weighted maximum intensity projection image from a magnetic resonance urogram performed to evaluate urinary obstruction identified in a patient with an obstructing soft tissue mass in the pelvis on unenhanced computed tomography (CT). The patient had acute renal failure; therefore, contrast-enhanced CT was not performed.

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.

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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