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Figure 2 Normal postcontrast T1-weighted MR urogram. (A) A series of six coronal post-contrast T1-weighted images during the cortical, excretory, and delayed phases show normal cortical enhancement and excretion bilaterally. Urinary bladder appears normal. (B) A series of six coronal MIP images rotated through 360° show the normal appearance of both kidneys and the bladder. Abbreviations: MIP, maximum intensity projection; MR, magnetic resonance.

and T2-weighted images are acquired just after furosemide administration (8,27). Gadolinium is then injected 15 minutes after the furosemide, and dynamic imaging commences immediately. A high-resolution volumetric sequence is then obtained with the patient prone, to promote mixing of the excreted contrast material with urine as well as drainage of contrast material into dilated collecting systems (if dilatation is present). The F-15 technique dramatically shortens the imaging time. It also eliminates the appearance of any gadolinium-related magnetic susceptibility artifacts. In addition, the F-15 protocol makes it possible to quantitatively trace the passage of contrast material from its appearance in the calyces to the renal pelvis and into the ureter. In this fashion, the diagnosis of obstruction can be based on functional asymmetry in excretion rather than solely on such morphologic abnormalities as pelvocaliectasis, ureterectasis, or persistent ureteral narrowing (8,27). Also, differential renal function can be estimated based on the volume of enhancing renal par-enchymal volume during the corticomedullary phase prior to appearance of contrast material in the collecting systems. In the future, using assessments of differential function that are based on time-activity analysis of corticomedullary gadolinium transit will likely be more precise than those that rely on morphologic assessments of functioning renal parenchymal volume alone (8,27).

MRU performed without gadolinium enhancement using heavily T2-weighted sequences generates images in which static fluid in the renal collecting systems has a very high signal intensity (4,8,9,14-16,22-29). On these images, signal intensity in the renal parenchyma is relatively suppressed because it normally has a shorter T2 relaxation time. One of the most widely used T2-weighted urographic techniques is performing a series of single-shot, fast spin-echo sequences with half-Fourier acquisitions, with reconstruction of multiple thin slice images or a single thick section image. Steady-state free precession sequences are also becoming more popular in adults.

T2-weighted MRU provides excellent visualization of the urinary tract in patients with dilated collecting systems and ureters (22-24,28) and is, therefore, usually able to identify the precise site of any obstruction when such dilatation is present. T2-weighted MRU is also useful for evaluating patients with impaired renal function because imaging is not dependent upon excretion of gadolinium by the kidney. When used in conjunction with contrast-enhanced imaging, ureteric anatomy can be delineated in almost every case, even when there are nonfunctioning systems. The most significant limitations of this technique are its inability to consistently demonstrate nondilated systems and its inability to provide any direct functional data. Superimposition of fluid-filled extra-urinary structures, such as bowel loops and the gallbladder, can also be a problem. Some investigators have suggested that visualization of nondilated collecting systems and ureters can be improved with the administration of furosemide (15,16). It must be remembered that visualization of urinary calculi is also limited on MRI, because these appear as filling defects or signal voids on both conventional MRI and on MRU techniques (14,23,24,26,28).

MR Angiography

The intrinsic sensitivity of MRI to motion allows for the depiction of vessels and for the assessment of flow both quantitatively and qualitatively when several specific sequences are employed (6,20,25,30). Ironically, vascular flow phenomena were initially considered to represent undesirable artifacts, and considerable effort was aimed at eliminating them. Subsequently, the diagnostic importance of these flow-related signal changes was realized and rapidly led to the development of MR angiographic imaging protocols that permitted direct imaging of vascular morphology and quantification of flow (even in the absence of contrast material administration). Flow-sensitive techniques, such as phased contrast MR angiography (MRA) and time-of-flight MRA, were developed, although both techniques do have some limitations, including artifactual signal dropout caused by turbulent blood flow, degradation of image quality secondary to respiratory motion, and saturation of in-plane blood flow. These effects limit the visualization of small vascular structures, which can particularly be a problem in infants and smaller children. 3-D gadolinium (3D-Gd) MRA overcomes the limitations by acquiring a 3-D dataset within a single breath hold. Continuing improvements in 3D-Gd MRA will likely reduce the necessity of utilizing imaging modalities that require intravascular administration of potentially nephrotoxic iodinated contrast material, such as CT angiography or conventional catheter angiography (25,30).

CONGENITAL ANOMALIES OF THE KIDNEYS AND URETERS Variations in Renal Shape

Nonpathological variations in renal shape can be developmental or can result from the compression or displacement of the kidney by adjacent normal or abnormal structures (13,31). The spleen can flatten the upper pole of the left kidney and produce a prominent lateral renal bulge, commonly referred to as a dromedary hump. In patients with splenomegaly, the displacement and compression of the left kidney can be quite pronounced, but the resulting renal deformity should not be mistaken for a renal mass.

Persistent fetal lobation is a common developmental variant in renal shape that should be readily recognized and should not be confused either with a renal mass or with cortical scarring. The indentations in the renal contour due to fetal lobation lie between the medullary rays and occur throughout the kidney, in contrast to cortical scars due to chronic atrophic pyelonephritis, where the areas of parenchymal loss directly overlie the pyramids and also have a strong polar predilection (32).

The junctional cortical defect is a common sonographic finding on the lateral surface of the upper third of the kidney, usually extending inferomedially for a variable distance into the renal sinus (33-35). It is also occasionally visible on other modalities and should not be mistaken for a cortical scar or infarct. This common normal defect represents the anterior site of fusion of the superior and inferior reniculi, the two primitive nephrogenic masses that combine to form the kidney.

Anomalies of Renal Position, Rotation, and Fusion

During nephrogenesis, the developing kidneys normally ascend from the pelvis to the retroperitoneum until they reach their normal positions in the flanks. As each kidney ascends, it rotates along its sagittal axis such that the renal hilum is redirected ante-romedially where the advancing ureteral bud invaginates into the developing renal sinus. Renal ascent is normally completed by the ninth gestational week. When a kidney fails to ascend normally, in addition to being abnormal in location, it is also frequently malrotated and the renal sinus is poorly developed (36,37).

The sagittal renal axis in the supine patient is normally at an approximately 30° angle relative to the long axis of the body, with the lower poles located anterior to the upper poles. On either a frontal radiograph from an intravenous urogram or on a coronal image from an MRU, lines representing the longitudinal axes of the kidneys should intersect superiorly at approximately the level of the 10th to 11th thoracic vertebra.

There are a wide range of anomalies of renal rotation, from nonrotation (in which the renal pelvis is located anteriorly) to incomplete rotation (in which the renal pelvis is located between 30° and 90° from horizontal), to reverse rotation (in which the renal pelvis is located laterally), to transverse rotation (in which the renal pelvis is located superiorly or inferiorly) (36-39). Whereas ectopic kidneys are frequently malrotated, otherwise normally positioned kidneys can also occasionally be malro-tated. Whereas the position and orientation of the kidney are usually readily apparent on MRI, differentiation between anterior and posterior malrotation and between superior and inferior transverse malrotation depends on the identification of the courses of the main renal vessels as they emerge from the renal hilum and continue along the external surface of the kidney to reach the aorta and IVC.

Renal ectopia occurs in 1 in 500 to 1200 individuals (13,36,37). Most ectopic kidneys lie caudal to the normal renal fossa. Although all caudally ectopic kidneys are often referred to generically as pelvic kidneys, the actual degree of ectopia is quite variable. It is generally accepted that ptotic kidneys are located only slightly caudal to the normal renal position, lumbar kidneys located somewhat more caudally, but still intra-abdominal, and sacral kidneys located completely caudal to the lumbosa-cral junction. In patients who have anterior abdominal wall defects, such as gastro-schisis or omphalocele, the kidneys can be located cephalad to the normal renal fossa, just beneath the diaphragm. In patients with congenital diaphragmatic hernia, the ipsilateral kidney rarely is intrathoracic (13,37,40,41). Ectopic kidneys typically have an anomalous arterial supply related to their final position at the point of arrest of normal renal ascent (13,36,37,42,43). Horseshoe, crossed ectopic, and pelvic kidneys typically receive their blood supply from renal arteries originating from the midabdominal aorta and also from the distal abdominal aorta, iliac vessels, or even branches of the inferior mesenteric artery. Multiple renal arteries are frequently present in these patients.

Horseshoe kidney is the most common renal fusion anomaly, with an incidence of 1 in 400 to 1800 individuals (Fig. 3) (44,45). In nearly all cases, the kidneys lie on either side of the midline, and their lower poles are joined by an isthmus composed of renal parenchyma and fibrous tissue in variable proportions. Upper pole fusion is exceedingly rare. The horseshoe kidney is typically located somewhat more caudally than normal, with the isthmus passing between the inferior mesenteric artery and the aortic bifurcation. The renal axes are reversed, with the lower poles being located more medially than the upper poles. Also, the kidneys are anteriorly malrotated (13,31). As a result, the pelves and ureteropelvic junctions (UPJs) are located anteriorly, and the ureters are displaced laterally by the isthmus. UPJ obstruction occurs in 30% of patients with horseshoe kidney and can be caused by intrinsic stenosis, by a high ureteral insertion, or by extrinsic compression by the isthmus or an anomalous vessel (46).

Crossed renal ectopia represents a spectrum of anomalies of renal ascent in which the ectopic kidney is located both caudal and medial to its normal location, either overlying the midline or completely on the opposite side of the abdomen (13,36,37,43,47,48). Usually the ectopic kidney is fused to its contralateral mate, a condition referred to as "crossed-fused renal ectopia.'' Crossed ectopia without fusion accounts for less than 10% of cases. Solitary crossed renal ectopia, i.e., crossed

Renal Ascent Caudal Growth

ectopia with contralateral renal agenesis and bilateral crossed renal ectopia, are very rare. In all cases of crossed renal ectopia, whether fused or unfused, the ureter draining the ectopic kidney crosses the midline to insert on its proper side on the trigone of the bladder.

Variations in the extent of renal fusion, as well as in the timing of the fusion in relation to renal ascent and rotation, result in a wide spectrum of potential anatomic configurations in crossed renal ectopia (13,36,37,47). The most common form is the unilateral fused type, in which the upper pole of the ectopic kidney is fused to the lower pole of its normally or nearly normally positioned contralateral mate. In another common configuration, the ectopic kidney assumes a transverse, inferior position, spanning the midline, with its upper pole fused to the medial aspect of the lower pole of the more vertically oriented contralateral kidney resulting in an "L-shaped kidney.'' Occasionally, the extensive fusion of the kidneys in the midline results in a single, amorphous renal structure, usually located in the lower abdomen or pelvis, referred to variably in the literature as a "lump," "cake," or "disk" kidney (Fig. 4). Although crossed renal ectopia is reported to be less common than horseshoe kidney, crossed ectopia and horseshoe kidney very likely represent a continuum of anomalies in renal ascent, position, and fusion rather than being truly distinct entities.

In children with myelomeningocele and severe thoracolumbar kyphoscoliosis, the lower poles of the kidneys can be oriented more medially, thereby falsely simulating a horseshoe kidney. A mistaken diagnosis of horseshoe kidney is thus most likely in children with caudal regression syndrome, in which the two kidneys lie directly apposed to one another, mimicking the appearance of a single, fused, midline kidney. This problem was originally described on intravenous urography (49); however, the appearance can also be confusing when other imaging modalities are used, including renal scintigraphy, CT, or even potentially MRI (13,31,50).

Patients with ectopic, malrotated, or fused kidneys are usually asymptomatic. However, renal ectopia is frequently associated with other congenital urinary tract malformations, including duplication anomalies, vesicoureteral reflux, multicystic dysplastic kidney, and UPJ obstruction (36,37). Symptoms, when present, are usually related to collecting-system dilatation, infection, or urolithiasis. Renal ecto-pia, renal agenesis, and multicystic dysplastic kidney are present in half of patients with cloacal malformation and are also the most common renal malformations that are associated with the VACTERL [vertebral anomalies, anal atresia, cardiac defects, tracheoesophageal fistulae, renal abnormalities, limb abnormalities (usually radial dysplasia)] anomalad (13,37,51). Anomalies of renal position and fusion, including horseshoe kidney, crossed renal ectopia, and pelvic kidney, are also the most common renal anomalies in Turner syndrome (52).

Figure 3 (Figure on facing page) A two-year-old with horseshoe kidney. (A) Coronal post-contrast T1-weighted images during the cortical phase (left) and excretion phase (right) show the typical morphology of a horseshoe kidney with abnormally vertical renal axes and a well-defined isthmus extending between the two lower poles. Note that the isthmus overlies the aortic bifurcation and proximal common iliac vessels. (B) Coronal 3-D postcontrast T1-weighted MIP image showing the typical morphology of a horseshoe kidney with abnormally vertical renal axes and a well-defined isthmus extending between the two lower poles. Note that the collecting systems are rotated laterally and the proximal ureters overlie the lower poles of the kidneys. Abbreviation: MIP, maximum intensity projection.

Figure 4 Infant with crossed fused renal ectopia in the pelvis (lump kidney). Coronal postcontrast T1-weighted images during the cortical phase (left) and excretion phase (right) show both kidneys fused, with a single renal structure lying in the right pelvis and separate right and left ureters, each inserting normally into the trigone on their respective sides. Note the vascular supply to the right kidney arises from the origin of the right common iliac artery and the vascular supply to the left kidney from the left common iliac artery.

Figure 4 Infant with crossed fused renal ectopia in the pelvis (lump kidney). Coronal postcontrast T1-weighted images during the cortical phase (left) and excretion phase (right) show both kidneys fused, with a single renal structure lying in the right pelvis and separate right and left ureters, each inserting normally into the trigone on their respective sides. Note the vascular supply to the right kidney arises from the origin of the right common iliac artery and the vascular supply to the left kidney from the left common iliac artery.

Renal Agenesis

Bilateral renal agenesis is a rare, lethal condition that is typically associated with oligohydramnios, pulmonary hypoplasia, and musculoskeletal and facial deformation anomalies secondary to severe intrauterine fetal compression—i.e., Potter syndrome (36,37,53,54). The patient is characteristically anuric and rapidly develops clinical and biochemical characteristics consistent with severe renal insufficiency. Diagnosis is usually readily established based upon portable ultrasonographic evaluation and confirmed with renal scintigraphy (13,31,54). Renal scintigraphy can confirm the absence of functioning renal parenchyma, although this is usually unnecessary when the diagnosis is straightforward based on clinical and sono-graphic features. Affected newborns usually die of respiratory failure within a few hours or days following birth, due to associated pulmonary hypoplasia. Given the invariably lethal nature of this disorder and the availability of bedside ultrasound (US) examinations, MRI plays no role in the diagnosis of this condition in the newborn (13,31).

Unilateral renal agenesis, on the other hand, is by comparison relatively common (1 in 1000 live births) (Fig. 5) (13,31,37,54,55). In this disorder, only a solitary functioning kidney is identifiable on all imaging studies. Unilateral renal agenesis is likely not a single disorder, because it has become clear that a number of embryo-logical abnormalities can result in unilateral absence of a kidney. Unilateral renal

Figure 5 Infant with left renal agenesis. A series of six coronal T2-weighted images show no evidence of a left kidney anywhere in the abdomen or pelvis. The left renal artery is also not identified. The right kidney appears normal.

agenesis can result from a spectrum of embryological defects during nephrogenesis as well as from vascular insults to the developing fetal kidney (37). Although multi-cystic dysplastic kidney (MCDK) is discussed in greater detail in section "Renal Dysplasia and MCDK,'' it is important to mention in the current context that involution of MCDK can be indistinguishable from unilateral renal agenesis in the absence of earlier documentation of a cystic dysplastic kidney (13,31,54,55).

Because there is no function on the affected side in patients with unilateral renal agenesis and MCDK, scintigraphy is not helpful in distinguishing between these entities, and there is probably considerable overlap in diagnosis as a result. Occasionally, MRI can be used to detect a small, poorly functioning or nonfunction-ing kidney in a child who appears to have unilateral agenesis on US and renal scinti-graphy (13,31). In such cases, however, the reason for the atrophic appearance and poor function of the kidney frequently cannot be established through imaging in the absence of associated known lower genitourinary anomalies, such as an ectopic ureter (that might suggest MCDK) or a history of a previous severe vascular renal insult. Additionally, severe congenital or infantile renal artery stenosis can have a similar imaging appearance.

MRI in patients with an absent kidney may not be justifiable in the absence of any symptoms. On the other hand, in a child who presents with a history of an "absent kidney'' and incontinence or hypertension, MRI can be invaluable in

identifying a tiny renal remnant. Nephrectomy in these symptomatic patients can, in some instances, lead to resolution of incontinence or improvement or even complete resolution of hypertension (Fig. 6) (13,31).

Compensatory renal hypertrophy is a well-established, albeit poorly understood, trophic response of the solitary kidney in a child to the lack of a contralateral functioning organ (31,56,57). In 1971, Griscom et al. (56) showed that in neonates and young infants born with a solitary functioning kidney, this kidney will usually be larger than normal, as a consequence of an acceleration in the rate of renal growth that begins early in life. This period of accelerated growth is time limited and is followed by a restoration of the normal rate of renal growth that parallels the growth curve for normally paired kidneys. Subsequent prenatal and postnatal studies with US have confirmed the generally larger size of solitary kidneys in children in a variety of congenital and acquired disorders that result in the absence of a contralateral functioning kidney.

Failure to visualize a kidney in its normal location can also be suspected to be the result of a congenital abnormality when the ipsilateral adrenal gland has an abnormal configuration. The normal "arrowhead" configuration of the adrenal gland results from its elevation and deformation by the subjacent kidney. When a kidney is absent or ectopic, the ipsilateral adrenal gland assumes an elongated, linear configuration. In some cases, its appearance can superficially mimic that of a small, hypoplastic, atrophic, or dysplastic kidney (58,59). This is particularly true in the fetus and neonate, who have relatively larger-sized adrenal glands (compared with the other abdominal visceral organs) than do adults. There is a useful trick to avoid this mistake: The adrenal gland will always be present even when the kidney is absent. Therefore, if the adrenal gland cannot be identified separately from a retroperitoneal structure that is initially misidentified as a small kidney, the structure in question can be assumed to represent the adrenal gland instead, with the ipsilateral kidney either being absent or being located somewhere else in the abdomen or pelvis.

Renal Dysplasia and MCDK

MCDK is a nonhereditary, developmental disorder of the kidney in which atresia of the ureteral bud at or below the UPJ results in a severely dysplastic, nonfunctioning, cystic kidney (Fig. 7) (23,36,37,54,60). This entity was first described in 1836 by the French pathologist Cruveilhier based on an autopsy of a three-year-old boy. Before the development of US and renal scintigraphy, reliable noninvasive diagnosis of MCDK was not possible. Because most patients presented with a palpable flank mass that could not be distinguished from a renal tumor based on the then available imaging techniques, nearly all MCDK patients underwent nephrectomy, as much for diagnosis as for treatment (31,61). Today, the presence of the abnormal multicystic

Figure 6 (Figure on facing page) A nine-year-old girl with malignant hypertension and secondary cardiomyopathy secondary to severe right congenital renal artery stenosis (renal angio-dysplasia). Previous US and MAG-3 renal scans showed no right kidney and were interpreted as being consistent with right renal agenesis. (A) Axial and (B) coronal noncontrast-enhanced T1-weighted fast spin-echo images show a normal left kidney and a very tiny right kidney (arrows) in the right renal fossa. Following laparascopic right nephrectomy, her blood pressure normalized. Abbreviation: US, ultrasound.

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