Tulisan ini saya kutip dari web
A variety of techniques have been developed for imaging the urinary tract. Of these techniques, only two—computed tomographic (CT) urography and magnetic resonance (MR) urography—have the potential to provide a comprehensive assessment of the urinary collecting system, renal parenchyma, and surrounding structures. Although CT urography is nearing its potential in terms of spatial resolution, tissue differentiation, and elucidation of the renal anatomy, MR urography is a more nascent technology. MR urography is an evolving group of techniques with the potential to noninvasively provide the most comprehensive and specific imaging test available for many urinary tract abnormalities without the use of ionizing radiation (1,2). At the same time, formidable limitations and challenges remain for MR urography, including its relative insensitivity for renal calculi, relatively long imaging times, sensitivity to motion, and lower spatial resolution compared with CT and radiography. In this article, we review the most common MR imaging techniques used to image the urinary tract and discuss special considerations (pediatric patients, pregnant patients, renal insufficiency, imaging at 3 T) related to MR urography. In addition, we discuss and illustrate potential clinical applications of MR urography with respect to urolithiasis, urinary tract obstruction unrelated to urolithiasis, hematuria, congenital anomalies, and pre- and postoperative assessment. We also describe various pitfalls and artifacts associated with this modality.
MR Urographic Techniques
The most common MR urographic techniques used to display the urinary tract can be divided into two categories: (a) static-fluid MR urography (also known as static MR urography, T2-weighted MR urography, or MR hydrography), and (b) excretory MR urography (also known as T1-weighted MR urography) (1,3,4).
Static-Fluid MR Urography
T2-weighted techniques were the first clinically relevant means of visualizing the urinary tract with MR imaging (5–10). Static-fluid MR urography treats the urinary tract as a static column of fluid, using one of a variety of T2-weighted sequences that exploit the long T2 relaxation time of urine (11). Therefore, static-fluid MR urographic techniques closely resemble those used for T2-weighted MR cholangiopancreatography. Breath-hold T2-weighted MR urograms can be obtained with either thick-slab single-shot fast spin-echo techniques or similar thin-section techniques (eg, half-Fourier rapid acquisition with relaxation enhancement, single-shot fast spin-echo, single-shot turbo spin-echo). The signal intensity of background tissues can be adjusted by modifying the echo time or using fat suppression. Three-dimensional (3D) respiratory-triggered sequences can be used to obtain thin-section data sets that can then be postprocessed to create volume-rendered (VR) or maximum-intensity-projection (MIP) images of the entire urinary tract (11,12). Heavily T2-weighted static-fluid MR urograms resemble conventional excretory urograms and are useful for quickly identifying the level of urinary tract obstruction. However, identifying the cause of obstruction often requires additional sequences (Fig 1) (8). Static-fluid MR urography does not require the excretion of contrast material and is therefore useful for demonstrating the collecting system of an obstructed, poorly excreting kidney (10).
Static-fluid MR urograms can be obtained with single-shot fast spin-echo techniques in 1–2 seconds, which allows multiple images to be obtained sequentially in a relatively short period of time and played as a cine loop (13). Such image series ensure that both ureters are distensible along their entire lengths and that no fixed narrowings or standing columns exist (Fig 2; see also Movie 1 at radiographics.rsnajnls.org/cgi/content/full/28/1/23/DC1). Cine MR urography is particularly helpful in confirming the existence of urinary tract stenosis (13). When acquiring a series of static-fluid MR urograms to be viewed in cine mode, one should allow 5–10 seconds between acquisitions to prevent radiofrequency saturation of the tissues, which causes progressive signal intensity loss on the images. Because cine MR urography is quick and easy to perform, we have made it a routine part of our MR urography protocol.
The T2 shortening effect of gadolinium prevents successful application of static-fluid MR urography during the excretory phase after the intravenous administration of gadolinium-based contrast material (Fig 3). Because static-fluid MR urography depends on the presence of urine within the collecting systems rather than the excretory function of the kidneys, it is ideally suited for patients with dilated, obstructed collecting systems (Fig 4; see also Movie 2 at radiographics.rsnajnls.org/cgi/content/full/28/1/23/DC1). For patients with nondilated systems, the use of hydration, diuretics, or compression may enhance the quality of MR urography (9). Normal and abnormal fluid-filled structures can interfere with static-fluid MR urography, since the T2-weighted techniques used to display the urinary tract are not specific for urine. For this reason, intravenous hydration may be preferable to oral hydration prior to static-fluid MR urography in patients with nondistended ureters. Alternatively, acquisition planes or postprocessing reconstruction volumes can be adjusted to exclude bowel or other fluid-containing structures. At our institution, we do not use compression during MR urography.
Excretory MR Urography
Excretory MR urography is roughly analogous to CT urography and conventional intravenous urography. A gadolinium-based contrast agent is administered intravenously, and the collecting systems are imaged during the excretory phase. Gadolinium shortens the T1 relaxation time of the urine, allowing the urine to initially appear bright on T1-weighted images. At standard doses of 0.1 mmol/kg, gadolinium-based contrast material quickly becomes concentrated in the urine, and sufficiently concentrated contrast material reduces the signal intensity of the urine due to T2* effects (Fig 5). This effect may be overcome with the use of low-dose gadolinium-based contrast material administration (as low as 0.01 mmol/kg), although such a technique does nothing to distend the collecting systems (14). Low-dose gadolinium-based contrast material administration has also been combined with oral hydration in an attempt to improve dilution and dispersion of excreted gadolinium-based contrast material throughout the collecting systems while improving ureteral distention (15). Unfortunately, MR urography performed with any amount of gadolinium-based contrast material without a pharmacologic means of enhancing urine flow tends to be suboptimal (16).
Diuretic administration can improve the quality of excretory MR urography by enhancing urine flow, resulting in dilution and uniform distribution of gadolinium-based contrast material throughout the urinary tract (17–19). One additional benefit of diuretic administration is expansion of the temporal window during which one may obtain images after gadolinium administration, since T2* effects become less limiting. A relatively low dose of furosemide on the order of 0.1 mg/kg (ie, 5–10 mg for adults) is typically used for MR urography provided no contraindications exist (1,3,20–22). For average-sized adults, we have found that a 5-mg dose of furosemide typically yields excellent image quality while permitting the patient to finish the examination without having to void. Symptoms of acute ureteral obstruction may be exacerbated by the administration of a diuretic, although such occurrences seem to be rare. In a report by Sudah et al (23), only one of 26 patients who presented with acute flank pain due to calculi developed exacerbation of symptoms after the administration of 0.1 mg/kg of furosemide for excretory MR urography. Contraindications for furosemide administration include anuria and hypersensitivity to furosemide, and electrolyte imbalance or hypotension should be corrected before administering furosemide. Patients who are allergic to sulfonamides may also be allergic to furosemide.
The optimal dose of gadolinium-based contrast material for diuretic-augmented MR urography has yet to be established. Nolte-Ernsting et al (1) advocated a gadolinium-based contrast material dose of 0.05 mmol/kg for diuretic-augmented excretory MR urography. Although doses of contrast material of less than 0.05 mmol/kg may yield satisfactory urographic images, concern exists that soft-tissue imaging will be compromised if the gadolinium dose is not sufficient.
The primary imaging sequence for excretory MR urography is the 3D gradient-echo sequence (3,23). Fat suppression enhances the conspicuity of the ureters and is recommended. Depending on the degree of background suppression desired, either a 3D soft-tissue imaging type sequence such as VIBE (volumetric interpolated breath-hold examination), FAME (fast acquisition with multiphase Efgre 3D), THRIVE (T1-weighted high-resolution isotropic volume examination), or liver acquisition with volume acceleration (LAVA) or a sequence normally used for MR angiography will suffice (Fig 6). Most modern imagers are capable of imaging the kidneys, ureters, and bladder in their entirety with a coronal 3D gradient-echo sequence during a single breath hold. Motion suppression is critical for MR urographic sequences, and breath-hold acquisitions have been shown to better demonstrate the pelvicaliceal systems compared with respiratory triggering (22). A coronal through-plane resolution of 2–4 mm is generally possible on newer imagers depending on the breath-holding ability of the patient (see Movie 3 at radiographics.rsnajnls.org/cgi/content/full/28/1/23/DC1). For patients with a limited capacity to hold their breath, adequate spatial resolution can be achieved by imaging the urinary tract in segments. Imaging the urinary tract in segments with a smaller field of view and thinner sections also allows the acquisition of high-detail images of the collecting systems, although the degree of detail obtainable is limited by the signal-to-noise ratio (SNR). The use of echoplanar sequences for excretory MR urography has been described, although aside from reduced acquisition time, such techniques appear to offer few advantages over more conventional 3D gradient-echo techniques (22).
Excretory MR urography requires the excretion of gadolinium into the renal collecting systems to be effective. Therefore, excretory MR urography has no role in the evaluation of patients with severely compromised renal function and may require significantly delayed imaging in patients with urinary tract obstruction (Fig 4). In the case of a markedly dilated ureter, static-fluid MR urography is usually sufficient, although the use of gadolinium-based contrast material will occasionally help distinguish between high-grade partial and complete ureteral obstruction.
Comprehensive MR Urography Protocol
In many patients, static-fluid and excretory MR urography and conventional MR imaging sequences are complementary techniques, yielding different types of information that together can be helpful in establishing the correct diagnosis (24). A comprehensive MR urography protocol can be developed that facilitates evaluation of the renal parenchyma, upper urinary tracts, renal vasculature, urinary bladder, and surrounding structures (1,2). A comprehensive “one-stop shop” type of protocol, such as the one shown in the Table, can take between 30 and 60 minutes for an average technologist to complete depending on the choice of sequences and the available equipment. An abbreviated study that eliminates some components of the comprehensive examination can easily be completed in less than 30 minutes. We do not advocate our protocol as the only approach to a comprehensive MR urographic examination. Indeed, with the rapid pace of developments in the field of abdominal MR imaging, our own protocols are constantly evolving and change from year to year.
View this table:
Hardware and Accessories
It would be impractical to address every possible commercially available hardware configuration in an article such as this one. Therefore, we will speak primarily from our own experience regarding hardware. Satisfactory MR urograms can be obtained at either 1.5 T or 3 T; we do not have experience performing MR urography at field strengths below 1.5 T. All studies described in this article were performed on a 1.5-T imager with an eight-channel phased-array torso coil unless otherwise specified. Although most of the newly developed torso coils allow coverage of the entire abdomen and pelvis in the axial plane with a single acquisition, we image the abdomen and pelvis separately using the maximum number of available coil elements for each acquisition to maximize SNR and to allow high-resolution breath-hold imaging. Most of the new, commercially available torso coils are compatible with sensitivity-encoding parallel imaging techniques. Use of parallel imaging reduces imaging time and the potential for respiratory motion artifacts. The improvement in image quality related to fewer respiratory artifacts usually more than compensates for the loss in SNR related to the use of parallel imaging. We limit our parallel imaging to acceleration factors of 2, since higher acceleration factors result in poor image quality on our current imagers. Mechanical compression has been used by some technologists to aid in urinary tract distention, although we have not found compression to be necessary (25).
Having patients void prior to entering the imager improves their comfort and prevents interruption of the study at an inopportune time. If no contraindications (eg, fluid restriction, congestive heart failure) exist, our patients are given 250 mL of normal saline solution intravenously at the start of imaging. Bowel contents are often bright with the T1- and T2-weighted sequences used for MR urography. We have found the use of oral negative contrast agents helpful in reducing the signal intensity of bowel contents, although the use of such agents is not required for MR urography. In most cases, imaging can be performed successfully with the patient supine.
T2-weighted imaging can be performed with a variety of different sequences depending on the available equipment. For fat-suppressed T2-weighted imaging of the renal parenchyma and pelvic organs, we prefer a respiratory-triggered fast spin-echo sequence. For standard non-fat-suppressed T1-weighted imaging, in-phase and opposed-phase gradient-echo sequences can be useful for detecting intracellular lipid in incidental adrenal masses and clear cell carcinoma of the kidney as well as for characterizing some angiomyolipomas. For cine imaging of the ureters, a thick-slab, heavily T2-weighted single-shot fast spin-echo sequence similar to sequences used for MR cholangiopancreatography is performed. This sequence is typically performed 10–15 times with 5–10 seconds between acquisitions to prevent tissue saturation. The total number of thick-slab acquisitions can be varied to fit the circumstances.
For contrast material–enhanced T1-weighted imaging of the kidneys, a 3D interpolated fat-suppressed gradient-echo sequence combined with parallel imaging suffices. By obtaining pre- and postcontrast images using identical imaging parameters and respiratory cessation, a subtracted data set can be obtained that is useful for assessing the enhancement of solid masses. Acquiring a postcontrast data set during the arterial phase allows assessment of the renal arteries. After two postcontrast acquisitions, we immediately image through the urinary bladder to ensure that we obtain images with bladder wall enhancement prior to the arrival of gadolinium-based contrast material via the ureters (Fig 7). This procedure prevents mixing artifacts, which may obscure bladder tumors (Fig 8). Excretory phase images can be obtained approximately 5 minutes after contrast material injection in nonobstructed patients with normal or mildly impaired renal function. We routinely image the urinary tract in the axial and coronal planes during the excretory phase. It is beneficial to have patients raise the arms over the head during coronal imaging to prevent wraparound artifact.
The pediatric patient presents unique technical challenges for MR urography (26–30), including smaller physical size, inconsistent breath holding, and increased cardiac and respiratory rates (29). The majority of our pediatric patients are less than 6 years old and require sedation (27–29). Sedated patients can be successfully imaged during quiet respiration, although the use of respiratory-gated acquisitions has been described (31). Our pediatric MR urography protocol has evolved considerably over time. For excretory MR urography, we currently hydrate patients with 10 mL/kg of normal saline solution and administer furosemide at a dose of 0.1 mg/kg up to a maximum of 5 mg prior to the administration of gadolinium-based contrast material (standard dose of 0.1 mmol/kg). In pediatric patients with high-grade obstructions, static-fluid MR urography can be used to assess nonfunctioning systems. Static-fluid MR urography has a distinct advantage over excretory urography, which routinely presents problems in documenting the course and insertion of ureters when there is obstruction or poor function (26,28–30). In pediatric patients, performing dynamic contrast-enhanced imaging in the coronal plane allows improved assessment of vascular structures, such as crossing vessels in the setting of UPJ obstruction (28). This approach also allows contemporaneous imaging of the kidneys, ureters, and bladder, given the small size of many pediatric patients. Time–signal-intensity curves have been successfully used to assess renal obstruction in an effort to duplicate the curves generated with diuretic-enhanced renal scintigraphy, although the generation of curves based on segmentation of the renal cortex and medulla may be time consuming in the absence of software automation (32–34). Preliminary studies have also shown the potential of MR urography to suggest the diagnosis of vesicoureteral reflux on the basis of time–signal-intensity curves generated from diuretic-augmented excretory MR urographic images obtained over a period of 40 minutes (35).
Contrast-enhanced MR urography is generally unnecessary in pregnant women. Instead, T2-weighted (static-fluid) urography is performed. Multiple acquisitions (cine MR urography) may be necessary to visualize the entire ureters and exclude fixed narrowings or filling defects. In the latter stages of pregnancy, imaging with the patient in the left lateral decubitus position helps reduce pressure exerted on the inferior vena cava by the gravid uterus. Roy et al (36) reported excellent results with T2-weighted MR urography in identifying urinary tract dilatation and level of obstruction in 17 pregnant patients. The challenge of interpreting MR urographic images obtained during pregnancy remains the differentiation of physiologic hydronephrosis from pathologic obstruction (36–38). The MR urographic findings of physiologic hydronephrosis that have been described include compression of the mid-ureter with tapering at the pelvic brim and no discernable filling defect (Fig 9). Tapering at another level suggests an alternative diagnosis, such as ureteral stone. The ureter below the level of compression should be relatively collapsed, although this segment can be seen to intermittently fill and empty at cine urography. A standing column of urine between the site of physiologic compression and the ureterovesical junction suggests the presence of a distal ureteral stone. In cases of acute calculous obstruction, renal and perirenal edema are often present.
The success of static-fluid MR urography depends on the presence of fluid within the urinary collecting system irrespective of renal function. Any patient who can undergo MR imaging can potentially undergo static-fluid MR urography, although the latter may be of limited value for nondilated collecting systems. The success of excretory MR urography depends on the excretion of gadolinium into the renal collecting systems. Consequently, patients with severely compromised renal function are poor candidates for excretory MR urography. In the past, excretory MR urography has been advocated for use in patients with less severe renal insufficiency as a means of avoiding the use of iodinated contrast material, given the reported low nephrotoxicity of gadolinium chelates at standard clinical doses (39–42). Relatively recent reports linking gadolinium administration to a disorder known as nephrogenic systemic fibrosis have resulted in new recommendations to avoid (whenever possible) the use of gadolinium-based contrast material in patients with moderate to severe renal insufficiency (43–48). It is important to note that the factors contributing to the development of nephrogenic systemic fibrosis remain an area of intense investigation, and physicians are encouraged to stay abreast of new developments and recommendations regarding the use of gadolinium-based contrast material in patients with renal insufficiency.
Imaging at 3 T
MR urography can be successfully performed at 3 T (Fig 10). The improved SNR available at 3 T allows contiguous imaging from the top of the kidneys through the pelvis without switching coil configurations. We are routinely able to image the entire urinary tract in the coronal plane with an excellent SNR on our 3-T imager using partitions of 2 mm (see Movie 4 at radiographics.rsnajnls.org/cgi/content/full/28/1/23/DC1). To our knowledge, there have been no direct comparisons between 1.5-T and 3-T MR urography. Therefore, it remains to be seen whether 3-T imaging offers significant advantages in terms of lesion detection. Individuals performing MR urography at 3 T should be aware of the potential limitations of abdominal and pelvic imaging performed at higher field strengths (49). Prolongation of the T1 relaxation time could potentially have a negative impact on image contrast and lesion conspicuity on T1-weighted images, and chemical shift and susceptibility artifacts are accentuated at higher field strengths. Standing wave and conductivity artifacts may also degrade the quality of images obtained with sequences that are often performed for MR urography (eg, single-shot fast spin-echo) (49).
In the United States, most patients with acute symptoms thought to be related to urolithiasis undergo unenhanced multidetector CT rather than MR urography. Regardless, calculi are common and will likely be encountered on occasion by anyone performing MR urography. Most urinary tract calculi appear as signal voids with T1-and T2-weighted sequences. At both static-fluid and excretory MR urography, calculi appear as filling defects when surrounded by urine or contrast material (Figs 11, 12). However, low-signal-intensity filling defects in the urinary tract are not specific for calculi. The most common causes of noncalculous filling defects within the urinary tract at static-fluid MR urography are blood clots and tumor. Urinary tract calculi can often be distinguished from blood clots, since the latter typically exhibit high-signal-intensity elements on unenhanced T1-weighted images (Fig 13). A calculus can also usually be distinguished from neoplasm, since the latter typically enhances after intravenous contrast material administration. MR imaging findings of acute stone colic include increased perinephric fluid on T2-weighted images, ureteral dilatation proximal to the stone, and a filling defect on T2-weighted or excretory MR urograms. The presence of perirenal fluid can be helpful in distinguishing acute from chronic ureteric obstruction (8). Sudah et al (23) demonstrated improved sensitivity for ureteral calculi for diuretic-augmented excretory MR urography (96.2%–100%) compared with a T2-weighted technique (53.8%–57.7%). In their study, perirenal high signal intensity was present on T2-weighted images in 92% of patients with ureteral calculi (23). Karabacakoglu et al (21) successfully found 26 of 28 collecting system calculi with diuretic-augmented excretory MR urography.
A variety of studies have addressed the detection of urinary tract calculi with MR urography compared with other modalities. Regan et al (8) compared a combination of static-fluid MR urography and abdominal radiography with unenhanced spiral CT and found that the former was more sensitive in the detection of secondary findings of acute ureteral obstruction such as perirenal fluid and ureteric dilatation, although MR urography combined with abdominal radiography showed only 72% of the ureteral calculi seen at CT. In a study by Jung et al (50), diuretic-augmented excretory MR urography helped make the correct diagnosis in 64 of 72 patients with ureteric calculi, compared with 49 of 72 patients for conventional intravenous urography. In two patients with calculi in this study, an incorrect diagnosis of tumor was made at MR urography (50). In a study of 149 patients with ureteral obstruction, MR urography was significantly inferior to unenhanced CT in identifying the site of stone impaction (sensitivity, 69% vs 100%) but was superior to unenhanced CT in identifying ureteral strictures (83% vs 28%) and neoplastic obstruction (42 of 43 neoplasms vs 22 of 43) (51). In this study, the sensitivity of unenhanced CT for noncalculous obstruction was 40%, compared with 89% sensitivity for MR urography and 18% for a combination of KUB (kidneys, ureters, and bladder) and ultrasonography (US) (51).
Urinary Tract Obstruction Unrelated to Urolithiasis
MR urography is more sensitive and specific for noncalculous urinary tract obstruction than is unenhanced CT (51,52). Benign strictures of the ureter may complicate abdominal and pelvic inflammatory processes (eg, appendicitis, Crohn disease, endometriosis), infection (eg, tuberculosis), radiation therapy, surgical or interventional procedures, or stone disease. Benign strictures of the ureter are typically smoothly tapering and not associated with a soft-tissue mass (Fig 14). Cine or excretory MR urography is helpful in gauging the severity of a stricture. In cases of partial obstruction, cine MR urography will demonstrate intermittent distention and collapse of the ureter below the level of narrowing, whereas excretory MR urography will demonstrate contrast enhancement of the ureter distal to the narrowing (see Movie 5 at radiographics.rsnajnls.org/cgi/content/full/28/1/23/DC1). High-grade obstruction will result in delayed excretion of gadolinium-based contrast material on the affected side. Ureteral jets can also be demonstrated with cine urography in the absence of ureteral obstruction.
Extrinsic narrowing of the ureters may occur with entities such as uterine fibroids, fluid collections, retroperitoneal fibrosis, and vascular abnormalities (Fig 15). Benign extrinsic processes may cause deviation of one or both ureters and often result in smooth tapering of the ureter at the site of compression. As with other modalities, retroperitoneal fibrosis can result in medial deviation of the ureters at MR urography. Benign extrinsic compression of the ureter rarely results in complete obstruction.
Neoplastic obstruction of the urinary tract can result from benign or malignant processes. Benign urothelial tumors such as fibroepithelial polyps appear as filling defects at MR urography (53). Malignant processes that can obstruct the ureters include intrinsic urothelial tumors such as TCC of the ureters or bladder, metastatic tumors to the ureters or periureteric tissues, neoplastic lymph nodes, and direct invasion from extraureteral neoplasms (Figs 1, 16). When neoplastic obstruction is suspected, it is important to include sequences to evaluate the soft tissues in addition to MR urographic sequences optimized to visualize the lumen of the urinary tract. Tumor that arises directly from or invades the ureter often results in irregularity of the ureter at the point of obstruction or appears as an irregular filling defect. Unlike calculi, most neoplasms will enhance after intravenous contrast material administration.
The role of MR urography for screening patients at risk for urothelial malignancy has yet to be defined. In one study, static-fluid MR urography performed with a high-resolution technique in two planes through the level of ureteral obstruction was successful in demonstrating eight ureteral and five renal pelvic TCCs in 23 high-risk patients who were poor candidates for other types of imaging examinations (54).
Bladder cancer, cervical cancer, and prostate cancer are relatively common causes of malignant ureteral obstruction (Fig 1). Most malignant tumors of the urinary epithelium are TCCs (Fig 17). TCCs can appear as sessile filling defects or wall thickening (55). Proximal ureteral dilatation may be present. As with other forms of urography, the “goblet” sign can occasionally be seen at MR urography in the setting of TCC of the ureter (3). Urethelial tumors generally have intermediate signal intensity at MR imaging and demonstrate enhancement that is not present with calculi or blood clots (53). It is important to evaluate the entire urinary tract in the setting of TCC, given the propensity of TCC for multifocal involvement (Fig 18).
The evaluation of hematuria with MR imaging requires the use of routine imaging sequences in addition to MR urography. This approach facilitates the detection of renal parenchymal and vascular lesions as well as urothelial abnormalities (Fig 19). MR imaging cannot currently match the spatial resolution of CT, although it is excellent for the detection, characterization, and staging of renal neoplasms (56–60). Small nonobstructive calculi and calcifications will likely be missed at MR urography performed for the evaluation of hematuria. Furthermore, the actual sensitivity of MR urography for the detection of small urothelial neoplasms is unknown.
MR urography can be used to evaluate patients with absent kidney, abnormally positioned or rotated kidney, renal duplication, renal dysplasia, ectopic ureter, retrocaval ureter, primary megaureter, or UPJ obstruction (Fig 20) (26–30,53). In our practice, complicated renal duplication and congenital UPJ obstruction constitute the two most common indications in this category.
Renal duplication can be partial, with the ureters joining above the bladder, or complete, with two ureters inserting separately on one side. Complete or complicated (eg, obstructed moiety) renal duplication is more common in females than in males, and MR urography has been shown to be superior to intravenous urography and US in evaluating the complicated duplex kidney (12,26,28). In cases of complete duplication, the upper pole ureter typically inserts inferior and medial to the lower pole ureter and is more prone to obstruction (Fig 21). The upper pole ureter can insert either ectopically, with development of a ureterocele, or in an extravesicular location. Excretion function of the kidney is not a limiting factor in the diagnosis of an ectopic ureter, since static-fluid MR urographic images are usually sufficient (26,28–30). The lower pole ureter of a duplicated kidney has a tendency to demonstrate reflux, although this phenomenon may be difficult to appreciate at MR urography performed only to evaluate the anatomy.
The UPJ is the most common site of urinary tract obstruction in children. Historically, intravenous urography and lasix (furosemide) renography have been the examinations of choice for the assessment and grading of obstruction, although MR urography can provide both the anatomic information of intravenous urography and the functional data of renal scintigraphy in a single test without exposure to ionizing radiation (28–30,61). A variety of theories have been proposed to explain the etiology of UPJ obstruction, including abnormal smooth muscle arrangement, abnormal innervation of the ureter, crossing vessel, and fibrotic scar. The prognosis for a patient with congenital UPJ obstruction is best predicted on the basis of renal function, and declining function is cause for intervention. In older children, up to 50% of symptomatic UPJ obstructions will be associated with a crossing vessel, the presence of which alters the surgical approach (Fig 22) (29). Therefore, including an MR angiographic sequence is advisable when assessing for UPJ obstruction.
Pre- and Postoperative Assessment
MR urography is easily combined with MR angiography and standard MR imaging for the pre-operative assessment of the arterial supply, collecting system, and renal parenchyma in potential renal transplant donors (25). Likewise, MR imaging can be used to evaluate the renal vasculature, renal parenchyma, collecting system, and peritransplant tissues in renal transplant recipients in a single test, making it a useful adjunct to US, which will likely remain the first-line imaging examination for renal transplant patients (Fig 23) (62–64). When examining renal transplants with MR urography, it is advantageous to combine static-fluid with excretory phase sequences, provided sufficient renal function is present. Schubert et al (65) studied nine renal transplant recipients with static-fluid MR urography and found the lack of functional information to be limiting. In their small series, static-fluid MR urography revealed a definite cause of obstruction in only two of five cases (65). Static-fluid MR urography may not be capable of helping to distinguish between ureteral obstruction and vesicoureteral reflux in some cases, and ureteral leak cannot be demonstrated in the absence of excreted contrast material. Excretory MR urography provides some functional information, and findings are not likely to be obscured by peritransplant fluid collections, which are relatively common after kidney transplantation. MR imaging can also be used to demonstrate postoperative anatomy or to screen for complications after urinary diversion or neobladder reconstruction (Figs 24, 25; see also Movie 6 at radiographics.rsnajnls.org/cgi/content/full/28/1/23/DC1) (24,66).
Pitfalls and Artifacts
As with any MR imaging technique, one must be aware of potential pitfalls when interpreting findings at MR urography (67). When reviewing MR urographic images created with MIP or VR algorithms, one should always consult the original data (source images) to ensure that small filling defects are not obscured by surrounding high-signal-intensity urine. Thick-slab acquisitions may also mask filling defects and should be used primarily to document the presence and level of obstruction (Fig 26).
Whereas small intrarenal calculi are usually inconspicuous at MR imaging, large calculi can mimic a dilated, poorly functioning collecting system on T1-weighted images (Fig 27). However, this pitfall is easily avoided because urine is typically bright and calculi are typically dark on unenhanced T2-weighted images. Of course, correlation of the MR urographic findings with other available imaging findings such as radiographic or CT findings is always a good idea. Another mimic of a dilated intrarenal collecting system is the renal sinus cyst (Fig 28). When viewed on T1- or T2-weighted images obtained prior to the intravenous administration of contrast material, renal sinus cysts have the same signal intensity as urine. Therefore, renal sinus cysts are best differentiated from hydronephrosis on postcontrast excretory phase images.
Susceptibility artifact from metallic objects such as surgical clips can interfere with the visualization of ureteral segments or create the appearance of a ureteral stenosis. As with renal calculi, correlation with radiographic or CT findings can be helpful, although most regions of susceptibility artifact can be correctly identified on gradient-echo source images. Air from recent intervention or an indwelling nephrostomy tube may result in filling defects that simulate calculi.
Flow-related artifacts are relatively common with single-shot fast spin-echo sequences (eg, half-Fourier rapid acquisition with relaxation enhancement, single-shot turbo spin-echo). With such sequences, fluid in motion will demonstrate varying degrees of signal intensity loss that may simulate filling defects within the collecting systems (Fig 29). Girish et al (68) reported filling defect artifacts at heavily T2-weighted diuretic-augmented imaging in 23 of 45 patients with neurogenic bladder dysfunction, with the majority of these artifacts seen in the pelvicaliceal system and upper ureters. The authors did not identify the cause of these artifacts; however, the artifacts were likely caused by a variety of processes, including flow. In our experience, flow-related artifacts tend to be transient or change appearance between sequences. Unlike true filling defects, flow-related artifacts will not have a corresponding abnormality at excretory MR urography.
Hemorrhage into the renal collecting systems may appear bright on gradient-echo T1-weighted images and may be obscured by gadolinium-based contrast material. Therefore, we always perform precontrast gradient-echo T1-weighted imaging in at least one plane. Hemorrhage can also potentially interfere with static-fluid MR urography by reducing the signal intensity of urine. Decreasing the echo time at T2-weighted imaging may help overcome this limitation to some extent. Ureteral peristalsis may occasionally result in ghost artifacts on 3D gradient-echo images, although these artifacts rarely interfere substantially with interpretation (22).
When properly performed, MR urography can be a valuable means of noninvasively assessing the urinary tract. Static-fluid and excretory MR urography can be combined with conventional MR imaging to provide a comprehensive evaluation of the kidneys, ureters, bladder, vasculature, and soft tissues in patients with symptoms referable to the urinary tract. T2-weighted techniques are excellent for demonstrating dilated or obstructed collecting systems, whereas excretory MR urography provides excellent visualization of nonobstructed systems. MR urography can be useful for assessing patients with a variety of urinary tract disorders and allows the evaluation of pediatric and pregnant patients without the use of ionizing radiation. The successful interpretation of MR urographic examinations requires familiarity with the numerous potential pitfalls and artifacts that may be encountered.