JR Basford MD, TR Jenkyn MD, KN Ane MD , RL Ehman MD , G Heers MD , KR Kaufman MD
Magnetic resonance (MR) urography comprises an evolving group of techniques with the potential for allowing optimal noninvasive evaluation of many abnormalities of the urinary tract. MR urography is clinically useful in the evaluation of suspected urinary tract obstruction, hematuria, and congenital anomalies, as well as surgically altered anatomy, and can be particularly beneﬁcial in pediatric or pregnant patients or when ionizing radiation is to be avoided. The most common MR urographic techniques for displaying the urinary tract can be divided into two categories: static-ﬂuid MR urography and excretory MR urography. Static-ﬂuid MR urography makes use of heavily T2weighted sequences to image the urinary tract as a static collection of ﬂuid, can be repeated sequentially (cine MR urography) to better demonstrate the ureters in their entirety and to conﬁrm the presence of ﬁxed stenoses, and is most successful in patients with dilated or obstructed collecting systems. Excretory MR urography is performed during the excretory phase of enhancement after the intravenous administration of gadolinium-based contrast material; thus, the patient must have sufﬁcient renal function to allow the excretion and even distribution of the contrast material. Diuretic administration is an important adjunct to excretory MR urography, which can better demonstrate nondilated systems. Static-ﬂuid and excretory MR urography can be combined with conventional MR imaging for comprehensive evaluation of the urinary tract. The successful interpretation of MR urographic examinations requires familiarity with the many pitfalls and artifacts that can be encountered with these techniques.
Key words– Static fluid MR urography, Cine MR urography, Excretory MR urogrphy.
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, T2weighted 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 spinecho, 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 (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. 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. 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, 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). Lowdose 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.
Fig 1: Excretion urography following subtraction.
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 breathhold 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. 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.
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. 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.
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 T2weighted imaging of the renal parenchyma and pelvic organs, we prefer a respiratory-triggered fast spin-echo sequence. For standard non-fatsuppressed 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 thickslab 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. This procedure prevents mixing artifacts, which may obscure bladder tumors. Excretory phase images can be obtained approximately 5 minutes after contrast material injection in nonobstructed patients with normal or mildly impaired renal function.
Fig 2: MR urography T2 weighted sequences.
Table 1: Imaging protocol for MR Urography sequences.
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 gadoliniumbased 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. Staticﬂuid 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, T2weighted (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 midureter with tapering at the pelvic brim and no discernable ﬁlling defect. 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 calculus 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.
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 highsignal-intensity urine. Thick-slab acquisitions may also mask ﬁlling defects and should be used primarily to document the presence and level of obstruction.
Whereas small intrarenal calculi are usually inconspicuous at MR imaging, large calculi can mimic a dilated, poorly functioning collecting system on T1-weighted images. 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.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.
Hemorrhage into the renal collecting systems may appear bright on gradient-echo T1-weighted images and may be obscured by gadoliniumbased 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.
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