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Magnetic Resonance Imaging of the Urinary Tract in the Fetal and Pediatric Population
Magnetic Resonance Imaging of the Urinary Tract in the Fetal and Pediatric Population D. Alicia Morales Ramos, MD, Pedro A.B. Albuquerque, MD, Lucia Carpineta, MD, and Ricardo Faingold, MD
Magnetic resonance imaging (MRI) has become an excellent modality for the assessment of renal pathologies in children; its multiplanar capability and soft-tissue contrast resolution allows for exquisite demonstration of the renal anatomy and its abnormalities. In this article, we illustrate and discuss MRI techniques and findings of the most commonly seen renal anomalies, including congenital, inflammatory, neoplastic, posttransplant, and miscellaneous conditions.
Magnetic resonance imaging (MRI) is an excellent imaging modality for the assessment of the fetal and pediatric urinary tract, allowing for an effective evaluation of a wide spectrum of renal diseases without the use of ionizing radiation. Even though MRI is not the modality of choice for the primary imaging evaluation of renal pathologies in children, with the recent technical developments of higher resolution coils and faster sequences, renal MRI has become a valuable alternative for complementary imaging assessment of the renal system in children.1 Our purpose was to review and discuss the most useful MRI techniques as well as to illustrate the characteristic MRI findings of the most frequent anomalies of the urinary tract in the fetal and pediatric population, including congenital, inflammatory, neoplastic, posttransplant, and miscellaneous conditions.
From the Department of Medical Imaging, Montreal Children’s Hospital/ McGill University Health Centre, Montreal, PQ, Canada. Reprint requests: Ricardo Faingold, MD, Department of Medical Imaging, Montreal Children’s Hospital/McGill University Health Center, 2300 Tupper Street Montreal, Local C343, Montreal, PQ H3H 1P3, Canada. E-mail: [email protected]. Curr Probl Diagn Radiol 2007;36:153-63. © 2007 Mosby, Inc. All rights reserved. 0363-0188/2007/$32.00 ⫹ 0 doi:10.1067/j.cpradiol.2007.02.002
Curr Probl Diagn Radiol, July/August 2007
MR Imaging Techniques Several different strategies and techniques are proposed in the literature, which will vary depending on the patient’s age group, clinical history, radiologist’s experience as well as different MRI scanners and even different vendors.1-8 Moreover, MRI is a modality very well known for providing exquisite details of the renal morphology, but nowadays it has evolved into a modality that can also provide functional information of the renal system.1,8,9 Careful and tailored preimaging planning is crucial for the success of the examination and takes different aspects under consideration. Correct selection of the most appropriate phased array coil for the specific examination plays an important role in the final image quality, as the use of localized coils significantly increases the signal-to-noise ratio. Different sedation protocols are used in the pediatric population. The eventual need and type of sedation also play an important role in this equation, since such variables will determine whether or not breath-hold sequences can be acquired. In older/cooperative children, we commonly encounter artifacts secondary to physiologic motion such as respiration, cardiac pulsations, and bowel peristalsis. In such cases, fast imaging and single-shot pulse sequences in conjunction with breath-holding techniques are most effective in eliminating respiratory motion artifacts. For ventilated patients, temporary suspension of the respiration often yields motion-free quality images. In younger/noncooperative patients where breath-hold imaging is not possible, fast sequences, single-shot imaging, or respiratory correction techniques can be performed. Magnetization-prepared gradient-echo (MagPrep-GRE) imaging is used for fast T1-weighted images, as it allows for very short acquisition times on a per-slice basis, eliminating the requirement for breath-holding to achieve motion-free images. Furthermore, conven-
153
tional spin-echo T1-weighted images can also be acquired on cooperative patients or on scanners that are unable to achieve good quality breath-hold sequences. This type of T1 sequence is known to provide excellent details of the renal anatomy. Fat-suppression techniques are widely used to increase contrast and reduce artifacts caused by respiration and other motion. Even though many fat suppression strategies increase the acquisition time, they can still be applied to motion insensitive non-breath-hold techniques to improve image contrast. Several centers have relied on fat suppression with half-Fournier acquisition singleshot turbo spin-echo and single-shot fast spin echo (SSFSE) to improve the image contrast with T2weighted sequences. Echo-train imaging commonly uses vendor-acronyms and are also used to obtain T2-weighted images in a shorter time than conventional spin-echo sequences. The use of in-phase and out-of-phase sequences is also described as useful for the detection of focal or diffuse intracellular lipid within renal masses. Different protocols are proposed for renal MR angiography (MRA), but an ideal approach should include high spatial resolution including the smallest field-of-view possible. Flow-sensitive sequences can be used to complement the morphological and contrast-enhanced sequences. However, they tend to overestimate the severity of the disease. The introduction of gadolinium-enhanced MRA has virtually replaced flow-sensitive sequences for abdominal imaging, as it improves the evaluation of the renal vascular anatomy. Gadolinium is also widely used in contrast-enhanced dynamic imaging, usually using fast gradient echo (GRE) sequences with fat suppression. These protocols often include nonenhanced MRI sequences followed by postcontrast imaging during arterial, venous, and nephrographic phases. Conventional T1 with fat saturation (FS) is also used in some institutions as the last sequence acquired after the dynamic study. More recently, the improved spatial and temporal resolution of current MR scanners allows the visualization of gadolinium contrast material within distinct time phases in distinct intrarenal regions, such as the cortex, the medulla, and the collecting system. These features are believed to be a valuable tool for noninvasive evaluation of multiple parameters of renal function, such as glomerular filtration, tubular concentration, regional perfusion, water movements, and oxygenation. In our institution, we use a 1.5-Tesla magnet (GE Signa LX release 9.0; GE Medical Systems, Milwau-
154
FIG 1. Horseshoe kidney. Seven-day-old boy with Tetralogy of Fallot and horseshoe kidney. Coronal SPGR gadolinium-enhanced MRA acquired in arterial phase shows normal enhancement of the parenchymal isthmus of the horseshoe kidney.
kee, WI) and the generic renal MRI protocol consists of the following: axial and coronal T1; axial, coronal, and sagittal T2 with fat saturation; pre- and postgadolinium-enhancement coronal 3D fast spoiled gradient echo (FSPGR); post-gadolinium-enhancement axial and coronal T1 with fat saturation (breath-hold spoiled gradient echo (SPGR) if the child is cooperative). The renal transplant protocol consists of the following: coronal SSFSE, sagittal T1, axial T2 with fat saturation, gadolinium-enhanced coronal 3D MRA, axial 3D phase contrast, and axial 2D time-of-flight. Our fetal MR protocol includes three orthogonal planes through the fetal region-of-interest using primarily SSFSE-T2, with one plane of FSPGR in maternal breath-hold if needed.
Fetal and Congenital Horseshoe kidney (Fig 1) is the most common type of renal fusion, where there is fusion of the lower poles across the midline by an isthmus, which usually lies anterior to the aorta. MRI evaluates the structural details of the renal parenchyma, isthmus, collecting systems, and ureters and also delineates the vascular anatomy with MRA. It is also the best modality for the evaluation of associated renal tumors.10 Renal agenesis occurs in 1 in 600 (unilateral) to 1 in 3,000 to 10,000 births (bilateral).11 Unilateral agenesis
Curr Probl Diagn Radiol, July/August 2007
FIG 2. Fraser syndrome. Thirty-four-week-gestation fetus with Fraser syndrome. Coronal oblique SSFSE-T2 (A and B) and axial oblique SSFSE-T2 (C). Coronal images (A and B) show small left kidney. In the right renal fossa, no kidney was identified. There is severe oligohydramnios. Axial SSFSE view through the head (C) shows hypoplastic ocular globes left more than right with deformed contour of the left one.
Curr Probl Diagn Radiol, July/August 2007
155
FIG 3. Hydronephrosis. Thirty-one-week-gestation fetus with dilated left renal collecting system. On coronal oblique SSFSE-T2, we note mild to moderate hydronephrosis without hydroureter, suggestive of ureteropelvic junction obstruction.
is known to be part of numerous syndromes, with the best known being the syndrome characterized by vertebral, anorectal, cardiac, tracheoesophageal, renal, and limb abnormalities (VACTERL). When bilateral, renal agenesis is incompatible with life, with pulmonary hypoplasia responsible for the neonate’s demise in the first few days of life. Prenatally, MRI signs that often accompany renal hypoplasia or agenesis include oligohydramnios and varying degrees of decreased signal in fetal lungs on T2-weighted sequences. Elements of the Potter sequence may also be present. In cases of bilateral agenesis, there is also nonvisualization of the fetal bladder. Fraser syndrome (Fig 2) is a rare autosomal-recessive disorder with multiple malformation, including cryptohthalmos; syndactyly; genital abnormalities; congenital malformations of the nose, ears, larynx; cleft lip and/or palate; skeletal defects; umbilical hernia; renal agenesis; and mental retardation.12
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FIG 4. Hydronephrosis. Three-year-old male with hydronephrosis. Coronals T2 FS (A) and coronal gadolinium-enhanced T1 FS (B) showing right hydronephrosis with dilation of the collecting system and mild blunting of the calices within the upper pole.
Curr Probl Diagn Radiol, July/August 2007
FIG 5. Hydronephrosis. Five-year-old girl with hydronephrosis. Coronal T2 FS (A), coronal T1 FS postcontrast (B), and axial T2 FS (C) show significant hydronephrosis with dilation of the collecting system, blunting of the calices, and parenchymal thinning of the left kidney.
Curr Probl Diagn Radiol, July/August 2007
Hydronephrosis (Figs 3-5) is a very common pathology of the urogenital system, characterized by dilation of the collecting system. MRI has proven to be an excellent modality to evaluate normal renal anatomy.13. The most common cause of hydronephrosis is stenosis or obstruction of the ureteropelvic junction.4 MRI is not the modality of choice for the initial detection of hydronephrosis, but it is the only modality that allows complete evaluation of the entire urinary system (renal parenchyma, collecting system, ureters, and bladder), renal vascular supply, and renal function in a single examination.6 Prune belly syndrome (PBS) (Fig 6) is also known as Eagle–Barret syndrome or triad syndrome. It occurs in 1 in 35,000 to 500,000 live births. PBS presents almost exclusively in males, with less than 3% occurring in female patients.14 The cause of PBS is unknown. The syndrome is named for the wrinkled appearance of the distended and lax abdominal wall that results from the absence of rectus muscles. Bilateral, nonpalpable undescended testes are present and there is an abnormal urinary tract characterized by tortuous, dilated ureters and renal dysmorphism. A variety of respiratory, gastrointestinal, musculoskeletal, and cardiovascular anomalies are also associated with the PBS.15 Nowadays, magnetic resonance urography (MRU) utilizes two different imaging strategies, which can be used in complementarity to assess nearly all aspects of upper urinary tract disease. The first technique is the unenhanced, heavily T2-weighted sequence to obtain static-fluid images of the urinary tract. T2-weighted MR urograms have beautifully demonstrated dilation of the urinary tract, even if the renal excretory function is minimal. The second technique is analogous to an intravenous pyelogram and is therefore also referred to as excretory MRU. Gadolinium is administered and, following excretion, the contrast-containing urine is imaged using fat-suppressed T1-weighted GRE sequences. Use of low-dose furosemide helps to achieve uniform distribution of contrast and to avoid signal loss for high concentrations of gadolinium. Together these techniques help assess both nondilated and obstructed urinary systems in patients with compromised renal function.16
157
FIG 6. Prune belly syndrome. Seventeen-year-old male with prune belly syndrome. Coronal T2 (A and B) axial T2 (C) show the coronal SSFSE T2 (D) showing a paucity of abdominal wall musculature, with slightly abnormally located kidneys. Bilateral hydronephrosis, right greater than left.
158
Curr Probl Diagn Radiol, July/August 2007
FIG 8. Postrenal transplant. Seventeen-year-old girl with renal transplant presenting with increased creatinine. Coronal T2 FS showing a large and heterogeneous hyperintense subcapsular collection compatible with subcapsular hematoma.
FIG 7. (A) Left pyelonephritis and vesico-ureteral reflux. Five-month-old female (A) with left pyelonephritis and vesico-ureteral reflux. Axial T2 FS shows a mildly swollen left kidney. (B and C) Evolving renal abscess. Eleven-year-old boy with evolving renal abscess on the right side (B and C). Sagittal T2 FS (B), coronal T1 FS with gadolinium (C) MRI shows an area of abnormal signal involving the upper pole of the right kidney. The postgadolinium image demonstrates abnormal enhancement in the upper pole.
the infectious/inflammatory process toward adjacent soft tissues. Enteric Gram-negative bacteria are the usual infecting organisms among all age groups. Approximately 66% of patients have a history of recurrent urinary tract infections. Since MRI does not involve the use of ionizing radiation, it is indeed a good tool for evaluation of renal abscess and perinephric collections and follow-up.17
Infectious and Inflammatory Even though MRI is not the modality of choice for initial detection of pyelonephritis (Fig 7A), its exquisite anatomic resolution allows for accurate depiction and evaluation of associated complications, such as renal vessel thrombosis, intrarenal abscess (Fig 7B and C), perinephric collections, and even the extension of
Curr Probl Diagn Radiol, July/August 2007
Miscellaneous Renal transplantation (Fig 8) is a common surgical procedure, with approximately 11,000 procedures performed annually in the United States. Causes of graft dysfunction can be categorized as vascular
159
FIG 9. Tuberous sclerosis. Sixteen-year-old girl with tuberous sclerosis. Appearance of polycystic kidneys bilaterally associated with small angiomyolipomas. Coronal T2 FS (A), coronal T1 FS with gadolinium (B) showing bilaterally enlarged kidneys, with multiple cysts of different sizes, most of them with low signal on T1- and high signal on T2- and a couple of other lesions with high signal intensity on enhanced T1- and T2-weighted images, corresponding to the angiomyolipomas.
(stenosis or occlusion of the transplant artery or vein), parenchymal (acute tubular necrosis, rejection, and medication toxicity), or extrinsic (perinephric collections or ureteral obstruction). Several studies have demonstrated the usefulness of MRA angiography in the evaluation of the transplant renal artery.5 Other studies have shown the usefulness of MRI in the evaluation of the transplanted kidney and peritransplant region.18 Lee has also described the use of MRI in the assessment of renal transplant dysfunction.19 Both angiomyolipomas and cysts occur commonly in pediatric patients with tuberous sclerosis (TS) (Fig 9) and both tend to increase in size and number with increasing age. It has been shown that angiomyolipomas continue to grow in adults and can distort renal architecture, which compromises function and can also lead to hemorrhage. Renal failure is the leading cause of death in adults with TS. Because of these reasons, imaging surveillance is often performed in patients to identify such lesions and monitor their progression.20
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Tumors Wilms tumor (Figs 10 and 11) accounts for 87% of pediatric renal masses and occurs in approximately 1:10,000 children. Its peak incidence is at 3 to 4 years of age and 80% of patients present before 5 years of age. It is rare in neonates, with less than 0.16% of cases manifesting in this age group. Wilms tumor is bilateral in 4 to 13% of children. Metastases are most commonly found in the lungs (85%), liver, and regional lymph nodes; metastatic disease may also produce vascular invasion. Wilms tumors demonstrate low signal intensity on T1- and high signal intensity on T2-weighted images. MRI also permits assessment of vascular patency and multifocal disease. Determining whether there is direct invasion of the inferior vena cava or adjacent structures may be extremely difficult, but MRI has been reported to be the most sensitive modality for evaluation of such findings.21 Nephroblastomatosis (Fig 12) consists of diffuse or multifocal areas of nephrogenic rests (NR) within the kidneys. NR are foci of metanephric blastema that
Curr Probl Diagn Radiol, July/August 2007
FIG 10. Wilms tumor. Twelve-month-old girl with large bilateral renal masses. Axial T1 (A), enhanced T1 FS (B), and T2 FS (C) images show low signal intensity masses that appear heterogeneous but mildly hyperintense on T2-weighted images, with heterogeneous enhancement postgadolinium. Internal hypointense areas on enhanced T1 images are suggestive of necrotic degeneration. Pathology proved the masses to be mainly nephroblastomatosis with associated malignant features, compatible with Wilms tumors.
Curr Probl Diagn Radiol, July/August 2007
FIG 11. Wilms tumor. Three-year-old girl with Wilms tumor presenting with retroperitoneal bleed. Axial T1 (A), T2 (B), and sagittal T2 (C) images show a large and heterogeneous mass arising from the lower pole of the left kidney. The MRI clearly shows the renal origin of the mass as well as its relationship with the adjacent organs, mass effect, and perinephric bleed.
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found in up to 99% of patients with bilateral Wilms tumors.21
Conclusions The ability to provide exquisite anatomic details of the renal system is a well-recognized asset of MRI. More recently, the introduction of new and improved MRI applications has also added the ability to provide valuable functional information on the renal system. Even though MRI does not play a role as the initial modality for assessing the renal system, its use has been gaining acceptance throughout the world as a valuable tool for assessing renal pathology. The lack of ionizing radiation is also an important advantage, especially within the pediatric population, as it allows us to perform initial and follow-up exams without the undesired radiation exposure. Pediatric renal MRI has a wide spectrum of clinical applications, with this spectrum constantly increasing. With the new advent of functional imaging tools, it is also providing new insights into renal pathology and pathophysiology.
REFERENCES
FIG 12. Nephroblastomatosis. Five-year-old boy with focal nephroblastomatosis. Axial T2 FS (A), coronal inversion recovery (IR) (B), and axial gadolinium-enhanced T1 FS (C) show focal heterogeneous lesion within the interpolar region of the right kidney.
persist beyond 36 weeks of gestation. They have the potential for malignant transformation into Wilms tumors and it is currently believed that NR give rise to approximately 30 to 40% of Wilms tumors. They are
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1. Zhang J, Pedrosa I, Rofsky N. MR techniques for renal imaging. Radiol Clin N Am 2003;41:877-907. 2. Krestin GP. Genitourinary MR: Kidneys and adrenal glands. Eur Radiol 1999;9:1705-14. 3. Zhang H, Prince R. Renal MR Angiography. Magn Reson Imaging Clin N Am 2004;12:487-503. 4. Ertl-Wagner B, Lienemann A, Strauss A, et al. Fetal magnetic resonance imaging indications, technique, anatomical considerations and a review of fetal abnormalities. Eur Radiol 2002;12:1931-40. 5. Hussain SM. MR Imaging: A “one stop shop” modality for preoperative evaluation of potential living kidney donors. Radiographics 2003;23:505-20. 6. Prayer D, Christian BP, Prayer L. Fetal MRI: Techniques and protocols. Pediatr Radiol 2004;34:685-93. 7. Levine D, Barnes P, Edelman RR. Obstetric MR imaging. Radiology 1999;211:609-17. 8. Grenier N, Basseau F, Ries M, et al. Functional MRI of the kidney. Abdom Imaging 2003;28:164-75. 9. Huang AJ, Lee VS, Rusinek H. Functional renal MR imaging. Magn Reson Imaging Clin N Am 2004;12:469-86. 10. Dyer RB, Chen MY, Zagoria RJ. Classic signs in uroradiology. Radiographics 2004;24:S247-80. 11. Dahnert W. Radiology Review Manual. Philadelphia, PA: Lippincott, Williams & Wilkins, 2003. 12. Berg C, Geipel A, Germer U, et al. Prenatal detection for Fraser syndrome without cryptophthalmos: Case report and
Curr Probl Diagn Radiol, July/August 2007
13.
14. 15.
16.
review of the literature. Ultrasound Obstet Gynecol 2001;18: 76-80. Perez-Brayfield MR, Kirssh AJ, Jones RA, et al. A prospective study comparing ultrasound, nuclear scintigraphy and dynamic contrast enhanced magnetic resonance imaging in the evaluation of hydronephrosis. J Urol 2003;170: 1330-4. Wood BP. Prune Belly Syndrome, emedicine 2005. Available at: http://www.emedicine.com/radio/topic 575.htm Berrocal T, Lopez-Pereira P, Arjonilla A, et al. Anomalies of the distal ureter, bladder, and urethra in children: Embryologic, radiologic and pathologic features. Radiographics 2002; 22:1139-64. Nolte-Ernsting CCA, Adam GB, Gunther RW. MR urography: Examination techniques and clinical applications. Eur Radiol 2001;11:355-72.
Curr Probl Diagn Radiol, July/August 2007
17. Willard TB, Teague JL, Steinbecker D. Renal Corticomedullary Abscess, emedicine 2005. Available at: http://www. emedicine.com/med/topic 2848.htm. 18. Hohenwalter MD, Skowlund CJ, Erickson SJ, et al. Renal transplant evaluation with MR angiography and MR imaging. Radiographics 2001;21:1505-17. 19. Lee V. Renal MRI: Novel Techniques and Contrast Agents, PhD, Medscape 2003. Available at: http://www.medscape.com/ viewarticle/471043. 20. Casper KA, Donnelly LF, Chen B, et al. Tuberous sclerosis complex: Renal imaging findings. Radiology 2002;225: 451-6. 21. Lowe LH, Lusani B, Heller RM, et al. Pediatric renal masses: Wilms tumor and beyond. Radiographics 2000;20: 1585-603.
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Magnetic resonance imaging (MRI) has become an excellent modality for the assessment of renal pathologies in children; its multiplanar capability and soft-tissue contrast resolution allows for exquisite demonstration of the renal anatomy and its abnormalities. In this article, we illustrate and discuss MRI techniques and findings of the most commonly seen renal anomalies, including congenital, inflammatory, neoplastic, posttransplant, and miscellaneous conditions.
Magnetic resonance imaging (MRI) is an excellent imaging modality for the assessment of the fetal and pediatric urinary tract, allowing for an effective evaluation of a wide spectrum of renal diseases without the use of ionizing radiation. Even though MRI is not the modality of choice for the primary imaging evaluation of renal pathologies in children, with the recent technical developments of higher resolution coils and faster sequences, renal MRI has become a valuable alternative for complementary imaging assessment of the renal system in children.1 Our purpose was to review and discuss the most useful MRI techniques as well as to illustrate the characteristic MRI findings of the most frequent anomalies of the urinary tract in the fetal and pediatric population, including congenital, inflammatory, neoplastic, posttransplant, and miscellaneous conditions.
From the Department of Medical Imaging, Montreal Children’s Hospital/ McGill University Health Centre, Montreal, PQ, Canada. Reprint requests: Ricardo Faingold, MD, Department of Medical Imaging, Montreal Children’s Hospital/McGill University Health Center, 2300 Tupper Street Montreal, Local C343, Montreal, PQ H3H 1P3, Canada. E-mail: [email protected]. Curr Probl Diagn Radiol 2007;36:153-63. © 2007 Mosby, Inc. All rights reserved. 0363-0188/2007/$32.00 ⫹ 0 doi:10.1067/j.cpradiol.2007.02.002
Curr Probl Diagn Radiol, July/August 2007
MR Imaging Techniques Several different strategies and techniques are proposed in the literature, which will vary depending on the patient’s age group, clinical history, radiologist’s experience as well as different MRI scanners and even different vendors.1-8 Moreover, MRI is a modality very well known for providing exquisite details of the renal morphology, but nowadays it has evolved into a modality that can also provide functional information of the renal system.1,8,9 Careful and tailored preimaging planning is crucial for the success of the examination and takes different aspects under consideration. Correct selection of the most appropriate phased array coil for the specific examination plays an important role in the final image quality, as the use of localized coils significantly increases the signal-to-noise ratio. Different sedation protocols are used in the pediatric population. The eventual need and type of sedation also play an important role in this equation, since such variables will determine whether or not breath-hold sequences can be acquired. In older/cooperative children, we commonly encounter artifacts secondary to physiologic motion such as respiration, cardiac pulsations, and bowel peristalsis. In such cases, fast imaging and single-shot pulse sequences in conjunction with breath-holding techniques are most effective in eliminating respiratory motion artifacts. For ventilated patients, temporary suspension of the respiration often yields motion-free quality images. In younger/noncooperative patients where breath-hold imaging is not possible, fast sequences, single-shot imaging, or respiratory correction techniques can be performed. Magnetization-prepared gradient-echo (MagPrep-GRE) imaging is used for fast T1-weighted images, as it allows for very short acquisition times on a per-slice basis, eliminating the requirement for breath-holding to achieve motion-free images. Furthermore, conven-
153
tional spin-echo T1-weighted images can also be acquired on cooperative patients or on scanners that are unable to achieve good quality breath-hold sequences. This type of T1 sequence is known to provide excellent details of the renal anatomy. Fat-suppression techniques are widely used to increase contrast and reduce artifacts caused by respiration and other motion. Even though many fat suppression strategies increase the acquisition time, they can still be applied to motion insensitive non-breath-hold techniques to improve image contrast. Several centers have relied on fat suppression with half-Fournier acquisition singleshot turbo spin-echo and single-shot fast spin echo (SSFSE) to improve the image contrast with T2weighted sequences. Echo-train imaging commonly uses vendor-acronyms and are also used to obtain T2-weighted images in a shorter time than conventional spin-echo sequences. The use of in-phase and out-of-phase sequences is also described as useful for the detection of focal or diffuse intracellular lipid within renal masses. Different protocols are proposed for renal MR angiography (MRA), but an ideal approach should include high spatial resolution including the smallest field-of-view possible. Flow-sensitive sequences can be used to complement the morphological and contrast-enhanced sequences. However, they tend to overestimate the severity of the disease. The introduction of gadolinium-enhanced MRA has virtually replaced flow-sensitive sequences for abdominal imaging, as it improves the evaluation of the renal vascular anatomy. Gadolinium is also widely used in contrast-enhanced dynamic imaging, usually using fast gradient echo (GRE) sequences with fat suppression. These protocols often include nonenhanced MRI sequences followed by postcontrast imaging during arterial, venous, and nephrographic phases. Conventional T1 with fat saturation (FS) is also used in some institutions as the last sequence acquired after the dynamic study. More recently, the improved spatial and temporal resolution of current MR scanners allows the visualization of gadolinium contrast material within distinct time phases in distinct intrarenal regions, such as the cortex, the medulla, and the collecting system. These features are believed to be a valuable tool for noninvasive evaluation of multiple parameters of renal function, such as glomerular filtration, tubular concentration, regional perfusion, water movements, and oxygenation. In our institution, we use a 1.5-Tesla magnet (GE Signa LX release 9.0; GE Medical Systems, Milwau-
154
FIG 1. Horseshoe kidney. Seven-day-old boy with Tetralogy of Fallot and horseshoe kidney. Coronal SPGR gadolinium-enhanced MRA acquired in arterial phase shows normal enhancement of the parenchymal isthmus of the horseshoe kidney.
kee, WI) and the generic renal MRI protocol consists of the following: axial and coronal T1; axial, coronal, and sagittal T2 with fat saturation; pre- and postgadolinium-enhancement coronal 3D fast spoiled gradient echo (FSPGR); post-gadolinium-enhancement axial and coronal T1 with fat saturation (breath-hold spoiled gradient echo (SPGR) if the child is cooperative). The renal transplant protocol consists of the following: coronal SSFSE, sagittal T1, axial T2 with fat saturation, gadolinium-enhanced coronal 3D MRA, axial 3D phase contrast, and axial 2D time-of-flight. Our fetal MR protocol includes three orthogonal planes through the fetal region-of-interest using primarily SSFSE-T2, with one plane of FSPGR in maternal breath-hold if needed.
Fetal and Congenital Horseshoe kidney (Fig 1) is the most common type of renal fusion, where there is fusion of the lower poles across the midline by an isthmus, which usually lies anterior to the aorta. MRI evaluates the structural details of the renal parenchyma, isthmus, collecting systems, and ureters and also delineates the vascular anatomy with MRA. It is also the best modality for the evaluation of associated renal tumors.10 Renal agenesis occurs in 1 in 600 (unilateral) to 1 in 3,000 to 10,000 births (bilateral).11 Unilateral agenesis
Curr Probl Diagn Radiol, July/August 2007
FIG 2. Fraser syndrome. Thirty-four-week-gestation fetus with Fraser syndrome. Coronal oblique SSFSE-T2 (A and B) and axial oblique SSFSE-T2 (C). Coronal images (A and B) show small left kidney. In the right renal fossa, no kidney was identified. There is severe oligohydramnios. Axial SSFSE view through the head (C) shows hypoplastic ocular globes left more than right with deformed contour of the left one.
Curr Probl Diagn Radiol, July/August 2007
155
FIG 3. Hydronephrosis. Thirty-one-week-gestation fetus with dilated left renal collecting system. On coronal oblique SSFSE-T2, we note mild to moderate hydronephrosis without hydroureter, suggestive of ureteropelvic junction obstruction.
is known to be part of numerous syndromes, with the best known being the syndrome characterized by vertebral, anorectal, cardiac, tracheoesophageal, renal, and limb abnormalities (VACTERL). When bilateral, renal agenesis is incompatible with life, with pulmonary hypoplasia responsible for the neonate’s demise in the first few days of life. Prenatally, MRI signs that often accompany renal hypoplasia or agenesis include oligohydramnios and varying degrees of decreased signal in fetal lungs on T2-weighted sequences. Elements of the Potter sequence may also be present. In cases of bilateral agenesis, there is also nonvisualization of the fetal bladder. Fraser syndrome (Fig 2) is a rare autosomal-recessive disorder with multiple malformation, including cryptohthalmos; syndactyly; genital abnormalities; congenital malformations of the nose, ears, larynx; cleft lip and/or palate; skeletal defects; umbilical hernia; renal agenesis; and mental retardation.12
156
FIG 4. Hydronephrosis. Three-year-old male with hydronephrosis. Coronals T2 FS (A) and coronal gadolinium-enhanced T1 FS (B) showing right hydronephrosis with dilation of the collecting system and mild blunting of the calices within the upper pole.
Curr Probl Diagn Radiol, July/August 2007
FIG 5. Hydronephrosis. Five-year-old girl with hydronephrosis. Coronal T2 FS (A), coronal T1 FS postcontrast (B), and axial T2 FS (C) show significant hydronephrosis with dilation of the collecting system, blunting of the calices, and parenchymal thinning of the left kidney.
Curr Probl Diagn Radiol, July/August 2007
Hydronephrosis (Figs 3-5) is a very common pathology of the urogenital system, characterized by dilation of the collecting system. MRI has proven to be an excellent modality to evaluate normal renal anatomy.13. The most common cause of hydronephrosis is stenosis or obstruction of the ureteropelvic junction.4 MRI is not the modality of choice for the initial detection of hydronephrosis, but it is the only modality that allows complete evaluation of the entire urinary system (renal parenchyma, collecting system, ureters, and bladder), renal vascular supply, and renal function in a single examination.6 Prune belly syndrome (PBS) (Fig 6) is also known as Eagle–Barret syndrome or triad syndrome. It occurs in 1 in 35,000 to 500,000 live births. PBS presents almost exclusively in males, with less than 3% occurring in female patients.14 The cause of PBS is unknown. The syndrome is named for the wrinkled appearance of the distended and lax abdominal wall that results from the absence of rectus muscles. Bilateral, nonpalpable undescended testes are present and there is an abnormal urinary tract characterized by tortuous, dilated ureters and renal dysmorphism. A variety of respiratory, gastrointestinal, musculoskeletal, and cardiovascular anomalies are also associated with the PBS.15 Nowadays, magnetic resonance urography (MRU) utilizes two different imaging strategies, which can be used in complementarity to assess nearly all aspects of upper urinary tract disease. The first technique is the unenhanced, heavily T2-weighted sequence to obtain static-fluid images of the urinary tract. T2-weighted MR urograms have beautifully demonstrated dilation of the urinary tract, even if the renal excretory function is minimal. The second technique is analogous to an intravenous pyelogram and is therefore also referred to as excretory MRU. Gadolinium is administered and, following excretion, the contrast-containing urine is imaged using fat-suppressed T1-weighted GRE sequences. Use of low-dose furosemide helps to achieve uniform distribution of contrast and to avoid signal loss for high concentrations of gadolinium. Together these techniques help assess both nondilated and obstructed urinary systems in patients with compromised renal function.16
157
FIG 6. Prune belly syndrome. Seventeen-year-old male with prune belly syndrome. Coronal T2 (A and B) axial T2 (C) show the coronal SSFSE T2 (D) showing a paucity of abdominal wall musculature, with slightly abnormally located kidneys. Bilateral hydronephrosis, right greater than left.
158
Curr Probl Diagn Radiol, July/August 2007
FIG 8. Postrenal transplant. Seventeen-year-old girl with renal transplant presenting with increased creatinine. Coronal T2 FS showing a large and heterogeneous hyperintense subcapsular collection compatible with subcapsular hematoma.
FIG 7. (A) Left pyelonephritis and vesico-ureteral reflux. Five-month-old female (A) with left pyelonephritis and vesico-ureteral reflux. Axial T2 FS shows a mildly swollen left kidney. (B and C) Evolving renal abscess. Eleven-year-old boy with evolving renal abscess on the right side (B and C). Sagittal T2 FS (B), coronal T1 FS with gadolinium (C) MRI shows an area of abnormal signal involving the upper pole of the right kidney. The postgadolinium image demonstrates abnormal enhancement in the upper pole.
the infectious/inflammatory process toward adjacent soft tissues. Enteric Gram-negative bacteria are the usual infecting organisms among all age groups. Approximately 66% of patients have a history of recurrent urinary tract infections. Since MRI does not involve the use of ionizing radiation, it is indeed a good tool for evaluation of renal abscess and perinephric collections and follow-up.17
Infectious and Inflammatory Even though MRI is not the modality of choice for initial detection of pyelonephritis (Fig 7A), its exquisite anatomic resolution allows for accurate depiction and evaluation of associated complications, such as renal vessel thrombosis, intrarenal abscess (Fig 7B and C), perinephric collections, and even the extension of
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Miscellaneous Renal transplantation (Fig 8) is a common surgical procedure, with approximately 11,000 procedures performed annually in the United States. Causes of graft dysfunction can be categorized as vascular
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FIG 9. Tuberous sclerosis. Sixteen-year-old girl with tuberous sclerosis. Appearance of polycystic kidneys bilaterally associated with small angiomyolipomas. Coronal T2 FS (A), coronal T1 FS with gadolinium (B) showing bilaterally enlarged kidneys, with multiple cysts of different sizes, most of them with low signal on T1- and high signal on T2- and a couple of other lesions with high signal intensity on enhanced T1- and T2-weighted images, corresponding to the angiomyolipomas.
(stenosis or occlusion of the transplant artery or vein), parenchymal (acute tubular necrosis, rejection, and medication toxicity), or extrinsic (perinephric collections or ureteral obstruction). Several studies have demonstrated the usefulness of MRA angiography in the evaluation of the transplant renal artery.5 Other studies have shown the usefulness of MRI in the evaluation of the transplanted kidney and peritransplant region.18 Lee has also described the use of MRI in the assessment of renal transplant dysfunction.19 Both angiomyolipomas and cysts occur commonly in pediatric patients with tuberous sclerosis (TS) (Fig 9) and both tend to increase in size and number with increasing age. It has been shown that angiomyolipomas continue to grow in adults and can distort renal architecture, which compromises function and can also lead to hemorrhage. Renal failure is the leading cause of death in adults with TS. Because of these reasons, imaging surveillance is often performed in patients to identify such lesions and monitor their progression.20
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Tumors Wilms tumor (Figs 10 and 11) accounts for 87% of pediatric renal masses and occurs in approximately 1:10,000 children. Its peak incidence is at 3 to 4 years of age and 80% of patients present before 5 years of age. It is rare in neonates, with less than 0.16% of cases manifesting in this age group. Wilms tumor is bilateral in 4 to 13% of children. Metastases are most commonly found in the lungs (85%), liver, and regional lymph nodes; metastatic disease may also produce vascular invasion. Wilms tumors demonstrate low signal intensity on T1- and high signal intensity on T2-weighted images. MRI also permits assessment of vascular patency and multifocal disease. Determining whether there is direct invasion of the inferior vena cava or adjacent structures may be extremely difficult, but MRI has been reported to be the most sensitive modality for evaluation of such findings.21 Nephroblastomatosis (Fig 12) consists of diffuse or multifocal areas of nephrogenic rests (NR) within the kidneys. NR are foci of metanephric blastema that
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FIG 10. Wilms tumor. Twelve-month-old girl with large bilateral renal masses. Axial T1 (A), enhanced T1 FS (B), and T2 FS (C) images show low signal intensity masses that appear heterogeneous but mildly hyperintense on T2-weighted images, with heterogeneous enhancement postgadolinium. Internal hypointense areas on enhanced T1 images are suggestive of necrotic degeneration. Pathology proved the masses to be mainly nephroblastomatosis with associated malignant features, compatible with Wilms tumors.
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FIG 11. Wilms tumor. Three-year-old girl with Wilms tumor presenting with retroperitoneal bleed. Axial T1 (A), T2 (B), and sagittal T2 (C) images show a large and heterogeneous mass arising from the lower pole of the left kidney. The MRI clearly shows the renal origin of the mass as well as its relationship with the adjacent organs, mass effect, and perinephric bleed.
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found in up to 99% of patients with bilateral Wilms tumors.21
Conclusions The ability to provide exquisite anatomic details of the renal system is a well-recognized asset of MRI. More recently, the introduction of new and improved MRI applications has also added the ability to provide valuable functional information on the renal system. Even though MRI does not play a role as the initial modality for assessing the renal system, its use has been gaining acceptance throughout the world as a valuable tool for assessing renal pathology. The lack of ionizing radiation is also an important advantage, especially within the pediatric population, as it allows us to perform initial and follow-up exams without the undesired radiation exposure. Pediatric renal MRI has a wide spectrum of clinical applications, with this spectrum constantly increasing. With the new advent of functional imaging tools, it is also providing new insights into renal pathology and pathophysiology.
REFERENCES
FIG 12. Nephroblastomatosis. Five-year-old boy with focal nephroblastomatosis. Axial T2 FS (A), coronal inversion recovery (IR) (B), and axial gadolinium-enhanced T1 FS (C) show focal heterogeneous lesion within the interpolar region of the right kidney.
persist beyond 36 weeks of gestation. They have the potential for malignant transformation into Wilms tumors and it is currently believed that NR give rise to approximately 30 to 40% of Wilms tumors. They are
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1. Zhang J, Pedrosa I, Rofsky N. MR techniques for renal imaging. Radiol Clin N Am 2003;41:877-907. 2. Krestin GP. Genitourinary MR: Kidneys and adrenal glands. Eur Radiol 1999;9:1705-14. 3. Zhang H, Prince R. Renal MR Angiography. Magn Reson Imaging Clin N Am 2004;12:487-503. 4. Ertl-Wagner B, Lienemann A, Strauss A, et al. Fetal magnetic resonance imaging indications, technique, anatomical considerations and a review of fetal abnormalities. Eur Radiol 2002;12:1931-40. 5. Hussain SM. MR Imaging: A “one stop shop” modality for preoperative evaluation of potential living kidney donors. Radiographics 2003;23:505-20. 6. Prayer D, Christian BP, Prayer L. Fetal MRI: Techniques and protocols. Pediatr Radiol 2004;34:685-93. 7. Levine D, Barnes P, Edelman RR. Obstetric MR imaging. Radiology 1999;211:609-17. 8. Grenier N, Basseau F, Ries M, et al. Functional MRI of the kidney. Abdom Imaging 2003;28:164-75. 9. Huang AJ, Lee VS, Rusinek H. Functional renal MR imaging. Magn Reson Imaging Clin N Am 2004;12:469-86. 10. Dyer RB, Chen MY, Zagoria RJ. Classic signs in uroradiology. Radiographics 2004;24:S247-80. 11. Dahnert W. Radiology Review Manual. Philadelphia, PA: Lippincott, Williams & Wilkins, 2003. 12. Berg C, Geipel A, Germer U, et al. Prenatal detection for Fraser syndrome without cryptophthalmos: Case report and
Curr Probl Diagn Radiol, July/August 2007
13.
14. 15.
16.
review of the literature. Ultrasound Obstet Gynecol 2001;18: 76-80. Perez-Brayfield MR, Kirssh AJ, Jones RA, et al. A prospective study comparing ultrasound, nuclear scintigraphy and dynamic contrast enhanced magnetic resonance imaging in the evaluation of hydronephrosis. J Urol 2003;170: 1330-4. Wood BP. Prune Belly Syndrome, emedicine 2005. Available at: http://www.emedicine.com/radio/topic 575.htm Berrocal T, Lopez-Pereira P, Arjonilla A, et al. Anomalies of the distal ureter, bladder, and urethra in children: Embryologic, radiologic and pathologic features. Radiographics 2002; 22:1139-64. Nolte-Ernsting CCA, Adam GB, Gunther RW. MR urography: Examination techniques and clinical applications. Eur Radiol 2001;11:355-72.
Curr Probl Diagn Radiol, July/August 2007
17. Willard TB, Teague JL, Steinbecker D. Renal Corticomedullary Abscess, emedicine 2005. Available at: http://www. emedicine.com/med/topic 2848.htm. 18. Hohenwalter MD, Skowlund CJ, Erickson SJ, et al. Renal transplant evaluation with MR angiography and MR imaging. Radiographics 2001;21:1505-17. 19. Lee V. Renal MRI: Novel Techniques and Contrast Agents, PhD, Medscape 2003. Available at: http://www.medscape.com/ viewarticle/471043. 20. Casper KA, Donnelly LF, Chen B, et al. Tuberous sclerosis complex: Renal imaging findings. Radiology 2002;225: 451-6. 21. Lowe LH, Lusani B, Heller RM, et al. Pediatric renal masses: Wilms tumor and beyond. Radiographics 2000;20: 1585-603.
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