K M Yeon

Gachon University, Seongnam, Gyeonggi, South Korea

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Publications (138)371.37 Total impact

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    ABSTRACT: We report a case of thymic hyperplasia accompanied by pericardial lipomatosis and right facial hemihypertrophy in an 8-year-old boy. On imaging studies, the hyperplastic thymus had prominent curvilinear and nodular fatty areas simulating a fat-containing anterior mediastinal mass, which is an unusual finding in children. To our knowledge, this is the first report on a child with a combination of thymic hyperplasia, pericardial lipomatosis, and right facial hemihypertrophy. The radiologic findings are presented with a brief discussion.
    Korean journal of radiology: official journal of the Korean Radiological Society 05/2011; 12(3):376-81. · 1.32 Impact Factor
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    ABSTRACT: Sinus pericranii is a rare venous anomaly, which shows abnormal communication between the intracranial and extracranial venous systems. Sonograms of sinus pericranii in 4 children were retrospectively reviewed to evaluate the following imaging findings: the overall sonographic appearance, change in size and shape, Doppler flow pattern, and associated anomalies. All 4 patients had irregularly shaped anechoic lesions with lobulating and bulging contours. They all showed intracranial and extracranial venous communication through a calvarial defect. Two patients had changes in the sizes and shapes of the lesions, and 2 patients had bidirectional turbulent flow on Doppler images. Two patients had isolated sinus pericranii, and the other 2 were associated with Crouzon syndrome. By identifying a venous scalp mass with intracranial and extracranial venous communication and changes in its size, shape, and venous flow pattern, radiologists will be able to make a definitive diagnosis of sinus pericranii by using gray scale and Doppler sonography.
    Journal of ultrasound in medicine: official journal of the American Institute of Ultrasound in Medicine 03/2011; 30(3):411-7. · 1.40 Impact Factor
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    ABSTRACT: We report a case of isolated periostitis associated with Takayasu arteritis in a 10-year-old boy presenting with calf pain. Radiographs revealed a localized, thick periosteal reaction with irregular margin in the proximal left fibula. MRI revealed irregular thickening with enhancement of the periosteum in the proximal fibula and heterogeneous enhancement along vascular bundles in the calf. Isolated periostitis is a rare skeletal manifestation of systemic vasculitis that could be misdiagnosed as neoplastic or traumatic periosteal reaction.
    Pediatric Radiology 12/2010; 40 Suppl 1:S116-9. · 1.57 Impact Factor
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    ABSTRACT: To determine whether there is a correlation between liver MR findings and the clinical manifestations and severity of liver dysfunction in patients with Wilson's disease. Two radiologists retrospectively evaluated MR images of the liver in 50 patients with Wilson's disease. The Institutional Review Board approved this retrospective study and informed consent was waived. MR images were evaluated with a focus on hepatic contour abnormalities and the presence of intrahepatic nodules. By using Fisher's exact test, MR findings were compared with clinical presentations (neurological and non-neurological) and hepatic dysfunction, which was categorized by the Child-Pugh classification system (A, B and C). Follow-up MR images were available for 17 patients. Contour abnormalities of the liver and intrahepatic nodules were observed in 31 patients (62%) and 25 patients (50%), respectively. Each MR finding showed a statistically significant difference (p < 0.05) among the three groups of Child-Pugh classifications (A, n = 36; B, n = 5; C, n = 9), except for splenomegaly (p = 0.243). The mean age of the patients with positive MR findings was higher than that of patients with negative MR findings. For patients with Child-Pugh class A (n = 36) with neurological presentation, intrahepatic nodules, surface nodularity, and gallbladder fossa widening were more common. Intrahepatic nodules were improved (n = 8, 47%), stationary (n = 5, 29%), or aggravated (n = 4, 24%) on follow-up MR images. MR imaging demonstrates the contour abnormalities and parenchymal nodules of the liver in more than half of the patients with Wilson's disease, which correlates with the severity of hepatic dysfunction and clinical manifestations.
    Korean journal of radiology: official journal of the Korean Radiological Society 11/2010; 11(6):665-72. · 1.32 Impact Factor
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    ABSTRACT: The purpose of this study was to examine the clinical applicability of Doppler sonography by evaluating Doppler sonographic findings in an experimental rabbit model of necrotizing enterocolitis (NEC). Necrotizing enterocolitis was experimentally induced using a combination of endotoxin, hypoxia, and cold stress in 23 rabbits. Doppler sonography was performed to obtain the Doppler spectrum of the superior mesenteric artery. From the flow profile, the peak systolic velocity (PSV) and resistive index (RI) were calculated at 5 time sessions: initial and 1 to 2, 3 to 4, 5 to 6, and 20 to 24 hours. Animals were divided into 2 groups based on pathologic NEC scores (NEC-positive [NEC+] group versus NEC-negative [NEC-] group). Differences between the groups with regard to RI and PSV values were evaluated for each time session. Comparison of RI and PSV values between the NEC+ and NEC- groups revealed a significant increase in the PSV in the NEC+ group during the 1- to 2-hour session (P = .0199). Comparison of RI and PSV differences revealed a significant increase in RI and PSV differences in the NEC+ group during the 1- to 2-hour session (P = .0095 and .0013, respectively). In the other time sessions, there was no difference between the groups. The NEC+ group showed a significant increase in the PSV and RI during the 1- to 2-hour period.
    Journal of ultrasound in medicine: official journal of the American Institute of Ultrasound in Medicine 03/2010; 29(3):379-86. · 1.40 Impact Factor
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    ABSTRACT: We retrospectively evaluated the effectiveness of the esophageal balloon dilatation (EBD) in children with a corrosive esophageal stricture. The study subjects included 14 patients (M:F = 8:6, age range: 17-85 months) who underwent an EBD due to a corrosive esophageal stricture. The causative agents for the condition were glacial acetic acid (n = 9) and lye (n = 5). A total of 52 EBD sessions were performed in 14 patients (range 1-8 sessions). During the mean 15-month follow-up period (range 1-79 months), 12 patients (86%) underwent additional EBD due to recurrent esophageal stricture. Dysphagia improved after each EBD session and oral feeding was possible between EBD sessions. Long-term success (defined as dysphagia relief for at least 12 months after the last EBD) was achieved in two patients (14%). Temporary success of EBD (defined as dysphagia relief for at least one month after the EBD session) was achieved in 17 out of 52 sessions (33%). A submucosal tear of the esophagus was observed in two (4%) sessions of EBD. Only a limited number of children with corrosive esophageal strictures were considered cured by EBD. However, the outcome of repeated EBD was sufficient to allow the children to eat per os prior to surgical management.
    Korean journal of radiology: official journal of the Korean Radiological Society 03/2010; 11(2):203-10. · 1.32 Impact Factor
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    ABSTRACT: Congenital esophageal stenosis (CES) can be associated with esophageal atresia/tracheoesophageal fistula (EA/TEF). Because there are a variety of degrees of obstruction and symptoms of CES, it is frequently difficult to make a pre- and post-operative diagnosis of the distal CES associated with EA/TEF. To evaluate the clinical and radiologic features of congenital esophageal stenosis associated with esophageal atresia/tracheoesophageal fistula. We retrospectively reviewed postoperative esophagograms and medical records of 187 children (107 boys, 80 girls) who had primary repair of EA/TEF from 1992 to 2009 at our institution. We evaluated the incidence of CES, clinical findings, radiologic features and management of CES in these children. CES was diagnosed in 22 of 187 EA/TEF children (12%); one child had double CES lesions, for a total of 23 lesions. Ten of those 22 children (45%) had presented with significant symptoms of esophageal obstruction. The diagnosis of CES was delayed in 10 children (45%) until 1-10 years of age. On esophagogram, CES (n = 23) was located in the distal esophagus (n = 20, 87%) or mid-esophagus (n = 3, 13%). The degree of stenosis was severe (n = 6, 26%), moderate (n = 10, 43%), or mild (n = 7, 30%). Eight children, including two with unsuccessful esophageal balloon dilatation of CES, were treated surgically. Histologic examination revealed tracheobronchial remnant (n = 7) or fibromuscular hyperplasia (n = 1). One child with surgically treated CES developed achalasia at the age of 3 years 9 months. Esophagography after EA/TEF repair should be performed with a high index of suspicion for the presence of distal CES, because the diagnosis and adequate management of CES can often be delayed.
    Pediatric Radiology 03/2010; 40(8):1353-9. · 1.57 Impact Factor
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    ABSTRACT: OBJECTIVE: The purpose of this study was to evaluate retrospectively the CT and sonographic features in nine children with pathologically proven inflammatory myofibroblastic tumors of the abdomen. CONCLUSION: Although inflammatory myofibroblastic tumors occur in various sites and the imaging characteristics are variable, tumors showed different imaging patterns that were dependent on the site at which the tumor had originated.
    American Journal of Roentgenology 11/2009; 193(5):1419-24. · 2.90 Impact Factor
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    ABSTRACT: The purpose of our study was to determine whether CT provides additional information for children with a suspected residual foreign body in the airway after bronchoscopy. We reviewed the CT findings and medical records of nine patients (five girls and four boys; mean age, 17.9 months) who underwent CT between March 1999 and February 2007 for the evaluation of a clinically suspected residual foreign body in the airway after bronchoscopy. We evaluated the location and pattern of bronchial obstruction and associated parenchymal abnormalities on CT. CT findings were compared with a second bronchoscopy in five patients. The remaining four patients were followed clinically. Association between CT findings and results on second bronchoscopy was evaluated with Fisher's exact test. CT after bronchoscopy (n = 9) showed bronchial obstruction in eight patients with focal complete (n = 3), diffuse (n = 3), or combined type (n = 2) bronchial obstruction. These obstructions were not seen at chest radiography. CT revealed unilateral or lobar emphysema (n = 6), atelectasis (n = 6), and consolidation (n = 1). The remaining one patient showed no abnormality on CT. A second bronchoscopy (n = 5) confirmed the focal complete bronchial obstruction at CT (n = 3) as retained foreign body fragments (n = 2) or mucus plug (n = 1) and the combined type at CT (n = 2) as retained foreign body fragments, granulation tissue, and edema of the bronchus (n = 1) or retained foreign body fragments, granulation tissue, and mucus plug (n = 1). There was a significant association between CT findings of type of bronchial obstruction and intrabronchial obstructive lesion on the second bronchoscopy (p = 0.036). CT after bronchoscopy can provide additional information regarding the presence and pattern of bronchial obstruction in children with a suspected residual foreign body.
    American Journal of Roentgenology 07/2009; 192(6):1744-51. · 2.90 Impact Factor
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    ABSTRACT: To evaluate whether perfusion magnetic resonance (MR) imaging can depict hemodynamic status after revascularization surgery and whether changes at perfusion MR imaging after revascularization surgery correspond with clinical outcome in moyamoya disease. An institutional review board approved this retrospective study; informed consent was waived. Pre- and postoperative perfusion MR imaging data in 67 children with moyamoya disease (mean age, 7.2 years; range, 2-13 years) were included. Regional time to peak (rTTP) and regional cerebral blood volume (rCBV) were calculated by adjusting cerebral time to peak (TTP) and cerebral blood volume (CBV) values by using cerebellar reference values. For quantitative regional analysis, pixel values were divided into five categories (>0, >2, >4, >6, and >8 seconds), and percentages of pixels in rTTP meeting these time conditions were calculated. Changes in the values after revascularization were calculated. Postoperative clinical outcomes were categorized as follows: 1 indicated excellent; 2, good; 3, fair; and 4, poor. Pre- and postoperative perfusion parameters were compared by using a paired t test; relationships between perfusion parameters and clinical outcomes were investigated by using one-way analysis of variance, with a significance level of .05. rTTP, rCBV, and percentage of pixels of rTTP decreased significantly after revascularization surgery. Pre- and postoperative rTTP were significantly different for the clinical outcome categories. Change in rTTP and change in percentage of pixels of rTTP (>0 seconds to >6 seconds) were significantly different for the clinical outcome categories. TTP and CBV perfusion maps can depict hemodynamic status after revascularization surgery in moyamoya disease. Furthermore, changes in TTP perfusion maps after revascularization surgery correspond with clinical outcome in patients with moyamoya disease.
    Radiology 05/2009; 251(1):216-23. · 6.34 Impact Factor
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    ABSTRACT: Patients with complex congenital heart defects palliated by connecting the systemic veins directly to the pulmonary circulation are known to develop hepatopulmonary syndrome (HPS). Although rare, HPS can develop following the Fontan operation. To present and analyse the CT findings of HPS in patients with a Fontan circulation. From May to December 2005, six patients with HPS following the completion of a Fontan circulation were evaluated. CT findings were reviewed and were compared with angiographic findings. All six patients showed unilateral involvement. All patients except one had inferior vena cava (IVC) interruption with azygos continuation. CT scans showed abnormal vascular dilatation in one lung, and properly demonstrated the anatomy causing the hepatic venous blood to flow preferentially into one lung. These CT findings correlated well with the angiography findings. HPS that develops after the Fontan procedure is typically unilateral and is often associated with IVC interruption and azygos or hemiazygos continuation. CT demonstrates dilatation of pulmonary vessels in the affected lung and may be able to demonstrate the underlying anatomical cause for the predilection of hepatic venous flow to the contralateral lung.
    Pediatric Radiology 03/2009; 39(4):336-42. · 1.57 Impact Factor
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    ABSTRACT: This study was designed to assess the usefulness of transperineal ultrasonography (US) for the determination of imperforate anus (IA) type. From January 2000 to December 2004, 46 of 193 patients with an IA underwent transperineal US prior to corrective surgery. Sonographic findings were reviewed to identify the presence of internal fistulas and to determine "distal rectal pouch to perineum (P-P)" distances. IA types were determined based on the sonographic findings, and the diagnostic accuracy of transperineal US was evaluated based on surgical findings. Of the 46 patients, 17 patients were surgically confirmed as having a high-type IA, three patients were confirmed as having an intermediate-type IA and 26 patients were confirmed as having a low-type IA. The IA type was correctly diagnosed by the use of transperineal US in 39 of the 46 patients (85%). In 14 of the 17 patients with a high-type IA, internal fistulas were correctly identified. All cases with a P-P distance > 16 mm were high-type IAs and all cases with a P-P distance < 5 mm were low-type IAs. Transperineal US is a good diagnostic modality for the identification of internal fistulas in cases of high-type IA and for defining the IA level.
    Korean journal of radiology: official journal of the Korean Radiological Society 01/2009; 10(4):355-60. · 1.32 Impact Factor
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    ABSTRACT: Central skull base tumours are rare in children. To characterize the imaging features of central skull base lymphoma in children to aid its diagnosis. The authors reviewed the CT findings (available in three patients) and MR findings (available in four patients) in four patients (three boys, one girl; age 2-12 years) with pathologically proven lymphoma (two with Burkitt lymphoma, two with B-cell lymphoma) involving the central skull base. Tumour location and extent, MRI signal intensities, noncontrast CT attenuation, patterns of enhancement, and changes in adjacent structures were evaluated. All four tumours involved the sphenoid bone. In three patients, the tumour extended to the ethmoid sinus and both cavernous sinuses. All lesions were isointense solid masses on T2-weighted images and showed avid enhancement, except for one lesion with a focal necrosis. All tumours were associated with adjacent dural thickening. Non-contrast CT showed highly attenuating masses and permeative bone destruction in the central skull base. Lymphoma involving the central skull base in children is visualized as an isointense mass on T2-weighted MRI with marked contrast enhancement and adjacent dural thickening, and as a highly attenuating mass on noncontrast CT with permeative or erosive bone destruction patterns.
    Pediatric Radiology 09/2008; 38(8):863-7. · 1.57 Impact Factor
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    ABSTRACT: We present two cases of granulocytic sarcoma of the cervical lymph nodes with central necrosis as an initial manifestation of childhood leukaemia, focusing on the imaging features. Recognition of the CT and MR imaging findings of granulocytic sarcoma involving the cervical lymph nodes assists the differential diagnosis of noninfective lymphadenopathy in children.
    Pediatric Radiology 07/2008; 38(6):685-7. · 1.57 Impact Factor
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    ABSTRACT: To evaluate the dynamic changes of the power Doppler (PD) in acute renal vein occlusion and recanalization in a canine model. We performed a PD of the kidney during graded renal vein occlusion and recanalization induced by balloon inflation and deflation in nine dogs. The PD images were transferred to a personal computer, and the PD signals were quantified. We observed the temporal change of the PD signal during renal vein occlusion and recanalization, with a decrease in the PD signal during occlusion and an increase during recanalization. The mean PD signal decreased gradually as the renal vein was occluded, and conversely increased gradually with sequential relief of occlusion. The sequential change of the mean value of the PD signal was statistically significant. The PD can detect a change in renal blood flow during acute renal vein occlusion and recanalization in a canine model. The PD may be used as a helpful tool for the early detection of acute renal vein thrombosis and the monitoring of renal perfusion.
    Korean Journal of Radiology 01/2008; 9(2):128-33. · 1.56 Impact Factor
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    ABSTRACT: The experimental or theoretical method that can visualize or quantify the architecture of bio-cake formed on the surface of membrane is of great interest in understanding biofouling mechanisms, also controlling biofouling in a membrane bioreactor (MBR). This paper summarizes the methods for determining the porosity of bio-cake experimentally or theoretically and also correlates the bio-cake architecture with membrane filterability in a submerged MBR under various operating conditions.
    Desalination. 01/2008; 231(1):115-123.
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    ABSTRACT: To prospectively evaluate the accuracy of hepatic artery diameter and hepatic artery diameter-to-portal vein diameter ratio for ultrasonographic (US) diagnosis of biliary atresia, with cholangiographic or clinical information as reference standard. Institutional review board approval and informed consent were obtained. US was performed in 68 neonates and infants with cholestatic jaundice (mean age, 61 days; male-to-female ratio, 38:30). Biliary atresia (n = 38) was confirmed with cholangiography, and hepatitis (n = 30) was diagnosed with clinical (n = 24) or cholangiographic (n = 6) findings. Diameter of the right hepatic artery was measured with US. Right hepatic artery diameter-to-right portal vein diameter ratio was measured to determine relative enlargement of the hepatic artery. As a control group, 17 neonates and infants (mean age, 67 days; male-to-female ratio, 12:5) without jaundice underwent US of the porta hepatis. Statistical analysis was performed to compare US parameters among three groups with one-way analysis of variance. Optimal cutoff values of the hepatic artery diameter and hepatic artery diameter-to-portal vein diameter ratio for biliary atresia diagnosis were obtained with receiver operating characteristic analysis. The diameter of the right hepatic artery in biliary atresia group (1.9 mm +/- 0.4 [standard deviation]) was significantly larger than that in the hepatitis (1.4 mm +/- 0.3) and control (1.2 mm +/- 0.2) groups (P < .001). Hepatic artery diameter-to-portal vein diameter ratio in the biliary atresia group (0.52 +/- 0.12) was larger than that in hepatitis (0.40 +/- 0.07) and in control (0.40 +/- 0.10) groups (P < .001). Optimum cutoff values for diagnosis of biliary atresia were 1.5 mm (sensitivity, 92%; specificity, 87%; accuracy, 89%) for hepatic artery diameter and 0.45 for hepatic artery diameter-to-portal vein diameter ratio (sensitivity, 76%; specificity, 79%; accuracy, 78%). Measurement of hepatic artery diameter can be helpful in the US diagnosis of biliary atresia.
    Radiology 11/2007; 245(2):549-55. · 6.34 Impact Factor
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    ABSTRACT: To evaluate sonographic findings in ischemic enterocolitis (IEC) and correlate with pathologic findings in an experimental study. Ischemic enterocolitis was induced with ligation of the superior mesenteric artery in 20 rabbits. Plain radiography and ultrasonography (US) were performed. US was done hourly after the ligation using 10 MHz linear probe. US findings were categorized into 2 groups according to the bowel wall echogenicity; the echogenic dots (ED) group and the circumferential granular echogenicity (CGE) group. US findings were compared with the specimen radiography and the histopathology. On US, ED were seen in the bowel of all rabbits after SMA ligation (2.2 +/- 1.3 hours [standard deviation]) and CGE in 16 rabbits (4.1 +/- 0.9 hours). On the specimen radiographs, multiple radiolucent air bubbles were present. Comparing the ED and CGE group, histopathological findings revealed the CGE group had severer injury of the bowel wall than the ED group. On plain radiography, there was progressive bowel distention, but pneumatosis intestinalis (PI) was not evident. ED or CGE are the sonographic findings of ischemic enterocolitis, and bowel wall echogenicity might reflect the degree of ischemic injury.
    Investigative Radiology 06/2007; 42(5):312-8. · 5.46 Impact Factor
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    ABSTRACT: A hepatic mesenchymal hamartoma is an uncommon benign tumor in children and little is known about the spectrum of its radiological features. The purpose of this study is to describe the spectrum of radiological features of a hepatic mesenchymal hamartoma in children. Thirteen children with a pathologically confirmed hepatic mesenchymal hamartoma (M:F = 7:6; mean age, 3 years 2 months) were included in our study. Ultrasonography (US) was performed in nine patients including color and power Doppler US (n = 7). CT scans were performed in all patients. We evaluated the imaging findings of the hepatic mesenchymal hamartomas and the corresponding pathological features. Each patient had a single tumor (mean diameter: 13 cm [1.8-20 cm]). On CT and/or US, four patients (31%) had a "multiseptated cystic tumor", five patients (38%) had a "mixed solid and cystic tumor", and four patients (31%) had a "solid tumor." The septa of the cystic portion were thin in the multiseptated cystic tumors and irregularly thick in the mixed solid and cystic tumors as seen on US. On a post-contrast CT scan, solid portions or thick septa of the tumors showed heterogeneous enhancement. The amount of hepatocytes was significantly different among the three tumor groups according to the imaging spectrum (p = 0.042). A hepatic mesenchymal hamartoma in children can show a wide spectrum of radiological features, from a multiseptated cystic tumor to a mixed solid and cystic tumor, and even a solid tumor.
    Korean Journal of Radiology 01/2007; 8(6):498-505. · 1.56 Impact Factor
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    ABSTRACT: To evaluate prospectively the hemodynamic state of the left renal vein with Doppler ultrasonography (US) and venography in pediatric patients with varicocele to assess the "nutcracker phenomenon." The institutional review board considered this study ethically sound, and parental informed consent was obtained for all subjects. Doppler US of the left renal vein was performed in 27 consecutive boys with varicocele (age range, 7-15 years; mean, 11.9 years) and in 20 boys without varicocele as control subjects (age range, 7-17 years; mean, 11.4 years). Doppler US was used to evaluate left renal vein diameters and peak velocities in the proximal left renal vein near the renal hilum and in the left renal vein between the aorta and superior mesenteric artery (aortomesenteric portion). The diameter ratios and peak velocity ratios between two sites were obtained. For statistical comparison of results, the t test was used. Left renal venography and renocaval pressure measurement were performed in 13 patients with varicocele. The Fisher exact test was used to evaluate the associations between the nutcracker phenomenon (renocaval pressure gradient >/=3 mm Hg) and the development of collateral veins. The diameters of the proximal left renal vein and the peak velocities in the aortomesenteric portion of the left renal vein were significantly different between the varicocele group and the control group (P < .001). The diameter ratios (5.7 +/- 1.8 [standard deviation]) and peak velocity ratios (5.2 +/- 2.6) in patients with varicocele were significantly higher than those in control subjects (3.5 +/- 1.0 and 3.1 +/- 0.8, respectively) (P < .005). According to findings at left renal venography (n = 13), 10 patients (77%) met the criteria for the nutcracker phenomenon. The nutcracker phenomenon was significantly associated with the development of collateral veins (P = .035). Doppler US and venography of the left renal vein can show hemodynamic changes of the left renal vein and depict the presence of the nutcracker phenomenon in pediatric varicocele.
    Radiology 11/2006; 241(1):228-34. · 6.34 Impact Factor

Publication Stats

1k Citations
371.37 Total Impact Points

Institutions

  • 2011
    • Gachon University
      • Department of Radiology (College of Medical Science)
      Seongnam, Gyeonggi, South Korea
  • 1992–2010
    • Seoul National University Hospital
      • Department of Radiology
      Seoul, Seoul, South Korea
  • 1989–2008
    • Seoul National University
      • • Department of Chemical and Biological Engineering
      • • College of Medicine
      Sŏul, Seoul, South Korea
  • 2000
    • Hallym University Medical Center
      • Department of Radiology
      Seoul, Seoul, South Korea
    • Eulji University
      • Department of Radiology
      Seoul, Seoul, South Korea
  • 1998
    • Korea Institute of Science and Technology
      Sŏul, Seoul, South Korea
    • Inha University Hospital
      Sinhyeon, South Gyeongsang, South Korea
    • Ewha Womans University
      Sŏul, Seoul, South Korea
  • 1997
    • National Cancer Center Korea
      Kōyō, Gyeonggi Province, South Korea
  • 1995
    • International St. Mary's Hospitals
      Chemulpo, Incheon, South Korea