Magnetic resonance urography by breath-hold contrast-enhanced three-dimensional FISP

MRI tools for assessment of microstructure and nephron function of the kidney

MRI can provide excellent detail of renal structure and function. Recently, novel MR contrast mechanisms and imaging tools have been developed to evaluate microscopic kidney structures including the tubules and glomeruli. Quantitative MRI can assess local tubular function and is able to determine the concentrating mechanism of the kidney noninvasively in real time. Measuring single nephron function is now a near possibility. In parallel to advancing imaging techniques for kidney microstructure is a need to carefully understand the relationship between the local source of MRI contrast and the underlying physiological change. The development of these imaging markers can impact the accurate diagnosis and treatment of kidney disease. This study reviews the novel tools to examine kidney microstructure and local function and demonstrates the application of these methods in renal pathophysiology.

3D excretory MR urography: Improved image quality with intravenous saline and diuretic administration

PURPOSE

To assess the effect of diuretic administration on the image quality of excretory magnetic resonance urography (MRU) obtained following intravenous hydration, and to determine whether intravenous hydration alone is sufficient to produce diagnostic quality studies of nondilated upper tracts.

MATERIALS AND METHODS

A total of 22 patients with nondilated upper tracts were evaluated with contrast-enhanced MRU. All patients received 250 mL of saline intravenously immediately prior to the examination. A total of 11 patients received 10-20 mg furosemide in addition to saline. Imaging was performed with a three-dimensional (3D) and two-dimensional (2D) breathhold spoiled gradient-echo sequences. Excretory MRU images were acquired five minutes after the administration of 0.1 mmol/kg gadolinium and were independently reviewed by two radiologists, who were blinded to the MRU technique. Readers evaluated the calyces, renal pelvis, and ureters qualitatively for degree of opacification, distention, and artifacts on a four-point scale. Statistical analysis was performed using a permutation test.

RESULTS

There was no significant disagreement between the two readers (P=0.14). Furosemide resulted in significant improvement in calyceal and renal pelvis distention (P<0.005), and significant artifact reduction in all upper tract segments (P<0.001) compared to the effect of saline alone.

CONCLUSION

Intravenous furosemide significantly improves the image quality of excretory MRU studies obtained following intravenous hydration. Intravenous saline alone is insufficient to produce diagnostic quality studies of the non-dilated upper tracts.

Functional renal imaging: nonvascular renal disease

Functional renal imaging-a fast-growing field of MR-imaging-applies different sequence types to gather information about the kidneys other than morphology and angiography. This update article presents the current status of different functional imaging approaches and presents current and potential clinical applications. Apart from conventional in-phase and opposed-phase imaging, which already yields information about the tissue composition, BOLD (blood-oxygenation level dependent) sequences, DWI (diffusion-weighted imaging) sequences, perfusion measurements, and dedicated contrast agents are used.

Magnetic resonance voiding cystography in the diagnosis of vesicoureteral reflux: comparative study with voiding cystourethrography

PURPOSE

To evaluate the feasibility of magnetic resonance voiding cystography (MRVC) compared with voiding cystourethrography (VCUG) for detecting and grading vesicoureteral reflux (VUR).

MATERIALS AND METHODS

MRVC was performed upon 20 children referred for investigation of reflux. Either coronal T1-weighted spin-echo (SE) or gradient-echo (GE) (fast multiplanar spoiled gradient-echo (FMPSPGR) or turbo fast low-angle-shot (FLASH)) images were obtained before and after transurethral administration of gadolinium solution, and immediately after voiding. The findings of MRVC were compared with those of VCUG and technetium-99m ((99m)Tc) dimercaptosuccinic acid (DMSA) single-photon emission computed tomography (SPECT) performed within 6 months of MRVC.

RESULTS

VUR was detected in 23 ureterorenal units (16 VURs by both methods, 5 VURs by VCUG, and 2 VURs by MRVC). With VCUG as the standard of reference, the sensitivity of MRVC was 76.2%; the specificity, 90.0%; the positive predictive value, 88.9%; and the negative predictive value, 78.3%. There was concordance between two methods regarding the grade of reflux in all 16 ureterorenal units with VUR detected by both methods. Of 40 kidneys, MRVC detected findings of renal damage or reflux nephropathy in 13 kidneys, and (99m)Tc DMSA renal SPECT detected findings of reflux nephropathy in 17 kidneys.

CONCLUSION

Although MRVC is shown to have less sensitivity for VUR than VCUG, MRVC may represent a method of choice offering a safer nonradiation test that can additionally evaluate the kidneys for changes related to reflux nephropathy.

Magnetic resonance imaging as a sole method for the morphological and functional evaluation of live kidney donors

OBJECTIVE

To evaluate gadolinium-enhanced dynamic magnetic resonance imaging (MRI) as the sole method for the anatomical and functional assessment of potential live-kidney donors.

SUBJECTS AND METHODS

The study included 50 consecutive kidney donors; in addition to routine donor evaluation, the kidney was imaged with Gd-enhanced dynamic MRI, which was also used for selectively determining the glomerular filtration rate (GFR) of each kidney. All donors had a m99Tc-mercaptoacetyltriglycine (MAG3) renal scan as the reference standard to measure GFR. The anatomical results of MRI were compared with the findings at donor nephrectomy, and the GFR estimated from MRI compared with that from MAG3 scintigraphy.

RESULTS

MR angiography had 100% sensitivity, 94% specificity and 96% overall accuracy for detecting the number of renal arteries, and 100% sensitivity, 98% specificity and 98% overall accuracy for the number of renal veins. There was a close correlation (r = 0.54, P < 0.01) between the GFR of each kidney estimated by MRI or MAG3. For the right and left kidneys the mean isotope clearance was not significantly different from that of mean MRI clearance. MR urography allowed visualization of the urinary tract and the detection of any abnormality.

CONCLUSION

Gd-enhanced dynamic MRI can provide accurate information about the anatomy of the urinary tract and vasculature of the kidney, and can be used to accurately estimate the selective GFR of each kidney. Therefore, we recommend MRI as a single imaging diagnostic method for assessing potential live kidney donors.

Diagnostic value of diuretic-enhanced excretory MR urography in patients with obstructive uropathy

PURPOSE

Ultrasonography and conventional intravenous urography are most common methods in diagnosis of obstructive uropathies. The disadvantage of ultrasonography is inability of visualizing middle and lower one thirds of ureter, while intravenous urography is using radiation, also functionally extra loading effect on kidneys. In this study, the diagnostic value of MR urography on obstructive uropathy were investigated.

MATERIALS AND METHODS

Forty five patients who were suffered from obstructive uropathy examined by ultrasonography, intravenous urography and diuretic-enhanced excretory MR urography by using MR-contrast-agent.

RESULTS

MR urography established accuracy rate of 92.8% for stone diseases which formed the largest group in this study, however, in other causes of obstructive uropathy, MR urography provide 100% correct diagnosis.

CONCLUSION

MR urography provide high quality images for diagnosing and determining causes of urinary obstruction defining position and severity of dilatations as well as showing localization of the pathology. We think that MR urography should be a primary investigation in patients with obstructive uropathy who have contrast agent and X-ray contrindication.

Patients with acute flank pain: comparison of MR urography with unenhanced helical CT

PURPOSE

To compare unenhanced helical computed tomography (CT) and magnetic resonance (MR) urography, by using T2-weighted and contrast material-enhanced T1-weighted imaging to examine patients with acute flank pain, with reference to excretory urography and final clinical diagnosis.

MATERIALS AND METHODS

Forty-nine patients underwent CT, MR urography (with T2-weighted and gadopentetate dimeglumine-enhanced T1-weighted sequences), and excretory urography. CT and MR urographic findings were evaluated separately and independently by two radiologists each (CT, observers A and B; MR urography, observers C and D) for the presence, cause, level, and degree of obstruction. The final conclusive diagnosis was based on the combination of excretory urographic, clinical, and interventional results.

RESULTS

At final diagnosis, 32 (65%) patients were found to have ureteral stones causing unilateral obstruction. In ureteral stone detection, the sensitivity and specificity of CT were 90.6% (29 of 32 patients) and 100.0% (17 of 17 patients), respectively (observer A) and 90.6% (29 of 32 patients) and 94.1% (16 of 17 patients), respectively (observer B), while those of MR urography were 93.8% (30 of 32 patients) and 100.0% (17 of 17 patients), respectively (observer C) and 100.0% (32 of 32 patients) and 100.0% (17 of 17 patients), respectively (observer D). Spearman correlation coefficients for stone size at CT were 0.76 (P <.001) and 0.75 (P <.001) and at MR urography, 0.49 (P =.005) and 0.51 (P =.004).

CONCLUSION

In routine clinical practice, CT is the modality of choice in the evaluation of patients with acute flank pain. MR urography is an accurate and suitable alternative imaging technique in selected patients.

Functional, dynamic, and anatomic MR urography: feasibility and preliminary findings

RATIONALE AND OBJECTIVES

The authors assessed the feasibility of using magnetic resonance (MR) urography to acquire functional, dynamic, and anatomic information in human subjects with normal and hydronephrotic kidneys.

MATERIALS AND METHODS

In subjects known to have or suspected of having hydronephrosis, split renal filtration fractions were measured with a customized magnetization-prepared, inversion-prepared gradient-recalled echo sequence to determine the T1 of flowing blood in the inferior vena cava and aorta before and after contrast medium administration and in the renal veins and arteries after contrast medium administration. Multiple timed sets of coronal fast spoiled gradient-echo 70 degrees flip-angle images were acquired before and after contrast medium administration to derive MR renograms from changes in the signal intensity of the cortex and medulla. Precontrast T2-weighted images were obtained with a three-dimensional fast spoiled gradient-echo maximum intensity projection pulse sequence, and postcontrast T1 maximum intensity projection images were also obtained to depict the renal anatomy.

RESULTS

Split filtration fraction differentiated normal from hydronephrotic kidneys. MR renograms depicted vascular, tubular, and ductal phases and differentiated between normal and hydronephrotic kidneys (P < .05, n = 20). Contrast medium dose correlated with the peak of the cortical signal intensity curves on the renogram (r = 0.7, P < .0005; n = 20). The sensitivities for the visual determination of hydronephrosis and unilateral delayed excretion of contrast material were both 100%, and the specificities were 64% and 85%, respectively.

CONCLUSION

The preliminary findings show promise for the use of MR urography in the comprehensive assessment of renal function, dynamics, and anatomy.

MR urography in evaluation of acute flank pain: T2-weighted sequences and gadolinium-enhanced three-dimensional FLASH compared with urography. Fast low-angle shot

OBJECTIVE

The aim of this study was to compare the usefulness of breath-hold heavily T2-weighted sequences with gadolinium-enhanced three-dimensional fast low-angle shot (3D FLASH) MR urography in the evaluation of patients with acute flank pain.

SUBJECTS AND METHODS

Forty consecutive patients with symptoms of acute flank pain underwent MR urography followed immediately by excretory urography. Heavily T2-weighted (combined thin-slice half-Fourier acquisition single-shot turbo spin-echo [HASTE] and thick-slab single-shot turbo spin-echo) and 3D FLASH sequences were evaluated separately and independently by two experienced radiologists for the presence, cause, level, and degree of obstruction. Interobserver agreement was calculated using the kappa statistic. Excretory urography and the final clinical diagnosis were used as reference.

RESULTS

Twenty-six patients were found to have unilateral obstruction caused by ureteral stones. Both MR urography methods were excellent for detecting obstruction. In the detection of stones 3D FLASH was superior, with a sensitivity of 96.2% and 100% and specificity of 100% and 100% for observers A and B, respectively, compared with a sensitivity of 57.7% and 53.8% and a specificity of 100% and 100%, respectively, for T2-weighted sequences. The best degree of obstruction was seen with 3D FLASH, and the interobserver agreement was excellent for stone detection (kappa = 0.97).

CONCLUSION

T2-weighted sequences alone are not sufficient for examining patients with acute flank pain. However, the combined use of both T2-weighted and 3D FLASH sequences will ensure better confidence in the evaluation of acute suspected renal colic. MR urography can replace conventional excretory urography when the latter is contraindicated or undesirable.

MR virtual endoscopy of the upper urinary tract

OBJECTIVE

We investigated the feasibility of applying surface-rendered virtual endoscopy to the visualization of the upper urinary tract by processing unenhanced MR urography data sets.

SUBJECTS AND METHODS

Twenty-six patients, having neoplastic lesions (n = 9), calculi (n = 8), pelviureteric junction stenoses (n = 4), postoperative fibrotic strictures (n = 3), and extrinsic compressions of the ureter (n = 2), underwent unenhanced MR urography. Virtual endoscopy of the upper urinary tract was obtained using a thresholding technique and surface-rendering MR urography data sets.

RESULTS

Virtual endoscopy of the renal pelvis and calices was feasible in all cases on the side of the urinary obstruction. Virtual endoscopy of the ureter was obtained for a diameter of at least larger than 5 mm. The nondilated side could be partially explored in 11 cases (43%). The mean virtual endoscopy threshold required for the visualization of the urinary tract was 157.36-159.94. The mean time for virtual endoscopy was 13.8 min. Endoluminal masses were found in three (12%) of 26 cases on the renal pelvis (corresponding to neoplastic lesions), and occlusions, in 23 (88%) of 26 on the pelviureteric junction and ureter (neoplastic lesions and other abnormalities).

CONCLUSION

Virtual endoscopy of MR urography data sets is feasible in patients with urinary tract dilatation. Virtual endoscopy displays the renal pelvis, calices, and ureter and, moreover, can show endoluminal changes caused by abnormalities.

Captopril MR renography in a swine model: toward a comprehensive evaluation of renal arterial stenosis

PURPOSE

To test the feasibility of captopril magnetic resonance (MR) renography and to validate the technique in an animal model of renal arterial stenosis.

MATERIALS AND METHODS

Seven pigs with induced renal arterial stenosis were studied. MR renography was performed with a T1-weighted approach by using three-dimensional fast imaging with steady-state precession, or FISP, sequences after administration of a bolus of 0.1 mmol of gadopentetate dimeglumine per kilogram of body weight. Captopril was administered to improve the specificity.

RESULTS

The results demonstrate that differences in renographic curves and indices are observed only if an anatomically substantial stenosis, typically a diameter reduction of more than 70%, is present and captopril is administered.

CONCLUSION

In this preliminary experience in an animal model, captopril MR renography provided data consistent with expectations based on conventional renographic results.

Feasibility and utility of respiratory-gated, gadolinium-enhanced T1-weighted magnetic resonance urography in children

RATIONALE AND OBJECTIVES

To evaluate the feasibility and utility of respiratory-gated, gadolinium-enhanced T1-weighted magnetic resonance (MR) urography in children.

METHODS

In a prospective study, 30 consecutive children, aged 3 weeks to 13.8 years, underwent MR urography. After intravenous injection of low-dose furosemide and gadopentetate dimeglumine, respiratory-gated, coronal, T1-weighted 3D-gradient-echo sequences were obtained at 1.5 T 5 to 30 minutes after contrast administration. Postprocessing of the data was performed using a maximum-intensity projection algorithm. In addition, precontrast half-Fourier rapid acquisition with relaxation enhancement MR urograms were obtained in 29 children.

RESULTS

Respiratory-gated, T1-weighted MR urography was successfully performed in all children without the need for sedation. Compared with the final diagnosis, prospective analysis of MR urography images revealed the correct diagnosis in 56 of 58 pelvicaliceal systems (96%). The ureteral morphology was correctly evaluated in 59 of 64 ureters (92%). The method showed limited efficiency for evaluating nonfunctioning renal units.

CONCLUSIONS

Respiratory-gated, gadolinium-enhanced T1-weighted MR urography is a feasible and effective diagnostic tool in the assessment of upper urinary tract morphology in children. It is especially useful in depicting nondilated collecting systems and ureters.

Effectiveness of MR urography in the evaluation of kidney which failed to opacify during excretory urography: comparison with ultrasonography

OBJECTIVE

The purpose of this study was to compare the effectiveness of MR urography (MRU) with that of ultrasonography (US) in the evaluation of urinary tract when this failed to opacify during excretory urography (EXU).

MATERIALS AND METHODS

Twelve urinary tracts in 11 patients were studied. In each case, during EXU, the urinary system failed to opacify within one hour of the injection of contrast media, and US revealed dilatation of the pelvocalyceal system. Patients underwent MRU, using a HASTE sequence with the breath-hold technique; multi-slice acquisition was then performed, and the images were reconstructed using maximal intensity projection. Each set of images was evaluated by three radiologists to determine the presence, level, and cause of urinary tract obstruction.

RESULTS

Obstruction was present in all twelve cases, and in all of these, MRU accurately demonstrated its level. In this respect, however, US was successful in only ten. The cause of obstruction was determined by MRU in eight cases, but by US in only six. In all of these six, MRU also successfully demonstrated the cause.

CONCLUSION

MRU is an effective modality for evaluation of the urinary tract when this fails to opacify during EXU, and appears to be superior to US in demonstrating the level and cause of obstruction.

State-of-the-art magnetic resonance imaging of the kidneys and upper urinary tract

State-of-the-art magnetic resonance imaging (MRI) is an excellent examination in several clinical contexts of interest to endourologists. First, it offers a one-stop imaging examination of prospective renal donors, obviating the need for arteriography and conventional excretory urography. Second, it reliably depicts urinary tract obstruction and can usually distinguish acute from chronic obstruction, although it is not as sensitive as helical CT in detecting small, nonobstructing stones. Third, it is an excellent examination for characterizing renal masses, especially complex cystic masses, and for surgical planning. Because MRI does not use ionizing radiation and because gadolinium contrast agents are essentially non-nephrotoxic, MRI is especially useful in children, women of childbearing age, and patients with renal insufficiency or renal allografts. Future developments will likely include MR "fluoroscopy," which will provide real-time imaging guidance for interventional procedures in the urinary tract.

Functional imaging of the kidneys with fast MRI techniques

Availability of faster and stronger gradient systems have given rise to a multitude of fast MRI data acquisition strategies which have tremendously increased the scope of MRI applications. These have led to the realization of long desired comprehensive approaches to evaluate anatomy and function using a single modality. In this work, we describe some of our own experiences with functional evaluation of the kidneys using MRI. Examples that suggest the feasibility of comprehensive approaches for evaluation of renal disease are also provided. We also introduce BOLD renal MRI, a method that may allow basic understanding of human renal physiology and pathophysiology in a way that has not been previously possible.

Three-dimensional low dose gadolinium-enhanced peripheral MR venography

Our initial experience with low dose contrast-enhanced (LCE) peripheral MR venography (MRV) is presented. Five subjects were studied using three-dimensional (3D) fast imaging with a steady-state precession (FISP) sequence. A dose of 60 ml of gadopentetate dimeglumine diluted 1:20 was used. A tourniquet was applied during lower extremity MR venography. The venous anatomy was well depicted with the 3D LCE technique in all subjects. Compared to the two-dimensional (2D) time-of-flight (TOF) technique, acquisition time of 3D LCE MRV was much shorter, images looked sharper, and more veins could be seen. It is not affected by in-plane saturation and can be performed repeatedly because of the low dose of contrast. This technique holds promise for the detection of venous thrombosis and other disorders.