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Original Research |
1 Department of Radiology, Scott and White Clinic and Hospital, Temple,
TX.
2 Present address: Medical Prevention Center Hamburg, Medical University Center
Hamburg-Eppendorf, Falkenried 88, 20251 Hamburg, Germany.
Received November 24, 2006;
accepted after revision July 6, 2007.
Address correspondence to C. U. Herborn.
Abstract
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SUBJECTS AND METHODS. Twenty-two patients (14 men, eight women; mean age, 66.5 years) underwent two MR angiographic examinations of the renal arteries separated by at least 24 hours on whole-body 1.5- and 3-T MRI systems with a phase-encoded 3D spoiled breath-hold pulse sequence. Two radiologists blinded to the dose of contrast material assessed all image data in consensus for renal arterial disease and for image quality on a five-point Likert-type scale. Quantitative evaluation (vessel signal-to-noise ratio and vessel-muscle contrast-to-noise ratio) was performed by a third radiologist.
RESULTS. Five renal arterial stenoses were detected with both techniques. The difference in mean image quality for the two doses and field strengths was not statistically significant. Overall vessel length and intraparenchymal branches, however, were better visualized with the double dose at 1.5 T. Signal-to-noise and contrast-to-noise ratios were significantly higher (both, p < 0.05) with the double dose at 1.5 T (125.7 and 64.2, respectively) compared with the standard dose at 3 T (112.3 and 59.7).
CONCLUSION. MR angiography can be performed with high diagnostic image quality at 3 T with 0.1 mmol/kg of gadodiamide. Signal-to-noise and contrast-to-noise ratios are higher with a double dose at 1.5 T.
Keywords: high magnetic field strength • MR angiography • renal arterial stenosis
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The worldwide installation of new MRI systems with higher field strength, particularly systems operating at 3 T, is constantly increasing. One of the most favorable features of MR angiography at 3 T is the general gain in signal-to-noise ratio (SNR), which may also translate into an improved contrast-to-noise ratio (CNR) between enhancing vessels and nonenhancing background [10]. The longitudinal TR of T1-weighted imaging of most tissues is substantially longer at higher field strength. This prolongation is tissue-dependent, however, and influences the contrast behavior during T1-weighted imaging. Several publications [11-14] comparing TRs at 1.5 T and 3 T have reported changes between 5% for skeletal muscle and nearly 40% for white matter in the brain and for liver. With the pro-longed T1-weighted TR of tissues at higher field strength, the T1 shortening effect of gadolinium-based contrast agents will become more effective, because the relaxivity of such contrast agents is only slightly affected between 1.5 and 3 T [15]. Changes in signal intensity caused by vascular contrast enhancement observable on the rapid T1-weighted spoiled gradient-recalled echo sequences used for contrast-enhanced MR angiography should be stronger at 3 T than at 1.5 T.
The optimal contrast dose required for contrast-enhanced MR angiography of the renal arteries at 3 T has remained undefined. Imaging can be performed with a double dose (0.2 mmol/kg) or a standard-dose (0.1 mmol/kg). We sought to prospectively compare standard-dose 3D contrast-enhanced MR angiography at 3 T with double-dose contrast-enhanced MR angiography at 1.5 T in patients with suspected renal arterial stenosis.
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MRI
All MRI examinations were performed with superconductive whole-body magnets
operating at 1.5 T and 3 T (Magnetom Sonata and Trio, Siemens Medical
Solutions) with a circular polarized phased-array surface coil with eight
receiving elements at 1.5 T and six elements at 3 T. Examinations with both
MRI systems were performed at random and were separated by at least 24 hours
(maximum, 29.5 hours; mean, 25.25 hours).
After performance of a localizer sequence consisting of three imaging stacks oriented in the axial, sagittal, and coronal planes, a coronal 2D steady-state precession sequence was performed for localization of the origin of the renal arteries. Coronal breath-hold fat-saturated spoiled gradient-recalled echo sequences (fast low-angle shot) with centric k-space filling were performed before and after IV bolus administration of paramagnetic contrast material. The sequences were characterized by the following parameters for both MRI systems: TR/TE, 3.44/1.17; flip angle, 25°; field of view, 280 mm2; matrix size, 288 x 384; number of slices per slab, 56; true spatial resolution, 1.0 x 1.7 x 1.8 mm; acquisition time, 21 seconds; no parallel imaging. Gadodiamide (Omniscan, GE Healthcare) was used for all contrast-enhanced MR angiographic examinations. The contrast agent was administered at a dose of 0.2 mmol/kg for examinations at 1.5 T and at a dose of 0.1 mmol/kg for studies at 3 T. The agent was administered with an automatic injector (Solaris, Medrad) at a rate of 2.0 mL/s and flushed with 30 mL of saline solution at the same flow rate. The sequence was immediately started when bolus tracking showed contrast agent in the abdominal aorta. The injection rate was kept constant to keep the bolus as compact as possible.
For in vitro measurements, increasing concentrations of gadodiamide were prepared in 10-mL vials with saline solution and thoroughly stirred before imaging of the vials at 1.5 T and 3 T in the iso-center of the magnet with the gradient-recalled echo sequence and the phased-array surface coil described earlier.
Image Analysis
All data were transferred to and evaluated on a dedicated postprocessing
workstation (Leonardo, Siemens Medical Solutions). In a side-by-side reading,
images of the same patient acquired at 1.5 T and 3 T were viewed and assessed
at random. To eliminate recognition bias, all patient-related data were masked
on the digitally stored images. Maximum vessel length on the maximum intensity
projection was recorded for both contrast-enhanced MR angiographic data sets
by one investigator with 6 years of experience in MRI evaluation of the renal
arteries. For in vitro measurements, circular and equally sized regions of
interest (ROIs) of 40 mm2 were positioned inside each vial on
cross-sectional source images. SNR was measured on the source images within
the first third of the main renal artery on each site with equally sized (size
range, 40-60 pixels) and locally adapted ROIs according to the following
equation: SIrenal artery/SDnoise, where noise is the SD
from a signal intensity (SI) measurement in a circular ROI in the background.
With the ROI in paravertebral muscle, blood over soft-tissue CNR was
calculated according to the following equation: (SIrenal artery -
SImuscle)/SDnoise. These measurements were performed by
another investigator (3 years of MRI experience). Signal variations of the
surface coils were not further addressed before the respective
calculations.
Image quality of the maximum intensity projection was assessed in consensus
by two radiologists (> 10 years of MRI experience) on the following
four-point Likert-type scale: 1, nondiagnostic (no enhancement within vessel
lumen, complete venous overlay); 2, moderate (enhancement within vessel lumen
but still inhomogeneous, incomplete delineation of vessel border, slight
venous overlay, evaluation possible with low diagnostic confidence); 3, good
visualization (good enhancement within vessel lumen, almost completely
homogeneous, almost no venous overlay, incomplete delineation of vessel
border, evaluation possible with satisfactory diagnostic confidence); 4,
excellent visualization (high and completely homogeneous signal enhancement
within vessel lumen, optimal delineation of vessel border, no venous overlay,
evaluation possible with high diagnostic confidence). No examples of the
grading system were presented to the readers before-hand. Maximum reduction of
the luminal diameter for each lesion (< 50%, 
50%, or occlusion) was
assessed in consensus by two experienced readers on maximum intensity
projections of both MR angiographic data sets at random.
Statistical Analysis
All parameters were recorded on an electronic data sheet (Excel XP,
Microsoft) and subjected to statistical analysis with SPSS version 12 for
Windows software (SPSS). SNR, CNR, and maximal visible vessel length were
analyzed with a nonparametric Wilcoxon's test. Image quality of both sets of
contrast-enhanced MR angiograms was compared by use of a Mantel-Haenszel test
for ordinal data. A value of p < 0.05 was considered to indicate a
statistically significant difference in all cases.
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SNR and CNR with the higher dose at 1.5 T showed a statistically significant difference from the SNR and CNR with the lower dose at 3 T. The mean SNR of the renal arteries on the images obtained at 1.5 T with the double dose were higher than on the data sets obtained at 3 T at the standard dose (125.7 ± 12 vs 112.3 ± 11; p < 0.05). Likewise, the mean CNR was significantly higher on the images obtained at 1.5 T than on those obtained at 3 T (mean, 35 ± 9 vs 23 ± 13; p < 0.02). Mean image quality and field strength also were not significantly different at the two doses. Intraparenchymal branches, however, were more visible with the full dose at 1.5 T. Overall, contrast-enhanced MR angiographic data sets at 1.5 T had an image quality score of 3.6 ± 0.3 versus 3.5 ± 0.4 at 3 T, which did not amount to a statistically significant difference (p > 0.05). For both techniques, there was hardly any overlay from the renal veins or the inferior vena cava.
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Our dose comparison included analysis of both quantitative and qualitative imaging features. Definition of the latter was based on the premise that adequate visualization is a precondition for any meaningful diagnostic assessment. Although relevant disease was correctly detected with the reduced dose, the substantial increase in both SNR and CNR on double-dose contrast-enhanced MR angiographic studies at 1.5 T was paralleled by no-table improvements in absolute vessel length and the visibility rating of intraparenchymal branches. This finding is important because full diagnostic evaluation of the renal arteries is predicated on full visualization of the proximal and distal segments of the renal arteries [24]. However, parenchymal enhancement of the kidneys might have obscured display of vessel length at 3 T. Whereas atherosclerotic changes generally affect the more proximal parts, fibromuscular dysplasia is more frequently encountered in the more distal portions of the renal arteries. This distinction is important because in several instances standard-dose contrast-enhanced MR angiography at 3 T had insufficient contrast enhancement and failed to delineate the distal aspects of the renal arteries.
As for other vascular territories, the concentration of gadolinium-based contrast agents in the renal arteries is influenced by factors well beyond contrast dose. In addition to timing and acquisition parameters, which were equal for examinations at both field strengths, these factors include body mass, blood volume, and circulation time [9]. In turn, these factors are affected by hemodynamic factors such as heart rate, blood pressure, and cardiac ejection fraction. The intrasubject design of this study compensated for variations introduced by these factors because different contrast doses were administered with standardized injection rates. However, the injection volume at 3 T was halved compared with the dose given at 1.5 T. This factor might have contributed to significant differences in quantitative image parameters because the smaller bolus might not have fully matched the central k-space acquisition. To counteract this potential limitation, image acquisition was always timed so that the peak gadolinium concentration in the renal arteries coincided with sampling of the central orders of k-space for both field strengths with bolus tracking. That is, data acquisition was manually started when the bolus was visually detected in the abdominal aorta at the level of the origins of the renal arteries. Titration of the contrast material to equal volumes for both examinations would have undesirably lengthened and diluted the compact standard bolus before pulmonary passage.
Despite its overall substantial role in the diagnostic evaluation of suspected renal arterial stenosis, contrast-enhanced MR angiography of the renal arteries at 1.5 T faces recognized challenges. Several studies have shown much lower sensitivities and specificities than initially reported for the detection of renal arterial disease in addition to a relatively high interobserver variability in grading of renal arterial stenosis [25, 26]. Therefore, the search for technical refinements of current methods appears justified. In this regard, contrast-enhanced MR angiography at 3 T is promising because the doubled magnetic field strength is most likely to translate into improved signal intensity and thereby improved vessel conspicuity. In our study, however, these improvements were not observed with a standard dose of contrast material. The MR angiography sequence at 3 T might have been suboptimal, because the same parameters were used for 1.5 T. In addition, the duration of injection was halved because of the bisected dose administered with the same flow rate, which might have caused more pronounced artifacts on the 3 T data sets. Despite the limitations, contrast-enhanced MR angiography of the renal arteries at 3 T with high spatial and temporal resolution has been described as robust and highly accurate [27-29].
One patient in our study group experienced partial extravasation of the contrast agent. This subject did not undergo the 3 T examination and withdrew from the study. This situation brings up a rarely mentioned but nevertheless serious issue, which is the need to administer IV contrast agents to perform these studies. Some gadolinium-based contrast agents are reported to have a high tissue toxicity that can cause severe damage to interstitial tissue when extravasated from the vascular bed [30].
Our study had several limitations. Although the examination protocol had been optimized and homogenized for both MRI systems used in the study, it is likely that improvements in technique, particularly parallel imaging and time-resolved 3D MR angiography, will further improve contrast-enhanced renal MR angiography at 3 T. In addition, the spatial resolution described may be further improved with parallel imaging techniques such as simultaneous acquisition of spatial harmonics and sensitivity encoding [31-35]. Although these features have become available in the meantime, the 3-T MRI system used in this study was not equipped with such features at the time of data acquisition. However, a large reduction in SNR inherent to parallel imaging techniques must be considered and may be balanced only with the use of the full dose of paramagnetic contrast agents at even higher field strength.
A substantial limitation involved the standard of reference as defined with contrast-enhanced MR angiography at 1.5 T. However, catheter angiography of the renal arteries is deemed clinically necessary only when contrast-enhanced MR angiography depicts severe stenosis and color-coded duplex sonography or functional studies corroborate the finding. Choosing multiple doses, such as 0.15 mmol/kg and 0.2 mmol/kg, to evaluate contrast-enhanced renal MR angiography at 3 T would have been desirable.
Regarding limitations of the statistics reported for our study, one has to keep in mind that evaluation of disease detection was based on the renal artery as the unit of analysis. Our results might have been biased if within-patient clustering occurred; that is, if the results for arteries from the same patient were more closely related than arteries from different patients.
Despite the limitations, our study showed that a dose of 0.1 mmol/kg of gadodiamide is sufficient for acquisition of diagnostic images of the renal arteries at 3 T to exclude atherosclerotic abnormalities in the vessel wall. For delineation of intraparenchymal branches and to fully exploit the potential advantages of higher magnetic field strength, double-dose contrast-enhanced MR angiography may still be necessary. Future hardware and software improvements are likely to help reduce the contrast dose necessary for diagnostic contrast-enhanced MR angiography of the renal arteries.
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