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Objectives: The objective of this study was to evaluate the performance of a highly accelerated breath-hold 3-dimensional noncontrast-enhanced steady-state free precession thoracic magnetic resonance angiography (NC-MRA) technique in a clinical population, including assessment of image quality, aortic dimensions, and aortic pathology, compared with electrocardiographically gated gadolinium-enhanced MRA (Gd-MRA)., Materials and Methods: After approval from the institution board and informed consent were obtained, 30 patients (22 men; mean age, 53.4 years) with known or suspected aortic pathology were imaged with NC-MRA followed by Gd-MRA at a single examination at 1.5 T. Images were made anonymous and reviewed by 2 readers for aortic pathology and diagnostic confidence on a 5-point scale (1, worst; 5, best) on a patient basis. Image quality and artifacts were also evaluated in 10 vascular segments: aortic annulus, sinuses of Valsalva, sinotubular junction, ascending aorta, aortic arch, descending aorta, diaphragmatic aorta, great vessel origins, and the left main and right coronary artery origins. Finally, aortic dimensions were measured in each of the 7 aortic segments. The Wilcoxon signed rank test was used to compare diagnostic confidence, image quality, and artifact scores between NC-MRA and Gd-MRA. The paired Student t test and Bland-Altman analysis were used for comparison of aortic dimensions., Results: All patients completed NC-MRA and Gd-MRA successfully. Vascular pathologic findings were concordant with Gd-MRA in 29 of 30 (96.7%) patients and 28 of 30 (93.3%) patients for readers 1 and 2, respectively, with high diagnostic confidence (mean [SD], 4.35 [0.77]) not significantly different from Gd-MRA (4.38 [0.64]; P = 0.74). The image quality and artifact scores were comparable with Gd-MRA in most vascular segments. Notable differences were observed at the ascending aorta, where Gd-MRA had superior image quality (4.13 [0.73]) compared with NC-MRA (3.80 [0.88]; P = 0.028), and at the coronary artery origins where NC-MRA was considered superior (NC-MRA vs Gd-MRA, 3.38 [1.47] vs 2.78 [1.21] for the left main artery and NC-MRA vs Gd-MRA, 3.55 [1.40] vs 2.32 [1.16] for the right coronary artery; P < 0.05, both comparisons). The aortic dimensions were comparable, with the only significant difference observed at the ascending aorta, where NC-MRA dimension (4.05 [0.76]) was less than 1 mm smaller than that of Gd-MRA (4.12 [0.7]; P = 0.043)., Conclusions: Breath-hold NC-MRA of the thoracic aorta yields good image quality, comparable to Gd-MRA, with high accuracy for aortic dimension and pathology. It can be considered as an alternative to Gd-MRA in patients with relative contraindications to gadolinium contrast or problems with intravenous access.

Purpose: To examine the feasibility of flow-independent T2-prepared inversion recovery (T2IR) black-blood (BB) magnetization preparation for three-dimensional (3D) balanced steady-state free precession (SSFP) vessel wall MRI of the popliteal artery, and to evaluate its performance relative to flow-dependent double inversion recovery (DIR), spatial presaturation (SPSAT), and motion-sensitizing magnetization preparation (MSPREP) BB techniques in healthy volunteers., Materials and Methods: Eleven subjects underwent 3D MRI at 1.5 Tesla with four techniques performed in a randomized order. Wall and lumen signal-to-noise ratio (SNR), wall-to-lumen contrast-to-noise ratio (CNR), vessel wall area, and lumen area were measured at proximal, middle, and distal locations of the imaged popliteal artery. Image quality scores based on wall visualization and degree of intraluminal artifacts were also obtained., Results: In the proximal region, DIR and SPSAT had higher wall SNR and wall-to-lumen CNR than both MSPREP and T2IR. In the middle and distal regions, DIR and SPSAT failed to provide effective blood suppression, whereas MSPREP and T2IR provided adequate black blood contrast with comparable wall-to-lumen CNR and image quality., Conclusion: The feasibility of 3D SSFP imaging of the popliteal vessel wall using flow-independent T2IR was demonstrated with effective blood suppression and good vessel wall visualization. Although DIR and SPSAT are effective for thin slab imaging, MSPREP and T2IR are better suited for 3D thick slab imaging., (Copyright © 2011 Wiley-Liss, Inc.)

Purpose: To evaluate the accuracy of magnetic resonance angiography (MRA) for preoperative mapping of rectus and gluteal muscle perforating arteries prior to autologous flap breast reconstruction., Materials and Methods: Preoperative MRA on 25 consecutive patients undergoing perforator artery-based autologous breast reconstruction was performed at 1.5 T using 3D liver accelerate volume acquisition (LAVA) of abdominal or gluteal regions acquired during injection of 20 mL of gadobenate dimeglumine with bolus timing optimized using MR fluoroscopy or SmartPrep. Perforator artery size and coordinates relative to umbilicus or top of gluteal crease on 3D MRA were compared to findings at surgery. Reconstructed breast volume estimates from MRA were also compared to weights at harvesting., Results: In all, 132 perforator arteries were found at surgery to be located within 1 cm of the coordinates measured on MRA and were surgically verified to be suitable for flap perfusion. Surgery verified the arterial course and caliber through the rectus and gluteal muscles visualized on MRA in 48 of 49 arteries. Volume rendering of 3D MRA predicted a breast reconstruction volume with a mean difference of 47 g compared to measurements at harvesting., Conclusion: MRA accurately maps rectus and gluteal muscle perforator arteries for preoperative planning of autologous flaps for breast reconstruction., (Copyright 2010 Wiley-Liss, Inc.)

Blood suppression in the lower extremities using flow-reliant methods such as double inversion recovery may be problematic due to slow blood flow. T(2) mapping using fast spin echo (FSE) acquisition was utilized to quantitate the effectiveness of double inversion recovery blood suppression in 13 subjects and showed that 25 +/- 12% of perceived vessel wall pixels in the popliteal arteries contained artifactual blood signal. To overcome this problem, a flow-insensitive T(2)-prepared inversion recovery sequence was implemented and optimal timing parameters were calculated for FSE acquisition. Black blood vessel wall imaging of the popliteal and femoral arteries was performed using two-dimensional T(2)-prepared inversion recovery-FSE in the same 13 subjects. Comparison with two-dimensional double inversion recovery-FSE showed that T(2)-prepared inversion recovery-FSE reduced wall-mimicking blood artifacts that inflated double inversion recovery-FSE vessel wall area measurements in the popliteal artery., ((c) 2010 Wiley-Liss, Inc.)

Renal artery stenosis can be diagnosed with multiple imaging modalities, each one having different risk vs accuracy tradeoffs. Catheter angiography with pressure gradient measurements is the definitive gold standard but also the most invasive and thus reserved primarily for imaging at the time of renal revascularization. Ultrasonography is the safest and least expensive but also the least accurate and most operator-dependent. Contrast-enhanced computed tomographic angiography and magnetic resonance angiography are intermediate (between ultrasound and catheter angiography) with respect to accuracy and expense. Exciting new advances in magnetic resonance that include new contrast agents, which eliminate nephrogenic systemic fibrosis risk, and techniques to characterize the hemodynamic significance of renal artery stenoses are now becoming available. In addition, magnetic resonance angiography without any contrast has become more accurate and rivals contrast-enhanced techniques in some patients. This review explores these techniques for renal artery stenosis imaging.

Objective: Our objective was to evaluate the diagnostic accuracy of time-resolved 2D projection MR angiography in detecting calf and pedal artery occlusive disease., Materials and Methods: Time-resolved MR angiography of calf and pedal arteries was performed on 59 symptomatic legs of 52 patients using the head coil and bolus injections of 6 mL of gadolinium contrast medium. Selective X-ray digital subtraction angiography was performed within 30 days after MR angiography. Calf and pedal arteries were divided into 10 segments. X-ray digital subtraction angiography and MR angiography images were retrospectively interpreted by three expert observers, who graded segments as having no significant stenosis, significant stenosis (> 50%), or occlusion. The accuracy of MR angiography interpretations was compared with the accuracy of consensus X-ray digital subtraction angiography interpretations as the standard of reference. Arterial segments with discrepant grading on X-ray digital subtraction angiography and MR angiography were reviewed again to determine the reasons for disagreement., Results: Arterial phase MR angiography images free of venous contamination were obtained in every case. The agreement between MR angiography and X-ray digital subtraction angiography in depicting infrapopliteal arterial disease was fair to good (kappa = 0.44-0.92). Overall sensitivity and specificity were 83% and 87%, respectively, for detecting significant stenosis of calf and pedal arteries and 86% and 93%, respectively, for detecting occlusions. Accuracy was higher in the larger vessels-for example, calf (84%) compared with foot (71%). In 21% (22/105) of the segments graded differently on MR angiography than on X-ray digital subtraction angiography, it was believed that MR angiography was more likely to be correct than X-ray digital subtraction angiography because of visualization of late-filling arteries on MR angiography that did not opacify on X-ray digital subtraction angiography., Conclusion: Time-resolved 2D projection MR angiography accurately evaluates calf and pedal arteries without degradation from venous contamination.

Purpose: To evaluate visualization inside platinum stents with three-dimensional contrast-enhanced magnetic resonance angiography (CE-MRA)., Materials and Methods: Breath-hold three-dimensional gadolinium (Gd) MRA was performed on 18 patients with 22 platinum stents in the renal (n = 18), celiac (n = 1), superior mesenteric (n = 1), and iliac (n = 2) arteries. Electronic calibers were used to measure the lumen diameter within the stent and just distal to the stent to calculate percent stenosis. MRA accuracy was determined from the difference between percent stenosis measured on MRA and digital subtracted angiography (DSA). The patients were imaged at flip angles of 45 degrees , 60 degrees , 75 degrees , 90 degrees , and 150 degrees ., Results: MRA demonstrated the stent lumen in all of the patients, with a mean difference between MRA and DSA of 21%. For stents oriented parallel to B0 (iliac arteries) the difference was only 10%, as compared to 22% for stents perpendicular to B0. The flip angle with the best agreement between MRA and DSA was 75 degrees (16%)., Conclusion: The lumen of a platinum stent can be imaged with three-dimensional CE-MRA, although grading of restenosis has limited accuracy. The best results were obtained with a flip angle of 75 degrees and for stents in the iliac arteries parallel to B0., (2004 Wiley-Liss, Inc.)

Purpose: To evaluate the incidence and locations of soft tissue enhancement on time-resolved two-dimensional projection magnetic resonance angiography (MRA) of the calf and foot., Materials and Methods: Time-resolved two-dimensional projection MRA of the knee, calf, and foot, performed at 1.5 Tesla using the head coil, was retrospectively reviewed in 326 patients. Soft tissue enhancement of the foot was identified and graded by blinded review. Subsequently, patient medical records were reviewed to determine the presence or absence of diabetes, cellulitis, gangrene, and ulceration of the foot and presence of neuropathic joints., Results: Enhancing spots were identified in 228 patients. For those feet without clinical abnormalities, diabetic patients had an average of 1.2 enhancing spots per foot, while nondiabetics had only 0.6 (P < 0.001). Higher-grade lesions were more prevalent in diabetics and in heavier patients. Of 64 patients with follow-up, 8 (13%) developed cellulitis (N = 4) or ulceration (N = 4) at the location of an enhancing spot, including 5 diabetic and 3 nondiabetic patients., Conclusion: Pedal soft tissue enhancement frequently occurs on time-resolved gadolinium (Gd):MRA of the feet. The etiology is uncertain, but the high frequency in diabetic patients and observation of progression to cellulitis/ulceration suggest this soft tissue enhancement may identify sites of subclinical pedal soft tissue injury., (Copyright 2004 Wiley-Liss, Inc.)

Fast arterial-venous transit in the carotid arteries requires accurate, reliable timing of the acquisition to the bolus transit to maximize arterial signal and minimize venous artifacts. The rising edge of the bolus is not utilized in conventional elliptical-centric view-ordering because the critical k-space center must be acquired with full arterial enhancement. In this study, a recessed elliptical-centric view-ordering scheme is introduced in which the k-space center is acquired a few seconds following scan initiation. The recessed view-ordering is shown to be more robust to timing errors than the conventional scheme in a study of 37 patients., (Copyright 2002 Wiley-Liss, Inc.)

Rationale and Objectives: To investigate the phenomena and causes for undesired venous signal in the distal station of bolus chase 3D MRA., Methods: Consecutive patients (in 8 months) undergoing peripheral MRA consisting of 2D projection MRA of the tibial trifurcation and 3D bolus chase MRA were retrospectively evaluated. Venous contamination in mid-calf in bolus chase 3D MRA was correlated to the arterial phase duration, the time between the contrast bolus arrival and venous return measured on time resolved 2D images. Statistical analyses were performed to identify the clinical parameters indicative of venous contamination., Results: The arterial phase durations at the mid-calf were 49 +/- 8 seconds on 101 legs without venous signal in the bolus chase 3D MRA, 35 +/- 9 seconds on 13 legs with moderate venous signal, and 20 +/- 4 seconds on 40 legs with substantial venous signal; the differences were significant among different venous signal levels (P < 0.001 for all pairs). Legs with cellulitis had shorter arterial phase and more venous contamination than legs without cellulitis (P < 0.05). Patients with myocardial infarction had longer arterial phase and less venous contamination than patients without myocardial infarction (P < 0.01)., Conclusion: Venous signal in the distal calf station of bolus chase 3D peripheral MRA is caused by fast arterial-venous transit. It is worse in legs with cellulitis and less in patients with a history of myocardial infarction.

Purpose: To compare combined three-dimensional (3D) and two-dimensional (2D) contrast material-enhanced magnetic resonance (MR) angiography with x-ray angiography for planning treatment of peripheral vascular disease., Materials and Methods: Three radiologists retrospectively reviewed the pretreatment x-ray angiographic and MR angiographic studies obtained in 30 consecutive patients: 15 patients (15 limbs) evaluated for limb salvage and 15 patients (20 limbs) evaluated because of claudication. MR angiography included acquisition of 2D contrast-enhanced MR digital subtraction angiograms of the area from the adductor canal to the feet and 3D spoiled gradient-recalled-echo bolus chase MR angiograms obtained in three stations from the aorta to the middle portion of the calf. Each reader reviewed the x-ray and MR angiograms to determine the inflow and outflow segments for a hypothetical bypass graft placement., Results: The three readers selected identical segments for inflow at MR angiography and x-ray angiography in 32, 32, and 35 of the 35 limbs evaluated (mean percentages of agreement [95% CI ]: 91% [77%, 98%], 91% [77%, 98%], and 95% [90%, 100%], respectively). The readers selected identical segments for outflow in 32, 32, and 34 of the 35 limbs evaluated (mean percentages of agreement [95% CI]: 91% [77%, 98%], 91% [77%, 98%], and 97% [85%, 100%], respectively)., Conclusion: Preliminary data support the combining of 2D MR digital subtraction angiography with 3D bolus chase MR angiography to extend the utility of 3D MR angiography in treatment planning to include patients being evaluated for limb salvage, as well as those being evaluated for claudication.

Purpose: To retrospectively review time-resolved two-dimensional projection magnetic resonance (MR) angiographic data to characterize the passage of gadolinium-based contrast material down the leg arteries in patients undergoing peripheral MR angiography., Materials and Methods: Data of 87 consecutive standardized peripheral MR angiographic examinations were retrospectively reviewed to determine contrast material arrival times to the common femoral artery (CFA), the popliteal and tibial arteries, and the corresponding veins. Travel times were correlated with clinical data obtained with retrospective chart review and predictive multiparameter models developed with stepwise linear regression., Results: The mean travel time of contrast material to the CFA was 24 seconds +/- 6 (SD), with additional 5 seconds +/- 2 to reach the popliteal artery and 7 seconds +/- 4 to reach the ankle artery. The mean time window of arterial enhancement was 49 seconds +/- 10 in the pelvis, 45 seconds +/- 10 in the thigh, and 35 seconds +/- 14 in the calf. The travel time to CFA was correlated with aortic aneurysm (r = 0.41; P <.001), increasing age (r = 0.31; P =.003), male sex (r = 0.3; P =.005), myocardial infarction (r = 0.26; P =.016), and type 2 diabetes mellitus (r = -0.22; P =.041). Predicted travel time to CFA was 10.6 seconds, plus 0.143 seconds times patient age, plus 4.8 seconds if aneurysm was present, plus 3.8 seconds if male sex, plus 2.8 seconds if history of myocardial infarction., Conclusion: Contrast material injected intravenously traveled rapidly down the peripheral arteries at approximately 6 seconds per station, but a long arterial phase time window allowed bolus-chase peripheral MR angiography to function with slower table stepping. Patient-to-patient variations in contrast material kinetics may be anticipated on the basis of age, sex, and clinical parameters.

The purpose of this study was to improve dynamic two-dimensional projection magnetic resonance digital subtraction angiography by using remasking and filtering postprocessing techniques. Four methods were evaluated in 50 patients: default mask subtraction, remasked subtraction, filtering based on the SD, and linear filtering. The results demonstrated that postprocessing techniques such as linear filtering can reduce background motion artifacts and improve arterial contrast-to-noise ratio.

Rationale and Objectives: The purpose of this study was to identify the cross-sectional location of collateral vessels in patients with peripheral vascular disease on three-dimensional magnetic resonance angiograms (3D MRAs) to suggest sites for intravascular or transcutaneous angiogenesis gene delivery in the lower extremity., Methods: The axial locations were measured and categorized by tissue compartments, as well as by radial coordinates with respect to the femur., Results: Collateral vessels in the thigh were identified in 24 of 93 consecutive patients who underwent peripheral 3D MRA. Ninety-one percent (99/109) of the observed collaterals were located near the adductor canal level of the thigh, with 78% (31/46) of these collaterals located in the fat in or surrounding the posterior muscle., Conclusions: The majority of collateral vessels in the thigh are located in the fat or muscle within the posterior compartment near the femur at the level of the adductor canal.

Introduction: Intra-arterial thrombolysis is commonly used as the initial treatment of acute or subacute lower extremity ischemia., Methods: To evaluate the efficacy and cost of thrombolysis, we retrospectively analyzed 100 consecutive cases (87 patients) in which intra-arterial lysis (urokinase) was used as the initial treatment for native arterial lower extremity occlusive disease. The mean age of patients was 67 years, 57% of the patients were male, and preexisting peripheral vascular disease was present in 74%. Presenting symptoms were limb-threatening ischemia (53%) and claudication (47%). Acute symptoms (< 2 weeks' duration) were present in 48%., Results: The 30-day morbidity rate was 31%, and four patients died. Complications were significant bleeding (23%), ischemic stroke (1%), and renal failure with (2%) and without (2%) dialysis. Concomitant angioplasty was performed in 63%. Complete or significant lysis as demonstrated with angiography was achieved in 75% of iliac, 58% of femoropopliteal, and 41% of crural vessels (P <.001). Within 30 days of lysis, 9% of patients underwent major amputation and 20% surgical revascularization (in 3 patients the extent of revascularization was lessened by the lytic therapy). Amputation-free survival was 83% and 75% at 6 months and 2 years, respectively. Relief of ischemia (defined as relief of claudication or limb salvage without major surgical intervention) was achieved in only 70% and 43% of patients at 30 days and 2 years, respectively (Kaplan-Meier analysis; mean follow-up, 31 months). Patients with aortoiliac disease had significantly better outcomes than those with infrainguinal disease (P =.03). Duration or type of presenting symptoms did not predict outcome. The cost of the initial hospitalization per patient for thrombolysis was $18,490., Conclusion: Thrombolysis can be as or more costly than surgery and is associated with a suboptimal outcome in a significant number of patients. These data lead us to caution against a uniform policy of initial thrombolysis for patients who present with lower extremity ischemia.

The purpose of this study was to evaluate a variable k-space sampling method for bolus-chase three-dimensional magnetic resonance digital subtraction angiography in the arterial system from the abdomen to the calf. Imaging time was minimized by tailoring the acquisition according to the vascular anatomy of the station. For images obtained in 30 patients, results with the modified protocol were compared to those with the previously published protocol. For all stations, significant improvement (P <.001) was found with the modified protocol.

Unlabelled: Wang Y, Winchester PA, Khilnani NM, et al. Contrast-enhanced peripheral MR angiography from the abdominal aorta to the pedal arteries: Combined dynamic two-dimensional and bolus-chase three-dimensional acquisitions. Invest Radiol 2001;36:170-177., Rationale and Objectives: To obtain reliable contrast-enhanced peripheral MR angiography for imaging peripheral vascular disease from the abdominal aorta to the pedal arteries., Methods: A protocol consisting of contrast-enhanced, dynamic two-dimensional (2D) acquisition at the feet and calf and bolus-chase three-dimensional (3D) acquisition from the abdominal aorta to the calf was developed and applied in patients with peripheral vascular disease. The performance of this integrated protocol was assessed in 89 consecutive patients., Results: The bolus-chase 3D acquisition was of diagnostic quality in 100% of the acquisitions in the abdomen, 96% in the thigh, and 43% in the calf. The poor quality of the calf acquisitions was due to insufficient spatial resolution, poor arterial signal, and venous contamination. Diagnostic-quality images were obtained in 100% of the dynamic 2D acquisitions of the calf and 98% of the feet., Conclusions: The combined dynamic 2D and bolus-chase 3D contrast-enhanced MR angiography technique provides diagnostic images of the entire lower extremity.

A view order that matches k-space sampling to cardiac motion within the acquisition window was developed for breath-hold three-dimensional contrast material-enhanced coronary magnetic resonance angiography. In vivo experiments in seven volunteers demonstrated that blurring was substantially reduced with this motion-matched view order as compared with the standard centric view order. Coronary arteries were well delineated.

Purpose: To perform a preliminary evaluation of the diagnostic accuracy of contrast-enhanced, two-dimensional (2D) magnetic resonance (MR) digital subtraction angiography (DSA) of the lower extremity by comparison with x-ray angiography (XRA)., Materials and Methods: Forty lower extremities in 22 patients were imaged at multiple levels with both XRA and 2D MR DSA. Images were retrospectively analyzed by three radiologists in a randomized blinded manner. Seventeen vascular segments were graded as an insignificant lesion, a significant lesion, or as an occlusion. With the use of segments well depicted with XRA as the gold standard, the sensitivity, specificity, and accuracy of 2D MR DSA, as compared with XRA, were evaluated. The McNemar-Stuart-Maxwell test was performed to determine the significance of any differences found., Results: Three hundred eighty-three arterial segments were evaluated with both techniques. Three hundred one segments were well depicted with XRA. There was no significant difference between 2D MR DSA and XRA for assessing the degree of occlusive disease in these 301 segments (.25 < P < .5). The sensitivity, specificity, and diagnostic accuracy of 2D MR DSA were found to be 90%, 98%, and 93%, respectively., Conclusion: Two-dimensional MR DSA is an accurate method for assessing arterial lesions in the lower extremity.

Purpose: To compare the rates of thrombolysis produced by forced intrathrombic injections of saline versus urokinase, as well as automated versus manual injections of urokinase, with use of an in vitro model of a vascular occlusion., Materials and Methods: The rates of thrombolysis produced by forced intrathrombic injections of saline and urokinase were compared in an in vitro radiometric model utilizing I-125-labeled thrombus. Similar experiments were performed to compare manual and automated injections of urokinase. The dissolution of the thrombus was quantitatively monitored with use of a scintillation detector. Averaged time activity data for each type of experiment were fit to exponential functions and half times of lysis calculated. The differences in the half times for the experiments being compared were evaluated for significance with use of the Student t test., Results: The half times of lysis produced by forced intrathrombic injections of urokinase were substantially and significantly shorter than those produced by forced saline injections. The half time of lysis produced by automated injections was not significantly different than that produced by manual injections., Conclusions: Forced intrathrombic injections of urokinase produce faster and substantially more thrombolysis when compared with similarly administered saline. Also, for forced intrathrombic injections of lytic agents, an automated injector is an equivalent alternative to manual injections.

To acquire multiple longitudinal locations in the lower extremity after a single contrast injection, appropriate table translation and contrast injection are required. An approximate model based on constant bolus velocity was developed to describe the space-time course of a contrast bolus in the lower extremity. This model was verified in dynamic MR angiograms acquired in a group of patients using time-resolved 2D MR digital subtraction angiography (MRDSA). From this contrast bolus passage model, a timing algorithm for table translation and contrast injection was developed for bolus chase MRDSA, subsequently validated in bolus chase 2D MRDSA experiments. All targeted major peripheral arteries were well depicted in bolus chase 2D MRDSA using this timing algorithm and a single 15-ml contrast dose.

A bolus-chase magnetic resonance (MR) angiographic technique performed with a prototypic stepping table and coil holder and a 15-20-mL injection of contrast material was developed to depict the entire lower extremity. Image acquisition was synchronized with passage of the contrast medium bolus through the lower extremity. Ten subjects underwent the examination, which was performed in less than 1 minute. All major arteries were well demonstrated in all cases.