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Patient population

Between November 2013 and February 2015, 30 consecutive patients (19 men, 11 women;
mean age 71.96?±?7.6, age range 51–76 years) with confirmed internal carotid artery
(ICA) stenosis were enrolled. Institutional Review Board (IRB) (Sapienza – University
of Rome) and patient informed consent were obtained. For a patient to be included
confirmation of at least one of the following criteria was needed:

1. Doppler-ultrasonography (DUS) showing a hemodynamically significant stenosis of
the internal carotid artery (PSV 125 cm/s);

2. Ultrasound examination showing a stenosis of 30 % according to NASCET criteria
17];

3. Ultrasound examination showing a heterogeneous plaque, irregular surface, intra-plaque
hemorrhage or ulceration;

Patients with a general contraindication to MRA/CTA examination and/or with a known
allergy to contrast agents and/or laboratory signs of renal failure were excluded
from the study.

Imaging technique

Patients were randomly assigned to group A or B, undergoing MRA after the administration
of 0.1 mmol/kg Gadobutrol (0.1 mL/kg) and 0.1 mmol/kg Gadoteridol (0.2 ml/kg), respectively;
randomization was performed according to a 2:1 ratio with 2/3 of patients undergoing
Gadoteridol enhanced MRA.

All examinations were performed on a 3T system (Discovery MR750, GE Healthcare, Milwaukee-WIS,
USA; with peak gradient strength 50 mT/m, peak slew rate 200 mT/m/ms, HD Neurovascular
Array configuration 8-channel, 12-element) with the same technical parameters (T1-weighted
3D SPGRE sequence, TR 4.8 ms, TE 1.8, FA 25°, thickness 0.8 mm, matrix 418?×?418)
in both groups. A contrast agent bolus was administered with an automatic injector
at a rate of 1 ml/s through an 18-gauge cannula placed in the antecubital vein of
the right arm, followed by 15 ml of saline solution. The optimal delay between injection
and MRA acquisition was visually evaluated by the bolus tracking technique. CTA was
performed on a 128-MDCT scanner (Somatom Definition, Siemens Medical System Erlangen,
Germany) using a dual-energy protocol (80 kV and 140 kV, 200 mAs, pitch 0.8, slice-thickness
1 mm, recon increment 0.9 mm, matrix 512×512). The optimal delay between contrast
administration and scan was evaluated with the bolus-tracking technique, with a region
of interest (ROI) placed at the level of the aortic arch and automatic scan triggering
with enhancement threshold set at 150 HU. The acquisition was performed after administration
of 50 ml of nonionic iodinated contrast material (Iomeprol 400 mgI/ml, Iomeron 400,
Bracco, Milan, Italy), followed by the injection of 30 ml of saline solution at a
rate of 4 ml/s with the use of a dual-head injector.

Image analysis

An independent observer (B.S. with 8 years of experience in cardiovascular imaging),
blinded to the contrast agent used, assessed the quality of all MRA datasets using
a 3-point scale (poor: inhomogeneous vessel enhancement, poor intraluminal signal
and wall delineation; adequate: homogenous vessel enhancement, sufficient intraluminal
signal and wall delineation, motion artifacts that did not impair measurements; excellent:
homogenous vessel enhancement, high intraluminal signal, precise wall delineation,
no motion artifacts). Quantitative measurements of signal-to-noise ratio (SNR; signal
Intensity in the vessel/Standard Deviation (SD) outside the body) were also performed
on each dataset by the same observer. More in detail, in each case a vessel ROI was
placed in the terminal common carotid artery just proximal to carotid bifurcation,
whereas the background-ROIs were obtained for 3 different images, in ghosts-free areas.
Two readers (F.Z. with 9 years’ experience and M.A. with 13 years’ experience in MRA
of the carotid arteries), who were blinded to the contrast agent used, independently
evaluated MRA datasets for the presence and degree of steno-occlusive disease (according
to NASCET criteria) and plaque characteristics. All image sets were presented in random
order to each reader. An independent reader (I.C. with 18 years’ experience in cardiovascular
imaging) evaluated the stenosis degree and plaque characteristics on CTA images.

Statistical analysis

Statistical analysis was performed using dedicated software (STATA SE 12 for Macintosh;
Stata Corporation; College Station, Texas, USA). The normality of each continuous
variable group was tested using the Kolmogorov-Smirnov Z test. Continuous data are
described as the mean value?±?Standard Deviation [95 % confidence interval] or [minimum
– maximum] as appropriate; categorical data are expressed as number (percentage).
The Mann–Whitney test was applied to determine significant differences in image quality
between the two contrast agents. CTA was used as the reference standard for ROC curve
analysis and to calculate sensitivity, specificity, positive predictive value (PPV),
negative predictive value (NPV) and accuracy for stenosis degree and plaque morphology
(i.e. plaque ulceration or surface irregularity); a stenosis of 70 % was used as threshold
level for stenosis degree. Areas under the curve (AUC) were compared using the Z-statistic
to determine differences in diagnostic performance between the two contrast agents
18]. Pearson’s correlation coefficient was used to assess performance in the evaluation
of plaque length. Agreement between the two readers was tested using Cohen’s kappa
test and was deemed poor for kappa values of 0.21–0.40, fair for values of 0.41–0.60,
good for values of 0.61–0.80 and excellent for values of 0.81–1.00 19]. SNR values were compared using the Independent-Samples T Test. A p value??0.05 was considered as statistically significant for all tests used.