Abstract
Accurate quantification and characterization of atherosclerotic plaques with MRI requires high spatial resolution acquisitions with excellent image quality. The intrinsically better signal-to-noise ratio (SNR) at high-field clinical 7T compared to the widely employed lower field strengths of 1.5 and 3T may yield significant improvements to vascular MRI. However, 7T atherosclerosis imaging also presents specific challenges, related to local transmit coils and B1 field inhomogeneities, which may overshadow these theoretical gains. We present the development and evaluation of 3D, black-blood, ultra-high resolution vascular MRI on clinical high-field 7T in comparison lower-field 3T. These protocols were applied for in vivo imaging of atherosclerotic rabbits, which are often used for development, testing, and validation of translatable cardiovascular MR protocols. Eight atherosclerotic New Zealand White rabbits were imaged on clinical 7T and 3T MRI scanners using 3D, isotropic, high (0.63 mm3) and ultra-high (0.43 mm3) spatial resolution, black-blood MR sequences with extensive spatial coverage. Following imaging, rabbits were sacrificed for validation using fluorescence imaging and histology. Image quality parameters such as SNR and contrast-to-noise ratio (CNR), as well as morphological and functional plaque measurements (plaque area and permeability) were evaluated at both field strengths. Using the same or comparable imaging parameters, SNR and CNR were in general higher at 7T compared to 3T, with a median (interquartiles) SNR gain of +40.3 (35.3–80.1)%, and a median CNR gain of +68.1 (38.5–95.2)%. Morphological and functional parameters, such as vessel wall area and permeability, were reliably acquired at 7T and correlated significantly with corresponding, widely validated 3T vessel wall MRI measurements. In conclusion, we successfully developed 3D, black-blood, ultra-high spatial resolution vessel wall MRI protocols on a 7T clinical scanner. 7T imaging was in general superior to 3T with respect to image quality, and comparable in terms of plaque area and permeability measurements.
Highlights
Cardiovascular disease due to atherosclerosis is the main cause of morbidity and mortality worldwide [1]
In several rabbit studies we have demonstrated that the area under the contrast agent uptake curve (AUC) from in vivo vessel wall dynamic contrast enhanced (DCE)-magnetic resonance imaging (MRI) is a good surrogate measure of atherosclerotic plaque permeability [17, 18, 32], as validated by histology [17, 32], and by measuring vessel wall permeability to Evans Blue (EB) dye using near infra-red fluorescence (NIRF) [18]
At the same spatial resolution, we found signal-to-noise ratio (SNR) to be significantly higher (p
Summary
Cardiovascular disease due to atherosclerosis is the main cause of morbidity and mortality worldwide [1]. Efficient “black blood” preparations, which suppress the bright signal from flowing blood, and perivascular fat signal suppression are needed to ensure optimal contrast and delineation of the arterial wall with respect to the vessel lumen and other neighboring structures [20, 22, 23] To satisfy all these requirements while maintaining high scan time efficiency, signal-to-noise ratio (SNR) and spatial resolution often need to be compromised when imaging on 1.5 and 3T scanners. This significantly affects the accuracy of atherosclerosis burden measurements and the characterization of plaque composition, especially when using advanced quantitative MR techniques [24]. The measurement of functional parameters relevant to high-risk plaque phenotypes, such as enhanced vessel wall permeability and neovascularization using high temporal resolution dynamic contrast enhanced (DCE) MRI [25], may be significantly impacted by low SNR measurements
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