The development and potential of acoustic radiation force impulse (ARFI) imaging for carotid artery plaque characterization.

Stroke is the third leading cause of death and long-term disability in the USA. Currently, surgical intervention decisions in asymptomatic patients are based upon the degree of carotid artery stenosis. While there is a clear benefit of endarterectomy for patients with severe (> 70%) stenosis, in those with high/moderate (50-69%) stenosis the evidence is less clear. Evidence suggests ischemic stroke is associated less with calcified and fibrous plaques than with those containing softer tissue, especially when accompanied by a thin fibrous cap. A reliable mechanism for the identification of individuals with atherosclerotic plaques which confer the highest risk for stroke is fundamental to the selection of patients for vascular interventions. Acoustic radiation force impulse (ARFI) imaging is a new ultrasonic-based imaging method that characterizes the mechanical properties of tissue by measuring displacement resulting from the application of acoustic radiation force. These displacements provide information about the local stiffness of tissue and can differentiate between soft and hard areas. Because arterial walls, soft tissue, atheromas, and calcifications have a wide range in their stiffness properties, they represent excellent candidates for ARFI imaging. We present information from early phantom experiments and excised human limb studies to in vivo carotid artery scans and provide evidence for the ability of ARFI to provide high-quality images which highlight mechanical differences in tissue stiffness not readily apparent in matched B-mode images. This allows ARFI to identify soft from hard plaques and differentiate characteristics associated with plaque vulnerability or stability.

Comparison of Acoustic Radiation Force Impulse Imaging Derived Carotid Plaque Stiffness With Spatially Registered MRI Determined Composition.

Measurements of plaque stiffness may provide important prognostic and diagnostic information to help clinicians distinguish vulnerable plaques containing soft lipid pools from more stable, stiffer plaques. In this preliminary study, we compare in vivo ultrasonic Acoustic Radiation Force Impulse (ARFI) imaging derived measures of carotid plaque stiffness with composition determined by spatially registered Magnetic Resonance Imaging (MRI) in five human subjects with stenosis > 50%. Ultrasound imaging was implemented on a commercial diagnostic scanner with custom pulse sequences to collect spatially registered 2D longitudinal B-mode and ARFI images. A standardized, multi-contrast weighted MRI sequence was used to obtain 3D Time of Flight (TOF), T1 weighted (T1W), T2 weighted (T2W), and Proton Density Weighted (PDW) transverse image stacks of volumetric data. The MRI data was segmented to identify lipid, calcium, and normal loose matrix components using commercially available software. 3D MRI segmented plaque models were rendered and spatially registered with 2D B-mode images to create fused ultrasound and MRI volumetric images for each subject. ARFI imaging displacements in regions of interest (ROIs) derived from MRI segmented contours of varying composition were compared. Regions of calcium and normal loose matrix components identified by MRI presented as homogeneously stiff regions of similarly low (typically ≈ 1 µm) displacement in ARFI imaging. MRI identified lipid pools > 2 mm(2), found in three out of five subjects, presented as softer regions of increased displacement that were on average 1.8 times greater than the displacements in adjacent regions of loose matrix components in spatially registered ARFI images. This work provides early evidence supporting the use of ARFI imaging to noninvasively identify lipid regions in carotid artery plaques in vivo that are believed to increase the propensity of a plaque to rupture. Additionally, the results provide early training data for future studies and aid in the interpretation and possible clinical utility of ARFI imaging for identifying the elusive vulnerable plaque.

Diabetes status differentiates endothelial function and plasma nitrite response to exercise stress in peripheral arterial disease following supervised training.

AIMS: To determine if type 2 diabetes mellitus (T2D) differentiates endothelial function and plasma nitrite response (a marker of nitric oxide bioavailability) during exercise in peripheral arterial disease (PAD) subjects prior to and following 3 months supervised exercise training (SET). METHODS: In subjects with T2D+PAD (n = 13) and PAD-only (n = 14), endothelial function was measured using brachial artery flow-mediated dilation. On a separate day, venous blood draws were performed at rest and 10 min following a symptom-limited graded treadmill test (SL-GXT). Plasma samples were snap-frozen for analysis of nitrite by reductive chemiluminescence. All testing was repeated following 3 months of SET. RESULTS: Prior to training both groups demonstrated endothelial dysfunction, which was correlated with a net decrease in plasma nitrite following a SL-GXT (p ≤ 0.05). Following SET, the PAD-only group demonstrated an improvement in endothelial function (p ≤ 0.05) and COT (p ≤ 0.05), which was related to a net increase in plasma nitrite following the SL-GXT (both p ≤ 0.05). The T2D+PAD group had none of these increases. CONCLUSIONS: T2D in the presence of PAD attenuated improvements in endothelial function, net plasma nitrite, and COT following SET. This suggests that T2D maybe associated with an inability to endogenously increase vascular NO bioavailability to SET.

Dietary nitrate supplementation enhances exercise performance in peripheral arterial disease.

Peripheral arterial disease (PAD) results in a failure to adequately supply blood and oxygen (O(2)) to working tissues and presents as claudication pain during walking. Nitric oxide (NO) bioavailability is essential for vascular health and function. Plasma nitrite (NO(2)(-)) is a marker of vascular NO production but may also be a protected circulating "source" that can be converted to NO during hypoxic conditions, possibly aiding perfusion. We hypothesized that dietary supplementation of inorganic nitrate in the form of beetroot (BR) juice would increase plasma NO(2)(-) concentration, increase exercise tolerance, and decrease gastrocnemius fractional O(2) extraction, compared with placebo (PL). This was a randomized, open-label, crossover study. At each visit, subjects (n = 8) underwent resting blood draws, followed by consumption of 500 ml BR or PL and subsequent blood draws prior to, during, and following a maximal cardiopulmonary exercise (CPX) test. Gastrocnemius oxygenation during the CPX was measured by near-infrared spectroscopy. There were no changes from rest for [NO(2)(-)] (152 ± 72 nM) following PL. BR increased plasma [NO(2)(-)] after 3 h (943 ± 826 nM; P ≤ 0.01). Subjects walked 18% longer before the onset of claudication pain (183 ± 84 s vs. 215 ± 99 s; P ≤ 0.01) and had a 17% longer peak walking time (467 ± 223 s vs. 533 ± 233 s; P ≤ 0.05) following BR vs. PL. Gastrocnemius tissue fractional O(2) extraction was lower during exercise following BR (7.3 ± 6.2 vs. 10.4 ± 6.1 arbitrary units; P ≤ 0.01). Diastolic blood pressure was lower in the BR group at rest and during CPX testing (P ≤ 0.05). These findings support the hypothesis that NO(2)(-)-related NO signaling increases peripheral tissue oxygenation in areas of hypoxia and increases exercise tolerance in PAD.

Potential mechanisms for reduced delivery of nitric oxide to peripheral tissues in diabetes mellitus.

Nitric oxide (NO) bioavailability is crucial for normal vascular endothelial function and health. Recent studies have demonstrated an endocrine role for NO equivalents that may be transported in the blood to peripheral tissue beds, where under hypoxic conditions they can liberate NO and cause vasodilation. Exercise training improves endothelial function but its effect on NO bioavailability in peripheral tissues during acute exercise stress in CVD is unclear. This paper will present evidence and discuss possible mechanisms by which NO delivery to peripheral tissues may be dysfunctional in diabetic subjects.

Plasma nitrite flux predicts exercise performance in peripheral arterial disease after 3months of exercise training.

Plasma nitrite is a major oxidation product of nitric oxide. It has also recently been suggested to perform an endocrine-like function as a nitric oxide donor in hypoxic tissues, allowing vasodilation. Exercise performance is limited in peripheral arterial disease because of an inadequate blood supply to working tissues. We hypothesized that exercise training in peripheral arterial disease subjects will improve "plasma nitrite flux" and endothelial function, to accompany increased exercise performance. Peripheral arterial disease subjects were tested at baseline and after 3 months supervised or home exercise training. Venous blood (arm) was drawn at rest and 10 min after a maximal graded treadmill test. Samples were added to heparin and centrifuged and plasma was snap-frozen for analysis by reductive chemiluminescence. Brachial artery endothelial function was measured in response to a hyperemic stimulus (flow-mediated dilation). At 3 months the peripheral arterial disease-supervised exercise group showed increases in claudication onset pain time (+138 s, p< or =0.05), peak walking time (+260 s, p< or =0.01), VO(2peak) (1.3 ml/kg/min, p< or =0.05), brachial artery flow-mediated dilation (+2%, p< or =0.05), and plasma nitrite flux (+33% p< or =0.05). There were no changes in the peripheral arterial disease-home exercise group. The change in plasma nitrite flux predicted the change in claudication onset pain (r(2)=0.59, p< or =0.01). These findings suggest that changes in plasma nitrite are related to endothelial function and predict exercise performance in peripheral arterial disease.