Alessandra Borlotti

Noninvasive determination of local pulse wave velocity and wave intensity: changes with age and gender in the carotid and femoral arteries of healthy human

We recently introduced noninvasive methods to assess local pulse wave velocity (PWV) and wave intensity ((n)dI) in arteries based on measurements of flow velocity (U) and diameter (D). Although the methods were validated in an experimental setting, clinical application remains lacking. The aim of this study was therefore to investigate the effect of age and gender on PWV and (n)dI in the carotid and femoral arteries of an existing population. We measured D and U in the carotid and femoral arteries of 1,774 healthy subjects aged 35-55 yr, a subgroup of the Asklepios population. With the use of the lnDU-loop method, we calculated local PWV, which was used to determine arterial distensibility ((n)Ds). We then used the new algorithm to determine maximum forward and backward wave intensities ((n)dI(+max) and (n)dI(-min), respectively) and the reflection index ((n)RI). On average, PWV was higher, and (n)Ds was lower in the femoral than at the carotid arteries. At the carotid artery, PWV increased with age, but (n)Ds, (n)dI(+max), and (n)dI(-min) decreased; (n)RI did not change with age. At the femoral artery, PWV was higher, and (n)Ds was lower in male, but all parameters did not change significantly with age in both women and men. We conclude that the carotid artery is more affected by the aging process than the femoral artery, even in healthy subjects. The new techniques provide mechanical and hemodynamic parameters, requiring only D and U measurements, both of which can be acquired using ultrasound equipment widely available today, hence their advantage for potential use in the clinical setting.

CMR Native T1 Mapping Allows Differentiation of Reversible Versus Irreversible Myocardial Damage in ST-Segment-Elevation Myocardial Infarction: An OxAMI Study (Oxford Acute Myocardial Infarction).

Background— CMR T1 mapping is a quantitative imaging technique allowing the assessment of myocardial injury early after ST-segment–elevation myocardial infarction. We sought to investigate the ability of acute native T1 mapping to differentiate reversible and irreversible myocardial injury and its predictive value for left ventricular remodeling. Methods and Results— Sixty ST-segment–elevation myocardial infarction patients underwent acute and 6-month 3T CMR, including cine, T2-weighted (T2W) imaging, native shortened modified look-locker inversion recovery T1 mapping, rest first pass perfusion, and late gadolinium enhancement. T1 cutoff values for oedematous versus necrotic myocardium were identified as 1251 ms and 1400 ms, respectively, with prediction accuracy of 96.7% (95% confidence interval, 82.8% to 99.9%). Using the proposed threshold of 1400 ms, the volume of irreversibly damaged tissue was in good agreement with the 6-month late gadolinium enhancement volume (r=0.99) and correlated strongly with the log area under the curve troponin (r=0.80) and strongly with 6-month ejection fraction (r=−0.73). Acute T1 values were a strong predictor of 6-month wall thickening compared with late gadolinium enhancement. Conclusions— Acute native shortened modified look-locker inversion recovery T1 mapping differentiates reversible and irreversible myocardial injury, and it is a strong predictor of left ventricular remodeling in ST-segment–elevation myocardial infarction. A single CMR acquisition of native T1 mapping could potentially represent a fast, safe, and accurate method for early stratification of acute patients in need of more aggressive treatment. Further confirmatory studies will be needed.

Experimental evaluation of local wave speed in the presence of reflected waves.

Wave speed (also called pulse wave velocity) is the speed by which disturbance travels along the medium and it depends on the mechanical and geometrical properties of the vessel and on the density of the blood. Wave speed is a parameter of clinical relevance because it is an indicator of arterial stiffness and cardiovascular diseases. The aim of this work is to compare different methods for the determination of local wave speed in bench experiments and investigate their relative accuracy when reflections are present. Pressure (P), flow (Q) and diameter (D) were measured along a flexible tube far and close to three positive and three negative reflection sites. Wave speed was calculated using PU-loop, (lnD)U-loop, QA-loop, D(2)P-loop, sum of squares and characteristic impedance methods. Results were compared to the foot-to-foot method. We found that far from the reflections almost all methods give uniform results. Close to positive reflections the methods that rely on P and Q (or U) overestimate the wave speed value, while techniques based on D (or A) and Q (or U) underestimate it. On the contrary, close to negative reflections the methods that rely on P and Q (or U) underestimate the wave speed value, while techniques based on D (or A) and Q (or U) overestimate it. The D(2)P-loop does not seem to be affected by positive or negative reflections. Most of the methods currently used to determine local wave speed are affected by reflections, but the (lnD)U-loop remains the easiest technique to use in the clinic.

Reservoir and reservoir-less pressure effects on arterial waves in the canine aorta

Background: A time-domain approach to couple the Windkessel effect and wave propagation has been recently introduced. The technique assumes that the measured pressure in the aorta (P) is the sum of a reservoir pressure (Pr), due to the storage of blood, and an excess pressure (Pe), due to the waves. Since the subtraction of Pr from P results in a smaller component of Pe, we hypothesized that using the reservoir-wave approach would produce smaller values of wave speed and intensities. Therefore, the aim of this study is to quantify the differences in wave speed and intensity using P, wave-only, and Pe, reservoir-wave techniques. Method: Pressure and flow were measured in the canine aorta in the control condition and during total occlusion at four sites. Wave speed was determined using the PU-loop (c) and PeU-loop (ce) methods, and wave intensity analysis was performed using P and separately using Pe; the magnitude and time of the main waves and the reflection index were calculated. Results: Both analyses produced similar wave intensity analysis curves, and no significant differences in the timing of the waves, except onset of the forward expansion wave, indicated that distal occlusions have little effect on haemodynamics in the ascending aorta. We consistently found lower values of wave speed and intensities when the reservoir-wave model was applied. In particular, the magnitude of the backward waves was markedly smaller, even during proximal occlusions. Conclusion: In the absence of other independent techniques or evidence, it is not currently possible to decide which of the two models is more correct.

A comparison between local wave speed in the carotid and femoral arteries in healthy humans: Application of a new method

The wave speed (c) and the arrival time of reflected wave (Trw) in the common left carotid artery and common left femoral artery have been evaluated in 70 healthy subjects, aged 35–55 years with a non-invasive method.

Myocardial scar quantification using SLIC supervoxels - parcellation based on tissue characteristic strains

Abnormal myocardial motion occurs in many cardiac pathologies, though in different ways, depending on the disease, some of which can result in negative clinical outcomes. Therefore, a better understanding of the contractile capability of the tissue is crucial in providing an improved and patient-specific clinical outcome [4]. Cardiovascular Magnetic Resonance Imaging (CMR) is considered the gold standard for the assessment of cardiac function and has the potential to also be used for routine tissue strain analysis because of its high availability in clinical practice. In this study we estimate the local strain in myocardial tissue over a cardiac cycle using cine MRI imaging to perform the analysis. To quantify the tissue displacement, we use the diffeomorphic demons registration algorithm [15] in a multi-step 3D registration, for the minimization of cumulative errors propagation. Using the displacement gradient of the deformation, individual voxel strain curves are computed. We present a novel method for parcellating the myocardium into regions based on the strain behaviour of clusters of voxels. We define the supervoxels using the Simple Linear Iterative Clustering (SLIC) algorithm [1] inside a predefined mask. The results are consistent with late gadolinium enhancement scar identification.

Wave speed and intensity in the canine aorta: Analysis with and without the Windkessel-wave system

The Windkessel model, coupled with the wave propagation theory, was applied to data measured in the ascending aorta of 11 anaesthetised dogs during total aortic occlusion at the thoracic and diaphragm levels. Wave speed and wave intensity were calculated using the measured pressure (P) and velocity (U), and separately using the pressure due to the wave (Pex) and U in the aorta approximately 1 cm distal to the aortic valve. Results show that wave speed, determined using the PU-loop method, is higher during thoracic than in diaphragm occlusion (p<0.001). On average wave speed calculated using P (c) is higher than that determined using Pex (cWK) in both occlusion sites (p<0.001). During aortic occlusion at the thoracic level, the intensity of backward waves was almost negligible using the Windkessel-wave system. Backward waves were observed during the occlusion at the diaphragm level, but their magnitude is lower compared to that determined with P. The Windkessel-wave system seems to reduce the magnitude of reflected waves during total aorta occlusion, notably if the occlusion sites are close to the ascending aorta.

Using magnetic resonance imaging measurements for the determination of local wave speed and arrival time of reflected waves in human ascending aorta

Wave speed is one of the key factors describing wave propagation in arteries [1]. Local wave speed is directly related to the arterial wall properties [2]. With aging, arterial wave speed increases due to the stiffening of arterial wall, and also related to arterial disease.

The ATI score (age-thrombus burden-index of microcirculatory resistance) determined during primary percutaneous coronary intervention predicts final infarct size in patients with ST-elevation myocardial infarction: a cardiac magnetic resonance validation study

AIMS The age-thrombus burden-index of microcirculatory resistance (ATI) score is a diagnostic tool able to predict suboptimal myocardial reperfusion before stenting, in patients with ST-elevation myocardial infarction (STEMI). We aimed to validate the ATI score against cardiac magnetic resonance imaging (cMRI). METHODS AND RESULTS The ATI score was calculated prospectively in 80 STEMI patients. cMRI was performed within 48 hours in all patients and in 50 patients at six-month follow-up to assess the extent of infarct size (IS%) and microvascular obstruction (MVO%). The ATI score was calculated using age (>50=1 point), pre-stenting index of microcirculatory resistance (IMR) (>40 and <100=1 point; ≥100=2 points) and angiographic thrombus score (4=1 point; 5=3 points). ATI score was closely related to final IS% (ATI.

Variation of wave speed determined by the PU-loop with proximity to a reflection site

Wave speed is directly related to arterial distensibility and is widely used by clinicians to assess arterial stiffness. The PU-loop method for determining wave speed is based on the water hammer equation for flow in flexible tubes and artery using the method of characteristics. This technique determines wave speed using simultaneous measurements of pressure and velocity at a single point. The method shows that during the early part of systole, the relationship between pressure and velocity is generally linear, and the initial slope of the PU-loop is proportional to wave speed. In this work, we designed an in-vitro experiment to investigate the effect of proximity to a reflection site on the wave speed determined by the PU-loop through varying the distance between the measurement and reflection sites. Measurements were made in a flexible tube with a reflection site at the distal end formed by joining the tube to another tube with a different diameter and material properties. Six different flexible tubes were used to generate both positive and negative reflection coefficients of different magnitudes. We found that the wave speed determined by the PU-loop did not change when the measurement site was far from the reflection site but did change as the distance to the reflection site decreased. The calculated wave speed increased with positive reflections and decreased with negative reflections. The magnitude of the change in wave speed at a fixed distance from the reflection site increased with increasing the value of the reflection coefficient.