Kelly Jarvis

Advanced flow MRI: emerging techniques and applications.

Magnetic resonance imaging (MRI) techniques provide non-invasive and non-ionising methods for the highly accurate anatomical depiction of the heart and vessels throughout the cardiac cycle. In addition, the intrinsic sensitivity of MRI to motion offers the unique ability to acquire spatially registered blood flow simultaneously with the morphological data, within a single measurement. In clinical routine, flow MRI is typically accomplished using methods that resolve two spatial dimensions in individual planes and encode the time-resolved velocity in one principal direction, typically oriented perpendicular to the two-dimensional (2D) section. This review describes recently developed advanced MRI flow techniques, which allow for more comprehensive evaluation of blood flow characteristics, such as real-time flow imaging, 2D multiple-venc phase contrast MRI, four-dimensional (4D) flow MRI, quantification of complex haemodynamic properties, and highly accelerated flow imaging. Emerging techniques and novel applications are explored. In addition, applications of these new techniques for the improved evaluation of cardiovascular (aorta, pulmonary arteries, congenital heart disease, atrial fibrillation, coronary arteries) as well as cerebrovascular disease (intra-cranial arteries and veins) are presented.

Four-dimensional flow magnetic resonance imaging-based characterization of aortic morphometry and haemodynamics: Impact of age, aortic diameter, and valve morphology

AIMS Four-dimensional (4D) flow magnetic resonance imaging (MRI) was employed for the simultaneous assessment of morphometry and flow parameters along the thoracic aorta to investigate associations between flow, age, aorta diameter, and aortic valve morphology. METHODS AND RESULTS One hundred and sixty-five subjects, 65 controls, 50 patients with bicuspid aortic valve (BAV), and 50 patients with a dilated aorta, and a tricuspid aortic valve (TAV) underwent 4D flow MRI. Following 3D segmentation of the aorta, a vessel centreline was calculated and used to extract aorta diameter, peak systolic velocity, and normalized systolic flow displacement. Validation of 4D flow MRI-based morphometric measurements compared with manual diameter measurements from standard contrast-enhanced MR angiography in 20 controls showed good agreement (mean difference = 0.4 mm, limits of agreement = ±1.31 mm) except at the sinus of valsalva. BAV showed significant differences in average peak velocity (PV; P < 0.016) compared with TAV and controls between the left ventricle outflow tract to sino-tubular junction (BAV: 1.3 ± 0.3 m/s; TAV: 1.2 ± 0.2 m/s; controls: 1.0 ± 0.1 m/s) and the ascending aorta for average normalized flow displacement (BAV: 0.11 ± 0.02; TAV: 0.09 ± 0.02; controls: 0.06 ± 0.01, P < 0.016) despite similar average aortic dimensions for BAV (37 ± 1 mm) and TAV (39 ± 1 mm). Multivariate linear regression showed a significant correlation of maximal aortic diameter to age, PV, and normalized flow displacement (R(2) = 0.413, P < 0.001). CONCLUSION A single acquisition of 4D flow MRI characterized local morphological and haemodynamic differences between groups along the aorta. BAV showed altered haemodynamics when compared with TAV in spite of having similar aorta dimensions. Maximal aorta diameter was associated with age, PV, and normalized flow displacement.

Efficient method for volumetric assessment of peak blood flow velocity using 4D flow MRI

To test the feasibility and effectiveness of using maximum intensity plots (MIPs) based on 4D flow magnetic resonance imaging (MRI) velocity data to assess systolic peak velocities in a cohort of bicuspid aortic valve (BAV) patients.

The effect of resolution on viscous dissipation measured with 4D flow MRI in patients with Fontan circulation: Evaluation using computational fluid dynamics

Viscous dissipation inside Fontan circulation, a parameter associated with the exercise intolerance of Fontan patients, can be derived from computational fluid dynamics (CFD) or 4D flow MRI velocities. However, the impact of spatial resolution and measurement noise on the estimation of viscous dissipation is unclear. Our aim was to evaluate the influence of these parameters on viscous dissipation calculation. Six Fontan patients underwent whole heart 4D flow MRI. Subject-specific CFD simulations were performed. The CFD velocities were down-sampled to isotropic spatial resolutions of 0.5mm, 1mm, 2mm and to MRI resolution. Viscous dissipation was compared between (1) high resolution CFD velocities, (2) CFD velocities down-sampled to MRI resolution, (3) down-sampled CFD velocities with MRI mimicked noise levels, and (4) in-vivo 4D flow MRI velocities. Relative viscous dissipation between subjects was also calculated. 4D flow MRI velocities (15.6 ± 3.8 cm/s) were higher, although not significantly different than CFD velocities (13.8 ± 4.7 cm/s, p=0.16), down-sampled CFD velocities (12.3 ± 4.4 cm/s, p=0.06) and the down-sampled CFD velocities with noise (13.2 ± 4.2 cm/s, p=0.06). CFD-based viscous dissipation (0.81 ± 0.55 mW) was significantly higher than those based on down-sampled CFD (0.25 ± 0.19 mW, p=0.03), down-sampled CFD with noise (0.49 ± 0.26 mW, p=0.03) and 4D flow MRI (0.56 ± 0.28 mW, p=0.06). Nevertheless, relative viscous dissipation between different subjects was maintained irrespective of resolution and noise, suggesting that comparison of viscous dissipation between patients is still possible.

Thoracic Aorta 3D Hemodynamics in Pediatric and Young Adult Patients With Bicuspid Aortic Valve

To evaluate the 3D hemodynamics in the thoracic aorta of pediatric and young adult bicuspid aortic valve (BAV) patients.

Distribution of blood flow velocity in the normal aorta: Effect of age and gender

To apply flow distribution analysis in the entire aorta across a wide age range from pediatric to adult subjects.

Comparison of 4D flow and 2D PC MRI blood flow quantification in children and young adults with congenital heart disease

Background Echocardiography (echo) is the primary imaging modality for assessment of aortic and pulmonary blood flow velocities. 2D phase contrast (PC) MRI provides better access to all segments of the aortic and pulmonary system and is considered the standard for evaluating blood flow. Both techniques are limited by velocity analysis in 2D planes and by single-direction velocity measurement which may be inadequate to characterize the complex 3D hemodynamics in congenital heart disease (CHD). 4D flow MRI provides simultaneous assessment of 3D blood flow characteristics of all vessels within a 3D volume and offers the ability to retrospectively quantify blood flow parameters at selectable regions of interest. The aim of this study is to test the potential of 4D flow for accuracy of quantification of aortic and pulmonary flow parameters compared to the reference standards echo and 2D PC MRI in children and young adults with CHD. Methods 32 patients with CHD who underwent simultaneous 4D flow and 2D PC MRI and echo within 9 months of MRI were retrospectively included. 2D PC MRI flow quantification in the aortic root (Ao), pulmonary trunk (PT), and right and left pulmonary arteries (RPA, LPA) was analyzed using Medis (Medis, Leiden, The Netherlands). 4D flow data analysis included calculation of a 3D-PC-angiogram which was used to position analysis planes in the Ao, PT, LPA and RPA (EnSight, CEI, Apex, NC) for quantification of net flow, regurgitant fraction, Qp:Qs, and peak velocities. Ao peak velocities were assessed by echo. Linear regression analysis was performed. Pearson’ sc orrelation coefficient (r) was calculated. A correlation with p<0.05 was considered significant. Results

Evaluating the disease progression of pediatric bicuspid aortic valve patients using 4D flow MRI data

Background Understanding the natural history of bicuspid aortic valve (BAV) disease from childhood through adulthood may aid in determining if and when these patients are at risk for disease progression. 4D flow MRI can be used to visualize blood flow patterns in BAV patients and can be used to assess valvular function. In this study, we track the progression of pediatric BAV disease by assessing flow pattern and velocity changes over the course of two 4D flow MRI exams separated by at least 7 months between baseline and follow-up scan.

Enhanced segmentation improves 4D blood flow quantification in patients with tetralogy of Fallot and pulmonary regurgitation

Background 2D phase contrast (PC) MRI provides reliable quantification of blood flow in pts with tetralogy of Fallot (TOF). While 2D PC is the standard for evaluating pulmonary regurgitant fraction, it is limited by single direction velocity measurement and may be inadequate to characterize 3D hemodynamics. Multiple planar acquisitions are often required. While 4D flow MRI provides simultaneous assessment of 3D flow characteristics of all vessels within a volume and offers the ability to retrospectively quantify blood flow parameters at selectable regions of interest, these exams require substantial postprocessing and adjacent structures may be difficult to analyze. We compared traditional 2D PC and 4D flow quantification in patients with TOF using both traditional and enhanced segmentation techniques.

Incorporating time-resolved three-dimensional phase contrast (4D flow) MRI in clinical workflow: initial experiences at a large tertiary care medical center

Background Time-resolved three-dimensional phase contrast (4D flow) MRI allows for visualization of three-dimensional cardiovascular anatomy and pulsatile flow with full volumetric coverage in a single, easy to prescribe 3D acquisition. The technique provides comprehensive flow visualization and permits retrospective flow quantification at any user-defined region of interest. [1] To our knowledge, no center has incorporated 4D flow MRI as a part of standard clinical cardiovascular MRI (CMR). The goals of this study include: 1) reporting on the incorporation of 4D flow MRI acquisition and processing as part of clinical CMR workflow and 2) better understanding the clinical impact of 3D flow visualization and retrospective flow quantification derived from 4D flow MRI in CMR.