Highly accelerated,real‐time phase‐contrast MRI using radial k‐space sampling and GROG‐GRASP reconstruction: a feasibility study in pediatric patients with congenital heart disease

Retrospective electrocardiogram‐gated, 2D phase‐contrast (PC) flow MRI is routinely used in clinical evaluation of valvular/vascular disease in pediatric patients with congenital heart disease (CHD). In patients not requiring general anesthesia, clinical standard PC is conducted with free breathing for several minutes per slice with averaging. In younger patients under general anesthesia, clinical standard PC is conducted with breath‐holding. One approach to overcome this limitation is using either navigator gating or self‐navigation of respiratory motion, at the expense of lengthening scan times. An alternative approach is using highly accelerated, free‐breathing, real‐time PC (rt‐PC) MRI, which to date has not been evaluated in CHD patients. The purpose of this study was to develop a 38.4‐fold accelerated 2D rt‐PC pulse sequence using radial k‐space sampling and compressed sensing with 1.5 × 1.5 × 6.0 mm³ nominal spatial resolution and 40 ms nominal temporal resolution, and evaluate whether it is capable of accurately measuring flow in 17 pediatric patients (aortic valve, pulmonary valve, right and left pulmonary arteries) compared with clinical standard 2D PC (either breath‐hold or free breathing). For clinical translation, we implemented an integrated reconstruction pipeline capable of producing DICOMs of the order of 2 min per time series (46 frames). In terms of association, forward volume, backward volume, regurgitant fraction, and peak velocity at peak systole measured with standard PC and rt‐PC were strongly correlated (R² > 0.76; P < 0.001). Compared with clinical standard PC, in terms of agreement, forward volume (mean difference = 1.4% (3.0% of mean)) and regurgitant fraction (mean difference = ?2.5%) were in good agreement, whereas backward volume (mean difference = ?1.1 mL (28.2% of mean)) and peak‐velocity at peak systole (mean difference = ?21.3 cm/s (17.2% of mean)) were underestimated by rt‐PC. This study demonstrates that the proposed rt‐PC with the said spatial resolution and temporal resolution produces relatively accurate forward volumes and regurgitant fractions but underestimates backward volumes and peak velocities at peak systole in pediatric patients with CHD.     

Influence of beta-blocker therapy on aortic blood flow in patients with bicuspid aortic valve

In patients with bicuspid aortic valve (BAV), beta-blockers (BB) are assumed to slow ascending aorta (AAo) dilation by reducing wall shear stress (WSS) on the aneurysmal segment. The aim of this study was to assess differences in AAo peak velocity and WSS in BAV patients with and without BB therapy. BAV patients receiving BB (BB+, n = 30, age: 47 ± 11 years) or not on BB (BB?, n = 30, age: 46 ± 13 years) and healthy controls (n = 15, age: 43 ± 11 years) underwent 4D flow MRI for the assessment of in vivo aortic 3D blood flow. Peak systolic velocities and 3D WSS were calculated at the anterior and posterior walls of the AAo. Both patient groups had higher maximum and mean WSS relative to the control group (p = 0.001 to p = 0.04). WSS was not reduced in the BB+ group compared to BB? patients in the anterior AAo (maximum: 1.49 ± 0.47 vs. 1.38 ± 0.49 N/m², p = 0.99, mean: 0.76 ± 0.2 vs. 0.74 ± 0.18 N/m², p = 1.00) or posterior AAo (maximum: 1.45 ± 0.42 vs. 1.39 ± 0.58 N/m², p = 1.00; mean: 0.65 ± 0.16 vs. 0.63 ± 0.16 N/m², p = 1.00). AAo peak velocity was elevated in patients compared to controls (p < 0.01) but similar for BB+ and BB? groups (p = 0.42). Linear models identified significant relationships between aortic stenosis severity and increased maximum WSS (β = 0.186, p = 0.007) and between diameter at the sinus of Valsalva and reduced mean WSS (β = ?0.151, p = 0.045). Peak velocity and systolic WSS were similar for BAV patients irrespective of BB therapy. Further prospective studies are needed to investigate the impact of dosage and duration of BB therapy on aortic hemodynamics and development of aortopathy.     

Perioperative evaluation of regional aortic wall shear stress patterns in patients undergoing aortic valve and/or proximal thoracic aortic replacement

Objectives

To assess in patients with aortopathy perioperative changes in thoracic aortic wall shear stress (WSS), which is known to affect arterial remodeling, and the effects of specific surgical interventions.

Multidirectional flow analysis by cardiovascular magnetic resonance in aneurysm development following repair of aortic coarctation

Aneurysm formation is a life-threatening complication after operative therapy in coarctation. The identification of patients at risk for the development of such secondary pathologies is of high interest and requires a detailed understanding of the link between vascular malformation and altered hemodynamics. The routine morphometric follow-up by magnetic resonance angiography is a well-established technique. However, the intrinsic sensitivity of magnetic resonance (MR) towards motion offers the possibility to additionally investigate hemodynamic consequences of morphological changes of the aorta.We demonstrate two cases of aneurysm formation 13 and 35 years after coarctation surgery based on a Waldhausen repair with a subclavian patch and a Vosschulte repair with a Dacron patch, respectively. Comprehensive flow visualization by cardiovascular MR (CMR) was performed using a flow-sensitive, 3-dimensional, and 3-directional time-resolved gradient echo sequence at 3T. Subsequent analysis included the calculation of a phase contrast MR angiography and color-coded streamline and particle trace 3D visualization. Additional quantitative evaluation provided regional physiological information on blood flow and derived vessel wall parameters such as wall shear stress and oscillatory shear index.The results highlight the individual 3D blood-flow patterns associated with the different vascular pathologies following repair of aortic coarctation. In addition to known factors predisposing for aneurysm formation after surgical repair of coarctation these findings indicate the importance of flow sensitive CMR to follow up hemodynamic changes with respect to the development of vascular disease.