Noninvasive 4D pressure difference mapping derived from 4D flow MRI in patients with repaired aortic coarctation: comparison with young healthy volunteers

Four-dimensional flow cardiovascular magnetic resonance aortic cross-sectional pressure changes and their associations with flow patterns in health and ascending thoracic aortic aneurysm

BACKGROUND

Ascending thoracic aortic aneurysm (ATAA) is a silent and threatening dilation of the ascending aorta (AscAo). Maximal aortic diameter which is currently used for ATAA patients management and surgery planning has been shown to inadequately characterize risk of dissection in a large proportion of patients. Our aim was to propose a comprehensive quantitative evaluation of aortic morphology and pressure-flow-wall associations from four-dimensional (4D) flow cardiovascular magnetic resonance (CMR) data in healthy aging and in patients with ATAA.

METHODS

We studied 17 ATAA patients (64.7 ± 14.3 years, 5 females) along with 17 age- and sex-matched healthy controls (59.7 ± 13.3 years, 5 females) and 13 younger healthy subjects (33.5 ± 11.1 years, 4 females). All subjects underwent a CMR exam, including 4D flow and three-dimensional anatomical images of the aorta. This latter dataset was used for aortic morphology measurements, including AscAo maximal diameter (iDMAX) and volume, indexed to body surface area. 4D flow MRI data were used to estimate 1) cross-sectional local AscAo spatial (∆PS) and temporal (∆PT) pressure changes as well as the distance (∆DPS) and time duration (∆TPT) between local pressure peaks, 2) AscAo maximal wall shear stress (WSSMAX) at peak systole, and 3) AscAo flow vorticity amplitude (VMAX), duration (VFWHM), and eccentricity (VECC).

RESULTS

Consistency of flow and pressure indices was demonstrated through their significant associations with AscAo iDMAX (WSSMAX:r = -0.49, p < 0.001; VECC:r = -0.29, p = 0.045; VFWHM:r = 0.48, p < 0.001; ∆DPS:r = 0.37, p = 0.010; ∆TPT:r = -0.52, p < 0.001) and indexed volume (WSSMAX:r = -0.63, VECC:r = -0.51, VFWHM:r = 0.53, ∆DPS:r = 0.54, ∆TPT:r = -0.63, p < 0.001 for all). Intra-AscAo cross-sectional pressure difference, ∆PS, was significantly and positively associated with both VMAX (r = 0.55, p = 0.002) and WSSMAX (r = 0.59, p < 0.001) in the 30 healthy subjects (48.3 ± 18.0 years). Associations remained significant after adjustment for iDMAX, age, and systolic blood pressure. Superimposition of ATAA patients to normal aging trends between ∆PS and WSSMAX as well as VMAX allowed identifying patients with substantially high pressure differences concomitant with AscAo dilation.

CONCLUSION

Local variations in pressures within ascending aortic cross-sections derived from 4D flow MRI were associated with flow changes, as quantified by vorticity, and with stress exerted by blood on the aortic wall, as quantified by wall shear stress. Such flow-wall and pressure interactions might help for the identification of at-risk patients.

Validation of 4D Flow based relative pressure maps in aortic flows

While the clinical gold standard for pressure difference measurements is invasive catheterization, 4D Flow MRI is a promising tool for enabling a non-invasive quantification, by linking highly spatially resolved velocity measurements with pressure differences via the incompressible Navier-Stokes equations. In this work we provide a validation and comparison with phantom and clinical patient data of pressure difference maps estimators. We compare the classical Pressure Poisson Estimator (PPE) and the new Stokes Estimator (STE) against catheter pressure measurements under a variety of stenosis severities and flow intensities. Specifically, we use several 4D Flow data sets of realistic aortic phantoms with different anatomic and hemodynamic severities and two patients with aortic coarctation. The phantom data sets are enriched by subsampling to lower resolutions, modification of the segmentation and addition of synthetic noise, in order to study the sensitivity of the pressure difference estimators to these factors. Overall, the STE method yields more accurate results than the PPE method compared to catheterization data. The superiority of the STE becomes more evident at increasing Reynolds numbers with a better capacity of capturing pressure gradients in strongly convective flow regimes. The results indicate an improved robustness of the STE method with respect to variation in lumen segmentation. However, with heuristic removal of the wall-voxels, the PPE can reach a comparable accuracy for lower Reynolds' numbers.

4D flow imaging of the thoracic aorta: is there an added clinical value?

Four-dimensional (4D) flow MRI has emerged as a powerful non-invasive technique in cardiovascular imaging, enabling to analyse in vivo complex flow dynamics models by quantifying flow parameters and derived features. Deep knowledge of aortic flow dynamics is fundamental to better understand how abnormal flow patterns may promote or worsen vascular diseases. In the perspective of an increasingly personalized and preventive medicine, growing interest is focused on identifying those quantitative functional features which are early predictive markers of pathological evolution. The thoracic aorta and its spectrum of diseases, as the first area of application and development of 4D flow MRI and supported by an extensive experimental validation, represents the ideal model to introduce this technique into daily clinical practice. The purpose of this review is to describe the impact of 4D flow MRI in the assessment of the thoracic aorta and its most common affecting diseases, providing an overview of the actual clinical applications and describing the potential role of derived advanced hemodynamic measures in tailoring follow-up and treatment.

4D flow MRI applications in congenital heart disease

Advances in the diagnosis and management of congenital heart disease (CHD) have resulted in a growing population of patients surviving well into adulthood and requiring lifelong follow-up. Flow quantification is a central component in the assessment of patients with CHD. 4D flow magnetic resonance imaging (MRI) has emerged as a tool that enables comprehensive study of flow. It involves the acquisition of a three-dimensional time-resolved volume with velocity encoding in all three spatial directions along the cardiac cycle. This allows flow quantification and visualization of blood flow patterns as well as the study of advanced hemodynamic parameters as kinetic energy and wall shear stress. 4D flow MRI-based study of flow has given insight into the altered hemodynamics in CHD particularly in bicuspid aortic valve disease and Fontan circulation. The aim of this review is to discuss the expanding clinical and research applications of 4D flow MRI in CHD as well its limitations.Key Points• Three-dimensional velocity encoding allows not only flow quantification but also the visualization of multidirectional flow patterns and the study of advanced hemodynamic parameters.• 4D flow MRI has added insight into the abnormal hemodynamics involved in congenital heart disease in particular in bicuspid aortic valve and Fontan circulation.• The main limitation of 4D flow MRI in congenital heart disease is the relatively long scan duration required for the complete coverage of the heart and great vessels with adequate spatiotemporal resolution.

Impact of sequence type and field strength (1.5, 3, and 7T) on 4D flow MRI hemodynamic aortic parameters in healthy volunteers

PURPOSE

4D flow magnetic resonance imaging (4D-MRI) allows time-resolved visualization of blood flow patterns, quantification of volumes, velocities, and advanced parameters, such as wall shear stress (WSS). As 4D-MRI enters the clinical arena, standardization and awareness of confounders are important. Our aim was to evaluate the equivalence of 4D flow-derived aortic hemodynamics in healthy volunteers using different sequences and field strengths.

METHODS

4D-MRI was acquired in 10 healthy volunteers at 1.5T using three different prototype sequences, at 3T and at 7T (Siemens Healthineers). After evaluation of diagnostic quality in three segments (ascending-, descending aorta, aortic arch), peak velocity, flow volumes, and WSS were investigated. Equivalence limits for comparison of field strengths/sequences were based on the limits of Bland-Altman analyses of the intraobserver variability.

RESULTS

Non-diagnostic quality was found in 10/144 segments, 9/10 were obtained at 7T. Apart for the comparison of forward flow between sequence 1 and 3, the differences in measurements between field strengths/sequences exceeded the range of agreement. Significant differences were found between field strengths/sequences for forward flow (1.5T vs. 3T, 3T vs. 7T, sequence 1 vs. 3, 2 vs. 3 [P < .001]), WSS (1.5T vs. 3T [P < .05], sequence 1 vs. 2, 1 vs. 3, 2 vs. 3 [P < .001]), and peak velocity (1.5T vs. 7T, sequence 1 vs. 3 [P > .001]). All parameters at all field strengths/with all sequences correlated moderately to strongly (r ≥ 0.5).

CONCLUSION

Data from all sequences could be acquired and resulting images showed sufficient quality for further analysis. However, the variability of the measurements of peak velocity, flow volumes, and WSS was higher when comparing field strengths/sequences as the equivalence limits defined by the intraobserver assessments.

4D Flow with MRI

Magnetic resonance imaging (MRI) has become an important tool for the clinical evaluation of patients with cardiac and vascular diseases. Since its introduction in the late 1980s, quantitative flow imaging with MRI has become a routine part of standard-of-care cardiothoracic and vascular MRI for the assessment of pathological changes in blood flow in patients with cardiovascular disease. More recently, time-resolved flow imaging with velocity encoding along all three flow directions and three-dimensional (3D) anatomic coverage (4D flow MRI) has been developed and applied to enable comprehensive 3D visualization and quantification of hemodynamics throughout the human circulatory system. This article provides an overview of the use of 4D flow applications in different cardiac and vascular regions in the human circulatory system, with a focus on using 4D flow MRI in cardiothoracic and cerebrovascular diseases.

Three-dimensional and four-dimensional flow assessment in congenital heart disease

Congenital heart disease (CHD) is the most common form of congenital defects, with an incidence of 8 per 1000 births. Due to major advances in diagnostics, perioperative care and surgical techniques, the survival rate of patients with CHD has improved dramatically. Conversely, although 70%-95% of infants with CHD survive into adulthood, the rate of long-term morbidity, which often requires (repeat) intervention, has increased. Recently, the role of altered haemodynamics in cardiac development and CHD has become a subject of interest. Patients with CHD often have abnormal blood flow patterns, either due to the primary cardiac defect or as a consequence of the surgical intervention(s). Research suggests that these abnormal blood flow patterns may contribute to diminished cardiac and vascular function. Serial assessment of haemodynamic parameters in patients with CHD may allow for improved understanding of the often complex haemodynamics in these patients and thereby potentially guide the timing and nature of interventions with the aim of preventing progression of cardiovascular deterioration. In this article we will discuss two novel non-invasive four-dimensional (4D) techniques to evaluate cardiovascular haemodynamics: 4D-flow cardiac magnetic resonance and computational fluid dynamics. This review focuses on the additional value of these two modalities in the evaluation of patients with CHD with abnormal flow patterns, who could benefit from advanced haemodynamic evaluation: patients with coarctation of the aorta, bicuspid aortic valve, tetralogy of Fallot and patients after Fontan palliation.

Neopulmonary Outflow Tract Obstruction Assessment by 4D Flow MRI in Adults With Transposition of the Great Arteries After Arterial Switch Operation

BACKGROUND

The main complication in adult patients with transposition of the great arteries (TGA) treated by an arterial switch operation (ASO) is neopulmonary outflow tract stenosis (NPOTS). However, pulmonary flow velocity measurements cannot always be performed with transthoracic echocardiography (TTE) due to complex anatomical features. 4D flow MRI allows detection, quantification, and location of the obstruction site along the NPOTS.

PURPOSE AND HYPOTHESIS

To investigate the accuracy of 4D flow for the diagnosis of NPOTS in adults with TGA corrected by ASO.

STUDY TYPE

Prospective.

POPULATION

Thirty-three adult patients with TGA treated by ASO (19 men, mean age 25.5 years old).

FIELD STRENGTH/SEQUENCE

Accelerated 4D flow research sequence at 3T.

ASSESSMENT

Maximum NPOTS velocities on TTE and 4D flow MRI done the same day.

STATISTICAL TESTS

Pearson correlation coefficient, paired t-test, and Bland-Altman analysis were used to investigate the relationship between TTE and MRI data.

RESULTS

In 16 patients (48.5%), evaluation of NPOTS anatomy was not obtained by TTE, while it was always possible by 4D flow. Peak flow velocity (PV) measurements in Doppler and 4D flow were highly correlated (r = 0.78; P < 0.001). PV >350 cm.s^-1 was detected in only one patient (3%) by TTE vs. five patients (15%) by 4D flow. Moreover, a high correlation was found between PV and the right ventricle (RV) mass index to body surface area when using 4D flow (r = 0.63; P < 0.001). The location of NPOTS was determined in all patients using 4D flow and concerned the main pulmonary artery in 42%.

DATA CONCLUSION

Compared to TTE, 4D flow MRI provides better sensitivity to detect and locate NPOTS in patients with TGA treated by ASO. 4D flow PV measurements in NPOTS were well correlated with TTE PV and RV mass.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2020;51:1699-1705.

Imaging-Based 4D Aortic Pressure Mapping in Marfan Syndrome Patients: A Matched Case-Control Study

BACKGROUND

Marfan syndrome predisposes to aortic aneurysm, dissection, and rupture. We sought to investigate aortic 4-dimensional (4D) relative pressure maps derived from 4D flow cardiovascular magnetic resonance to identify disease characteristic alterations of the intraaortic pressure field in Marfan patients with aortic root dilation compared with age- and sex-matched healthy controls.

METHODS

This prospective case-control study included 11 Marfan patients with aortic root dilation (31 ± 5 years, 5 female) and 11 age- and sex-matched healthy controls (31 ± 8 years, 5 female) undergoing 4D flow cardiovascular magnetic resonance of the thoracic aorta. 4D relative pressure maps were computed and compared between groups for 8 aortic regions.

RESULTS

Aortic root diameters were significantly larger in patients compared with controls (43 vs 31 mm, P < .001), but not in the proximal descending aorta (23 vs 21 mm, P = .19). Regional pressure gradients over the cardiac cycle were significantly altered in Marfan patients with significantly higher minimum pressure gradients in the proximal ascending aorta (-44.3 vs -97.0 mm Hg/m, P < .001) and significantly lower maximum pressure gradients in the proximal descending aorta (55.1 vs 82.3 mm Hg/m, P < .01). The latter finding was associated with pathologic vortical flow patterns. Regional pressure gradient at mid systole significantly correlated with aortic diameter (proximal ascending aorta: r = 0.73, P < .001; proximal descending aorta: r = -0.59, P = .004).

CONCLUSIONS

Noninvasive 4D pressure mapping derived from 4D flow cardiovascular magnetic resonance revealed significant alterations of spatiotemporal pressure characteristics in the thoracic aorta of Marfan patients. These alterations were most pronounced in the proximal ascending aorta and the proximal descending aorta, corresponding to the regions where aortic dissections often originate in Marfan patients.

Pelvic Blood Flow Predicts Fibroid Volume and Embolic Required for Uterine Fibroid Embolization: A Pilot Study With 4D Flow MR Angiography

OBJECTIVE

We report here an initial experience using 4D flow MRI in pelvic imaging-specifically, in imaging uterine fibroids. We hypothesized that blood flow might correlate with fibroid volume and that quantifying blood flow might help to predict the amount of embolic required to achieve stasis at subsequent uterine fibroid embolization (UFE).

MATERIALS AND METHODS

Thirty-three patients with uterine fibroids and seven control subjects underwent pelvic MRI with 4D flow imaging. Of the patients with fibroids, 10 underwent 4D flow imaging before UFE and seven after UFE; in the remaining 16 patients with fibroids, UFE had yet to be performed. Four-dimensional flow measurements were performed using Arterys CV Flow. The flow fraction of the internal iliac artery was expressed as the ratio of internal iliac artery flow to external iliac artery flow and was compared between groups. The flow ratios between the internal iliac arteries on each side were calculated. Fibroid volume versus internal iliac flow fraction, embolic volume versus internal iliac flow fraction, and embolic volume ratio between sides versus the ratio of internal iliac artery flows between sides were compared.

RESULTS

The mean internal iliac flow fraction was significantly higher in the 26 patients who underwent imaging before UFE (mean ± standard error, 0.78 ± 0.06) than in the seven patients who underwent imaging after UFE (0.48 ± 0.07, p < 0.01) and in the seven control patients without fibroids (0.48 ± 0.08, p < 0.0001). The internal iliac flow fraction correlated well with fibroid volumes before UFE (r = 0.7754, p < 0.0001) and did not correlate with fibroid volumes after UFE (r = -0.3051, p = 0.51). The ratio of embolic required to achieve stasis between sides showed a modest correlation with the ratio of internal iliac flow (r = 0.6776, p = 0.03).

CONCLUSION

Internal iliac flow measured by 4D flow MRI correlates with fibroid volume and is predictive of the ratio of embolic required to achieve stasis on each side at subsequent UFE and may be useful for preprocedural evaluation of patients with uterine fibroids.

Creating hemodynamic atlases of cardiac 4D flow MRI

Purpose

Hemodynamic atlases can add to the pathophysiological understanding of cardiac diseases. This study proposes a method to create hemodynamic atlases using 4D Flow magnetic resonance imaging (MRI). The method is demonstrated for kinetic energy (KE) and helicity density (H_d).

Materials and Methods

Thirteen healthy subjects underwent 4D Flow MRI at 3T. Phase‐contrast magnetic resonance cardioangiographies (PC‐MRCAs) and an average heart were created and segmented. The PC‐MRCAs, KE, and H_d were nonrigidly registered to the average heart to create atlases. The method was compared with 1) rigid, 2) affine registration of the PC‐MRCAs, and 3) affine registration of segmentations. The peak and mean KE and H_d before and after registration were calculated to evaluate interpolation error due to nonrigid registration.

Results

The segmentations deformed using nonrigid registration overlapped (median: 92.3%) more than rigid (23.1%, P < 0.001), and affine registration of PC‐MRCAs (38.5%, P < 0.001) and affine registration of segmentations (61.5%, P < 0.001). The peak KE was 4.9 mJ using the proposed method and affine registration of segmentations (P = 0.91), 3.5 mJ using rigid registration (P < 0.001), and 4.2 mJ using affine registration of the PC‐MRCAs (P < 0.001). The mean KE was 1.1 mJ using the proposed method, 0.8 mJ using rigid registration (P < 0.001), 0.9 mJ using affine registration of the PC‐MRCAs (P < 0.001), and 1.0 mJ using affine registration of segmentations (P = 0.028). The interpolation error was 5.2 ± 2.6% at mid‐systole, 2.8 ± 3.8% at early diastole for peak KE; 9.6 ± 9.3% at mid‐systole, 4.0 ± 4.6% at early diastole, and 4.9 ± 4.6% at late diastole for peak H_d. The mean KE and H_d were not affected by interpolation.

Conclusion

Hemodynamic atlases can be obtained with minimal user interaction using nonrigid registration of 4D Flow MRI.

Level of Evidence: 2

Technical Efficacy: Stage 1

J. Magn. Reson. Imaging 2017;46:1389–1399.

Unravelling cardiovascular disease using four dimensional flow cardiovascular magnetic resonance

Knowledge of normal and abnormal flow patterns in the human cardiovascular system increases our understanding of normal physiology and may help unravel the complex pathophysiological mechanisms leading to cardiovascular disease. Four-dimensional (4D) flow cardiovascular magnetic resonance (CMR) has emerged as a suitable technique that enables visualization of in vivo blood flow patterns and quantification of parameters that could potentially be of prognostic value in the disease process. In this review, current image processing tools that are used for comprehensive visualization and quantification of blood flow and energy distribution in the heart and great vessels will be discussed. Also, imaging biomarkers extracted from 4D flow CMR will be reviewed that have been shown to distinguish between normal and abnormal flow patterns. Furthermore, current applications of 4D flow CMR in the heart and great vessels will be discussed, showing its potential as an additional diagnostic modality which could aid in disease management and timing of surgical intervention.

Ability of noninvasive criteria to predict hemodynamically significant aortic obstruction in adults with coarctation of the aorta

OBJECTIVE

Coarctation of the aorta (CoA) is a common condition. Adult patients with newly diagnosed CoA and patients with recurring or residual CoA require evaluation of the severity of aortic obstruction. Cardiac catheterization is considered the gold standard for the evaluation of hemodynamically significant CoA. The European Society of Cardiology (ESC) Guidelines for the management of grown-up congenital heart disease (GUCH) include noninvasive criteria for identifying significant CoA. Our aim was to investigate the ability of the Class I and Class IIa ESC recommendations to identify significant CoA at cardiac catheterization.

DESIGN

Sixty-six adult patients with native or recurrent CoA underwent diagnostic cardiac catheterization at the GUCH unit at the Sahlgrenska University Hospital in Gothenburg from October 1998 to November 2013. Clinical and imaging data, as well as data about cardiac catheterization were retrospectively collected from patient records.

RESULTS

The Class I ESC recommendations predicted significant CoA with a sensitivity of 0.57, a specificity of 0.63, a positive predictive value of 0.67, and a negative predictive value of 0.53. The combination of Class I and Class IIa recommendations predicted significant CoA with a sensitivity of 0.75, a specificity of 0.42, a positive predictive value of 0.66 and a negative predictive value of 0.52.

CONCLUSIONS

the noninvasive criteria proposed by the ESC guidelines to identify subjects with significant CoA performed poorly in our dataset. Further research is needed to develop more accurate, noninvasive criteria to evaluate CoA severity and thereby reduce the number of unnecessary cardiac catheterizations.

Imaging in vascular diseases of the lung

PURPOSE OF REVIEW

The present review focuses on the recent developments in the field of pulmonary angiography and perfusion by means of computed tomography and magnetic resonance imaging.

RECENT FINDINGS

Computed tomography allows for a detailed analysis of large and small pulmonary vascular pathologies and simultaneously allows for evaluation of the lung parenchyma. Magnetic resonance imaging allows for large and small vessel evaluation as well as noninvasive pressure assessment. Furthermore, recently non-breath-hold contrast-enhanced and noncontrast-enhanced techniques have been developed making magnetic resonance imaging an ideal tool for comprehensive thoracic imaging, even in challenging patients.

SUMMARY

Noninvasive imaging using computed tomography and magnetic resonance imaging further increases their value in daily clinical practice when it comes to assessment of large and small pulmonary artery disease. As computed tomography is more easy to use and widely available, it remains the diagnostic modality of choice. However, magnetic resonance imaging is the modality of choice when a comprehensive angiographic and functional assessment is deemed necessary.