Imaging Insights on the Aorta in Aging

Abstract

The aorta has two main functions: conduit and cushion, and is designed to transmit blood to the periphery and buffer pulsatile stress from ventricular contraction. In the interaction between the structural and functional changes of the aorta, aging and disease processes impact on aortic material properties and hemodynamics. For a comprehensive assessment of changes in aortic structure and function associated with aging and disease, noninvasive cardiovascular imaging techniques, especially magnetic resonance imaging (MRI), have recently been developed. MRI allows for direct and accurate measurement of different aortic characteristics including structural measures such as aortic area or volume, aortic length, curvature, and aortic wall thickness, as well as functional measures such as aortic strain, distensibility, and pulse wave velocity (PWV). Excellent reproducibility of MRI methods allows us to assess the response of the whole aorta to both pharmacologic and non-pharmacologic therapy. Aortic flow and functional assessment could be added to clinical routine CMR as a comprehensive imaging modality primarily performed for the noninvasive evaluation of LV function, LV load, and vascular/ventricular coupling. New techniques such as 4D flow could provide further elucidate the combined age-related effects of altered aortic geometry and function. This following review will describe the pathophysiological aspects of the aorta, and the ability, value, and prospects of cardiovascular imaging, especially MRI, to study age-related changes in aortic structure and function and assess the relationship between these alterations and cardiovascular disease (CVD).

Introduction

Aging is one of the most important determinants of cardiovascular risk and is associated with progressive alterations in the structure and function of the cardiovascular (CV) system and foremost of the aorta, referred to by Sir William Osler as our “vital rubber”^1-3. With advancing age, the arterial wall thickens, elastic and collagen components of the media undergo drastic degeneration leading the aorta to dilate, elongate and become tortuous with stiffened walls^{4, 5}, a process also known as arteriosclerosis. This process has been of interest to many illustrious physicians and researchers over the years but new technological advances may help to untangle conflicting concepts but also shed new light on the pathophysiology of aortic aging and its relevance to health outcomes. This review is focused on the ability and value of cardiovascular imaging, especially magnetic resonance imaging (MRI), to study age-related changes in aortic structure and function and assess the relationship between these alterations and cardiovascular disease (CVD).

Background

Key pathophysiological aspects of the aorta in aging

Although the aorta is often thought of as a neutral conduit vessel, it actually plays a pivotal role as the primary cushion in buffering and smoothing the pulsatile pressure blood flow as it travels from the heart to the periphery⁶. In young healthy adults, the proximal aorta is highly distensible and stores about 60% of the stroke volume from the heart during systole, and, thanks to elastic recoil, redistributes part of this volume to the coronary arteries and periphery during diastole, thus paving the way for a more continuous peripheral circulation (Windkessel effect)⁷. The proximal aorta is exposed to such repeated pulsatile stress about 3 billion times over an average lifetime so that the elastic components of the aortic media progressively fragment and eventually break down to be partially replaced by mostly fibrotic non-distensible tissue, such that the orderly arrangement of elastin fibers in youth is lost with advancing age^{6, 8}. These histological processes lead to stiffening of the aortic wall and increased aortic pulse pressure, and finally to elongation and dilation of the aorta^{9, 10}.

Aortic structure and function are heterogeneous along the arterial tree. Aortic diameter decreases from the sino-tubular junction to the aortic bifurcation defining aortic tapering. The proximal aorta media has a higher elastin-to-collagen ratio compared to the distal aorta and peripheral arteries, so that the most important characteristic of the proximal aorta is its initially high compliance or distensiblity¹¹. Conversely, the distal aorta media such as in the abdominal aorta contains a higher proportion of collagen and smooth muscle cells relative to elastin, leading to a stiffer and more resistive conduit¹¹. This structural heterogeneity in the aortic wall entails different functional responses to mechanical stress, leading to diverse structural and functional changes over its thoracic and abdominal length; i.e. the ascending aorta is more likely to dilate and lengthen compared to the descending aorta¹⁰. On the basis of natural rubber fatigue, fracture of the elastin component in the proximal aorta considering a fixed 10% extension per cycle occurs at 30 years (corresponding to 8 × 10⁸ cycles at heart rate 70 beats/min)⁶. For peripheral arteries with a 3% extension per cycle, elastin fracture would appear later, at an expected 100 years⁶.

The healthy elastic aorta in youth is able to optimize work efficiency of the left ventricle over a wide range of vascular loading conditions, commonly referred to as optimal vascular-ventricular coupling¹². In the stiffened aorta, the pressure wave propagation down to the vascular tree is faster, so that the wave reflection of such forward wave returns earlier from the periphery, thus augmenting systolic pressure within the proximal aorta earlier in the cardiac cycle. This leads to an increase in pulse pressure which subsequently increases left ventricular (LV) load¹³. Increased arterial stiffness is also associated with lower diastolic blood pressure and reduced coronary perfusion¹⁴. Increased aortic afterload leads to secondary LV hypertrophy associated with diastolic dysfunction and increased oxygen demand. Higher systolic cardiac work and mismatch between oxygen supply and demand may result in myocardial ischemia and further reduced cardiac systolic and diastolic performance. Moreover, a stiff aorta also transmits greater pulsatile energy to the microcirculation which may lead to peripheral organ damage especially in highly vascularized organs such as the brain and the kidneys¹⁵. These mechanisms underlie the strong relationship between age-associated aortic stiffness and cardiovascular risk over and above the influence of traditional risk factors including hypertension^16-19.

Imaging of aortic structure and function

Alterations of aortic structure with aging interfere with the favorable relationship between aortic pressure and flow that are seen in youth. For a comprehensive characterization of changes in aortic structure and function associated with aging and disease, we need accurate and reproducible methods that allow for the quantification of morphology, flow, hemodynamics and tissue composition. In clinical practice and clinical investigation, non-invasive cardiovascular imaging, such as echocardiography, computed tomography (CT), and magnetic resonance imaging (MRI) are commonly used to assess aortic structure, function and material properties.

Echocardiography is the most used modality to assess aortic root dilatation and flow, because of its availability and low cost²⁰, for large cross-sectional^{21, 22} and longitudinal studies^{23, 24}, as well as in routine clinical evaluation. Aortic root diameter is commonly measured from M-mode tracings with the leading edge to leading edge technique, as recommended by the American Society of Echocardiography²⁵. Changes in aortic diameter throughout the cardiac cycle as maximal to minimal change in aortic diameter represent unidimensional measures of aortic strain or distensibility that reflect local aortic function²⁶. Reference values for aortic diameter by 2-dimensional (2D) echocardiography have been reported in children²⁰, young adults²⁴ and over a broad range of age²⁷. Echocardiography could also allow for measurements of aortic flow using Doppler imaging, however, its main drawbacks include the narrow acoustic window which is frequently insufficient for the comprehensive assessment of aortic material properties, especially of complex blood flow patterns and the necessity to study aortic flow along its main flow direction through the aortic valve. CT provides excellent spatial resolution to accurately assess aortic area or volume instead of simple diameters but radiation exposure during the entire cardiac cycle and inability to quantify flow represent the main drawbacks of CT based techniques for the purposes of quantifying aortic structure and function. Conversely, MRI allows for direct and accurate measurement of different aortic characteristics including structural measures such as aortic area or volume, aortic length, curvature, and aortic wall thickness, as well as functional measures such as aortic strain, distensibility, and pulse wave velocity (PWV). ²⁸. The precision and good reproducibility of MRI measurements could allow for the assessment of longitudinal variations of aortic parameters with aging as well as the effect of pharmacological and non-pharmacological interventions on aortic function.

Aortic Structure Assessed by MRI

Aortic diameter, area, and elongation

MRI has the advantage of being able to measure aortic cross-sectional areas accurately at any plane using a 2D or a 3D approach compared to the 1D assessment by echocardiography, which is prone to measurement bias²⁹. The cross-sectional area of the aorta can be used to derive the average aortic diameter.

Recent cross-sectional studies from the Multi-Ethnic Study of Atherosclerosis (MESA) report ascending aortic luminal size assessed by MRI in a multi-ethnic population aged 45-85 years³⁰. MESA is a prospective study designed to evaluate risk factors and mechanisms that underlie the development and progression of subclinical CVD among asymptomatic individuals using MRI to assess cardiovascular structure and function³¹. In that study, the ascending aortic (AA) diameter increased on average 1.1 mm per decade for both men and women. Age, gender (men had larger AA diameter than women), body surface area, ethnicity, and blood pressure were determinants of change in AA diameter. The influence of diastolic blood pressure on AA diameter was more than double that of systolic blood pressure (0.73 mm vs. 0.32 mm increase in AA diameter per 1SD increase in DBP and SBP, respectively). Reference values using the sub-cohort without risk factors were also reported; median and upper limits (95%CI) of AA diameter were 31mm and 38mm. Reference median values of AA diameter for the age categories of 45-54, 55-64, 65-74, and 75-84 years were 29, 30, 31, 31 mm in women, and 32, 33, 34, 35 mm in men, respectively. Other studies have provided normal reference values of aortic area assessed by MRI for children and young adults (3 to 28 years)³².

MRI also allows the assessment of the whole thoracic aorta geometry in 2D sagittal oblique planes acquired using black blood spin echo or steady state free precession (SSFP) sequences, as well as in 3D SSFP or contrast-enhanced MR angiography (Figure 1). A previous study demonstrates that the aorta lengthens with age, due primarily to the elongation of the ascending aorta, presumably because the proximal aorta is likely to experience the material fatigue to cyclical longitudinal stretch⁹. Another recent study demonstrates the close relationship between alterations in proximal aortic geometry, namely aortic unfolding (elongation and widening of the aortic arch) and increased LV mass and concentric remodeling among subjects 20 to 84 years of age³³. In this study, increases in length and diameter of the ascending aorta were predominant over similar changes in the descending aorta. Taken together, these findings suggest that ascending aorta dilation and lengthening, resulting in aortic unfolding and the augmentation of proximal aortic volume, may help to compensate for wall stiffening and loss of distensibility by increasing the local storage capacity of systolic blood volume albeit at the expense of decreased cardiovascular efficiency.

Figure 1.

Morphological evolution of the human aorta with age. Note that early modification of the aorta include widening of the aorta, particularly the ascending segment accompanied by widening of the arch and finally marked tortuosity. Automated segmentation of aortic volume on isotropic 3D-SSFP MRI ECG and respiratory gated acquisitions.

In this regard, aortic dilatation has been associated with future risk of heart failure among middle-aged and older adults in the Framingham study using echocardiography³⁴. This association was attenuated after adjustments for LV mass, suggesting that LV hypertrophy mediates the progression to HF in the presence of aortic dilatation³⁴. No study has assessed the relationship of geometrical changes such as elongation or widening of the aortic arch with incident CVD events.

Aortic wall thickness

MRIalso allows quantification of aortic architecture in any plane orientation. Black-blood spin echo T1-weighted ECG-gated acquisitions are used in most studies due to their sharp contrast between the signal from the wall and blood (Figure 2). Cross-sectional studies from MESA have reported that the average ascending aorta wall thickness was 2.8mm and showing no significant association with age, whereas an age-related increase in average wall thickness in the descending aorta was reported in a subsample of 1053 MESA participants aged 45 to 85 years^{30, 35}. Longitudinal observations using the MESA cohort have also reported that the wall thickness increase in the descending aorta was more pronounced for those with a baseline age of 45 to 54 years and reached a plateau as age increased³⁶.

Figure 2.

Black-blood spin echo T1-weighted image to measure aortic wall thickness. The thickness of descending aortic wall was measured using electronic calipers at 4 standard positions: 12, 3, 6, and 9 o'clock, and calculated as the average value of these 4 measurement.

The main limitation of measurements of aortic wall thickness from MRI images is the spatial resolution－the in plane spatial resolution in most acquisitions (0.7-1 mm) is about half of the expected aortic wall thickness and slice thickness is usually >5mm, being therefore susceptible to partial volume effects. Higher spatial resolution and refined automated quantification methods are needed for more accurate measurements of aortic wall thickness by MRI, particularly for the ascending aorta and among younger individuals. Future studies are also needed to assess the relationship of aortic wall thickness with future CVD events.

Aortic Function Assessed by MRI

Aortic strain and distensibility

Ascending and descending aortic strain represent the relative change in luminal area during the cardiac cycle, measured from a plane positioned at the mid-ascending aorta perpendicular to the aortic wall at the level of the center of the right pulmonary artery. A validated semi-automated software can accurately determineaortic cross-sectional area variations throughout the entire cardiac cycle and represent them as area over time curves ³⁷ (Figure 3). Aortic strain is then calculated as follows³⁸:

Figure 3.

Measurement of aortic distensibility in MRI. (A) Magnetic resonance imaging (MRI) shows 3-dimensional reconstruction of the thoracic aorta with transverse aortic dynamic acquisition plane (red). (B) Result of the semi-automated segmentation of the ascending aorta (green contour). (C) Resulting cross-sectional area to cardiac cycle time curve; Reprinted from Redheuil et al³⁷ with permission of the publisher.

$$\mathit{\text{Aortic strain}}(\%)=\left[\right(\mathit{\text{maximum aortic area}}-\mathit{\text{minimum aortic area}})/(\mathit{\text{minimum aortic area}}\left)\right]\times \mathit{100}$$

Distensibility is then calculated as aortic strain normalized by the pulse pressure changes that drive aortic systolic wall stretch and diastolic recoil³⁸.

$$\mathit{\text{Aortic distensibility}}({\mathit{\text{mmHg}}}^{-\mathit{1}}\cdot {\mathit{10}}^{-\mathit{3}})=[\mathit{\text{aortic strain}}/\mathit{\text{pulse pressure}}]\times \mathit{10}$$

Central pulse pressure is preferred for the calculation of distensibility instead of brachial pulse pressure, especially in young adults, due to the peripheral pressure amplification phenomenon which is marked in young adults. Several noninvasive methods to estimate central blood pressure have been developed using pressure waveforms from the carotid, brachial or radial arteries and calibrated to cuff pressures. Distensibility is commonly used as a measure of local aortic stiffness in population studies^{30, 35, 37}.

For the purpose of measuring aortic strain or distensibility, SSFP cine imaging is most commonly used and validated due to its high spatial and temporal resolution and flow compensation minimizing artifacts³⁹. Prior studies have also demonstrated the ability to substitute the modulus images of phase contrast MRI (PC-MRI) for SSFP imaging, which has the advantage of decreasing the scanning time since it does not require an additional acquisition for the measurement of PWV by PC-MRI ⁴⁰, as further discussed in this review.

Aging and strain/distensibility

Aortic distensibility decreases non-linearly with aging⁴¹. Recent studies have demonstrated a marked reduction in local aortic strain and distensibility determined by MRI before 50 years of age among asymptomatic general population volunteers⁴¹. Ascending aortic distensibility measured using central pulse pressure is on average 7.4 ± 2.3mmHg^-1·10^-3 between 20 and 30 years of age, 1.8 ± 0.7 mmHg^-1·10^-3 between 50 and 60 years of age and 1.0 ± 0.6 mmHg^-1·10^-3 after 70 years (Figure 4). In addition, aortic distensibility correlates more strongly with aging than more peripheral aortic stiffness measures such as carotid distensibility, carotid-femoral PWV (cf-PWV) that is measured by tonometry and considered a gold standard method for arterial stiffness assessment ³⁸, and the Augmentation Index (AIx) that is a measure that represents wave reflection³⁸. This interesting finding of a dramatic decrease in ascending aortic strain and distensibility early in life when blood pressure changes are not yet apparent is consistent with the mechanical hypothesis of elastin degradation in the first 40 years of life, because of fatigue and fracture of elastin fibers due to repeated deformation and stretch induced by pulsatile stress⁶.

Figure 4.

The change in ascending aortic distensibility with aging. (A) Ascending aorta strain (dot) and distensiblity (squares) by decades of age. (B) Ascending aortic distensibility (age<50=red dots and >50 years=blue dots);Reprinted from Redheuil et al⁴¹ with permission of the publisher.

Longitudinal studies from MESA demonstrate that age, baseline and change in blood pressure, and a history of smoking were associated with increased local aortic stiffening over 10 years⁴². In this study, the use of calcium channel blockers was associated with less decrease in ascending aortic distensibility compared to other drugs including ACE-I/ARB, beta-blockers, and diuretics, while other drugs were not associated with change in aortic stiffness. Further interventional studies are needed to assess the beneficial effect of different pharmacological and non-pharmacological interventions on aortic function.

Prognostic value of aortic distensibility

Importantly, recent studies from MESA showed that decreased ascending aortic distensibility assessed by MRI predicted all-cause mortality in a multi-ethnic population free of overt CV disease³⁷. The hazard ratio for death over 8.5 years was 2.3 for the 20% of individuals with the stiffest aorta independent of all established CV risk factors, LV mass and common measures of subclinical atherosclerosis (coronary artery calcium, carotid intima-media thickness, ankle-brachial index). Ascending aortic distensibility predicted CV events independent of age and CVD risk factors in those with low-to-moderate baseline individual risk.

Aortic pulse wave velocity

The most documented index of aortic stiffness is pulse wave velocity (PWV), defined as the propagation speed of the pressure or the blood flow waves along the artery⁴³. PWV is calculated as the ratio between the distance separating two anatomical locations along the artery and the transit time needed for the pressure or flow wave to cover that distance. Arterial PWV is commonly measured using applanation tonometry, an indirect method which divides the distance between the carotid artery and the femoral artery measured over the body surface, by the time delay of the arterial pulse between the two sites. cf-PWV has been regarded as the gold standard method because of its relative ease in determination and its perceived reliability⁴⁴. However, cf-PWV by tonometry is affected by a measurement error related to the distance estimation over the body surface, which neglects the torturous vessel routes ³⁸. Such errors can be increased in obese (increased body surface) or elder (increased aortic tortuosity) individuals ⁴⁵. In addition, cf-PWV ignores the proximal ascending aorta segment. Because the majority of the buffering capacity of the arterial system resides in the proximal aorta, this omission may have important consequences in deciphering the impact of age and disease in the aorta.

MRI aortic PWV assessment is a novel method that substantially reduces measurement error by using accurate aortic length and corresponding transit time between flow wave measurements^{46, 47} in precise locations and has excellent reproducibility²⁸. The assessment of aortic PWV using PC-MRI derived flow waves is well validated by comparisons with invasive intra-aortic pressure assessment⁴⁶ and established cf-PWV⁴⁸. MRI also allows the measurement of PWV in different segments of the aorta such as in the ascending aorta, aortic arch, thoracic descending, and abdominal descending aorta⁴⁹. Arch PWV for example, is most commonly measured using 2D PC-MRI due to its simple acquisition during a single breath-hold of one PC-MRI plane with a velocity encoding gradient in the through-plane direction^{41, 42} making it suitable for routine protocols. To measure aortic arch PWV, images of the ascending and descending aorta are obtained simultaneously in the transverse plane perpendicular to the aortic centerline, at the level of the right pulmonary artery. Specific software for aortic analysis provides transit time estimation between the two flow waves, after automated segmentation of the modulus and velocity PC-MRI images⁵⁰ (Figure 5). One of the main disadvantages of MRI is the relatively lower temporal resolution (10-20ms) compared to echocardiography or especially applanation tonometry, although recent MRI technical developments and post-processing techniques have the potential to overcome such disadvantages (See details in supplemental materials).

Figure 5.

Measurement of arch PWV in MRI using ARTFUN software (LIB, INSERM 1146, France). (A) Phase contrast cine transverse view. (B) Aortic arch view with steady state free precession sequence. (C) Measurement of the transit distance in the aortic arch. Numbers correspond to those in A and B. (D) Flow wave curves of ascending aorta and descending aorta after peak flow normalization. Transit time is measured as the average time shift that minimizes the least squares difference between systolic upslope data points of the ascending and descending aortic flow curves;Reprinted from Ohyama et al⁵⁰ with permission of the publisher.

Aging and PWV

cf-PWV increases progressively with aging–from approximately 7 m/s at 30 years of age to 12 m/s at 70 years of age⁴³. Data from MESA demonstrated that the increase in arch PWV assessed by PC-MRI was particularly marked in participants of 45-54 years of age compared to those over 54 years of age ⁵¹(Figure 6). Another recent study using PC-MRI showed that age differentially affects regional aortic stiffness⁴⁹. The greatest age-related increase in regional PWV was observed in the distal aorta and the least in the aortic arch, whereas the greatest increase in aortic diameter and length was seen in the aortic arch, which may, in part, be compensatory, maintaining capacitance to face the increased wall stiffness. Heterogeneity of age-related structural and functional modifications as well as of atherosclerotic burden across the arterial tree cause differential regional aortic stiffness.

Figure 6.

The change in arch PWV with aging in MESA study. Dots indicate median values and bars indicate 25%ile and 75%ile for each age decade. Lines connect each dots;Reprinted from Ohyama et al⁵¹ with permission of the publisher.

With regard to vascular-ventricular interaction, a recent study shows the relationship of aortic arch PWV assessed by MRI with LV deformation assessed by tagged MRI as well as LV structure in the MESA cohort. In that study, greater arch PWV was associated with concentric LV remodeling and reduced LV systolic and diastolic function, and this association had a significant gender interaction⁵⁰. Among women, greater arch PWV was associated with LV concentric remodeling, reduced LV diastolic function and preserved EF with maintained torsion. In men, greater arch PWV was associated with reduced LV systolic function demonstrated as impaired LV circumferential strain and torsion, with less LV concentric remodeling. This gender difference in the relationship of aortic arch stiffness with LV structure and function may contribute to the higher prevalence of heart failure with preserved EF among women. Another recent study demonstrated that greater aortic stiffness is associated with reduced aortic longitudinal stretch-related aortic work assessed by MRI, which might lead to lower early diastolic LV filling⁵².

Prognostic value of regional aortic stiffness

cf-PWV has been demonstrated to be an independent risk factor for first CV event and to improve risk prediction^{17, 19}. A recent meta-analysis of over 17,000 participants showed that cf-PWV was an independent predictor of CV events and overall mortality in the general population⁵³.

The analysis using the MESA cohort showed that arch PWV predicted future incidence of CVD events in 45-54 year-old participants, but not in those above 54 years in the general population. These findings on the age interaction of arch PWV with events were consistent with what we have previously documented regarding the association of PWV with age; i.e. the marked increase in predictive power among MESA participants 45-54 years of age compared to those over 54 years of age may reflect the greater predictive value of arch PWV in younger versus older individuals, similarly to distensibility. Moreover, given that the structure of the aorta is regionally heterogeneous, different prognostic values maybe associated with regional differences in material properties of the aortic wall components but outcome data on such regional differences is lacking.

Advanced methodology to assess aortic property using MRI

Flow analysis

PC-MRI analysis allows for the separation of global aortic flow into two components: forward and backward flow. In this regard, a recent study demonstrated that ascending aorta backward flow magnitude increases and its onset occurs earlier with aging⁵⁴. Such changes were strongly associated with alterations of aortic arch geometry such as dilation and elongation resulting in aortic unfolding, that lead to changes in local pressure gradients and local rotating blood vortices⁵⁴. These flow modifications, which might lead to increased LV load and a loss in circulatory efficiency, could in part explain age-related vascular alterations and their potential clinical implications in terms of increased CVD risk.

Combining pressure and flow

Ascending aortic impedance is the most comprehensive measure of aortic stiffness for LV afterload determination according to the American Heart Association (AHA) statement on scientific principles of arterial stiffening⁵⁵. Aortic impedance is derived from simultaneous pressure and flow assessments, and could be estimated in the time domain using applanation tonometry and PC-MRI or Doppler echocardiography velocity data. Flow assessment by MRI with noninvasive pressure assessment could provide more accurate aortic impedance compared to measures derived using ultrasound flow assessment sampling the LV outflow tract instead of ascending aortic flow. The ability to measure ascending aortic flow and impedance more accurately would likely provide new insight into aortic function and vascular/ventricular interactions, and provide the opportunity to elucidate the differences and similarities between patients with systolic and diastolic LV dysfunction as mechanisms of heart failure⁵.

4D flow

Recently, more comprehensive 3D cine techniques in combination with three-directionally encoded velocities (known as 3D cine PC or 4D flow MRI) over the cardiac cycle (3D+t=4D) have been reported and have greatly expanded research possibilities on the aorta ^56-59. The most interesting feature of 4D flow MRI is its full 4D spatial-temporal coverage of the vessel of interest and acquisition of multidirectional flow at all locations within a predefined volume (Figure 7). Taking into account the unique anatomy of the thoracic aorta in individuals with complex aortic geometry without a priori is one of the advantages of the 4D flow technique. 4D flow also permits the assessment of regional PWV at any location of interest in the aorta⁵⁷ and 4D magnitude images allow to compute local distensibility in multiple locations. Thus, 4D flow MRI could provide a comprehensive assessment of aortic structure, function and flow simultaneously, and further elucidate the combined age-related effects of altered aortic geometry, compliance and pressure on changes in local flow patterns and partly explain aortic complications. The major drawback of 4D flow is its currently low temporal resolution (30-40ms), long scan times (5-20min), and the complexity of post-processing requiring specific software. However, improving 4D flow pulse sequences and post-processing methods with expanded calculation power may rapidly lead to increased use in clinical investigation and potential translation to routine clinical utilization.

Figure 7.

4D flow images of an 80-year-old subject free from overt cardiovascular disease.

Conclusions

The aorta has two main functions: conduit and cushion, and is efficiently designed to transmit blood to the periphery and buffer pulsatile ventricular contraction. The composition and structure of the aorta is optimized to achieve these two main functions. In the interaction between the structural and functional changes of the aorta, aging and disease processes impact on aortic material properties and hemodynamics. Noninvasive imaging techniques, especially MRI, have recently been developed to enable quantification of the changes in aortic structure and function that accompany aging and disease for the purposes of medical diagnosis and risk prediction improvement. Excellent reproducibility of MRI methods allow us to assess the response of the whole aorta to both pharmacologic and non-pharmacologic therapy. Practically, aortic flow and functional assessment could be added to clinical routine CMR as a comprehensive imaging modality primarily performed for the noninvasive evaluation of LV function, LV load, and vascular/ventricular coupling. Further development and research including longitudinal studies are needed for a more complete understanding of changes in aortic function and material properties in response to aging and disease processes.