Aortic vessel wall magnetic resonance imaging at 3.0 Tesla: a reproducibility study of respiratory navigator gated free-breathing 3D black blood magnetic resonance imaging

How to improve epidemiological trustworthiness concerning abdominal aortic aneurysms

BACKGROUND

Research on degenerative abdominal aortic aneurysms (AAA) is hampered by complex pathophysiology, sub-optimal pre-clinical models, and lack of effective medical therapies. In addition, trustworthiness of existing epidemiological data is impaired by elements of ambiguity, inaccuracy, and inconsistency. Our aim is to foster debate concerning the trustworthiness of AAA epidemiological data and to discuss potential solutions.

METHODS

We searched the literature from the last five decades for relevant epidemiological data concerning AAA development, rupture, and repair. We then discussed the main issues burdening existing AAA epidemiological figures and proposed suggestions potentially beneficial to AAA diagnosis, prognostication, and management.

RESULTS

Recent data suggest a heterogeneous scenario concerning AAA epidemiology with rates markedly varying by country and study cohorts. Overall, AAA prevalence seems to be decreasing worldwide while mortality is apparently increasing regardless of recent improvements in aortic-repair techniques. Prevalence and mortality are decreasing in high-income countries, whereas low-income countries show an increase in both. However, several pieces of information are missing or outdated, thus systematic renewal is necessary. Current AAA definition and surgical criteria do not consider inter-individual variability of baseline aortic size, further decreasing their reliability.

CONCLUSIONS

Switching from flat aortic-size thresholds to relative aortic indices would improve epidemiological trustworthiness regarding AAAs. Aortometry standardization focusing on simplicity, univocity, and accuracy is crucial. A patient-tailored approach integrating clinical data, multi-adjusted indices, and imaging parameters is desirable. Several novel imaging modalities boast promising profiles for investigating the aortic wall. New contrast agents, computational analyses, and artificial intelligence-powered software could provide further improvements.

Pediatric magnetic resonance angiography: to contrast or not to contrast

Magnetic resonance (MR) angiography and MR venography imaging with contrast and non-contrast techniques are widely used for pediatric vascular imaging. However, as with any MRI examination, imaging the pediatric population can be challenging because of patient motion, which sometimes requires sedation. There are multiple benefits of non-contrast MR angiographic techniques, including the ability to repeat sequences if motion is present, the decreased need for sedation, and avoidance of potential risks associated with gadolinium administration and radiation exposure. Thus, MR angiography is an attractive alternative to CT or conventional catheter-based angiography in pediatric populations. Contrast-enhanced MR angiographic techniques have the advantage of increased signal to noise. Blood pool imaging allows long imaging times that result in high-spatial-resolution imaging, and thus high-quality diagnostic images. This article outlines the technique details, indications, benefits and downsides of non-contrast-enhanced and contrast-enhanced MR angiographic techniques to assist in protocol decision-making.

Advanced-sectioned images obtained by microsectioning of the entire male body

Realistic two-dimensional (2D) and three-dimensional (3D) applications for anatomical studies are being developed from true-colored sectioned images. We generated advanced-sectioned images of the entire male body and verified that anatomical structures of both normal and abnormal shapes could be visualized in them. The cadaver was serially sectioned at constant intervals using a cryomacrotome. The sectioned surfaces were photographed using a digital camera to generate horizontal advanced-sectioned images in which normal and abnormal structures were classified. Advanced-sectioned images of the entire male body were generated. The image resolution was 3.3 × 3.3 fold better than that of the first sectioned images obtained in 2002. In the advanced-sectioned images, normal and abnormal structures ranging from microscopic (≥0.06 mm × 0.06 mm; pixel size) to macroscopic (≤473.1 mm × 202 mm; body size) could be identified. Furthermore, the real shapes and actual sites of lung cancer and lymph node enlargement were ascertained in them. Such images will be useful because of their true color and high resolution in digital 2D and 3D applications for gross anatomy and clinical anatomy. In future, we plan to generate new advanced-sectioned images of abnormal cadavers with different diseases for clinical anatomy studies.

On the impact of vessel wall stiffness on quantitative flow dynamics in a synthetic model of the thoracic aorta

Aortic wall stiffening is a predictive marker for morbidity in hypertensive patients. Arterial pulse wave velocity (PWV) correlates with the level of stiffness and can be derived using non-invasive 4D-flow magnetic resonance imaging (MRI). The objectives of this study were twofold: to develop subject-specific thoracic aorta models embedded into an MRI-compatible flow circuit operating under controlled physiological conditions; and to evaluate how a range of aortic wall stiffness impacts 4D-flow-based quantification of hemodynamics, particularly PWV. Three aorta models were 3D-printed using a novel photopolymer material at two compliant and one nearly rigid stiffnesses and characterized via tensile testing. Luminal pressure and 4D-flow MRI data were acquired for each model and cross-sectional net flow, peak velocities, and PWV were measured. In addition, the confounding effect of temporal resolution on all metrics was evaluated. Stiffer models resulted in increased systolic pressures (112, 116, and 133 mmHg), variations in velocity patterns, and increased peak velocities, peak flow rate, and PWV (5.8–7.3 m/s). Lower temporal resolution (20 ms down to 62.5 ms per image frame) impacted estimates of peak velocity and PWV (7.31 down to 4.77 m/s). Using compliant aorta models is essential to produce realistic flow dynamics and conditions that recapitulated in vivo hemodynamics.

Magnetic resonance multitasking for multidimensional assessment of cardiovascular system: Development and feasibility study on the thoracic aorta

PURPOSE

To develop an MR multitasking-based multidimensional assessment of cardiovascular system (MT-MACS) with electrocardiography-free and navigator-free data acquisition for a comprehensive evaluation of thoracic aortic diseases.

METHODS

The MT-MACS technique adopts a low-rank tensor image model with a cardiac time dimension for phase-resolved cine imaging and a T₂ -prepared inversion-recovery dimension for multicontrast assessment. Twelve healthy subjects and 2 patients with thoracic aortic diseases were recruited for the study at 3 T, and both qualitative (image quality score) and quantitative (contrast-to-noise ratio between lumen and wall, lumen and wall area, and aortic strain index) analyses were performed in all healthy subjects. The overall image quality was scored based on a 4-point scale: 3, excellent; 2, good; 1, fair; and 0, poor. Statistical analysis was used to test the measurement agreement between MT-MACS and its corresponding 2D references.

RESULTS

The MT-MACS images reconstructed from acquisitions as short as 6 minutes demonstrated good or excellent image quality for bright-blood (2.58 ± 0.46), dark-blood (2.58 ± 0.50), and gray-blood (2.17 ± 0.53) contrast weightings, respectively. The contrast-to-noise ratios for the three weightings were 49.2 ± 12.8, 20.0 ± 5.8 and 2.8 ± 1.8, respectively. There were good agreements in the lumen and wall area (intraclass correlation coefficient = 0.993, P < .001 for lumen; intraclass correlation coefficient = 0.969, P < .001 for wall area) and strain (intraclass correlation coefficient = 0.947, P < .001) between MT-MACS and conventional 2D sequences.

CONCLUSION

The MT-MACS technique provides high-quality, multidimensional images for a comprehensive assessment of the thoracic aorta. Technical feasibility was demonstrated in healthy subjects and patients with thoracic aortic diseases. Further clinical validation is warranted.

Simultaneous 3D whole-heart bright-blood and black blood imaging for cardiovascular anatomy and wall assessment with interleaved T2 prep-IR

PURPOSE

To develop a motion-corrected 3D flow-insensitive imaging approach interleaved T₂ prepared-inversion recovery (iT₂ prep-IR) for simultaneous lumen and wall visualization of the great thoracic vessels and cardiac structures.

METHODS

A 3D flow-insensitive approach for simultaneous cardiovascular lumen and wall visualization (iT₂ prep) has been previously proposed. This approach requires subject-dependent weighted subtraction to completely null the arterial blood signal in the black-blood volume. Here, we propose an (T₂ prep-IR) approach to improve wall visualization and remove need for weighted subtraction. The proposed sequence is based on the acquisition and direct subtraction of 2 interleaved 3D whole-heart data sets acquired with and without T₂ prep-IR preparation. Image navigators are acquired before data acquisition to enable 2D translational and 3D non-rigid motion correction allowing 100% respiratory scan efficiency. The proposed approach was evaluated in 10 healthy subjects and compared with the conventional 2D double inversion recovery (DIR) sequence and the 3D iT₂ prep sequence. Additionally, 5 patients with congenital heart disease were acquired to test the clinical feasibility of the proposed approach.

RESULTS

The proposed iT₂ prep-IR sequence showed improved blood nulling compared to both DIR and iT₂ prep techniques in terms of SNR (SNR_blood = 6.9, 12.2, and 18.2, respectively) and contrast-to-noise-ratio (CNR_myoc-blood = 28.4, 15.4, and 15.3, respectively). No statistical difference was observed between iT₂ prep-IR, iT₂ prep and DIR atrial and ventricular wall thickness quantification.

CONCLUSION

The proposed interleaved T₂ prep-IR sequence enables the simultaneous lumen and wall visualization of cardiac structures and shows promising results in terms of SNR, CNR, and wall thickness measurement.

The Growing Field of Imaging of Atherosclerosis in Peripheral Arteries

In the past decades, peripheral arteries have represented a model for the comprehension of atherosclerosis as well as for the development of new diagnostic imaging modalities and therapeutic strategies. Peripheral arteries may represent a window to study atherosclerosis. Pathology has prominently contributed to move the clinical and research attention from the arterial lumen stenosis and angiography to morphological and functional imaging techniques. Evidence from large and prospective cohort or randomized controlled studies is still modest. Nevertheless, several emerging imaging investigations represent a potential tool for a comprehensive "in vivo" evaluation of the entire natural history of peripheral atherosclerosis. This constitutes a demanding assignment, as it would be desirable to obtain both single-lesion focused and extensive arterial system views to achieve the most accurate prognostic information. Our narrative review rests upon the fundamental pathological evidence, summarizing the rapidly growing field of imaging of atherosclerosis in peripheral arteries and presenting a selection of both currently available and emerging imaging techniques.

Magnetic Resonance Imaging of Cardiovascular Function and the Brain: Is Dementia a Cardiovascular-Driven Disease?

The proximal aorta acts as a coupling device between heart and brain perfusion, modulating the amount of pressure and flow pulsatility transmitted into the cerebral microcirculation. Stiffening of the proximal aorta is strongly associated with age and hypertension. The detrimental effects of aortic stiffening may result in brain damage as well as heart failure. The resulting cerebral small vessel disease and heart failure may contribute to early cognitive decline and (vascular) dementia. This pathophysiological sequence of events underscores the role of cardiovascular disease as a contributory mechanism in causing cognitive decline and dementia and potentially may provide a starting point for prevention and treatment. Magnetic resonance imaging is well suited to assess the function of the proximal aorta and the left ventricle (eg, aortic arch pulse wave velocity and distensibility) as well as the various early and late manifestations of cerebral small vessel disease (eg, microbleeds and white matter hyperintensities in strategically important regions of the brain). Specialized magnetic resonance imaging techniques are explored for diagnosing preclinical changes in white matter integrity or brain microvascular pulsatility.

Vessel wall characterization using quantitative MRI: what's in a number?

The past decade has witnessed the rapid development of new MRI technology for vessel wall imaging. Today, with advances in MRI hardware and pulse sequences, quantitative MRI of the vessel wall represents a real alternative to conventional qualitative imaging, which is hindered by significant intra- and inter-observer variability. Quantitative MRI can measure several important morphological and functional characteristics of the vessel wall. This review provides a detailed introduction to novel quantitative MRI methods for measuring vessel wall dimensions, plaque composition and permeability, endothelial shear stress and wall stiffness. Together, these methods show the versatility of non-invasive quantitative MRI for probing vascular disease at several stages. These quantitative MRI biomarkers can play an important role in the context of both treatment response monitoring and risk prediction. Given the rapid developments in scan acceleration techniques and novel image reconstruction, we foresee the possibility of integrating the acquisition of multiple quantitative vessel wall parameters within a single scan session.

Fast 3D isotropic imaging of the aortic vessel wall by application of 2D spatially selective excitation and a new way of inversion recovery for black blood imaging

PURPOSE

Aortic vessel wall imaging requires large coverage and a high spatial resolution, which makes it prohibitively time-consuming for clinical use. This work explores the feasibility of imaging the descending aorta in acceptable scan time, using two-dimensional (2D) spatially selective excitation and a new way of inversion recovery for black blood imaging.

METHODS

The excitation pattern and field of view in a 3D gradient echo sequence are reduced in two dimensions, following the aorta's anisotropic geometry. Black blood contrast is obtained by partially inverting the blood's magnetization in the heart at the start of the cardiac cycle. Imaging is delayed until the inverted blood has filled the desired part of the aorta. The flip angle and delay are determined such that the blood signal is nulled upon arrival in the aorta.

RESULTS

Experiments on eight volunteers showed that the descending aortic vessel wall could be imaged over more than 15 cm at a maximal resolution of 1.5 × 1.5 × 1.5 mm(3) in less than 5 min minimal scan time.

CONCLUSION

This feasibility study demonstrates that time-efficient isotropic imaging of the descending aorta is possible by using 2D spatially selective excitation for motion artifact reduction and a new way of inversion recovery for black blood imaging.

Site-specific coupling between vascular wall thickness and function: an observational MRI study of vessel wall thickening and stiffening in hypertension

OBJECTIVES

The objective of this study was to evaluate associations between aortic pulse wave velocity (PWV) and aortic and carotid vessel wall thickness (VWT) using cardiovascular magnetic resonance imaging (MRI) in patients with hypertension as compared with healthy adult volunteers.

MATERIALS AND METHODS

Local medical ethics approval was obtained and the participants gave informed consent. Fifteen patients with hypertension (5 men and 10 women; mean [SD] age, 49 [14] years) and 15 age- and sex-matched healthy volunteers were prospectively included and compared. All participants underwent MRI examination for measuring aortic and carotid VWT and aortic PWV with well-validated MRI techniques at 1.5- and 3-T MRI systems: PWV was assessed from velocity-encoded MRI and VWT was assessed by using dual-inversion black-blood gradient-echo imaging techniques. Paired t tests were used for testing differences between the volunteers and the patients and Pearson correlation (r) and univariable and multivariable stepwise linear regression analyses were used to test associations between aortic and carotid arterial wall thickness and stiffness.

RESULTS

Mean values for aortic PWV and aortic and carotid VWT (indexed for body surface area [BSA]) were all significantly higher in patients with hypertension as compared with the healthy volunteers (ie, aortic PWV, 7.0 ± 1.4 m/s vs 5.7 ± 1.3 m/s; aortic VWT/BSA, 0.12 ± 0.03 mL/m vs 0.10 ± 0.03 mL/m; carotid VWT/BSA, 0.04 ± 0.01 mL/m vs 0.03 ± 0.01 mL/m; all P < 0.01). Aortic PWV was highly correlated with aortic VWT/BSA (r = 0.76 and P = 0.002 in the patients vs r = 0.63 and P = 0.02 in the volunteers), and in the patients, aortic PWV was moderately correlated with carotid VWT/BSA (r = 0.50; P = 0.04). In the volunteers, correlation between aortic PWV and carotid VWT/BSA was not significant (r = 0.40; P = 0.13). In addition, aortic VWT/BSA was significantly correlated with carotid VWT/BSA, in both the patients (r = 0.60; P = 0.005) and volunteers (r = 0.57; P = 0.007).

CONCLUSIONS

In the patients with hypertension and the healthy volunteers, the aortic PWV is associated more strongly with aortic wall thickness than with carotid wall thickness, reflecting site-specific coupling between vascular wall thickness and function.

Three-dimensional imaging of the aortic vessel wall using an elastin-specific magnetic resonance contrast agent

OBJECTIVE

The aim of this study was to demonstrate the feasibility of high-resolution 3-dimensional aortic vessel wall imaging using a novel elastin-specific magnetic resonance contrast agent (ESMA) in a large animal model.

MATERIALS AND METHODS

The thoracic aortic vessel wall of 6 Landrace pigs was imaged using a novel ESMA and a nonspecific control agent. On day 1, imaging was performed before and after the administration of a nonspecific control agent, gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA; Bayer Schering AG, Berlin, Germany). On day 3, identical scans were repeated before and after the administration of a novel ESMA (Lantheus Medical Imaging, North Billerica, Massachusetts). Three-dimensional inversion recovery gradient echo delayed-enhancement imaging and magnetic resonance (MR) angiography of the thoracic aortic vessel wall were performed on a 1.5-T MR scanner (Achieva; Philips Medical Systems, the Netherlands). The signal-to-noise ratio and the contrast-to-noise ratio of arterial wall enhancement, including the time course of enhancement, were assessed for ESMA and Gd-DTPA. After the completion of imaging sessions, histology, electron microscopy, and inductively coupled plasma mass spectroscopy were performed to localize and quantify the gadolinium bound to the arterial vessel wall.

RESULTS

Administration of ESMA resulted in a strong enhancement of the aortic vessel wall on delayed-enhancement imaging, whereas no significant enhancement could be measured with Gd-DTPA. Ninety to 100 minutes after the administration of ESMA, significantly higher signal-to-noise ratio and contrast-to-noise ratio could be measured compared with the administration of Gd-DTPA (45.7 ± 9.6 vs 13.2 ± 3.5, P < 0.05 and 41.9 ± 9.1 vs 5.2 ± 2.0, P < 0.05). A significant correlation (0.96; P < 0.01) between area measurements derived from ESMA scans and aortic MR angiography scans could be found. Electron microscopy and inductively coupled plasma mass spectroscopy confirmed the colocalization of ESMA with elastic fibers.

CONCLUSION

We demonstrate the feasibility of aortic vessel wall imaging using a novel ESMA in a large animal model under conditions resembling a clinical setting. Such an approach could be useful for the fast 3-dimensional assessment of the arterial vessel wall in the context of atherosclerosis, aortic aneurysms, and hypertension.

Coronary artery distensibility assessed by 3.0 Tesla coronary magnetic resonance imaging in subjects with and without coronary artery disease

Coronary vessel distensibility is reduced with atherosclerosis and normal aging, but direct measurements have historically required invasive measurements at cardiac catheterization. Therefore, we sought to assess coronary artery distensibility noninvasively using 3.0 Telsa coronary magnetic resonance imaging (MRI) and to test the hypothesis that this noninvasive technique can detect differences in coronary distensibility between healthy subjects and those with coronary artery disease (CAD). A total of 38 healthy, adult subjects (23 men, mean age 31 ± 10 years) and 21 patients with CAD, diagnosed using x-ray angiography (11 men, mean age 57 ± 6 years) were studied using a commercial whole-body MRI system. In each subject, the proximal segment of a coronary artery was imaged for the cross-sectional area measurements using cine spiral MRI. The distensibility (mm Hg(-1) × 10(3)) was determined as (end-systolic lumen area - end-diastolic lumen area)/(pulse pressure × end-diastolic lumen area). The pulse pressure was calculated as the difference between the systolic and diastolic brachial blood pressure. A total of 34 healthy subjects and 19 patients had adequate image quality for coronary area measurements. Coronary artery distensibility was significantly greater in the healthy subjects than in those with CAD (mean ± SD 2.4 ± 1.7 mm Hg(-1) × 10(3) vs 1.1 ± 1.1 mm Hg(-1) × 10(3), respectively, p = 0.007; median 2.2 vs 0.9 mm Hg(-1) × 10(3)). In a subgroup of 10 patients with CAD, we found a significant correlation between the coronary artery distensibility measurements assessed using MRI and x-ray coronary angiography (R = 0.65, p = 0.003). In a group of 10 healthy subjects, the repeated distensibility measurements demonstrated a significant correlation (R = 0.80, p = 0.006). In conclusion, 3.0-Tesla MRI, a reproducible noninvasive method to assess human coronary artery vessel wall distensibility, is able to detect significant differences in distensibility between healthy subjects and those with CAD.

Current techniques for MR imaging of atherosclerosis

Vessel wall imaging of large vessels has the potential to identify culprit atherosclerotic plaques that lead to cardiovascular events. Comprehensive assessment of atherosclerotic plaque size, composition, and biological activity is possible with magnetic resonance imaging (MRI). Magnetic resonance imaging of the atherosclerotic plaque has demonstrated high accuracy and measurement reproducibility for plaque size. The accuracy of in vivo multicontrast MRI for identification of plaque composition has been validated against histological findings. Magnetic resonance imaging markers of plaque biological activity such as neovasculature and inflammation have been demonstrated. In contrast to other plaque imaging modalities, MRI can be used to study multiple vascular beds noninvasively over time. In this review, we compare the status of in vivo plaque imaging by MRI to competing imaging modalities. Recent MR technological improvements allow fast, accurate, and reproducible plaque imaging. An overview of current MRI techniques required for carotid plaque imaging including hardware, specialized pulse sequences, and processing algorithms are presented. In addition, the application of these techniques to coronary, aortic, and peripheral vascular beds is reviewed.

Noncontrast-enhanced three-dimensional magnetic resonance aortography of the thorax at 3.0 T using respiratory-compensated T1-weighted k-space segmented gradient-echo imaging with radial data sampling: preliminary study

OBJECTIVE

To evaluate the feasibility of a respiratory-compensated three-dimensional (3D) T1-weighted k-space segmented gradient-echo imaging sequence with radial data sampling for noncontrast-enhanced 3D magnetic resonance (MR) aortography of the thorax at 3.0 T.

MATERIALS AND METHODS

Twenty-two subjects, including healthy volunteers (n = 6) and patients with suspected diseases of the thoracic aorta (n = 16), underwent noncontrast-enhanced 3D MR aortography at 3.0 T acquired using a navigator- or respiratory-gated 3D T1-weighted k-space segmented gradient-echo imaging sequence with radial data sampling (TR, 4.8 milliseconds; TE, 1.5 milliseconds; flip angle, 20 degrees; spatial resolution 0.66 x 0.76 x 5.6-6.4 mm) in the sagittal oblique imaging plane. ECG gating, fat-suppression, and T2-prepared pulses were employed. The vascular contrast of the thoracic aorta and the contrast ratio between the aorta and the superior vena cava or pulmonary artery were compared between the noncontrast-enhanced 3D MR aortography and transverse two-dimensional (2D) steady-state free precession. Image quality of the noncontrast-enhanced 3D MR aortography was rated on a 4 point scale (1, nondiagnostic, to 4, diagnostic and excellent image quality).

RESULTS

The noncontrast-enhanced 3D MR aortography provided vascular contrast of the thoracic aorta comparable to, and contrast ratio between the aorta and superior vena cava higher than, those of 2D steady-state free precession. The mean score of image quality of the noncontrast-enhanced 3D MR aortography was 3.0 (diagnostic with no or few artifact), and some major branch arteries were visualized by this imaging.

CONCLUSION

Respiratory-compensated 3D T1-weighted k-space segmented gradient-echo imaging with radial data sampling are feasible for the noncontrast-enhanced 3D MR aortography of the thorax at 3.0 T.