Magnetic resonance angiography: current status and future directions

MRI Gadolinium-Based Contrast Media: Meeting Radiological, Clinical, and Environmental Needs

Gadolinium-based contrast agents (GBCAs) are routinely used in magnetic resonance imaging (MRI). They are essential for choosing the most appropriate medical or surgical strategy for patients with serious pathologies, particularly in oncologic, inflammatory, and cardiovascular diseases. However, GBCAs have been associated with an increased risk of nephrogenic systemic fibrosis in patients with renal failure, as well as the possibility of deposition in the brain, bones, and other organs, even in patients with normal renal function. Research is underway to reduce the quantity of gadolinium injected, without compromising image quality and diagnosis. The next generation of GBCAs will enable a reduction in the gadolinium dose administered. Gadopiclenol is the first of this new generation of GBCAs, with high relaxivity, thus having the potential to reduce the gadolinium dose while maintaining good in vivo stability due to its macrocyclic structure. High-stability and high-relaxivity GBCAs will be one of the solutions for reducing the dose of gadolinium to be administered in clinical practice, while the development of new technologies, including optimization of MRI acquisitions, new contrast mechanisms, and artificial intelligence may help reduce the need for GBCAs. Future solutions may involve a combination of next-generation GBCAs and image-processing techniques to optimize diagnosis and treatment planning while minimizing exposure to gadolinium. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY: Stage 3.

Simultaneous brain and neck time-of-flight MRA using spiral multiband with localized quadratic encoding

PURPOSE

To develop a method that achieves simultaneous brain and neck time-of-flight (ToF) magnetic resonance angiography (MRA) within feasible scan timeframes.

METHODS

Localized quadratic (LQ) encoding is efficient for both signal-to-noise ratio (SNR) and in-flow enhancement. We proposed a spiral multiband LQ method to enable simultaneous intracranial and carotid ToF-MRA within a single scan. To address the venous signal contamination that becomes a challenge with multiband (MB) ToF, tilt-optimized non-saturated excitation (TONE) and partial-Fourier slice selection (PFSS) were further introduced in the LQ framework to mitigate the venous signal and improve artery contrast. A sequential spiral MB and LQ reconstruction pipeline was employed to obtain the brain-and-neck image volumes.

RESULTS

The proposed MB method was able to achieve simultaneous brain and neck ToF-MRA within a 2:50-min scan. The complementarily boosted SNR-efficiency by MB and LQ acquisitions allows for the increased spatial coverage without increase in scan time or noticeable compromise in SNR. The incorporation of both TONE and PFSS effectively alleviated the venous contamination with improved small vessel sensitivity. Selection of scan parameters such as the LQ factor and flip angle reflected the trade-off among SNR, blood contrast, and venous suppression.

CONCLUSIONS

A novel MB spiral LQ approach was proposed to enable fast intracranial and carotid ToF-MRA with minimized venous corruption. The method has shown promise in MRA applications where large spatial coverage is necessary.

Hypersensitive MR Angiography for Diagnosis of Ischemic Stroke and Reperfusion Subarachnoid Hemorrhage

Stroke is an acute injury of the central nervous system caused by the disorders of cerebral blood circulation, which has become one of the major causes of disability and death. Hemorrhage, particularly subarachnoid hemorrhage (SAH), is one of the poorest prognostic factors in stroke, which is related to the thrombolytic therapy, and has been considered very dangerous. In this context, the MR angiography with high sensitivity and resolution has been developed based on biocompatible paramagnetic ultrasmall NaGdF4 nanoprobes. Owing to the appropriate hydrodynamic diameter, the nanoprobe can be confined inside the blood vessels and it only extravasates at the vascular injury site when the bleeding occurs. Relying on this property, the three-dimensional (3D) anatomic structures of artery occlusion of stroke rat can be precisely visualized; reperfusion-related SAH has been successfully visualized and identified. Benefiting from the long blood half-life of the nanoprobe, the observation window of MR angiography can last for the whole period of reperfusion, thereby monitoring the probable SAH in real time during thrombolytic therapy. More importantly, through reconstruction of multiparametric MRI, the arterial occlusion, cerebral ischemic region, and SAH can be simultaneously visualized in vivo in a 3D manner for the first time. Therefore, the current study provides a novel approach for both noninvasive 3D vascular visualization and hemorrhage alert, which possesses great prospects for clinical translation.

Progress in ultrasmall ferrite nanoparticles enhanced T1 magnetic resonance angiography

Contrast-enhanced magnetic resonance angiography (CE-MRA) plays a critical role in diagnosing and monitoring various vascular diseases. Achieving high-sensitivity detection of vascular abnormalities in CE-MRA depends on the properties of contrast agents. In contrast to clinically used gadolinium-based contrast agents (GBCAs), the new generation of ultrasmall ferrite nanoparticles-based contrast agents have high relaxivity, long blood circulation time, easy surface functionalization, and high biocompatibility, hence showing promising prospects in CE-MRA. This review aims to comprehensively summarize the advancements in ultrasmall ferrite nanoparticles-enhanced MRA for detecting vascular diseases. Additionally, this review also discusses the future clinical translational potential of ultrasmall ferrite nanoparticles-based contrast agents for vascular imaging. By investigating the current status of research and clinical applications, this review attempts to outline the progress, challenges, and future directions of using ultrasmall ferrite nanoparticles to drive the field of CE-MRA into a new frontier of accuracy and diagnostic efficacy.

Ultrasmall catechol-PEG-anchored ferrite nanoparticles for highly sensitive magnetic resonance angiography

Highly sensitive iron oxide nanoparticles with stable, safe and efficient surface functionalization, as potential substitutes for gadolinium-based contrast agents (GBCAs) with increasing biosafety concerns, exhibit great potential for high-performance magnetic resonance angiography (MRA). Herein, we developed ultrasmall catechol-PEG-anchored ferrite nanoparticles (PEG-UMFNPs) for highly sensitive MRA. The obtained nanoprobe has a high T1 relaxivity value (7.2 mM-1 s-1) due to its ultrasmall size and Mn doping. It has a suitable hydrodynamic size of 20 nm, which prevents rapid vascular extravasation and renal clearance and prolongs its blood circulation time. In vivo MRA at 3.0 T using the nanoprobe shows that the arteries and veins of rats, even blood vessels as small as 0.32 mm, are distinctly visible, and the contrast enhancement can last for at least 1 h. In addition, due to the outstanding contrast enhancement and long circulation time, the stenosis and recanalization process of the rat's carotid artery can be continuously monitored with a single injection of the nanoprobe. Our study indicates that PEG-UMFNPs are outstanding MR imaging nanoprobes that can be used to diagnose vascular diseases without the biosafety issues of GBCAs.

Evaluation of the celiac trunk and its branching pattern by magnetic resonance angiography

OBJECTIVE

The study aimed to examine the celiac trunk (CT) in detail by magnetic resonance angiography, to determine the locations, positions and variations of the CT, to provide detailed information for surgeons and interventional radiologists, and to reduce the time spent in the catheter angiography.

MATERIAL AND METHODS

MR angiography images of 185 patients with abdominal imaging in PACS (Picture Archiving Communication Systems) were retrospectively analyzed. The level of origin of CT, according to the vertebral column, angle of origin, distance between CT and branches of the abdominal aorta (AA), and the branching pattern of CT were evaluated. Parameters were evaluated according to gender and age.

RESULTS

The most common origin site for CT, according to the vertebral column, was found to be at T12-L1 in both genders. There was a low, positive correlation between age and CT-Sagittal angle (SA) in the whole study group (p<0.05). The most common CT branching pattern was the hepatogastrosplenic trunk in both genders, according to Adachi and Uflkacker's classification. The distance between CT and the inferior mesenteric artery (IMA) and CT and the aortic bifurcation (AB) of males was greater than in females, and the differences were statistically significant (p<0.05).

CONCLUSION

Knowledge of the locations, positions, and variations of CT is essential in the diagnosis, differential diagnosis and decision-making mechanisms regarding the type of intervention to be performed for this vessel and related structures. In addition, the fact that data on these vessels can be obtained by MR angiography due to the improved image quality will prevent patients and physicians from the problems caused by the ionizing radiation of computed tomography. The data presented will constitute a basis for detailed and individualized interpretation and evaluation of each patient, as they provide important details about the configuration of the CT concerning gender and age using MRA.

Cardiothoracic Magnetic Resonance Angiography

Catheter-based angiography is regarded as the clinical reference imaging technique for vessel imaging; however, it is invasive and is currently used for intervention or physiologic measurements. Contrast enhanced magnetic resonance angiography (MRA) with gadolinium-based contrast agents can be performed as a three-dimensional (3D) MRA or as a time resolved 3D (4D) MRA without physiologic synchronization, in which case cardiac and respiratory motion may blur the edges of the vessels and cardiac chambers. Ferumoxytol has recently been a popular contrast agent for MRA in patients with chronic renal failure. Noncontrast 3D MRA with ECG gating and respiratory navigation are safe and accurate noninvasive cross-sectional imaging techniques for the visualization of great vessels of the heart and coronary arteries in a variety of cardiovascular disorders including complex congenital heart diseases. Noncontrast flow dependent MRA techniques such as time of flight, phase contrast, and black-blood MRA techniques can be used as complementary or primary techniques. Here we review both conventional and relatively new contrast enhanced and non-contrast enhanced MRA techniques including ferumoxytol enhanced MRA, and bright-blood and water-fat separation based noncontrast 3D MRA techniques.

ACR Appropriateness Criteria® Pulsatile Abdominal Mass, Suspected Abdominal Aortic Aneurysm: 2023 Update

Abdominal aortic aneurysm (AAA) is defined as abnormal dilation of the infrarenal abdominal aortic diameter to 3.0 cm or greater. The natural history of AAA consists of progressive expansion and potential rupture. Although most AAAs are clinically silent, a pulsatile abdominal mass identified on physical examination may indicate the presence of an AAA. When an AAA is suspected, an imaging study is essential to confirm the diagnosis. This document reviews the relative appropriateness of various imaging procedures for the initial evaluation of suspected AAA. The American College of Radiology Appropriateness Criteria are evidence-based guidelines for specific clinical conditions that are reviewed annually by a multidisciplinary expert panel. The guideline development and revision process support the systematic analysis of the medical literature from peer reviewed journals. Established methodology principles such as Grading of Recommendations Assessment, Development, and Evaluation or GRADE are adapted to evaluate the evidence. The RAND/UCLA Appropriateness Method User Manual provides the methodology to determine the appropriateness of imaging and treatment procedures for specific clinical scenarios. In those instances where peer reviewed literature is lacking or equivocal, experts may be the primary evidentiary source available to formulate a recommendation.

Hypersensitive MR angiography based on interlocking stratagem for diagnosis of cardiac-cerebral vascular diseases

Magnetic resonance (MR) angiography is one of the main diagnostic approaches for cardiac-cerebral vascular diseases. Nevertheless, the non-contrast-enhanced MR angiography suffers from its intrinsic problems derived from the blood flow-dependency, while the clinical Gd-chelating contrast agents are limited by their rapid vascular extravasation. Herein, we report a hypersensitive MR angiography strategy based on interlocking stratagem of zwitterionic Gd-chelate contrast agents (PAA-Gd). The longitudinal molar relaxivity of PAA-Gd was 4.6-times higher than that of individual Gd-chelates as well as appropriate blood half-life (73.8 min) and low immunogenicity, enabling sophisticated micro-vessels angiography with a resolution at the order of hundred micrometers. A series of animal models of cardiac-cerebrovascular diseases have been built for imaging studies on a 7.0 T MRI scanner, while the clinical translation potential of PAA-Gd has been evaluated on swine on a 3.0 T clinical MRI scanner. The current studies offer a promising strategy for precise diagnosis of vascular diseases.

SpeckleCam: high-resolution computational speckle contrast tomography for deep blood flow imaging

Laser speckle contrast imaging is widely used in clinical studies to monitor blood flow distribution. Speckle contrast tomography, similar to diffuse optical tomography, extends speckle contrast imaging to provide deep tissue blood flow information. However, the current speckle contrast tomography techniques suffer from poor spatial resolution and involve both computation and memory intensive reconstruction algorithms. In this work, we present SpeckleCam, a camera-based system to reconstruct high resolution 3D blood flow distribution deep inside the skin. Our approach replaces the traditional forward model using diffuse approximations with Monte-Carlo simulations-based convolutional forward model, which enables us to develop an improved deep tissue blood flow reconstruction algorithm. We show that our proposed approach can recover complex structures up to 6 mm deep inside a tissue-like scattering medium in the reflection geometry. We also conduct human experiments to demonstrate that our approach can detect reduced flow in major blood vessels during vascular occlusion.

Comparison of bone subtraction CT angiography with standard CT angiography for evaluating circle of Willis in normal dogs

BACKGROUND

Bone subtraction computed tomography angiography (BSCTA) is a useful alternative technique for improving visualization of vessels surrounded by skull bone. However, no studies have compared computed tomography angiography (CTA) and BSCTA for improving the visibility of canine cerebral blood vessels.

OBJECTIVES

To evaluate the potential benefit of BSCTA for better delineation of brain arteries of the circle of Willis (CoW) in dogs by comparing BSCTA with non-subtraction computed tomography angiography (NSCTA).

METHODS

Brain CTA was performed for nine healthy beagle dogs using a bolus tracking method with saline flushing. A total dose of 600 mgI/kg of contrast agent with an iodine content of 370 mgI/mL was injected at a rate of 4 ml/s. Bone removal was achieved automatically by subtracting non-enhanced computed tomography (CT) data from contrast CT data. Five main intracranial arteries of the CoW were analyzed and graded on a scale of five for qualitative evaluation.

RESULTS

Scores of basilar artery, middle cerebral artery, and rostral cerebral artery in the BSCTA group were significantly higher than those in the NSCTA group (p = 0.001, p = 0.020, and p < 0.0001, respectively). Scores of rostral cerebellar artery (RcA) and caudal cerebral artery (CCA) did not differ significantly between the two groups. However, scores of RcA and CCA in the BSCTA group were higher than those in the NSCTA group.

CONCLUSIONS

BSCTA improved visualization of intracranial arteries of the CoW with close contact to bone. Thus, it should be recommended as a routine scan method in dogs suspected of having brain vessel disease.

Respiratory-Triggered Flow-Independent Noncontrast Non-ECG-Gated MRV (REACT) Versus CE-MRV for Central Venous Evaluation in Children and Young Adults: A Six-Reader Study

BACKGROUND. Contrast-enhanced MRI is commonly used to evaluate thoracic central venous patency in children and young adults. A flow-independent noncontrast non-ECG-gated 3D MRA-MR venography (MRV) technique described in 2019 as "relaxation-enhanced angiography without contrast and triggering (REACT)" may facilitate such evaluation. OBJECTIVE. The purpose of our study was to compare image quality, diagnostic confidence, and interreader agreement between respiratory-triggered REACT and 3D Dixon-based contrast-enhanced MRV (CE-MRV) for evaluating thoracic central venous patency in children and young adults. METHODS. This retrospective study included 42 consecutive children and young adults who underwent MRI of the neck and chest to evaluate central venous patency between August 2019 and January 2021 (median age, 5.2 years; IQR, 1.4-15.1 years; 22 female patients and 20 male patients). Examinations included respiratory-triggered REACT and navigator-gated CE-MRV sequences based on the institution's standard-of-care protocol. Six pediatric radiologists from four different institutions independently reviewed REACT and CE-MRV sequences; they assessed overall image quality (scale, 1-5; 5 = excellent), diagnostic confidence (scale, 1-5; 5 = extremely confident), and presence of clinically relevant artifact(s). Readers classified seven major central vessels as normal or abnormal (e.g., narrowing, thrombosis, or occlusion). Analysis used Wilcoxon signed rank and McNemar tests and Fleiss kappa coefficients. RESULTS. The distribution of overall image quality scores was higher (p = .02) for REACT than for CE-MRV for one reader (both sequences: median score, 5). Image quality scores were not significantly different between the sequences for the remaining five readers (all p > .05). Diagnostic confidence scores and frequency of clinically relevant artifact(s) were not significantly different between sequences for any reader (all p > .05). Interreader agreement for vessel classification as normal or abnormal was similar between sequences for all seven vessels (REACT: κ = 0.37-0.81; CE-MRV: κ = 0.34-0.81). Pooling readers and vessels, 65.4% of vessels were normal by both sequences; 18.7%, abnormal by both sequences; 9.8%, abnormal by REACT only; and 6.1%, abnormal by CE-MRV only. CONCLUSION. Respiratory-triggered REACT, in comparison with CE-MRV, showed no significant difference in image quality (aside from for one of six readers), diagnostic confidence, or frequency of artifact(s), with similar interreader agreement for vessel classification as normal or abnormal. CLINICAL IMPACT. High-resolution 3D MRV performed without IV contrast material can be used to assess central venous patency in children and young adults.

Development and Evaluation of a Multifrequency Ultrafast Doppler Spectral Analysis (MFUDSA) Algorithm for Wall Shear Stress Measurement: A Simulation and In Vitro Study

Cardiovascular pathology is the leading cause of death and disability in the Western world, and current diagnostic testing usually evaluates the anatomy of the vessel to determine if the vessel contains blockages and plaques. However, there is a growing school of thought that other measures, such as wall shear stress, provide more useful information for earlier diagnosis and prediction of atherosclerotic related disease compared to pulsed-wave Doppler ultrasound, magnetic resonance angiography, or computed tomography angiography. A novel algorithm for quantifying wall shear stress (WSS) in atherosclerotic plaque using diagnostic ultrasound imaging, called Multifrequency ultrafast Doppler spectral analysis (MFUDSA), is presented. The development of this algorithm is presented, in addition to its optimisation using simulation studies and in-vitro experiments with flow phantoms approximating the early stages of cardiovascular disease. The presented algorithm is compared with commonly used WSS assessment methods, such as standard PW Doppler, Ultrafast Doppler, and Parabolic Doppler, as well as plane-wave Doppler. Compared to an equivalent processing architecture with one-dimensional Fourier analysis, the MFUDSA algorithm provided an increase in signal-to-noise ratio (SNR) by a factor of 4-8 and an increase in velocity resolution by a factor of 1.10-1.35. The results indicated that MFUDSA outperformed the others, with significant differences detected between the typical WSS values of moderate disease progression (p = 0.003) and severe disease progression (p = 0.001). The algorithm demonstrated an improved performance for the assessment of WSS and has potential to provide an earlier diagnosis of cardiovascular disease than current techniques allow.

Recent Advances in Explainable Artificial Intelligence for Magnetic Resonance Imaging

Advances in artificial intelligence (AI), especially deep learning (DL), have facilitated magnetic resonance imaging (MRI) data analysis, enabling AI-assisted medical image diagnoses and prognoses. However, most of the DL models are considered as "black boxes". There is an unmet need to demystify DL models so domain experts can trust these high-performance DL models. This has resulted in a sub-domain of AI research called explainable artificial intelligence (XAI). In the last decade, many experts have dedicated their efforts to developing novel XAI methods that are competent at visualizing and explaining the logic behind data-driven DL models. However, XAI techniques are still in their infancy for medical MRI image analysis. This study aims to outline the XAI applications that are able to interpret DL models for MRI data analysis. We first introduce several common MRI data modalities. Then, a brief history of DL models is discussed. Next, we highlight XAI frameworks and elaborate on the principles of multiple popular XAI methods. Moreover, studies on XAI applications in MRI image analysis are reviewed across the tissues/organs of the human body. A quantitative analysis is conducted to reveal the insights of MRI researchers on these XAI techniques. Finally, evaluations of XAI methods are discussed. This survey presents recent advances in the XAI domain for explaining the DL models that have been utilized in MRI applications.

A hybrid hierarchical strategy for registration of 7T TOF-MRI to 7T PC-MRI intracranial vessel data

PURPOSE

7T time-of-flight (TOF) MRI provides high resolution for the evaluation of cerebrovascular vessels and pathologies. In combination with 4D flow fields acquired with phase-contrast (PC) MRI, hemodynamic information can be extracted to enhance the analysis by providing direct measurements in the larger arteries or patient-specific boundary conditions. Hence, a registration between both modalities is required.

METHODS

To combine TOF and PC-MRI data, we developed a hybrid registration approach. Vessels and their centerlines are segmented from the TOF data. The centerline is fit to the intensity ridges of the lower resolved PC-MRI data, which provides temporal information. We used a metric that utilizes a scaled sum of weighted intensities and gradients on the normal plane. The registration is then guided by decoupled local affine transformations. It is applied hierarchically following the branching order of the vessel tree.

RESULTS

A landmark validation over Monte Carlo simulations yielded an average mean squared error of 184.73 mm and an average Hausdorff distance of 15.20 mm. The hierarchical traversal that transforms child vessels with their parents registers even small vessels not detectable in the PC-MRI.

CONCLUSION

The presented work combines high-resolution tomographic information from 7T TOF-MRI and measured flow data from 4D 7T PC-MRI scan for the arteries of the brain. This enables usage of patient-specific flow parameters for realistic simulations, thus supporting research in areas such as cerebral small vessel disease. Automatization and free deformations can help address the limiting error measures in the future.

Ferumoxytol-Enhanced Cardiac Magnetic Resonance Angiography and 4D Flow: Safety and Utility in Pediatric and Adult Congenital Heart Disease

Cardiac magnetic resonance imaging and angiography have a crucial role in the diagnostic evaluation and follow up of pediatric and adult patients with congenital heart disease. Although much of the information required of advanced imaging studies can be provided by standard gadolinium-enhanced magnetic resonance imaging, the limitations of precise bolus timing, long scan duration, complex imaging protocols, and the need to image small structures limit more widespread use of this modality. Recent experience with off-label diagnostic use of ferumoxytol has helped to mitigate some of these barriers. Approved by the U.S. FDA for intravenous treatment of anemia, ferumoxytol is an ultrasmall superparamagnetic iron oxide nanoparticle that has a long blood pool residence time and high relaxivity. Once metabolized by macrophages, the iron core is incorporated into the reticuloendothelial system. In this work, we aim to summarize the evolution of ferumoxytol-enhanced cardiovascular magnetic resonance imaging and angiography and highlight its many applications for congenital heart disease.

Effect of Contrast Agent Dose Reduction on Vascular Enhancement and Image Quality in Thoracoabdominal Dynamic 3-Dimensional Magnetic Resonance Angiography: A Systematic Intraindividual Analysis in Pigs

OBJECTIVE

High spatial and temporal resolution contrast-enhanced magnetic resonance angiography (MRA) with gadolinium-based contrast agents (GBCAs) at standard dose offers both detailed anatomic information on both arterial and venous vessels and hemodynamic characteristics. Several preclinical and clinical dynamic 3-dimensional (3D) MRA studies that focused on arterial vessels only proposed that high image quality may also be achieved with significantly reduced GBCA doses, calling into question the need to use standard doses. A systematic analysis of GBCA doses and resulting image quality for both arteries and veins has not yet been performed. The purpose of this study was therefore to systematically analyze dose-dependent vascular enhancements in dynamic 3D-MRA of the thoracoabdominal vasculature at 1.5 T in an animal model to determine the optimal contrast agent protocol for optimized vascular assessment.

MATERIALS AND METHODS

The vascular enhancement in thoracoabdominal dynamic 3D-MRA (time-resolved angiography with interleaved stochastic trajectories, TWIST at 1.5 T) was interindividually and intraindividually compared in 5 anesthetized Göttingen minipigs using gadobutrol at the standard dose (0.1 mmol/kg body weight, ie, 0.1 mL/kg) and at reduced doses (0.08, 0.06, 0.04, 0.02 mmol/kg) in a randomized order. All injections were performed at 2 mL/s followed by 20 mL saline. Images were quantitatively analyzed, measuring signal intensities in 5 regions that covered the passage of the GBCA through the body at different representative stages of circulation (pulmonary, arterial, and venous system). The evaluation of GBCA dose-dependent signal intensity changes in the different vascular regions was performed by linear regression analysis.The qualitative image analysis of dynamic 3D-MRA by 3 independent radiologists included the visibility of 25 arterial and venous vessel segments at different stages of GBCA passage. Possible quality losses were statistically tested by comparing image quality ratings at the reduced dose with that of the standard dose using Friedman test followed by Dunn post hoc test for multiple comparison. Significance was stated at P < 0.05.

RESULTS

Quantitative analysis revealed shorter time-to-peak intervals and bolus durations in line with decreasing GBCA dose and volume in all vessels. Although the peak signal was almost independent of the administered GBCA dose at the level of the pulmonary trunk, a linear signal decrease in the abdominal aorta ( r2 = 0.96), the renal arteries ( r2 = 0.99), the inferior vena cava ( r2 = 0.99), and the portal vein ( r2 = 0.97) was observed. Cumulative analysis of arterial segments revealed significantly lower image quality at doses below 40% of the standard dose, whereas in venous segments, significantly lower image quality was observed at doses below 60% of the standard dose.

CONCLUSIONS

In dynamic 3D-MRA at 1.5 T, dose reduction leads to a signal loss that is most pronounced in the venous system and results in significantly lower image quality according to the dose and vessels of interest. Careful dose reduction is thus required according to the specific diagnostic needs. For dynamic 3D-MRA of the arterial and venous system, GBCA doses of at least 60% of the standard dose up to the full dose are preferable, whereas 40% of the standard dose seems feasible if only the arterial system is to be imaged.

Diagnostic yield of TOF-MRA for detecting incidental vascular lesions in patients with cognitive impairment: An observational cohort study

Objectives

The role of three-dimensional (3D) TOF-MRA in patients with cognitive impairment is not well established. We evaluated the diagnostic yield of 3D TOF-MRA for detecting incidental extra- or intracranial artery stenosis and intracranial aneurysm in this patient group.

Methods

This retrospective study included patients with cognitive impairment undergoing our brain MRI protocol from January 2013 to February 2020. The diagnostic yield of TOF-MRA for detecting incidental vascular lesions was calculated. Patients with positive TOF-MRA results were reviewed to find whether additional treatment was performed. Logistic regression analysis was conducted to identify the clinical risk factors for positive TOF-MRA findings.

Results

In total, 1,753 patients (mean age, 70.2 ± 10.6 years; 1,044 women) were included; 199 intracranial aneurysms were detected among 162 patients (9.2%, 162/1,753). A 3D TOF-MRA revealed significant artery stenoses (>50% stenosis) in 162 patients (9.2%, 162/1,753). The overall diagnostic yield of TOF-MRA was 16.8% (294/1,753). Among them, 92 patients (31.3%, 92/294) underwent either medical therapy, endovascular intervention, or surgery. In total, eighty-one patients with stenosis were prescribed with either antiplatelet medications or lipid-lowering agent. In total, fifteen patients (aneurysm: 11 patients, stenosis: 4 patients) were further treated with endovascular intervention or surgery. Thus, the “number needed to scan” was 19 for identifying one patient requiring treatment. Multivariate logistic regression analysis showed that being female (odds ratio [OR] 2.05) and old age (OR 1.04) were the independent risk factors for intracranial aneurysm; being male (OR 1.52), old age (OR 1.06), hypertension (OR 1.78), and ischemic heart disease history (OR 2.65) were the independent risk factors for significant artery stenosis.

Conclusions

Our study demonstrated the potential benefit of 3D TOF-MRA, given that it showed high diagnostic yield for detecting vascular lesions in patients with cognitive impairment and the considerable number of these lesions required further treatment. A 3D TOF-MRA may be included in the routine MR protocol for the work-up of this patient population, especially in older patients and patients with vascular risk factors.

NMR measurement of biomolecular translational and rotational motion for evaluating changes of protein oligomeric state in solution

Defining protein oligomeric state and/or its changes in solution is of significant interest for many biophysical studies carried out in vitro, especially when the nature of the oligomeric state is crucial in the subsequent interpretation of experimental results and their biological relevance. Nuclear magnetic resonance (NMR) is a well-established methodology for the characterization of protein structure, dynamics, and interactions at the atomic level. As a spectroscopic method, NMR also provides a compelling means for probing both molecular translational and rotational motion, two predominant measures of effective molecular size in solution, under identical conditions as employed for structural, dynamic and interaction studies. Protein translational diffusion is readily measurable by pulse gradient spin echo (PGSE) NMR, whereas its rotational correlation time, or rotational diffusion tensor when its 3D structure is known, can also be quantified from NMR relaxation parameters, such as 15N relaxation parameters of backbone amides which are frequently employed for probing residue-specific protein backbone dynamics. In this article, we present an introductory overview to the NMR measurement of bimolecular translational and rotational motion for assessing changes of protein oligomeric state in aqueous solution, via translational diffusion coefficients measured by PGSE NMR and rotational correlation times derived from composite 15N relaxation parameters of backbone amides, without need for the protein structure being available.

3 T: the good, the bad and the ugly

It is around 20 years since the first commercial 3 T MRI systems became available. The theoretical promise of twice the signal-to-noise ratio of a 1.5 T system together with a greater sensitivity to magnetic susceptibility-related contrast mechanisms, such as the blood oxygen level dependent effect that is the basis for functional MRI, drove the initial market in neuroradiology. However, the limitations of the increased field strength soon became apparent, including the increased radiofrequency power deposition, tissue-dependent changes in relaxation times, increased artifacts, and greater safety concerns. Many of these issues are dependent upon MR physics and workarounds have had to be developed to try and mitigate their effects. This article reviews the underlying principles of the good, the bad and the ugly aspects of 3 T, discusses some of the methods used to improve image quality and explains the remaining challenges and concerns.

Measuring Arterial Pulsatility With Dynamic Inflow Magnitude Contrast

The pulsatility of blood flow through cerebral arteries is clinically important, as it is intrinsically associated with cerebrovascular health. In this study we outline a new MRI approach to measuring the real-time pulsatile flow in cerebral arteries, which is based on the inflow phenomenon associated with fast gradient-recalled-echo acquisitions. Unlike traditional phase-contrast techniques, this new method, which we dub dynamic inflow magnitude contrast (DIMAC), does not require velocity-encoding gradients as sensitivity to flow velocity is derived purely from the inflow effect. We achieved this using a highly accelerated single slice EPI acquisition with a very short TR (15 ms) and a 90° flip angle, thus maximizing inflow contrast. We simulate the spoiled GRE signal in the presence of large arteries and perform a sensitivity analysis. The sensitivity analysis demonstrates that in the regime of high inflow contrast, DIMAC shows much greater sensitivity to flow velocity over blood volume changes. We support this theoretical prediction with in-vivo data collected in two separate experiments designed to demonstrate the utility of the DIMAC signal contrast. We perform a hypercapnia challenge experiment in order to experimentally modulate arterial tone within subjects, and thus modulate the arterial pulsatile flow waveform. We also perform a thigh-cuff release challenge, designed to induce a transient drop in blood pressure, and demonstrate that the continuous DIMAC signal captures the complex transient change in the pulsatile and non-pulsatile components of flow. In summary, this study proposes a new role for a well-established source of MR image contrast and demonstrates its potential for measuring both steady-state and dynamic changes in arterial tone.

Dark blood cardiovascular magnetic resonance of the heart, great vessels, and lungs using electrocardiographic-gated three-dimensional unbalanced steady-state free precession

BACKGROUND

Recently, we reported a novel neuroimaging technique, unbalanced T1 Relaxation-Enhanced Steady-State (uT1RESS), which uses a tailored 3D unbalanced steady-state free precession (3D uSSFP) acquisition to suppress the blood pool signal while minimizing bulk motion sensitivity. In the present work, we hypothesized that 3D uSSFP might also be useful for dark blood imaging of the chest. To test the feasibility of this approach, we performed a pilot study in healthy subjects and patients undergoing cardiovascular magnetic resonance (CMR).

MAIN BODY

The study was approved by the hospital institutional review board. Thirty-one adult subjects were imaged at 1.5 T, including 5 healthy adult subjects and 26 patients (44 to 86 years, 10 female) undergoing a clinically indicated CMR. Breath-holding was used in 29 subjects and navigator gating in 2 subjects. For breath-hold acquisitions, the 3D uSSFP pulse sequence used a high sampling bandwidth, asymmetric readout, and single-shot along the phase-encoding direction, while 3 shots were acquired for navigator-gated scans. To minimize signal dephasing from bulk motion, electrocardiographic (ECG) gating was used to synchronize the data acquisition to the diastolic phase of the cardiac cycle. To further reduce motion sensitivity, the moment of the dephasing gradient was set to one-fifth of the moment of the readout gradient. Image quality using 3D uSSFP was good-to-excellent in all subjects. The blood pool signal in the thoracic aorta was uniformly suppressed with sharp delineation of the aortic wall including two cases of ascending aortic aneurysm and two cases of aortic dissection. Compared with variable flip angle 3D turbo spin-echo, 3D uSSFP showed improved aortic wall sharpness. It was also more efficient, permitting the acquisition of 24 slices in each breath-hold versus 16 slices with 3D turbo spin-echo and a single slice with dual inversion 2D turbo spin-echo. In addition, lung and mediastinal lesions appeared highly conspicuous compared with the low blood pool signals within the heart and blood vessels. In two subjects, navigator-gated 3D uSSFP provided excellent delineation of cardiac morphology in double oblique multiplanar reformations.

CONCLUSION

In this pilot study, we have demonstrated the feasibility of using ECG-gated 3D uSSFP for dark blood imaging of the heart, great vessels, and lungs. Further study will be required to fully optimize the technique and to assess clinical utility.

Free-breathing high resolution modified Dixon steady-state angiography with compressed sensing for the assessment of the thoracic vasculature in pediatric patients with congenital heart disease

BACKGROUND

Cardiovascular magnetic resonance angiography (CMRA) is a non-invasive imaging modality of choice in pediatric patients with congenital heart disease (CHD). This study was aimed to evaluate the diagnostic utility of a respiratory- and electrocardiogram-gated steady-state CMRA with modified Dixon (mDixon) fat suppression technique and compressed sensing in comparison to standard first-pass CMRA in pediatric patients with CHD at 3 T.

METHODS

In this retrospective single center study, pediatric CHD patients who underwent CMR with first-pass CMRA followed by mDixon steady-state CMRA at 3 T were analyzed. Image quality using a Likert scale from 5 (excellent) to 1 (non-diagnostic) and quality of fat suppression were assessed in consensus by two readers. Blood-to-tissue contrast and quantitative measurements of the thoracic vasculature were assessed separately by two readers. CMRA images were reevaluated by two readers for additional findings, which could be identified only on either one of the CMRA types. Paired Student t test, Wilcoxon test, and intraclass correlation coefficients (ICCs) were used for statistical analysis.

RESULTS

32 patients with CHD (3.3 ± 1.7 years, 13 female) were included. Overall image quality of steady-state mDixon CMRA was higher compared to first-pass CMRA (4.5 ± 0.5 vs. 3.3 ± 0.5; P < 0.001). Blood-to-tissue contrast ratio of steady-state mDixon CMRA was comparable to first-pass CMRA (7.85 ± 4.75 vs. 6.35 ± 2.23; P = 0.133). Fat suppression of steady-state mDixon CMRA was perfect in 30/32 (94%) cases. Vessel diameters were greater in first-pass CMRA compared to steady-state mDixon CMRA with the greatest differences at the level of pulmonary arteries and veins (e.g., right pulmonary artery for reader 1: 10.4 ± 2.4 vs. 9.9 ± 2.3 mm, P < 0.001). Interobserver agreement was higher for steady-state mDixon CMRA for all measurements compared to first-pass CMRA (ICCs > 0.92). In 9/32 (28%) patients, 10 additional findings were identified on mDixon steady-state CMRA (e.g., partial anomalous venous return, abnormalities of coronary arteries, subclavian artery stenosis), which were not depicted using first-pass CMRA.

CONCLUSIONS

Steady-state mDixon CMRA offers a robust fat suppression, a high image quality, and diagnostic utility for the assessment of the thoracic vasculature in pediatric CHD patients.

Hierarchical imaging and computational analysis of three-dimensional vascular network architecture in the entire postnatal and adult mouse brain

The formation of new blood vessels and the establishment of vascular networks are crucial during brain development, in the adult healthy brain, as well as in various diseases of the central nervous system. Here, we describe a step-by-step protocol for our recently developed method that enables hierarchical imaging and computational analysis of vascular networks in postnatal and adult mouse brains. The different stages of the procedure include resin-based vascular corrosion casting, scanning electron microscopy, synchrotron radiation and desktop microcomputed tomography imaging, and computational network analysis. Combining these methods enables detailed visualization and quantification of the 3D brain vasculature. Network features such as vascular volume fraction, branch point density, vessel diameter, length, tortuosity and directionality as well as extravascular distance can be obtained at any developmental stage from the early postnatal to the adult brain. This approach can be used to provide a detailed morphological atlas of the entire mouse brain vasculature at both the postnatal and the adult stage of development. Our protocol allows the characterization of brain vascular networks separately for capillaries and noncapillaries. The entire protocol, from mouse perfusion to vessel network analysis, takes ~10 d.

Fast three-dimensional time-of-flight magnetic resonance angiography: Should it be used in routine neuroimaging for headaches?

OBJECTIVES

We aimed to evaluate the diagnostic efficacy of fast three-dimensional (3D) time-of-flight (TOF) magnetic resonance angiography (MRA) in the detection of cerebral aneurysms.

METHODS

We screened 1589 patients over a 3-year period, who underwent a magnetic resonance imaging for headache. Fast 3D TOF MRA images taken in addition to routine neuroimaging sequences, which were examined by two independent observers to assess cerebral aneurysms.

RESULTS

Sixty-nine aneurysms were detected in 63 patients. The locations of the aneurysms were as follows: Middle cerebral artery, 27.5% (n = 19); internal carotid artery, 53.6% (n = 37); anterior cerebral artery, 5.8% (n = 4); posterior cerebral artery, 5.8% (n = 4); anterior communicating artery, 5.8% (n = 4); and ophthalmic artery, 1.4% (n = 1). Thirty-five (50.7%) were 7 mm or less, 23 (33.3%) were 8-10 mm, and 11 (15.9%) were 11 mm or larger. About 88% of pathological cases (n = 61) were saccular and 12 (n = 8) were fusiform type aneurysms. The interobserver compliance was high (K = 0.83) for detecting the aneurysms.

CONCLUSION

Considering the mortality and morbidity of subarachnoid hemorrhages, 3D TOF MRA is a successful, non-invasive method for detecting cerebral aneurysms. Results of the present study have shown that adding 3D TOF MRA to the routine brain imaging protocol is a beneficial tool for diagnosis.

The Usefulness of Magnetic Resonance Angiography to Analyze the Variable Arterial Facial Anatomy in an Effort to Reduce Filler-Associated Blindness: Anatomical Study and Visualization Through an Augmented Reality Application

BACKGROUND

The use of soft tissue fillers for facial rejuvenation is increasing rapidly and the complications, unfortunately, follow the same path. Blindness caused by intravascular filler injections is a rare but devastating complication. Knowledge of the individual arterial anatomy may aid the injector in avoiding injecting into an artery and thus to prevent blindness.

OBJECTIVES

To evaluate if the use of magnetic resonance angiography (MRA) may visualize the arterial facial anatomy in a contrast- and radiation-free way and study the individual arterial variations using an augmented reality (AR) image.

METHODS

The individual arterial anatomy of the 3 terminal branches of the ophthalmic artery (supraorbital [SO]; supratrochlear [STr]; and dorsal nasal [DN] arteries) of 20 volunteers was studied by a 3-Tesla MRI, combining infrared (IR) facial warming and 3-dimensional time-of-flight multiple overlapping thin slab acquisition MRA. The resulting visualization of the facial arteries was shown on the patient's face through AR technology.

RESULTS

The MRA was able to visualize the SO in 90.0%, STr in 92.5%, and DN arteries in 75% of the examined patients, as well as numerous variations in both vessel localization and path. Furthermore, a proof-of-concept of the AR visualization of the individual arterial anatomy was successfully implemented.

CONCLUSIONS

Dermal filler injectors should be aware of the risk of filler-induced blindness and familiarize themselves with the visualization of the variable facial vascular anatomy. The implementation of a one-time MRA and subsequent AR visualization may be useful in the accurate planning of minimally invasive facial rejuvenation procedures.

Optimal placement of the region of interest for bolus tracking on brain computed tomography angiography in Beagle dogs

This study aimed to determine the optimal placement of the region of interest (ROI) among four anatomical sites-pulmonary artery (PA), pulmonary vein (PV), aortic arch (AA), and carotid artery (CA)-in computed tomography (CT) brain angiography with automatic bolus tracking in healthy beagle dogs. Six beagles were included, and CT brain angiography was performed four times for each dog, to cover each ROI. The scan parameters, amount, and injection rate of the contrast medium were the same. The major intracranial arteries were selected for quantitative and qualitative evaluation: caudal cerebellar artery (CcA), basilar artery (BA), rostral cerebellar artery (RcA), caudal cerebral artery (CCA), middle cerebral artery (MCA), and rostral cerebral artery (RCA). Quantitative evaluation showed significantly higher CT attenuation values for the RcA, CCA, and MCA in the PA group and RcA and MCA in the PV group than in the CA group. Qualitative analysis revealed significantly higher scores for the BA, CCA, and MCA in the PA and PV groups than in the CA group. Venous contamination did not differ significantly among the ROIs, but the mean scores of the AA and CA groups were higher than those of the PA and PV groups. CT brain angiography using bolus tracking in the beagle dogs showed that the ROI should be placed at the PA or PV rather than at the CA for optimal images with strong contrast enhancement of the BA, RcA, CCA, and MCA and minimal venous contamination.

Multimodality Imaging in Acute Ischemic Stroke

Stroke is the most common cause of mortality and morbidity worldwide. The prognosis of stroke depends upon the area affected and its early treatment. Time is of the essence in the care of stroke patients as it is estimated that approximately 1.9 million neurons, 14 billion synapses, and 12 km myelinated nerve fibers are lost per minute. Therefore, early diagnosis and prompt treatment are necessary. The primary goal of imaging in acute stroke is to diagnose the underlying cause, estimate the area affected, predict response towards thrombolytic therapy and to exclude the conditions mimicking stroke. With advancements in radiology, multiple imaging modalities are available for diagnosis and predicting prognosis. None of them is considered alone to be perfect. In this era of multimodality imaging, the decision of choosing appropriate techniques depends upon purpose and availability. Non-Contrast Computed Tomography is time effective, and helps in excluding other causes, Trans Cranial Doppler is time-effective and cost-effective with wide availability, however, is operator dependent and less sensitive. It holds a great future in sonothrombolysis. Magnetic Resonance Imaging is so far considered to be the most superior one in terms of early diagnosis, planning for interventional treatment and predicting the response of treatment. However, it is limited due to high cost and lack of availability. The current review gives a detailed account of all imaging modalities available for imaging stroke and their associated pros and cons.

The diagnostic value of non-contrast magnetic resonance coronary angiography in the assessment of coronary artery disease: A systematic review and meta-analysis

Purpose

The current literature reports a wide range of diagnostic accuracy of non-contrast magnetic resonance coronary angiography (NC-MRCA) for the assessment of coronary artery disease (CAD). We aimed to compare the clinical effectiveness of NC-MRCA with that of invasive coronary angiography (ICA) in patients with suspected CAD using a systematic review and meta-analysis.

Methods

Two investigators independently extracted 36 published manuscripts between 2010 and 2019. Databases including Medline, Web of Knowledge, Google Scholar, Scopus, and Cochrane were searched using pre-established keywords. Analysis of the data followed the PRISMA statement for reporting systematic reviews and meta-analyses and primary analysis followed the Mantel-Hansel methodology. Correctness of classification for detecting coronary artery stenosis ≥50% (CAS) was measured using ICA as the gold standard.

Results

A total of five studies met inclusion criteria, with a total of 417 patients and 2883 coronary segments. The pooled per patient sensitivity and specificity of NC-MRCA for CAS in suspected patients was 90.3% (95% CI 85.6–95.1%) and 77.9% (95% CI 69.5–86.3%). Pooled per vessel assessment of NC- MRCA revealed a sensitivity of 83.7% (95%CI 79.7–87.8%) and specificity of 90.0% (95%CI 86.7–93.4%). Per-segment assessment of NC-MRCA showed a pooled sensitivity of 81.6% (95% CI 76.8–86.4) and specificity of 97.0% (95% CI 95.5–98.5). Mild to moderate heterogeneity was noted in most diagnostic parameters with larger heterogeneity noted in the per-segment analyses. There was less heterogeneity in sensitivity and NPV than specificity and PPV.

Conclusion

According to this meta-analysis, non-contrast coronary MRA resulted in adequate screening in patients with suspected CAD with high sensitivity and specificity. This result was true for per-patient, per-vessel, and per-segment assessment.

Major cerebral vessels involvement in patients with MELAS syndrome: Worth a scan? A systematic review

Major cerebral vessels have been proposed as a target of defective mitochondrial metabolism in patients with mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes syndrome (MELAS). Cerebral angiographic techniques are not routinely performed in MELAS patients. A systematic literature review was performed to identify studies describing major vessel caliber alterations in MELAS. Twenty-three studies reporting on 46 MELAS patients were included. Alterations in major caliber vessels were present in 59% (27/46) of patients. Dilation occurred in 37% (17/46) of patients, and in 88% (15/17) of them during a stroke-like episode (SLE). Stenosis was reported in 24% (11/46) of patients: 36% (4/11) related to an SLE and 64% (7/11) to dissections or degenerative changes. During an SLE, identification of intracranial vessels dilation or stenosis could be a selection tool for new treatment protocols. Outside SLE, identification of major cerebral vessels dissections and degenerative changes may help to prevent subsequent complications.

Visualization techniques of the facial arteries

BACKGROUND

As the face is known for its extreme variation in vascular anatomy and the number of filler-associated complications due to intra-arterial injection is increasing, we are in need of a method to visualize anyone's individual arterial anatomy of the face in a completely harmless way.

AIMS

The different medical imaging methods and a recently developed MRA protocol are reviewed.

METHODS

The literature of the last twenty years-with special attention for the last five years-concerning the different medical imaging modalities of the facial arteries was reviewed.

RESULTS

A harmless visualisation of the facial arteries is currently only possible with US or MRA. US may identify single vessels but never the complete arterial network. A combination of IR "heat enhancement" and a MRA 3D-TOF sequence might make it feasible to visualize a large number of facial arteries in a risk-free, radiation-free, contrast-free and non-invasive way.

CONCLUSION

Currently, a new combination of IR "heat enhancement" and a MRA 3D-TOF sequence might be the only method to visualize a large number of facial arteries.

Surveillance Imaging following Endovascular Aneurysm Repair: State of the Art

Endovascular aneurysmal repair (EVAR) has become a prominent modality for the treatment of abdominal aortic aneurysm. Surveillance imaging is important for the detection of device-related complications, which include endoleak, structural abnormalities, and infection. Currently used modalities include ultrasound, X-ray, computed tomography, magnetic resonance imaging, and angiography. Understanding the advantages and drawbacks of each modality, as well available guidelines, can guide selection of the appropriate technique for individual patients. We review complications following EVAR and advances in surveillance imaging modalities.

Quantifying Intra-Arterial Verapamil Response as a Diagnostic Tool for Reversible Cerebral Vasoconstriction Syndrome

BACKGROUND AND PURPOSE

There is mounting evidence supporting the benefit of intra-arterial administration of vasodilators in diagnosing reversible cerebral vasoconstriction syndrome. We prospectively quantified the degree of luminal diameter dilation after intra-arterial administration of verapamil and its accuracy in diagnosing reversible cerebral vasoconstriction syndrome.

MATERIALS AND METHODS

Patients suspected of having intracranial arteriopathy on noninvasive imaging and referred for digital subtraction angiography were enrolled in a prospective registry. Intra-arterial verapamil was administered in vascular territories with segmental irregularities. The caliber difference (Caliber_post - Caliber_pre) and the proportion of caliber change ([(Caliber_post - Caliber_pre)/Caliber_pre] × 100%) were used to determine the response to verapamil. The diagnosis of reversible cerebral vasoconstriction syndrome was made on the basis of clinical and imaging features at a follow-up appointment, independent of the reversibility of verapamil. Receiver operating characteristic curve analysis was performed to determine the best threshold.

RESULTS

Twenty-six patients were included, and 9 (34.6%) were diagnosed with reversible cerebral vasoconstriction syndrome. A total of 213 vascular segments were assessed on diagnostic angiography. Every patient with a final diagnosis of reversible cerebral vasoconstriction syndrome responded to intra-arterial verapamil. The maximal proportion of change (P < .001), mean proportion of change (P = .002), maximal caliber difference (P = .004), and mean caliber difference (P = .001) were statistically different between patients with reversible cerebral vasoconstriction syndrome and other vasculopathies. A maximal proportion of change  ≥32% showed a sensitivity of 100% and a specificity of 88.2% to detect reversible cerebral vasoconstriction syndrome (area under the curve = 0.951). The Reversible Cerebral Vasoconstriction Syndrome-2 score of  ≥5 points achieved a lower area under the curve (0.908), with a sensitivity of 77.8% and a specificity of 94.1%.

CONCLUSIONS

Objective measurement of the change in the arterial calibers after intra-arterial verapamil is accurate in distinguishing reversible cerebral vasoconstriction syndrome from other vasculopathies. A proportion of change  ≥32% has the best diagnostic performance.

Non-contrast-enhanced abdominal MRA at 3 T using velocity-selective pulse trains

PURPOSE

Most existing non-contrast-enhanced methods for abdominal MR arteriography rely on a spatially selective inversion (SSI) pulse with a delay to null both static tissue and venous blood, and are limited to small spatial coverage due to the sensitivity to slow arterial inflow. Velocity-selective inversion (VSI) based approach has been shown to preserve the arterial blood inside the imaging volume at 1.5 T. Recently, velocity-selective saturation (VSS) pulse trains were applied to suppress the static tissue and have been combined with SSI pulses for cerebral MR arteriography at 3 T. The aim of this study is to construct an abdominal MRA protocol with large spatial coverage at 3 T using advanced velocity-selective pulse trains.

METHODS

Multiple velocity-selective MRA protocols with different sequence modules and 3D acquisition methods were evaluated. Sequences using VSS only as well as SSI+VSS and VSI+VSS preparations were then compared among a group of healthy young and middle-aged volunteers. Using MRA without any preparations as reference, relative signal ratios and relative contrast ratios of different vascular segments were quantitatively analyzed.

RESULTS

Both SSI+VSS and VSI+VSS arteriograms achieved high artery-to-tissue and artery-to-vein relative contrast ratios above aortic bifurcation. The SSI+VSS sequence yielded lower signal at the bilateral iliac arteries than VSI+VSS, reflecting the benefit of the VSI preparation for imaging the distal branches.

CONCLUSION

The feasibility of noncontrast 3D MR abdominal arteriography was demonstrated on healthy volunteers using a combination of VSS pulse trains and SSI or VSI pulse.

Intracranial vasculature 3D printing: review of techniques and manufacturing processes to inform clinical practice

Background

In recent years, three-dimensional (3D) printing has been increasingly applied to the intracranial vasculature for patient-specific surgical planning, training, education, and research. Unfortunately, though, much of the prior literature regarding 3D printing has focused on the end-product and not the process. In addition, for 3D printing/manufacturing to occur on a large scale, challenges and bottlenecks specific to each modeled anatomy must be overcome.

Main body

In this review article, limitations and considerations of each 3D printing processing step, as they relate to printing individual intracranial vasculature models and providing an active clinical service for a quaternary care center, are discussed. Relevant advantages and disadvantages of the available acquisition techniques (computed tomography, magnetic resonance, and digital subtraction angiography) are reviewed. Specific steps in segmentation, processing, and creation of a printable file may impede the workflow or degrade the fidelity of the printed model and are, therefore, given added attention. The various available printing techniques are compared with respect to printing the intracranial vasculature. Finally, applications are discussed, and a variety of example models are shown.

Conclusion

In this review we provide insight into the manufacturing of 3D models of the intracranial vasculature that may facilitate incorporation into or improve utility of 3D vascular models in clinical practice.

Subtractive NCE-MRA: Improved background suppression using robust regression-based weighted subtraction

PURPOSE

To correct the intensity difference of static background signal between bright blood images and dark blood images in subtractive non-contrast-enhanced MR angiography using robust regression, thereby improving static background signal suppression on subtracted angiograms.

METHODS

Robust regression (RR), using iteratively reweighted least squares, is used to calculate the regression coefficient of background tissues from a scatter plot showing the voxel intensity of bright blood images versus dark blood images. The weighting function is based on either the Euclidean distance from the estimated regression line or the deviation angle. Results from RR using the deviation angle (RRDA), conventional RR using the Euclidean distance, and ordinary leastsquares regression were compared with reference values determined manually by two observers. Performance was evaluated over studies using different sequences, including 36 thoracic flow-sensitive dephasing data sets, 13 iliac flow-sensitive dephasing data sets, and 26 femoral fresh blood imaging data sets.

RESULTS

RR deviation angle achieved robust and accurate performance in all types of images, with small bias, small mean absolute error, and high-correlation coefficients with reference values. Background tissues, such as muscle, veins, and bladder, were suppressed while the vascular signal was preserved. Euclidean distance gave good performance for thoracic and iliac flow-sensitive dephasing, but could not suppress background tissues in femoral fresh blood imaging. Ordinary least squares regression was sensitive to outliers and overestimated regression coefficients in thoracic flow-sensitive dephasing.

CONCLUSION

Weighted subtraction using RR was able to acquire the regression coefficients of background signal and improve background suppression of subtractive non-contrast-enhanced MR angiography techniques. RR deviation angle has the most robust and accurate overall performance among three regression methods.

Zeolites for theranostic applications

Theranostic platforms bring about a revolution in disease management. During recent years, theranostic nanoparticles have been utilized for imaging and therapy simultaneously. Zeolites, because of their porous structure and tunable properties, which can be modified with various materials, can be used as a delivery agent. The porous structure of a zeolite enables it to be loaded and unloaded with various molecules such as therapeutic agents, photosensitizers, biological macromolecules, MRI contrast agents, radiopharmaceuticals, near-infrared (NIR) fluorophores, and microbubbles. Furthermore, theranostic zeolite nanocarriers can be further modified with targeting ligands, which is highly interesting for targeted cancer therapies.

Cardiovascular magnetic resonance imaging for structural heart disease

Cardiovascular magnetic resonance (CMR) has increasingly become a powerful imaging technique over the past few decades due to increasing knowledge about clinical applications, operator experience and technological advances, including the introduction of high field strength magnets, leading to improved signal-to-noise ratio. Its success is attributed to the free choice of imaging planes, the wide variety of imaging techniques, and the lack of harmful radiation. Developments in CMR have led to the accurate evaluation of cardiac structure, function and tissues characterisation, so this non-invasive technique has become a powerful tool for a broad range of cardiac pathologies. This review will provide an introduction of magnetic resonance imaging (MRI) physics, an overview of the current techniques and clinical application of CMR in structural heart disease, and illustrated examples of its use in clinical practice.

Infrared Thermally Enhanced 3-Dimensional Time of Flight Magnetic Resonance Angiography Imaging for the Visualization of the Arteries of the Face

Background

The face is known for its extreme variation in vascular anatomy. Furthermore, the rapidly increasing number of filler treatments leads to an increase in severe filler-associated complications (such as skin necrosis and blindness) due to intra-arterial injection. Visualizing a patient’s individual complete facial arterial anatomy in a contrast- and radiation-free way has not been published before. This innovative imaging technique could, therefore, enhance the safety of minimally invasive surgical procedures as it provides a harmless way to map the arteries of the face.

Objectives

Evaluate a newly developed imaging technique to visualize the arteries of the face in a noninvasive and radiation-free manner.

Methods

The individual arterial facial anatomy of 20 volunteers was studied by an imaging technique, combining infrared (IR) facial warming and 3-dimensional (3D) time of flight (TOF) magnetic resonance angiography (MRA). The source and maximum intensity projection images were assessed by 2 investigators, familiar with the anatomy of the face.

Results

The MRA technique visualized most of the main facial arteries, albeit in a variable way. The main facial branches of the external carotid artery (facial, angular, supralabial, and superficial temporal arteries) were illustrated well, whereas the visualization of the internal carotid branches (supratrochlear and supraorbital arteries) and nasal branches (dorsal nasal and lateral nasal arteries) was less consistent.

Conclusions

The combination of IR “heat-induced enhancement” and a 3D-TOF MRA sequence may actually be an important step toward the visualization of the variable facial vascular anatomy in a noninvasive, radiation-free, and contrast-free manner.

Tumefactive perivascular space demonstrated with post-contrast time-of-flight MR angiography

The cerebral perivascular spaces are interstitial fluid-filled anatomic structures surrounding the perforating arteries. They appear as small, round or curvilinear structures on magnetic resonance (MR) imaging. Occasionally, these structures may become very large and cause mass effect. In this case, they may imitate malignant processes and are referred to as tumefactive perivascular spaces. In this study, we present a case of tumefactive perivascular space demonstrated with post-contrast time-of-flight (TOF) MR angiography. To our knowledge, there have been no previous clear demonstrations of the perforating artery in tumefactive perivascular space with contrast-enhanced TOF MR angiography. The purpose of this study was to describe advanced imaging findings in this unusual condition.

Molecular and Nonmolecular Magnetic Resonance Coronary and Carotid Imaging

Atherosclerosis is the leading cause of cardiovascular morbidity and mortality. Over the past 2 decades, increasing research attention is converging on the early detection and monitoring of atherosclerotic plaque. Among several invasive and noninvasive imaging modalities, magnetic resonance imaging (MRI) is emerging as a promising option. Advantages include its versatility, excellent soft tissue contrast for plaque characterization and lack of ionizing radiation. In this review, we will explore the recent advances in multicontrast and multiparametric imaging sequences that are bringing the aspiration of simultaneous arterial lumen, vessel wall, and plaque characterization closer to clinical feasibility. We also discuss the latest advances in molecular magnetic resonance and multimodal atherosclerosis imaging.

Improving the Otsu method for MRA image vessel extraction via resampling and ensemble learning

Accurate extraction of vessels plays an important role in assisting diagnosis, treatment, and surgical planning. The Otsu method has been used for extracting vessels in medical images. However, blood vessels in magnetic resonance angiography (MRA) image are considered as a sparse distribution. Pixels on vessels in MRA image are considered as an imbalanced data in classification of vessels and non-vessel tissues. To extract vessels accurately, a novel method using resampling technique and ensemble learning is proposed for solving the imbalanced classification problem. Each pixel is sampled multiple times through multiple local patches within the image. Then, vessel or non-vessel tissue is determined by the ensemble voting mechanism via a p-tile algorithm. Experimental results show that the proposed method is able to outperform the traditional Otsu method by extracting vessels in MRA images more accurately.

In Vivo Tracking of Tissue Engineered Constructs

To date, the fields of biomaterials science and tissue engineering have shown great promise in creating bioartificial tissues and organs for use in a variety of regenerative medicine applications. With the emergence of new technologies such as additive biomanufacturing and 3D bioprinting, increasingly complex tissue constructs are being fabricated to fulfill the desired patient-specific requirements. Fundamental to the further advancement of this field is the design and development of imaging modalities that can enable visualization of the bioengineered constructs following implantation, at adequate spatial and temporal resolution and high penetration depths. These in vivo tracking techniques should introduce minimum toxicity, disruption, and destruction to treated tissues, while generating clinically relevant signal-to-noise ratios. This article reviews the imaging techniques that are currently being adopted in both research and clinical studies to track tissue engineering scaffolds in vivo, with special attention to 3D bioprinted tissue constructs.

Pharmacokinetics and biodistribution study of self-assembled Gd-micelles demonstrating blood-pool contrast enhancement for MRI

Magnetic resonance angiography (MRA) requires the use of contrast agents (CAs) to enable accurate diagnosis. There are currently no CAs on the market with appropriate pharmacokinetic (PK) parameters, namely long persistence in the blood, that can be easily used for MRA. We have recently synthesized amphiphilic building blocks loaded with gadolinium (Gd), which self-assemble into Gd-micelles in aqueous media, and have evaluated their potential as a blood-pool contrast agent (BPCA) in vivo. To assess the short and long term PK of Gd-micelles, the blood and organs of the mice were analyzed at t = 30 min, 1, 2, 3 h, 7, 14 and 21 days. Gd-DOTA was used as a control because it is the gold-standard CA for MRA despite its rapid clearance from the blood compartment. Gd-micelles circulated in the blood for more than 3 h postinjection whereas Gd-DOTA was eliminated less than half an hour postinjection. No side effects were observed in the mice up to the end of the study at 21 days and no accumulation of Gd was observed in the brain or bones. The Magnetic Resonance Imaging (MRI) parameters and the results of this in vivo study indicate the true BCPA properties of Gd-micelles and warrant further development.

Feeding arteries and arteriovenous shunt for discrimination of soft tissue tumors

Time resolved magnetic resonance angiography with interleaved stochastic trajectories (TWIST) allows for identification of tumor feeding arteries and arteriovenous shunt (AVS). We used TWIST to obtain number of feeding arteries (NFA) and detect AVS for 43 cases of pathology-confirmed soft tissue tumors. We compared normalized number of feeding arteries (nNFA) and AVS between malignant and benign tumors, and found nNFA was significantly greater in malignant tumors versus benign tumors (2.1 vs 1.3, P < .05). The incidence of AVS was significantly higher in malignant tumors versus benign tumors (87.5% vs 10.5%, P < .05). TWIST derived nNFA and AVS could be useful in the discrimination of benign and malignant soft tissue tumors.

Studying the cardiovascular system of a marine crustacean with magnetic resonance imaging at 9.4 T

OBJECTIVES

An approach is presented for high-field MRI studies of the cardiovascular system (CVS) of a marine crustacean, the edible crab Cancer pagurus, submerged in highly conductive seawater.

MATERIALS AND METHODS

Structure and function of the CVS were investigated at 9.4 T. Cardiac motion was studied using self-gated CINE MRI. Imaging protocols and radio-frequency coil arrangements were tested for anatomical imaging. Haemolymph flow was quantified using phase-contrast angiography. Signal-to-noise-ratios and flow velocities in afferent and efferent branchial veins were compared with Student's t test (n = 5).

RESULTS

Seawater induced signal losses were dependent on imaging protocols and RF coil setup. Internal cardiac structures could be visualized with high spatial resolution within 8 min using a gradient-echo technique. Variations in haemolymph flow in different vessels could be determined over time. Maximum flow was similar within individual vessels and corresponded to literature values from Doppler measurements. Heart contractions were more pronounced in lateral and dorso-ventral directions than in the anterior-posterior direction.

DISCUSSION

Choosing adequate imaging protocols in combination with a specific RF coil arrangement allows to monitor various parts of the crustacean CVS with exceptionally high spatial resolution despite the adverse effects of seawater at 9.4 T.

Intercostal artery pseudoaneurysm following thoracentesis: multi-modal imaging and treatment

Background

A pseudoaneurysm occurs as the result of a contained rupture of an arterial wall, yielding a perfused sac that communicates with the arterial lumen. Pseudoaneurysm of an intercostal artery is an extremely rare event but it carries with it a significant risk of rupture and subsequent hemothorax. It must be considered as a potential complication of thoracentesis.

Case presentation

Here, we report a rare case of an intercostal artery pseudoaneurysm following thoracentesis in an 82-year old male. The patient presented with respiratory distress 1 day after a therapeutic thoracentesis had been performed. Computed tomography (CT) with contrast revealed a left intercostal pseudoaneurysm with hemothorax and adjacent compressive atelectasis. Doppler ultrasound revealed bidirectional blood flow in the pseudoaneurysm sac. An intercostal arteriogram and thoracic aortogram aided in confirmation of the pseudoaneurysm and successful treatment with coil embolization.

Conclusions

An intercostal pseudoaneurysm complication following thoracentesis is very rare but important to rule out as a possible cause of hemothorax after the procedure. Capturing this finding with the aid of multiple imaging modalities allowed for diagnostic certainty and rapid treatment with coil embolization, leading to a successful patient recovery.