Abdominal applications of quantitative 4D flow MRI

Assessment of four-dimensional flow MRI for prediction of varices risk in cirrhotic patients

OBJECTIVE

This study aims to validate the application of abdominal four-dimensional flow magnetic resonance imaging (MRI) for cirrhotic patients and quantify its effectiveness in assessing the hemodynamic impacts of cirrhosis to evaluate varices.

METHODS

All consecutive patients who underwent MRIs between September 2022 and June 2023 were enrolled. Groups were divided into varicose, non-varicose, and healthy groups. ANOVA and post hoc LSD-t tests were used for statistical analysis. The correlation between hemodynamic parameters and liver function grade was evaluated using Kendall's correlation coefficient.

RESULTS

A total of 80 patients were included (53 cirrhotic, 27 healthy). Significant disparities were found in main portal vein flow (MPV-FR), splenic vein flow (SV-FR), and vessel diameters (MPV-VD, SV-VD) among the groups (p < 0.05). MPV-FR was higher in the varicose group (24.81 ± 8.52) compared to non-varicose (19.52 ± 5.07) and healthy groups (17.26 ± 5.48). The most robust assessment of variceal risk was achieved by combining the flow rates (FRs) and VDs of MPV and SV (AUC 0.83, 95% CI 0.72-0.94).

CONCLUSION

The combined indices of FRs and VDs of MPV and SV effectively predict the occurrence of varicose veins in cirrhotic patients.

KEY POINTS

Question Non-invasive prediction of variceal risk is essential for the clinical management of advanced chronic cirrhosis, yet existing clinical examinations are inadequate. Findings The effective assessment of variceal risk was achieved by combining the flow rates and vessel diameters of the main portal vein and splenic vein. Clinical relevance Four-dimensional flow MRI can reveal hemodynamic changes in cirrhotic patients and assist in identifying gastroesophageal varices, serving as a marker for varices risk prediction.

Non-contrast MR angiography: physical principles and clinical applications in chest, abdomen and pelvis imaging

This review article focuses on the advancements in non-contrast magnetic resonance angiography (NC-MRA) and its increasing importance in body imaging, especially for patients with renal complications, pregnant women, and children. It highlights the relevance of NC-MRA in chest, abdominal, and pelvis imaging and details various bright-blood NC-MRA techniques like cardiac-gated 3D Fast Spin Echo (FSE), balanced Steady-State Free Precession (bSSFP), Arterial Spin Labeling (ASL), and 4D flow methods. The article explains the operational principles of these techniques, their clinical applications, and their advantages over traditional contrast-enhanced methods. Special attention is given to the utility of these techniques in diverse imaging scenarios, including liver, renal, and pelvic imaging. The article underscores the growing importance of NC-MRA in medical diagnostics, offering insights into current practices and potential future developments. This comprehensive review is a valuable resource for radiologists and clinicians, emphasizing NC-MRA's role in enhancing patient care and diagnostic accuracy across various medical conditions.

Quantifying and visualizing abdominal hemodynamics in patients with Fontan circulation by 4D phase-contrast flow magnetic resonance imaging at 1.5 T

BACKGROUND

Liver fibrosis has been recognized as a long-term morbidity associated with Fontan circulation (Fontan-associated liver disease, FALD). The pathophysiology of FALD is not completely understood and abnormal flow dynamics may be associated with this condition. Liver hemodynamics can be quantitatively evaluated with four-dimensional phase-contrast flow magnetic resonance imaging (4D PC flow MRI). The study aimed to evaluate suitability of liver 4D PC flow MRI in Fontan patients and relate flow measurements to normal values and FALD severity.

PATIENTS AND METHODS

Twenty-two Fontan patients were examined by 4D PC flow MRI at 1.5 Tesla to assess mesenteric, portal, splenic, and hepatic venous blood flow. Severity of FALD was graded based on routine screening, including abdominal ultrasound and laboratory tests.

RESULTS

Median age was 18.5 (interquartile range, IQR 15.5-20.2) years. FALD was graded as "none or mild" in 16 and as "moderate to severe" in six cases. Ten patients presented at least one feature of portal hypertension (ascites, splenomegaly, or thrombocytopenia). For the entire cohort, blood flow in the superior mesenteric, splenic, and portal vein was lower than reported in the literature. No significant differences were observed in relation to FALD severity. Features of portal hypertension were associated with a higher splenic vein blood flow (0.34 ± 0.17 vs. 0.20 ± 0.07 l/min, p = 0.046). Splenic vein blood flow was negatively correlated to platelet count (r = -0.590, p = 0.005).

CONCLUSIONS

4D PC flow MRI appears suitable to assess liver hemodynamics in Fontan patients and integration into clinical follow-up might help to improve our understanding of FALD.

Under pressure: a head-to-toe review of vascular compression syndromes

Vascular compression syndromes are a group of conditions resulting from mechanical compression of blood vessels by adjacent structures leading to compromised blood flow and various associated symptoms. They frequently affect young, otherwise healthy individuals and are often underdiagnosed due to their rarity and vague clinical manifestations. Achieving an accurate diagnosis depends on the integration of clinical presentation and imaging findings. Imaging modalities including color doppler ultrasound, computed tomography angiography, magnetic resonance angiography, and catheter-directed digital subtraction angiography are essential for diagnosis and management. Dynamic imaging is crucial in eliciting findings due to the positional nature of many of these syndromes. In this paper, we will present a "head-to-toe" overview of vascular compression syndromes including Vascular Eagle Syndrome, Vascular Thoracic Outlet Syndrome, Quadrilateral Space Syndrome, Hypothenar Hammer Syndrome, Median Arcuate Ligament Syndrome, Renal Artery Entrapment Syndrome, Left Renal Vein Compression/Nutcracker Syndrome, May-Thurner Syndrome, Adductor Canal Syndrome, and Popliteal Artery Entrapment Syndrome. Treatment is variable but typically involves a combination of conservative and surgical management. Surgical approaches focus on decompression of affected neurovascular structures. Endovascular treatment alone is rarely recommended. We aim to equip general radiologists with the knowledge needed to accurately diagnose patients with vascular compression syndromes, allowing for timely treatment.

Quantitative Flow Measurements of Pelvic Venous Vasculature Using 4D Flow MRI

RATIONALE AND OBJECTIVES

To evaluate 4D Flow magnetic resonance imaging (MRI) sequences for quantitative flow measurements of the pelvic venous vasculature.

MATERIALS AND METHODS

A prospective study of healthy volunteers was performed. After informed consent all subjects underwent 4D flow sequences at a 3 T MRI scanner with different isotropic resolution and different velocity encoding (Venc) settings: (sequence #1) voxel size (VS) 1.63 mm3, Venc 50 cm/s; (sequence #2) VS 1.63 mm3, Venc 100 cm/s and (sequence #3) VS 2.03 mm3, Venc 50 cm/s. Perfusion parameters were calculated for all venous vessel segments starting at the level of the inferior vena cava and extending caudally to the level of the common femoral vein. For reference, arterial flow was calculated using 1.63 mm3 isotropic resolution with a Venc of 100 cm/s.

RESULTS

Ten healthy subjects (median age 28 years, interquartile range [IQR]: 26.25-28 years) were enrolled in this study. Median scanning time was 12:12 minutes (IQR 10:22-13:32 minutes) for sequence #1, 11:02 minutes (IQR 9:57-11:19 minutes) for sequence #2 and 6:10 minutes (IQR 5:44-6:47 minutes) for sequence #3. Flow measurements were derived from all sequences. The venous pelvic vasculature showed similar perfusion parameters compared to its arterial counterpart, for example the right common iliac arterial segment showed a perfusion of 8.32 ml/s (IQR: 6.94-10.68 ml/s) versus 7.29 ml/s (IQR: 4.70-8.90 ml/s) in the corresponding venous segment (P = 0.218). The venous flow measurements obtained from the three investigated sequences did not reveal significant differences.

CONCLUSION

4D Flow MRI is suitable for quantitative flow measurement of the venous pelvic vasculature. To reduce the scanning time without compromising quantitative results, the resolution can be decreased while increasing the Venc. This technique may be utilized in the future for the diagnosis and treatment response assessment of iliac vein compression syndromes.

Fat mitigation strategies to improve image quality of radial 4D flow MRI in obese subjects

PURPOSE

This study addresses the challenges in obtaining abdominal 4D flow MRI of obese patients. We aimed to evaluate spectral saturation and inner volume excitation as methods to mitigating artifacts originating from adipose signals, with the goal of enhancing image quality and improving quantification.

METHODS

Radial 4D flow MRI acquisitions with fat mitigation (inner volume excitation [IVE] and intermittent fat saturation [FS]) were compared to a standard slab selective excitation (SSE) in a test-retest study of 15 obese participants. IVE selectively excited a cylindrical region of interest, avoiding contamination from peripheral adipose tissue, while FS globally suppressed fat based on spectral selection. Acquisitions were evaluated qualitatively based on expert ratings and quantitatively based on conservation of mass, test-retest repeatability, and a divergence free quality metric. Errors were evaluated statistically using the absolute and relative errors, regression, and Bland-Altman analysis.

RESULTS

IVE demonstrated superior performance quantitatively in the conservation of mass analysis in the portal vein, with higher correlation and lower bias in regression analysis. IVE also produced flow fields with the lowest divergence error and was rated best in overall image quality, delineating small vessels, and producing the least streaking artifacts. Evaluation results did not differ significantly between FS and SSE. Test-retest reproducibility was similarly high for all sequences, with data suggesting biological variations dominate the technical variability.

CONCLUSION

IVE improved hemodynamic assessment of radial 4D flow MRI in the abdomen of obese participants while FS did not lead to significant improvements in image quality or flow metrics.

Non-Contrast Magnetic Resonance Angiography: Techniques, Principles, and Applications

Several non-contrast magnetic resonance angiography (MRA) techniques have been developed, providing an attractive alternative to contrast-enhanced MRA and a radiation-free alternative to computed tomography (CT) CT angiography. This review describes the physical principles, limitations, and clinical applications of bright-blood (BB) non-contrast MRA techniques. The principles of BB MRA techniques can be broadly divided into (a) flow-independent MRA, (b) blood-inflow-based MRA, (c) cardiac phase dependent, flow-based MRA, (d) velocity sensitive MRA, and (e) arterial spin-labeling MRA. The review also includes emerging multi-contrast MRA techniques that provide simultaneous BB and black-blood images for combined luminal and vessel wall evaluation.

4D Flow cardiovascular magnetic resonance consensus statement: 2023 update

Hemodynamic assessment is an integral part of the diagnosis and management of cardiovascular disease. Four-dimensional cardiovascular magnetic resonance flow imaging (4D Flow CMR) allows comprehensive and accurate assessment of flow in a single acquisition. This consensus paper is an update from the 2015 '4D Flow CMR Consensus Statement'. We elaborate on 4D Flow CMR sequence options and imaging considerations. The document aims to assist centers starting out with 4D Flow CMR of the heart and great vessels with advice on acquisition parameters, post-processing workflows and integration into clinical practice. Furthermore, we define minimum quality assurance and validation standards for clinical centers. We also address the challenges faced in quality assurance and validation in the research setting. We also include a checklist for recommended publication standards, specifically for 4D Flow CMR. Finally, we discuss the current limitations and the future of 4D Flow CMR. This updated consensus paper will further facilitate widespread adoption of 4D Flow CMR in the clinical workflow across the globe and aid consistently high-quality publication standards.