Promising four-dimensional flow magnetic resonance imaging technique for vascular flow analysis in children

Abstract

Leesmidt et al present a comprehensive analysis of abdominal vascular flow in children using four-dimensional (4D) flow magnetic resonance imaging (MRI), aim to establish normal hemodynamic values for the abdominal visceral organs and to assess the feasibility of 4D flow MRI (4D-f-MRI) in this population. The researchers performed 4D-f-MRI on 9 pediatric patients with a history or suspicion of bowel pathology. Flow velocities were measured in the abdominal aorta and superior and inferior mesenteric arteries. The quality of the 4D-f-MRI images was evaluated, and the agreement between the measured flow velocities and those obtained from Duplex ultrasound was established. However, due to the specific limitations of this work, future studies should address the issues of small sample size and the specific age group design.

Key Words: Four-dimensional flow magnetic resonance imaging; Hemodynamic values; Imaging; Diagnosis; Vascular flow

Core Tip: The study demonstrates that four-dimensional (4D) flow magnetic resonance imaging (MRI) is feasible for assessing abdominal vascular flow in children, with good image quality and reproducibility. The flow velocities measured by 4D flow MRI (4D-f-MRI) showed good agreement with those obtained from Duplex ultrasound, suggesting that 4D-f-MRI may serve as a complementary tool to Duplex ultrasound in clinical practice for diagnosing and managing pediatric vascular diseases.

TO THE EDITOR

Leesmidt et al[1] present a comprehensive analysis of abdominal vascular flow in children using four-dimensional (4D) flow magnetic resonance imaging (MRI), aim to establish normal hemodynamic values for the abdominal visceral organs in children and to assess the feasibility of 4D flow MRI (4D-f-MRI) in this population. The researchers performed 4D-f-MRI on 9 pediatric patients (median age: 13 years; range: 7-14 years; 5 males, 4 females) with a history or suspicion of bowel pathology, including Crohn's disease, ulcerative colitis, and celiac disease. Flow velocities were measured in the abdominal aorta and superior and inferior mesenteric arteries. In addition, the quality of the 4D-f-MRI images was evaluated, along with the agreement between the measured flow velocities and those obtained from Duplex ultrasound.

In this study, the authors present several key findings as follows: (1) Normal hemodynamic values: The study provides normal hemodynamic values for the abdominal visceral organs in children, which may serve as a reference for future studies and clinical practice; (2) Feasibility of 4D-f-MRI: The study demonstrates the feasibility of 4D-f-MRI for assessing abdominal vascular flow in children. The imaging was performed using 3 T MRI scanners of 32-channel cardiac coil (GE Discovery MR750, WI, United States) by applying 4D-f-MR protocol and running the DV27 software. The results indicate good image quality and reproducibility; and (3) Agreement with Duplex ultrasound: The measured flow velocities obtained from 4D-f-MRI for the superior mesenteric vein (SMV) and superior mesenteric artery (SMA) valued with a mean bias (-1.2 cm/s vs -3.2 cm/s) and 95% limits of agreement (-9.4 to 7.0 cm/s vs -31.4 to 24.9 cm/s) demonstrated good agreement with those obtained from Duplex ultrasound on Bland-Altman analysis. These results suggest that 4D-f-MRI can serve as a complementary tool to Duplex ultrasound in clinical practice. Additionally, the inter-reader good agreement for flow velocity was founded according to a significant difference of the SMV and the SMA with a mean bias (-0.01 L/min vs -0.03 L/min) and 95% limits of agreement (-0.09 to 0.08 L/min vs -0.23 to 0.17 L/min).

These findings indicate significant potential for clinical diagnosis, which may include the following: (1) It can capture hemodynamic parameters and anatomical structural changes synchronously in intestinal lesion areas through 4D spatial and temporal resolution. This enables integrated analysis of both function and morphology, improving the accuracy of inflammatory activity assessment; (2) Combining 4D-f-MRI with magnetic resonance bowel imaging allows comprehensive bowel assessment under non-invasive conditions; and (3) Compared with traditional endoscopy and X-ray air enema, 4D-f-MRI involves no radiation exposure and no invasive procedures, making it particularly suitable for young children and patients with chronic bowel disease who require repeated examinations.

However, several key points challenged by current techniques must be addressed and strengthened.

Imaging methods for vascular flow measurement

Duplex ultrasound is a non-radiative tool for obtaining vascular flow velocity information but has limitations in larger patients due to reduced acoustic windows and operator dependency. Dynamic perfusion computed tomography (CT) imaging provides high-resolution images; however, it increases radiation exposure, which is a particular concern for pediatric patients. MRI offers non-invasive techniques such as two-dimensional and three-dimensional (3D) time-of-flight imaging, which do not involve ionizing radiation. Owning the characteristics of volumetric and time-resolved cine sequence, the 4D-f-MRI enables 3D velocity encoding with electrocardiographic gating, then provides comprehensive vascular flow assessment.

Challenges with 4D-f-MRI

Respiratory motion can adversely affect the quality of 4D-f-MRI, particularly in the abdominal region. Although respiratory suppression techniques have been developed, free-breathing 4D-f-MRI has also been reported. The respiratory inhibition technique, also known as navigation gating, triggers data acquisition by monitoring the position of the diaphragm, effectively reducing respiratory motion artifacts. However, this method requires a prolonged scanning time and patient cooperation through respiratory training.

By combining respiratory gating with k-space optimization, using adaptive k-space reordering technology, key data can be preferentially collected during the optimal phase of the respiratory cycle, balancing imaging speed and quality. The free-breathing method does not require patient breath-holding and provides high patient comfort and greater clinical feasibility. However, it necessitates compensation for respiratory motion, typically achieved through post-processing algorithms such as motion tracking or image registration. Additionally, natural marker point tracking, another free-breathing method, uses lung anatomy as an endogenous marker and achieves dynamic correction by tracking motion trajectories through local intensity features[2].

Nevertheless, respiratory gating in 4D-f-MRI remains a topic of ongoing debate.

The study provides valuable insights into the hemodynamics of abdominal vessels in children. It also highlights the potential of 4D-f-MRI as a non-invasive and radiation-free imaging modality for assessing vascular flow in this population. It reveals its potential role in evaluating bowel ischemia. Future research could focus on validating these findings in larger pediatric cohorts and exploring the clinical utility of 4D-f-MRI in diagnosing and managing pediatric vascular diseases.

Furthermore, subsequent investigations should address the following aspects to further validate and enhance the application of 4D-f-MRI in the evaluation of active intestinal diseases in children: (1) Expanding sample size: Conducting more extensive studies to verify the generalizability and reliability of the current findings. Increasing the sample size could help determine the applicability of 4D-f-MRI in children across different age groups, disease states, and geographic regions; (2) Long-term follow-up: Implementing long-term follow-up of enrolled children to assess the sustained effectiveness of 4D-f-MRI in monitoring disease progression and evaluating treatment efficacy; (3) Multicenter studies: Performing multicenter studies to evaluate the impact of different MRI systems and operators on 4D-f-MRI results, ensuring reproducibility and standardization across institutions; (4) Technical optimization: Further optimizing the imaging protocols and post-processing techniques of 4D-f-MRI to enhance image quality and improve flow quantification accuracy; (5) Comparison with other imaging techniques: The 4D-f-MRI has been compared with other imaging modalities, including Doppler ultrasound, CT scans, and conventional MRI, to evaluate its advantages and limitations in diagnosing and monitoring intestinal diseases; (6) Clinical application guidelines: Guidelines for the clinical application of 4D-f-MRI in pediatric intestinal diseases have been developed based on current findings. These include recommendations regarding indications, imaging protocols, data analysis methodologies, and clinical decision support strategies; (7) Potential side effects and risks: The potential side effects and risks associated with 4D-f-MRI in children have been assessed, with particular attention to the implications of long-term use and repeated imaging; (8) Cost-benefit analysis: A cost-benefit analysis has been conducted to evaluate the economic feasibility of integrating 4D-f-MRI into clinical practice for pediatric patients; and (9) Assessment of patient quality of life: Studies have examined the impact of 4D-f-MRI on the quality of life in children, including factors such as comfort during imaging procedures and the effects on daily activities.

By addressing these aspects, future studies could further consolidate the role of 4D-f-MRI in assessing active bowel disease in children, providing more robust decision-support tools for clinical practice.

Conclusion

The flow velocities measured using 4D-f-MRI demonstrated good agreement with those obtained from Duplex ultrasound. These findings suggest that 4D-f-MRI may serve as a complementary tool to Duplex ultrasound in clinical practice for diagnosing and managing pediatric vascular diseases. Future studies could further strengthen the role of 4D-f-MRI in assessing active bowel disease in children, providing more robust decision support tools for clinicians, and should expand the study to a broader range of conditions and incorporate long-term follow-up to assess the impacts of repeated scanning.