Blood Flow Imaging
David Saloner

Synopsis

MR imaging is the only effective modality for imaging hemodynamic conditions in the intracranial vasculature. The use of these methods for evaluating healthy and diseased vessels will be discussed.

Highlights

· Evaluation of hemodynamic conditions provides added information on intracranial vascular health by determining important parameters that govern physiological conditions such as perfusion and the likelihood of vascular disease progression.

· Typical MR flow methods can be separated into two categories, 2D acquisitions or 4D flow methods which evaluate flow in 3D space through the cardiac cycle.

· Improved MR methods are directed at providing high coverage in reduced acquisition times with standardized and validated imaging approaches.

Target Audience

· Clinicians and scientists interested in methods for evaluating hemodynamics in the intracranial vessels

Outcome/Objectives

· Learners will understand the methods used to acquire information on intracranial hemodynamics, challenges and opportunities.

Purpose

The techniques described in this presentation are aimed at providing tools for assessing blood flow conditions in normal and diseased vessels. While information on the structural morphology of the vasculature is informative in identifying the location and extent of vascular disease, it does not provide direct information on the impact of that disease on the distribution of blood which could be important in determining physiologically relevant parameters such as perfusion status, or the likelihood of disease progression. Investigations are being actively pursued to validate and standardize robust methods for quantification of blood flow.

Methods

The investigation of blood flow is typically pursued using one of two approaches: 2D determinations of flow through the cardiac cycle using slices that are transverse to targeted vessels; and 4D flow methods, which determine the velocity field in a three-dimensional volume through the cardiac cycle (1,2).

2D flow methods have been used to establish the relative flow in different hemispheres or territories, as well as to measure absolute flow values. Slice locations are prescribed from a prior MR angiographic study acquired in the same imaging session. Although prescription of a 4D flow study also requires some localizer information, precise placement is not as stringent as for the 2D study, as data can be reformatted post acquisition.

The details of the MR flow study must be matched to the presenting pathology. In stenotic atherosclerotic lesions, the dynamic range of velocities (or VENC) must be selected to capture the high velocity associated with the post-stenotic jet. However, if the lesion is flow-limiting that strategy can have negative consequences. In aneurysmal disease, velocity values in the aneurysm sac are generally reduced and the VENC needs to be correspondingly adjusted. 4D flow can provide novel insights into the prevailing hemodynamics as in the case of strongly vortical flow in the jugular venous outflow tract where other modalities have failed to identify these important flow features (3,4).

A challenge for any validation of flow measurements made in vivo is the absence of an appropriate standard-of-reference. Because of this, comparison between novel methods and standard methods may produce different results without establishing which is correct. Surrogate methods for determining the most favorable method include qualitative ratings, or selection of the method that has the lowest variation in measurement reproducibility. Another powerful method for selecting between candidate techniques is to use carefully calibrated and validated in vitro flow models.

Results

The evaluation of quantitative MR flow methods indicate that there are a number of important correction factors that must be taken into account to provide reliable and consistent data within and across vendor platforms. These include factors that are often built into the vendor product such as correction for Maxwell terms, eddy currents, and other approaches that require background phase correction and divergence-free flow.

Discussion

The major advantages of 2D flow relative to 4D flow are rapid acquisitions, and high in-plane resolution. Relative benefits of 4D flow are coverage and the inclusion of multiple target vessel segments that do not require pre-planning. Studies are being pursued that utilize heavily undersampled data acquisition with iterative reconstruction to substantially reduce the total acquisition time. In vitro models can be used to validate novel methods but do not always reflect important in vivo characteristics such as vascular compliance or beat-to-beat cardiac output variability. However, they do serve to determine baseline performance characteristics.

Conclusion

MR flow imaging methods are a powerful tool for evaluating hemodynamics in important vascular structures. They are of particular value in assessment of the intracranial circulation where the only competitive modality, Doppler Ultrasound, is of little to no value given the inability of sound waves to penetrate the skull. Large outcome studies are needed to determine the relationship between MR-derived hemodynamic descriptors and the evolution of disease in intracranial dysfunction.

Acknowledgements

No acknowledgement found.

References

1. Markl M, Schnell S, Wu C, Bollache E, Jarvis K, Barker AJ, Robinson JD, Rigsby CK. Advanced flow MRI: emerging techniques and applications. Clin Radiol. 2016 Review. PMID: 26944696

2. Schnell S, Wu C, Ansari SA. Four-dimensional MRI flow examinations in cerebral and extracerebral vessels - ready for clinical routine? Curr Opin Neurol. 2016 PMID: 27262148

3. Acevedo-Bolton G, Amans MR, Kefayati S, Halbach V, Saloner D. Four dimensional magnetic resonance velocimetry for complex flow in the jugular vein. Quant Imaging Med Surg. 2015 PMID: 26435930

4. Kefayati S, Amans M, Faraji F, Ballweber M, Kao E, Ahn S, Meisel K, Halbach V, Saloner D. The manifestation of vortical and secondary flow in the cerebral venous outflow tract: An in vivo MR velocimetry study. J Biomech. 2017 PMID: 27894675.

Proc. Intl. Soc. Mag. Reson. Med. 25 (2017)