MRI and Ultrasound Combo Opens Blood-brain Barrier

In a mouse model study of MRI-guided focused ultrasound-induced blood-brain barrier (BBB) opening at MRI field strengths ranging from ­approximately 0 T (outside the magnetic field) to 4.7 T, the static magnetic field dampened the detected microbubble cavitation signal and decreased the BBB opening volume. Credit: Washington University School of Medicine in St. Louis

Using a combination of ultrasound, MRI field strength and microbubbles can open the blood-brain barrier (BBB) and allow therapeutic drugs to reach the diseased brain location with MRI guidance. 

Using the physical phenomenon of cavitation, it is a promising technique that has been shown safe in patients with various brain diseases, such as Alzheimer’s diseases, Parkinson’s disease, ALS, and glioblastoma.
While MRI has been commonly used for treatment guidance and assessment in preclinical research and clinical studies, until now, researchers did not know the impact that MRI scanner’s magnetic field had on the BBB opening size and drug delivery efficiency.

Hong Chen, associate professor of biomedical engineering at Washington University in St. Louis, and her lab have found for the first time that the magnetic field of the MRI scanner decreased the BBB opening volume by 3.3-fold to 11.7-fold, depending on the strength of the magnetic field, in a mouse model. The findings were in Radiology.

Prof Chen conducted the study on four groups of mice. After they were injected microbubbles, three groups received focused-ultrasound sonication at different strengths of the magnetic field: 1.5 T (teslas), 3 T and 4.7 T, and one group was never exposed to the field. 

The researchers found that the microbubble cavitation activity, or the growing, shrinking and collapse of the microbubbles, decreased by 2.1 decibels at 1.5 T; 2.9 decibels at 3 T; and 3 decibels at 4.7 T, compared with those that had received the dose outside of the magnetic field. Additionally, the magnetic field decreased the BBB opening volume by 3.3-fold at 1.5 T; 4.4-fold at 3 T; and 11.7-fold at 4.7 T. No tissue damage from the procedure was seen.

Following focused-ultrasound sonication, the team injected a model drug, Evans blue dye, to investigate whether the magnetic field affected drug delivery across the BBB. The images showed that the fluorescence intensity of the Evans blue was lower in mice that received the treatment in one of the three strengths of magnetic fields compared with mice treated outside the magnetic field. The Evans blue trans-BBB delivery was decreased by 1.4-fold at1.5 T, 1.6-fold at 3.0 T and 1.9-fold at 4.7 T when compared with those treated outside of the magnetic field.

“The dampening effect of the magnetic field on the microbubble is likely caused by the loss of bubble kinetic energy due to the Lorentz force acting on the moving charged lipid molecules on the microbubble shell and dipolar water molecules surrounding the microbubbles,” said Yaoheng (Mack) Yang, a doctoral student in Prof Chen’s lab and the lead author of the study.

“Findings from this study suggest that the impact of the magnetic field needs to be considered in the clinical applications of focused ultrasound in brain drug delivery,” Prof Chen said.

In addition to brain drug delivery, cavitation is also used in several other therapeutic techniques, such as histotripsy, the use of cavitation to mechanically destroy regions of tissue, and sonothrombolysis, a therapy used after acute ischaemic stroke. The magnetic field’s damping effect on cavitation is expected to affect the treatment outcomes of other cavitation-mediated techniques when MRI-guided focused-ultrasound systems are used.

Source: Washington University in St. Louis

Journal information: Yang, Y., et al. (2021) Static Magnetic Fields Dampen Focused Ultrasound–mediated Blood-Brain Barrier Opening. Radiology. doi.org/10.1148/radiol.2021204441

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