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Opening BBB with MR-focused ultrasound affected by MR magnetic fields

John R. Fischer, Senior Reporter | July 23, 2021
MRI Ultrasound
The strength of an MR scanner's magnetic field can affect the size of which MR-focused ultrasound can open the brain blood barrier
Static magnetic fields generated by MR scanners can decrease the opening of a blood-brain barrier when applying MR-focused ultrasound.

The discovery was made by researchers at Washington University in St. Louis and suggests that MR-guided focused-ultrasound may affect treatment outcomes for such procedures and other cavitation-mediated techniques.

“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,” said Hong Chen, associate professor of biomedical engineering in the McKelvey School of Engineering and of radiation oncology in the School of Medicine, in a statement.

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Chen and her colleagues found that depending on the strength of the magnetic field, the BBB opening volume could drop by 3.3-fold to 11.7-fold in a mouse model. Injecting 30 mice with microbubbles, they performed focused-ultrasound sonication at 1.5T, 3T and 4.7T on three different groups. A fourth group never entered the magnetic field.

They found that the activity of microbubble cavitation — the expansion, contraction and collapse of the microbubbles — decreased by 2.1 decibels at 1.5T; 2.9 decibels at 3T; and 3 decibels at 4.7T, compared to those who received the dose outside of the magnetic field. The magnetic field decreased the BBB opening volume by 3.3-fold at 1.5T; 4.4-fold at 3T; and 11.7-fold at 4.7T. No mouse incurred any tissue damage.

The team then injected a model drug, Evans blue, to see if the static magnetic field affected trans-BBB drug delivery. The images produced showed that the fluorescence intensity of the Evans blue was lower in mice that received treatment in one of the three strengths of magnetic fields compared to those outside of them. The Evans blue trans-BBB delivery fell by 1.4-fold at 1.5T; 1.6-fold at 3T; and 1.9-fold at 4.7T.

Cavitation is the fundamental physical mechanism in brain drug delivery and several other therapeutic techniques, including histotripsy and sonothrombolysis.

The findings were published in Radiology.

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