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. 2010 Jan;57(1):145-54.
doi: 10.1109/TBME.2009.2034533. Epub 2009 Oct 20.

Microbubble-size Dependence of Focused Ultrasound-Induced Blood-Brain Barrier Opening in Mice in Vivo

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Free PMC article

Microbubble-size Dependence of Focused Ultrasound-Induced Blood-Brain Barrier Opening in Mice in Vivo

James J Choi et al. IEEE Trans Biomed Eng. .
Free PMC article

Abstract

The therapeutic efficacy of neurological agents is severely limited, because large compounds do not cross the blood-brain barrier (BBB). Focused ultrasound (FUS) sonication in the presence of microbubbles has been shown to temporarily open the BBB, allowing systemically administered agents into the brain. Until now, polydispersed microbubbles (1-10 microm in diameter) were used, and, therefore, the bubble sizes better suited for inducing the opening remain unknown. Here, the FUS-induced BBB opening dependence on microbubble size is investigated. Bubbles at 1-2 and 4-5 microm in diameter were separately size-isolated using differential centrifugation before being systemically administered in mice (n = 28). The BBB opening pressure threshold was identified by varying the peak-rarefactional pressure amplitude. BBB opening was determined by fluorescence enhancement due to systemically administered, fluorescent-tagged, 3-kDa dextran. The identified threshold fell between 0.30 and 0.46 MPa in the case of 1-2 microm bubbles and between 0.15 and 0.30 MPa in the 4-5 microm case. At every pressure studied, the fluorescence was greater with the 4-5 mum than with the 1-2 microm bubbles. At 0.61 MPa, in the 1-2 microm bubble case, the fluorescence amount and area were greater in the thalamus than in the hippocampus. In conclusion, it was determined that the FUS-induced BBB opening was dependent on both the size distribution in the injected microbubble volume and the brain region targeted.

Figures

Fig. 1
Fig. 1
In vivo FUS-induced BBB opening experimental setup.
Fig. 2
Fig. 2
Acoustic waves propagated through (a) left parietal bone and converged to a 1.32-mm diameter region (dotted circle). (b) Left hippocampus was targeted while the right region acted as a control.
Fig. 3
Fig. 3
Size distributions of microbubbles are depicted as (a) number-weighted percent of the total concentration of bubbles and (b) volume-weighted percent of the total volume of bubbles. Distinct peaks at approximately 1–2 μm (solid black) and 4–5 μm (solid blue) are visible. After sonication, the microbubbles were reanalyzed (dashed), depicting little deviation of its mean or median diameters.
Fig. 4
Fig. 4
Fluorescence images of the (a, c, and e) left and (b, d, and f) right brain regions. Delivery of 3 kDa dextran by sonication of the left region in the presence of 1–2 μm bubbles resulted in a fluorescence increase at (a) 0.61 and (c) 0.46 MPa, but not (e) 0.30 MPa.
Fig. 5
Fig. 5
Fluorescence images of the (a, c, e, and g) left and (b, d, f, and h) right brain regions. Delivery of 3 kDa dextran by sonication of the left region in the presence of 4–5 μm bubbles resulted in a fluorescence increase at (a) 0.61 (c) 0.46, and (e) 0.30 MPa, but not (g) 0.15 MPa.
Fig. 6
Fig. 6
Fluorescence increase of the left (a, b) hippocampi and (c, d) thalami relative to the right corresponding region at different acoustic pressures and microbubbles. With (a, c) 1–2 μm bubbles, no significant BBB opening was observed at 0.30 or 0.46 MPa while opening was observed at 0.61 MPa in both regions. With (b, d) 4–5 μm bubbles, no significant BBB opening was observed at 0.15 MPa while opening was observed at 0.30 and 0.46 MPa in both regions. A significant increase in fluorescence was also observed in the hippocampus at 0.46 MPa over 0.30 MPa.
Fig. 7
Fig. 7
Increase in average fluorescence of the left ROI relative to the right ROI due to sonication at 0.61 MPa after the intravenous injection of 1–2 μm bubbles. A significantly greater fluorescence was observed in the thalamus when compared with the hippocampus.
Fig. 8
Fig. 8
Paraffin-embedded 6-μm thick brain sections after sonicating the (a, c, and e) left ROI at 0.46 MPa in the presence of 1–2 μm bubbles. The (b, d, and f) right was the control. Increased fluorescence in the (a) left ROI indicates delivery of 3 kDa dextran. Hematoxylin and eosin sections reveal no macroscopic damage. White arrows indicate erythrocytes, but may be artifacts due to histological techniques since they were observed on both hemispheres. The black boxes in (c) and (d) refer to the ROIs of (e) and (f).
Fig. 9
Fig. 9
Paraffin-embedded 6-μm thick brain sections after sonicating the (a, c, and e) left ROI at 0.3 MPa in the presence of 4–5 μm bubbles. (b, d, and f) Right ROI was the control. Increased fluorescence in the (a) left ROI indicates delivery of 3 kDa dextran. Hematoxylin and eosin sections reveal no macroscopic damage. White arrows indicate erythrocytes, but may be artifacts due to histological techniques since they were observed on both hemispheres. The black boxes in (c) and (d) refer to the ROIs of (e) and (f).

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