Blood-brain barrier disruption induced by focused ultrasound and circulating preformed microbubbles appears to be characterized by the mechanical index

Abstract

This work investigated the effect of ultrasonic frequency on the threshold for blood-brain barrier (BBB) disruption induced by ultrasound pulses combined with an ultrasound contrast agent. Experiments were performed in rabbits using pulsed sonications at 2.04 MHz with peak pressure amplitudes ranging from 0.3 to 2.3 MPa. BBB disruption was evaluated using contrast-enhanced magnetic resonance imaging. The threshold for BBB disruption was estimated using probit regression. Representative samples with similar amounts of contrast enhancement were examined in light microscopy. Results from these experiments were compared with data from previous studies that used ultrasound frequencies between 0.26 and 1.63 MHz. We found that the BBB disruption threshold (value where the probability for disruption was estimated to be 50%) expressed in terms of the peak negative pressure amplitude increased as a function of the frequency. It appeared to be constant, however, when the exposures were expressed as a function of the mechanical index (peak negative pressure amplitude estimated in situ divided by square root of frequency). Regression of data from all frequencies resulted in an estimated mechanical index threshold of 0.46 (95% confidence intervals: 0.42 to 0.50). Histologic examination of representative samples with similar amounts of blood-brain barrier disruption found that the number of regions containing extravasated red blood cells per unit area was substantially lower on average for lower ultrasound frequencies. This data suggests that the mechanical index is a meaningful metric for ultrasound-induced blood-brain barrier disruption, at least for when other parameters that are not taken into account by the mechanical index are not varied. It also suggests that lower frequency sonication produces less red blood cell extravasation per unit area.

Figure 1

Diagram of the experimental setup.

Figure 2

A: Signal enhancement in contrast-enhanced T1-weighted MR images as a function of peak negative pressure amplitude for sonications applied at an ultrasound frequency of 2.04 MHz. B: Probability for BBB disruption (BBBD) as a function of peak negative pressure amplitude. The solid line indicates the probit regression of this data; dotted lines indicate 95% confidence intervals.

Figure 3

Examples of contrast-enhanced T1-weighted MR images of rabbit brains showing targeted BBB disruption, indicated by focal contrast enhancement, for the different frequencies tested in these studies. Four sonications were targeted in each brain. Note that two locations sonicated at 0.26 MHz did not result in BBB disruption in these examples.

Figure 4

A: BBB disruption (BBBD) threshold as a function of ultrasound frequency. B: BBB disruption threshold as a function of the mechanical index (MI), which appeared to be the same for the different frequencies tested. Solid line: regression of data to eq. (1); dotted lines: 95% confidence intervals.

Figure 5

Typical tissue effects as seen in histology. Histological changes were limited generally to minor vascular damage, as indicated by the presence of individual extravasated red blood cells or, as shown here, clumps of extravasated erythrocytes. (B: High magnification view of rectangle in A; Bar: 100 μm)

Figure 6

A: Number of regions containing extravasations as a function of ultrasound frequency. B: The density of extravasations vs. frequency. This density was defined as the number of areas with extravasation divided by πr², where r is the half width of beam plots measured in water with a needle hydrophone. Lower ultrasound frequencies had fewer cells per unit area on average, but substantial variation was present. C: MRI contrast enhancement. Mean ± standard deviation shown.