Use of ultrasound pulses combined with Definity for targeted blood-brain barrier disruption: a feasibility study

Abstract

We have developed a method to use low-intensity focused ultrasound pulses combined with an ultrasound contrast agent to produce temporary blood-brain barrier disruption (BBBD). This method could provide a means for the targeted delivery of drugs or imaging agents into the brain. In all our previous work, we used Optison as the ultrasound contrast agent. The purpose of this study was to test the feasibility of using the contrast agent Definity for BBBD. A total of 36 non-overlapping locations were sonicated through a craniotomy in experiments in the brains of nine rabbits (four locations per rabbit; ultrasound [US] frequency: 0.69 MHz; burst: 10 ms; pulse repetition frequency (PRF): 1 Hz; duration: 20 s). The peak negative pressure amplitude ranged from 0.2 to 1.5 MPa. An additional 11 locations were sonicated using Optison at pressure amplitude of 0.5 MPa. Definity and Optison dosages were the same as those used clinically for ultrasound imaging: 10 and 50 microl/kg, respectively. The probability for BBBD (determined using MRI contrast agent enhancement) as a function of pressure amplitude was similar to that found earlier with Optison. For both agents, the probability was estimated to be 50% at 0.4 MPa using probit regression. Histologic examination revealed small, isolated areas of extravasated erythrocytes in some locations. At 0.8 MPa and higher, these areas were sometimes accompanied by tiny (dimensions of 100 microm or less) regions of damaged brain parenchyma. The magnitude of the BBBD was larger with Optison than with Definity at 0.5 MPa (signal enhancement: 13.3% +/- 4.4% vs. 8.4% +/- 4.9%; p = 0.04). In addition, more areas with extravasated erythrocytes were observed with Optison (5.0 +/- 3.5 vs. 1.4 +/- 1.9 areas with extravasation in histology section with largest effect; p = 0.03). We concluded that BBBD is possible using Definity at the dosage of contrast agent and the acoustic parameters tested in this study. The probability for BBBD as a function of pressure amplitude and the type of acute tissue effects were similar to what has been observed using Optison. However, under the experimental conditions used in this study, Optison produced a larger effect for the same acoustic pressure amplitude.

Figures

Figure 1

MR image showing the signal intensity increase due to localized BBBD produced by focused ultrasound pulses combined with Definity® in the rabbit brain at four locations. The signal intensity increase was created from contrast-enhanced T1-weighted images normalized to a T1-weighted image acquired before baseline. The peak rarefactional pressure amplitude (estimated in brain) was 0.2, 1.1, 0.4, and 0.8 MPa for locations 1-4, respectively.

Figure 2

Probability for BBBD for the six pressure amplitudes tested for Definity® and Optison®. The Optison® data consists of the data from this study (at 0.50 MPa) and data from a previously published study ((Hynynen et al. 2005)).

Figure 3

a): Signal intensity enhancement – measured in contrast-enhanced T1-weighted images – for the six pressure amplitudes tested using Definity. The enhancement for Optison® after sonication at a pressure amplitude of 0.5 MPa is also shown. Mean values (± S.D.) of enhancement at all of the locations at each pressure amplitude are shown.

Figure 4

Microphotographs of H&E sections for four sonicated locations with BBBD produced with focused ultrasound combined with Definity® or Optison®. a) – b): histology score = 1; only three areas with extravasated erythrocytes were found in each of these examples. c): histology score = 2; twelve areas with extravasation were found in this example, as well as a vacuolated region (inset). d): histology score = 2; more than 20 regions with extravasations were found in this example, as well as damage (inset). In such cases with more than 20 areas with extravasated erythrocytes edema was evident as well in histology and in MRI. Bar: 100 μm.