Original contributionEffects of Acoustic Parameters and Ultrasound Contrast Agent Dose on Focused-Ultrasound Induced Blood-Brain Barrier Disruption
Introduction
Recently, several papers have described a method to temporarily disrupt the blood-brain barrier (BBB) using ultrasound pulses combined with a circulating ultrasound contrast agent (Hynynen et al 2001, Hynynen et al 2005, Hynynen et al 2006, Yang et al 2007, Sheikov et al 2004, McDannold et al 2005, McDannold et al 2006, McDannold et al 2007, Kinoshita et al 2006, Raymond et al 2006, Treat et al 2007, Choi et al 2007). Since these pulses can be applied at an ultrasound frequency that can readily be applied transcranially by a transducer located outside the body (Hynynen et al 2005, Hynynen et al 2006), the method represents a potentially noninvasive technique to produce targeted BBB disruption. This ability could facilitate the targeted delivery of drugs to the central nervous system.
The BBB is a functional and structural barrier in the vasculature of the central nervous system that limits or excludes the use of most therapeutic and imaging agents (Abbott and Romero 1996). It makes the development of new drugs difficult and prevents the use of many drugs that are effective in other parts of the body. Several strategies have been proposed or tested to circumvent the BBB, including creating drugs or drug carriers that can penetrate the barrier (Pardridge 2002), directly infusing agents via a catheter implanted in the brain tissue (Bobo et al. 1994), implanting devices that slowly release drugs to the brain tissue after surgery (Guerin et al. 2004) and introducing a catheter into the brain vasculature and infusing a solution such as mannitol to produce diffuse disruption of the BBB downstream (Doolittle et al 2000, Neuwelt et al 1979). Focused ultrasound offers several advantages over these methods: it is noninvasive, targeted and does not necessarily require the development of new drugs. By steering the beam to multiple locations, one could potentially conform the area where the BBB is disrupted only in a desired target volume, thereby avoiding dose-limiting side effects. Alternatively, one may be able to steer the beam to cover the entire brain, if desired.
The mechanism by which ultrasound causes BBB disruption is currently unknown. Presumably, it is related to the interaction between the ultrasound beam, the preformed microbubbles that make up the ultrasound contrast agent and the vessel walls. Previous studies have indicated that it is at least partially a physiological response in contrast to simply a direct physical modification of the endothelial cells or the tight junctions between them (Sheikov et al 2004, Raymond et al 2006). Other works have suggested that the mechanism is not inertial cavitation (McDannold et al. 2006) and that the exposure levels are substantially below the threshold for bulk heating-induced effects (Hynynen et al. 2001).
To further understand the mechanisms for BBB disruption and to optimize the procedure, the present study was performed to investigate the effects of different ultrasound parameters. We investigated the effects of ultrasound frequency, burst length, pulse repetition frequency and dosage of contrast agent on the threshold and magnitude of the BBB disruption and the associated tissue effects.
Section snippets
Animals
The experiments were approved by our institutional animal committee. Sonications were targeted at two nonoverlapping locations in each hemisphere of the brains of male New Zealand white rabbits (weight: approximately 4 kg). The targets were one cm deep in the thalamus and approximately 3 mm lateral to the midline. The animals were anesthetized using i.m. injections of a mixture of 12 mg of sodium xylazine (Xyla-ject; Phoenix Pharmaceuticals, St Joseph, MO, USA) and 48 mg of ketamine
General MRI and histology observations
BBB disruption was observed in contrast-enhanced MRI as localized regions of signal enhancement at the sonication target (Fig. 1). As multiple images were acquired after contrast injection, the magnitude of the enhancement at the focal coordinate increased initially and was then constant after a few minutes over the time examined.
Histologic changes were small and generally limited to minor vascular damage, as indicated by extravasated erythrocytes that were found in the sonicated regions. These
Discussion
Of the parameters tested, only the burst length had a significant effect on the magnitude of the BBB disruption as determined by the percent increase in signal enhancement measured in contrast-enhanced T1-weighted MRI. Thus, for the range of parameters tested, this work suggests that the magnitude of BBB disruption is insensitive to the number of pulses delivered and the number of microbubbles present in the vasculature. The data also suggests that these parameters as well the burst length do
Conclusions
This data suggests that for the range of parameters tested, the magnitude of the BBB disruption induced by ultrasound pulses does not depend on the pulse repetition frequency or the dosage of ultrasound contrast agent. However, the magnitude of the disruption decreases and the threshold for BBB disruption increases as the burst length is shortened. The data also suggest that none of the parameters that were varied will produce different histologic changes in the brain, at least for sonications
Acknowledgments
This study was supported by NIH (R01EB003268, R33EB000705 and U41RR019703). The authors thank Yongzhi Zhang for his help with these experiments.
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