The physiology of the vasculature in the central nervous system (CNS), which includes the blood-brain barrier (BBB) and other factors, complicates the delivery of most drugs to the brain. Different methods have been used to bypass the BBB, but they have limitations such as being invasive, non-targeted or requiring the formulation of new drugs. Focused ultrasound (FUS), when combined with circulating microbubbles, is a noninvasive method to locally and transiently disrupt the BBB at discrete targets. The method presents new opportunities for the use of drugs and for the study of the brain.
A transrectal MRgFUS system was tested in a canine prostate model. Focal volumes in each half of the prostate were
targeted, with high energy in one half of the gland for ablation and in the other with lower-energy sonications to test our
ability to localize the focal spot before causing thermal tissue damage. All sonications (n=155) were readily observed
with proton resonance frequency (PRF) MR temperature imaging, contrast enhanced MRI and histology. The prostate
gland moved during the experiments, demonstrating the need for motion tracking. The resultant focal temperature
changes during the experiments were 24.2 ± 8.2°C.
KEYWORDS: Tissues, Phased arrays, Acoustics, Ultrasonics, Magnetic resonance imaging, Staring arrays, In vivo imaging, Thermal modeling, Temperature metrology, Ultrasonography
A 256 element ultrasonic phased array was constructed from thermal treatment of deep seated tissue. The 1.1 MHz array had a 10 cm radius of curvature and a 12 cm diameter. The elements formed a planar projection grid of 0.65 X 0.65 cm2 such that the focal range of the array was approximately +/- 1 cm from the natural focus of the array both in the focal plane and +/- 2 cm along the array axis. The array was driven with phased continuous wave signals to both shift individual foci and to create multiple focus patterns. The goals of this study were to demonstrate that an array with many elements has the ability to coagulate large volumes of deep seated tissue in a single sonication and to experimentally compare the in vivo thermal measurements of large focal volume sonications to those predicted in a simple simulation model. It was found that the array could coagulate thigh muscle volumes of 3 - 5 cm3 in a twenty second sonication.
Magnetic resonance (MR) imaging was used to guide and monitor the thermal tissue coagulation of in vivo porcine tissue using a 256 element ultrasonic phased array. The array could coagulate tissue volumes greater than 2 cm3 in liver and 0.5 cm3 in kidney using a single 20 second sonication. This sonication used multiple focus fields which were temporally cycled to heat large tissue volumes simultaneously. Estimates of the coagulated tissue using a thermal dose threshold compare well with T2-weighted images of post sonication lesions. The overlapping large focal volumes could aid in the treatment of large tumor volumes which require multiple overlapping sonications. The ability of MR to detect the presence and undesirable thermal increases at acoustic obstacle such as cartilaginous and bony ribs is demonstrated. This could have a significant impact on the ability to monitor thermal treatments of the liver and other organs which are acoustically blocked.
Practical high power phased arrays have been developed and tested in our laboratory. The results show that they offer significant advantages over conventional single focused ultrasound transducers for noninvasive ultrasound surgery. These advantages include but are not limited to: compensation of phase distortion induced by overlying tissues, potential for inducing optimal energy delivery patterns, and generation of large focal spots for tumor coagulation. This paper will describe some of our results with the therapy phased array.
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