Gosh, you're both making this harder than it needs to be. Just use a drill. Drill all the way to the target point, push the transmitter to the end of the borehole, and fill the rest with tofu. This way you minimize acoustic artifacts not just from the test equipment, but also from shattered skull bone fragments. This would also let you take a continuous linear single-axis scan - just keep the transmitter running as you pull it out slowly, letting tofu flow around it.
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Gosh, you're both making this harder than it needs to be. Just use a drill. Drill all the way to the target point, push the transmitter to the end of the borehole, and fill the rest with tofu. This way you minimize acoustic artifacts not just from the test equipment, but also from shattered skull bone fragments. This would also let you take a continuous linear single-axis scan - just keep the transmitter running as you pull it out slowly, letting tofu flow around it.