Inadequate coupling (which may result from air bubbles, fecal residues etc.) between the balloon and the
rectal wall may either reflect or absorb part of the ultrasound energy, as well as create thermal imaging
artifacts.
1. Carefully review the Bubble Detection images (either directly on the Exablate Workstation or on the
MR) and ensure no significant air bubbles are found in the interface between the endorectal balloon
and the rectal wall.
The Bubble Detection images are suitable for this objective as air bubbles in the balloon-
rectal wall interface cause an enlarged artifact and are thus easy to identify. See Figure
6-3 for an example.
[N-15]
2. In case significant air bubbles are detected, attempt the following techniques (noted here in an
escalating order):
◼ Without draining the balloon or extracting the probe, rotate the motion unit from side to side,
attempting to sweep the air bubbles outside of the acoustic window.
◼ Without draining the balloon or extracting the probe, cover your finger with ultra-sound gel, insert
the finger into the gap between the balloon and the rectal wall and sweep your finger in an attempt
to replace the air with fresh gel. Pay attention not to scratch or puncture the balloon in the process.
◼ Fill the balloon with another 30 CC of water, attempting to push the air bubbles into regions of lower
pressure, outside of the acoustic window.
◼ As a last resort, drain the probe back to reference (insertion) volume, completely extract the probe,
clean it, apply new US gel and re-insert the probe. Fill the balloon with 60cc of water.
After re-positioning the probe (also for the purpose of removing air bubbles), re-start the
Positioning verification procedure (see Section 6.4).
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