In Reply: First Experience With Postoperative Transcranial Ultrasound Through Sonolucent Burr Hole Covers in Adult Hydrocephalus Patients.

To the Editor: We are grateful to Rossitto et al1 for the positive outlook on our work and for continuing the dialogue on this topic.2 We look forward to the results of their clinical trial evaluating trans-burr hole ultrasound after intraparenchymal hemorrhage evacuation.3 While evaluating and progressing any new idea or technique, it is certainly helpful to understand its history and evolution. Rossitto et al provided a useful review and literature catalog of transcranioplasty ultrasound as it has been trialed and reported to date. Of note, several different cranioplasty materials allow ultrasonography beyond the clear polymethyl methacrylate discussed in our article. Much work is ahead to optimize which material is optimal. It may be that different materials are best suited for different scenarios. In our article, we focus on B-mode gray-scale imaging of brain and ventricle anatomy. This application of transcranial ultrasound is perhaps most ripe for a paradigm shift in imaging, particularly as a point-of-care application. However, other ultrasound techniques under investigation through cranioplasty material include Doppler, color duplex, and therapeutic. Of particular interest is the role of sonolucent cranioplasty in allowing a window for focused ultrasound, which is being studied for both parenchymal lesioning and blood-brain barrier disruption. Also understudied are the various effects and contributions of implant thickness, implant shape, and underlying/overlying biologic and nonbiologic tissues and materials. Although many initial series of transcranioplasty ultrasound are reported, this field is still in its infancy. We would also like to emphasize that excitement in postneonatal transcranial ultrasound should not be limited to transcranioplasty applications. Point-of-care imaging through supratentorial craniectomy defects (without cranioplasty) has been validated in the literature and can produce excellent images.4,5 The limitation is that adoption has not been widespread. We anticipate that decreases in ultrasound cost, increases in quality, further familiarization with ultrasound, and conversations like this will all promote exploration and utilization of this technique. This is especially relevant for intensive care unit-level patients where hospital transports to radiology can be resource-intensive and risky and when the rapid information from point-of-care ultrasound can be critical. This applies to decompressive craniectomy not only of the supratentorial space but also of the posterior fossa. In that same vein, our group is very interested in applications for transcranial ultrasound to evaluate posterior fossa and craniocervical junction anatomy after foramen magnum decompression in Chiari malformation. There may be instances when ultrasound is comparable with computed tomography (CT) or MRI to monitor for postoperative complications and assess for adequate decompression. Beyond that, ultrasound may be the key to reveal the cause for continued symptoms after Chiari decompression as it is much easier with ultrasound to obtain imaging in different head positions and body postures than traditional CT or MRI. And further, can ultrasound reveal dynamic information about pulsatility and cerebrospinal fluid flow? For these scenarios, ultrasound can be performed through a sonolucent cranioplasty or through a nonreconstructed defect. We look forward to publishing our initial experience with this technique soon. Overall, we are optimistic about the potential of transcranioplasty ultrasound in neurosurgery but are proceeding thoughtfully to ensure that it is thoroughly and rigorously evaluated for strengths and limitations compared with current standard of care. And finally, we find this to be a salient topic in a time of reckoning with health care costs as ultrasound can provide a less expensive alternative to CT and MRI, which we hope to evaluate with cost analysis as experience accrues.