Postoperative change in neurological function may herald the onset of hydrocephalus, be indicative of an intracranial hemorrhage or be due to ischemic stroke or seizure. Specifically, the presence or absence of midline shift, intracranial hematoma, ventricular size and visible and stable catheter tip position in implant cases provides essential insights into most acute neurosurgical complications[15–18 ].
Post-operative cranial imaging—specifically CT scan—are the cornerstone of decision-making protocols guiding treatment in the post-operative setting. However, in many scenarios there is little need for detailed total brain imaging and the ability to rule out major reversable complications such as acute hydrocephalus, hemorrhage or mass effect can be either confirmed or ameliorated by specific imaging features. Furthermore, while CT scan provides a comprehensive view of the head and is still very much essential for the evaluation of pre- and post-operative neurosurgical patients, the threshold for obtaining a CT scan, especially in the acute stage post-surgery in the ICU, is controversial, and subjective [(Gunnarsson and Hillman 2000; Wen et al. 2016),(Karanci and Oktay 2021; El Khoury et al. 2000),(Zimmermann et al. 2016),(Benveniste, Ferraro, and Tsimpas 2014)]. As a result, it may be over-used. Obtaining a head CT also consumes time while subjecting patients to unnecessary transport, radiation, and other possible complications [(D., S., and A. 2010; Sheppard et al. 2020)].
Assuming skull penetrance can be achieved, ultrasound-based imaging modalities can identify each of these potentially reversible post-operative events and contribute to post-operative patient management. This is exemplified in neonates who have open fontanelles that serve as natural acoustic windows for sonication [(Franco and Lewis 2013)]. However, the use of ultrasound as a diagnostic tool has been otherwise limited as ultrasound doesn’t travel well through air or bone due to ultrasonic wave attenuation, scattering and absorption [(Estrada et al. 2018; Robertson et al. 2018),(Pinton et al. 2012)].
Previous pre-clinical studies [(Belzberg, Shalom, Yuhanna, et al. 2019)] explored the sonolucency of native cranial bone compared to synthetic implants such as clear PMMA, PEEK, porous-polyethylene, and opaque PMMA using a 2- to 4-MHz US transducer. While cranial bone and porous-polyethylene were not sonolucent, clear PMMA, PEEK, and opaque PMMA cranial implants were found to be sonolucent, as imaging through each material displayed different tissue echogenicities.
Recently, cranial customized implants (CCIs) fabricated with translucent/clear PMMA became approved by the Food and Drug Administration within the United States, with the inherent benefits of visible transparency and sonolucent transmission via trans cranial ultrasound (ClearFit® Longeviti Neuro Solutions, Hunt Valley, MD, USA) [(Belzberg, Shalom, Lu, et al. 2019)].
In most neurosurgical procedures, an opportunity exists for the operator to create a synthetic acoustic window by replacing normal bone, or covering a missing drilled bone, with a cranial implant composed of sonolucent biomaterial, thus facilitating trans cranial ultrasound use. As implants can be customized, and can be as small as a burr-hole cover, the introduction of an ultrasonic-based imaging modalities into the neurosurgical arena may be realized beyond cranioplasties.
In this study we sought to characterize a number of post-operative settings where urgent ultrasound based imaging through novel sonolucent implants could be performed with diagnostic accuracy and clinical ease. We evaluated the clinical utility of bedside ultrasound coupled with sonolucent implants in a variety of representative pathologies encountered in our department.
As an imaging modality, ultrasound has been historically underused in neurosurgery [(Harary et al. 2018)]. As a result, it is not a tool routinely used during training and most aren’t comfortable interpreting or performing brain US scans as compared with MRI and CT [(Giussani et al. 2017; Müns et al. 2014)]. While bedside ultrasound was useful in the above series to rapidly and reliably rule out major post-operative complications, the potential introduction of ultrasound as a viable and readily available option in neurosurgical patients in the acute post-operative state is dependent on use of a sonolucent implant.
Additional challenges, such as scalp closure, drain placement, subgleal swelling and irrigation of blood and air underneath a sonolucent flap, all impact the image quality obtained by a bed-side US and should be taken account during interpretation.
Clinical judgment should still dictate treatment decisions and CT scan timing, but clinicians should be encouraged to use US handheld probe at the bedside as part of a routine neuro-focused exam. While concern over fresh incisions remains real, care should be used to place the sonolucent plate under uninterrupted skin, as well as to use sterile lubrication and gentle touch to avoid infections or pain.
Lastly, ultrasounds come in a variety of makes and models with various features. The US probe definitions that were used in this study were either abdominal or vascular for doppler purposes. This, however, may not be ideal, as specific brain tissue characteristics may require a more sub-specialized probe setting with specific settings for the cranium and the brain [(Selbekk et al. 2013)]. Future studies should help characterize the combination of flap/ultrasound/brain tissue in both normal and pathological states such as hydrocephalus or brain edema, in order to define best acoustic properties and implant shape.