The field of neurosonography has evolved in the past few decades facilitating detailed anatomical evaluation of the central nervous system (CNS) of the unborn fetus (fetal neurosonography)^1–2 and newborn child (neonatal neurosonography).³ During these early stages of life, neurosonography is possible due to the presence of physiologic acoustic windows such as the fontanelle and unfused sutures of the skull, allowing the ultrasound waves to penetrate the brain tissue. Both 2D and 3D ultrasound have been used to obtain the required images of neurosonography.⁴ This technique enables a high-resolution, safe, readily available, and relatively inexpensive modality to obtain a detailed evaluation of the intra-cranial anatomy and vascular system that may be comparable in quality to that of other imaging modalities such as MRI.⁵ As the fontanelle and cranial sutures close during early childhood and the skull bone thickens, ultrasound can no longer be used to generate images of the brain. Therefore, adult neurosonography has significant limitations with use limited to transcranial Doppler imaging.

Recently, full or partial replacements of skull composed of an ultrasound-penetrable synthetic substitute, ie sonolucent cranial implants, have been available for post craniotomy skull replacement. Early experience by neurosurgeons that elect to use this technique offer their patients the potential of postoperative bedside sonographic assessment of the CNS of these individuals.⁶

In this report, we share our collaborative experience and enthusiasm about the emerging field of post cranioplasty neurosonography. Craniotomy is one of the most common surgical procedures performed in the US with indications ranging from surgical evacuation of intracranial bleed, hydrocephalus, brain tumors, and vascular lesions. Sonolucent cranial implants may replace a portion of the native skull or be used as a fully customized implant to restore form and function for cranial reconstruction procedures. In addition to the different sizes and shapes, these next-generation implants allow for postoperative imaging with ultrasound.

The sonolucent implant provides an acoustic window via a synthetic adult “fontanelle”. Depending on the location and size of the device, the assessment of the brain may be facilitated in different planes and points of view. Most commonly, the implants are located above the pathology (figure 1) and off the skull midline with the sagittal sinus preserved. Thus, the acoustic window enables the ability to easily image the brain in the coronal (Figure 2) and axial (Figure 3) planes. In cases where the implant approaches or covers the midline, evaluation in the sagittal plane (paramedian and even midsagittal/median) is possible (Figure 4) in addition to that of the coronal plane. Moreover, the closer the implant is to the midline, the easier the access is to image both hemispheres.

Post cranioplasty neurosonography can be done with point-of-care ultrasound by the neurosurgical or neurology teams in the acute postoperative period, in-office surveillance visit, or as a detailed evaluation in a neuro-radiology unit set up. We have used this technique in these setups to assess several postoperative parameters such as evaluation for possible midline shift, lateral ventricles for size, shape, potential bleed, and location of a shunt or ventricular catheter, patency of vascular anastomosis, as well as the evaluation of the brain parenchyma for postoperative pathologies such as presence of tumor or intracranial hemorrhage. Sequential surveillance is facilitated by the fact that the acoustic window is fixed so images are easily obtained in the exact same anatomical plane on subsequent scans.

As the clinical utilization of sonolucent grafts and experience with post cranioplasty neurosonography expand, there is much to be determined on how to best incorporate this emerging technology into patient care. For example, identifying the ideal diagnostic probe as there is no designated probe currently in the market. We have used both a curvilinear probe with a high-resolution abdominal setting and a cardiac phased array probe, ie, small footprint probes, with success. Indeed, the probe characteristics may vary between different lesions requiring different levels of penetration. As the implant is hard, and in many cases convex and of small size, a wide-sector, small footprint, high-resolution probe may offer the best access.

Future utilization and study will determine how post cranioplasty neurosonography influences the utilization of other imaging modalities (CT and MRI). Additional benefits potentially include decreased exposure to radiation, better point-of-care access to imaging, and a direct impact on healthcare costs.

Lastly, it is still to be determined which clinicians (neurosurgeons, neurologists, radiologists, neuro-intensivists, ACPs), and in which setup (point of care vs radiology suite) will master the techniques and take the lead in this emerging field. Regardless, it appears that the ultrasound community has a new and exciting opportunity at hand. 

References:

1. Malinger G, Paladini D, Haratz KK, Monteagudo A, Pilu G, Timor-Tritsch IE. ISUOG Practice Guidelines (updated): sonographic examination of the fetal central nervous system. Part 1: performance of screening examination and indications for targeted neurosonography. Ultrasound Obstet Gynecol 2020; 56:476–484.
2. Paladini D, Malinger G, Birnbaum R, et al. ISUOG Practice Guidelines (updated): sonographic examination of the fetal central nervous system. Part 2: performance of targeted neurosonography. Ultrasound Obstet Gynecol 2021; 57:661–671. https://doi.org/10.1002/uog.23616.
3. Rossi A, Argyropoulou M, Zlatareva D, et al; ESNR Pediatric Neuroradiology Subspecialty Committee; ESPR Neuroradiology Taskforce. European recommendations on practices in pediatric neuroradiology: consensus document from the European Society of Neuroradiology (ESNR), European Society of Paediatric Radiology (ESPR) and European Union of Medical Specialists Division of Neuroradiology (UEMS). Pediatr Radiol 2023; 53:159–168. doi: 10.1007/s00247-022-05479-4.
4. Bornstein E, Monteagudo A, Santos R, et al. Basic as well as detailed neurosonograms can be performed by offline analysis of three-dimensional fetal brain volumes. Ultrasound Obstet Gynecol 2010 Jul; 36(1):20–25. doi: 10.1002/uog.7527.
5. Malinger G, Paladini D, Pilu G, Timor-Tritsch IE. Fetal cerebral magnetic resonance imaging, neurosonography and the brave new world of fetal medicine. Ultrasound Obstet Gynecol 2017; 50:679–680.
6. Williams AL, Abu-Bonsrah N, Lee RP, et al. Letter: The role of sonolucent implants in global neurosurgery. Neurosurgery 2024; 94:e1–e5. doi: 10.1227/neu.0000000000002723.