Updates on Adult Transcranial Doppler, Gray-Scale, and Contrast-enhanced US Techniques

Transcranial US is a noninvasive, portable, real-time imaging technique that enables targeted evaluation of the intracranial circulation and select brain parenchymal structures through cranial bone windows. Traditionally, transcranial Doppler (TCD) has been used in well-established clinical scenarios, including vasospasm monitoring, stroke risk assessment, and ancillary testing for brain death. TCD encompasses two main techniques: nonimaging TCD (niTCD) and TCD imaging (TCDi). niTCD is a "blind" US technique that relies on standardized depths for vessel localization, guided by characteristic flow waveforms. In contrast, TCDi is a triplex approach that integrates gray-scale (B-mode) US, color Doppler US, and spectral Doppler US into an image-guided examination, enabling direct visualization of the intracranial anatomy and improving diagnostic confidence and reproducibility. In parallel, transcranial B-mode sonography (TCS), incorporated into modern TCDi, has demonstrated promising applications in adult patients, both in the neurointensive care unit and in outpatient practice. In addition, contrast-enhanced US (CEUS) has emerged as a powerful adjunct, markedly enhancing the Doppler signal in patients with poor bone windows, and has expanded the scope of transcranial US toward new exploratory vascular and cerebral perfusion applications. This article provides a comprehensive update on transcranial US in adults, including gray-scale imaging, Doppler techniques, and CEUS applications. With a focus on evidence-based indications and practical techniques, this article aims to support broader awareness and clinical adoption of transcranial US in contemporary adult neuroimaging, with a particular emphasis on TCDi, which remains underrecognized and underused despite its demonstrated clinical value in select indications. © The Author(s) 2026. Published by the Radiological Society of North America under a CC BY 4.0 license.