What are the indications, benefits, and limitations of intra‑operative ultrasound in neurosurgical procedures?

Intraoperative Ultrasound in Neurosurgery

Intraoperative ultrasound (ioUS) should be routinely integrated into neurosurgical practice as a real-time navigation tool for tumor localization, resection guidance, and assessment of extent of resection, particularly when combined with neuronavigation systems to compensate for brain shift. 1

Primary Indications for Intraoperative Ultrasound

Tumor Localization and Surgical Planning

• ioUS provides precise localization of deep-seated lesions and enables safe transcortical trajectories to intraparenchymal pathology including gliomas, metastases, hemangiomas, and intracerebral hemorrhages. 2
• Small subcortically located lesions are clearly visualized using high-frequency probes (10 MHz), while deeper structures require 5 MHz frequency for adequate penetration and delineation from surrounding anatomical structures. 3
• The technology successfully identifies all major intracranial pathologies with near-universal visibility—in one series of 162 patients, all pathologies except one aneurysm were visible on ioUS. 2

Real-Time Navigation During Glioma Resection

• When brain shift occurs during hemispheric tumor resection, navigated three-dimensional ultrasound serves to adjust intraoperative neuronavigation and define sampling localization during later stages of resection. 4
• Navigated ioUS provides point-of-care imaging that restores stereotactic accuracy after brain shift, with a median sweep time of 5.5 seconds and image processing time of 29.9 seconds. 1
• The sampling position should be electronically documented with the intraoperative neuronavigation system, and samples should be obtained early during resection before significant anatomical distortion occurs. 4

Assessment of Extent of Resection

• ioUS demonstrates 95% concordance with postoperative MRI for determining extent of resection, with 100% sensitivity and 94% specificity. 1
• The technology accurately checks extent of resection in over 80% of cases across various pathologies (gliomas, metastases, meningiomas, schwannomas). 2
• Surgical artifacts reduce sonographic visibility in approximately 15% of cases, representing a key limitation during the final stages of resection. 2

Tissue Characterization Capabilities

Structural Differentiation

• ioUS differentiates solid tumor components from cystic areas and necrosis, providing more structural detail within tissue compartments than CT or MRI. 3, 5
• Extra-axial tumors demonstrate superior echogenic visibility compared to intra-axial tumors: 83% of extra-axial lesions show grade 3 visibility (clear borders) versus only 48% of intra-axial lesions. 2
• The demarcation of infiltrative gliomas is less well-defined on ioUS compared to preoperative MRI, which accurately reflects the intraoperative reality of indistinct tumor margins. 5

Advanced Multiparametric Techniques

• Contrast-enhanced ultrasound (CEUS), ultrasensitive Doppler, and elastosonography provide functional characterization of lesions beyond standard B-mode imaging. 6
• These advanced modalities enable analysis of vascularization patterns and tissue stiffness in a qualitative and quantitative manner, particularly valuable for distinguishing low-grade gliomas, high-grade gliomas, meningiomas, and metastases. 6
• Dural sinuses and tumor invasion can be visualized using 10 MHz probes. 3

Specific Clinical Applications

Glioma Surgery

• For diffuse gliomas requiring tissue sampling from multiple tumor portions, ioUS guides identification of the tumor core, contrast-enhancing regions, and non-contrast-enhancing tumor portions visible on preoperative T2/FLAIR sequences. 7
• At least two samples per MRI abnormality should be obtained from opposing regions as distant from each other as possible, with ioUS facilitating this spatial sampling strategy. 4, 7
• High-resolution preoperative MRI (1×1×1 mm³) integrated with ioUS navigation enables accurate correlation between imaging findings and intraoperative anatomy. 7

Metastasis and Meningioma Resection

• ioUS successfully guides resection of brain metastases and meningiomas, with particularly clear visualization of extra-axial lesions. 6, 2
• The technology is effective across the full spectrum of neuro-oncological pathology encountered in routine practice. 6

Minimally Invasive Approaches

• ioUS guidance facilitates puncturing with catheters, needles, or endoscopes for minimally invasive procedures. 3
• The technology enables safe planning of mini-invasive approaches to deep-seated lesions. 2

Important Limitations and Pitfalls

Reduced Utility in Specific Scenarios

• ioUS provides limited benefit for approaching skull base tumors and aneurysms, where anatomical constraints and acoustic windows restrict visualization. 2
• Postoperative percutaneous ultrasound imaging through a burr hole has not proven useful in clinical practice. 3
• Proximity to vessels, air spaces, or bone can interfere with image quality. 8

Interpretation Challenges

• Correlation between ioUS findings and histopathology is not possible—the technology cannot distinguish tumor grade or molecular characteristics. 5
• Identification of small residual tumor fragments is difficult, particularly in the presence of surgical artifacts from cautery, blood products, and tissue manipulation. 2, 3
• Brain infiltration extends beyond MRI-visible borders into parenchyma that appears morphologically normal on all imaging sequences, including ioUS. 7

Technical Considerations

• The surgeon must obtain images early during resection before significant brain shift compromises navigational accuracy. 4
• Minimal cauterization should be used during sampling to preserve both tissue viability and ultrasound visibility. 4

Comparison with Alternative Technologies

Advantages Over Intraoperative MRI

• Although intraoperative MRI improves gross total resection rates, it is not routinely recommended due to increased false-positive rates requiring experience in interpretation. 4
• ioUS provides comparable real-time assessment without the cost, resource utilization, or workflow disruption of intraoperative MRI. 1
• The technology is more affordable, portable, and practical for routine neurosurgical practice. 6

Integration with Other Modalities

• ioUS complements rather than replaces other surgical adjuncts including fluorescence-guided surgery, functional monitoring, and awake mapping. 4
• The combination of neuronavigation with ioUS represents the optimal approach for maintaining spatial accuracy throughout the resection. 1