Impact of Navigable Ultrasound: How Much Credit Should We Give to the “Navigable” Component?

Dear Editor,

We read with interest the paper by Renovanz et al.[1] This is a retrospective study comparing the use of navigated versus non-navigated two-dimensional ultrasound (2D US) in patients operated for gliomas. It is unclear as to whether this was a consecutive series of gliomas or a selected group. Only 10% were eloquent region tumors and most were contrast enhancing, suggesting that this may be a selected cohort more amenable to radical resection. Further, a substantial number (24) had prior radiotherapy (that may actually dilute the results). We believe the major issue is how the decision to use non-navigated versus navigated 2D US was make. By their own admission, “navigated ultrasound was chosen for patients with better prognosis.” Hence any conclusion regarding survival and prognostic implication of the use of navigated US is impossible. The overall gross total resection (GTR) rate of 55% is therefore not representative for the cohort of “resectable” contrast-enhancing tumors. In fact, by excluding the 24 patients who had received radiotherapy, the GTR rate would increase to 75%. Further, the authors state that “impact of navigated ultrasound on the EOR [extent of resection] has not been evaluated so far.” We would beg to differ. The study by Solheim (which is already quoted by the authors) earlier reported EORs associated with various categories of gliomas (including “resectable” and prior treated ones).[2] We too have reported EORs with malignant gliomas using navigable US with GTRs of 47% in malignant gliomas, increasing to 88% in the “resectable” subgroup.[3] The only difference is that both Solheim and our studies have used navigable three-dimensional US.

The authors also discuss the image resolution, which is a very important aspect. It is known that a major obstacle to the widespread use of US in the intraoperative setting is the perception that image resolution is suboptimal. It is therefore very heartening to see that the reported “good” resolution rates were as high as 90%. We are surprised, however, that the resolution was poorer after opening the dura. Whereas decrease in resolution during the resection is well documented,[4] a well-performed US scan prior to beginning the resection should ideally be the best scan. In fact, often the dura is thickened or calcified leading to suboptimal insonation and inferior resolution prior to the dural opening. The justification that cerebrospinal fluid loss accounted for this loss of resolution is untenable. Meticulous attention to patient positioning and acoustic coupling is an absolute must to ensure the best scan. Moreover, before the start of the resection, the brain surface usually conforms well to the surface of the probe allowing a very good acoustic coupling. We therefore would disagree when the authors say that “a second ultrasound after opening the dura is not necessary.” In fact, it is always better to perform a second scan after the dura is opened.

The authors further report that although EOR was not affected, the use of navigated US resulted in a better sensitivity (41% versus 19%). This result is surprising. Rygh et al showed earlier that even in the subsequent phase of resection of glioblastomas, the sensitivity is still > 80%. It is the specificity that drops because of false positives due to echogenic artifacts. Therefore a low sensitivity points to a technical deficiency in the performance of the scan, an aspect that is entirely correctable. This low sensitivity may be a reason why the EOR rates were also low overall. Solheim et al showed that when image resolution was better, resection rates also improved.[2] Second, the difference in sensitivities between the two groups could be attributed to an unequal distribution of posttreatment cases in the two groups (the authors themselves admit to a selection bias), and hence attributing the improved sensitivity to the navigation component of the US is fallacious. Navigation certainly improves the orientation and will improve overall surgeon comfort. However it is difficult to accept that “navigation seems to lead to a better image quality.” If the US scanner and probe were the same across the two groups, the image resolution cannot be enhanced merely by the navigation function. The authors also do not separately report the resolution characteristics in the two groups, and we assume that there were no differences.

Nonetheless we would like to congratulate the authors on this article that highlights the many benefits of navigable US. It is important to reinforce the utility of this often forgotten imaging tool. Addition of navigation principles and improvements in US technology have together resulted in a resurrection of this invaluable tool. Whereas the addition of navigation may improve anatomical understanding and orientation, it cannot compensate for the resolution of the US itself. Every attempt must be made to obtain the best US scan, and it is this scan that actually determines the outcome of the surgery. As the authors rightly point out, navigable three-dimensional US overcomes a lot of the drawbacks of 2D US and provides a much more cost-effective alternative to intraoperative magnetic resonance imaging.

• References

• 1 Renovanz M, Hickmann AK, Henkel C, Nadji-Ohl M, Hopf NJ. Navigated versus non-navigated intraoperative ultrasound: is there any impact on the extent of resection of high-grade gliomas? A retrospective clinical analysis. J Neurol Surg A Cent Eur Neurosurg 2014; 75 (3) 224-230
• 2 Solheim O, Selbekk T, Jakola AS, Unsgård G. Ultrasound-guided operations in unselected high-grade gliomas—overall results, impact of image quality and patient selection. Acta Neurochir (Wien) 2010; 152 (11) 1873-1886
• 3 Moiyadi AV, Shetty PM, Mahajan A, Udare A, Sridhar E. Usefulness of three-dimensional navigable intraoperative ultrasound in resection of brain tumors with a special emphasis on malignant gliomas. Acta Neurochir (Wien) 2013; 155 (12) 2217-2225
• 4 Rygh OM, Selbekk T, Torp SH, Lydersen S, Hernes TA, Unsgaard G. Comparison of navigated 3D ultrasound findings with histopathology in subsequent phases of glioblastoma resection. Acta Neurochir (Wien) 2008; 150 (10) 1033-1041 ; discussion 1042