To the Editor:
We read with interest the recent report by Kantelhardt and colleagues1 on the use of image guidance for transcranial Doppler ultrasound (TCD). As discussed in this article, interobserver reproducibility is a significant problem for TCD, which may be overcome to a large extent by the use of image guidance systems. We certainly encourage the use of such tools to aid TCD operators, especially when computed tomography angiogram data are already available, as is often the case with subarachnoid hemorrhage patients.
In our institution, we have access to the same image guidance system as described by Kantelhardt and colleagues. However, we have not used it in the context of TCD. Our group recently described using imaging software alone to generate 3-dimensional representations of surface anatomy, allowing rapid freehand localization of more obvious entry points for ultrasound through the temporal bone.2 Surgical defects such as burr holes and the craniotomy edges represent ideal acoustic windows through which high-quality Doppler signals may be recorded, and their localization would certainly be expedited by computer-aided navigation tools. However, we feel that although the use of such systems may be possible in the intensive care unit setting as described by Kantelhardt et al, economic and ergonomic considerations may preclude their regular use in the general ward setting. We believe that broad use of TCD still ultimately depends on baseline operator skill and experience, which is acquired only after negotiating the steep learning curve and large number of examinations required to achieve proficiency. Moreover, this particular skill set requires maintenance, a goal achievable only via exposure to and experience with all methods of transcranial ultrasound scanning. We note with interest that the 3 sonographers in this article were experienced neurosurgeons, all with considerable expertise in TCD. We suggest that this may represent a unique situation in the broader context of TCD services worldwide.
The authors also address temporal window adequacy, which may remain problematic even when assisted by image guidance software. Its frequent impenetrability notwithstanding, skull bone is also known to unpredictably attenuate and distort ultrasound beam shape, which may complicate the assumption of the ultrasound beam propagating along a straight line.3,4 We speculate that these phenomena may have contributed to the need for slight adjustments of probe position as described by Kantelhardt et al. Ordinarily, such small adjustments may not be of significance; however, when encountering certain anatomic variants of the circle of Willis (see the Figure for an example), the need to be able to apply blind vessel identification techniques is paramount.
In summary, although we applaud Kantelhardt and colleagues’ application of image guidance systems for improving the reliability of TCD, we would like to emphasize that we feel such tools should not take the place of a solid grounding in blind Doppler scanning techniques. Accordingly, we would hesitate to recommend the regular use of image guidance systems by inexperienced TCD practitioners and suggest that they may be of limited practical value in the ward setting.
Lewis, Philip M.
Peter Y.K. Hwang
REFERENCES1. Kantelhardt S, Greke C, Keric N, Vollmer F, Thiemann I, Giese A. Image guidance for transcranial Doppler ultrasonography. Neurosurgery. 2011;68(suppl 2):ons257–ons266.2. Lewis PM, Goldschlager T, Rosenfeld JV. Image guidance for rapid temporal acoustic window localisation prior to transcranial Doppler ultrasound in the neurosurgical patient. Br J Neurosurg. 2010;24(3):303–305.3. White PJ, Clement GT, Hynynen K. Longitudinal and shear mode ultrasound propagation in human skull bone. Ultrasound Med Biol. 2006;32(7):1085–10964. Deverson S, Evans DH, Bouch DC. The effects of temporal bone on transcranial Doppler ultrasound beam shape. Ultrasound Med Biol. 2000;26(2)239–244.