With skilful and technical advancement, 3D directions of feeding arteries can be detected. After removal of the tumor, no residual tumor is easily confirmed with power Doppler image. Only weak point is required for skillfulness and rapid fading of arterial image. It had better re-observe record of power Doppler US with contrast-enhancing agent in every one frame. Although subtraction method of power Doppler is also useful, B-mode image also provide background echo signals, and then power Doppler US without subtraction is enough to evaluate the tumor vasculature. The information obtained from contrast-enhanced power Doppler US is useful for surgical navigation since the appropriate approach route to access the tumor becomes quite apparent, and for an evaluation of peritumoral vessels, particularly for showing the vasculature of highly vascularized tumors such as hemangioblastomas.

As for CNS hemangioblastomas, Glasker S and Shah M, et al. indicate that power Doppler flow sonography is a reliable and useful tool to localize hemangioblastomas intraoperatively (Gläsker et al. 2011). On grayscale imaging B-mode, many hemangioblastomas were only visible if cystic. However, with the use of power Doppler US, all lesions were able to be localized intraoperatively. The distinct sonographic visualization of hemangioblastomas compared with other tumors in the posterior fossa is due to their vascular and cystic nature. To localize cerebellar hemangioblastomas, power Doppler US surpasses MRI navigation since there is no brain shift. Furthermore, US- guided puncture of large posterior fossa cysts was useful for rapid release of pressure in cases where cerebellar tissue herniated after dural opening. In surgery for intra-medullary hemangioblastomas, power Doppler US is the only possibility of navigation. This is specifically important in patients with VHL disease who frequently undergo multiple surgeries. Such minimal approaches do not allow extensive searching of a tumor that is not directly visible after dural opening. Despite the fact that Avila and coauthors (Avila et al. 1993) used an older power Doppler technique with inferior resolution compared with the current technique, the authors feel that power Doppler US is helpful for surgery of hemangioblastomas and facilitates the localization of the tumors. The use of power Doppler has advantages and disadvantages compared with other methods of neuronavigation. The advantages include the possibility of real-time imaging, fast availability, non brain shift, and the possibility to visualize vascular structures. It is the only well-established navigation method in the surgery of intra-axial tumors. The disadvantages include limitations in its resolution. More than all other navigation methods, the usefulness of USs depends on the experience of the surgeon. The image planes may be unfamiliar for inexperienced surgeons. The imaging artifacts are different from other imaging methods. In addition, based on their findings, Kanno H and co-authors suggested that data on vascularity obtained using DSA correlated with those obtained using power Doppler US, but that the US contrast- enhancing effect obtained using Levovist were different from that revealed on MR or CT images (Kanno et al. 2005). In vitro studies, animal experiments, and clinical studies have revealed that a US contrast agent Levovist, did not have toxicity or side effects. Furthermore, Levovist improves the signal intensity and the signal- to-nose ratio for several minutes. Nevertheless, there have been few studies on intraoperative power Doppler US for intracranial lesions, in which both non-enhanced and contrast-enhanced images have been examined. In addition, it is not clear what kinds of intracranial tumors might display the enhancing effect. Otsuki H and co-authors demonstrated that intraoperative harmonic subtraction images of intracranial lesions obtained using a US contrast agent were almost equal to MR angiograms and DSAs (Otsuki et al. 2001). Previously, we examined the US contrast-enhancing effect on the tumor parenchyma and assessed the relationship between vascularization seen on DSAs and the contrast enhancement of intracranial tumors seen on power Doppler USs. Our results revealed that most meningiomas, metastatic tumors, and, particularly, hemangioblastomas reflected strong signals on contrast-enhanced power Doppler US images, while some gliomas and malignant lymphomas displayed weak signals despite their contrast enhancement on CT and MR images. Bogdahn U and co-authors found that for all Grade IV gliomas contrast-enhanced transcranial color-coded US revealed not only color Doppler flow signals within the tumor parenchyma, but also atypical arterial and venous spectra (Bogdahn et al. 1994). The mechanism of US contrast enhancement has not yet been fully elucidated, but depends on the microbubble concentration in the blood pool in the tumor parenchyma, which is considered to be related to tumor vascularity and not to the destruction of the blood–brain barrier. On the other hand, contrast enhancement on CT or MR images is considered to be related to destruction of the blood–brain barrier rather than to tumor vascularity. Various abdominal tumors, such as liver cell carcinoma, liver hemangioma, metastatic liver carcinoma, gallbladder carcinoma, pancreatic carcinoma, and, in particular, liver cell carcinoma and metastatic liver carcinoma, display a high level of US contrast enhancement. In addition, these findings indicate that the US contrast-enhancing effect in abdominal tumors depends on vascular enrichment. It is demonstrated that hemangioblastomas, meningiomas, metastatic brain tumors, and some high-grade gliomas also show high-grade contrast enhancement during power Doppler US when a contrast agent is used, and that the angiographic grade was correlated with this level of contrast enhancement. In particular, hemangioblastomas, even if small, demonstrate the US contrast-enhancement effect, which is useful to determine the orientation of a tumor during an operation. Thus, the use of power Doppler US with a contrast-enhancing agent has several advantages to facilitate tumor removal. The correct approach route to access the deep-seated tumor becomes immediately apparent because feeding arteries, draining veins, and tumor vasculature are clearly shown in color images. In addition, the extent of resection in parenchymal brain tumors is easily evaluated, particularly in tumors that have abundant vessels, and finally total removal of the tumor is confirmed with the disappearance of power Doppler signals in the tumor vasculature. Although we used the US contrast agent as a single bolus injection, other enhancement protocols such as repeated bolus injections or continuous infusion should be tried. In addition, other imaging modes such as pulse inversion and 3D US may be practical when performing power Doppler US with the contrast agent. Although a neuronavigation system based on data obtained from MR or CT images makes errors because of brain shift, US provides true real-time imaging without any brain shift. A neuronavigation system that could be revised by data obtained using power Doppler US with contrast enhancement would provide more superior information during surgery.