# Vascular Structure Identification in Intraoperative 3D Contrast-Enhanced Ultrasound Data
## 1 Introduction  
Intraoperative ultrasound imaging is nowadays commonly used in neurosurgery during brain tumor operations [1]. At the beginning of the intervention, the ultrasound images show the surgeon the intraoperative state of the tumor (Figure 1b) [2]. The tumor size or position can be possibly slightly different at the operation time point from the preoperative state depicted in the preoperative MR data (Figure 1a). During the operation ultrasound imaging is a valuable tool to detect the residuals of tumor with the goal to optimize the tumor removal (Figure 1c,d) [3]. However, the interpretation of the ultrasound can be complex [4; 5]. The acquisition of the images through the skull opening, called craniotomy, requires skill and experience. Firstly, the sweep of the ultrasound probe is limited by thesmall opening. The presence of air between the probe and the brain surface blocks the propagation of ultrasound burst. Secondly, the scanned brain area in the images is limited. The orientation of the image and the interpretation of the information it shows can be complex, mostly for images acquired at the end of the operation. A common method to assist the neurosurgeon in the visual analysis of the intraoperative ultrasound images is to visualize them overlapped to the preoperative MR data in the navigation system because the preoperative image data offers an overview of the entire head [2; 6; 7]. This technique has limitations for ultrasound images acquired at the end of the operation [5]. The brain structures are so largely deformed, mainly because of tissue resection, so that the information included in the visualization planes does not correspond at all anymore [8]. The evaluation and correction of the brain shift has been extensively studied. Methods are based on similarity measures based on image intensities [9; 10; 11; 12], anatomical landmarks [13; 14; 15] or biomedical deformation models [16; 17]. An interesting previous work proposed to use the vascular structures surrounding the tumor as landmarks in order to estimate the brain deformations [14; 18]. In this work, the extraction of the blood vessels was performed in preoperative MR angiographic (MRA) data and in intraoperative Doppler ultrasound images. MRA examination is in routine not acquired in case of brain tumor. Doppler ultrasound can be performed intraoperatively but has limitations to depict the diameters with accuracy and to provide 3D visualization. Moreover, information about brain tissue itself is missing. Several medical studies were conducted to evaluate contrast-enhanced ultrasound imaging for the visualization of brain tumors and the detection of residuals of tumor [19; 20; 21]. This imaging modality enables to enhance vascularized structures, like lesions and tumors, but also blood vessels [22; 23]. In this paper, an alternative method of vascular structures identification in contrast-enhanced ultrasound data is presented with a possible application in the brain shift estimation. Instead of considering the preoperative MRA and intraoperative Doppler ultrasound as previously mentioned, this work introduces the possibility of using cT1 MR and 3D-iCEUS data which are routinely involved in brain tumor operations. The method has three main steps _i.e._, the selection of a vascular segment in cT1 MR, the identification of this pattern firstly in 3D-iCEUS\({}_{start}\), then in the 3D-iCEUS\({}_{end}\)  
Figure 1: Patient image data acquired during tumor resection. The brain tumor (white arrow) is represented in the preoperative cT1 MR data (**a**) and in the 3D-iCEUS\({}_{start}\) (**b**) acquired at the beginning of the operation. After tumor removing, the resection cavity (indicated by two white arrows in (**c**) and (**d**) is well visible in the B-mode ultrasound image (**e**). In the 3D-iCEUS\({}_{end}\) acquired at the end of the operation (**d**); the borders of the cavity can be easily interpreted as a blood vessel.