Abstract
PurposeLaparoscopic ultrasound (LUS) enhances the safety of laparoscopic liver resection by enabling real-time imaging of internal structures such as vessels. However, LUS probes can be difficult to use, and many tumours are iso-echoic and hence are not visible. Registration of LUS to a pre-operative CT or MR scan has been proposed as a method of image guidance. However, the field of view of the probe is very small compared to the whole liver, making the registration task challenging and dependent on a very accurate initialisation.MethodsWe propose the use of a subject-specific planning framework that provides information on which anatomical liver regions it is possible to acquire vascular data that is unique enough for a globally optimal initial registration. Vessel-based rigid registration on different areas of the pre-operative CT vascular tree is used in order to evaluate predicted accuracy and reliability.ResultsThe planning framework is tested on one porcine subject where we have taken 5 independent sweeps of LUS data from different sections of the liver. Target registration error of vessel branching points was used to measure accuracy. Global registration based on vessel centrelines is applied to the 5 datasets. In 3 out of 5 cases registration is successful and in agreement with the planning. Further tests with a CT scan under abdominal insufflation show that the framework can provide valuable information in all of the 5 cases.ConclusionsWe have introduced a planning framework that can guide the surgeon on how much LUS data to collect in order to provide a reliable globally unique registration without the need for an initial manual alignment. This could potentially improve the usability of these methods in clinic.
Highlights
Liver cancer is a major health problem and 150,000 patients per year could benefit from liver resection [1]
Besides being moved by breathing motion, the liver is compressed by the Laparoscopic ultrasound (LUS) probe upon contact and deformed by pneumoperitoneum
The highest values are obtained at the liver edges, which is expected given that vasculature is less present in the liver periphery
Summary
Liver cancer is a major health problem and 150,000 patients per year could benefit from liver resection [1]. Solutions for freehand US registration have been proposed either by matching US to a combination of the CT signal and a simulated US signal [4], or by registering vessel probability maps from both US and CT [5] These methods have been demonstrated using images that capture large sections of liver and our preliminary tests suggest such intensity based methods do not work well on the restricted volume captured by the LUS probe. Besides being moved by breathing motion, the liver is compressed by the LUS probe upon contact and deformed by pneumoperitoneum (abdominal insufflation required for laparoscopy) This makes globally rigid solutions such as the three aforementioned ones not sufficient. In [16], we attempted vessel-based registration without establishing landmark correspondences, but the results still depended on the accuracy of the initialisation
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