**8. Live imaging**

Live imaging is an answer to capturing tissue deformation as it allows real-time dynamic information on the kidney and removes the need to 'estimate' structural changes in the tissue. Ultrasound is one live imaging modality that has shown high sensitivity and specificity at identifying tumour margins [32, 33]. There have been several studies using live USS to aid AR. Kang et al. [34] merged live laparoscopic ultrasound images on stereoscopic video and showed accuracy of image-to-video correlation of up to 2.76 mm. Kang et al. claim this aids in depth perception and better visualisation of internal structures. Cheung et al. demonstrated that a fused video-USS model for phantom partial nephrectomy allowed for a 1.1 mm tumour resection margin (with 2D fusion) for endophytic tumours [35].

Singla et al. [36] showed in their study, that simulated healthy renal tissue excised was reduced from an average 30.6–17.5 cm3 using intra-operative USS based AR. This technique would be especially beneficial in critical structures like endophytic renal tumours where most of the tumour lies below the organ surface—(endophytic tumours currently have complication rate of nearly 50%).

These are all however preliminary studies and are based on phantom models which does not represent the true nature of the operation in vivo. A majority of studies have involved manual registration with labour intensive methods that are unrealistic to be currently used in-vivo. Cheung et al. found that although there was 29% reduction in planning time with the USS-fused model, the tumour required longer operative times (being up to 39% slower than the conventional system) [35].

Some projects have combined all three aspects of AR named above. An example of this is PARIS (projector-based AR intracorporeal system)—a method by Edgcumbe et al. [37] where there is a combination of a tracked projector, tracked marker and laparoscopic ultrasound transducer. This has been used in 16 simulated laparoscopic partial nephrectomies, where cancerous tumours were projected onto the kidney surface and this projection moved with kidney. An ultrasound allowed live imaging of the intra-operative environment. This study showed better identification of underlying anatomy and tumour boundaries to show signification reduction in healthy tissue excised.
