# Augmented Reality in Surgical Navigation: A Review of Evaluation and Validation Metrics
## 2 State of the Art AR Based Surgical Systems
### Neurosurgery  
The first surgical field to employ navigation and successfully incorporate it into regular practice was neurosurgery. In the last decade, a growing number of scientific and commercial systems for neurosurgical applications have emerged [44; 45]. The visual perception and human-computer interface features of computer graphics for supplementing the display in neurosurgery are the main focus here [10]. AR has already been evaluated for the training of tumor resection planning in terms of perceptual and spatial reasoning ability [46]. Their technology was specifically created and tested with human aspects in mind, with the purpose of reducing cognitive stress. In comparison to traditional planning scenarios, their proposed method significantly increased novice performance, regardless of the sensorimotor tasks performed (as shown in Figures 3 and 4). Furthermore, when practitioners used the technology, they were able to complete clinically relevant activities in  
Figure 2: Pie chart showing the percentage division of the publications from various fields of surgery being discussed in this paper.  
significantly less time. This showed that augmented reality systems can help residents gain the spatial reasoning abilities they require to plan neurosurgical treatments and enhance patient outcomes.  
Augmented reality systems represent a significant advancement over current neuros navigation systems. In particular, the wide range of technical implementations gives the neurosurgeon viable options for various operations (generally neuro-oncological and neurosvascular), treatment methods (endo-vascular, endo-nasal, open), and various stages of the same operation (microscopic and macroscopic part). Although no prospective randomized research has been published, available literature [9] confirms that this technology in neurosurgery is a reliable, versatile, and a promising tool. In the work presented by [47], a target registration error of 2.5 mm was achieved using augmented reality neuro-navigation (data from 5 publications), with no significant difference (2.6 mm) when compared to conventional infrared neuro-navigation systems (data from 23 publications).  
According to the review of 12 publications on intra-operative medical applications in [48], the injection of 3D images with AR enables successful image integration in vascular, oncological, and other lesions without the need to look away from the surgical field which improves safety, surgical experience, and clinical outcome. According to the results of clinical trials presented in [44], the augmented reality system proved to be accurate and reliable for intra-operative image projection to the head, skull, and brain surface, with a mean registration time of 3.8 min and a mean projection error of 0.8 \(\pm\) 0.25 mm. Based on the evaluation completed in [45], better visualization techniques which differentiate between arteries and veins and ascertain the absolute depth of a vessel of interest are required for more complicated anomalies such as arteriovenous malformations (AVM) and arteriovenous fistula (AVF); however, AR is favourable technology in neuro-vascular surgery. To conclude, there is no universal method for reporting navigation accuracy in augmented reality neuro-navigation; comparative studies, however, still need to be assessed to identify its effectiveness [49].  
Figure 4: Visualization data in (**a**) AR; (**b**) AR with grid lines to promote the sensation of depth; and (**c**) VR [46].  
Figure 3: Planning environments for neurosurgery (**a**) 2D view of axial/coronal/sagittal slices; (**b**) XP representation of 2D slices; and (**c**) 3D volume rendering [46].