Five unique 3D models were produced using a CT quality control phantom (model 137856101, GE Healthcare, Waukesha WI USA) with fiducial markers (CT/MRI 2 mm center hole Multi-modality marker, MM3002, Izi Medical Products, Owings Mills, MD USA). The fiducial markers had an adhesive to allow for adjustment on the surface of the CT phantom. For each trial of the CT phantom, fiducial markers were adjusted to provide new distances in each of the orthogonal directions.
The distances were measured using electronic digital calipers (model 01407A, Neiko Tools, Henan, China). These measurements were set as the gold standard (GS), aiming to reproduce measurements gathered in clinical settings such as those via surgical ruler or calipers. The calipers’ product information reports precise measurements with resolution of 0.01mm and accurate measurements to 0.02mm. FIGURES 1,2. A total of six measurements between fiducial markers were made for each model CT phantom: two in the x-direction, two in the y-direction and two in the z-direction by one observer. The observer was trained to measure the shortest distance between fiducial markers set in the orthogonal directions (x, y, z) using the inside calipers.
CT scans of the phantom were obtained using the Head CT quality control phantom settings (5mm slice thickness, 134mAs, 22.7cm DFOV, 0.516:1 pitch) with 0.625cm x 0.625cm x 0.625cm voxels on a GE Lightspeed CT scanner (GE Healthcare, Waukesha WI USA) and stored as DICOM files within our AGFA IMPAX version 6.7.0.3502 “site” Picture Archiving and Communication System (PACS), (AGFA, Mortsel, Belgium). The DICOM files obtained were converted into 3D phantom models, and measurements were made on the AGFA-PACS. The observer was trained to measure the shortest distance between the outer edges fiducial markers using only the “ruler” found within PACS. FIGURE 3.Additionally, the observer had access to axial, sagittal and coronal views to project views of fiducial markers in their most appropriate planes.
Open-source software programs Horos (Purview, Annapolis, MD USA) and Blender (Stichting Blender Foundation, Amsterdam, Netherlands) were used to translate DICOM files into a 3D image. The 3D images were adapted and loaded onto the AR HMD platform using a C# programming language-based code on the Unity Platform (Unity Technologies, San Francisco, CA USA).The AR HMD used for this study was the Hololens 1 (Microsoft, Redmond, WA USA) OS version 10.0.14393.1358. The settings for HoloLens projections were set to default factory settings with the exception of distance of initial hologram projection set to 30cm from HoloLens. This was done to project the hologram within approximately arm’s length of the user. The hologram was manipulated via rotation and translation without scaling size to obtain best views of markers. FIGURE 4 (with supplemental video) demonstrates a side-by-side comparison of the CT phantom and its hologram. The 3D models were projected as holograms and the distances between the projected fiducial markers were measured by the same observer as GS measurements. Using the built-in AR HMD capabilities, the hologram was pinned to a table where the calipers could be laid flatly to make the measurements, FIGURE 5. These measurements are known as Holo measurements.
A power analysis was performed using SAS University Edition (SAS Institute Inc., Cary, NC). This demonstrated that for paired t-test (ɑ=0.05) with a sample size of n=30 per group, the power would be 80% in detecting a difference of 0.3mm. In a previous study assessing accuracy and reliability of CT measurement, max difference was observed in this range (11); the authors opted to detect a difference of this magnitude using this sample size in consideration of available time, resources and training in both PACS and holo measurements.
Since DICOM and PACS have become standard in the clinical radiology workspace, the assessment of accuracy and precision of the Holo measurements were set against this standard (12). To determine accuracy, the absolute error from the GS was gathered for PACS and Holo measurements. To compare the means and distributions of the absolute error, a two-tailed t-test was employed. For the assessment of precision, a right-tailed chi-square test of variance was used, where the PACS absolute error variance is set as the null. Additionally, a root-mean-square error was calculated for a series of repeated measurements made in each orthogonal direction (n=15).