# 3D Ultrasound Mosaic of the Whole Shoulder: A Feasibility Study
## 4 Discussion  
A total of 51 3D US images of a healthy subject's shoulder were acquired from all shoulder directions (anteroinferior, posteroinferior, anterosuperior and posterosuperior), and registered and combined into a single, complete ultrasound volume of the entire rotator cuff, a US shoulder mosaic. The US mosaic was also registered with an MRI that is segmented for bones and muscles. Visual comparison of the two modalities based on anatomical positions deemed the registration satisfactory. The anatomical geometry was preserved in the US mosaic when compared to the MRI or a single US volume. The humeral head surface was found to be only marginally thicker overall, by 0.65 mm, when segmented in the US mosaic than in a single US volume due to built-up artefacts and malalignments in the image registrations. The thickness of the humeral head surfaces significantly matched in both US data sets on the coronal plane, differing by only 0.235 mm. However, the reconstruction error was most notable across the transverse plane with the humeral head surface being 0.988 mm thicker in the US mosaic. For distance measures, the humeral head diameter perfectly matched in the MRI and US segmentation and the ACHD exhibited a marginal deviation in the US mosaic by 1.2 mm. The performance of the manual registration protocol developed in terms of preserving the anatomical distance measurements showed a significantly better performance than the block-based rigid-body registration used in [16], with a mean error of 2.7 mm. This is despite their approach being assessed on a footus phantom with a US mosaic consisting of only 10 volumes. Further evaluation of the US mosaic in this study was performed by two expert MSK clinical sonographers, who assessed  
Figure 8: Measured corresponding distances between the 2 points, in the acromion and the humeral head, in the US mosaic (**left**) and MRI segmentation (**right**) across the transverse plane. Figure 7: Measured diameter between the 2 points in the humeral head US mosaic segmentation corresponding to the MRI segmentation. it as a standalone model, and were subsequently provided with the registered MRI bone segmentations, followed by the muscle segmentations. Evaluated anatomical structures included anatomies that are typically imaged in a shoulder US examination [8; 9; 10] as well as structures listed as surgery-relevant by two experienced shoulder surgeons. Having access to the MRI bone segmentation improved only the bone grades for the first sonographer. For the second sonographer, it helped better demonstrate the ACHD; infraspinatus and supraspinatus muscles; and subscapularis, supraspinatus and infraspinatus tendons. The further overlay of the MRI muscle segmentations had negligible impact on the structure grades for both sonographers, possibly because identifying soft tissue is the conventional way of using US for experienced sonographers. Sonographers, on average, agreed on the demonstration of all five assessed anatomies in the US mosaic when provided with the MRI bone segmentations (graded bones = 2, muscles = 3, tendons = 3, ligaments = (4-5) and labrum = (4-5)), with a substantial agreement between their individual structure ratings: [PERSON]'s weighted kappa of 0.7 (95% CI: 0.526-0.877). There was consensus on the clinical viability of visualising the bony anatomy. They were the best demonstrated anatomy, likely because they are rigid and were considered as reference structures for the rigid alignment between volumes. The sonographers also agreed on the partial demonstration of the muscles and tendons and the failure to identify most of the ligaments and labrum. The partial demonstration of tendons was partly due to the subject's physique as the sonographers found the scanned tendons small and difficult to examine. They noted a clear visualisation of the ACHD, ACJ, subacromial/subdeltoid bursa and supraspinatus.