Validation of a three dimensional musculoskeletal ultrasound system for use in hands and wrists

Abstract

Purpose: The effect of chronic synovitis on the progression of osteoarthritis pathology is a relatively new but supported concept in the fields of rheumatology and hand surgery that has been linked to pain and disability in patients. Currently, the gold standard for diagnosing synovitis in the wrists and hands is a clinical examination and the use of 2 dimensional (2D) musculoskeletal ultrasound imaging (US). Major limitations associated with this method are the lack of quantitative measurement tools available and the fact that physicians have to be able to mentally reconstruct the patient’s 3-dimensional anatomy from the 2-dimensional images provided. These limitations can be overcome by using a 3-dimensional ultrasound (3DUS) system to quantitatively measure the size of synovial hypertrophy, effusions, ganglions and osteophytes as well as having a 3D reconstruction of the patient’s hand. The aim of this work is to develop a 3D ultrasound system for imaging the hand and wrist. This paper reports on the development, geometric, and volumetric validation for wrist/hand 3D ultrasound imaging. Methods: A mechatronic wrist/hand imaging system was designed and created to operate with a conventional 2D US imaging system. The scanning system features a motor which contains a single integrated electromagnetic encoder that records the rotational position of the motor. This motor drives a lead screw which in turn moves the transducer, which is submerged in a water bath, along the linear axis to acquire a series of 2D images. The encoder information provides the software with the position of each 2D ultrasound frame. These conventional diagnostic 2D US images are then reconstructed into a 3D US images from a licensed ultrasound machine. The geometric validation of the system was done using a multi-layered monofilament grid phantom with spacings of 10 mm and the Euclidean distances between the wires were measured. The volumetric validation was conducted using a simulated synovial tissue phantom and the images were analyzed using 3D-Slicer to manually segment individual slices in the 3D image. These slices were then interpolated to construct a 3D segmentation of the phantom. This process was repeated over four days to test the intra-rater reliability of the system. The 3D volumes were compared to those found using magnetic resonance imaging (MRI) and the known volume of the agar phantom. Results: The largest error and variance recorded from the geometric validation measurements was 0.0856± 0.0087mm and was acquired from the reconstruction plane. The remaining linear measurement errors were all < 1%. The error between the mean phantom volume measurements and the known synovium phantom volume was 5.99% and 2.43% for the 3D US device and MRI respectively. Linear regression for volumetric measures revealed an R2 value of 0.4736 with a p-value of 0.20, revealing no significant correlation between the measurements made over the course of 4 days for the 3D US device. The percent difference in the volume measurements from the 3D US device and MRI was 0.814%. Conclusions: A mechatronic 3D ultrasound imaging system was developed for the wrist and hand. Successful geometric and volumetric validation demonstrates feasibility for implementation in a clinical setting. Current work evaluates system ability to accurately and quantitatively classify synovitis in first carpometacarpal joint and distal radial injury patients to investigate the relationship between chronic inflammation and the progression of osteoarthritis in patients.