Reliability of a freehand three-dimensional ultrasonic device allowing anatomical orientation “at a glance”: Study protocol for 3D measurements with Curefab CS®

Abstract

Background and Purpose: 3D ultrasonic measurement of carotid atherosclerotic lesions has emerged as an important tool for research and patient management. We sought to evaluate the accuracy and the reliability of a new easy-to-use magnetically tracked freehand 3D ultrasonic device with quick reconstruction time, allowing for anatomical orientation “at a glance” for calibration and distance measurements, with unique usability as a combined tool for image acquisition, registration and measurement within 3-D ultrasonic (US) volumes, which is furthermore connectable to every conventional US-machine. To enable a complete evaluation of the whole system, three different key points had to be addressed: the reproducibility of the calibration procedure, the absolute accuracy of the whole system and a direct comparison to CT and MRI imaging modalities.

Materials and Methods: For validating our calibration method, a set of 6 calibrations was performed; each consisting of 6 records of a pyramid phantom taken from different positions and angles. To evaluate the accuracy of the whole 3D-ultrasound system, the point reconstruction accuracy and the distance accuracy were determined in a point phantom made out of a single metal wire vertically attached to the bottom of a plastic tub filled with water. For distance measurements, a precisely manufactured plastic tube phantom was scanned and the length between fixed landmarks on the tube was measured. In a final step, 3D US records acquired with Curefab CS were compared to CT and MRI scans; for this purpose all ultrasonic data was manually registered to the CT/MRI data.

Results: Concerning calibration precision the tested Curefab CS system performs state of the art compared to reviews of recent freehand 3D-Ultrasound calibration methods. The point reconstruction measure for evaluation of system accuracy retrieved a mean point accuracy of 1.52 mm in contrast to values ranging from 1.67 to 3.63 mm. Mean total error of distance measurements was 0.9% with standard deviation 0.56% in our study, compared with values reaching from about 1% up to 2.3% in other studies on this subject. All quantitative measurement results are listed in a summarized form in Table 1. Besides quantitative evaluation, 3D-ultrasound records acquired with the Curefab CS system were also compared to CT and MRI scans of patients (see Figure 3 and Figure 4). The alignment of both image modalities showed promising results for future development of diagnostic tools using all image data.

Discussion and Conclusion: Our study demonstrates that 3D measurements with Curefab CS are feasible with satisfactory reliability and accuracy. From the results gathered in our study we conclude that 3D-imaging with Curefab CS might start off the possibility of accurate visualization, volume measurement, carotid plaque characterization and identification of vulnerable plaques in the very near future.