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Title: Development of a portable 3D ultrasound system for imaging and measurement of musculoskeletal body parts
Authors: Huang, Qinghua
Degree: Ph.D.
Issue Date: 2007
Abstract: Diagnostic ultrasound (US) has been widely used in a large variety of medical practices, including obstetrics, gynaecology, cardiology, abdomen, vascular imaging, surgery, musculoskeletal tissues, etc. The advantages offered by US including real-time imaging, moderate cost, physiologic measurement, nonionizing radiation and no known bioeffects, have made it an important imaging modality in comparison with computerized tomography (CT) and magnetic resonance imaging (MRI). Conventional 2-D US imaging has made tremendous progress in achieving anatomical information from organs and tissues of patients. However, there are difficulties for physicians to obtain a 3-D impression of the anatomy of interest. In the last decade, advanced technology allowed the mature of developing 3-D US imaging methods for acquiring an entire volume data in a single image, generating slices along planes in 3-D space, or surfaces of 3-D objects. Since the 1990's, 3-D US has proven to be a promising imaging modality for clinical diagnosis and treatment monitoring. It offers some unique features including: relatively lower cost in comparison with CT and MRI, no requirement of intensive training and radiation protection for its operation, along with movable and potentially portable hardware. In recent years, musculoskeletal US has become a challenging area because of the complexity of soft tissues and bones in limb parts. Although commercial 3-D US systems are now available, they are mainly applied for fetus, vascular and cardiac imaging with real-time 3-D volumetric probes. Because the relatively large size of human limb body parts always exceeds the imaging range of such 3-D probes, these systems are not widely used for musculoskeletal tissues. Accordingly, the objectives of this PhD study are to develop a portable freehand 3-D US imaging system with enough accuracy for assessing musculoskeletal tissues, to explore novel scanning methods to obtain more information of tissues, to improve the 3-D image quality, and to investigate new methods for accurate, easy and real-time 3-D measurements. A portable freehand 3-D US imaging system was developed in this study. It included a portable US scanner to obtain real-time 2-D B-mode images of musculoskeletal tissues and an electromagnetic spatial sensor fixed to the US probe to acquire the position and orientation of the B-mode images. The images were digitized with a video digitizer and displayed on screen with their corresponding orientations and positions read from the spatial locator in real-time. A software system was developed for data acquisition, volume reconstruction, visualization and measurement using Visual C++ and Visualization toolkit software. Temporal calibrations were conducted to find out the time delay between the collected raw images and the spatial data. Spatial calibrations were also performed using a cross-wire phantom. The system accuracy was validated by three tissue mimicking phantoms made of silicone. The average errors for distance measurement in three orthogonal directions in comparison with micrometer measurement were 0.1+-0.4 mm, -0.3+-0.3 mm, and 0.3+-0.4 mm, respectively. The average error for volume measurement was -0.18+-5.44% for the three phantoms. Based on the portable 3-D US imaging system, a new scanning approach using a water bag was proposed to collect the complete volumes of limb extremities. The water bag was used to contain the limb extremity and the scanning was conducted on its external surface in different directions. The recorded 2-D US images were used to form a full 3-D volume of the limb extremity. A corresponding algorithm was proposed to remove invalid image information within each sweep using a separating plane defined semi-automatically. Two phantoms were employed to test the accuracy of the imaging. The distance between two plastic bands which were attached to a plastic tube filled with US coupling gel measured by a micrometer and from four reconstructed volumes was 39.03+-0.36 mm and 39.2+-0.5 mm, respectively. This novel water bag scanning avoided the use of a bulky water tank or potential compression to tissues by the probe, which usually happens in the conventional 3-D US scanning for complete limb extremities. For volume reconstruction, several novel interpolation algorithms were proposed based on the concepts of weighted mean filter and median filter. Squared distance weighted (SDW) using squared inverse distance as the weighting factor for voxel calculation was applied to achieve a better trade-off between computation speed and image quality. An adaptive SDW algorithm was proposed for speckle reduction and edge preservation during the volume reconstruction. Furthermore, four median filters, including standard median (SM), distance weighted median (DWM-1 and DWM-2), and Gaussian weighted median (GWM) were employed for interpolation of voxel arrays. Qualitative and quantitative comparisons were made based on the freehand captured raw data from the forearm of a healthy subject. The results demonstrated good performances of the median filters for predicting missed B-scan pixels. Compared with the voxel nearest neighbourhood (VNN) and distance-weighted (DW) interpolation methods, the four median filters were able to reconstruct volume data sets with better capability of producing accurate reconstructions and preserving edges. In addition, an easy and rapid method was developed to perform 3-D measurements among the recorded B-scans using corresponding spatial data without the need to create a 3-D voxel array. Based on the 3-D US imaging system, the US probe was moved smoothly on the surface of the human body during data acquisition. When the screen of the US scanner illustrated useful anatomical information, the current B-scan was captured and displayed in a 3-D space with its corresponding location. The 3-D spatial locations of the collected B-scans could be used to determine the relative positions of points within different B-scan image planes. Therefore, the distance between two points as well as the angle between two lines could be rapidly computed. According to the measurements on a phantom, the mean error of distance measurement was -0.8+-1.7 mm (-2.3+-3.6%) and that of angle measurement was -0.3+-2.9o (-0.1+-4.1%) with reference of the results of a micrometer. The lengths of the first metatarsal and the angles between the first metatarsal and the middle part of the tibia of three subjects were measured in vivo using the new US method and MRI imaging. The overall percentage differences of the length and angle measurements between the two methods were 0.8+-2.2% and 2.5+-3.6%, respectively. The results also demonstrated that this measurement method had good repeatability and reproducibility (interclass correlation coefficient > 0.75). It is expected that this new method could potentially provide a quick and effective approach for the 3-D measurement of musculoskeletal tissues. In summary, a portable freehand 3-D US system was developed together with the novel scanning methods, accurate reconstruction algorithms and real-time rapid measurement techniques, which are particularly useful for the musculoskeletal tissue assessment. Further research and development work, including development of an improved version of the current portable system based on a notebook PC, investigation of techniques for dynamically recording tissue changes, measurement among multiple volumes, and study of new reconstruction, registration and segmentation algorithms, could be followed to strengthen the functions of this portable 3-D US system. Another future direction is the potential applications of this system for the assessment of various musculoskeletal body parts.
Subjects: Hong Kong Polytechnic University -- Dissertations.
Musculoskeletal system -- Ultrasonic imaging.
Pages: xxv, 167 leaves : ill. ; 31 cm.
Appears in Collections:Thesis

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