In this paper, we introduce a new algorithm to register images with rotation and image blurring. The characteristic
of this approach is that a set of Legendre moment invariants are used to establish the correspondence of corner points
between the reference image and the distorted image, after these points are extracted by Harris corner detector.
Transformation parameters can be estimated from those matched points.
Freehand 3D ultrasound allows intra-operative imaging of volumes of interest in a fast and flexible way. However, the ultrasound device must be calibrated before it can be registered with other imaging modalities. We present a needle-fiducial based electromagnetic localization approach for calibrating freehand 3D ultrasound as a prerequisite for creating an intra-operative navigation system. Although most existing calibration methods require a complex and tedious experiment using a customized calibration phantom, our method does not. The calibration set-up requires only a container of water and only several frames (three to nine) to detect an electromagnetically tracked needle tip in a 2D ultrasound image. The tracked needle is dipped into the water and moved freehand to locate the tip in the ultrasound imaging plane. The images that show the needle tip are recorded and the coordinates are manually or automatically identified. For each frame, the pixel indices, as well as the discrete coordinates of the tracker and the needle, are used as the inputs, and the calibration matrix is reconstructed. Three group positions, each with nine frames, are recorded for calibration and validation. Despite the lower accuracy of the electromagnetic tracking device compared to optical tracking devices, the maximum RMS error for calibration is 1.22mm with six or more frames, which shows that our proposed approach is accurate and feasible.
Direct volume rendering via consumer PC hardware has become an efficient tool for volume visualization. In particular, the volumetric ray casting, the most frequently used volume rendering technique, can be implemented by the shading language integrated with graphical processing units (GPU). However, to produce high-quality images offered by GPU-based volume rendering, a higher sampling rate is usually required. In this paper, we present an algorithm to generate high quality images with a small number of slices by utilizing displaced pixel shading technique. Instead of sampling points along a ray with the regular interval, the actual surface location is calculated by the linear interpolation between the outer and inner points, and this location is used as the displaced pixel for the iso-surface illumination. Multi-pass and early Z-culling techniques are applied to improve the rendering speed. The first pass simply locates and stores the exact surface depth of each ray using a few pixel instructions; then, the second pass uses instructions to shade the surface at the previous position. A new 3D edge detector from our previous research is integrated to provide more realistic rendering results compared with the widely used gradient normal estimator. To implement our algorithm, we have made a program named DirectView based on DirectX 9.0c and Microsoft High Level Shading Language (HLSL) for volume rendering. We tested two data sets and discovered that our algorithm can generate smoother and more accurate shading images with a small number of intermediate slices.
We present a methodology for alignment of X-Ray image and CT image, based on chamfer 3-4 distance transform and simulated annealing optimization algorithm. The proposed approach firstly segments object’s structure from X-Ray image. Using projection model and optimization method, we deduce the correct projection matrix. This method is also integrated into medical intra-operation, dealing with the data set acquired from 3D image workstation and active navigation.