Satoru Oishi

Determination and correction of the wobble of a C-arm gantry

Digital subtraction angiographic image sequences of a calibration phantom are acquired at 30 frames per second from a C-arm gantry covering an angular arc of more than 180 degrees rotating at 40 degrees/s. For each frame, after XRII distortion correction, the relation between the source and its image plane orientation in 3D space is estimated from fiducial markers in the calibration phantom. This gives a mapping between the three-dimensional calibration object and its two- dimensional projection at each gantry angle. We derive eleven mapping coefficients as a function of gantry angle. We use the coefficients to backproject the contribution to any physical voxel. Thus the wobble correction is incorporated directly into cone-beam backprojection. In the absence of gantry wobble, this method is equivalent to the short-scan Feldkamp algorithm, any deviation of the coefficients from those perfect values can be taken as a measure of the gantry wobble. The mapping method requires no special knowledge of the system geometry and any wobble, twisting of the C-arm or XRII during rotation is automatically included. A phantom with tungsten- carbide beads is reconstructed. Accuracy is obtained by comparing reprojections of the center of the tungsten beads with their known values.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Accuracy of distance measurements in biplane angiography

Distance measurements of the vascular system of the brain can be derived from biplanar digital subtraction angiography (2p-DSA). The measurements are used for planning of minimal invasive surgical procedures. Our 90 degree-fixed-angle G- ring angiography system has the potential of acquiring pairs of such images with high geometric accuracy. The sizes of vessels and aneurysms are estimated applying a fast and accurate extraction method in order to select an appropriate surgical strategy. Distance computation from 2p-DSA is carried out in three steps. First, the boundary of the structure to be measured is detected based on zero-crossings and closeness to user-specified end points. Subsequently, the 3D location of the center of the structure is computed from the centers of gravity of its two projections. This location is used to reverse the magnification factor caused by the cone-shaped projection of the x-rays. Since exact measurements of possibly very small structures are crucial to the usefulness in surgical planning, we identified mechanical and computational influences on the geometry which may have an impact on the measurement accuracy. A study with phantoms is presented distinguishing between the different effects and enabling the computation of an optimal overall exactness. Comparing this optimum with results of distance measurements on phantoms whose exact size and shape is known, we found, that the measurement error for structures of size of 20 mm was less than 0.05 mm on average and 0.50 mm at maximum. The maximum achievable accuracy of 0.15 mm was in most cases exceeded by less than 0.15 mm. This accuracy surpasses by far the requirements for the above mentioned surgery application. The mechanic accuracy of the fixed-angle biplanar system meets the requirements for computing a 3D reconstruction of the small vessels of the brain. It also indicates, that simple measurements will be possible on systems being less accurate.

Automated detection of small spherical pellets with gradient filtering on digitized XRII images

Small pellets are often used as fiducial markers in a calibration phantom to estimate the geometrical parameters in 3D (three-dimensional) reconstruction. But calibration accuracy depends on the accuracy of locating the pellet centers. Here we describe a technique for fast and accurate detection of these centers. The phantom consists of tungsten carbide pellets arranged in a helical trajectory. The plastic holder mounting the pellets may cause unequal distribution of attenuation around edge pellets compared to the center ones. After log subtraction with flood frames the grayscale gradient in the background is derived within the mask for every point for a reliable background correction. The pellets are identified from the amplitude projections of each frame and a mask is used to refine its position. The grayscale gradient of the background is suitably estimated at each point by the equation of a plane. The center obtained after gradient filter correction is compared with manual measurement, and to measurement using a single background value for each mask. Gradient correction gives centers within 0.3 +/- 0.1 pixel of the manual measurements for the edge pellets, while a single value for background correction yields results within 0.6 +/- 0.3 pixel.

Reconstruction of Pediatric 3D Blood Vessel Images from Biplane Angiograms

In pediatric cardiac angiography, there are several peculiarities such as limitation of both x-ray dose and the amount of contrast medium in comparison with conventional angiography. Due to these peculiarities, the catheter examinations are accomplished in a short time with biplane x- ray apparatus. Thus, it is often difficult to determine 3D structures of blood vessels, especially those of pediatric anomalies. Then a new 3D reconstruction method based on selective biplane angiography was developed in order to support diagnosis and surgical planning. The method was composed of particular reconstruction and composition. Individual 3D image is reconstructed with the particular reconstruction, and all 3D images are composed into standard coordinate system in the composition. This method was applied to phantom images and clinical images for evaluation of the method. The 3D image of the clinical data was reconstructed accurately as its structures were compared with the real structures described in the operative findings. The 3D visualization based on the method is helpful for diagnosis and surgical planning of complicated anomalies in pediatric cardiology.