Biomedical subjects

Robert N Rohling

Publications and source records attributed to Robert N Rohling.

5 recordsLinked to original sources

PubMed2006

Methods for segmenting curved needles in ultrasound images.

Ultrasound-guided percutaneous needle insertions are widely used techniques in current clinical practice. Some of these procedures have a high degree of difficulty because of poor observability of the needle in the ultrasound image. There have been recent efforts to improve guidance by computer assisted needle detection. These software techniques are often limited by not representing needle curvature. We present two methods to detect the needle in 2D ultrasound that specifically address needle curvature. Firstly, we demonstrate a real-time needle segmentation algorithm based on the Hough transform which detects the needle and represents its curved shape. Secondly, we demonstrate how a new coordinate transformation can transform detection of a curved needle to a linear fit. These methods are demonstrated on ultrasound and photographic images.

Algorithms↗

PubMed2006

Three-dimensional extended field-of-view ultrasound.

Three-dimensional (3-D) extended field-of-view ultrasound creates a mosaic view from a set of volumes acquired from a dedicated 3-D ultrasound machine combined with a position tracker. A simple compounding technique can be used to combine the volumes together using only the position measurements, but some misalignment remains. Two different registration methods were developed to correct these errors in the overlapping regions. The first method divides the overlap into smaller blocks and warps the blocks to best align the features. The second method is similar, but uses rigid body registration of the blocks. Experiments in vitro and in vivo showed that block-based registration with warping produced the most reproducible results and the greatest increase in similarity among the overlapping regions. It also produced the best reconstruction accuracy, with a mean distance error of 0.4 mm measured across 101.78 mm in a phantom, representing 0.4% error.

Female↗

PubMed2005

Comparison of calibration methods for spatial tracking of a 3-D ultrasound probe.

Real-time 3-D ultrasound (US) is a new-generation US system that uses a dedicated probe to create volume data sets instead of standard 2-D cross-sectional images. For applications in image-guided surgery and radiation therapy, a position tracker is added to the probe so that the volumes can be located in space. Calibration plays a critical role in determining the overall accuracy of an US volume-tracking system. In this paper, three calibration methods are developed specifically for 3-D probes. The three methods are based on a IXI-shaped wire phantom, a cube phantom and a stylus. The performance of each method was evaluated in terms of calibration reproducibility, point accuracy and reconstruction accuracy by distance measurement. The mean errors in the reproducibility tests were 1.50 mm (IXI-wire), 1.16 mm (cube) and 5.13 mm (stylus). The root mean square errors of the point accuracy measure were 2.15 mm (IXI-wire), 4.91 mm (cube) and 2.36 mm (stylus). The root mean square errors of the reconstruction accuracy by distance measure were 1.52 mm (IXI-wire), 1.59 mm (cube) and 1.85 mm (stylus). Overall, the IXI-wire phantom achieved the best results.

Algorithms↗

PubMed2004

Two-dimensional spatial compounding with warping.

Spatial compounding aims to improve image quality through signal averaging, but speed-of-sound (SoS) and refraction errors can misalign the component frames and blur the compound image. A 2-D compounding system is demonstrated that uses a nonrigid registration (warping) to realign the frames before compounding. Block-based estimates of local misalignments are interpolated smoothly to compute the warp vectors. Simulations and a specialized phantom, both with a 9% SoS distortion, were created, and compound images with and without warping were compared to the conventional image. Image sharpness was compared by measuring the diameter of point targets and directional edge sharpness. The average registration accuracy was 0.06 to 0.07 mm (approximately one pixel). The diameter of point targets increased only 2% with warping vs. 32% without warping and directional edge sharpness dropped 3.7% vs. 20.0%. Furthermore, most of the speckle reduction due to compounding is retained when warping is used. The tests on simulated and phantom data demonstrate that the method is capable of making a small, but significant, improvement to image quality. The examinations in vitro and in vivo show the correct operation of the method with real tissue features. Further clinical studies should be performed to compare spatial compounding with and without warping to see which applications would benefit from the small improvement.

Animals↗

PubMed2004

Surface extraction with a three-dimensional freehand ultrasound system.

This paper presents a system for acquiring three-dimensional ultrasound data and extracting surfaces of the examined structures. The acquisition is performed freehand with a PC-based two dimensional ultrasound machine and an optical tracker. The extraction of surfaces from ultrasound data are normally inhibited by speckle, shadowing and gaps in the acquired data. A new method is developed that extracts a surface directly from the irregularly spaced, noisy freehand ultrasound data. The freehand data are first interpolated with radial basis functions and then a mesh is formed along an isosurface of the functional interpolation. The ability of radial basis functions to smooth speckle and interpolate across gaps is demonstrated on a series of experiments with phantoms and human tissue in a water bath. The geometry of the extracted surface matches the external measurements with an average difference ranging from 0.8 to 2.9 mm. These differences are within the range of errors from calibration, resolution and landmark localization. The experiments also show the ability to create continuous and realistic surfaces from scans that require multiple sweeps over a structure.

Animals↗