Bjorn Olstad

Volume rendering of 3D medical ultrasound data using direct feature mapping

The authors explore the application of volume rendering in medical ultrasonic imaging. Several volume rendering methods have been developed for X-ray computed tomography (X-CT), magnetic resonance imaging (MRI) and positron emission tomography (PET). Limited research has been done on applications of volume rendering techniques in medical ultrasound imaging because of a general lack of adequate equipment for 3D acquisitions. Severe noise sources and other limitations in the imaging system make volume rendering of ultrasonic data a challenge compared to rendering of MRI and X-CT data. Rendering algorithms that rely on an initial classification of the data into different tissue categories have been developed for high quality X-CT and MR-data. So far, there is a lack of general and reliable methods for tissue classification in ultrasonic imaging. The authors focus on volume rendering methods which are not dependent on any classification into different tissue categories. Instead, features are extracted from the original 3D data-set, and projected onto the view plane. The authors found that some of these methods may give clinically useful information which is very difficult to get from ordinary 2D ultrasonic images, and in some cases renderings with very fine structural details. The authors have applied the methods to 3D ultrasound images from fetal examinations. The methods are now in use as clinical tools at the National Center of Fetal Medicine in Trondheim, Norway.

Dynamic three-dimensional freehand echocardiography using raw digital ultrasound data.

In this paper, we present a new method for simple acquisition of dynamic three-dimensional (3-D) ultrasound data. We used a magnetic position sensor device attached to the ultrasound probe for spatial location of the probe, which was slowly tilted in the transthoracic scanning position. The 3-D data were recorded in 10-20 s, and the analysis was performed on an external PC within 2 min after transferring the raw digital ultrasound data directly from the scanner. The spatial and temporal resolutions of the reconstruction were evaluated, and were superior to video-based 3-D systems. Examples of volume reconstructions with better than 7 ms temporal resolution are given. The raw data with Doppler measurements were used to reconstruct both blood and tissue velocity volumes. The velocity estimates were available for optimal visualization and for quantitative analysis. The freehand data reconstruction accuracy was tested by volume estimation of balloon phantoms, giving high correlation with true volumes. Results show in vivo 3-D reconstruction and visualization of mitral and aortic valve morphology and blood flow, and myocardial tissue velocity. We conclude that it was possible to construct multimodality 3-D data in a limited region of the human heart within one respiration cycle, with reconstruction errors smaller than the resolution of the original ultrasound beam, and with a temporal resolution of up to 150 frames per second.

Efficient partitioning of sequences

We consider the problem of partitioning a sequence of n real numbers into p intervals such that the cost of the most expensive interval, measured with a cost function f is minimized. This problem is of importance for the scheduling of jobs both in parallel and pipelined environments. We develop a straightforward and practical dynamic programming algorithm that solves this problem in time O(p(n-p)), which is an improvement of a factor of log p compared to the previous best algorithm. A number of variants of the problem are also considered. >

Real-time display of ultrasound in slow motion

A system and method for acquiring ultrasound information at an acquisition rate and displaying at least a portion of the acquired ultrasound information at a display rate that is slower than the acquisition rate is disclosed. Ultrasound information may be continuously acquired and stored at a frame-rate that is greater than the perception rate of the human eye. At least a portion of the acquired ultrasound information is displayed at a frame-rate that allows human perception. Acquisition and display are synchronized from time-to-time upon satisfaction of a synchronization condition. The synchronization condition may be related to a predetermined time interval or a triggering event generated by or through triggering generated by, for example, a physiological event detected in, for example, an ECG trace. Acquired ultrasound information is, thus, displayed in a real-time slow motion manner that maintains real-time synchrony and yet provides a display rate that is lower than the acquisition rate and preferably lower than the maximum perception rate of the human eye.

Visualizing 4-D medical ultrasound data

Different standard rendering methods applied to 4-D medical ultrasound data are discussed. In particular, maximum value projection, sum of values projection, transparent gray level gradient shading, and surface shading have been tested. Due to the fact that ultrasound data suffer from a low signal to noise ratio, image processing and image analysis are used to enhance and classify the volumetric data set.<<ETX>>

Combined visualization of tumor and vessel geometry

In vivo studies of the liver function have been conducted. The data acquisitions have utilized the TomTec Echoscan system to obtain 3D tissue data in addition to ECG and respiration triggered 4D angio and tissue data. Several different algorithmic tools were developed for visualization of tumor and vessel geometry. A fuzzy region growing technique was developed to segment the liver into different anatomical parts. A filtering algorithm was developed to smooth out local intensity variations within the vessels. Different volume rendering techniques were evaluated for visualization

Annular array multiplanar TEE probe allowing 3D reconstruction of cineloops of the heart

Present transesophagus probes are giving image sections that are either normal to or along the scope axis [1]. By bending the scope tip, this gives some flexibility in selection of the scan plane, but many scan plane directions are still not possible to obtain from the esophagus. A transesophagus probe where the scanplane can be rotated continuously will solve much of this problem. To match these needs we have designed an annular array TEE probe, where the scanplane can be rotated continuously within 180 degrees. This also allows collection of 3D ultrasound data within a cone with adjustable opening angle up to 90 degrees. The annular array do also have some advantages for CW Doppler measurements, and the symmetric focus gives improved lateral resolution as discussed in [2].

Detection of the myocardial boundary in the left ventricle from simultaneously acquired triplane ultrasound images using multi view active appearance motion models

We report a new algorithm for detecting the LV myocardial boundary from simultaneously acquired triplane US image sequences using Multi View Active Appearance Motion Models. Coupled boundary detection in three planes can po- tentially increase the accuracy of LV volume measurements, and also increase the robustness of the boundary detection over traditional methods. A database of triplane image sequences from full cardiac cycles, including the standard A4CH, A2CH, and ALAX views were established from 20 volunteers, including 12 healthy persons and 8 persons suffering from heart disease. For each dataset the LV myocardial boundary was manually outlined, and the ED and ES frames were determined visually for phase normalization of the cycles. The evaluation of the MVAAMM was performed using a leave one out approach. The mean point distance between manually and automatically determined contours were 4.1±1.9 mm, the volume error was 7.0±14 ml, and fractional volume error was 8.5±16 %. Volume detection using the automatic method showed excellent correlation to the manual method (R 2 =0.87). Common ultrasound artefacts such as dropouts were handled well by the MVAAMM since the detection in the three image planes were coupled. The views with the largest point distance had one or more foreshortened views. A larger training database may improve the performance in such cases.

Knowledge based extraction of the left ventricular endocardial boundary from 2D echocardiograms

Extraction of the endocardial boundary of the left ventricle is a key challenge in cardiac ultrasound imaging. The cardiac anatomy may be difficult to determine automatically without incorporating knowledge of both wall shape and intensity signature into the detection algorithm. The aim of this study is to establish a framework for knowledge based extraction of the left ventricular endocardial boundary. The method is based upon the Snake algorithm where internal and external energy terms are combined into a Snake energy. Instead of using the patient image directly for calculation of the external energy we propose to use the correlation between geometrically normalized images, from the patient and from the database. The ventricular shapes from the database cases are used to compute the internal energy term. One boundary is detected for each case, hence a selection criterion is required. The total Snake energy is evaluated for this purpose and compared to manual selection of the best case. As a preliminary verification of the framework, the ventricular end diastolic and end systolic areas and the ventricular ejection fraction were calculated from the detected boundaries for a set of patient cases, using both manual and automatic database case selection. Using manual case selection, the results are encouraging, but the total Snake energy did not provide a sufficiently robust selection criterion. The strength of the proposed method is its ability to utilize expert knowledge directly for extraction of the endocardial boundary from ultrasound data. Using manual selection of the best case, the calculated parameters from detected boundaries were in good agreement with manual delineation. Further work is required to find a robust selection criterion.

Image filtering techniques and VLSI architectures for efficient data extraction in shell rendering

This paper presents an approach to volume rendering based on real-time and interactive data reduction prior to volume visualization. The underlying hardware design of a PCI based search engine is introduced including the architecture of a full custom VLSI chip that based on combinations of general range queries performs a real-time classification/opacity assignment of the multi-spectral voxel data. Various image preprocessing techniques are presented. These techniques are used to enhance the potential of the subsequent real-time data extraction, In particular, we describe how the real-time data extraction facility can be utilized to develop interactive inspection procedures for 3D imagery. The numerical experiments include 3D ultrasonics and 3D MRI studies.