L.S. Wilson

Spectral tissue strain: a new technique for imaging tissue strain using intravascular ultrasound.

Spectral tissue strain (STS) is a new technique for measuring and imaging tissue strain from a set of images using intravascular ultrasound. The technique is based on the Fourier scaling property and uses the chirp z-transform (CZT) to estimate strain within the vessel walls. Some preliminary results, both in vitro and in vivo, are described. A novel display technique has also been developed for encoding radial strain and displaying the resulting colour map as an overlay on the original image.

Preliminary results from attenuation-slope mapping of plaque using intravascular ultrasound

Excised femoral and iliac artery segments have been examined with 20 MHz intravascular ultrasound followed by histological assessment. During the ultrasound examinations, radio-frequency (RF) data were recorded digitally, and used for calculating local values of attenuation slope throughout the tissue, using a frequency-domain technique. The RF data were also reconstructed as conventional ultrasound images, and the attenuation-slope information presented as a threshold colour overlay. Areas identified as degenerative plaque in the histological assessments were usually found to correspond to areas of high attenuation slope, and were clearly identified from the pattern of colours on the combined image. Some examples are presented, illustrating the appearance of various pathologies imaged by this technique.

Elastography - the movement begins

The advent of real-time ultrasound in the 1970s, together with a growing interest in tissue characterization, led to a number of investigators using the nature of tissue motion to distinguish healthy from diseased tissue. Our group at the (then) Ultrasonics Institute demonstrated the use of phase methods for detecting very small tissue motions, using natural stimuli. The method could also be applied in the lag (autocorrelation) domain to directly measure the amount of deformation to high accuracy. This method was also applied to measuring the amount of dilatation of blood vessels using both conventional and intravascular ultrasound. A basic limitation of these techniques was the poor spatial resolution, and quasistatic methods soon replaced this method of measuring tissue deformation. However, a new way of assessing the health of tissues had been established.

Pulse-echo ultrasound speed measurements: Progress and prospects

The data on the relationship of sound speed to tissue condition, and the development of methods for measurement of sound speed in vivo, are outlined. The methods developed by the authors are discussed. These include methods which use the spatial shift in images of targets viewed from different directions, time-of-flight measurements along incrementally tracked beams and echo tracking during transaxial compression of tissue. The reported sound-speed values are discussed, discrepancies noted and suggestions made on the potential clinical applications of in vivo sound-speed estimation.

The effect of variations in transducer position and sound speed in intravascular ultrasound: A theoretical study

The intravascular insonation of a blood vessel in the presence of an impedance interface between blood and the inner vessel wall is studied theoretically. The model, which uses a ray approximation, is three dimensional and allows consideration of arbitrary noncircular lumen shapes. Model results are presented for the image geometry, and the insonating intensity over the vessel wall. It is shown that the inner lumen can be imaged accurately with the transducer at any position within the lumen, and at any forward viewing angle, provided the point of origin of the beam is stationary. If it is not stationary but rotating with the same angular velocity as the beam itself, the inner vessel wall is not mapped accurately. A particular geometric distortion which has been observed in practice is predicted if the transducer is near vessel wall. Acoustic impedance interfaces will be encountered in vascular disease because the speed of sound in fatty plaque is less than in blood, whereas the speed of sound in fibrous and calcified plaque is greater than in blood. A simplified model representation of an atherosclerotic lumen in developed using a cardioid-like curve and a single impedance interface. Model results show that refraction at this interface leads to an intensity distribution which is not uniform around the lumen, and which depends on lumen shape and transducer position. The exception is the special case of a circular lumen with a centrally positioned transducer. Noncircular impedance interfaces encountered in vivo in vascular disease may cause considerable intensity distortion, particularly if the transducer is close to the wall in an irregularly shaped lumen.

Model-based assessment of lung structures: inferencing and control system

A general methodology has been developed for computer interpretation of medical images, based on an explicit anatomical model. A test system for analyzing posterior- anterior (PA) chest x-rays has been implemented. The inferencing and control system identifies the major lung structures in the image, and then flags any suspected abnormalities. Image and model data are transformed into a feature space where they are represented in terms of edge descriptions. The inference engine compares the image and model in feature space to label the edges anatomically, and check for normality. The control system schedules events within the inference engine and coordinates interaction with the model and image processing routines. The control architecture is blackboard-based, with a separate data frame for each structure to be identified. The anatomical model uses fuzzy sets to provide ranges of feature values which are considered normal or indicative of a particular abnormality. This allows the inference engine to give a confidence score and linguistic description to each decision. Mediastinum, cardiac border, domes of the diaphragm, ribs and lung outline have been modeled. Their automatic identification allows diagnostic checks such as the cardiothoracic ratio, comparison of right and left lungs to identify lobular collapse and inspection of interfaces in terms of shape and clarity. The inference engine provides simple comments on its findings, making it suitable for pre- and double-checking of images.

Development of a Multi-Feature Cad System for Mammography

Routine screening mammography for the early detection of breast cancer in women over the age of 50 is now widespread in many countries including Australia. The introduction of these screening programs has stimulated research into computer systems which may assist radiologists examining these mammogram images.

Automatic vessel tracking and measurement for Doppler studies.

In this initial report, an imaging technique is described for automatically recognising the walls of a blood vessel during Doppler measurements, and hence automatically adjusting the position of the sample volume. The method reduces the inherently two-dimensional edge detection problem to a one-dimensional one, and iterates over several imaging frames to optimise the information. The orientation and diameter of the vessel are also measured. The technique has been clinically tested off-line and is particularly applicable to Doppler volumetric flow studies.

The CSIRO hospital without walls home telecare system

Home telecare uses a number of telemedicine technologies used for caring for patients in their homes rather than in the hospital system. To provide continuous monitoring of vital signs with minimal disruption of a normal lifestyle, we have developed an ultra-low power wireless and sensing system which may be installed in the home. The system is built around a two-way, 2.4 GHz radio system including miniature patient-worn units. A PC and small base station in the home collect and upload information to a monitoring centre at regular intervals, or in response to defined events. The system is designed to be installed in a home for periods of monitoring as a replacement for more expensive institutional care. The initial clinical application is a group of elderly patients who have presented following a number of idiopathic falls. In order to both detect a fall and determine its cause (which may be cardiovascular), the wearable radio system is equipped with accelerometers and equipment for measuring heart rate. A web-based distributed information system for caregivers incorporates automatic recognition of events or trends requiring intervention. The system is currently undergoing preliminary clinical trials.

Segmentation of medical images using explicit anatomical knowledge

Knowledge-based image segmentation is defined in terms of the separation of image analysis procedures and representation of knowledge. Such architecture is particularly suitable for medical image segmentation, because of the large amount of structured domain knowledge. A general methodology for the application of knowledge-based methods to medical image segmentation is described. This includes frames for knowledge representation, fuzzy logic for anatomical variations, and a strategy for determining the order of segmentation from the modal specification. This method has been applied to three separate problems, 3D thoracic CT, chest X-rays and CT angiography. The application of the same methodology to such a range of applications suggests a major role in medical imaging for segmentation methods incorporating representation of anatomical knowledge.