Paul R. Detmer

3D ULTRASONIC IMAGE FEATURE LOCALIZATION BASED ON MAGNETIC SCANHEAD TRACKING: IN VITRO CALIBRATION AND VALIDATION

The basis of a three-dimensional (3D) ultrasound imaging system was constructed from a commercially available magnetometer-based position and orientation measurement (POM) device, a standard B-Mode ultrasound instrument and a personal computer. To evaluate the systems performance, a novel method was devised using an iterative, least-squares technique to simultaneously determine the systems calibration parameters and measure its precision in locating points in three-dimensional space. When tested separately, the POM system located single points with a root mean squared (RMS) uncertainty of from 1.4 mm to 3.2 mm over the 610 mm working radius of the system. When combined with the ultrasound instrument, the RMS uncertainty in locating point targets varied from 2.1 mm to 3.5 mm. These results establish the lower limits of variability to be expected from this system when locating fiducial anatomical landmarks for repeated examinations of the same region of the body, and when making numerical 3D reconstructions from multiple planar images.

Carotid Plaque Morphology and Clinical Events

BACKGROUND AND PURPOSE Studies have suggested that B-mode ultrasonography can be used to determine carotid plaque composition and that specific plaque characteristics are associated with a worse clinical outcome. However, histological studies examining the relationship between carotid plaque morphology and clinical outcome have reported conflicting findings. Furthermore, few investigators have described plaque morphology in quantifiable terms. This study examines the association between the volume of carotid plaque constituents and preoperative ischemic neurological symptoms. Constituents examined were chosen based on their potential for identification by current diagnostic imaging modalities such as ultrasound or MRI. METHODS Atherosclerotic plaques from 43 patients undergoing carotid endarterectomy were examined histologically, with sections obtained every 0.5 to 1 mm. The lesions were examined for the presence and quantity of fibrous intimal tissue, intraplaque hemorrhage, lipid core, necrotic plaque core, and calcification. The quantity of each constituent was compared in plaques removed from symptomatic patients with those excised from asymptomatic individuals. Differences were analyzed with a Kolmogorov-Smirnov statistic. RESULTS There was no difference between plaques removed from asymptomatic and symptomatic patients with regard to the presence and volume of fibrous intimal tissue, intraplaque hemorrhage, the lipid core, the necrotic core, or calcification. CONCLUSIONS In patients with highly stenotic carotid lesions who are undergoing carotid endarterectomy, gross plaque composition is similar regardless of preoperative symptom status. Given this similarity, it is unlikely that differences in the volume of intraplaque hemorrhage, lipid core, necrotic core, or calcification in atherosclerotic carotid plaques explain their embolic history.

Performance of a miniature magnetic position sensor for three-dimensional ultrasound imaging

A miniature magnetic position sensor used for three-dimensional ultrasound imaging was tested for precision and accuracy in vitro. The sensor alone was able to locate points with root-mean-square (rms) uncertainty of 1.7 mm and accuracy of 0.05 +/- 0.62 mm over its specified operating range of 50 cm. With an ultrasound imaging system, a point was located from arbitrary viewing windows with 2.4-mm rms uncertainty and 0.06 +/- 0.68 mm accuracy. If viewing windows were limited to those representative of a typical ultrasound examination, the system could achieve rms uncertainty in point location of < 1 mm. Performance was not affected by operation of the imaging system when the sensor was mounted on an ultrasound scanhead. Sensitivity to metals in the operating environment was also measured.

Matched filter identification of left-ventricular endocardial borders in transesophageal echocardiograms

The use of one-dimensional spatial matched filtering for identifying the left ventricular endocardial borders in human transesophageal echocardiograms recorded during surgery is investigated. A maximum-likelihood method was used to choose the endocardial intensity profiles centered within the ventricle. The computer-generated border points were compared to those identified by an experienced ultrasonographer. A 16-pixel step template located 63.2% of the border points within 2 mm of the manual border. Median prefiltering of the images reduced detection accuracy by 3% to 6%. No statistically significant difference in accuracy was found between longer and shorter templates or between data-derived and step templates. Compared to manual estimates, computer generated cross-sectional area determinations were correlated with a coefficient of 0.93. Matched filtering executes rapidly, does not require prefiltering, and performs as well as other reported methods in estimating ventricular area.

Three-dimensional ultrasound imaging using multiple magnetic tracking systems and miniature magnetic sensors

The authors have used several configurations of a commercial 6D magnetic position and orientation measurement system to track an ultrasound scanhead and reconstruct 2D ultrasound images in 3D space. They have measured precision and accuracy with two magnetic sensors working in parallel, and with a miniaturized magnetic sensor. The use of two standard size sensors has shown limited improvement in precision (1.5 mm RMS uncertainty compared with 1.8 mm for a single sensor). The miniature sensor has an average precision of 2.1 mm over its operating range. The miniature sensor has been calibrated with an imaging system and has been used in 3D reconstructions of the stomach and the carotid artery.

Ventricular volume measurement from a multiplanar transesophageal ultrasonic imaging system: an in vitro study

A system to assess the feasibility of using multiple transesophageal ultrasonic images to measure left-ventricular volume is discussed. The system includes a special transesophageal probe with a micromanipulator for acquiring cardiac images in multiple planes with known interplanar spatial relationship and an offline processing system to compute the volume. In vitro studies with the probe demonstrated that the distance between two targets in space can be identified within 2 mm ( sigma =0.4 mm) for points in the imaging plane and 3.4 mm ( sigma =0.5 mm) for points not lying in the imaging plane. This gives an average accuracy of +or-6.5% for distances greater than 4.5 cm. Comparison of ultrasonic measurements of the volume of water-filled balloons and excised hearts to the volume required to fill them revealed a correlation coefficient of 0.992, a regression line with a slope of 1.0 and an ordinate intercept at 0.2 mL, and a standard error of the estimate of 8 mL.<<ETX>>

Echolucent regions in carotid plaque: Preliminary analysis comparing three-dimensional histologic reconstructions to sonographic findings☆

This study compares sonographic and histologic findings within defined spatial regions in carotid artery plaque, using computer generated three-dimensional reconstructions. Twenty-four patients (14 asymptomatic, 10 symptomatic) with angiographically documented 70% to 99% carotid artery stenosis were examined with ultrasonic B-mode imaging prior to endarterectomy. Using a standardized protocol for instrument set-up and scanning technique, echolucent regions in the plaque were identified. After endarterectomy, each plaque was sectioned at 0.5 to 1.0 millimeter increments throughout its length. Sites containing intraplaque hemorrhage, cholesterol clefts, foam cells, necrotic cores, dense calcification and speckled calcification were identified. These areas were outlined on a template, digitized and imported into a computer program that created three-dimensional reconstructions of the histologic findings. Each carotid plaque was divided into quadrants for analysis: (1) lateral wall proximal to the common carotid bifurcation (flow divider); (2) medial wall proximal to the flow divider; (3) lateral wall distal to the flow divider; and (4) medial wall distal to the flow divider. The odds of finding intraplaque hemorrhage, foam cells, necrotic cores and speckled calcification were significantly higher in quadrants with an echolucent region identified by ultrasonography (odds ratio (95% confidence interval) for intraplaque hemorrhage = 3.5 (1.4-8.6); foam cells = 4.0 (1.6-9.9); necrotic cores = 3.2 (1.2-8.4); speckled calcification = 4.0 (1.6-9.8). This preliminary analysis demonstrates the potential of these newly developed techniques for comparing ultrasonic imaging to histology.

Carotid artery intraplaque hemorrhage and stenotic velocity.

Background and Purpose One of the proposed mechanisms for sudden expansion of a carotid bifurcation plaque is hemorrhage within the lesion. It has been postulated that the sudden increase in plaque size will acutely reduce blood flow to the ipsilateral hemisphere and induce either a transient ischemic attack or a stroke. In this study, the relation between peak systolic velocity at the site of narrowing and its potential role in the development of intraplaque hemorrhage were investigated. Methods Ten patients who had carotid endarterectomy were examined by duplex Doppler sonography before surgery to determine the peak systolic velocity at the site of maximal narrowing. The excised carotid plaques were sectioned at 1-mm intervals and examined for histological evidence of intraplaque hemorrhage. The recorded peak systolic velocities in patients with intraplaque hemorrhage were compared with the velocities in cases in which no hemorrhage was identified. Results Five of the ten patients had intraplaque hemorrhage. Four of the five patients with intraplaque hemorrhage had a peak systolic velocity of >420 cm/sec and diastolic velocities of >16 0 cm/sec; none of the patients without intraplaque hemorrhage had such high values. Conclusions Peak systolic velocity is significantly higher in patients with intraplaque hemorrhage. The specificity and sensitivity of a peak systolic velocity of >420 cm/sec in predicting intraplaque hemorrhage remains to be determined.

A miniature position and orientation locator for three dimensional echocardiography

The accuracy of a new miniature prototype (6 mm/spl times/6 mm/spl times/9 mm) six dimensional (6D) locator sensor for three dimensional (3D) echocardiography was investigated. This new sensor identifies its 3D position and 3D orientation in space with respect to the position of a magnetic field generator. The sensor when attached to an ultrasound imaging transducer allows identifying and tracking the position and orientation in space of the ultrasonic imaging plane. The accuracy of this new miniature sensor was tested by rotating it on a lever arm apparatus which maintained a constant distance (R) from the sensor to the fixed point of rotation. At each rotational spatial position the sensors readings were used to calculate the fixed points position in space. The mean (MRE) and standard deviation (STDRE) of the radial error in this determination were calculated for N trials. The results with the apparatus: for R=10.6 cm were MRE=1.3 mm, STDRE=0.06 mm, N=500; and for R=18 cm they were MRE=1.8 mm, STDR=0.7 mm, N=500. It was then tested with a transcutaneous 5 MHz echo probe on 3D imaging a fixed position string target. The results in locating the position of the target was MRE=2.2 mm and STDRE=1.5 mm for N=45.<<ETX>>

Ultrasound determination of total arterial wall thickness

PURPOSE Ultrasonic measurement techniques for determining intima-media thickness and total arterial wall thickness have been described. The intima-media thickness measurements are currently in use in large epidemiologic trials. Intima-media thickness does not evaluate extramedial atherosclerotic change and so may not fully reflect pathologic changes in the arterial wall. METHODS After we performed variability studies of B-mode image acquisition and measurement, we measured total wall thickness and intima-media thickness of the common carotid arteries in 60 adult subjects in three groups: a control group aged 20 to 29 years, a control group aged 60 to 79 years, and a claudication group aged 60 to 79 years. Measurements were made with B-mode ultrasound images. RESULTS No statistical difference between sexes was noted. A statistically significant (p < or = 0.05) increase in intima-media thickness and wall thickness was found with increasing age, and an additional increase was observed with clinically significant lower extremity arterial occlusive disease (p < or = 0.05). Image quality had an effect on measurement accuracy. CONCLUSIONS The finding that the wall thickness of common carotid arteries is increased in those patients with clinically significant lower extremity disease supports the theory that atherosclerosis affects the arterial wall in a systemic fashion. Because intima-media thickness also increases across subject groups without change in its proportional contribution to the total arterial wall thickness, extramedial arterial changes also occur with aging and the development of atherosclerosis. We propose that because increases in wall thickness measurements of common carotid arteries follow intima-media thickness increase (but do not necessarily measure the same physiologic change) and the wall thickness method can be used in cases when the intima-media thickness cannot be measured, arterial wall thickness measurement may serve as an alternate or confirmatory test of peripheral artery atherosclerotic severity.