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Richard Prager

Publications and source records attributed to Richard Prager.

5 recordsLinked to original sources
PubMed2006

Modeling ultrasound imaging as a linear, shift-variant system.

We solve the equation that governs acoustic wave propagation in an inhomogeneous medium to show that the radio-frequency (RF) ultrasound signal can be expressed as the result of filtering the scatterer field with a point-spread function. We extend the analysis to make the link between the RF ultrasound signal and the representation of ultrasound scatterers as vectors with small magnitude and random phase in the complex plane. Others have previously performed parts of this analysis. The contribution of the present paper is to provide a single, coherent treatment emphasizing the assumptions that have to be made and the physical consequences of the models derived. This leads to insights into the interaction of monopole and dipole scattering, useful techniques for simulating and analyzing speckle statistics in the complex plane and a new expression for the normalized covariance of the analytic RF ultrasound signal in terms of the complex envelope of the point-spread function.

Algorithms↗
PubMed2005

Ultrasound attenuation measurement in the presence of scatterer variation for reduction of shadowing and enhancement.

Pulse-echo ultrasound display relies on many assumptions that are known to be incorrect. Departure from these makes interpretation of conventional ultrasound images difficult, and three-dimensional (3-D) visualizations harder still. For instance, shadowing and enhancement are the result of an incorrect assumption that sound attenuation is a function only of depth. Attempts to reduce such artefacts by estimating attenuation locally have been frustrated by large statistical variations and the influence of scatterer type. We address the latter by examining the influence of scatterer type on two existing attenuation estimation algorithms. This analysis is novel for one of the algorithms, and contains a correction to previously published work for the other. We then propose a novel algorithm that is less sensitive to scatterer variation. We also present a novel technique for handling large statistical variations based on combined assumptions of monotonicity and smoothness. We then assess the performance of each algorithm for correcting shadowing and enhancement in in vitro data, using a real time 3-D radio frequency (RF) ultrasound acquisition system developed for this purpose. The results show visible differences in attenuation estimates from each technique, which are supported by the theoretical analysis. The novel attenuation estimation algorithm does show less sensitivity to scatterer variation, though it results in a more noisy estimate. Nevertheless, the novel technique for reducing statistical variations is sufficient to allow some degree of correction of shadowing and enhancement in each case.

Algorithms↗
PubMed2005

Statin initiation following coronary artery bypass grafting: outcome of a hospital discharge protocol.

STUDY OBJECTIVES: To evaluate the outcome of a hospital discharge statin drug therapy initiation protocol following coronary artery bypass graft (CABG) surgery. Our goal was to measure the percentage of patients receiving statin drugs at hospital discharge and at a post-hospital discharge assessment following the implementation of the statin drug therapy initiation protocol. We also evaluated low-density lipoprotein cholesterol (LDL-C) goal attainment (ie, < 100 mg/dL), safety monitoring, and tolerability of the statin drug. DESIGN: Single-center, observational study with a historical control group. SETTING: University-affiliated health system with a comprehensive heart care program that included a 14-bed cardiac surgery ICU. Approximately 400 CABG procedures are performed annually. PATIENTS: Patients who underwent CABG surgery were eligible for inclusion in the study. The exclusion criteria were as follows: contraindications to statin therapy; refusal to take a statin drug; refusal to give informed consent; and age < 18 years. INTERVENTION: A protocol was implemented to recommend treatment with a statin drug at hospital discharge in all post-CABG surgery patients if the presurgical LDL-C level was > 100 mg/dL or the patient was receiving a statin prior to hospital admission. The protocol also included a presurgical assessment of lipoprotein levels and hepatic function. All cardiac surgery staff were educated regarding the specifics of the protocol. RESULTS: A total of 403 patients were included in the study. The historical control group (202 subjects) and the intervention group (201 subjects) were similar with respect to gender, age, and baseline lipoprotein levels. The follow-up assessment interval was approximately 6 months in both groups. Overall, patients were more likely to receive a statin at hospital discharge in the intervention group compared to the control group (relative risk [RR], 1.6; 95% confidence interval [CI], 1.3 to 2.0). Attainment of the goal for LDL-C level was similar between the intervention and control groups in the overall sample. Patients who were not at their LDL-C goal at baseline were more likely to have a follow-up LDL-C level of < 100 mg/dL in the intervention group (RR, 1.9; 95% CI, 1.0 to 3.5). The rate of liver function assessment was similar in the control and intervention groups. No patients in either group experienced elevations of alanine aminotransferase levels that were more than three times the upper limit of normal, and no cases of muscle toxicity were noted. CONCLUSION: The initiation of therapy with a statin drug at hospital discharge following CABG surgery was associated with increased utilization rates. The LDL-C goal attainment improved in patients who were not at their goal prior to surgery. However, the persistence of medication use declined within 6 months. Statin therapy initiation was well-tolerated in this cohort of patients.

Adult↗
PubMed2002

Freehand 3D ultrasound without voxels: volume measurement and visualisation using the Stradx system.

This paper describes recent developments in the Stradx freehand 3D ultrasound system. Unlike other systems, Stradx works directly from the raw B-scans and their relative positions, without first resampling this unstructured data onto a regular voxel array. The paper discusses the many advantages of this unique approach, and presents new developments in the visualisation and quantitative analysis of freehand 3D ultrasound data, including novel volume rendering schemes and a robust method to compensate for the effects of varying probe pressure.

Artifacts↗