Ronald A. Robinson

Hyperbaric Dye Solution Distribution Characteristics after Pencil-point Needle Injection in a Spinal Cord Model

Background: The flow-rate limiting and directional characteristics of caudally directed microcatheters, which lead to intrathecal maldistribution of hyperbaric 5% lidocaine, are believed to have contributed to at least 11 cases of cauda equina syndrome. The authors investigated the distribution characteristics of hyperbaric dye solutions via caudally directed side-port needles at various rates of injection in a spinal cord model to determine the potential for maldistribution. Methods: Using a digital video image processing technique, we injected a hyperbaric solution of phthalocyanine blue dye through caudally directed side-port needles into a supinely oriented transparent spinal canal model filled with simulated cerebrospinal fluid. Injections via commonly used spinal needles (24-gauge and 25-gauge Sprotte, and 25-gauge and 27-gauge Whitacre) were recorded using five injection rates (2, 4, 6, 8, and 16 ml/ruin). Results: For all needles tested, injection rate had a significant effect on the peak dye concentration (P < 0.0001). Injection rates 6 ml/ruin (2 ml/20 s) resulted in peak dye concentrations of less than 168 mg/1 (extrapolated concentration of 1% lidocaine). Injection via the 24-gauge Sprotte needle, which has a larger orifice area and internal diameter, resulted in significantly lower peak dye concentrations than via the smaller Whitacre needles tested (P < 0.05). Conclusions: Sacral maldistribution could be minimized by using injection rates < 6 ml/min (2 ml/20 s), for all of the side-port spinal needles used in this model study. When very slow injection rates (2 ml/min) are used, peak dye concentrations varied inversely and significantly with needle internal diameter and orifice area.

In vitro modeling of spinal anesthesia. A digital video image processing technique and its application to catheter characterization.

BackgroundMaldistribution of intrathecal local anesthetic has recently been implicated as a contributor to neurotoxic injury. In vitro modeling can be used to understand the distribution of anesthetic agents within the subarachnoid space. We describe an in vitro modeling technique that uses digital video image processing and its application to catheter injection of local anesthetic. MethodsA clear plastic model of the subarachnoid space, including a simulated spinal cord and cauda equina, was filled with lactated Ringers solution. Phthalocyanine blue dye of known concentration was injected into the model through small-bore (28-G) and large-bore (18-G) catheters. Injections were performed at a variety of controlled rates and sacral catheter positions, and the propagation of dye throughout the model was recorded on videotape, digitized by computer, and converted to a two-dimensional image of dye concentration. A subset of data was compared with results obtained from spectrophotometric analysis. ResultsThere was a strong correlation (r = 0.98) between data obtained with analysis by digital video image processing and those obtained spectrophotometrically. Catheter size, catheter angle, and injection rate significantly influenced the distribution and peak concentration of simulated anesthetic. No major differences in distribution or peak concentration were observed with the two types of 28-G catheters. ConclusionsThe digital video image processing technique can be used to quantify anesthetic distribution rapidly within a model of the subarachnoid space without disturbing the distribution. The current results demonstrate a strong dependence of anesthetic distribution on catheter angle, catheter size, and injection rate. Comparisons between 28-G catheters suggest that the difference in reported incidence of cauda equlna syndrome associated with different 28-G catheters cannot be explained on the basis of differences in anesthetic distribution

Characterization of high intensity focused ultrasound transducers using acoustic streaming

A new approach for characterizing high intensity focused ultrasound (HIFU) transducers is presented. The technique is based upon the acoustic streaming field generated by absorption of the HIFU beam in a liquid medium. The streaming field is quantified using digital particle image velocimetry, and a numerical algorithm is employed to compute the acoustic intensity field giving rise to the observed streaming field. The method as presented here is applicable to moderate intensity regimes, above the intensities which may be damaging to conventional hydrophones, but below the levels where nonlinear propagation effects are appreciable. Intensity fields and acoustic powers predicted using the streaming method were found to agree within 10% with measurements obtained using hydrophones and radiation force balances. Besides acoustic intensity fields, the streaming technique may be used to determine other important HIFU parameters, such as beam tilt angle or absorption of the propagation medium.

Portable ultrasonic radiometer

A portable ultrasonic radiometer, for use as a field survey instrument of ultrasonic therapy equipment, employs the radiation force principle and comprises a solenoid balance arm nulling device. One end of the balance arm includes a movable helical coil which moves inside of a fixed helical coil, and the other end supports an airbacked target. Substantially frictionless rotation of the balance arm is achieved through use of a jewel fulcrum. A reference position of the target is predetermined as the null point, and upon application of the ultrasonic power, deflection of the target occurs. Voltage across the moving coil is used to reposition the target at the null point, and the voltage required is an analogue of the total ultrasonic power. A digital read-out system with manual zeroing capability directly displays this power output.

Quantitative estimation of ultrasound beam intensities using infrared thermography—Experimental validation

Infrared (IR) thermography is a technique that has the potential to rapidly and noninvasively determine the intensity fields of ultrasound transducers. In the work described here, IR temperature measurements were made in a tissue phantom sonicated with a high-intensity focused ultrasound (HIFU) transducer, and the intensity fields were determined using a previously published mathematical formulation relating intensity to temperature rise at a tissue/air interface. Intensity fields determined from the IR technique were compared with those derived from hydrophone measurements. Focal intensities and beam widths determined via the IR approach agreed with values derived from hydrophone measurements to within a relative difference of less than 10%, for a transducer with a gain of 30, and about 13% for a transducer with a gain of 60. At axial locations roughly 1 cm in front (pre-focal) and behind (post-focal) the focus, the agreement with hydrophones for the lower-gain transducer remained comparable to that in the focal plane. For the higher-gain transducer, the agreement with hydrophones at the pre-focal and post-focal locations was around 40%.

Time-Resolved Particle Image Velocimetry Measurements with Wall Shear Stress and Uncertainty Quantification for the FDA Nozzle Model

Abstract We present advanced particle image velocimetry (PIV) processing, post-processing, and uncertainty estimation techniques to support the validation of computational fluid dynamics analyses of medical devices. This work is an extension of a previous FDA-sponsored multi-laboratory study, which used a medical device mimicking geometry referred to as the FDA benchmark nozzle model. Experimental measurements were performed using time-resolved PIV at five overlapping regions of the model for Reynolds numbers in the nozzle throat of 500, 2000, 5000, and 8000. Images included a twofold increase in spatial resolution in comparison to the previous study. Data was processed using ensemble correlation, dynamic range enhancement, and phase correlations to increase signal-to-noise ratios and measurement accuracy, and to resolve flow regions with large velocity ranges and gradients, which is typical of many blood-contacting medical devices. Parameters relevant to device safety, including shear stress at the wall and in bulk flow, were computed using radial basis functions. In addition, in-field spatially resolved pressure distributions, Reynolds stresses, and energy dissipation rates were computed from PIV measurements. Velocity measurement uncertainty was estimated directly from the PIV correlation plane, and uncertainty analysis for wall shear stress at each measurement location was performed using a Monte Carlo model. Local velocity uncertainty varied greatly and depended largely on local conditions such as particle seeding, velocity gradients, and particle displacements. Uncertainty in low velocity regions in the sudden expansion section of the nozzle was greatly reduced by over an order of magnitude when dynamic range enhancement was applied. Wall shear stress uncertainty was dominated by uncertainty contributions from velocity estimations, which were shown to account for 90–99% of the total uncertainty. This study provides advancements in the PIV processing methodologies over the previous work through increased PIV image resolution, use of robust image processing algorithms for near-wall velocity measurements and wall shear stress calculations, and uncertainty analyses for both velocity and wall shear stress measurements. The velocity and shear stress analysis, with spatially distributed uncertainty estimates, highlights the challenges of flow quantification in medical devices and provides potential methods to overcome such challenges.

Evaluation of an In Vitro Cardiac Pulse Duplicator

A modified Dynatek model MP-1 cardiac pulse duplicator was evaluated at the FDA laboratories. It was tested to determine the inherent cycle-to-cycle variability in measuring aortic and ventricular pressure, bulk flow and fluid velocities. Four different prosthetic heart valves were used in the aortic position. Measurements were performed to quantify the percent variability, R, and its standard deviation. The results of this evaluation indicated that the variability, R, decreased for pressure and bulk flow as the number, N, of cycles averaged approached 20. Variability then remained constant at approximately 1%. For the fluid velocity measurements, the variability was much larger and decreased exponentially to 3–15% at N = 100 (depending on valve type, heart rate, and probe/valve position). Preliminary testing of a new duplicator design indicated an improved cycle-to-cycle variability for all measured parameters. In addition, reduced ambient noise and vibration (six fold) were noted, along with a highly stable cycle period. As compared to the Dynatek model MP-1, overall reduction of the variability for the new duplicator varied by a factor ranging from 1 to 16. Average value of improvement was a factor of 4:1, depending on the parameter measured and BPM used.

A novel study of mechanical heart valve cavitation in a pressurized pulsatile duplicator.

Submission of data regarding the cavitation potential of a mechanical heart valve is recommended by the U.S. Food and Drug Administration in the device-review process. An acoustic method has long been proposed for cavitation detection. However, the question as to whether such a method can differentiate the cavitation noise from the mechanical closing sound has not been sufficiently addressed. In this study, cavitation near a Medtronic Hall tilting disc valve was investigated in a pressurized pulsatile duplicator. The purpose of pressurizing the testing chambers was to prevent cavitation under a normally cavitating loading condition to isolate the mechanical closing sound. By comparing the sound signals before and after pressurization, some noticeable differences were found between them. In the time domain, the intensity of the sound under a cavitating condition was much higher. In the frequency domain, the energy distribution of a sound signal was distinctively different depending on whether cavitation occurred or not. The valve closing sound had a large amount of energy in the low-frequency range (less than about 25 kHz). When cavitation took place, the sound energy shifted toward the high-frequency range (from 25 to 500 kHz).

Uncertainty Estimations for Particle Image Velocimetry in a Medical Device Analog (Nozzle) Model

Particle Image Velocimetry (PIV) is currently the most widely used and well-established tool for non-invasive flow field velocity measurements, and a valuable method for validating computational fluid dynamics (CFD) models of medical devices. One of the critical steps in the CFD validation process is quantification of the experimental uncertainties. This work utilizes a new uncertainty estimation methodology developed by Charonko et al.1 for quantifying the PIV cross-correlation uncertainties. Uncertainties from experimental sources, including image magnification and acquisition timing, were propagated using Taylor series expansion for PIV data within the FDA benchmark Nozzle model2.Copyright

Techniques for performing quantitative measurements of high intensity focused ultrasound intensity distributions noninvasively.

Estimation of the HIFU intensity field at clinically relevant power levels can be difficult due to the possibility of hydrophone damage or of sensor interference with the focused beam. To address this issue, we have developed two non‐invasive methods for determining the intensity in free field and in tissue‐mimicking material. In the first method, streaming field generated by the absorption of acoustic energy in liquid is measured using particle image velocimetry. The intensity distribution giving rise to the velocity field is computed by performing the operations of the Navier–Stokes equations upon the experimentally measured streaming field. The second technique (IR thermography) involves the use of an infrared camera to measure the temperature within a tissue‐phantom. An air interface is required at the phantom boundary in order for the IR camera to observe the temperature rise. Quantitative determination of the intensity can be difficult due to the presence of the air interface (at which the intensity...