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T Christopher

Publications and source records attributed to T Christopher.

8 recordsLinked to original sources
PubMed1999

Computing the mechanical index.

A computational nonlinear beam propagation model was used to compute the water path and in situ fields of a phased array transducer operating at 2 MHz. The computational source was matched to the transducer's z = 10 cm focal plane field. Subsequent computed propagations considered this source operating at source amplitudes up to 1.49 MPa in a water medium and in a tissue medium with an attenuation of 0.3 dB cm(-1) MHz(-1). The mechanical index was calculated in three ways based on these computations: extrapolated from one low amplitude water path propagation, extrapolated from a series of water path propagations using the existing Output Display Standard protocol, and directly from a series of tissue path propagations. These computed results suggest that extrapolation from derated measurements of a low level water path field can provide mechanical index estimates which progressively overestimate the in situ values. At the highest source amplitude considered, the linearly extrapolated mechanical index was 29% higher than the mechanical index computed by the tissue path propagations. The Output Display Standard protocol offered improved accuracy but consistently underestimated the in situ values. The maximum error for the Output Display Standard protocol was 8%. A variation of the Output Display Standard protocol in which mechanical index estimates were obtained from the on-axis spatial peak in the derated temporal peak rarefactional curve was also considered. The maximum error for this method was 3%. The results considered here also demonstrated the feasibility of computational investigations of high intensity clinical propagations.

Linear Models↗
PubMed1999

Nonlinear propagation and the output indices.

By ignoring the effects of nonlinear propagation, current exposimetry protocols may yield significant underestimates of the acoustic pressure in situ. This problem can be avoided simply by (1) extrapolating pressures linearly from low amplitude measurements in water and (2) linearly derating these values to obtain estimates of fields in situ. The mechanical index was designed to provide an indication of temporal peak acoustic fields for use in prediction of nonthermal biological effects in tissues. At low outputs, the mechanical index, together with the frequency, gives the peak negative pressure near the focus of the field. As currently formulated, however, the pressure used in the mechanical index may be far from the focus at high output levels. Recommendations of the World Federation of Ultrasound in Medicine and Biology avoid the underestimate associated with nonlinear propagation as well as other problems with the mechanical index and may be preferable in dealing with non-thermal bioeffects. The thermal indices that are implemented currently in the Output Display Standard (American Institute of Ultrasound in Medicine/National Electrical Manufacturers' Association) are affected less seriously by nonlinear propagation.

Acoustics↗
PubMed1998

Increasing the dose and rate of Albunex infusion leads to superior left ventricular contrast effect.

In routine clinical use, the efficacy of Albunex in producing clinically useful opacification may be lower than in initial clinical studies. We hypothesized that increasing either the rate of injection or amount of Albunex administered would increase left ventricular opacification. Fifty adult volunteers were each injected with Albunex in five volume/rate combinations. Blinded reviewers evaluated left ventricular opacification and endocardial border delineation compared with the baseline (noncontrast) echocardiogram. In addition, captured digitized images were analyzed with video-densitometric techniques. Injected at the highest volume/rate tested (20 ml at 3.0 ml/sec), Albunex provided the greatest improvement in left ventricular opacification, endocardial border delineation, and quality of the echocardiogram. The administration of Albunex caused no serious adverse events at any volume/rate regimen tested. Our data indicate that faster injection rates and larger dose volumes than those currently recommended by the package insert significantly improve Albunex ultrasound contrast without compromising safety.

Adult↗
PubMed1996

Finite amplitude distortion and its relationship to linear derating formulae for diagnostic ultrasound systems.

Formulation of indices that can be used as predictors of biological effects of ultrasound involves a process called derating, in which measurements of the sound field made in water are extrapolated to estimates of the magnitude of the sound fields in the tissues of the body. All indices that have been formulated up to the present time assume that the propagation of ultrasound is linear. In fact, under most exposure conditions for which biological effects may be a concern, sound propagation is highly nonlinear. A nonlinear propagation model has been used in this study to evaluate the nature of the effects that occur under realistic exposure conditions encountered in diagnostic procedures. Because of the way that the thermal index is defined, it turns out that ignoring nonlinear propagation leads to underestimates of tissue temperature increments that typically are less than 40%. As currently implemented, the mechanical index may be underestimated by more than a factor of two because it ignores the saturation of the sound fields that result from nonlinear propagation. For large propagation distances in soft tissues (e.g., 10 cm at 3 MHz in liver); however, it is physically difficult to exceed tissue pressures corresponding to MI > 2 because of these same saturation phenomena.

Adipose Tissue↗
PubMed1994

Modeling the Dornier HM3 lithotripter.

The computational modeling of a Dornier HM3 electrohydraulic, extracorporeal shock wave lithotripter is considered. In order to produce large amplitude shock waves for the purpose of pulverizing renal and ureteric calculi (stones), the HM3 uses a hemi-ellipsoidal bowl to focus the spherical field generated by a high-voltage spark gap. The initial propagation of the bowl-focused field is accomplished using a recently developed nonplanar source algorithm. An updated version of an existing nonlinear acoustic beam propagation model is then used to consider the subsequent propagation of the field. The resulting modeling sequence accounts for the effects of diffraction, attenuation, dispersion, nonlinearity, and (planar) reflection and refraction. The water path computed predictions agree well with existing measurements. The computed in vivo predictions suggest that the Dornier HM3's clinical performance is not significantly different than its water path performance.

Equipment Design↗