A known method for the determination of the attenuation coefficient of tissues involves the measurement of the center frequency downshift of Gaussian spectra. The tissue is generally assumed to have linear frequency dependent attenuation. This assumption results in a simple relationship between the spectral downshift and the attenuation coefficient. However, recent studies have shown that many tissues exhibit nonlinear frequency dependent attenuation. In this paper we investigate the consequences of applying the linear assumption to non-linear cases. These studies show that even a small deviation from linearity results in significant errors in the determination of the attenuation parameters.

Mathematics↗

PubMedDate not supplied

Parametric vs nonparametric measurements in tissues with nonlinear frequency dependent attenuation.

The relationship between the attenuation parameters of tissue and the linear regression slope derived from nonparametric estimates of tissue attenuation is investigated. It was found that if the attenuation in the tissue possesses a nonlinear frequency dependence, the linear regression slope parameter becomes dependent on center frequency and on the bandwidth of the measurement. Moreover, it was found that the slope estimate is numerically close to the parameter derived from parametric attenuation measurements in such cases.

Humans↗

PubMedDate not supplied

The measurement of attenuation in nonlinearly attenuating media by the zero crossing method.

A closed form expression relating the zero crossing density to the material parameters of media with nonlinear frequency dependent attenuation is derived for the case of Gaussian excitation. This expression indicates that for tissues which exhibit a small degree of such nonlinearity, significant errors in the determination of the material parameter result if the nonlinearity is ignored. This result is similar to that shown earlier [Narayana and Ophir (1983) Ultrasound in Med. & Biol. 9, 357-361] for frequency shift techniques.

Humans↗

PubMedDate not supplied

The effects of digitization on the elastographic signal-to-noise ratio.

In elastography, the tissue under investigation is compressed and the resulting strain is estimated from the gradient of the displacement (time-delay) estimates. The displacements are typically estimated by cross-correlating the radiofrequency (RF) ultrasound signals of the pre- and postcompressed tissue. One of the parameters used to quantify the resulting quality of the elastogram is the elastographic signal-to-noise ratio (SNR(e)). For a uniformly elastic target (a single elastic modulus), the dependence of the SNR(e) on the applied strain has a bandpass characteristic that has been termed the strain filter. Theoretical expressions for the upper bound on the strain filter were developed earlier. Yet, simulated as well as experimental strain filters derived from uniformly elastic phantoms deviate from these upper bounds. The failure to achieve the upper bounds could be partially attributed to the fact that, in both simulations and experiments, the RF signals used to compute the TDEs are sampled and quantized. Using simulated models of uniformly elastic phantoms, a study of the dependence of the strain filter on the quantization and sampling rates was performed. The results indicated that the strain filter improves with both the sampling rate and the quantization, as expected. A theoretical analysis was done to incorporate quantization as a derating factor to the strain filter.

Elasticity↗

PubMedDate not supplied

Clinical significance of markedly elevated serum creatine kinase levels in patients with acne on isotretinoin.

Muscle-related complaints and high creatine kinase (CK) blood levels have been reported in 16-51% of patients with acne treated with isotretinoin. It has been suggested that this retinoid and exercise have a synergistic effect on muscle. The presence of marked hyperCKemia during the treatment raises concern about rhabdomyolysis. The objective of this report was to evaluate the incidence, course and clinical significance of severe hyperCKemia in isotretinoin therapy for acne. Out of 442 patients on isotretinoin, we reviewed 7 patients (1.58%) with CK values above 5,000 IU/l. Only two of them had myalgia. Physical activity or intramuscular injection prior to blood testing was reported in 6 patients. CK values returned to normal within 2 weeks and all subjects except 2, completed treatment. In conclusion, marked hyperCKemia with or without muscle-related complaints in isotretinoin-treated patients with acne is a benign phenomenon.

Acne Vulgaris↗