Modified Substitution Method Research Articles

Boundary element simulation of backscattering properties for red blood with high frequency ultrasonic transducers

High frequency ultrasonic imaging (e.g. >30 MHz) from blood is difficult due to its tenuous backscattered pressure and the interference from adjacent tissues as well. To increase the sensitivity focused transducer has to be used, thus raising the complexity of interpreting the received signals. A numerical simulation of the ultrasonic scattering property from erythrocyte and rouleaux based on boundary element method was performed with experimental results based on a modified substitution method. The results (proportional relationship between backscattered pressure and frequency and the frequency limit for Rayleigh scattering) closely coincide with experimental data for erythrocyte. Rouleaux model results also show the dependence of degree of red cell aggregation on backscattering properties. The boundary element method serves as a good means to calculate the acoustic scattering from blood cells under arbitrary incident waves.

High-frequency backscatter and attenuation measurements of porcine erythrocyte suspensions between 30–90 MHz

There are now diagnostic ultrasonic imaging devices that operate at very high frequencies (VHF) of 20 MHz and beyond for clinical applications in ophthalmology, dermatology and vascular surgery. To be able to better interpret these images and to further the development of these devices, knowledge of ultrasonic attenuation and scattering of biologic tissues, such as blood, in the high-frequency range is crucial. VHF attenuation and backscatter experiments were made on porcine red blood cell (RBC) suspensions, for which much data on attenuation and backscatter can be found in the literature in the lower frequency range. Attenuation and backscatter at hematocrits of 6%, 10%, 15%, 20%, 25% and 30% from 30 to 90 MHz were measured using a modified substitution method that allows the utilization of focused transducers. The results show that the attenuation coefficient from all suspensions increased linearly with frequency and the backscatter coefficient for low hematocrit suspensions was found to have a maximum between 10% and 15%. At higher hematocrits, a decrease in the frequency-dependence was observed, possibly indicating that Rayleigh scattering is no longer valid because the wavelength in the VHF range is comparable to the size of a porcine RBC.

High frequency ultrasonic scattering by biological tissues

High frequency (HF) diagnostic ultrasonic imaging devices at frequencies higher than 20 MHz have found applications in ophthalmology, dermatology, and vascular surgery. To be able to interpret these images and to further the development of these devices, a better understanding of ultrasonic scattering in biological tissues such as blood, liver, myocardium in the high frequency range is crucial. This work has previously been hampered by the lack of suitable transducers. With the availability of HF transducers going to 90 MHz, HF attenuation and backscatter experiments have been made on porcine red blood cell (RBC) suspensions, for which much data on attenuation and backscatter can be found in the literature in the lower frequency range for frequencies, from 30 to 90 MHz and on bovine tissues for frequencies from 10 to 30 MHz using a modified substitution method that allow the utilization of focused transducers. These results will be reviewed in this talk along with relevant theoretical models that could be applied to interpreting them. The relevance of the parameter that has been frequently used in the biomedical ultrasound literature to describe backscattering, the backscattering coefficient, will be critically examined.

High-frequency backscatter and attenuation measurements of selected bovine tissues between 10 and 30 MHz

There are now diagnostic ultrasonic imaging devices that operate at very high frequencies (VHF) of 20 MHz and beyond for clinical applications in ophthalmology, dermatology, vascular surgery, endoluminal imaging and small animal imaging. To be able to better interpret these images and to further the development of these devices, knowledge of ultrasonic attenuation and scattering of biological tissues in this frequency range is crucial. Attenuation and backscatter coefficients (BSCs) of bovine tissues in the frequency range of 10 to 30 MHz were measured, respectively, using a standard substitution method for attenuation measurements and a modified narrow-band substitution method for scattering measurements. A modified substitution method for scattering measurements has to be used at high frequencies because unfocused transducers due to their decreased sensitivity cannot be used in the simple substitution method. In the modified method, the flat reflector is substituted by a particulate reference medium whose BSC is well-known and documented; in this case, a red cell suspension. In this paper, experimental results on BSC and attenuation coefficient measured between 10 and 30 MHz are reported. The frequency dependence of backscatter of the selected bovine tissues ranges from 2.4 to 3.5, whereas attenuation is observed to be still approximately linearly proportional to frequency. The BSC measured with the modified method is in good agreement with those obtained with the standard method between 10 and 20 MHz.

Molecular structure and centrifugal distortion constants of isocyanic acid from the microwave, millimeter wave, and far-infrared spectra

The pure rotational spectra of HNCO and DNCO were measured in the far-infrared region from 80 to 350 cm −1 by a Fourier transform spectrometer with a resolution of 0.1 cm −1. The r R K branches were measured and assigned for HNCO from K = 1 to 6, and for DNCO from K = 3 to 8. The measured transition wavenumbers were analyzed together with the microwave and millimeter wave data reported by Hocking, Gerry, and Winnewisser [ Can. J. Phys. 53, 1869–1901 (1975)] and with the far-infrared data of Krakow, Lord, and Neely [ J. Mol. Spectrosc. 27, 148–176 (1968)] in the low-wavenumber region. The microwave and millimeter wave data of H 15NCO, HN 13CO, and HNC 18O reported by Hocking et al. were reanalyzed assuming several centrifugal distortion constants to be identical with those of the normal species. The molecular structure of HNCO was reevaluated using a modified substitution method from the rotational constants obtained in this work. The molecule has a bent structure in a trans configuration with r (NH) = 0.995 A ̊ , r (NC) = 1.314 A ̊ , r (CO) = 1.668 A ̊ , ∠HNC = 123.9°, and ∠NCO = 172.6°.

Ground state spectroscopic constants of H 15NCS, HN 13CS, and HNC 34S, and the molecular structure of isothiocyanic acid

The rotational spectrum of isothiocyanic acid was measured for the isotopically enriched species H 15NCS and HN 13CS as well as HNC 34S in natural abundance. In the frequency range from 8 to 240 GHz the a-type R-branch transitions were measured for all three isotopic species. The q Q 1 transitions were identified in the microwave region for H 15NCS and HN 13CS. The rotational and centrifugal distortion constants were determined using Watson's Hamiltonian in the S reduction, extended empirically to higher order terms in the angular momentum. The molecular structure of isothiocyanic acid was reevaluated using a modified substitution method and the NCS chain was found to be bent: r( NH) = 0.993 A ̊ , r( NC) = 1.207 A ̊ , r( CS) = 1.5665 A ̊ , ∠HNC = 131.7°, and ∠NCS = 173.8°. The molecule has the trans conformation.