Parametric modeling with beamspread compensation and MIMO frequency domain inversion applied to fine saturated sands.

A system identification technique is applied to estimate the intrinsic absorption and dispersion of two fine sands. The method is based on the parametric modeling of the wave propagation through a Plexiglas tank filled with the sediment under investigation. The applicability of various porous models is discussed. The viscoelastic constant Q model and viscoelastic rational form model are applied and compared. Closed form expressions describing the wave propagation are replaced by Debye series expansions with correction coefficients that consider the beamspread caused by the finite aperture of the emitter. A multiple input multiple output (MIMO) representation is used in conjunction with the maximum likelihood estimator (MLE) for the estimation of the sediment parameters. The estimated absorption and dispersion curves are depicted.

Experimental validation for the diffraction effect in the ultrasonic field of piston transducers and its influence on absorption and dispersion measurements.

The aim of this work was to study the diffraction effects in the ultrasonic field of piston source transducers and their importance for accurate measurements of attenuation and dispersion in viscoelastic materials. In laboratory measurements, the diffraction phenomena are mainly due to the beam spread of the ultrasonic wave propagating in viscoelastic materials. This effect is essentially related to the estimated attenuation and dispersion in the material. In this work, a frequency domain system identification approach, using the maximum likelihood estimator (MLE), was applied to the measured data in order to determine a function for correcting the diffraction losses in both normal and oblique incidences for a large frequency band (300 kHz to 3 MHz). The effective radius of the used transmitter was determined by the inverse problem when ultrasonic beam propagation was investigated in a water medium. Using the estimated radius, the propagation through viscoelastic materials was established, and the acoustic parameters of these materials were estimated. Attention was paid to the determination of the attenuation and dispersion in the materials. These quantities were compared to those obtained without diffraction correction in order to see the influence of introducing the diffraction correction into the propagation model.

Theoretical comparison of the SAR distributions from arrays of modified current sheet applicators with that of lucite cone applicators using Gaussian beam modelling.

The technical comparison of Current Sheet Applicator (CSA) and Lucite Cone Applicator (LCA) arrays covering an area of approximately 20 x 20 cm2 is investigated based on Gaussian beam (GB) predicted SAR distributions. The comparison is made in muscle equivalent tissue at 1 cm depth (maximum SAR normalized to 100%) and over a volume of 3 cm depth under the aperture of the antennae. The planar SAR distribution is tested on field sizes (FSx: area covering x% SAR), penetration depth (PD) and homogeneity coefficient (HC = FS75/FS25). From the SAR volume, a SAR-Volume histogram (volume enclosing y% SAR/total volume) is calculated as well as the volumetric HC. First, the prototype CSA (aperture 58 x 67 mm2, FS50 = 21 cm2) is technically modified to assure clinical safety and load independence. The modified CSA, the D-CSA, has an aperture of 66 x 75 mm2 with an FS50 = 28 cm2 and a PD of 10 mm, the LCA (aperture 105 x 105 mm2) has an FS50 = 76 cm2 and PD = 12 mm. The HC equals 0.21 (D-CSA), respectively 0.22 (LCA). Secondly, a 3 x 3 D-CSA array is compared with a 2 x 2 LCA array. The FS50s equals 72% (D-CSA), respectively 75% (LCA). The SAR-volume histograms, planar and volumetric HC show no significant difference; however, the planar HCs for D-CSA and LCA increase from 0.2-0.3, indicating that incoherently powered arrays from these antennae build SAR distributions constructively.

Quantitative evaluation of 2 x 2 arrays of Lucite cone applicators in flat layered phantoms using Gaussian-beam-predicted and thermographically measured SAR distributions.

SAR distributions from four different E-field-orientated 2 x 2 arrays of incoherently driven Lucite cone applicators (LCAs) were investigated. The LCAs operated at 433 MHz with an aperture of 10.5 cm x 10.5 cm each. Two techniques were used to obtain SAR distributions in flat layered phantoms: Gaussian beam (GB) predictions and thermographical (TG) imaging. The GB predictions showed that the effective field size of the different array configurations varied by up to 3%. The TG-measured SAR distribution showed significant deviations from the GB-predicted SAR distributions (maximum 34.6%). The difference between GB-predicted and TG-measured SAR levels (averaged per 10% GB-predicted SAR intervals) equalled less than 11.3% for the parallel E-field orientated array and respectively 15.1% for the clockwise-orientated array. When antennae in the clockwise-orientated array were more widely spread (array aperture 23 cm x 23 cm) in order to diminish their mutual interactions, these differences decreased to 12.4%. However, the overall difference within the 50% SAR or higher range decreased from 14% to 9%. The results lead us to conclude that LCAs can be used clinically and their antenna interactions are not considered to be a problem under clinical conditions.

Effectiveness of the Gaussian beam model in predicting SAR distributions from the lucite cone applicator.

The Gaussian beam model (GBM) has been shown to be a successful tool in the development of the current sheet applicator. As a result, the effectiveness of the GBM is investigated in single and dual array applications of the lucite cone applicator (LCA). The LCA is a modified water-filled waveguide applicator with an improved effective field size (EFS > 64 cm2, aperture 10 cm x 10 cm). The GB-source parameters were calculated from the emanating E-field of a single LCA. The SAR distribution from a single LCA was measured by E-field scanning and thermographic (TG) imaging, and compared with the GB-predicted SAR distribution. Deviations in the principal planes were found to be less than 5%. TG-measured and GB-predicted SAR distributions from three different dual LCA configurations were compared and evaluated. When water was used as intermedium between LCAs and phantom, a maximum SAR difference of 27% was calculated. In the absence of water as intermedium, this difference increased to 44%. These large deviations were only found in areas where the measured SAR distribution was disturbed due to antenna interactions. The average SAR differences with and without water as intermedium were 7% respectively 11%, indicating that the GBM can provide good qualitative information about the SAR distribution of dual LCA-arrays.

The use of a current sheet applicator array for superficial hyperthermia: incoherent versus coherent operation.

There are a number of potential advantages to be gained by using an array of applicators in hyperthermia treatments compared with single applicator systems. These advantages include the possibility of greater spatial control of power deposition and conformability to nonplanar sites. Arrays of applicators can be driven either coherently or incoherently. In the case of coherent operation, an added advantage is the ability to steer power deposition by varying the phases of the antennas. In this study, we investigated the relative merits of the two modes of operation when a 2 x 2 planar array of current sheet applicators is used. The effective field size (EFS) of the array was calculated using a Gaussian beam representation of the applicators on a layered model in which the fat layer had its thickness varied. Good agreement was obtained between the square of the electric field distribution (E2) and quantitative experimental results. It is shown that when the planar array is used with a fat layer greater than about 2 mm present, it should be driven incoherently as this results in a significantly larger EFS than that obtained when the array is driven coherently.

Current sheet applicator arrays for superficial hyperthermia of chestwall lesions.

The current sheet applicator is an electromagnetic heating device whose size may be chosen virtually independent of frequency even though practical limitations may restrict it to VHF and UHF bands. In this paper we investigate absorbed power distributions in muscle tissue from current sheet applicators when used as elements of a planar array intended for superficial hyperthermic treatment of tumours. Advantages offered by current sheet applicators for tissue heating include compact size, a linear polarization of the induced electric field and relatively large heating area. It is shown that the effective field produced by a pair of these elements is continuous regardless of whether the common edges of the elements are perpendicular or parallel to the direction of impressed current. The feasibility of customizing the shape and size of the field is also illustrated. The absorbed power distribution patterns due to a coherently driven array operating around 434 MHz is relatively insensitive to phase variations of about 20 degrees but is sensitive to relative power level variations as low as 10%. Mutual coupling between array elements may be reduced to acceptable levels by incorporating suitable spacing between them. It is also demonstrated that there is good agreement between measurements of absorbed power distributions and predictions using the Gaussian beam model.

Experimental assessment of phased-array heating of neck tumours.

An investigation of phased-array microwave systems (PAMS) for non-invasively inducing hyperthermia, primarily in neck lesions, has been done with implications for applications at other sites such as lung and pelvis. Our general approach was to combine numerical and analytical approaches with parallel experimental studies. In this paper we will concentrate only on the experimental aspects. The object, such as a homogeneous cylindrical phantom or a neck phantom, was encircled with several standard applicators driven by a single source, but with relative phase and amplitude control over each applicator. The relative phases of the applicators were adjusted by using an implanted monopole antenna connected to an HP network analyser. Power was applied and the specific absorption rate (SAR) was determined by using split phantoms and thermography or by measuring temperature transients dT/dt, recorded by implanted thermometer probes. We found that at 915 MHz for our applicators (SMA Co.) the centre of an 11 cm diameter muscle-like phantom heated to about 33% of the value at the surface in front of the applicator. Similarly, we were able to show significant SAR at the centre of realistically sized neck phantoms using four phased apertures of 915 MHz. Furthermore, substantial improvement was observed if the frequency was lowered to about 400 MHz.

Use of Gaussian beam model in predicting SAR distributions from current sheet applicators.

The Gaussian beam model is shown to be a good predictor of SAR distributions due to current sheet applicators (CSAs). It is fast, efficient and adaptable. SAR distributions from a single applicator and from simple arrays of CSAs in homogeneous and layered lossy media are computed at 434 and 450 MHz at CPU times of less than 60 s. The good agreement between theory and experiment justifies the use of the Gaussian beam model to predict SAR distributions from CSAs.