Optimization of a beam shaping bolus for superficial microwave hyperthermia waveguide applicators using a finite element method

Enhancing the properties of beam forming bolus in hyperthermia: numerical simulation and empirical verification

In this paper we present a simulation study of the induced specific absorption rate (SAR) within the phantom produced by radiofrequency radiation from a 8 MHz capacitive applicator. The main focus of the current study is on demonstrating the beam shaping properties of the bolus system as well as its effect on controlling the therapeutic area. Different electrical conductivities and geometries of the bolus were considered in the simulation of induced SAR distributions in a muscle-equivalent model with uniform dielectric properties. To validate the presented model, we carried out a comparison between the SAR simulation results and the temperature measurements in an agar split-phantom and an excellent agreement was observed.

Volume and distances of the maxillary sinus in craniofacial deformities with midfacial hypoplasia

OBJECTIVE

Craniofacial deformities (CFDs) frequently accompany midfacial hypoplasia. The authors evaluated characteristics of maxillary sinuses that had CFDs with variable degrees of midfacial hypoplasia.

STUDY DESIGN

Cross-sectional survey with chart review.

SETTING

Department of Plastic and Reconstructive Surgery, Severance Hospital, Yonsei University College of Medicine, Seoul, Republic of Korea.

SUBJECTS AND METHODS

We investigated 40 patients with CFDs having midfacial hypoplasia. Study group 1 (SG 1) consisted of eight patients with Crouzon syndrome (16 maxillary sinuses). Study group 2 (SG 2) consisted of 10 patients with midfacial hypoplasia after palatoplasty (20 maxillary sinuses). Study group 3 (SG 3) consisted of 22 patients with Pruzansky grade I hemifacial microsomia (22 maxillary sinuses on the affected sides). Data on volume and three-dimensional distances (height, width, and depth) from computed tomography were collected and compared with each corresponding control group. Correlation coefficient between volume and the three distances was also calculated.

RESULTS

The volume, height, width, and depth of the maxillary sinus were significantly decreased in SG 1 (P < 0.01). In SG 2, only the depth was significantly decreased (P < 0.05). In SG 3, there were no significant differences in any parameters. A multiple-regression analysis between the volume and the three distances showed a statistically significant relationship for width in SG 1, width and height in SG 2, and all distances in SG 3.

CONCLUSION

There were differences in the structure of the maxillary sinuses among patients with different CFDs.

Flow patterns and heat convection in a rectangular water bolus for use in superficial hyperthermia

This paper investigates both numerically and experimentally the spatio-temporal effects of water flow in a custom-made water bolus used for superficial hyperthermia generated by a 915-MHz, 4 x 3 microwave applicator array. Similar hyperthermia models referenced in the literature use a constant water temperature and uniform heat flux to describe conduction and convection energy exchange within the heating apparatus available to cool the tissue surface. The results presented in this paper show that the spatially varying flow pattern and rate are vital factors for the overall heat control applicability of the 5 mm thick bolus under study. Regions with low flow rates and low heat convection clearly put restrictions on the maximum microwave energy possible within the limits of skin temperature rise under the bolus. Our analysis is illustrated by experimental flow front studies using a contrast liquid set-up monitored by high definition video and complemented by numerical analysis of liquid flow and heat exchange within the rectangular water bolus loaded by malignant tissue. Important factors for the improvement of future bolus designs are also discussed in terms of diameter and configuration of the water input and output tubing network.

Paradoxical conducting airway responses and heterogeneous regional ventilation after histamine inhalation in rabbit studied by synchrotron radiation CT

We studied both central conducting airway response and changes in the distribution of regional ventilation induced by inhaled histamine in healthy anesthetized and mechanically ventilated rabbit using a novel xenon-enhanced synchrotron radiation computed tomography (CT) imaging technique, K-edge subtraction imaging (KES). Images of specific ventilation were obtained using serial KES during xenon washin, in three axial lung slices, at baseline and twice after inhalation of histamine aerosol (50 or 125 mg/ml) in two groups of animals (n = 6 each). Histamine inhalation caused large clustered areas of poor ventilation, characterized by a drop in average specific ventilation (sV(m)), but an increase in sV(m) in the remaining lung zones indicating ventilation redistribution. Ventilation heterogeneity, estimated as coefficient of variation (CV) of sV(m) significantly increased following histamine inhalation. The area of ventilation defects and CV were significantly larger with the higher histamine dose. In conducting airways, histamine inhalation caused a heterogeneous airway response combining narrowing and dilatation in individual airways of different generations, with the probability for constriction increasing peripherally. This finding provides further in vivo evidence that airway reactivity in response to inhaled histamine is complex and that airway response may vary substantially with location within the bronchial tree.

Attenuated total reflection Fourier transform infrared spectroscopy analysis of human hair fiber structure

The structure of human hair was studied by attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopy. The use of Ge, ZnSe, and Si internal reflection elements, various incident light angles, and difference spectra allowed detailed analysis of the cuticle, cortex, and cuticle-cortex intercellular regions without physically or mechanically removing the cuticle of the hair fiber. The ATR-FT-IR data showed the cuticle to be composed of protein having predominantly beta-sheet and disorder and beta-turn configurations. In contrast, the cortex spectra showed alpha-helical structures due to the presence of intermediate filaments of alpha keratin plus beta-sheet, beta-turn and disorder structures. In the cuticle-cortex interface region the protein structures were primarily disorder and beta-turn with small amounts of beta-sheet configurations. The spectral analyses are consistent with the general information on hair fiber structure proposed in the literature.

Can we settle with single-band radiometric temperature monitoring during hyperthermia treatment of chestwall recurrence of breast cancer using a dual-mode transceiving applicator?

The total thermal dose that can be delivered during hyperthermia treatments is frequently limited by temperature heterogeneities in the heated tissue volume. Reliable temperature information on the heated area is thus vital for the optimization of clinical dosimetry. Microwave radiometry has been proposed as an accurate, quick and painless temperature sensing technique for biological tissue. Advantages include the ability to sense volume-averaged temperatures from subsurface tissue non-invasively, rather than with a limited set of point measurements typical of implanted temperature probes. We present a procedure to estimate the maximum tissue temperature from a single radiometric brightness temperature which is based on a numerical simulation of 3D tissue temperature distributions induced by microwave heating at 915 MHz. The temperature retrieval scheme is evaluated against errors arising from unknown variations in thermal, electromagnetic and design model parameters. Whereas realistic deviations from base values of dielectric and thermal parameters have only marginal impact on performance, pronounced deviations in estimated maximum tissue temperature are observed for unanticipated variations of the temperature or thickness of the bolus compartment. The need to pay particular attention to these latter applicator construction parameters in future clinical implementation of the thermometric method is emphasized.

Differences in the time course of proximal and distal airway response to inhaled histamine studied by synchrotron radiation CT

We studied the kinetics of proximal and distal bronchial response to histamine aerosol in healthy anesthetized and mechanically ventilated rabbits up to 60 min after histamine administration using a novel xenon-enhanced synchrotron radiation computed tomography imaging technique. Individual proximal airway constriction was assessed by measuring the luminal cross-sectional area. Distal airway obstruction was estimated by measuring the ventilated alveolar area after inhaled xenon administration. Respiratory system conductance was assessed continuously. Proximal airway cross-sectional area decreased by 57% of the baseline value by 20 min and recovered gradually but incompletely within 60 min. The ventilated alveolar area decreased immediately after histamine inhalation by 55% of baseline value and recovered rapidly thereafter. The results indicate that the airway reaction to inhaled histamine and the subsequent recovery are significantly slower in proximal than in distal bronchi in healthy rabbit. The findings suggest that physiological reaction mechanisms to inhaled histamine in the airway walls of large and small bronchi are not similar.

Investigation of a scanned cylindrical ultrasound system for breast hyperthermia

This paper investigates the feasibility of a scanned cylindrical ultrasound system for producing uniform heating from the central to the superficial portions of the breast or localized heating within the breast at a specific location. The proposed system consists of plane ultrasound transducer(s) mounted on a scanned cylindrical support. The breast was immersed in water and surrounded by this system during the treatment. The control parameters considered are the size of the transducer, the ultrasound frequency, the scan angle and the shifting distance between the axes of the breast and the system. Three-dimensional acoustical and thermal models were used to calculate the temperature distribution. Non-perfused phantom experiments were performed to verify the simulation results. Simulation results indicate that high frequency ultrasound could be used for the superficial heating, and the scan angle of the transducer could be varied to obtain an appropriate high temperature region to cover the desired treatment region. Low frequency ultrasound could be used for deep heating and the high temperature region could be moved by shifting the system. In addition, a combination of low and high frequency ultrasound could result in a portion treatment from the central to the superficial breast or an entire breast treatment. Good agreement was obtained between non-perfused experiments and simulation results. The findings of this study can be used to determine the effects of the control parameters of this system, as well as to select the optimal parameters for a specific treatment.

Infrared thermographic SAR measurements of interstitial hyperthermia applicators: errors due to thermal conduction and convection

Thermal conduction and convection were examined as sources of error in thermographically measured SAR patterns of an interstitial microwave hyperthermia applicator. Measurements were performed in a layered block of muscle-equivalent phantom material using an infrared thermographic technique with varying heating duration. There was a 52.7% reduction in maximum SAR and 75.5% increase in 50% iso-SAR contour area for a 60-s heating duration relative to a 10-s heating duration. A finite element model of heat transfer in an homogeneous medium was used to model conductive and convective heat transfer during the thermographic measurement. Thermal conduction artefacts were found to significantly distort thermographically measured SAR patterns. Convective cooling, which occurs when phantom layers are exposed for thermal image acquisition, was found to significantly affect the magnitude, but not the spatial distribution, of thermographically measured SAR patterns. Results from this investigation suggest that the thermal diffusion artefacts can be minimized if the duration of the applied power pulse is restricted to 10 s or less.

Fourier transform infrared attenuated total reflection analysis of human hair: comparison of hair from breast cancer patients with hair from healthy subjects

A comparative study of Fourier transform infrared attenuated total reflection (FTIR-ATR) spectra of 32 scalp and pubic hair samples from individuals diagnosed with breast cancer and those who were negative for breast cancer showed increases in the beta-sheet/disorder structures (relative to alpha-helix structures) and C-H lipid content of hair from breast cancer patients. Thus, the presence of breast cancer appears to alter the hair growth process, resulting in changes in the composition and conformation of cell membrane and matrix materials of hair fiber. These appear to be consistent with the changes observed in X-ray diffraction patterns for hair from breast cancer patients. A blind study of 12 additional hair samples using these FTIR-ATR spectral differences as markers correctly identified all four hair samples from cancer patients (100%). Two of these samples were from breast cancer patients. Of the remaining two samples analyzing positive for cancer, one was from a prostate cancer patient and one from a lung cancer patient. Thus, it appears that the mechanism that alters hair fiber synthesis in the three types of cancer may be similar. The blind study incorrectly identified as positive for cancer three hair samples from two apparently healthy individuals and one patient considered cured from prostate cancer.

Two-dimensional acoustic attenuation mapping of high-temperature interstitial ultrasound lesions

Acoustic attenuation change in biological tissues with temperature and time is a critical parameter for interstitial ultrasound thermal therapy treatment planning and applicator design. Earlier studies have not fully explored the effects on attenuation of temperatures (75-95 degrees C) and times (5-15 min) common in interstitial ultrasound treatments. A scanning transmission ultrasound attenuation measurement system was devised and used to measure attenuation changes due to these types of thermal exposures. To validate the approach and to loosely define expected values, attenuation changes in degassed ex vivo bovine liver, bovine brain and chicken muscle were measured after 10 min exposures in a water bath to temperatures up to 90 degrees C. Maximum attenuation increases of approximately seven, four and two times the values at 37 degrees C were measured for the three tissue models at 5 MHz. By using the system to scan over lesions produced using interstitial ultrasound applicators, 2D contour maps of attenuation were produced. Attenuation profiles measured through the centrelines of lesions showed that attenuation was highest close to the applicator and decreased with radial distance, as expected with decreasing thermal exposure. Attenuation values measured in profiles through lesions were also shown to decrease with reduced power to the applicator. Attenuation increases in 2D maps of interstitial ultrasound lesions in ex vivo chicken breast, bovine liver and bovine brain were correlated with visible tissue coagulation. While regions of visible coagulation corresponded well to contours of attenuation increase in liver and chicken, no lesion was visible under the same experimental conditions in brain, due primarily to the heterogeneity of the tissue. Acoustic and biothermal simulations were employed to show that attenuation models taking into account these attenuation changes at higher temperatures and longer times were better able to fit experimental data than previous models. These simulations also indicated that the characterization of tissue acoustic and thermal properties over a large range of temperatures is critical for accurate treatment planning or design studies involving high-temperature interstitial ultrasound.