Monitoring structural changes in cells with high-frequency ultrasound signal statistics.

We investigate the use of signal envelope statistics to monitor and quantify structural changes during cell death using an in vitro cell model. Using a f/2.35 transducer (center frequency 20 MHz), ultrasound backscatter data were obtained from pellets of acute myeloid leukemia cells treated with a DNA-intercolating chemotherapy drug, as well as from pellets formed with mixtures of treated and untreated cells. Simulations of signals from pellets of mixtures of cells were generated as a summation of point scatterers. The signal envelope statistics were examined by fitting the Rayleigh and generalized gamma distributions. The fit parameters of the generalized gamma distribution showed sensitivity to structural changes in the cells. The scale parameter showed a 200% increase (p<0.05) between untreated and cells treated for 24 h. The shape parameter showed a 50% increase (p<0.05) over 24 h. Experimental results showed reasonable agreement with simulations. The results indicate that high-frequency ultrasound signal statistics can be used to monitor structural changes within a very low percentage of treated cells in a population, raising the possibility of using this technique in vivo.

High-frequency ultrasound scattering from microspheres and single cells.

Assessing the proportion of biological cells in a volume of interest undergoing structural changes, such as cell death, using high-frequency ultrasound (20-100 MHz), requires the development of a theoretical model of scattering by any arbitrary cell ensemble. A prerequisite to building such a model is to know the scattering by a single cell in different states. In this paper, a simple model for the high-frequency acoustic scattering by one cell is proposed. A method for deducing the backscatter transfer function from a single, subresolution scatterer is also devised. Using this method, experimental measurements of backscatter from homogeneous, subresolution polystyrene microspheres and single, viable eukaryotic cells, acquired across a broad, continuous range of frequencies were compared with elastic scattering theory and the proposed cell scattering model, respectively. The resonant features observed in the backscatter transfer function of microspheres were found to correspond accurately to theoretical predictions. Using the spacing of the major spectral peaks in the transfer functions obtained experimentally, it is possible to predict microsphere diameters with less than 4% error. Such good agreement was not seen between the cell model and the measured backscatter from cells. Possible reasons for this discrepancy are discussed.

Changes in the dielectric properties of rat prostate ex vivo at 915 MHz during heating.

Changes in the tissue dielectric properties at 915 MHz of rat prostate tissue due to heating at temperatures in the range 45-75 degrees C were measured. The changes were found to be reversible with temperature and independent of the time of heating. The dielectric properties at 23 +/- 1 degrees C were measured as epsilon' = 62.8 +/- 2.7 and sigma = 1.17 +/- 0.07 S/m, while the linear temperature coefficients for reversible changes were 1.10 +/- 0.11%/ degree C for conductivity and -0.31 +/- 0.05% /degree C for relative permittivity. These properties and their temperature coefficients can be utilized in microwave treatment planning programmes to provide insight into the effects of dielectric changes that arise during microwave thermal therapy of prostate cancer.

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.

Interstitial microwave thermal therapy and its application to the treatment of recurrent prostate cancer.

Interstitial microwave thermal therapy may be an effective alternative to surgery for the treatment of some solid tumours. Arrays of helical antennae can produce complex heating patterns which when combined with active cooling of normal tissue structures can provide conformal heating for thermal coagulation of tumours. The development of a clinical protocol involving phantom and animal model studies, treatment planning, tissue property measurement and methods for on-line treatment monitoring is reviewed. The technology developed has been applied to the problem of recurrent prostate cancer following failed radiation treatment where available curative options are associated with high normal tissue morbidity. The purpose was to develop a treatment option for this group of patients with a very low side-effect profile that would not preclude further treatment if the disease progressed. Results of a Phase I/II trial demonstrate safety, promising efficacy and a low complication rate. As the technology for delivering this treatment matures, larger multi-institutional trials should be considered.

Measurement of the thermal conductivity of polyacrylamide tissue-equivalent material.

The purpose of this work was to measure the thermal conductivity of polyacrylamide (PAG) and compare it with previously reported values. Polyacrylamide phantoms play an important role in the development of hyperthermia and high-temperature thermal therapies based on electromagnetic (EM) radiation by providing a material that mimics the electrical and thermal properties of human tissue. The thermal properties of PAG have, up until now, not been thoroughly investigated and at least two significantly different values have been published. In this study, the thermal conductivity of polyacrylamide was measured from the steady state temperature drop across samples exposed to a known heat flux. The measured conductivity was 0.56 +/- 0.047 W m(-1) degrees C(-1). To validate the correct set of thermal properties for polyacrylamide, simple heating experiments were performed in a PAG phantom and then simulated using a finite element numerical model that incorporated the measured thermal conductivity along with literature values for specific heat and density. Temperature predictions from the model agreed with average temperatures measured in the phantom to within 1 SD of the measured temperatures.

The use of CT perfusion to monitor the effect of hypocapnia during laser thermal therapy in a rabbit model.

One of the limiting factors in the thermal therapy of tumours is the dissipation of heat by blood flow. The current study investigates the use of hyperventilation (hypocapnia) to decrease tumour blood flow during laser thermal therapy. Rabbits with implanted VX2 thigh tumours were treated using a diode laser (805 nm) as the heating source. One group of rabbits (n = 8) was treated with the new hypocapnia protocol and another group (n = 8) with the conventional (normocapnia) protocol. The mean tumour volume and blood flow were the same for the two groups prior to treatment. The laser power temporal profile (3.0-1.5 W) and the duration of treatment (60 min) were also the same for both treatment protocols. Blood flow maps were calculated from a series of contrast-enhanced CT images. The average change in thermal lesion area at 60 min post-laser thermal therapy from pre-treatment normalized to the pre-treatment tumour area was significantly different between the two treatment protocols: 0.52+/-0.13 (hypocapnia) vs 0.33+/-0.12 (normocapnia) (p < 0.001). Similarly, the average fractional decrease in global tumour blood flow 60 min post-treatment from pre-treatment was also significantly different between the two protocols: 0.64+/-0.10 (hypocapnia) vs 0.41+/-0.14 (normocapnia) (p < 0.001). The hypocapnia protocol produced larger thermal lesion area and greater decrease in tumour blood flow post-treatment than the normocapnia protocol. These results support the further investigation of the use of hypocapnia to increase the therapeutic effect of laser thermal therapy.

Edge-element based finite element analysis of microwave hyperthermia treatments for superficial tumours on the chest wall.

Several three-dimensional hyperthermia treatment planning systems for deep regional hyperthermia have been successfully utilized for improving the performance of applicators such as the BSD Sigma 60. Treatment planning systems for superficial heating in contrast have been less utilized. This paper presents a study of the applicability of the finite element method that has been developed for modelling hyperthermia treatments of recurrent chest wall cancer using a patient geometry. The patient model was created by reconstructing the tissue geometry of a patient using a series of axial CT scans. Tetrahedral grids were generated from this geometry for use in finite element simulations of the SAR profile using edge-elements and in finite element simulations of the steady-state temperature profile using scalar elements. The predicted temperature profile was well correlated with thermometry readings taken after 30 min of heating during a hyperthermia treatment. The model predicted the presence of hot-spots in regions that were not monitored. Simulations also showed that the hot-spots can be manipulated by rotating the applicator by 90 degrees. This study demonstrates the ability of the model to provide detailed and accurate heating profiles in a patient specific model for superficial microwave hyperthermia of the chest wall.

Ultrasound properties of human prostate tissue during heating.

Changes in the ultrasound (US) properties of tissue during heating affect the delivery of US thermal therapy and may provide a basis for US image monitoring of thermal therapy. The US attenuation coefficient and backscatter power of fresh human prostate tissue were measured as the tissue was heated. Samples of human prostate were obtained directly from autopsies and heated rapidly to final temperatures of 45 degrees C, 50 degrees C, 55 degrees C, 60 degrees C and 65 degrees C. A 5.0-MHz transducer was scanned in a raster pattern over the tissue and radiofrequency (RF) data were collected at 36 uncorrelated positions. Both attenuation and backscatter were measured over the frequency range 3.5 to 7.0 MHz at each min of a 30-min heating. Little change was observed in attenuation or backscatter at 55 degrees C or less. The attenuation coefficient and backscatter power increased by factors of 1.25 and 5, respectively, during the 60 degrees C heating. During the 65 degrees C heating, the same properties showed increases by factors of 2.7 and 9.

An edge-element based finite element model of microwave heating in hyperthermia: application to a bolus design.

Heating of superficial tumours with microwave waveguide applicators has been shown in phase III trials to significantly improve the local control of small lesions when combined with radiation therapy. This success has not yet translated to the treatment of larger tumours, due to difficulty in adequately heating the entire tumour region. Several modifications to the water bolus used with external waveguide applicators have been made in the past in order to increase the heating area. One such modification consisted of a large, microwave-absorbing patch placed inside the bolus, which flattens out the beam profile produced by the applicator. Using this bolus instead of a conventional one resulted in a 30% increase in the effective heating volume produced by the BSD MA120 applicator. This paper describes an optimization procedure for this bolus design which utilises a new finite element model of microwave heating described in an accompanying paper. The optimization procedure resulted in a further 28% increase in the effective heating volume.

An edge-element based finite element model of microwave heating in hyperthermia: method and verification.

Hyperthermia has been shown to improve local tumour control of superficial and deep seated lesions when combined with radiotherapy. There remains difficulty in heating larger tumours with conventional applicators, but this is being addressed by several new applicator designs. This paper presents a new numerical model of microwave heating which is designed to aid in the development of new applicators for superficial heating. The model is based on a finite element method which utilises vector valued basis functions instead of the more conventional scalar valued basis functions. These basis functions were chosen since they are inherently suited for the solution of Maxwell's equations due to their vector nature. The model was successfully verified against an analytic solution to the Mie scattering problem as well as against previously published measurements of heating from a modified water bolus attached to a conventional waveguide applicator. An accompanying paper describes an application of this model to the design optimization of this modified bolus.

Ultrasonic spectral parameter characterization of apoptosis.

Ultrasound (US) spectral analysis methods are used to analyze the radiofrequency (RF) data collected from cell pellets exposed to chemotherapeutics that induce apoptosis and other chemicals that induce nuclear transformations. Calibrated backscatter spectra from regions-of-interest (ROI) were analyzed using linear regression techniques to calculate the spectral slope and midband fit. Two f/2 transducers, with operating frequencies of 30 and 34 MHz (relative bandwidths of 93% and 78%, respectively) were used with a custom-made imaging system that enabled the collection of the raw RF data. For apoptotic cells, the spectral slope increased from 0.37 dB/MHz before drug exposure to 0.57 dB/MHz 24 h after, corresponding to a change in effective scatterer radius from 8.7 to 3.2 microm. The midband fit increased in a time-dependent fashion, peaking at 13dB 24 h after exposure. The statistical deviation of the spectral parameters was in close agreement with theoretical predictions. The results provide a framework for using spectral parameter methods to monitor apoptosis in in vitro and in in vivo systems and are being used to guide the design of system and signal analysis parameters.

Changes in relative light fluence measured during laser heating: implications for optical monitoring and modelling of interstitial laser photocoagulation.

Dynamic changes in internal light fluence were measured during interstitial laser heating of tissue phantoms and ex vivo bovine liver. In albumen phantoms, the results demonstrate an unexpected rise in optical power transmitted approximately I cm away from the source during laser exposure at low power (0.5-1 W), and a decrease at higher powers (1.5-2.5 W) due to coagulation and possibly charring. Similar trends were observed in liver tissue, with a rise in interstitial fluence observed during 0.5 W exposure and a drop in interstitial fluence seen at higher powers (1-1.5 W) due to tissue coagulation. At 1.5 W irradiation an additional, later decrease was also seen which was most likely due to tissue charring. Independent spectrophotometric studies in Naphthol Green dye indicate the rise in fluence observed in the heated albumen phantoms may have been primarily due to light exposure causing photobleaching of the absorbing chromophore. and not due to heat effects. Experiments in liver tissue demonstrated that the observed rise in fluence is dependent on the starting temperature of the tissue. Correlating changes in light fluence with key clinical endpoints/events such as the onset of tissue coagulation or charring may be useful for on-line monitoring and control of laser thermal therapy via interstitial fluence sensors.

Interstitial microwave thermal therapy for prostate cancer: method of treatment and results of a phase I/II trial.

PURPOSE: Interstitial microwave thermal therapy is experimental treatment for prostate cancer with the goal of curing disease, while causing fewer complications than standard treatment options. We present a method for delivering interstitial microwave thermal therapy using microwave radiating helical antennae inserted percutaneously under transrectal ultrasound guidance. We report the results of a trial of this method in 25 patients in whom primary external beam radiation therapy had previously failed. This patient group currently has limited curative options that are associated with a high complication rate. However, these recurrent tumors often remain localized to the prostate, and so they may be amenable to localized therapy. MATERIALS AND METHODS: Patients with proved prostatic adenocarcinoma were candidates for treatment when prostate specific antigen (PSA) was 15 ng./ml. or less and prostate volume was 50 cc. or less. Followup included PSA measurement, digital rectal examination, urinalysis, and documentation of adverse events at 4, 8, 12 and 24 weeks. Sextant biopsy was performed at week 24. The procedure involved the insertion of 5 antennae percutaneously through a modified brachytherapy template. The antenna arrangement was determined based on computer simulated predictions of temperature throughout the prostate. The prostate was dissected away from the rectum by an injection of sterile saline to provide a thermal barrier that protected the rectum from thermal damage. Temperatures were monitored using interstitial mapping thermistor probes that were also inserted through the template. A minimum peripheral target temperature of 55C but less than 70C was maintained for 15 to 20 minutes, while the urethra, rectum and hydrodissection space remained below 42C. The urethra and rectum were actively cooled in addition to hydrodissection. RESULTS: Peripheral target temperatures of 55C were achieved. The urethra and rectum remained at a safe temperature. The procedure, including setup and treatment, required approximately 2.5 hours of operating room time. At 24 weeks the PSA nadir was 0.5 ng./ml. or less in 52% of patients and 0.51 to 4 ng./ml. was achieved in an additional 40%. The negative biopsy rate at 24 weeks was 64%, assuming that 3 patients lost to followup would have had positive results. No major complications were observed and in most cases minor complications resolved within 3 months. CONCLUSIONS: Interstitial microwave thermal therapy for prostate cancer was developed to heat the prostate safely to a cytotoxic temperature. Experience with 25 patients in whom external beam radiation therapy for prostate cancer had failed indicates that the treatment is safe. Although our series indicates that this therapy may be effective, further studies and longer followup are required in larger patient groups to confirm the potential role of this therapy as an option for recurrent and primary prostate cancer.

B-scan ultrasound imaging of thermal coagulation in bovine liver: frequency shift attenuation mapping.

Thermal therapy is an experimental procedure to treat localised tumours in, for example, the prostate, liver, kidney or breast. Following work that demonstrated a marked increase in the ultrasound (US) attenuation of tissues heated above 65 degrees C, US attenuation estimation is proposed as a method to monitor the progression and extent of thermal therapy. In this study, B-scan attenuation mapping techniques, based on the shift in centre frequency of the US signal, are examined. A simple technique based on the change of phase of the quadrature-demodulated signal is presented and analysed. Autoregressive analysis is also examined. It is shown with simulated data that these methods are effective at differentiating and localizing the attenuation change associated with tissue coagulation, but that, in the analysis of phantom or ex vivo bovine liver data, electronic noise severely limits the effectiveness of these techniques.

Development of a radiofrequency based thermal therapy technique in an in vivo porcine model for the treatment of small renal masses.

PURPOSE: Incidentally detected small renal tumors appear to grow slowly and be localized to the kidney. Minimally invasive therapies are being investigated as alternatives to standard surgical techniques. Radiofrequency ablation has been reported for the treatment of small renal cell carcinomas. We developed a radiofrequency technique and established its efficacy and safety in a large animal model. METHODS AND METHODS: A total of 22 lesions were created in normal kidneys of 7 pigs. Radiofrequency energy was administered during open exposure of the kidneys or percutaneously under ultrasound guidance. Lesion development was monitored with gray-scale and power Doppler ultrasound. To avoid heating surrounding tissues new hydro-dissection and gas-dissection techniques were developed. Lesion sizes and characteristics were assessed by ultrasound and pathological examination. RESULTS: No complications were observed due to probe insertion and removal. Perirenal structures were thermally damaged before the development and application of the dissection techniques. Lesion size was accurately predicted by gray-scale ultrasound on day 7. Loss of perfusion in the ablated volume was confirmed by power Doppler ultrasound. Lesions were wedge-shaped, presumably due to the effects of heating on segmental blood flow distribution. Pathological examination revealed changes consistent with thermal injury and ischemic type infarction. CONCLUSIONS: Radiofrequency thermal therapy is an effective and efficient method for ablating normal renal tissue in the pig. It may be applied percutaneously under ultrasound guidance with minimal complications provided that vital adjacent structures are protected from thermal damage. Further studies are required in humans before adopting this technique as definitive treatment for small renal cell carcinoma.

Temperature measurement artefacts of thermocouples and fluoroptic probes during laser irradiation at 810 nm.

This study examined the artefact induced in temperature measurements made with thermocouples and Luxtron fluoroptic probes in the presence of infrared radiation. Localized heating was created using a continuous-wave, 810 nm diode laser system emitting 2.0 W from a cylindrical diffusing optical fibre, in air, water and an agar-albumin phantom. The temperature was measured every 1.0 s for 10 to 150 s, with both a thermocouple and a Luxtron fluoroptic probe at distances of 2, 3, 4, 5, 6 and 7 mm from the cylindrical diffusing tip. In all cases, the fluoroptic probe recorded a higher temperature than the thermocouple during laser irradiation. The difference in measured temperatures between the Luxtron probe and the thermocouple ranged from 1.6 degrees C to 18.8 degrees C in air, from 0.3 degrees C to 10.2 degrees C in water, and from 1.4 degrees C to 10.1 degrees C in phantom, depending on the distance of the probe from the laser source. The results suggest that in the presence of laser irradiation, self-heating of the Luxtron probe induces a significant artefact in temperature measurements at distances less than 4 mm from the source fibre. As a result, fluoroptic probes may not be suitable for monitoring tissue temperature for treatments when laser irradiation is present if sensors are located close to the fibre tip (<4 mm).

Changes in ultrasound properties of porcine kidney tissue during heating.

Changes in the ultrasound (US) attenuation and backscatter of fresh pig kidney were measured as the tissue was heated. The objective was to use these changes to predict how an US image would change in real-time with a view to its use as a monitoring tool for minimally invasive thermal therapy (MITT). Separate samples of fresh pig kidney were heated from 37 degrees C to temperatures of 45 degrees, 50 degrees, 55 degrees, 60 degrees and 65 degrees with warm water. Measurements were made over the frequency range from 3.5 MHz to 7.0 MHz during 30-min heating experiments. A general increase in attenuation magnitude (dB/cm) and slope (dB/cm-MHz) was observed at temperatures of 55 degrees C or greater. Little change in backscatter power was observed during heating to 45 degrees C. At higher temperatures, the changes in backscatter showed a more complex pattern throughout the experiments, but still showed a trend of increase to a greater value at the end of heating than at the start. This backscatter increase was greater at higher temperatures. The net effect of the changes in US properties suggests that it may be possible to use diagnostic US to monitor, in real-time, MITT in kidney.

B-scan ultrasound imaging of thermal coagulation in bovine liver: log envelope slope attenuation mapping.

Thermal therapy is an experimental procedure to treat localised tumours in the prostate, liver, kidney or breast. Monitoring is important to ensure complete tumour destruction while sparing adjacent sensitive structures. Previous work demonstrated that, when biological tissues are heated above 65 degrees C, ultrasound (US) attenuation increases due to thermal coagulation of tissue. This study examined the feasibility of a B-scan, envelope-based attenuation estimation method to monitor thermal therapy. The slope of the log of the signal envelope was used to estimate attenuation. It is shown that this method is able to demonstrate, from simulated and phantom data, a limited ability to differentiate attenuation levels that correspond to thermally coagulated and uncoagulated beef liver. In ex vivo bovine liver, however, issues due to signal-to-noise ratio (SNR) of the signal envelope prevented the visulisation of a 2-cm diameter thermally coagulated inclusion. The limitations of this method are extended to the multinarrow-band (MNB) technique.

Laser thermal therapy: utility of interstitial fluence monitoring for locating optical sensors.

Multipoint optical fluence measurements can potentially be used to detect coagulation-induced changes in optical propagation during interstitial laser thermal therapy. Estimating the dimensions of coagulation using on-line optical monitoring, which is applicable to treatments where the tip of the source fibre is not precharred, may be limited by the accuracy of the placement of optical sensors with respect to source fibres. A strategy has been developed to determine accurately the position of a four-sensor linear array, prior to treatment, using optical fluence data obtained from the sensors for low-power (< or = 0.5 W) irradiation. A minimum of four sensors in an array was required in order to develop a mathematical formulation for position determination that did not require tissue optical properties or laser power as input. Optical propagation was based on diffusion theory for homogeneous tissues in spherical geometry. Low input laser power is needed to ensure that there are no thermally induced changes in tissue optical properties not accounted for in the mathematical description. Experimental evaluation was performed in a tissue-equivalent liquid phantom using 0.5 W of 805 nm optical energy and a translatable isotropic optical sensor. For sensor locations with 2 mm spacing, placement accuracy of 0.67 mm was achieved. The accuracy improved to 0.13 mm as the sensor spacing increased to 5 mm.