Medical device development requires a phase of instrument testing and performance validation using experimental biologic targets including cells, tissues, organs and individuals in vitro and in vivo. In this development phase, qualitative and quantitative pathologic methods can be very useful to evaluate the mechanisms of detection/effect and to map the extent and severity of device energy/tissue interactions. This paper is a general overview of the philosophical and practical aspects of 1) choosing and preparing effective collaborations with biomedical experts, 2) choosing appropriate biologic target tissues and 3) preparing the tissues to obtain optimal pathologic results.

Minimally invasive cryothermic and hyperthermic therapies are being increasingly used to destroy dysfunctional and neoplastic tissues in several organ systems. This report morphologically compares the acute tissue response that follow cryothermic and microwave therapy in porcine kidneys. Three cryothermic and hyperthermic groups of treated kidneys were pooled from other studies for evaluation: 1) in vitro treated non-perfused, 2) in situ treated with 2-hour post in vivo perfusion, and 3) in situ treated with 3-day or 7-day post in vivo perfusion. The cryolesions showed uniform central coagulative-type necrosis and interstitial hemorrhage. The hyperthermic lesions showed central thermal fixation and a rim of coagulative necrosis. The cryothermic and hyperthermic lesions both had a similar narrow transition zone of partial cell injury. The cryothermic lesions developed a wound healing response that advanced into the central lesion. In contrast, the heat-treated tissues lacked a prominent wound healing response and appeared to resist breakdown/repair by the body. Thus, the tissue effects of and response to cryothermic and heat injury appear to be different.

In this review we have summarized the basic principles that govern the relationships between thermal exposure (temperature and time of exposure) and thermal damage, with an emphasis on normal tissue effects. We have also attempted to identify specific thermal dose information (for safety and injury) for a variety of tissues in a variety of species. We address the use, accuracy and difficulty of conversion of an individual time and temperature (thermal dose) to a standardized value (eg equivalent minutes at 43degC) for comparison of thermal treatments. Although, the conversion algorithm appears to work well within a range of moderately elevated temperatures (2-15degC) above normal physiologic baseline (37-39degC) there is concern that conversion accuracy does not hold up for temperatures which are minimally or significantly above baseline. An extensive review of the literature suggests a comprehensive assessment of the "thermal dose-to-tissue effect" has not previously been assembled for most individual tissues and never been viewed in a semi-comprehensive (tissues and species) manner. Finally, we have addressed the relationship of thermal dose-to-effect vs. baseline temperature. This issues is important since much of the thermal dose-to-effect information has been accrued in animal models with baseline temperatures 1-2 deg higher than that of humans.

Estimating effective thermal damage process coefficients for the first order model of damage processes is not difficult when the temperature is held constant for a substantial period. Laser coagulation experiments, however, are of short duration and, because of non uniform beam profiles, exhibit important heat transfer effects: the thermal histories are transient by nature. We obtain the activation energy, E, and collision frequency factor, A, directly from the transient history at the boundary of the zones of white coagulation and red hemorrhagic coagulation in liver in the rat, as identified in histologic studies. The estimates are obtained by testing a large number of coefficients and determining the "best fit" from a cost function. Useful values may obtained from a single experiment if the transient history used has a very high confidence level N i.e. a few excellent curves are preferable to single curves at a large number of durations of exposure.

The optimization of electrosurgical procedures requires a rigorous understanding of the electrical, thermal, mechanical and chemical events accompanying the ablation process. Modeling is indispensable and is needed to further advance this technology. This study introduces a novel tissue electrosurgical ablation model based on interstitial vapor nucleation and expansion. The model describes interstitial vapor nucleation and bubble growth using a homogeneous nulceation theory and Rayleigh equation. Electrosurgical incisions were made on beef muscle while equivalent electrical circuit patameters were monitored as a function of power settings and scalpel geometries. Thermal damage was measured using light and polarization microscopy. Results were compared with predictions produced by a numerical simulation, which modeled the tissue and electrosurgical scalpel interaction as a function of power settings and scalpel geometry.

Breast and liver cancers provide an ongoing challenge in regard to treatment efficacy and successful clinical outcomes. A variety of percutaneous technology has been applied for thermal treatment of the liver and breast, including laser, microwave, cryogenic and radiofrequency (RF) devices. When simplicity and cost are factored in, RF hardware and applicators offer the most cost-effective treatment pathway by interventional radiologists and surgeons. To model percutaneous RF treatments in liver and breast, simulations were done in 3D with a finite element model. Three RF systems were modeled, including 1) single needle; 2) clustered needle, cooled and uncooled; and 3) deployable, hook electrodes. The results show the limitations of the systems in percutaneous procedures, depending on temperature limits, duration of treatment, and whether the devices are cooled or uncooled. For thermal treatment, the isotherm of 55°C was considered the margin of coagulation necrosis. The 3-D volumes of 55°C and 65°C isotherm shells aid in the selection of the best method to improve clinical outcomes, while paying attention to the size and shape of the applicator and duration of treatment.

This study investigated the contribution of applied thermal energy during the nucleoplasty procedure by obtaining temperature maps throughout human cadaveric disc specimens (n=5) during a simulated treatment protocol. The procedure was performed using the ArthroCare SpineWand RF-Coblation device inserted through a 17 g needle into the cadaveric disc. The device uses a dual mode heating technique which employs a high voltage RF plasma field to vaporize tissue, followed by RF current heating for thermal coagulation. The device is manipulated to create a series of 6 channels at a 60 degree angular spacing within a period of 3 minutes. A computer-controlled, motorized translational system was used to mimic the insertion (coblation) and retraction (rf-coagulation) performed during clinical implementation. Rotation was performed manually between each coblation/rf-coagulation cycle. Transient temperature data were obtained using five multi-junction thermocouple probes (5 to 6- 0.05 mm diameter junctions spaced at either 2 or 5 mm) spaced throughout the desired heating volume. Temperature distributions and accumulated thermal doses calculated from the temperature-time history were used to define probable regions of thermal coagulation. Intra-discal temperatures of 60-65C were measured within 2 to 3 mm radial distance from the introducer with therapeutic thermal doses of >250 EM_43C achieved at radial distances of up to 5 mm from the introducer. Although appreciable regions of thermal coagulation within the nucleus are localized around the applicator, improper placement of the applicator during treatment may also generate undesirable hot spots in the bone endplate.

Cryogen spray cooling (CSC) is used to minimize the risk of epidermal damage during pulsed laser treatment of port wine stain (PWS) birthmarks. Unfortunately, the current approach to CSC does not provide the necessary epidermal protection for all patients, particularly those with darker skin types. Therefore, alternative approaches need to be sought to improve PWS laser therapy. On a previous numerical study we showed that using multiple-intermittent CSC spurts and laser pulses could permit, under certain conditions, the use of higher laser doses while providing sufficient epidermal protection. In this study we show some results of ongoing experimental to study the feasibility of implementing clinically such an approach.

An in vitro study was performed to investigate a more effective method of destroying malignant tissue during cyrosurgery, which is based on eutectic crystallization. Eutectic formation is a solidification process through which water and solutes form a hydrate and can be recognized by a secondary heat release in differential scanning calorimetry (DSC). We investigated whether it is possible to induce eutectic crystallization by infusing concentrated salt solutions into cell suspension and tissue systems. These systems included AT-1 rat prostate tumor and normal rat liver tissues. In cell suspensions, the post-thaw viability significantly drops at or below the temperatures where eutectic crystallization occurred. When eutectic crystallization is induced in tissues, histological analysis shows significantly enhanced freezing injury. These results imply that this method may be of benefit in cryosurgical applications particularly at the edge of the iceball where tumor cell survival is in question. The possible advantages of inducing eutectic crystallization are i) enhancement of direct cell injury; ii) enlargement of effective cryosurgical cell/tissue destruction zone by selecting a salt with a high eutectic temperature; and iii) improvement of the efficacy of monitoring during cryosurgery.

The effectiveness of cryosurgery in treating tumors is highly dependent on knowledge of freezing extent, and therefore relies heavily on real-time imaging techniques for monitoring. Electrical impedance tomography (EIT), which utilizes tissue impedance variation to construct an image, is very well-suited to cryosurgery since frozen tissue impedance is much higher than that of unfrozen tissue. In this study, we develop numerical models to evaluate the theoretical ability of EIT to image cryosurgery. We begin in the simplified 2D arena, and then extend this line of study to the more appropriate 3D realm. Our simulated finite element phantoms and pixel-based Newton-Raphson reconstruction algorithms were able to produce easily identifiable images of frozen regions within tissue. We hope that these findings will serve as a stepping stone in developing EIT as a promising supplement to existing cryosurgical monitoring techniques.

Cryosurgery, in which the whole gland is frozen, has a high rate of impotence, similar to non-nerve sparing radical prostatectomy. In this paper we will present a pilot study in which 9 patients treated with focal, unilateral nerve sparing cryosurgery were followed for up to 6 years. Methods- Prior to focal nerve sparing cryosurgery all patients were re-biopsied on the side opposite the previous positive biopsy. One neurovascular one spared on the side opposite the positive biopsy. Just prior to the start of freezing a 22 gauge spinal needle was placed into Denonvillier's fascia via a transperineal route and saline were injected to separate the rectum from the prostate. CHT was stopped in all patients postoperatively. PSA’s were obtained every 3 months for the first two years and then every 6 months thereafter. Patients were considered to have a stable PSA if they had two consecutive PSA’s without a rise. All patients were strongly encouraged to have routine biopsies despite a stable PSA. Results-Between 6/95 and 11/00, 9 patients had focal, nerve sparing cryosurgery. Follow up ranged from 6-72 months with a mean of 36 months. All patients have stable PSA’s at this time. Six patients routinely biopsied had negative biopsies. Potency (defined as erection sufficient to complete intercourse to the satisfaction of the patient) has been maintained in 7 of 9 patients. Conclusion-Focal nerve sparing cryosurgery, in which one NVB is spared, appears to preserve potency in a majority of patients without compromising cancer control. These preliminary results warrant further study.

In this paper, we report the characterization of microwave therapy in normal porcine kidneys both in vitro and in vivo. This technology is being developed for eventual use in the treatment of small renal cell carcinomas (RCC) using minimally invasive procedures. Microwave energy was applied through an interstitial microwave probe (Urologix, Plymouth, MN) to the kidney cortex with involvement of the medulary region. The thermal histories at several locations were recorded. After treatment, the kidneys were bisected and tissue sections were prepared for histologic study at approximately the same depth as the thermal probe. Histologic cellular injury and microvascular stasis were quantitatively evaluated. Absolute rate kinetic models of cellular injury and vascular stasis were fit to the thermal and histologic data to determine the kinetic parameters. A 3-D finite element thermal model based on the Pennes Bioheat equation was developed and solved using a commercial software package (ANSYS, V5.7). The specific absorption rate (SAR) of the microwave probe was measured experimentally. This is the first thermal model validated using measured in vitro thermal histories and then used to determine the blood perfusion term in vivo.

More than 200,000 hysterectomies are performed annually in the US due to abnormal uterine bleeding from excessive menstrual flow. A minimally invasive procedure has been developed using thermal treatment combined with pressure to the endometrial lining of the uterus. Results from a 3-D finite element model will be shown, as well as experimental data. Good correlation was seen between simulations and experiments. The study found similar results then temperatures were increased and times for treatment were shortened.More than 200,000 hysterectomies are performed annually in the US due to abnormal uterine bleeding from excessive menstrual flow. A minimally invasive procedure has been developed using a balloon-based thermal treatment combined with pressure to the endometrial lining of the uterus. A 3D finite element model was set up to simulate the balloon ablation device in the human uterus as used in over 150,000 patients to date. Several additional simulations were made at higher temperatures to seek alternative combinations with higher temperature and shorter time intervals for the same depth of penetration, or deeper penetration at longer times and elevated temperatures. A temperature range of 87 to 150°C was explored. The Bioheat Equation was used in the simulations to predict temperature distributions in tissue. The Damage Integral was also used to characterize the location at depth of irreversible damage in the uterus. Treatment safety issues were also analyzed as the simulations showed the depth of penetration into the myometrium, towards the serosa.

Theoretical and experimental approaches were used to develop and evaluate site-specific designs of internally cooled direct coupled (ICDC) and catheter-cooled (CC) ultrasound applicators for thermal coagulation of disease in the prostate, liver, brain, and uterus. The diameter of an interstitial applicator can influence its clinical practicality and effectiveness as well as application site. One purpose of this study was to determine whether the use of larger ultrasound transducers and the inherent increase in applicator size could be justified by potentially producing larger lesion diameters. A second purpose was to explore how the response of tissue acoustic attenuation to heating effects lesion size and preferred applicator configuration. Four applicator configurations and sizes were studied using ex vivo tissue experiments in liver and beef and using acoustic and biothermal simulations. Transmission attenuation measurements showed a 6 to 8 fold increase in baseline tissue attenution inside interstitial ultrasound lesions. Formation of these high attenuation zones in lesions reduced potential lesion size. Larger applicators produced lesions with radial penetration depths superior to their smaller counterparts at power levels in the 20-40W /cm range. The higher cooling rates along the outer surface of the larger diameter applicators due to their greater surface area was a dominant factor in increasing lesion size. The higher cooling rates pushed the maximum temperature farther from the applicator surface and reduced the formation of high acoustic attenuation tissue zones. Acoustic and biothermal simulations matched the experimental data well and were applied to model these applicators within sites of clinical interest such as prostate, uterine fibroid, brain, and normal liver. Lesions of 3.9 to 4.7cm diameter were predicted for moderately perfused tissues such as prostate and fibroid and 2.8 to 3.2cm for highly perfused tissues such as normal liver. Feedback control to reduce maximum tissue temperatures helped to decrease formation of sound-blocking high attenuation zones. This work was supported by a gift from the Oxnard Foundation and Johnson & Johnson.

Microwave imaging has been investigated as a method of non-invasively estimating tissue electrical properties especially the conductivity, which is highly temperature dependent, as a means of monitoring thermal therapy. The technique we have chosen utilizes an iterative Gauss-Newton approach to converge on the correct property distribution. A previous implementation utilizing the complex form (CF) of the electric fields along with a sub-optimal phantom experimental configuration resulted in imaging temperature accuracy of only 1.6°C. Applying the log-magnitude/phase form (LMPF) of the algorithm has resulted in imaging accuracy on the order of 0.3°C which is a significant advance for the area of treatment monitoring. The LMPF algorithm was originally introduced as a way to reconstruct images of large, high-contrast scatterers as is the case in breast imaging. However, recent analysis of the Jacobian matrices for the comparable implementations has shown that the reconstruction problem in the new formulation more closely resembles a linear task as is the case in x-ray computed tomography. The comparisons were performed by examining plots of the Jacobian matrix terms for fixed transmit and receive antennas which demonstrated higher sensitivity in the center of the imaging zone along with narrower paths of senstivity between the atnenna pair for the LMPF algorithm. Animal model experiments have also been performed to validate these capabilities in a more realistic setting. Finally, the overall computational efficiency has been significantly enhanced through the use of the adjoint image reconstruction approach. This enables us to reconstruct images in roughly one minute which is essential if the approach is to be used as a therapy feedback mechanism.

Nonlinear ultrasonic imaging methods are used in visualization of lesion formation in freshly excised tissue. Both of these methods are more sensitive to nonlinear echoes than standard B-mode imaging. While all three methods typically show increased echogenicity at the lesion location, the nonlinear methods exhibit more localized echo enhancement than B-mode imaging. Therefore, nonlinear methods are potentially better suited to lesion mapping for purposes of image guidance. Quadratic images have the added advantage of a significant increase in image dynamic range and noise reduction. The results shown in this report continue to support the hypothesis that micro-bubbles play an important role of lesion formation. In this paper, we present imaging results before and after volumetric lesion formation in ex vivo tissue. The results illustrate the advantage of nonlinear imaging methods compared to conventional B-scan imaging in terms of accurate mapping of lesion size and location.

Two catheter-based transurethral ultrasound applicators designed for selective thermal coagulation of prostate tissue were evaluated. The first applicator utilized two 3.5 mm piezoelectric sectored tubes with the active transducer surface forming 90°. The second applicator's transducer assembly consisted of a linear array of 3.5 x 10 mm planar transducer elements. Both applicators operated at 8 MHz and were positioned on a 4 mm diameter catheter within an integrated expandable balloon (10 mm). Manual rotation of the transducer assembly within the balloon allowed for angular control and/or sweeping of the treatment volume. Ambient temperature degassed cooling water (~120 ml/min) was circulated inside the balloon to preserve the urethral mucosa. Acoustic efficiencies of 20-54% and acoustic beam distributions were measured. The thermal treatment characteristics of the applicator were investigated in vivo (canine prostate) under MRI guidance in an interventional open magnet (0.5 T). Magnetic resonance thermal imaging (MRTI) monitored the treatments (GRE phase mapping, multiple planes, 15 sec update intervals). Post-treatment imaging (T1 w/contrast) and TTC staining of the prostate were used to verify zones of thermal damage. Single sonications lasting 8-15 min produced coagulated zones of tissue extending to the outer boundary of the prostate while preserving 2-3 mm of urethral mucosa. Multiple sonications in sequence produced larger contiguous sectors of coagulated tissue (~ 3/4 of the gland). In summary, highly directional transurethral applicators under MRI guidance were able to produce selective and controllable thermal coagulation.

Fiber optically delivered laser energy may be ideal in treating small intracerebral lesions with minimal invasiveness. We have continued development of a laser-computer system for automated magnetic resonance thermal imaging (MRTI) guidance and control of intracerebral laser interstitial thermal therapy (LITT). The system consists of a workstation which is interfaced to a clinical MR scanner via Ethernet and to a compact high power diode laser via hardware interface. The system analyzes MRTI data to compute temperature changes based on the proton resonance frequency (PRF) shift, and constructs two-dimensional maps of temperature and estimated chronic thermal damage during therapy. Images are obtained approximately every 4.5 seconds allowing near-real-time tracking of LITT progress. A graphical user interface allows specification of temperature constraints on the image which regulate delivery of thermal energy. We have tested the ability of the system to create small focal intracranial lesions of specified dimension in both normal canine brain (n = 6 animals, 15 lesions) and in an intracerebral tumor model grown from inoculum (n = 11 animals, 15 lesions). Histological analysis was used to assess the accuracy of MRTI-derived predictions of lesion size and to assess effectiveness of reaching prescribed tumor boundaries.

Recent interest in using High Intensity Focused Ultrasound (HIFU) for surgical applications such as hemostasis and tissue necrosis has stimulated the development of image-guided systems for non-invasive HIFU therapy. Seeking an all-ultrasound therapeutic modality, we have developed a clinical HIFU system comprising an integrated applicator that permits precisely registered HIFU therapy delivery and high quality ultrasound imaging using two separate arrays, a multi-channel signal generator and RF amplifier system, and a software program that provides the clinician with a graphical overlay of the ultrasound image and therapeutic protocol controls. Electronic phasing of a 32 element 2 MHz HIFU annular array allows adjusting the focus within the range of about 4 to 12 cm from the face. A central opening in the HIFU transducer permits mounting a commercial medical imaging scanhead (ATL P7-4) that is held in place within a special housing. This mechanical fixture ensures precise coaxial registration between the HIFU transducer and the image plane of the imaging probe. Recent enhancements include development of an acoustic lens using numerical simulations for use with a 5-element array. Our image-guided therapy system is very flexible and enables exploration of a variety of new HIFU therapy delivery and monitoring approaches in the search for safe, effective, and efficient treatment protocols.

MRI compatible, multi-element ultrasound applicators were fabricated using cylindrical piezoceramic transducers sectored to 180 degrees to provide angular directional heating. The applicators were designed to be inserted into standard 13 or 14 gage brachytherapy catheters integrated with water-cooling. Two applicators were inserted transperinealy into the posterior region of a canine prostate. Power output ranged from 5-15 W per element during the 15 minute heating period. Phase-sensitive gradient-recalled MR imaging was used to monitor the treatment in real-time on a 0.5 Tesla MRT system. Gadolinium-enhanced T1 weighted images and diffusion-weighted images were obtained to view the regions which had been ablated during the heating procedure. Upon euthanasia, the prostate was removed, axially sectioned, and stained with TTC to reveal any regions of remaining viable tissue. Results from this study indicated a large volume of ablated tissue within the prostate which was highly correlated to the regions in the T1-weighted and diffusion-weighted images which had decreased intensity, and to the 52C contour displayed in the images obtained during the treatment. This study demonstrates the ability to control thermal coagulation within a targeted tissue volume while protecting surrounding tissue from thermal damage.

The development of therapeutic ultrasound requires specially adapted transducers for the generation of high intensity focused beams. New piezocomposite technologies have been developed to take into account the high power requirements and constraints like the high degree of focussing, the capability to withstand thermal shocks, the requirement for safe and reliable transducers, the compatibility with MRI and Ultrasonic imaging systems. This approach enables a wide variety of shapes and the design of array transducers for electronic focusing, scanning and steering of the therapeutic beam. Recent results will be presented on power and efficiency aspects. The feasibility of highly focused transducers with several array structures as well as the combination of these technologies with MRI and Ultrasonic imaging systems will be illustrated through various examples.

At high intensity levels, ultrasound energy focused into remote tissue targets in human body has shown to produce thermal necrosis in circumscribed regions with sub-millimeter accuracy. The non-invasive modality known as HIFU has enormous potential for thermal ablation of cancers/tumors of the human body without any adverse effects in the surrounding normal tissue. In this paper, empirical results for parametric assessment and interdependence of several exposure variables are presented for producing thermal necrosis as well as hemostasis. Multiple HIFU transducers in selective spatial configuration have been deployed using a suitably designed experiemntal harness, with and without motorized jig scanning. The pre-planning and on-line procedure for treatment and specified instrumentation is described. Custom designed 25mm aperture HIFU probes resonating at 2 MHz focused at 64 and 80 mm are used. Results have been obtained in ex-vivo animal tissue and in vitro biological phantoms for hemostasis.

Cryogen spray cooling (CSC) is used to pre-cool the epidermis during laser dermatological procedures such as treatment of port wine stain (PWS) birthmarks. It is known that PWS patients with medium to high epidermal melanin concentrations are at a high risk of epidermal thermal damage after laser irradiation. To avoid this complication, it is necessary to maximize CSC efficiency and, thus, essential to understand the mechanical and thermal interactions of cryogen droplets with the sprayed surface. It has been observed that cryogen sprays exhibit droplet rebound as droplets impinge on the skin surface. Studies of water droplet impact on hard surfaces have shown that droplet rebound may be suppressed by dissolving small amounts (a few percent) of diverse polymer or surfactant solutions prior to atomization. To investigate the possibility of suppressing the rebound of cryogen droplets in a similar way, we have constructed a device that allows observation of the impact, spreading, and rebound of individual water and cryogen droplets with and without these solutions, and their influence on cryogen/surface dynamics and heat transfer. Our preliminary studies show that dissolving a 4% non-ionic surfactant in water reduces droplet rebound and thickness of the residual liquid layer. The maximum spread of water droplets after impact can be described within 20% accuracy by a previously developed theoretical model. The same model provides an even more accurate prediction of the maximum spread of cryogen droplets. This study will aid the analysis of future results and design conditions of new studies, which will recreate conditions to determine if added surfactant solutions suppress droplet rebound and lead to improved CSC efficiency.

For various soft tissues (e.g., liver, breast, etc.), we are developing the ultrasonic strain measurement-based mechanical properties (shear modulus, visco-shear modulus, etc.) reconstruction/imaging technique. To clarify the limitation of our quantitative reconstruction/imaging technique as a diagnostic tool for differentiating malignancies, together with improving the spatial resolution and the dynamic range we are collecting the clinical reconstruction image data. Furthermore, we are applying our technique as a monitoring technique for the effectiveness of chemical therapy (e.g., anticancer drug, ethanol, etc.), thermal therapy (e.g., micro, and rf electromagnetic wave, HIFU, LASER, etc.), and cryotherapy. As soft tissues are deformed in 3-D space due to externally situated quasi-static and/or low frequency mechanical sources, multidimensional signal processing improves strain measurement accuracy and reduces inhomogeneity-dependent modulus reconstruction artifacts. These have been verified by us through simulations and phantom/animal in vitro experiments. Briefly, here we discuss the limitations of low dimensional signal processing. Moreover, we exhibit the superiority both on differential diagnosis for these human in vivo malignancies and monitoring for these therapies of our quasi-real time imaging (using conventional US equipment) to conventional B-mode imaging. Our technique is available as a clinical visualization technique both for diagnosis and treatment, and monitored mechanical properties data can also be effectively utilized as the measure for controlling the therapy, i.e., the exposure energy, the foci, the exposure interval, etc. In the near future, suitable combination of various simple and low-invasive therapy techniques with our imaging technique will open up a new clinical style allowing diagnosis and the subsequently immediate treatment. This must substantially reduce the total medical expenses.