Monitoring cryosurgery in the brain and in the prostate with proton NMR

Optimization of prostatic cryosurgery with multi-cryoprobe based on refrigerant flow

Cryosurgery is a promising novel minimally invasive surgical technique to eradicate carcinoma and non-carcinoma tissues by freezing. In this research, we applied a transient 3D two-phase refrigerant flow model inside the LN₂ boiling chamber as well as a bioheat transfer model inside the tissues to evaluate the optimized ablation outcome during prostatic cryosurgery. For the evaluation of the scenarios, a defect function was used that considers non-ablated target tissue (prostate/cancer tissue) as well as ablated healthy tissue, in which the ablated tissue was evaluated using a temperature threshold. Three different configurations using three LN₂ cryoprobes were analyzed during the modeling study, and the best configuration with the three LN₂ cryoprobes positioned isoscelesly was found. For this configuration, temperature distributions and temperature profiles at specific points within the tissue were investigated numerically. Owing to its low computational cost, the 3D coupled model has an advantage in accurate modeling cryosurgery for curing numerous diseases.

Computerized Planning of Prostate Cryosurgery and Shape Considerations

The current study aims to explore possible relationships between various prostate shapes and the difficulty in creating a computer-based plan for cryosurgery. This research effort is a part of an ongoing study to develop computational means in order to improve cryosurgery training and education. This study uses a computerized planner-a key building block of a recently developed prototype for cryosurgery training. The quality of planning is measured by the overall defect volume, a proprietary concept which refers to undercooled areas internal to the target region and overcooled areas external to it. Results of this study numerically confirm that the overall defect volume decreases with an increasing number of cryoprobes, regardless of the geometry of the prostate. However, the number of cryoprobes required to achieve the smallest possible defect may be unrealistically high (<30). Results of this study also demonstrate that the optimal cryoprobe layout is associated with a smaller defect for symmetric prostate geometries and, independently, for prostate models that better resemble a sphere. Furthermore, a smaller defect is typically achieved when the urethra passes through the center of the prostate model. This study proposes to create a cryoprobe convex hull for the purpose of initial planning, which is a subdomain similar in shape to the prostate but at a reduced size. Parametric studies indicate that a cryoprobe convex hull contracted by 7 to 9 mm in all directions from the prostate capsule serves as a quasi-optimal initial condition for planning, that is, a preselected number of cryoprobes placed in the cryoprobe convex hull yields favorable results for optimization. The cryoprobe convex hull could accelerate computer-based planning, while also being adopted as a concept for traditional cryosurgery training, when computerized means are absent.

Cryoelectrolysis-electrolytic processes in a frozen physiological saline medium

Background

Cryoelectrolysis is a new minimally invasive tissue ablation surgical technique that combines the ablation techniques of electrolytic ablation with cryosurgery. The goal of this study is to examine the hypothesis that electrolysis can take place in a frozen aqueous saline solution.

Method

To examine the hypothesis we performed a cryoelectrolytic ablation protocol in which electrolysis and cryosurgery are delivered simultaneously in a tissue simulant made of physiological saline gel with a pH dye. We measured current flow, voltage and extents of freezing and pH dye staining.

Results

Using optical measurements and measurements of currents, we have shown that electrolysis can occur in frozen physiological saline, at high subzero freezing temperatures, above the eutectic temperature of the frozen salt solution. It was observed that electrolysis occurs when the tissue resides at high subzero temperatures during the freezing stage and essentially throughout the entire thawing stage. We also found that during thawing, the frozen lesion temperature raises rapidly to high subfreezing values and remains at those values throughout the thawing stage. Substantial electrolysis occurs during the thawing stage. Another interesting finding is that electro-osmotic flows affect the process of cryoelectrolysis at the anode and cathode, in different ways.

Discussion

The results showing that electrical current flow and electrolysis occur in frozen saline solutions imply a mechanism involving ionic movement in the fluid concentrated saline solution channels between ice crystals, at high subfreezing temperatures. Temperatures higher than the eutectic are required for the brine to be fluid. The particular pattern of temperature and electrical currents during the thawing stage of frozen tissue, can be explained by the large amounts of energy that must be removed at the outer edge of the frozen lesion because of the solid/liquid phase transformation on that interface.

Conclusion

Electrolysis can occur in a frozen domain at high subfreezing temperature, probably above the eutectic. It appears that the most effective period for delivering electrolytic currents in cryoelectrolysis is during the high subzero temperatures stage while freezing and immediately after cooling has stopped, throughout the thawing stage.

Issues Critical to the Successful Application of Cryosurgical Ablation of the Prostate

The techniques of present-day cryosurgery performed with multiprobe freezing apparatus and advanced imaging techniques yield predictable and encouraging results in the treatment of prostatic and renal cancers. Nevertheless, and not unique to cryosurgical treatment, the rates of persistent disease demonstrate the need for improvement in technique and emphasize the need for proper management of the therapeutic margin. The causes of persistent disease often relate to a range of factors including selection of patients, understanding of the extent of the tumor, limitations of the imaging techniques, and failure to freeze the tumor periphery in an efficacious manner. Of these diverse factors, the one most readily managed, but subject to therapeutic error, is the technique of freezing the tumor and appropriate margin to a lethal temperature [Baust, J. G., Gage, A. A. The Molecular Basis of Cryosurgery. BJU Int 95, 1187–1191 (2005)]. This article describes the recent experiments that examine the molecular basis of cryosurgery, clarifies the actions of the components of the freeze-thaw cycle, and defines the resultant effect on the cryogenic lesion from a clinical perspective. Further, this review addresses the important issue of management of the margin of the tumor through adjunctive therapy. Accordingly, a goal of this review is to identify the technical and future adjunctive therapeutic practices that should improve the efficacy of cryoablative techniques for the treatment of malignant lesions.

Progress toward Optimization of Cryosurgery

Cryosurgery for diverse neoplastic and non-neoplastic diseases has expanded in applicability in recent years, especially since intraoperative ultrasound became available as a method of monitoring the process of tissue freezing. However, persistence of disease after presumably adequate cryosurgical treatment has disclosed deficiencies in the technique, perhaps due to faulty application of the freeze-thaw cycles or due to shortcomings in the imaging method. Clearly cryosurgical technique is less than optimal. The optimal dosimetry for tissue freezing, the recent improvements in imaging techniques, and the need for adjunctive therapy are defined in this review, which assesses the progress toward improving the efficacy of cryosurgery.

Improved Cryosurgery by Use of Thermophysical and Inflammatory Adjuvants

In the present article, recent research efforts in our laboratory to improve cryosurgery by use of mechanistically derived adjuvants are reviewed. Our research has been focused on enhancing two freezing induced injury mechanisms - i) direct cell injury by use of thermophysical adjuvants, and ii) vascular injury by use of an inflammatory adjuvant. The thermophysical adjuvants are chemicals, usually salts, which can induce secondary crystallization, called eutectic solidification, in a cryolesion; thereby enhancing direct cell injury. The inflammatory adjuvant is a cytokine, tumor necrosis factor-alpha (TNF-α), which upregulates inflammation of microvasculature in tumors prior to freezing to promote vascular injury in the cryolesion. Even though the individual mechanism of injury enhancement within the cryolesion of each adjuvant requires further study, both adjuvants are envisioned to enlarge the complete killing zone so that the boundary of the cryolesion matches more closely with the edge of ice-ball. By bringing the edge of the cryolesion closer to the edge of iceball, the adjuvants hold promise for improvement of image guidance and outcome of cryosurgery.

An efficient technique for estimating the two-dimensional temperature distributions around multiple cryo-surgical probes based on combining contributions of unit circles

This study presents an efficient, fast and accurate method for estimating the two-dimensional temperature distributions around multiple cryo-surgical probes. The identical probes are inserted into the same depth and are operated simultaneously and uniformly. The first step in this method involves numerical derivation of the temporal performance data of a single probe, embedded in a semi-infinite, tissue-like medium. The results of this derivation are approximated by algebraic expressions that form the basis for computing the temperature distributions of multiple embedded probes by combining the data of a single probe. Comparison of isothermal contours derived by this method to those computed numerically for a variety of geometrical cases, up to 15 inserted probes and 2-10 min times of operation, yielded excellent results. Since this technique obviates the solution of the differential equations of multiple probes, the computational time required for a particular case is several orders of magnitude shorter than that needed for obtaining the full numerical solution. Blood perfusion and metabolic heat generation rates are demonstrated to inhibit the advancement of isothermal fronts. Application of this method will significantly shorten computational times without compromising the accuracy of the results. It may also facilitate expeditious consideration of the advantages of different modes of operation and the number of inserted probes at the early design stage.

A new method for temperature-field reconstruction during ultrasound-monitored cryosurgery using potential-field analogy

The current study aims at developing computational tools in order to gain information about the thermal history in areas invisible to ultrasound imaging during cryosurgery. This invisibility results from the high absorption rate of the ultrasound energy by the frozen region, which leads to an apparent opacity in the cryotreated area and a shadow behind it. A proof-of-concept for freezing-front estimation is demonstrated in the current study, using the new potential-field analogy method (PFAM). This method is further integrated with a recently developed temperature-field reconstruction method (TFRM) to estimate the temperature distribution within the frozen region. This study uses prostate cryosurgery as a developmental model and trans-rectal ultrasound imaging as a choice of practice. Results of this study indicate that the proposed PFAM is a viable and computationally inexpensive solution to estimate the extent of freezing in the acoustic shadow region. Comparison of PFAM estimations and experimental data shows an average mismatch of less than 2 mm in freezing-front location, which is comparable to the uncertainty in ultrasound imaging. Comparison of the integrated PFAM + TFRM scheme with a full-scale finite-elements analysis (FEA) indicates an average mismatch of 0.9 mm for the freezing front location and 0.1 mm for the lethal temperature isotherm of -45 °C. Comparison of the integrated PFAM + TFRM scheme with experimental temperature measurements show a difference in the range of 2 °C and 6 °C for selected points of measurement. Results of this study demonstrate the integrated PFAM + TFRM scheme as a viable and computationally inexpensive means to gain information about the thermal history in the frozen region during ultrasound-monitored cryosurgery.

Temperature field reconstruction for minimally invasive cryosurgery with application to wireless implantable temperature sensors and/or medical imaging

There is an undisputed need for temperature-field reconstruction during minimally invasive cryosurgery. The current line of research focuses on developing miniature, wireless, implantable, temperature sensors to enable temperature-field reconstruction in real time. This project combines two parallel efforts: (i) to develop the hardware necessary for implantable sensors, and (ii) to develop mathematical techniques for temperature-field reconstruction in real time-the subject matter of the current study. In particular, this study proposes an approach for temperature-field reconstruction combining data obtained from medical imaging, cryoprobe-embedded sensors, and miniature, wireless, implantable sensors, the development of which is currently underway. This study discusses possible strategies for laying out implantable sensors and approaches for data integration. In particular, prostate cryosurgery is presented as a developmental model and a two-dimensional proof-of-concept is discussed. It is demonstrated that the lethal temperature can be predicted to a significant degree of certainty with implantable sensors and the technique proposed in the current study, a capability that is yet unavailable.

Percutaneous ablation in the kidney

Percutaneous ablation in the kidney is now performed as a standard therapeutic nephron-sparing option in patients who are poor candidates for resection. Its increasing use has been largely prompted by the rising incidental detection of renal cell carcinomas with cross-sectional imaging and the need to preserve renal function in patients with comorbid conditions, multiple renal cell carcinomas, and/or heritable renal cancer syndromes. Clinical studies to date indicate that radiofrequency ablation and cryoablation are effective therapies with acceptable short- to intermediate-term outcomes and with a low risk in the appropriate setting, with attention to pre-, peri-, and postprocedural detail. The results following percutaneous radiofrequency ablation and cryoablation in the treatment of renal cell carcinoma are reviewed in this article, including those of several larger scale studies of ablation of T1a tumors. Clinical and technical considerations unique to ablation in the kidney are presented, and potential complications are discussed.

MRI-guided cryoablation: In vivo assessment of focal canine prostate cryolesions

PURPOSE

To analyze the appearance of acute and chronic canine prostate cryolesions on T1-weighted (T1w) and T2-weighted (T2w) magnetic resonance imaging (MRI) and compare them with contrast-enhanced (CE) MRI and histology for a variety of freezing protocols.

MATERIALS AND METHODS

Three different freezing protocols were used in canine prostate cryoablation experiments. Six acute and seven chronic (survival times ranging between 4-53 days) experiments were performed. The change in T2w signal intensity was correlated with freezing protocol parameters. The lesion area on T2w MRI was compared to CE-MRI. Histopathologic evaluation of the cryolesions was performed and visually compared to the appearance on MRI.

RESULTS

The T2w signal increased from pre- to postfreeze at the site of the cryolesion, and the enhancement was higher for smaller freeze area and duration. The T2w lesion area was between the CE nonperfused area and the hyperenhancing CE rim. The appearance of the lesion on T1w and T2w imaging over time correlated with outcome on pathology.

CONCLUSION

T1w and T2w MRI can potentially be used to assess cryolesions and to monitor tissue response over time following cryoablation.

Thermal ablation of prostate diseases: advantages and limitations

Both benign and malignant conditions affecting prostate gland are very common in elderly men. However, the conventional treatment of these conditions can be associated with significant side effects and complications, and less invasive treatment alternative has been always searched for. Because of the anatomical location and easy accessibility of prostate, many newer treatment modalities using thermal ablation have been applied to the organ. These include not only heating of the pathological tissue but also freezing. Some of such treatment techniques have shown to be effective and safe and been clinically used widely. In this review article, various tissue ablation techniques using temperature change applied to prostate gland are covered. Each procedure's advantages and disadvantages are compared and discussed.

Computerized Planning of Cryosurgery: From Model Reconstruction to Cryoprobe Placement Strategies

As a part of an ongoing program to develop computerized tools for surgery, the current study focuses on the design of optimal cryoprobe layouts for prostate cryosurgery. Once a decision to treat the prostate with cryosurgery has been made, its application can be presented as a four-stage process: (i) 3D reconstruction of the target region; (ii) evaluation of the optimum number of cryoprobes and their layout; (iii) insertion of cryoprobes according to that plan; and, (iv) orchestrating cryoprobe operation to achieve the optimum match between the target region and the forming frozen region. Cryosurgical success equals the sum of the successes of each of the above stages. To date, this four-stage process is performed manually, relying upon the cryosurgeon's experience and "rules of thumb". This manuscript reviews recent efforts to develop the necessary building blocks for an integrated computerized surgical tool for prostate cryosurgery, which includes methods for prostate model reconstruction, schemes for bioheat transfer simulation, and optimization techniques for cryoprobe placement; experimental verification of these building blocks are also presented. The emphasis in this line of development is on performing a full-scale planning in less than one minute, while the patient is on the operation table. It can be concluded from the current manuscript that the above goals are achievable. The current manuscript concludes with a review of current challenges in the development of related computerized means.

MR imaging-guided percutaneous cryoablation of the prostate in an animal model: in vivo imaging of cryoablation-induced tissue necrosis with immediate histopathologic correlation

PURPOSE

To evaluate the feasibility of magnetic resonance (MR) imaging-guided percutaneous cryoablation of normal canine prostates and to identify MR imaging features that accurately predict the area of tissue damage at a microscopic level.

MATERIALS AND METHODS

Six adult male mixed-breed dogs were anesthetized, intubated, and placed in a 0.5-T open MR imaging system. A receive-only endorectal coil was placed, and prostate location and depth were determined on T1-weighted fast spin-echo (FSE) MR imaging. After placement of cryoprobes and temperature sensors, three freezing protocols were used to ablate prostate tissue. Ice ball formation was monitored with T1-weighted FSE imaging. Tissue necrosis area was assessed with contrast-enhanced weighted MR imaging and compared with histopathologic findings.

RESULTS

A total of 12 cryolesions (mean size, 1.2 cm) were bilaterally created in six prostates. Ice ball formation was oval and signal-free on T1-weighted FSE sequences in all cases. Postprocedural contrast-enhanced MR imaging typically showed a nonenhancing area of low signal intensity centrally located within the frozen area, surrounded by a bright enhancing rim in all cases. On histopathologic examination, two distinct zones were identified within cryolesions. Centrally, a necrotic zone with complete cellular destruction and hemorrhage was found. Between this necrotic zone and normal glandular tissue, a zone of fragmented and intact glands, interstitial edema, and rare acute inflammatory cells was seen. Correlation between nonenhancement on contrast-enhanced weighted MR images and tissue necrosis on pathologic examination was consistent within all six dogs.

CONCLUSIONS

MR imaging-guided cryoablation of the prostate is technically feasible. The nonenhancing area on postablation contrast-enhanced weighted MR imaging accurately predicts the area of cryoablation-induced tissue necrosis on pathologic analysis.

Computerized planning of prostate cryosurgery using variable cryoprobe insertion depth

The current study presents a computerized planning scheme for prostate cryosurgery using a variable insertion depth strategy. This study is a part of an ongoing effort to develop computerized tools for cryosurgery. Based on typical clinical practices, previous automated planning schemes have required that all cryoprobes be aligned at a single insertion depth. The current study investigates the benefit of removing this constraint, in comparison with results based on uniform insertion depth planning as well as the so-called "pullback procedure". Planning is based on the so-called "bubble-packing method", and its quality is evaluated with bioheat transfer simulations. This study is based on five 3D prostate models, reconstructed from ultrasound imaging, and cryoprobe active length in the range of 15-35 mm. The variable insertion depth technique is found to consistently provide superior results when compared to the other placement methods. Furthermore, it is shown that both the optimal active length and the optimal number of cryoprobes vary among prostate models, based on the size and shape of the target region. Due to its low computational cost, the new scheme can be used to determine the optimal cryoprobe layout for a given prostate model in real time.

Prostate cryotherapy monitoring using vibroacoustography: preliminary results of an ex vivo study and technical feasibility

The objective of this research is to prospectively evaluate the feasibility of vibroacoustography (VA) imaging in monitoring prostate cryotherapy in an ex vivo model. Baseline scanning of an excised human prostate is accomplished by a VA system apparatus in a tank of degassed water. Alcohol and dry ice mixture are used to freeze two prostate tissue samples. The frozen prostates are subsequently placed within the water tank at 27 degrees C and rescanned. VA images were acquired at prescribed time intervals to characterize the acoustic properties of the partially frozen tissue. The frozen prostate tissue appears in the images as hypoemitting signal. Once the tissue thaws, previously frozen regions show coarser texture than prior to freezing. The margin of the frozen tissue is delineated with a well-defined rim. The thawed cryolesions show a different contrast compared with normal unfrozen prostate. In conclusion, this pilot study shows that VA produces clear images of a frozen prostate at different temperature stages. The frozen tissue appears as a uniform region with well-defined borders that are readily identified. These characteristic images should allow safer and more efficient application of prostatic cryosurgery. These results provide substantial motivation to further investigate VA as a potential modality to monitor prostate cryotherapy intraoperatively.

Computerized Planning of Cryosurgery Using Bubble Packing: An Experimental Validation on a Phantom Material

The current study focuses on experimentally validating a planning scheme based on the so-called bubble-packing method. This study is a part of an ongoing effort to develop computerized planning tools for cryosurgery, where bubble packing has been previously developed as a means to find an initial, uniform distribution of cryoprobes within a given domain; the so-called force-field analogy was then used to move cryoprobes to their optimum layout. However, due to the high quality of the cryoprobes' distribution, suggested by bubble packing and its low computational cost, it has been argued that a planning scheme based solely on bubble packing may be more clinically relevant. To test this argument, an experimental validation is performed on a simulated cross-section of the prostate, using gelatin solution as a phantom material, proprietary liquid-nitrogen based cryoprobes, and a cryoheater to simulate urethral warming. Experimental results are compared with numerically simulated temperature histories resulting from planning. Results indicate an average disagreement of 0.8 mm in identifying the freezing front location, which is an acceptable level of uncertainty in the context of prostate cryosurgery imaging.

Experimental verification of numerical simulations of cryosurgery with application to computerized planning

As a part of an ongoing effort to develop computerized planning tools for cryosurgery, an experimental study has been conducted to verify a recently developed numerical technique for bioheat transfer simulations. Experiments were performed on gelatin solution as a phantom material, using proprietary liquid-nitrogen cryoprobes. Urethral warming was simulated with the application of a cryoheater, which is a proprietary temperature-controlled electrical heater. The experimental design was aimed at creating a 2D heat transfer problem. Analysis of experimental results was based on reconstruction of the frozen region from video recordings, using a region-growing segmentation algorithm. Results of this study show an average disagreement of 2.9% in the size of the frozen region, between experimental data and numerical simulation of the same experiment, which validates both the recently developed algorithm for numerical simulations and the newly developed algorithm for segmentation from video recordings.

Irreversible electroporation: a new ablation modality--clinical implications

Irreversible electroporation (IRE) is a new tissue ablation technique in which micro to millisecond electrical pulses are delivered to undesirable tissue to produce cell necrosis through irreversible cell membrane permeabilization. IRE affects only the cell membrane and no other structure in the tissue. The goal of the study is to test our IRE tissue ablation methodology in the pig liver, provide first experience results on long term histopathology of IRE ablated tissue, and discuss the clinical implications of the findings. The study consists of: a) designing an IRE ablation protocol through a mathematical analysis of the electrical field during electroporation; b) using ultrasound to position the electroporation electrodes in the predetermined locations and subsequently to monitor the process; c) applying the predetermined electroporation pulses; d) performing histopathology on the treated samples for up to two weeks after the procedure; and e) correlating the mathematical analysis, ultrasound data, and histology. We observed that electroporation affects tissue in a way that can be imaged in real time with ultrasound, which should facilitate real time control of electroporation during clinical applications. We observed cell ablation to the margin of the treated lesion with several cells thickness resolution. There appears to be complete ablation to the margin of blood vessels without compromising the functionality of the blood vessels, which suggests that IRE is a promising method for treatment of tumors near blood vessels (a significant challenge with current ablation methods). Consistent with the mechanism of action of IRE on the cell membrane only, we show that the structure of bile ducts, blood vessels, and connective tissues remains intact with IRE. We report extremely rapid resolution of lesions, within two weeks, which is consistent with retention of vasculature. We also document tentative evidence for an immunological response to the ablated tissue. Last, we show that mathematical predictions with the Laplace equation can be used in treatment planning. The IRE tissue ablation technique, as characterized in this report, may become an important new tool in the surgeon armamentarium.

An efficient numerical technique for bioheat simulations and its application to computerized cryosurgery planning

As a part of ongoing efforts to develop computerized planning tools for cryosurgery, the current study focuses on developing an efficient numerical technique for bioheat transfer simulations. Our long-term goal is to develop a planning tool for cryosurgery that takes a 3D reconstruction of a target region, and suggests the best cryoprobe layout. Toward that goal, a planning algorithm, termed "force-field analogy," has been recently presented, based on a sequence of bioheat transfer simulations, which are by far the most computationally expensive part of the planning method. The objective in the current study is to develop a finite difference numerical scheme for bioheat transfer simulations, which reduces the overall run time of computerized planning, thereby making it clinically relevant. While the general concept of variable grid size and time intervals is not new, its application to the phase change problem of cryosurgery is the unique contribution of the current study.

Front-tracking image reconstruction algorithm for EIT-monitored cryosurgery using the boundary element method

The effectiveness of cryosurgery, treatment of tumors by freezing, is highly dependent on knowledge of transient freezing extent, and therefore relies heavily on real-time imaging techniques for monitoring. Electrical impedance tomography (EIT) holds much promise for this application. In cryosurgery there is a three order of magnitude change in impedance across the freezing boundary and there is a priori knowledge of the freezing origin. Furthermore, an EIT image of the tissue can be done prior to the cryosurgery. In this study, we have developed an EIT front tracking reconstruction algorithm which takes advantage of these particular attributes of cryosurgery. The method tracks the freezing interface rather than the impedance distribution in the freezing tissue. In addition to drastically reducing the number of parameters needed to define the image, the computational complexity is further reduced by using the more appropriate boundary element method (BEM) for solution to the forward problem. The front-tracking method was found to converge rapidly and accurately to a variety of simulated phantom images.

Cryoheater as a means of cryosurgery control

A new concept for cryosurgery control is presented in this paper, a concept which has the potential to dramatically change the outcome of cryosurgery. Unlike other cryosurgery control techniques, which are based on controlling the thermal performance of the cryoprobe, this new concept is based on heating the treated tissue as a means of shaping the frozen region. The new controlling heater is termed a 'cryoheater'. The cryoheater is a complementary device to the cryoprobe and can work with any cryosurgery cooling technique. In the current pilot study, the new cryoheater is demonstrated in a gelatin solution and, using heat transfer simulations, it is studied in simulated cases of prostate cryosurgery. It is suggested that cryosurgery planning tools and optimization techniques must be developed before the concept of the cryoheater can be applied in its full capacity.

Diffusion-weighted MRI after cryosurgery of the canine prostate. Magnetic resonance imaging

PURPOSE

To evaluate the acute lesion created by cryosurgery with diffusion-weighted magnetic resonance imaging (DWI).

MATERIALS AND METHODS

The appearance of the acute cryolesion was evaluated in four canine prostates DWI after they were warmed to original body temperature. The prostates were excised, stained with triphenyl tetrazolium chloride (TTC), photographed, prepared for hematoxylin and eosin (H&E) staining, and examined under a light microscope.

RESULTS

A marked decrease in apparent diffusion coefficient of 38% was evident in the center of the previously frozen tissue, but not in all of the previously frozen tissue. Histologic results confirm differences between the iceball core and the periphery of the iceball, which have markedly different imaging characteristics on DWI.

CONCLUSION

The core of the previously frozen tissue has a reduced apparent diffusion coefficient (ADC) compared to the periphery of the previously frozen tissue and previously unfrozen tissue.

The evolution and state of modern technology for prostate cryosurgery

Cryosurgery is the in situ ablation of a target tissue by application of extreme cold temperature. The ability of cryosurgery to ablate tissue is unquestioned. It is the controlled application of a cryoinjury in a manner to minimize morbidity that is problematic. Prostate cryosurgery is complicated by the proximity of the prostate to adjacent structures that are sensitive to a freeze injury, namely the urethra, rectal wall, and neurovascular bundles. Several recent technological advances have led to the development of an effective treatment protocol with acceptable morbidity. These include the advent of real-time transrectal ultrasound, cryomachines with almost instant freeze-thaw control through the use of the Joule-Thompson effect, and warming catheters to effectively preserve the integrity of the urethra and external sphincter. Further, temperature monitoring at the posterior margin of the prostate sometimes combined with an injection of saline solution into Denonvilliers fascia has reduced the occurrence of urethrorectal fistula formation to 0% to 0.5% in modern series. We review the key innovations of prostate cryosurgery that differentiate this state-of-the-art procedure from that used by early investigators to even that of the early 1990s. Potential future innovations, specifically related to image guidance of the procedure, are also addressed.

Cryosurgery

Cryosurgery is a surgical technique that employs freezing to destroy undesirable tissue. Developed first in the middle of the nineteenth century it has recently incorporated new imaging technologies and is a fast growing minimally invasive surgical technique. A historical review of the field of cryosurgery is presented, showing how technological advances have affected the development of the field. This is followed by a more in-depth survey of two important topics in cryosurgery: (a) the biochemical and biophysical mechanisms of tissue destruction during cryosurgery and (b) monitoring and imaging techniques for cryosurgery.

Quantitative measurement and prediction of biophysical response during freezing in tissues

Cryopreservation and cryosurgery are important biomedical applications used to selectively preserve or destroy cellular systems through freezing. Studies using cryomicroscopy techniques, which allow the visualization of the freezing process in single cells, have shown that a drop in viability correlates with the extent of two biophysical events during the freezing process: (a) intracellular ice formation and (b) cellular dehydration. These same biophysical events operate in tissue systems; however, the inability to visualize and quantify the dynamics of the freezing process in tissues has hampered direct correlation of these events with freezing-induced changes in viability. This review highlights two new techniques that use freeze substitution and differential scanning calorimetry to provide dynamic freezing data in tissue. Characteristic dimensions and parameters extracted from these new data are then used in a predictive model of biophysical freezing response in several tissues, including liver and tumor. This approach promises to help guide improved design of both cryopreservation and cryosurgical applications of tissue freezing.

MR-guided percutaneous cryotherapy of the liver: in vivo evaluation with histologic correlation in an animal model

The purpose of this study was to evaluate the feasibility of MR-guided percutaneous cryotherapy of the porcine liver and to correlate the resulting tissue necrosis with MR imaging and histology. Using an MR-compatible, argon-based cryotherapy system (CryoHit; Galil Medical Ltd., Israel) with 2- and 3-mm diameter tapered cryotherapy probes, MR-guided percutaneous cryotherapy was performed in seven pigs (mean body weight, 40 kg) under general anesthesia in a short-bore magnet (1.5 T ACS NT; Philips, The Netherlands) using an ultrafast T2-weighted single-shot LoLo TSE sequence and a T1-weighted gradient-echo sequence. The frozen liver tissue was depicted accurately on fast T2- and T1-weighted sequences, providing precise delineation of the ablated tissue volume. On follow-up postcontrast MR controls, the cryolesions appeared avascular. They decreased in size compared with the initially frozen volume down to 70% at a 2-week follow-up. Histologically, a coagulation necrosis with a close correlation to the MR follow-up examinations was objectified. No cryotherapy-related complications occurred. J. Magn. Reson. Imaging 2001;13:50-56.

Cryosurgical monitoring using bioimpedance measurements--a feasibility study for electrical impedance tomography

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 explore cryosurgical monitoring as a previously uninvestigated application for EIT. The feasibility of bio-impedance measurements to detect ice front propagation is demonstrated by freezing planar tissue samples one-dimensionally while measuring impedance along a linear array. The experimental results compare favorably to a simple finite element model designed to provide an electrical field visualization tool.

Suppression of high-density artefacts in x-ray CT images using temporal digital subtraction with application to cryotherapy

Image guidance in cryotherapy is usually performed using ultrasound. Although not currently in routine clinical use, x-ray CT imaging is an alternative means of guidance that can display the full 3D structure of the iceball, including frozen and unfrozen regions. However, the quality of x-ray CT images is compromised by the presence of high-density streak artefacts. To suppress these artefacts we applied temporal digital subtraction (TDS). This TDS method has the added advantage of improving the grey scale contrast between frozen and unfrozen tissue in the CT images. Two sets of CT images were taken of a phantom material, cryoprobes and a urethral warmer (UW) before and during the cryoprobe freeze cycle. The high density artefacts persisted in both image sets. TDS was performed on these two image sets using the corresponding mask image of unfrozen material and the same geometrical configuration of the cryoprobes and the UW. The resultant difference image had a significantly reduced artefact content. Thus TDS can be used to significantly suppress or eliminate high-density CT streak artefacts without reducing the metallic content of the cryoprobes. In vivo study needs to be conducted to establish the utility of this TDS procedure for CT assisted prostate or liver cryotherapy. Applying TDS in x-ray CT guided cryotherapy will facilitate estimation of the number and location of all frozen and unfrozen regions, potentially making cryotherapy safer and less operator dependent.

A semi-empirical treatment planning model for optimization of multiprobe cryosurgery

A model is presented for treatment planning of multiprobe cryosurgery. In this model a thermal simulation algorithm is used to generate temperature distribution from cryoprobes, visualize isotherms in the anatomical region of interest (ROI) and provide tools to assist estimation of the amount of freezing damage to the target and surrounding normal structures. Calculations may be performed for any given freezing time for the selected set of operation parameters. The thermal simulation is based on solving the transient heat conduction equation using finite element methods for a multiprobe geometry. As an example, a semi-empirical optimization of 2D placement of six cryoprobes and their thermal protocol for the first freeze cycle is presented. The effectiveness of the optimized treatment protocol was estimated by generating temperature-volume histograms and calculating the objective function for the anatomy of interest. Two phantom experiments were performed to verify isotherm locations predicted by calculations. A comparison of the predicted 0 degrees C isotherm with the actual iceball boundary imaged by x-ray CT demonstrated a spatial agreement within +/-2 mm.

The use of view angle tilting to reduce distortions in magnetic resonance imaging of cryosurgery

Susceptibility artifacts from magnetic resonance (MR)-compatible cryoprobes can distort MR images of iceballs. In this work, we investigate the ability of view angle tilting (VAT) to correct susceptibility induced distortions in MR images of cryosurgery. The efficacy of VAT was tested in an ex vivo bovine liver model of cryosurgery using MR-compatible cryoprobes. Artifacts on high bandwidth fast spin echo images of freezing obtained with and without VAT were compared with photographs of the actual iceball shape and size. In vivo imaging with VAT was demonstrated during percutaneous MR-guided cryosurgery of pig liver and brain. VAT was most successful in reducing probe and iceball distortions when the imaging plane was normal to the cryoprobe, and the cryoprobe was perpendicular to the main magnetic field of the scanner. VAT had the greatest benefit when used to correct MR images of freezing when the surface of the iceball was relatively near to the cryoprobe. For large iceballs, the artifact was small so the VAT correction was less important. We conclude that VAT significantly reduced distortions in the shape of the signal void corresponding to the extent of freezing visualized during MR-guided cryosurgery. This improved ability to visualize the exact location of the cryoprobe, as well as the precise shape of the iceball, particularly during initial freezing when the iceball is small, has potential to significantly improve the accuracy of MR-guided cryosurgery of small lesions, and the accuracy of MR-assisted temperature calculations that are based on precise imaging of the probe location, and boundary geometry of the iceball.

Molecular aspects of magnetic resonance imaging and spectroscopy

Magnetic resonance imaging (MRI) is a well known diagnostic tool in radiology that produces unsurpassed images of the human body, in particular of soft tissue. However, the medical community is often not aware that MRI is an important yet limited segment of magnetic resonance (MR) or nuclear magnetic resonance (NMR) as this method is called in basic science. The tremendous morphological information of MR images sometimes conceal the fact that MR signals in general contain much more information, especially on processes on the molecular level. NMR is successfully used in physics, chemistry, and biology to explore and characterize chemical reactions, molecular conformations, biochemical pathways, solid state material, and many other applications that elucidate invisible characteristics of matter and tissue. In medical applications, knowledge of the molecular background of MRI and in particular MR spectroscopy (MRS) is an inevitable basis to understand molecular phenomenon leading to macroscopic effects visible in diagnostic images or spectra. This review shall provide the necessary background to comprehend molecular aspects of magnetic resonance applications in medicine. An introduction into the physical basics aims at an understanding of some of the molecular mechanisms without extended mathematical treatment. The MR typical terminology is explained such that reading of original MR publications could be facilitated for non-MR experts. Applications in MRI and MRS are intended to illustrate the consequences of molecular effects on images and spectra.

Imaging of interstitial cryotherapy--an in vitro comparison of ultrasound, computed tomography, and magnetic resonance imaging

RATIONALE AND OBJECTIVES

To evaluate the imaging capabilities of ultrasound (US), computed tomography (CT), and magnetic resonance imaging (MRI) in monitoring interstitial cryotherapy and to compare them with visual control.

METHODS

An experimental MR-compatible, vacuum-insulated and liquid nitrogen-cooled cryoprobe was inserted under in vitro conditions into a porcine liver, which was kept at a temperature of 37 +/- 1 degrees C, in a water bath with continuous stirring. The freezing procedure was controlled macroscopically, by US (Toshiba Sonolayer, 7.5-MHz linear array transducer), by CT (Siemens Somatom Plus, slice thickness 2-8 mm, 165-210 mA at 120 kV), and by MRI (Philips Gyroscan ACS-NT, FFE TR/TE/FA = 15/5.4/25 degrees, T1-SE 550/20, T2-TSE 1800/100) after the iceball reached its maximum size.

RESULTS

The maximum iceball diameter around the probe tip was 12.0 mm by visual control, 12.4 mm by US, 12.7 mm by CT, and within 12.8 mm by spin echo sequences and 11 mm by gradient echo sequence. Due to the nearly signal-free appearance of the frozen tissue on MR images, the ice/tissue contrast on T1-weighted and gradient echo images was superior to T2-weighted images and CT images. Sonographically, the ice formation appeared as a hyperechoic sickle with nearly complete acoustic shadowing.

CONCLUSION

Due to the better ice/tissue contrast, T1-weighted or gradient echo MR images were superior to CT and US in monitoring interstitial cryotherapy. Gradient echo sequences generally underestimated the ice diameter by 15%.

An in vivo study of antifreeze protein adjuvant cryosurgery

Cryosurgery employs freezing to destroy undesirable tissue. However, under certain thermal conditions, frozen tissues survive. The survival of frozen undesirable tissue may lead to complications, such as recurrence of cancer. In a study of nude mice with subcutaneous metastatic prostate tumors, we showed that the preoperative injection of a phosphate-buffered saline solution with 10 mg/ml antifreeze protein of type I into the tumor prior to freezing enhances destruction under thermal conditions which normally yield cell survival. This suggests that the adjunctive use of antifreeze proteins in cryosurgery may reduce the complications from undesirable tissues that survive freezing.

Magnetic resonance imaging of frozen tissues: temperature-dependent MR signal characteristics and relevance for MR monitoring of cryosurgery

Previously, the magnetic resonance (MR) imaging appearance of frozen tissues created during cryosurgery has been described as a signal void. In this work, very short echo times (1.2 msec) allowed MR signals from frozen tissues to be measured at temperatures down to -35 degrees C. Ex vivo bovine liver, muscle, adipose tissue, and water were imaged at steady-state temperatures from -78 degrees to +6 degrees C. Signal intensity, T2*, and T1 were measured using gradient-echo imaging. Signal intensity and T2* decrease monotonically with temperature. In the future, these MR parameters may be useful for mapping temperatures during cryosurgery.

Outcome and safety of transrectal US-guided percutaneous cryotherapy for localized prostate cancer

OBJECTIVES

To assess the effectiveness and safety of ultrasound (US)-guided cryotherapy as a primary treatment for localized prostate cancer.

MATERIALS AND METHODS

A prospective study of percutaneous transrectal US (TRUS)-guided cryotherapy was performed on 71 patients with T1-T3, N0, M0 prostatic cancer: 10 patients underwent two or more procedures. All cases were newly diagnosed and patients had no previous treatment for cancer. For all patients, TRUS biopsies were performed at 5-6 months. Patients were monitored at 6 weeks; 3, 6, 9, and 12 months; and twice yearly thereafter for prostate specific antigen (PSA) levels, complications, and clinical evidence of residual disease.

RESULTS

Follow-up from 10 to 36 months was available for 70 of 71 patients; one patient died of unrelated disease. Initially, 10 of 69 patients had positive postcryotherapy biopsy results. After repeated treatment, nine of these 10 patients had negative biopsy results and one patient had no follow-up. Overall, 68 of 69 patients had negative biopsy results. At 1 year, 43 of 64 (67%) had an undetectable PSA level. Two patients had proven metastases. Complications include three cases with urethral sloughing requiring transurethral resection of the prostate (TURP). One patient had orchitis. Two patients had persistent incontinence, one as the result of a TURP. There was no death, acute serious morbidity, or fistula formation. Impotence was universal at 6 months, but many patients demonstrated late recovery.

CONCLUSION

Cryoablation is an imaging-guided percutaneous intervention for prostate cancer that can safely yield disease-free status in a high percentage of patients with localized disease.

Effect of thermal variables on human breast cancer in cryosurgery

There is a growing interest in the use of cryosurgery to treat breast cancer, following recent breakthroughs in noninvasive imaging and in cryotechnology, as well as the recent success of cryosurgery in treating various types of cancer. However, since haphazard freezing does not guarantee tissue destruction, in order to apply this technique effectively it is essential to determine the thermal parameters that produce complete destruction of malignant tissue. This study seeks to quantitatively identify the relationship between thermal variables and the degree of freezing damage to human breast cancer cells. In order to do this, human breast cancer and normal cells were frozen with controlled thermal parameters using a directional solidification apparatus. Cell viability was determined after thawing using trypan blue, and correlated to the thermal variables used during freezing. Cellular damage is observed to increase with increasing cooling rates, due to the higher probability of intracellular ice formation. A double freeze thaw cycle significantly increases the extent of cell damage, and is sufficient to ensure complete cell destruction at final freezing temperatures of -40 degrees C for a 25 degrees C/min cooling rate, and -20 degrees C for a 50 degrees C/min cooling rate. The correlations between cell death and thermal parameters are qualitatively identical for all the cell types in this study, although there is some variation from one cell type to another in the overall susceptibility to freezing damage. The correlations established in this study can be used to design systematic and optimal breast cryosurgery protocols.

Ice-Front Propagation Monitoring in Tissue by the use of Visible-Light Spectroscopy

For demonstrating that visible-light spectroscopy can be used for ice-front detection within freezing tissue, proton magnetic resonance images were correlated to time-evolving transmittance spectra as an ice front progressed across a tissue sample. The experimental apparatus was designed to be compatible with magnetic resonance imaging, to produce one-dimensional freezing, and to allow both reflectance and transillumination emitter-detector configurations about a normally progressing planar ice front in chicken muscle. This demonstration has potentially important medical applications in cryopreservation (freezing of biological materials for preservation) and cryosurgery (destruction of tissue by freezing).

MR-guided interstitial cryotherapy of the liver with a novel, nitrogen-cooled cryoprobe

The purpose of the study was to test a newly developed, MR-compatible, liquid nitrogen-cooled cryoprobe. The probe has an outer diameter of 3.5 mm and was specifically developed for percutaneous, MR-guided, interstitial cryotherapy. The probe was inserted percutaneously into the livers of 10 rabbits. The cryotherapy procedure was monitored with a surface coil in a 1.5 Tesla magnet using a gradient echo sequence. Follow-up examinations were performed 3 and 7 days after the freezing procedure using T1- and T2-weighted spin echo sequences. At 7 days the animals were sacrificed and the cryolesions were examined histologically. The cryoprobe enabled artifact-free MR imaging of the "iceball" formation during freezing of the rabbit liver. After 1 min of freezing, the iceball at the tip of the probe showed an average maximum diameter of 10.8 mm. No bleeding complications were observed during or after the freezing procedure. Histologic examination 7 days after cryotherapy confirmed that the liver lesions were the same size as had been predicted by the images of the acute iceball. This new, percutaneously inserted, MR-compatible, liquid-nitrogen cooled cryoprobe allows accurate, artifact-free MR imaging of interstitial cryotherapy.

History of cryosurgery

The use of freezing temperatures for the therapeutic destruction of tissue began in England in 1845-51 when James Arnott described the use of iced salt solutions (about-20 degrees C) to freeze advanced cancers in accessible sites, producing reduction in tumor size and amelioration of pain. Improved freezing techniques were possible early in the 1990s when solidified carbon dioxide came into use and later when liquid nitrogen and nitrous oxide became available. Nevertheless, cryotherapy was a minor technique, used only for the accessible lesions of skin and mucosa. With the development of modern cryosurgical apparatus by Cooper in 1961, a resurgence of interest in cryosurgery was initiated and techniques for diverse clinical conditions, including visceral cancer, evolved, After the initial widespread clinical trials matured in the 1970s, some applications of the technique fell into disuse while others became standard treatment. Late in the 1980s, further improvements in apparatus and imaging techniques have permitted increased clinical use in neoplastic disease, including visceral cancer.

MR-monitored LITT as a palliative concept in patients with high grade gliomas: preliminary clinical experience

The purpose of this study was to evaluate the clinical utility of laser-induced thermotherapy (LITT) as a palliative treatment for patients with high-grade gliomas. Four consenting patients with recurrent high grade III/IV gliomas near the primary language or motor areas were palliatively treated with LITT (2-5 W, 3-13 minutes; Neodym YAG Laser, Dornier, Friedrichshafen, Germany). Temperature monitoring was performed by T1-weighted turbo-fast low-angle shot (FLASH) imaging at 1.5 T (Siemens Magnetom SP 4000, Siemens, Erlangen, Germany). MRI studies before LITT included contrast-enhanced conventional scans and functional activation studies to localize the primary motor cortex or language areas using an echo-planar imaging (EPI) spin-echo (SE) sequence. Follow-up studies consisted of contrast-enhanced conventional scans as well as diffusion studies (contrast-enhanced Fourier-acquired steady-state technique and EPI-SE) and perfusion studies (EPI-SE with .2 mmol of gadolinium (Gd)/kg body weight) to differentiate post-therapeutic effects from residual or recurrent tumor growth. Local tumor control was achieved in areas with laser energy deposition with clinically stable conditions > or = 6 months. Conventional contrast-enhanced scans demonstrated strong enhancement surrounding ablated tumor components, which showed a reduction in CBV/CBF. Perfusion studies were useful to discriminate granulomatous tissue enhancement from residual or recurrent tumor growth. Careful application of LITT may evolve as an alternative palliative concept for patients with end-stage high-grade cerebral gliomas reducing clinical symptoms from circumscribed areas of pathology.

Chemical adjuvant cryosurgery with antifreeze proteins

BACKGROUND AND OBJECTIVES

Imaging monitored cryosurgery is emerging as an important minimally invasive surgical technique for treatment of cancer. Although imaging allows excellent control over the process of freezing itself, recent studies show that at high subzero temperatures cells survive freezing. Antifreeze proteins (AFP) are chemical compounds that modify ice crystals to needle-like shapes that can destroy cells in cellular suspensions. The goal of this study was to determine whether these antifreeze proteins can also destroy cells in frozen tissue and serve as chemical adjuvants to cryosurgery.

METHODS

Livers from six rats were excised, perfused with solutions of either phosphate-buffered saline (PBS) or PBS with 10 mg/ml AFP-I, and frozen with a special cryosurgery apparatus. Lobes were frozen with one or two freeze-thaw cycles and the cell viability was examined with a two stain fluorescent dye test and histological assessment.

RESULTS

A significant percentage of hepatocytes survive freezing on the margin of a frozen cryolesion. AFP significantly increase cellular destruction in that region apparently through formation of intracellular ice.

CONCLUSIONS

This preliminary study demonstrates that antifreeze proteins may be effective chemical adjuvants to cryosurgery.

Minimally invasive cryosurgery--technological advances

The technological advances which have caused renewed interest in cryosurgery are the development of intraoperative ultrasound to monitor the therapeutic process and the development of new cryosurgical equipment designed to use supercooled liquid nitrogen. The thin, highly efficient probes, available in several sizes, can be placed in diseased sites via endoscopy or percutaneously in minimally invasive procedures. The manner of use is to place the probe in the desired location in the diseased tissue with ultrasound guidance. If required by the size or location of the tumor, as many as five probes can be inserted and cooled to -195 degrees C simultaneously. The process of freezing is monitored by ultrasound which displays a hypoechoic (dark) image when the tissue if frozen. Rapid freezing, slow thawing, and repetition of the freeze/thaw cycle are standard features of technique. Clinical applications which have become common in the past 4 years include the treatment of prostatic cancer and liver tumors. The cases selected for cryosurgery are generally those for which no conventional treatment is possible. However, especially in prostatic cancer, the operative morbidity is so low and the results of therapy are sufficiently good in the short term to merit consideration of use in earlier stages of the disease. Diverse tumors in other sites, such as the brain, bronchus, bone, pancreas, kidney, and uterus, have also been treated in small numbers by cryosurgery. Judging from this experience, further expansion in the use of cryosurgical techniques seems certain.

MR appearance and spectral features of injected ethanol in the liver: implication for fast MR-guided percutaneous ethanol injection therapy

PURPOSE

Our goal was to evaluate several fast MR strategies for monitoring ethanol distributions so that percutaneous ethanol injection might be guided with MRI.

METHOD

Fast RF spoiled GRE sequences (SPGR) and T2-weighted rapid acquisition with relaxation enhancement (RARE) sequences with and without spectroscopic-quality water suppression techniques were assessed for their ability to depict the distribution of injected ethanol in ex vivo pig liver. A line scan Carr-Purcell-Meiboom-Gill spectroscopic imaging sequence was used to validate observations and measure spectral relaxation characteristics of the ethanol signal in liver. Injected deuterated ethanol was also tested as an alternative possibility to depict the distribution of ethanol.

RESULTS

The water-suppressed T2-weighted RARE sequence depicted the distribution of ethanol better than other sequences. Deuterated ethanol appeared as a signal void on all sequences.

CONCLUSION

Water-suppressed T2-weighted RARE sequences could be useful to rapidly monitor MR-guided PEI.

Temperature determination in the frozen region during cryosurgery of rabbit liver using MR image analysis

Cryosurgery currently is being used clinically to treat tumors in internal organs such as the liver and prostate. Although performed at present under ultrasound monitoring, magnetic resonance imaging (MRI)-guidance of these procedures not only permits monitoring of the frozen region during cryosurgery but also makes it possible to determine the temperature distribution in the frozen region, which is not possible using ultrasound monitoring. A good estimate of the region of destruction in the tissue can be obtained from correlating the temperature distribution and the time course of the freezing with the image of the frozen region. Unfortunately, MR pulse sequence-based temperature determination techniques such as diffusion, relaxation time, and chemical shift cannot be used for measuring the temperature in the frozen region because the T2 of the frozen regions is so short that there is effectively no RF signal from the frozen region. This paper describes a numerical technique for determining the two dimensional temperature distribution in the frozen region during MR image-guided cryosurgery of normal liver in rabbits. The technique involves solving the energy equation numerically in the frozen region to determine the temperature distribution there. The boundary conditions needed to solve the equation are determined from MR images of the frozen tissue during cryosurgery and from the measured temperature of the cryoprobe. The calculated temperature in the frozen region is then correlated with the damaged region (cryolesion) determined from post mortem histologic evaluation.

Effect of thermal variables on frozen human primary prostatic adenocarcinoma cells

OBJECTIVES

Recent advances in imaging technology and cryotechnology have rekindled interest in prostate cryosurgery. Cryosurgery, however, cannot be applied precisely without knowing how the thermal variables used during the procedure affect tissue destruction. The goal of this article is to provide quantitative values for the relationship between thermal variables during freezing and the destruction of human primary prostatic adenocarcinoma cells.

METHODS

Human primary prostatic adenocarcinoma cells were frozen with controlled thermal parameters, using a directional solidification apparatus. Cell viability was determined after thawing, using trypan blue and a two-dye fluorescent test and correlated to the thermal variables used during freezing.

RESULTS

Human primary prostatic adenocarcinoma cells are damaged by intracellular chemical damage when frozen with cooling rates lower than 5 degrees C/min and by intracellular ice formation when frozen with cooling rates higher than 25 degrees C/min. A double freeze/thaw cycle is required to ensure complete cell destruction at high subzero temperatures, which must be lower than -40 degrees C for the low cooling rates and lower than -19 degrees C for the higher cooling rate.

CONCLUSIONS

Haphazard freezing does not necessarily destroy tissue during cryosurgery; however, quantitative data on the relation between thermal variables and frozen cell destruction can provide the means for performing cryosurgery more precisely and with greater control over the outcome of the procedure.

MR image-guided control of cryosurgery

Cryoablation has recently become a useful procedure for the treatment of prostatic and hepatic tumors, primarily because of advances in the ability to monitor visually the freezing process with ultrasound. Success of the procedure depends in large part on how well the ice front can be positioned to destroy pathologic tissue, while sparing healthy tissue. This study describes a cryogen delivery system that can be used in conjunction with magnetic resonance (MR) image-guided cryoablation, and an automatic control system that uses MR image guidance in a feedback loop to control the ice front trajectory. Edge-detected MR images are used to determine the current ice front location at each time interval, providing feed-back to an automatic control system that adjusts the flow of cryogen to the cryoprobe. Numerical simulations and experimental results demonstrate that an ice front with cylindrical symmetry can be accurately controlled using this MR image-guided feedback control scheme.