Curvature-based nonfiducial registration for the Frameless Stereotactic Operating Microscope.

The Frameless Stereotactic Operating Microscope permits information from CT and MRI scans to be displayed in the operating microscope in the proper scale and perspective without using a mechanical frame in the process, when the microscope is positioned over the surgical field. This registration is currently done using fiducials that must be present when the imaging scans are taken. A new technique, based on the theory of curvatures, has been developed as an alternative to the use of fiducials for the registration of a patient's head position during surgery with diagnostic images from CT or MRI. Surface curvatures are estimated from a trace of points of the surface of the skin, obtained from both the diagnostic images and an intraoperative nonimaging ultrasonic rangefinder. The point traces need not be identical or evenly spaced. Plots of the resulting curvature fields are compared visually and the alignment determined. Phantom testing, using a human skull, has resulted in a median alignment error of 1.95 mm. Testing using a human subject with clinically obtained data has resulted in alignment errors on the order of 7 mm.

Brain hyperthermia: I. Interstitial microwave antenna array techniques--the Dartmouth experience.

PURPOSE: Microwave antennas of various designs were inserted into arrays of nylon catheters implanted in brain tumors with the goal of raising temperatures throughout the target volume to 43.0 degrees C. METHODS AND MATERIALS: All antennas were flexible, and included dipole, choke dipole, modified dipole, and helical designs driven at 915 or 2450 MHz. Antennas were tested in brain-equivalent phantom in arrays. Phase shifting and phase rotation techniques were incorporated into the treatment system to steer power in the tumor, assisted by a treatment planning computer that predicted power deposition patterns and temperature distributions. Choke antennas were designed and tested to reduce a dependence of the central power location on depth of insertion into tissue. Temperature data analysis used only central and orthogonal axes mapping data measured at 2.0 mm intervals. RESULTS: A total of 23 patients were treated, using from one to six microwave antennas. Minimum tumor temperatures, averaged over the 60 min treatment, ranged from 37.2-44.3 degrees C (mean 40.0 degrees C) and maximum average tumor temperatures ranged from 46.5-60.1 degrees C (mean 49.1 degrees C). The percentage of all measured temperatures reaching therapeutic levels (> or = 43.0 degrees C) was 70.9. T90, the temperature at which 90% of all measured temperatures equaled or exceeded, was 40.8 degrees C, and T50 was 44.2 degrees C. CONCLUSION: Patient data analysis showed that the array of four dipole antennas spaced 2.0 cm apart were capable of heating a volume of 5.9 cm (along the central array axis) x 2.8 cm x 2.8 cm.

Hyperthermia equipment evaluation.

This article looks briefly at the advances over the past 2 decades in the development of equipment for administering clinical hyperthermia, and observes that while there has been much progress, the equipment available today still does not meet the design criteria articulated 20 years ago. The assertation is made that the testing of hyperthermia equipment in animals has not addressed the questions most of interest in the clinic, and a suggestion is made for a more rigorous way to evaluate clinical equipment. Finally some areas where significant progress in equipment development seems possible are mentioned.

Pattern of response to interstitial hyperthermia and brachytherapy for malignant intracranial tumour: a CT analysis.

Interstitial microwave hyperthermia in combination with iridium-192 brachytherapy has been administered to 23 cases of malignant brain tumours in a phase one clinical trial to assess the feasibility and safety of this treatment. In order to quantify the acute and long-term response of tumour and surrounding brain to this treatment, a morphometric computed tomography scan analysis was performed in 18 evaluable patients. Volumes defined by the outer margin of the contrast-enhancing rim, by the hypodense necrotic region within the enhancing rim and by the surrounding hypodensity region were calculated from computer measurements. Hyperthermia equipment performance (HEP) was calculated for the evaluation of heating. After the treatments, the volume of the inner hypodensity region decreased in seven patients and the volume increased in 11 patients. In five patients, the outer margin of the contrast-enhancing lesion showed an initial increase in volume followed by a decrease and in these patients higher HEP and longer survival were observed significantly. The volume of the surrounding hypodensity region varied following treatments, but in most instances, the region subsequently increased in the interval immediately prior to death. Contribution of heat effect to these changes are discussed and the significance of aggressive heating, which provides transient opening of blood brain barrier, is shown.

Performance of an adaptive MIMO controller for a multiple-element ultrasound hyperthermia system.

A prototype adaptive automatic control algorithm was implemented to regulate temperatures measured at several points in a tumour by adjusting the power applied to several ultrasound transducers. The goal was to control the temperatures under the elements of a mosaic applicator individually without any priori knowledge of which probes are under which elements. The control algorithm was devised for clinical applications where the position of each probe with respect to the heat sources is difficult to determine precisely. Instead, the program 'learns' the relationship between the inputs (power levels) and the outputs (temperatures) automatically. Based on the observed transfer function relating the power at m sources to the temperatures n probes, where n and m are not necessarily the same, a new method was used to implement a feedback controller. This method simplifies the design of the controller for a multiple-input/multiple-output (MIMO) system, while taking into account the coupling that may exist between the various elements of the system. As a result of using an adaptive scheme, the regulator continuously tracks changes in the system, such as blood flow variations or patient motion, by modifying its control parameters. The algorithm performance has been tested in simulations as well as experiments in dog thigh and a perfused kidney model.

Extracranial application of the frameless stereotactic operating microscope: experience with lumbar spine.

The frameless stereotactic operating microscope has expanded the potential application of modern stereotaxis to procedures outside of the intracranial compartment by removing the constraint of a rigid frame. We studied seven patients all of whom had a history, examination, and imaging studies consistent with lumbosacral spinal pathology for which they subsequently underwent surgery with the operating microscope. The ability of the frameless stereotactic system with preoperative computed tomography data to locate the level of the lesion as well as define the boundary of the spinal pathology intraoperatively was assessed. In parallel with this application of the frameless system, we analyzed the relationship between the lumbar intervertebral disc spaces (L3-L4, L4-L5, L5-S1) and skin surface fiducials using lateral radiographs. In seven patients with extracranial cases (six herniated lumbar discs and one lumbar spondylolysis with Grade I spondylolisthesis) who underwent operations by this system, the accuracy of the digitization component of the system with respect to localization of an independent test fiducial was 3.28 mm (SD, 0.61). The accuracy of the entire system in locating the independent fiducial within the viewing plane was 6.05 mm (SD, 4.04). Disc space localization had a far greater error of 28.81 mm (SD, 7.49). There was no consistent pattern to the magnitude or direction of the displacement of the lumbar intervertebral discs with respect to the fiducial markers in the sagittal plane. Although accuracy at the level of the fiducial plane was similar to that of intracranial applications, paraspinal tissue and vertebral column deformations rendered poorer accuracy with deeper structures.

Techniques for intraoperative hyperthermia with ultrasound: the Dartmouth experience with 19 patients.

Over the course of 3 years, tumours of 19 patients were heated with ultrasound in the operating room during surgical resection. Immediately following intraoperative radiation therapy, thermocouples were inserted into tumour and adjacent normal structures. Patients were then given a 60-min heat treatment with ultrasound after a 10-15-min heatup period. Temperatures were measured at a total of 133 fixed locations for the 19 patient series. Temperature mapping was done in the tumour volume when logistically feasible. Treatment sites included colorectal (n = 3), portahepatus (n = 1), pancreas (n = 7), liver (n = 1), pelvis (n = 3), sacrum (n = 2), and abdomen (n = 2). A sterile, constant-volume water circulating system was utilized to control surface temperatures. Three generations of completely immersible transducers were designed over the course of this study with a 4-cm height specification. Since the ultrasound transducer was assembled on the sterile field during surgery, a 1, 2 or 3 MHz ceramic element was placed in either a 6, 8 or 10 cm diameter aluminium housing to conform the acoustic field to the tumour size. Average of the maximum temperatures attained was 46.6 degrees C. Temperature with which 90% of all measured points equalled or exceeded (T90) was 39.2 degrees C. The T50 was 42.9 degrees C. This compared favourably with T90 and T50 of 38.8 and 41.9 degrees C, respectively, in our outpatient clinic series, in which superficial tumours were treated with a similar external applicator, and patient tolerance was often a treatment limitation.

Three-dimensional theoretical SAR and temperature distributions created in brain tissue by 915 and 2450 MHz dipole antenna arrays with varying insertion depths.

Theoretical three-dimensional power deposition and temperature distributions were calculated for interstitial hyperthermia microwave antenna arrays driven at 915 and 2450 MHz in brain tissue. Four dipole antennas were assumed to be placed in a 2 x 2 cm array with varying insertion depths in cylindrical tumour models. The bioheat transfer equation was solved for the three-dimensional steady-state temperature distributions using a finite element method. Homogeneous and non-homogeneous blood flow models were considered. As a basis of comparison of the various temperature distributions, the volume of the tumour heated to greater than or equal to 43 degrees C was calculated. SAR distributions calculated for the 915 MHz antenna arrays in brain tissue were very similar to those calculated for muscle. The 2450 MHz arrays showed similar behaviour to the 915 MHz arrays; however, as the insertion depth increased from slightly less than a full-wavelength there was a single hotspot centred at the antenna junction. For the 2450 MHz arrays, the predicted therapeutic tumour volumes were relatively constant over the entire range of insertion depths considered, and in fact, for most insertion depths considered, the model predicted the 2450 MHz arrays would heat larger therapeutic volumes than the 915 MHz arrays. For the 915 MHz array, at insertion depths between 7.8 and 14.6 cm there was a sharp decrease in the predicted therapeutic volume due to a proximal secondary hotspot in the normal tissue causing overheating. However, when the same size tumour at the same insertion depth was heated with the 2450 MHz array, the hotspot was in the tumour, adding to the volume of tumour that was heated to therapeutic temperatures.

Three-dimensional theoretical temperature distributions produced by 915 MHz dipole antenna arrays with varying insertion depths in muscle tissue.

Interstitial microwave antenna array hyperthermia (IMAAH) systems are currently being used in the treatment of cancer. The insertion depth of an interstitial microwave antenna, defined as the length of the antenna from the tip to the point of insertion in tissue, affects its ability to produce uniform power deposition patterns in tumor volumes. The effect of varying insertion depths on the ability of an IMAAH system to heat two theoretical tumor models was examined. Four dipole microwave antennas were implanted in a 2 x 2 cm array and driven at 915 MHz in muscle tissue. The explicit power deposition patterns were calculated for each insertion depth using known theory. The bioheat transfer equation was solved for the 3-dimensional steady-state temperature distributions in cylindrical and ellipsoidal tumor models using a finite element method. Homogeneous and nonhomogeneous blood flow models were considered. As a basis of comparison of the various temperature distributions, the volume of tumor heated to greater than or equal to 43 degrees C was calculated. Under the conditions of this study, the insertion depth was shown to have a significant effect on the ability of an IMAAH system to heat the tumor volumes. A sharp decrease in the percentage of tumor volume heated to greater than or equal to 43 degrees C was seen for insertion depths between 7.8 and 14.6 cm. At an insertion depth of 11.7 cm (3/4 lambda) there was virtually no heating of the tumor. Regions of elevated power occurred outside of the desired treatment volume, stressing the importance of adequate thermometry techniques and demonstrating the need for hyperthermia treatment planning prior to implantation of an antenna array. Plots of the power deposition patterns and the corresponding temperatures produced in the diagonal plane of the antenna arrays are present.

A theoretical evaluation of the performance of the Dartmouth IMAAH system to heat cylindrical and ellipsoidal tumour models.

The Dartmouth interstitial microwave antenna array hyperthermia (IMAAH) system was evaluated based on its ability to heat idealized three-dimensional cylindrical and ellipsoidal tumour models with lengths between 2 and 16 cm. The evaluation was based on computer simulations of the three-dimensional temperature distributions produced by the explicit theoretical power deposition pattern of resonant dipole antenna arrays driven at 433, 915 and 2450 MHz. The theoretical calculations were performed using three-dimensional tumour models with constant thermal and dielectric tissue properties. Each 2 X 2 cm antenna array was evaluated on the basis of its ability to heat a specified tumour volume with various blood flow patterns. The bioheat transfer equation was solved in three dimensions using a Galerkin finite-element method. In general, under the conditions of this study, in all cases examined larger percentages of the ellipsoidal tumour volumes were heated to therapeutic temperatures (greater than or equal to 43 degrees C) than the cylindrical tumours. The 2450 MHz antenna array was the most effective at heating the shorter tumours, while the 433 MHz antenna array heated the longer tumours most effectively. The antennas driven at 915 MHz were most effective at heating the medium-length tumours, especially tumours with high blood flow. The results of this computerized comparative thermal dosimetry study provide information on the feasibility of modelling tumours as simple geometries, and serve to advance three-dimensional interstitial hyperthermia treatment planning.

Experimental brain hyperthermia: techniques for heat delivery and thermometry.

An experimental canine brain model was developed to assess the effects of hyperthermia for a range of time and temperature endpoints, delivered within a specified distance of an interstitial microwave antenna in normal brain. The target temperature location was defined radially at 5.0 or 7.5 mm from the microwave source at the longitudinal location of maximum heating along the antenna in the left cerebral cortex. Temperatures were measured with fiberoptic probes in a coronal plane at this location in an orthogonal catheter at 1.0 mm intervals. Six antennas were evaluated, including dipole, modified dipole, and four shorted helical antennas with coil lengths from 0.5 to 3.9 cm. Antenna performance evaluated in tissue equivalent phantom by adjusting frequency at a fixed insertion depth of 7.8 cm or adjusting insertion depth at 915 MHz showed dipoles to be much more sensitive to insertion depth and frequency change than helical antennas. Specific absorption rate (SAR) was measured in a brain/skull phantom and isoSAR contours were plotted. In vivo temperature studies were also used to evaluate antenna performance in large and small canine brain tissues. A helical antenna with a 2.0 cm coil length driven at 915 MHz was chosen for the beagle experiments because of tip heating characteristics, well-localized heating along the coil length, and heating pattern appropriate to the smaller beagle cranial vault. Verification of lesion dimensions in 3-D was obtained by orthogonal MRI scans and histology to document the desired heat effect, which was to obtain an imagable lesion with well-defined blood-brain-barrier breakdown and necrotic zones. The desired lesion size was between 1.5 to 2.5 cm diameter radially, in the coronal plane with the greatest diameter.

Absorbed power deposition for various insertion depths for 915 MHz interstitial dipole antenna arrays: experiment versus theory.

Dipole antennas are commonly used in interstitial clinical hyperthermia treatments because of their compatibility with brachytherapy techniques and their good power deposition patterns when used in arrays. For accurate treatment planning, however, there must be a comprehensive knowledge base to predict the power deposition patterns when insertion depth is a non-resonant length. This is especially true for insertion depths that result in significant power deposition outside of the antenna junction plane and presumably outside of the tumor volume. A computer controlled measurement system was used with a muscle equivalent phantom to make measurements of specific absorption rate (SAR) or absorbed power per unit mass of tissue at 598 points in a plane. The diagonal plane was the measurement plane of choice because it characterized the SAR profiles at the array center as well as areas in the proximity of the antennas. Dartmouth dipole antennas were used (0.9 mm O.D.) in brachytherapy catheters with inner catheters (2.2 mm O.D./1.2 mm I.D.). The resonant half-wavelength of this dipole antenna/catheter combination is 7.8 cm. A choke modification of the dipole was also investigated. Four antennas were used in a boxlike configuration with 2.0 cm separation. Insertion depths of 5.9, 7.8, 9.8, 12.7, 15.6 and 17.6 cm were used. The hA subsection (junction to tip) was held constant at 3.9 cm. Plots were made of the experimental SAR data normalized to the maximum SAR measured in the plane. Theoretical plots were calculated in the same plane for each of the insertion depths. SAR comparisons were also made longitudinally along the central axis of the array and through the antenna junctions in the diagonal plane for resonant half-wavelength insertion depth. Experimental results verified theoretical predictions of the existence of a secondary hot-spot in the center of the array, but outside of the antenna junction plane and approximately a quarter-wavelength from the insertion point. This secondary hot-spot appears for all insertion depths greater than 10 cm. At longer insertion depths approaching a full wavelength, however, this secondary peak is not dominant. Choke antennas demonstrated a solution to the problem of shifting SAR patterns with varying insertion depths by restricting the active length of the antenna.

Theoretical investigation of a phased-array hyperthermia system with movable apertures.

A four-applicator phased-array hyperthermia system with movable apertures (MA) is compared with an eight-applicator annular phased-array hyperthermia system with fixed apertures (AA) in terms of the HEP (hyperthermia equipment performance) values, based on two-dimensional models and the bioheat transfer equation. A hybrid element method is used to calculate the zeta-directed two-dimensional electric field with the inhomogeneities in tissue properties taken into account. The amplitudes and phases of each applicator are then optimized with the objective of uniform power deposition in the tumour and no power deposited in normal tissues. The temperature distributions under different blood flow conditions are obtained by solving the bioheat transfer equation using the finite element method. It is found that among the seven patient models studied, the MA and AA in general perform equally well when the tumour has zero blood flow, or equally poorly when the tumour has a blood flow larger than 5 ml/100 g per min. The performance of AA is often significantly better than that of MA when the tumour blood flow is 2.7 ml/100 g per min. The effects of different weighting functions are evaluated. We show that even if uniform absorbed power density (ARD = absorption rate density) could be achieved in the tumour volume with zero ARD in normal tissue the entire tumour would still not be brought to 43 degrees C or greater. However, it is found that the performance of uniform ARD in the tumour is on average far better than either the AA or MA, and choosing the uniform ARD as the objective function improved 35% of the cases for AA and 16% for MA. The optimization formula includes a weighting function that can be varied for different tissues. By decreasing the weights in regions of high blood flow the HEP values can sometimes be improved quite noticeably. Finally, the importance of the locations of applicators is studied. The results obtained indicate that the applicators should be placed about 5 cm or more away from the patient body (assuming water is the coupling medium) to ensure good HEP ratings.

The effect of insertion depth on the theoretical SAR patterns of 915 MHz dipole antenna arrays for hyperthermia.

Interstitial microwave antenna array hyperthermia (IMAAH) is presently used clinically in the treatment of cancer. This paper presents the theoretical specific absorption rate (SAR) patterns of 915 MHz microwave antenna arrays for varying insertion depths. The antennas were oriented in a 2 x 2 cm square array. Insertion depth, defined as distance from skin to antenna tip, ranged from 5.9 to 17.6 cm. Two different antenna configurations were considered. In the first the antenna had a distal section a quarter-wavelength long (resonant case), while the second had a distal section approximately 13% longer than a quarter-wavelength (non-resonant case). SAR patterns were calculated from theoretical expressions, and displayed as lines of constant SAR normalized to the maximum SAR value in the array. The results show that regions of concentrated power deposition or 'hotspots' occurred in the centre of the array and moved in a complex but predictable fashion as insertion depth was varied. For insertion depths shorter than a resonant half-wavelength, there occurred one hotspot distal to the antenna junctions. As insertion depth was increased beyond a resonant half-wavelength, the hotspot moved proximal to the antenna junctions and eventually split in two. For depths very much longer than a resonant half-wavelength a hotspot centred about the antenna junction dominated the SAR pattern. For the resonant case the maximum SAR was often along the central axis of the array, while for the non-resonant case the maximum SAR was at the antennas with a local maximum on the central axis.

A frameless stereotaxic operating microscope for neurosurgery.

A new system, which we call the frameless stereotaxic operating microscope, is discussed. Its purpose is to display CT or other image data in the operating microscope in the correct scale, orientation, and position without the use of a stereotaxic frame. A nonimaging ultrasonic rangefinder allows the position of the operating microscope and the position of the patient to be determined. Discrete fiducial points on the patient's external anatomy are located in both image space and operating room space, linking the image data and the operating room. Physician-selected image information, e.g., tumor contours or guidance to predetermined targets, is projected through the optics of the operating microscope using a miniature cathode ray tube and a beam splitter. Projected images superpose the surgical field, reconstructed from image data to match the focal plane of the operating microscope. The algorithms on which the system is based are described, and the sources and effects of errors are discussed. The system's performance is simulated, providing an estimate of accuracy. Two phantoms are used to measure accuracy experimentally. Clinical results and observations are given.

Thermal conduction effects associated with temperature measurements in proximity to radiofrequency electrodes and microwave antennas.

The smearing effects due to thermal conduction along various, nonenergized, interstitial devices were quantified in a flow cell-thermal step gradient. An insulated cylindrical flow cell with a high (ca 45 degrees C, 1.12 cm i.d., 1.6 cm o.d.) temperature region surrounded by a low (ca 37 degrees C) temperature region was used to compare temperature profiles measured with a thermocouple sensor inside a Stanford radiofrequency (RF) hyperthermia/brachytherapy catheter, a BSD instrumented microwave (MW) antenna (i.e., thermistor integrated into a dipole antenna) and a Dartmouth MW antenna with a juxtaposed optical sensor. Two parameters were used to quantify the thermal smearing of each interstitial device in the flow cell: (a) the maximum temperature difference (MTD) and (b) the full- width- half-maximum (FWHM) of the high temperature region. The "true" temperature maximum (45.4 degrees C) and distribution (FWHM = 1.65 +/- 0.06 cm) were measured with an optical sensor. These data indicate that the BSD instrumented MW antenna significantly smeared the true temperature profile (MTD = 2.7 degrees C, FWHM = 2.1 cm), as did the Dartmouth MW antenna (MTD = 1.5 degrees C, FWHM = 1.7 cm). The Stanford RF catheter, when insulated, resulted in minimal smearing (MTD = 0.3 degrees C, FWHM = 1.9 cm). Moreover, when the insulation was removed so the RF electrode was exposed to the thermal step gradient, smearing was again minimal (MTD = 0.3 degrees C, FWHM = 1.9 cm).

Interstitial thermoradiotherapy.

The more recent engineering and clinical aspects of interstitial hyperthermia are reviewed. The advantages and difficulties of microwave, radiofrequency, and ferromagnetic seeds are evaluated and some future directions for improvements are outlined.

Optimization of the absorbed power distribution for an annular phased array hyperthermia system.

One of the systems under investigation for producing hyperthermia noninvasively for treating deep-seated tumors is the annular phased array. This device consists of two rings of eight electromagnetic apertures that are placed concentrically about the long axis of the patient and radiate energy toward the center. Previous theoretical and clinical studies have concentrated primarily on systems where the amplitude and phase of the signal applied to each aperture were the same, and these studies have shown that the system is capable of depositing power deep within the patient. Nevertheless, in many situations the system was not capable of producing desirable temperature distributions in the tumor and normal tissue. In this paper we report on a 2-dimensional theoretical investigation where an optimization routine was used to select the amplitude and phases of each of eight apertures. The optimization procedure and resulting calculations were based on CT scans of patients with tumors. The electrical and thermal properties of the different organs and tissues were taken into account. The optimization routine tried to achieve uniform absorbed power in the tumor region with zero absorbed power outside. Using the optimized amplitudes and phases, the SAR (specific absorption rate, W/kg) was calculated for the array. The results show that in general the optimization procedure was successful in that the power deposited within the tumor volume was increased with less power deposited into normal tissue when compared to the equal amplitude and phase case. This SAR data was then used as the input to a program based on the bioheat transfer equation, which calculated the temperature distribution in the patient model for an assumed set of blood perfusion rates. Depending on the location, size of the tumor, and blood perfusion rates, the improvement in the percentage of the tumor brought to therapeutic temperature varied from 0% to as much as 80%.

The stereotactic operating microscope: accuracy refinement and clinical experience.

Accuracy of a stereotactic operating microscope, by which imaging data may be superimposed on the operative field without a stereotactic frame, has been most limited by the resolution of imaging information. Using newer algorithms and pilot pole calibration of the digitizer, an error in registration of 2 mm and in contour display of 3 mm has been demonstrated. Greatest utility of the system clinically has been in providing navigational guidance to small lesions undergoing resection.