Prospective image planning in radiation therapy for optimization of image quality and reduction of patient dose.

INTRODUCTION: CT simulation data in image-guided radiation therapy (IGRT) provides patient-specific subject contrast. This information can be exploited to establish, a priori, a suitable imaging goal and to select patient-specific imaging acquisition parameters that optimize the similarity between reference and daily set-up images and reduce imaging dose. This study aims to describe and clinically validate a computerized algorithm designed to provide such optimization. MATERIAL AND METHODS: An image planning system (IPS) was developed to assist in planar kV imaging technique selection for radiation therapy. The system's patient-specific image quality and dose reduction capabilities were validated herein. Anthropomorphic phantom and clinical data were acquired. Mutual information (MI) was used to compare simulated and measured images in both phantom and clinical tests. Variations in contrast resolution resulting from imaging panel underexposure, saturation and a contrast plateau were investigated. For evaluation of patient-specific imaging dose reduction, the IPS was used to modify acquisition settings for six patients. RESULTS: Phantom data confirmed the IPS's predictive capability regarding image contrast. Measured and simulated images showed similar progressions from under-exposure, image quality peak, and loss of contrast due to detector saturation. Clinical data demonstrated that contrast resolution and imaging dose could be prospectively improved without loss of image contrast. The algorithm reduced imaging dose by an average of 47%, and a maximum of 80%. CONCLUSIONS: Loss of image contrast resulting from under-exposure or over-exposure, as well as a contrast plateau can be predicted by use of a prospective image planning algorithm. Image acquisition parameters can be predicted that reduce patient dose without loss of useful contrast.

Benchmarking a novel ultrasound-CT fusion system for respiratory motion management in radiotherapy: assessment of spatio-temporal characteristics and comparison to 4DCT.

Management of respiratory motion during radiation therapy requires treatment planning and simulation using imaging modalities that possess sufficient spatio-temporal accuracy and precision. An investigation into the use of a novel ultrasound (US) imaging system for assessment of respiratory motion is presented, exploiting its good soft tissue contrast and temporal precision. The system dynamically superimposes the appropriate image plane sampled from a reference CT data set with the corresponding US B-mode image. An articulating arm is used for spatial registration. While the focus of the study was to quantify the system's ability to track respiratory motion, certain unique spatial calibration procedures were devised that render the software potentially valuable to the general research community. These include direct access to all transformation matrix elements and image scaling factors, a manual latency correction function, and a three-point spatial registration procedure that allows the system to be used in any room possessing a traditional radiotherapy laser localization system. Counter-intuitively, it was discovered that a manual procedure for calibrating certain transformation matrix elements produced superior accuracy to that of an algorithmic Levenberg-Marquardt optimization method. The absolute spatial accuracy was verified by comparing the physical locations of phantom test objects measured using the spatially registered US system, and using data from a 3DCT scan of the phantom as a reference. The spatial accuracy of the display superposition was also tested in a similar manner. The system's dynamic properties were then assessed using three methods. First, the overall system response time was studied using a programmable motion phantom. This included US video update, articulating arm update, CT data set resampling, and image display. The next investigation verified the system's ability to measure the range of motion of a moving anatomical test phantom that possessed both high and low contrast test objects. Finally, the system's performance was compared to that of a four-dimensional CT (4DCT) data set. The absolute spatial and display superposition accuracy was found to be better than 2 mm and typically 1 mm. Overall dynamic system response was adequate to produce a mean relative positional error of less than 1 mm if an empiric latency correction of 3 video frames was incorporated. The dynamic CT/US display mode was able to assess phantom motion for both high and low contrast test objects to within 1 mm, and compared favorably to the 4DCT data. The 4DCT movie loop accurately assessed the target motion for both of the high and low contrast objects tested, but the minimum intensity and average intensity reconstructions did not. This investigation demonstrated that this US system possesses sufficient spatio-temporal accuracy to properly assess respiratory motion. Future work will seek to demonstrate efficacy in its clinical application to respiratory motion assessment, particularly for sites in the upper abdomen, where low tissue contrast is evident.

Effects of external beam irradiation on the TRAM flap: an experimental model.

The rat model of the transverse rectus abdominis musculocutaneous (TRAM) flap was used in the present study to determine the effects of external beam radiation on myocutaneous flap histology and pathophysiology. A total of 57 adult Sprague-Dawley rats underwent a TRAM procedure. A pilot study with 17 animals was first performed to determine proper radiation dosages, and the remaining 40 rats were then used in the definitive study. In half of the definitive study group, the flaps were subjected to fractionated doses of external beam radiation, whereas the other half served as controls. Six weeks after the last radiation dose, all animals were killed and the flaps were harvested for mechanical assessment and histopathologic evaluation. All TRAM flaps survived in both groups. The irradiated and nonirradiated flaps were minimally distinguishable in viscoelastic properties, as well as by histopathologic examination. Growth of the flap in the irradiated animals was significantly diminished (48 percent average surface area increase in irradiated flaps, versus 92 percent increase in nonirradiated flaps, p < 0.05). These findings suggest that the myocutaneous flap is relatively resistant to some of the known adverse affects of radiation on living tissues.

Treatment planning considerations and quality assurance for CT-guided transischiorectal implantation of the prostate.

A CT-guided technique for prostate brachytherapy has been developed which can be performed on patients with large prostates and which allows real time evaluation and modification of implant geometry. The patient is positioned prone and radioactive Pd 103 or I 125 seeds are implanted through the transischiorectal space. Needles are inserted through a template whose plane is oriented at a nominal angle of 26 degrees away from the horizontal to facilitate needle penetration through the ischiorectal space. The CT gantry is tilted 26 degrees, so that its plane is orthogonal to that of the template. The oblique geometry between the CT gantry and direction of couch translation must be considered during source position planning, implantation, and post-treatment evaluation. This consideration is presented along with discussion of relevant quality assurance procedures and recommended tolerances. The mechanical and radiological tolerances of the CT scanner must be consistent with the high level of precision required in radiation therapy. Special emphasis was placed on gantry laser and image plane alignment, sensitivity and radiation profiles, and spatial accuracy of image reconstruction, table translation, and gantry tilt.

Thermodynamics of movable inductively heated seeds for the treatment of brain tumors.

A thermodynamic study is presented of temperature distributions created by an inductively heated 6-mm-diam Ni sphere imbedded in vivo and in vitro into porcine brain tissue. This study was performed in support of the development of a system that creates localized heat-induced lesions in deep-seated brain tumors. In this system, a magnetic "seed" will be remotely repositioned within the brain by an externally produced magnetic field. Convective effects of a hot moving seed will produce a different thermodynamic situation than that arising from an array of static implants. In this work, a study is presented of part of the expected change, in which a static sphere is heated to high temperature. Measurements were made of the temporal and spatial dependence of the temperature rise in the vicinity of the heated sphere, in vivo in four animals and in one that was euthanized immediately prior to experimentation. These results are used for parameter estimation with a theoretical model based on a point source solution to a form of the thermal diffusion equation, i.e., the "bioheat transfer equation." With this model thermal distributions from a power source of arbitrary geometry can be found using appropriate integration methods, and the method has widespread applicability. Estimates of blood flow rates, tissue thermal conductivity, and seed power absorption were found using the parameter estimation algorithm. The estimated blood perfusion exhibits a step increase following the first heating in multiple heating experiments. Thermal conductivity estimated using data from the nonperfused (in vitro) animal is 0.6 W/m degrees C. Seed power absorption is estimated correspondingly to be 0.9 W, a result confirmed independently with calorimetry. Statistical uncertainty is established for the radial decrease of the tissue temperature rise created by this method. This result allows estimation of a cell death boundary uncertainty of 0.6 mm, caused by fluctuations in power delivered to the seed, uncertainty in the temperature probe placements, and thermal properties such as blood perfusion and tissue thermal conductivity.

Magnetic stereotaxis: a technique to deliver stereotactic hyperthermia.

Advances in imaging techniques and computer software over the past decade now define brain abnormalities such as tumors in precise, three-dimensional images. We have taken advantage of these technological improvements in designing a system capable of performing magnetic manipulation of an object in a nonlinear trajectory and able to deliver hyperthermia to highly specific targets within the brain. This device relies on external magnets to pull a small metal pellet (thermoceptor) through the brain, and on biplane fluoroscopy to localize the thermoceptor with respect to previously obtained magnetic resonance images. A radiofrequency tuned circuit serves as the hyperthermia applicator and selectively heats the thermoceptor. This paper describes experiments conducted in a series of dogs showing that all three components of the system (magnetic drive, stereotactic real time imaging, and hyperthermia) can be achieved. Integration of the system was accomplished in one animal. These encouraging results need further detailed substantiation in each of the components, yet demonstrate the feasibility of such a device.

Nonlinear magnetic stereotaxis: three-dimensional, in vivo remote magnetic manipulation of a small object in canine brain.

In a series of in vivo experiments on five adult canines, a small cylindrical permanent magnet (approximately 5-mm diameter x 5 mm long) was magnetically moved under fluoroscopic guidance from an occipital-lobe burr hole to a predetermined destination within the brain and then removed. On three of the animals, dorsal and temporal skull markers were used to establish a coordinate system against which the motions of the seed were referenced. These procedures were sufficiently accurate to permit the guided motion of the seed along nonlinear paths within the brain, including traversal of the midline through the corpus callosum. For removal, the seed could be steered either to a frontal lobe location for extraction through an auxiliary burr hole, or back to the same burr hole through which it had been inserted. This article discusses the way in which stereotactic motions were obtained, the performance limits of the instrumentation and the precision of motion achieved.

Experimental determination of the force required for insertion of a thermoseed into deep brain tissues.

Our laboratories are developing a new technique for delivering localized hyperthermia to deep-seated brain tumors. In this technique, a spherical thermoseed is stereotactically navigated through the brain and tumour tissues via the noncontact application of an external magnetic force. The force required to produce motion of a 3 mm diameter sphere through in vitro brain tissues was measured to be 0.07 +/- 0.03 N. This result was obtained from a series of experiments performed on whole brain specimens extracted from adult canines. Data were also taken with a 3 mm x 3 mm cylinder and a 5 mm sphere. An experimental procedure simulating physiological conditions was developed prior to testing. Evaluations of systematic effects included determinations of the calibration uncertainties, tests of the dependence of the measured force on temperature, and studies of the effects of method of storage of the tissue specimens. The results obtained are compared with (and confirmed by) two different series of experiments performed in vivo on adult canines and with another series of experiments using brain phantom gelatin.

Preliminary experimental investigation of in vivo magnetic manipulation: results and potential application in hyperthermia.

The first in vivo experiments in support of a new technique for delivering stereotaxic hyperthermia have been conducted at the Experimental Surgery Facility of the University of Virginia's Medical Center. We call this technique the "Video Tumor Fighter." In each of twelve trials a single, small permanent magnet or train of small permanent magnets was implanted on the brain surface of adult canine models. In three of the trials, this "seed" (typically 6-mm diameter X 6-mm long) was moved by magnetic manipulation to different locations within the brain. In two other trials, the seed moved along the interface between the brain and the inner vault of the skull. The noncontact magnetic manipulation was accomplished by coupling the permanently magnetized seed to the large dc magnetic field gradient created by a water-cooled coil surrounding the animal's head. The seed's motions were monitored with x-ray fluoroscopy; its rate of movement was found to be approximately 0.8 mm s-1. The forces required to produce these motions were on the order of 0.07 N. We document here the instrumentation used in these trials, describe the experimental procedures employed, and discuss the technical aspects of the results.

Magnetic movement of a brain thermoceptor.

Hyperthermia has significant potential as an adjuvant form of brain tumor therapy. Current intracranial hyperthermia methods, however, are limited in their ability to control spatiotemporal thermal distribution. A stereotaxic magnetic movement system that may be capable of heating discrete regions of brain to a preselected temperature is described. With this system, a ferromagnetic object (referred to as a thermoceptor) is directed through the brain by an external drive magnet. Real time thermoceptor position is monitored with biplanar fluoroscopy and superimposed on a preoperative magnetic resonance imaging scan using a computer. Once in position, the thermoceptor can be inductively heated by externally generated radiofrequency signals. Experiments on the magnetic drive and imaging aspects of this system have been conducted in vitro and in vivo. Mechanical studies of cadaver dog brains revealed that a mean force of 0.07 +/- 0.03 N was required to move a 3-mm diameter sphere through brain at a speed of less than 1 cm/15 s. A cranial phantom with mechanical properties similar to brain was constructed of gelatin and Plexiglas. With the use of a "neck loop" design drive magnet with a maximum magnetic field strength of 0.10 T, a 3 x 3 mm cylindrical neodymium iron boron thermoceptor was smoothly directed through the phantom in two dimensions. Additional experiments were conducted with a larger drive magnet in five anesthetized dogs. Neodymium iron boron and samarium cobalt thermoceptors of various shapes and sizes were placed into the cerebral cortex through a burr hole, then directed with the drive magnet. Fluoroscopy was used to follow the thermoceptor movements.(ABSTRACT TRUNCATED AT 250 WORDS)