Biomedical subjects

G H Olivera

Publications and source records attributed to G H Olivera.

12 recordsLinked to original sources

PubMed2000

Radiotherapy for brain tumors.

Over the last 2 years, several advances have been made in the field of radiotherapy for brain tumors. Key advances are summarized in this review. Crucial technologic advances, such as radiosurgery, fractionated stereotactic radiotherapy, and intensity-modulated radiotherapy, are discussed. Better understanding of the interaction between the processes of angiogenesis, apoptosis, cell-cycle regulation, and signal transduction and the effects of ionizing radiation has made it clear that many of these "new agents" are, in fact, valuable modulators of the radiation response. Another exciting molecular discovery is the recognition of radiation-induced promoters that can be exploited to cause spatially and temporally configured expression of selected genes; this approach may represent the ideal application of conformal radiation techniques in the future, yielding well-defined genetic changes in specifically targeted tissues. The final "frontier" covered in this review is the newer categories of radiosensitizers, ranging from topoisomerase-I inhibitors, to expanded metalloporphyrins, to oxygen- dissociating agents.

Brain Neoplasms↗

PubMed2000

Iterative approaches to dose optimization in tomotherapy.

This paper will present the results of an investigation into three iterative approaches to inverse treatment planning. These techniques have been examined in the hope of developing an optimization algorithm suitable for the large-scale problems that are encountered in tomotherapy. The three iterative techniques are referred to as the ratio method, iterative least-squares minimization and the maximum-likelihood estimator. Our results indicate that each of these techniques can serve as a useful tool in tomotherapy optimization. As compared with other mathematical programming techniques, the iterative approaches can reduce both memory demands and time requirements. In this paper, the results from small- and large-scale optimizations will be analysed. It will also be demonstrated that the flexibility of the iterative techniques can be greatly enhanced through the use of dose-volume histogram based penalty functions and/or through the use of weighting factors assigned to each region of the patient. Finally, results will be presented from an investigation into the stability of the iterative techniques.

Algorithms↗

PubMed2000

Megavoltage CT image reconstruction during tomotherapy treatments.

An integrated tomotherapy system allows for improved radiotherapy verification by enabling the collection of megavoltage computed tomography (MVCT) images before or after treatment delivery. In this investigation, the possibility of collecting MV tomographic data and reconstructing images during a tomotherapy treatment is examined. By overcoming difficulties with the normalization of modulated treatment data and with the incompleteness of treatment data, it is possible to use data collected during tomotherapeutic treatments for MVCT reconstruction. The benefits of these techniques include potential increases in patient throughput, reductions in imaging dose, visualization of the patient in the treatment position and improvements in image contrast.

Animals↗

PubMed2000

Calibration of a tomotherapeutic MVCT system.

Megavoltage CT provides the ability to image the patient before, during or after a radiotherapy treatment. This allows one to verify not only the placement of a patient's external boundary, but also the locations of internal anatomy. In addition, the reconstructed MVCT values are potentially useful for treatment planning inhomogeneity corrections and dose reconstruction. To this end, dosimetric calibration of the University of Wisconsin Tomotherapy Benchtop MVCT system was investigated. It was found that MVCT values correlate extremely well with electron density and that unlike kilovoltage CT, this correlation is well maintained for higher atomic number materials. Improvements of the order of 1% in the dosimetric calculations of high atomic number materials should be possible by deriving input images from MVCT as opposed to kVCT, and calibrating in terms of electron density, as opposed to physical density.

Calibration↗

PubMed1999

Megavoltage CT on a tomotherapy system.

A megavoltage computed tomography (MVCT) system was developed on the University of Wisconsin tomotherapy benchtop. This system can operate either axially or helically, and collect transmission data without any bounds on delivered dose. Scan times as low as 12 s per slice are possible, and scans were run with linac output rates of 100 MU min(-1), although the system can be tuned to deliver arbitrarily low dose rates. Images were reconstructed with clinically reasonable doses ranging from 8 to 12 cGy. These images delineate contrasts below 2% and resolutions of 3.0 mm. Thus, the MVCT image quality of this system should be sufficient for verifying the patient's position and anatomy prior to radiotherapy. Additionally, synthetic data were used to test the potential for improved MVCT contrast using maximum-likelihood (ML) reconstruction. Specifically, the maximum-likelihood expectation-maximization (ML-EM) algorithm and a transmission ML algorithm were compared with filtered backprojection (FBP). It was found that for expected clinical MVCT doses enough imaging photons are used such that little benefit is conferred by the improved noise model of ML algorithms. For significantly lower doses, some quantitative improvement is achieved through ML reconstruction. Nonetheless, the image quality at those lower doses is not satisfactory for radiotherapy verification.

Algorithms↗

PubMed1999

Registration using tomographic projection files.

An algorithm has been developed and experimentally verified for tomographic registration--a patient positioning method using internal anatomy and standard external fiducial marks. This algorithm improves patient set-up and verification to an accuracy sufficient for tomotherapy. By implementation of this technique, the time-consuming reconstruction process is avoided. Instead, offsets in the x, y and z directions are determined directly from sinogram data by an algorithm that utilizes cross-correlations and Fourier transforms. To verify the efficiency and stability of the algorithm, data were collected on the University of Wisconsin's dedicated tomotherapy research workbench. The experiment indicates offset statistical errors of less than +/-0.8 mm for offsets up to 30 mm. With standard clinical techniques, initial patient offsets are expected to be less than 5 mm, so the 30 mm limitation is of no consequence. The angular resolution for the direction of patient translation is within the +/-2 degrees needed for tomotherapy.

Algorithms↗

PubMed1999

Delivery verification in sequential and helical tomotherapy.

Conformal and conformal avoidance radiation therapy are new therapeutic techniques that are generally characterized by high dose gradients. The success of this kind of treatment relies on quality assurance procedures in order to verify the delivery of the treatment. A delivery verification technique should consider quality assurance procedures for patient positioning and radiation delivery verification. A methodology for radiation delivery verification was developed and tested with our tomotherapy workbench. The procedure was investigated for two cases. The first treatment using a torus-shaped target was optimized for 72 beam directions and sequentially delivered as a single slice to a 33 cm diameter cylinder of homogeneous solid water. For the second treatment, a random pattern of energy fluence was helically delivered for two slices to a 9.0 cm diameter phantom containing inhomogeneities. The presented process provides the energy fluence (or a related quantity) delivered through the multileaf collimator (MLC) using the signal measured at the exit detector during the treatment delivery. As this information is created for every pulse of the accelerator, the energy fluence and state for each MLC leaf were verified on a pulse-by-pulse basis. The pulse-by-pulse results were averaged to obtain projection-by-projection information to allow for a comparison with the planned delivery. The errors between the planned and delivered energy fluences were concentrated between +/-2.0%, with none beyond +/-3.5%. In addition to accurately achieving radiation delivery verification, the process is fast, which could translate to radiation delivery verification in real time. This technique can also be extended to reconstruct the dose actually deposited in the patient or phantom (dose reconstruction).

Acrylates↗

PubMed1999

Image/patient registration from (partial) projection data by the Fourier phase matching method.

A technique for 2D or 3D image/patient registration, PFPM (projection based Fourier phase matching method), is proposed. This technique provides image/patient registration directly from sequential tomographic projection data. The method can also deal with image files by generating 2D Radon transforms slice by slice. The registration in projection space is done by calculating a Fourier invariant (FI) descriptor for each one-dimensional projection datum, and then registering the FI descriptor by the Fourier phase matching (FPM) method. The algorithm has been tested on both synthetic and experimental data. When dealing with translated, rotated and uniformly scaled 2D image registration, the performance of the PFPM method is comparable to that of the IFPM (image based Fourier phase matching) method in robustness, efficiency, insensitivity to the offset between images, and registration time. The advantages of the former are that subpixel resolution is feasible, and it is more insensitive to image noise due to the averaging effect of the projection acquisition. Furthermore, the PFPM method offers the ability to generalize to 3D image/patient registration and to register partial projection data. By applying patient registration directly from tomographic projection data, image reconstruction is not needed in the therapy set-up verification, thus reducing computational time and artefacts. In addition, real time registration is feasible. Registration from partial projection data meets the geometry and dose requirements in many application cases and makes dynamic set-up verification possible in tomotherapy.

Algorithms↗

PubMed1998

Maximum likelihood as a common computational framework in tomotherapy.

Tomotherapy is a dose delivery technique using helical or axial intensity modulated beams. One of the strengths of the tomotherapy concept is that it can incorporate a number of processes into a single piece of equipment. These processes include treatment optimization planning, dose reconstruction and kilovoltage/megavoltage image reconstruction. A common computational technique that could be used for all of these processes would be very appealing. The maximum likelihood estimator, originally developed for emission tomography, can serve as a useful tool in imaging and radiotherapy. We believe that this approach can play an important role in the processes of optimization planning, dose reconstruction and kilovoltage and/or megavoltage image reconstruction. These processes involve computations that require comparable physical methods. They are also based on equivalent assumptions, and they have similar mathematical solutions. As a result, the maximum likelihood approach is able to provide a common framework for all three of these computational problems. We will demonstrate how maximum likelihood methods can be applied to optimization planning, dose reconstruction and megavoltage image reconstruction in tomotherapy. Results for planning optimization, dose reconstruction and megavoltage image reconstruction will be presented. Strengths and weaknesses of the methodology are analysed. Future directions for this work are also suggested.

Algorithms↗

PubMed1998

Multiple ionization in the earlier stages of water radiolysis.

We have studied the fragmentation of water vapour molecules induced by collision with a Xe44+ beam at 6.7 MeV/u. From the measurement of the fragment time of flight, we show that the amount of fragmentation due to multiple ionization is very large. In the case of single ionization, we are able to reproduce accurately the experimental cross sections by calculating for each molecular level the single-ionization cross section in the framework of the CDW-EIS theory and with a diagram of dissociation modified with respect to the diagram obtained in the case of dipolar ionization. By using qualitative arguments based on the ability of the medium to neutralize a charged species, we tentatively extend our result to liquid water. From our analysis, we show that ionizations involving three or more ejected electrons could enhance the oxygen production. For the physicochemical phase we estimate that the rate of oxygen production by multiple ionization represents approximately 18% of the OH rate produced by single ionization.

Oxygen↗

PubMed1996

Contribution from the inner shell of water vapour to dose profiles under proton and alpha particle irradiation.

Doubly differential cross sections for electron emission calculated using the CDW-EIS model and a simple approximation for the electron transport in the target are used to obtain dose profiles around the ion path for proton and alpha particles in water vapour. The contribution from each initial molecular orbital is determined. At large distances from the track, discrepancies are found with other models and with the well known r-2 dependence.

Alpha Particles↗

PubMed1995

Theoretical calculation of electronic stopping power of water vapor by proton impact.

The energy loss of proton beams in water vapor is analyzed with a full quantum-mechanical treatment, the distorted-wave model. This model takes into account distortion effects due to the long-range Coulomb potential. Projectile energies from 10 keV up to 1 MeV are considered. Mean stopping power and equilibrium charge-state fractions are calculated and compared with experimental data. The validity of Bragg's additivity rule is investigated.

Gases↗