Sergio Gonzalez-Arias

Stereotactic target point verification of an X ray and CT localizer

Stereotactic radiosurgery with a linear accelerator requires the accurate determination of a target volume and an accurate match of the therapeutic radiation dose distribution to the target volume. X ray and CT localizers have been described that are used to define the target volume or target point from angiographic or CT data. To verify the accuracy of these localizers, measurements were made with a target point simulator and an anthropomorphic head phantom. The accuracy of determining a known, high contrast, target point with these localizers was found to be a maximum of +/- 0.5 mm and +/- 1.0 mm for the X ray and CT localizer, respectively. A technique using portal X rays taken with a linear accelerator to verify the target point is also described.

Radiosurgery target point alignment errors detected with portal film verification.

Stereotactic radiosurgery with a linear accelerator requires an accurate match of the therapeutic radiation distribution to the localized target volume. Techniques for localization of the target volume using CT scans and/or angiograms have been described. Alignment of the therapeutic radiation distribution to the intended point in stereotactic space is usually accomplished using precision mechanical scales which attach to the head ring. The present work describes a technique used to verify that the stereotactic coordinates of the center of the intended radiation distribution are in agreement with the localized target point coordinates. This technique uses anterior/posterior and lateral accelerator portal verification films to localize the stereotactic coordinates of the center of the radiation distribution with the patient in the treatment position. The results of 26 cases have been analyzed. Alignment errors of the therapeutic radiation distribution in excess of 1 mm have been found using the portal film verification procedure.

Improved linac dose distributions for radiosurgery with elliptically shaped fields

Stereotactic radiosurgery techniques for a linear accelerator typically use circular radiation fields to produce an essentially spherical radiation distribution with a steep dose gradient. Target volumes are frequently irregular in shape, and circular distributions may irradiate normal tissues to high dose as well as the target volume. Improvements to the dose distribution have been made using multiple target points and optimizing the dose per arc to the target. A retrospective review of 20 radiosurgery patients has suggested that the use of elliptically shaped fields may further improve the match of the radiation distribution to the intended target volume. This hypothesis has been verified with film measurements of the radiation distribution obtained using elliptical radiation beam in a head phantom. Reductions of 40% of the high dose volume have been obtained with elliptical fields compared to circular fields without compromising the dose to the target volume.

Stereotactic radiosurgery: Dose‐volume analysis of linear accelerator techniques

Stereotactic radiosurgery of the brain may be accomplished with a linear accelerator by performing several noncoplanar arcs of a highly collimated beam focused at a point. The shape of the radiation distribution produced by this technique is affected by the beam energy, field size, and the number and size of the arcs. The influence of these parameters on the resulting radiation distributions was analyzed by computing dose volume histograms for a typical brain. Dose volume functions were computed for: (a) the energy range of 4-24 MV x rays; (b) target sizes of 1-4 cm; and (c) 1-11 arcs and dynamic rotation. The dose volume histograms were found to be dependent on the number of arcs for target sizes of 1-4 cm. However, these differences were minimal for techniques with 4 arcs or more. The influence of beam energy on the dose volume histogram was also found to be minimal.

A probabilistic approach for pediatric epilepsy diagnosis using brain functional connectivity networks

BackgroundThe lives of half a million children in the United States are severely affected due to the alterations in their functional and mental abilities which epilepsy causes. This study aims to introduce a novel decision support system for the diagnosis of pediatric epilepsy based on scalp EEG data in a clinical environment.MethodsA new time varying approach for constructing functional connectivity networks (FCNs) of 18 subjects (7 subjects from pediatric control (PC) group and 11 subjects from pediatric epilepsy (PE) group) is implemented by moving a window with overlap to split the EEG signals into a total of 445 multi-channel EEG segments (91 for PC and 354 for PE) and finding the hypothetical functional connectivity strengths among EEG channels. FCNs are then mapped into the form of undirected graphs and subjected to extraction of graph theory based features. An unsupervised labeling technique based on Gaussian mixtures model (GMM) is then used to delineate the pediatric epilepsy group from the control group.ResultsThe study results show the existence of a statistically significant difference (p < 0.0001) between the mean FCNs of PC and PE groups. The system was able to diagnose pediatric epilepsy subjects with the accuracy of 88.8% with 81.8% sensitivity and 100% specificity purely based on exploration of associations among brain cortical regions and without a priori knowledge of diagnosis.ConclusionsThe current study created the potential of diagnosing epilepsy without need for long EEG recording session and time-consuming visual inspection as conventionally employed.

Stereotactic surgical planning using three dimensional reconstruction and artificial neural networks

Recent research into different artificial neural network structures and topologies suggests the possibility of implementing a particular application. The goal is for the neural network to represent the input function in a natural manner. The authors describe such an implementation in the field of neurosurgical planning, where a set of neural networks represents the lesion to be treated as well as the different functional regions of the brain. It is shown that this neural network structure can actively and effectively assist in the surgical planning. Emphasis is on stereotactic radiosurgery, whereby a high dose of radiation is delivered to the lesion. This modality allows for extensive implementation of the neural network features in a natural way, using Gaussian potential functions for the neural activation. The goal of decreasing the procedural risk factor in stereotactic surgery is accomplished by implementing the visual interface and a framework of artificial neural networks.<<ETX>>

Connectivity maps of different types of epileptogenic patterns

EEG functional connectivity maps, showing the interactions between brain areas in context to the placement of electrodes, were used for the investigation and comparison of three different types of epileptiform activity defined as single spike, spike followed by slow wave and repetitive spike. A nonlinear data-driven method was used to extract connectivity matrices that helped to identify network synchronization based on the number of connections for all brain regions, as represented by the 10-20 EEG system. This quantification was used to assess these three types of spike patterns in relation to the type of seizure, focal or generalized. Results showed some differences between connectivity patterns of single spikes related to focal epilepsy and connectivity patterns of repetitive spikes related to generalized epilepsy. The variance statistical analysis reported a significant difference (P - value ≪ 0.001) between single spike connectivity maps and other spike types. The results obtained, augment the prospects for diagnosis and enhance recognition of disease type via EEG-based connectivity maps.

Epileptogenic brain connectivity patterns using scalp EEG

This paper introduces a new method for the quantification and analysis of functional connectivity from electroencephalogram (EEG) by implementing cross-correlation method. The recorded clinical EEG signals are segmented and categorized into three types of epileptiform discharges (ED), interictal spike, spike and slow wave complex, and repetitive spike and slow wave complexes. The extracted EEG functional connectivity in different frequency bands were quantified and analyzed to identify characterizing patterns between each type of ED. Results revealed that the number of connections from different epileptogenic biomarkers and calculated for each frequency band were statistically different. Such functional connectivity maps reveal distinctive patterns that can be used to classify these types of ED.

Pediatric epilepsy: Clustering by functional connectivity using phase synchronization

This study proposes a nonlinear data-driven method to delineate Electroencephalogram (EEG) recordings as either coming from controls or patients with epilepsy. This method uses the probability of recurrence and the correlation between electrodes to extract the phase synchronization and the functional connectivity maps of the brain from interictal EEG data recordings. This newly proposed algorithm utilizes probabilistic clustering by extracting graph theoretical features from the calculated functional connectivity matrices. Results reveal that brain connectivity networks of epileptic and control populations show statistically significant differences (t (340) = -37.4771, p<;0.01) between them. Performance results show an accuracy of 92.8% with a sensitivity of 85.7% and a specificity of 100%, when tested on 14 subjects. These preliminary results confirm that this method can be used to enhance and validate diagnosis of epileptic patients from controls using non-invasive scalp EEG signals.

Stereotactic target point verification of an angiographic and CT localizer

This paper presents our experience in controlling Dupuytrens contraction of the palmar fascia. Between 1980 and 1988, 124 patients were treated with definitive radiotherapy. Treatment consisted of conventional 180 KV/ZO mAS beam, 3 Gy per fraction, 5 times per week for a total dose of 15 Gy in a first course. After an interval of six weeks a second series with the same fractionation was given. The total dose of the two series was 30 Gy. All cases were carefully documented. The findings were marked with a coloured pen and directly documented on a regular Xerox machine. This was done before, and six weeks after the end of treatment and at the time of follow up as well. 44 patients had Dupuytrens contracture of both hands and nearly all received the above treatment for both hands. In 75 % of the patients the disease was located exclusively jn the palm of the hand and 25 % showed an additional involvement of the fingers. All patients responded to the treatment. 57 % of them had a response with a 50 % reduction of the nodular formation and a release of contracture. 15 patients developped progress of the disease within or outside the treatment field. The interval between the end of treatment and the progress of disease was 6 51 months. At the time of presentation complete data for adjuvant and retreatment will be available and side effects discussed.