A method of stereotaxic localization adopted for conventional and digital radiography

Reproducibility and geometric accuracy of the Fixster system during hypofractionated stereotactic radiotherapy

Background

Hypofractionated radiotherapy has been used for the treatment of AVMs and brain metastases. Hypofractionation necessitates the use of a relocatable stereotactic frame that has to be applied on several occasions. The stereotactic frame needs to have a high degree of reproducibility, and patient positioning is crucial to achieve a high accuracy of the treatment.

Methods

In this study we have, by radiological means, evaluated the reproducibility of the isocenter in consecutive treatment sessions using the Fixster frame. Deviations in the X, Y and Z-axis were measured in 10 patients treated with hypofractionated radiotherapy.

Results

The mean deviation in the X-axis was 0.4 mm (range -2.1 – 2.1, median 0.7 mm) and in the Y-axis -0.3 mm (range -1.4 – 0.7, median -0.2 mm). The mean deviation in the Z-axis was -0.6 (range -1.4 – 1.4, median 0.0 mm).

Conclusion

There is a high degree of reproducibility of the isocenter during successive treatment sessions with HCSRT using the Fixster frame for stereotactic targeting. The high reducibility enables a safe treatment using hypofractionated stereotactic radiotherapy.

Localization of cerebral arterovenous malformations using digital angiography

Since 1989 we performed stereotactic radiotherapy treatments of cerebral arterovenous malformations (AVM), estimating three-dimensional (3-D) localization and shape of target volumes by the Leksell stereotactic helmet on two orthogonal radiographic projections. Due to the limitations of this method, we developed a new technique for the localization of the target volume using digital subtraction angiography (DSA) and digital image processing. To achieve this result we first developed a method to correct nonlinear distortion of DSA images using spatial relocation of image pixels based on a calibration grid. We then developed an algorithm for localization of the target volume using two independent DSA projections. Target volume coordinates in the helmet system are calculated using two DSA acquisitions taken with a free angle (approximately 90 degrees), one in the AP and the other in the LL direction. The helmet can be freely positioned between the x-ray source and the image plane. The projections of eight reference points inserted in the helmet at a known location, are used to calculate the transformation matrix between the two coordinate systems. We performed numerical and experimental validation of the system. A hypothetical random error (up to 2 mm) on image coordinates of the reference points allowed to determine that the error in target localization was less than 0.2 mm. Using DSA images of target points with a known location within a phantom, the error between calculated and actual location was, on average, 0.30+/-0.13 mm (mean+/-SD), with a maximum error of 0.49 mm. The results of numerical and experimental validations show that the system we have developed allows fast and accurate localization of the center of the target volume and it is suitable for efficient guiding during stereotactic radiosurgery of AVM.

A simple method for the correction of distorted digital angiographic images for stereotactic target localization

The most commonly used imaging modality for the diagnosis and localization of arteriovenous malformations (AVMs) treated with stereotactic radiotherapy is traditional angiography, but it would be desirable to also use digital subtraction angiography (DSA). However, DSA images are distorted due to the electron-optical characteristics of the X-ray image intensifier. For that reason, we have developed a method for the correction of the image distortion. The ISIS II Treatment Planning System (ISIS II TPS), developed at the Curie Institute, has been used for image acquisition and stereotactic localization. A grid phantom has been constructed for determining the distortion of the DSA images. The software developed for the correction has been implemented into the TPS and is based on a correction vector produced by matching the distorted and corrected grid points. The method has been tested for its ability to correct the position of all grid points as well as its effectiveness in real cases as compared to traditional angiography. The maximum displacement of the corrected grid points compared with their original position is measured to be 0.1 mm. The accuracy of the target localization using the corrected DSA images is comparable with traditional angiography localization and falls inside acceptable accuracy limits. In conclusion, this method offers the possibility of using DSA images for stereotactic localization without limiting the requested accuracy.

Three-dimensional localisation based on projectional and tomographic image correlation: an application for digital tomosynthesis

Accurate three-dimensional tumor localisation in Radiotherapy, is critical to the treatment outcome, particularly when high dose gradients are present. A number of techniques have been proposed for the localisation of anatomical structures or markers. The present study proposes an approach to a concurrent maximisation of localisation accuracy and efficiency by correlation of tomographic and projectional images. The method introduces an element of direct verification and interactive optimisation of the process. Tomographic images are used for the identification of a point of interest. Its position is computed within the treatment co-ordinate system and verification of this position is achieved by obtaining the beam's eye view of the identified point on two projection radiographs. The key element of the approach is that all images used should be part of one single image data set. The implementation of this localisation method, as part of the functionality of a Digital Tomosynthesis prototype, has provided an integrated facility for localisation, of optimised accuracy and precision, while easy and efficient to use. The considerations are general and apply in principle to any imaging system that can augment tomographic images with projections.

A simple method to verify in vivo the accuracy of target coordinates in linear accelerator radiosurgery

PURPOSE

A simple method that verifies the coincidence of the isocenter with the center of the target volume in radiosurgery treatment conditions is described. The accuracy is compared to that of accepted computerized procedures employing fiducial markers.

METHODS AND MATERIALS

The center of the beam is identified by a cylindrical localizer, fixed to the plate of the supplemental collimator, with a 2 x 50 mm tungsten rod coincident with the beam axis and is projected onto the x-ray portal verification films. Prior to irradiation, the coordinates of the intersection of the beams axes, which is in a known spatial relationship with the isocenter, are read directly on portal x-ray films and their coincidence with the coordinates set during patient positioning, is checked.

RESULTS

The mean displacement in AP, Lat, and Vert coordinates respectively, over 84 patients, between the coordinates calculated by the computerized procedure employing fiducial markers and the coordinates calculated by using the rulers was 0.3 +/- 0.4 mm.

CONCLUSIONS

From the results obtained with the two methods we can conclude that rulers method can be used as a fast indirect control of the position of the radiation isocenter. Moreover, the dimensions of the radiation field and the correct alignment of the tertiary circular collimator can be also documented.

Transfer of information between angiographic films and CT images: a technique to control the drawing of target volumes

BACKGROUND AND PURPOSE

This work was undertaken to improve the definition of target volumes for radiosurgery using the angiographic and CT data.

MATERIALS AND METHODS

The basis of this new method is to combine both imaging modalities and to compare them in each representation, i.e. to plot the volume obtained by angiography on CT images and also the contours defined by the CT on angiographic films. To obtain the angiographic volume, the radiographs are taken at several incidence angles. The X-ray sources position and the position of the films are determined using rectangular markers, then the intersection of all the loci of the target volume are calculated.

RESULTS

Verifications with a phantom show the accuracy of the procedure and the benefit obtained by increasing the number of angles of incidence in the angiographic imaging. The centre of gravity of the experimental target could be localized to an accuracy of better than 0.4 mm. The method was used in 11 clinical cases with excellent clinical results.

CONCLUSIONS

The method can be easily applied and improves the delineation of target volumes in radiosurgery. CT data counterbalances the relative weakness of angiography concerning the three-dimensional geometry. Angiography adds useful information on the blood flow that is not shown in CT. Almost all the presented clinical cases benefit from the technique described here.

A simple method for 3D lesion reconstruction from two projected angiographic images: implementation to a stereotactic radiotherapy treatment planning system

INTRODUCTION

The most used imaging modality for diagnosis and localisation of arteriovenous malformations (AVMs) treated with stereotactic radiotherapy is angiography. The fact that the angiographic images are projected images imposes the need of the 3D reconstruction of the lesion. This, together with the 3D head anatomy from CT images could provide all the necessary information for stereotactic treatment planning. We have developed a method to combine the complementary information provided by angiography and 2D computerized tomography, matching the reconstructed AVM structure with the reconstructed head of the patient.

MATERIALS AND METHODS

The ISIS treatment planning system, developed at Institute Curie, has been used for image acquisition, stereotactic localisation and 3D visualisation. A series of CT slices are introduced in the system as well as two orthogonal angiographic projected images of the lesion. A simple computer program has been developed for the 3D reconstruction of the lesion and for the superposition of the target contour on the CT slices of the head.

RESULTS AND CONCLUSIONS

In our approach we consider that the reconstruction can be made if the AVM is approximated with a number of adjacent ellipses. We assessed the method comparing the values of the reconstructed and the actual volumes of the target using linear regression analysis. For treatment planning purposes we overlapped the reconstructed AVM on the CT slices of the head. The above feature is to our knowledge a feature that the majority of the commercial stereotactic radiotherapy treatment planning system could not provide. The implementation of the method into ISIS TPS shows that we can reliably approximate and visualize the target volume.

Optimized target localization in stereotactic radiosurgery using real-time digital portal images

This paper presents a method for optimized estimation of target localization in stereotactic radiosurgery using real-time digital portal images. The positions of the two radiation sources and the two projection planes in radiosurgery can be chosen arbitrarily. The reconstruction of the target is characterized as a non-linear multi-variable optimization problem, which minimizes the difference between the actual target and the reconstructed target. This optimization problem is solved by the Gaussian least square differential correction (GLSDC) method. Due to the digitized errors and input data errors, the algorithm defines an indirect reference factor (the shortest distance between two x-rays pointing onto the target) as an indication of the accuracy of the method. Our experimental results show that the reference factor is less than 1.0 mm for the correct data entry. The proposed algorithm provides a method that supports on-line target localization which can be done directly on the digital portal imaging device.

Radiobiology of radiosurgery for refractory anxiety disorders

The neuroradiological manifestations of bilateral single-session gamma (gamma)-irradiation to normal tissue contained in the internal capsule after gamma knife capsulotomy for otherwise intractable anxiety disorders were studied. In nine consecutive patients, a target maximum dose of 200 Gy was administered in a target volume of 276 +/- 42 mm3 (mean +/- SD) within the 50% isodose level. Serial computed tomographic and magnetic resonance imaging scans were undertaken from 3 to 44 months after irradiation. After surgery, a necrotic lesion appeared on computed tomographic scans, reaching its maximum volume (900 +/- 800 mm3) at 6 to 9 months, then decreasing (to 457 +/- 400 mm3) over the first postoperative year. This volume correlated with the mean isodose level of 91 (range, 41-143) Gy. On T2-weighted magnetic resonance imaging scans, the reaction tissue volumes were considerably larger and took longer to disappear than expected. In 15 targets, maximum reaction volumes were recorded at 1 to (approximately) 2 years after irradiation. In the remaining seven targets, smaller reaction volumes were observed, with no clear maxima appearing during 3 years of observation. In a pilot case, a lower target maximum dose of 160 Gy and a radiation volume of 275 mm3 within the 50% isodose gave only minimal surrounding tissue reactions. This report serves to alert clinicians that the tissue reaction volumes and the time course of their development after high irradiation doses may be less predictable than expected from previous observations in smaller radiation volumes. For this reason, lower irradiation doses and smaller volumes should be used in the future, and the time factor should be taken into account when interpreting computed tomographic and magnetic resonance images of gamma-knife-induced lesions.

Superimposition of an average three-dimensional pattern of brain structures on CT scans

A method is described for the superimposition of an averaged telencephalic anatomical model and individual CT scans. The model is digitally available and derived from 3-D measurements of 30 post-mortem brains. It is averaged in size in relation to the midintercommissural point. The midintercommissural point in the individual brain is gained from reformed parasagittal projection images. The model is adjusted to the individual CT scan series by scaling and rotating according to the best fit and correspondence of the position of the central sulcus. The method needs no invasive neuroradiological techniques but is based on current computer algorithms.

An adaptor to the Leksell stereotaxic instrument for digital coordinate determination in radiography

Special perspex adaptors with radiopaque reference structures have been constructed to fit to the Leksell stereotaxic instrument. These structures are visualized on X-ray film in a radiographic examination like angiography or encephalography. The films obtained in two projections at arbitrary angles and focus-film-distances are placed on a digitizing table for the determination of the stereotaxic coordinates of selected targets. The reference structures and the targets are marked with a cursor whereupon a desk top computer performs the calculation of the stereotaxic coordinates. The system allows a rapid, simple and accurate coordinate determination in stereotaxic radiography using the Leksell stereotaxic instrument.

Stereotaxic localization of intracranial targets

We report on a useful clinical method for precisely locating intracranial targets. Utilizing the BRW system, the technique is currently used in stereotaxic irradiation of arteriovenous malformations. An intracranial localizer box, with four radio-opaque markers on each face, surrounds the patient's head and is attached to the BRW Head Ring. Two localization films are required. One film includes the target and the eight anterior and posterior markers, whereas the other film includes the target and the eight right and left markers. There are no constraints that the films be orthogonal or parallel to the box faces, only that the target and radio-opaque markers appear on the films. In addition, knowledge of the source-image and source-target distances are not required. Analysis of the projected target and radio-opaque markers gives both the target location and magnification. Simulation with the BRW Phantom Base demonstrates that point targets can be located with respect to the BRW system to within 0.3 mm and magnification determined to within 0.5%.

Stereotactic radiation therapy of intracranial lesions. Methodologic aspects

A technique for stereotactic radiation therapy of cerebral tumours and arteriovenous malformations using a linear accelerator (6 MV photons) is proposed. Treatment relies on a fixation system that permits a precise use of the coordinates estimated at stereotactic angiography or stereotactic computed tomography. The field of treatment can be exactly outlined in the CT images during repeat examinations, thus facilitating the recognition of changes induced by radiation. The system also allows the extent of the arteriovenous malformation, as seen at angiography, to be accurately traced in the CT sections thus enabling evaluation of possible radiation damage to surrounding brain structures. The precision of the method as well as its hypothetical merits and disadvantages are discussed. The number of patients treated is still small and the follow-up time is too short in the majority of cases to allow definite conclusions. Examples of preliminary results are given.