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0 Oncology Intelligence STEREOTACTIC
ANGIOGRAPHY: AN INADEQUATE FOR RADIOSURGERY?
University of Florida College of Medicine, Gainesville, FL Stereotactic angiography has long been the imaging database for the radiosurgical treatment of arteriovenous malformations (AVM). The following analysis reveals systematic shortcomings in the methodology, resulting in errors in determining target shape, errors in determining target size, and errors in the identification of the true AVM “nidus.” Radiosurgery, Arteriovenous malformations,
Stereotactic angiography, Stereotaxis.
from an orthogonal pair of radiographs. Figure 1 shows the projection of two such views used to reconstruct the object of interest. The second technique involves two stereoshifted views. This technique is usually associated with a fiducial jig (that is, a standard pattern of fixed radiopaque markers of known position), which allows the shift parameters to be derived from information contained on the films. While the latter technique has the advantage of only requiring a linear translation, instead of a rotation about the patient, there are potential errors when small shifts are used (2). For either of these localization techniques to define the 3-dimensional shape and size of an
In recent years, radiosurgery has gained tion in the treatment of some intracranial
increasing attenlesions for which conventional neurosurgical procedures are not generally suitable. When arteriovenous malformations (AVM’s) are treated radiosurgically, a stereotactic angiogram is performed to localize the center of the lesion and to determine its precise diameter. Although angiography has been the “gold standard” by which AVM’s have been diagnosed, its use as a stereotactic database has not been widespread. The ease of localization of either single points or spherical targets has led to the unfounded assumption that the size and shape of any target can be accurately determined from two stereotactic angiographic views. At the University of Florida, radiosurgical treatments with a radiosurgery system based on a linear accelerator were begun in May 1988 (3, 4). In the process of treating patients with AVM’s, certain problems inherent in stereotactic angiography became obvious. The purpose of this paper is to explain those problems in depth, as many institutions anticipate the adoption of radiosurgery as a treatment modality.
Errors in determining target shape For many years, two techniques have been used to define the size and shape of interstitial and intracavitary implants (5). The first and most accurate is reconstruction
on two radiographs. Correct reconstruction requires matching of unique points on the two views.
* Department of Radiation Oncology. + Division of Neurologic Surgery. Reprint requests to: Francis J. Bova, Ph.D., Dept. of Radiation
Oncology, Box J-385, J. Hillis Miller Health Center, Gainesville, FL 32610-0385. Accepted for publication 12 September 1990.
Fig. 1. Schematic representation
of the projection of an object
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Fig. 2. Anteroposterior
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(A) and lateral (B) angiographic views of an AVM show feeding arteries (small open arrow), A lack of identifiable structure within the nidus
nidus (large open arrow). and draining veins (closed arrow). prevents point-matching on the AP and lateral views.
object correctly, the “unique” points on each view must be matched. With most stereotactic angiograms. two problems prevent the identification of these unique points. The first is the inability to obtain two simultaneous angiographic views of the nidus. Even with the aid of biplanar angiography, the orthogonal views are staggered in time. For a filming sequence of four views per second. the time between the anteroposterior (AP) and lateral films is a quar-
ter of a second. During this time, a significant change in the target definition can occur. The second problem hindering the reconstruction process is the lack of structural detail within the nidus. Figure 2 shows the AP and lateral views of a typical AVM. Although some of the feeding arteries and draining veins can be identified and matched on each view, the lack of identifiable structures within the nidus prevents pointmatching.
Fig. 3. Projection of a spherical target on two angiographic views. In this unique case, the symmetry and lateral projection allows the deduction of the true lesion shape despite the lack of point-matching.
of the AP
Stereotactic angiography 0 F. J.
BOVA .ANDW. A. FRIEDMAN
Fig. 5. Lack of appreciation of the superior extent of the target on the lateral view would lead to an incorrect estimate of the target center.
true shape of the target, they might choose to treat it with multiple isocenters or other lesion-contouring techniques. The incomplete information provided by the stereotactic angiographs would usually result in a treatment plan that would target more normal tissue than necessary. Examples of other target types are shown in Figure 4B and 4C.
For many stereotactic radiosurgery procedures, the geometric center of the target volume is placed at isocenter. The target center can be located according to various techniques. The most reliable is to place the target at the center of the smallest sphere that fully circumscribes the target. This point can be found by first locating for each view the center of the smallest circle that would encompass the target. If the centers of these two circles are projected back toward the x-ray source. they intersect at the target’s geometric center. Take, for example, the AP and lateral views in Figure 3. The geometric center of the AP view locates the lateral and vertical coordinates of the target Fig. 4. These projections of these curved objects contain ficient information about their actual shape.
The product of the stereotactic angiographic localization procedure is 2-dimensional projections of a 3-dimensional object. As discussed above, these projections may not delineate the true shape and size of the target. For spherical targets, the projections are often adequate (Fig. 3). However, as the target shape deviates from a spherical geometry, the projections become more misleading. Figure 4A shows a curved target, which on the AP projection appears to be cylindrical; on the lateral view, however, the target appears to be bullet-shaped. With no prior knowledge of the target shape, these two views would lead one to believe that the target was cylindrical. On the other hand, if the treatment personnel knew the
Fig. 6. Projection of a target with its long axis at a 45” angle to the two angiographic views results in an underestimation of true target size when endpoint matching is not possible.
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Fig. 7. (A) Coronal MRI view ofan acoustic neuroma is in the same orientation as an AP angiogram. (B) Transaxial view. Because the long axis of the lesion is at a 45” angle to this view. the true lesion diameter would be underestimated.
center. The lateral view defines the AP coordinate of the target. This view also contains redundant information on the vertical coordinate. However, if the superoinferior extent of the target is interpreted differently in the AP and lateral views, the projection of the two lines will not meet, resulting in a “skew deviation,” which is the closest distance between the two projected rays (Fig. 5). Reasons for interpreting the nidus differently on the two views include the following: (a) differing overlap of arteries. veins, and nidus on the two views, (b) differences in temporal visualization of the two views, and (c) poor visualization of nidus on one view secondary to overlap with intervening skull structures. In theory, this error is independent of target shape, but in practice, the less spherical the target, the more difficult it is to determine its true shape on each projection.
Fig. 8. Two projections of an AVM with other normal difficulty in selecting the true nidus.
Another limitation of the two-view angiographic technique is the potential for errors in various target orientations. Figure 6 shows a potential target, length D 2, with its long axis at a 45” angle to two orthogonal angiographic views. The length projected on each plane is D. This error in size (4 1%‘) would go undetected. Figure 7 shows an actual lesion that further demonstrates this point.
Errors in ident~f~vingthe A VM nidus Despite the best angiographic techniques (subtraction, magnification, etc.), the identification of the AVM nidus remains subjective. It is usually difficult to delineate feeding arteries. nidus, and draining veins precisely, since they frequently fill almost simultaneously and since they overlap on the projected angiographic views (Fig. 8). AP and lateral angiogram films used for actual radio-
Stereotactic angiography 0 F. J. BOVAAND W. A. FRIEDMAN
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AP View as seen stereoscopically
Fig. 9. Stereoscopic view of a target fails to reveal the true shape of the lesion due to the limitation in view angle.
surgical treatment of AVM’s in four different patients were copied and distributed to four neurosurgeons who are knowledgeable in the treatment of this disorder. They were asked to outline the nidus of the AVM. The four sets of films were then digitized, and the coordinates of the geometric center were computed. The “disagreement” among the four neurosurgeons regarding the location of the center of the nidus ranged from 0.3 mm to 3.1 mm (4).
DISCUSSION Angiography has long been the procedure of choice for the delineation of cerebrovascular lesions, but it has substantial limitations. The accurate evaluation of AVM’s requires a computerized reconstruction in three dimensions. Stereotactic computed tomography (CT) is a po-
tential solution, although some AVM’s are not clearly visible on CT, nor is it easy to distinguish arteries, veins, and nidus. Stereotactic magnetic resonance imaging (MRI) generally shows the AVM components well, and some consider it the best imaging modality for this purpose (6, 8, 9). The limitations of MRI, however, include slice thickness (which limits the accuracy of the vertical COordinate) and disturbance of lesion geometry by the magnetic field (7). Some authors have suggested the use of stereoangiography to resolve some of these problems (7). This solution has two serious drawbacks. The first is the relationship between accuracy of the reprojection and the angular view used for the stereoshift. Fitzgerald and Mauderli (2) have shown that views of 26” are required to minimize these reconstruction errors. This exceeds the normal angle used in stereoradiography. The second limitation also results from the small angle used between the two views. Figure 9 shows how the limited view angle prevents the observer from viewing the posterior extent of the target. Stereotactic angiography allows 3-dimensional visualization of the target but from a grossly limited field of view. A second angiographic modality that has been suggested is digital imaging. Rapidly subtracted views can provide additional information by eliminating overlapping skull structures. One must, however, proceed with caution whenever image intensifiers and TV chains are used. Although the contrast and resolution of these systems are very good, the spatial resolution is susceptible to substantial errors because of nonuniformity in the image intensifier and camera optics ( 1). If this technique is used, rigorous checks on spatial uniformity of each view are necessary. Stereotactic angiography causes unavoidable errors in determining target shape, target size, and tumor nidus. It is hoped that the routine inclusion of 3-dimensional stereotactic images, such as CT or MRI, in the treatment planning of AVM radiosurgery will eliminate these problems.
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