In one patient in this series (case 9) stereotactic angiography was correlated with CT data to plan an avascular trajectory for biopsy o f a deeply situated lesion. The stereotactic angiogram was obtained by fixing the patients head in the Relocatable Stereotactic Frame with attached BRW base ring and the SGV Angiographic Localiser. The patient was positioned on a couch in the angiographic suite and the Relocatable Frame fixed to the BRW floor stand by a specially constructed interface bracket. Using fixed X-ray sources and radiographic plates, AP and lateral radiographs were taken during the injection of contrast (Omnipaque. Nycomed) into the right and left carotid arteries.
The CT derived stereotactic coordinates of the biopsy target within the tumour were transposed to the AP and lateral angiographic films with reference to radio-opaque markers on the angiographic localiser. This was achieved by overlaying a millimetre grid on the angiographic films and establishing coordinates for 8 reference points visible on each AP and lateral film. These X-ray coordinates were entered into a portable microcomputer which then calculated the mathematical transformations between BRW coordinates and the X-ray coordinates using the SGV software program. When the biopsy target coordinates are entered the computer then prints out the corresponding X-ray coordinates. The target can then be located on the films using the millimetre grid overlay. A biopsy trajectory that would avoid vessels was then planned from the AP and lateral angiograms and the coordinates of an entry point into the skull were then transformed from the X-ray coordinates to the BRW coordinates. The target and entry point coordinates were then used to calculate the arc settings and probe depth on the BRW stereoguide.
Image correlation
Image data was transferred from each imager to a Sun 3/260 computer (Sun Microsystems Inc. Mountain View Calif.) by magnetic tape. The software used for image correlation and analysis was Analyze Version 2 (Biodynamics Research Group. Mayo Clinic, Rochester Minn.)
A 3D computer matrix was constructed of voxel sizes identical to the first high resolution CT volume. Its total dimensions just incorporated the stereotactic space defined by the BRW stereotactic system such that each voxel position could be identified in terms o f BRW coordinates. The base line slice from the first CT volume was positioned precisely in the matrix so that the x,y and z coordinates matched. The vertical position o f the base line slice was known as it was the datum used in the set up procedure prior to image acquisition. The centre o f the image corresponded with the zero coordinates for x and y. These positions were confirmed by measuring the relationship between the vertical and oblique fiducials seen in section on the axial slice which allowed calculation of the x,y and z coordinates o f the central pixel on the image using a pre programmed portable computer (Epson HX-20). When the base line slice was positioned the subsequent axial slices in the CT volume were placed into the computer matrix. Correct alignment of both volumes were confirmed by calculating the x, y and z position of the central pixel in the top axial CT slice.
IMAGE CORRELATION - Method 103 Any misalignment could be corrected by rotating or translating the CT volume appropriately.
Image volumes to be correlated with the first CT volume (eg MRI, PET or additional CT images) were scaled to match the voxel size of the primary image volume and then placed into the 3D computer matrix as previously described. The accuracy of multimodality image correlation was checked by selecting corresponding axial slices from the top and bottom of each image volume and measuring the coordinate positions of the vertical and oblique fiducials to ensure that they were identical. Paired image slices could also be superimposed or subtracted from one another to ensure accurate matching of the fiducial positions.
Figure 36 G eom etrically co rrelated CT and T ran sm issio n PET volum es of p a tie n t in R elocatable Fram e.
A computer graphics reconstruction of the fiducials in the 3D computer matrix was used as a fiducial check of image correlation. The fiducials were represented by lines in space which corresponded with the central axes of each rod. Each imaging volume was resliced in a series of 3 vertical planes that passed through the central axes of each set of N shaped fiducials. By superimposing the graphic display of the fiducials over the reconstructed image taken through the same plane, any variation in rotation, translation or scaling could be measured and corrected for each image volume. It is of note that any computer manipulations performed on transmission PET images to achieve precise geometric correlation with other images were also performed on the PET emission data so that these were geometrically correlated by virtue of the fact that they were acquired with an identical set up to the transmission images. Geometrically correlated CT and transmission PET volumes are shown in fig.36.
IMAGE CORRELATION - Method 104
Planning stereotactic procedures
For planning stereotactic biopsy using geometrically correlated images, regions o f interest within the lesion were selected from the combined images. Trajectories were planned to avoid critical areas aided by the use o f 3 dimensional surface rendered views of the tumour volume (as defined on CT, M R or PET images) suspraded in stereotactic space in cut away CT surface projections of the skull. These give the surgeon an immediate appreciation of the 3D shape and size of the lesion and its relationships. The coordinates o f the chosen entry point and target could be read directly from the images. Along the axis o f this defined trajectory the images were reconstructed parallel to the sagittal and to the coronal planes for display. From these reconstructed views the apparent tumour boundaries seen on each imaging modality were outlined. The apparent tumour boundaries defined from different imaging modalities could then be compared by superimposition of the outlines.
In this series, serial stereotactic biopsies were performed through brain tumours using a Nashold side- cutting biopsy cannula that took cylinders of tissue 1cm in length and 2mm in diameter. The depth o f each biopsy sample was adjusted peroperatively using a microdrive. The positions of the serial biopsies were planned and displayed by subdividing the trajectory path on the images into a series o f 1cm long and 2mm wide rectangles which were numbered. The biopsy trajectory was transposed to the BRW stereotactic frame for serial biopsy by entering the coordinates of the entry point and the deepest target point into the portable pre-programmed computer that is supplied with the BRW system. This calculates the appropriate arc angle settings and probe depth on the stereoguide.
Correlated CT and PET data was used in one patient (case 7) to plan a stereotactically guided tumour resection via craniotomy. This was achieved by placing the PET defined tumour volume into a spatially matched CT volume depicting the skull. The BRW coordinates defining the height, width and length o f the tumour volume were established from the image volume and the location and dimensions of a rectangular craniotomy bone flap overlying the lesion planned from this information. Surgery was simulated by erasing the portion o f the skull that would be removed during craniotomy on the image volume. Using a 3D surface rendered projection o f the combined PET and CT volumes the tumour could be viewed through the simulated craniotomy flap to ensure adequate access. At surgery the stereoguide was set to the coordinates o f each comer o f the craniotomy flap in turn and these points were marked on the scalp. When the craniotomy bone flap had been reflected and the dura opened the stereoguide was set to a target at the deepest part of the PET defined tumour volume on a trajectory that passed through the centre o f the lesion. This was used peroperatively to guide surgical resection.
IMAGE CORRELATION - Results 105
Results:
The results of image correlation are illustrated with selected case studies:
Case 1
A 37 year old woman presented with a 3 month history of left leg weakness, left arm paraesthesia and headaches. A diagnostic CT scan demonstrated a deep callosal space occupying lesion, the extent of which was poorly defined despite contrast enhancement. CT and PET images were acquired under stereotactic conditions in the relocatable frame. PET imaging using '^FUDR as a tracer clearly delineated an extensive lesion infiltrating into both hemispheres. The extent of the (‘®FUDR) PET defined lesion did not appear to be due to simple diffusion across the damaged blood brain barrier because a PET study with Rubidiunf* demonstrated a less extensive lesion (fig. 37).
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Figure 37 Case 1; Correlated contrast CT, (F18-FUDR)PET and (Rb81)PET. Images reconstructed along biopsy tr^ ’ectory. (CT boundry overlaid on PET images)
IMAGE CORRELATION - Results 106 The histology obtained from serial stereotactic biopsy was consistent with an anaplastic astrocytoma (grade III) and its histological boundary showed a good correlation with the (*®FUDR) PET defined boundary (fig. 38) Biopsy sam ple CT
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i s o d e n s e h y p e r d e n s e t r a c e r u p t a k e n o u p t a k e H I S T O L . t u m o u r infiltration a b n o r m a l — ^ v e s s e l s t u m o u rFigure 38 Case 1. Comparison of contrast enhanced CT, (F18-FUDR)PET and (Rb81)PET with histology obtained from serial stereotactic biopsy.
IMAGE CORRELATION - Results 107
Case 3;
A 57 year old man presented with a 6 week history of headaches, left sensory inattention and a left homonymous hemianopia. A diagnostic CT head scan demonstrated a right occipito-parietal ring enhancing lesion. Serial stereotactic biopsy was planned from combined CT and (**FUDR) PET imaging. The extent of the lesion defined by each modality differed with CT demonstrating ring enhancement just outside the PET defined boundary and hypodensity in areas within the lesion in which there was a high uptake of ‘^FUDR on the PET image, (fig.39).
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Figure 39 Case 3: Correlated contrast enhanced CT and (F18-FTJDR)PET. Images reconstructed along biopsy trajectory.
IMAGE CORRELATION - Results 108
The histology obtained from the proximal part of sample 2 (fig. 40) which correlated with the area of ring enhancement on CT showed gliotic tissue with some tumour infiltration and abnormal vessels. It did not demonstrate areas of solid tumour in sample 2 (distal portion) or sample 3 which were hypodense and indistinguishable from areas of necrosis (eg sample 4). By contrast the (**FUDR)PET images showed increased uptake of tracer in areas where solid tumour was demonstrated histologically.
Biopsy sam ple 1 CT PET
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i s o d e n s e h y p e r d e n s e h y p o d e n s e t r a c e r u p t a k e n o u p t a k e L J r e a c t i v e g l i o s i s t u m o u r infiltration a b n o r m a i v e s s e l s t u m o u r n e c r o s i sFigure 40 Case 3: Comparison of contrast enhanced CT and (F18-FUDR)PET with histology obtained from stereotactic biopsy.
IMAGE CORRELATION - Results 109
Case 7:
A 39 year old woman presented with a three week history of sensory epilepsy involving the left arm and face. Examination revealed pyramidal weakness in the left arm. A CT scan demonstrated a right parietal ring enhancing lesion. She underwent a stereotactic biopsy and a craniotomy with guided excision of the lesion using integrated CT and (^®FUDR)PET data. The tumour proved to be a glioblastoma multiformé. The image simulated craniotomy is illustrated in fig.41.
Figure 41 Case 7: Image simulated craniotomy from correlated CT & (F18-FUDR)PET data.
She made a good post operative recovery with improvement in left arm weakness and was treated with radiotherapy. At follow up 7 months later she had developed a recurrence of left hand weakness. Follow up CT PET and MRI imaging was performed under stereotactic conditions, (fig. 42)
IMAGE CORRELATION - Results 1 1 0
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Figure 42 Case 7: Correlated CT, MRI and (F18-FUDR)PET demonstrating recurrence of tumour post craniotomy and radiotherapy
These showed recurrence of the tumour, the extent of which appeared to be most clearly depicted on the PET images. The use of the Rrelocatable Frame also allowed the precise geometric correlation of this data with the CT and PET images taken 7 months previously (fig. 43). Such correlations facilitate the objective measurement of changes in tumour volume.
Figure 43 Case 7: a. Pre treatm ent PET volume in planning CT volume for craniotomy
Post craniotom y & radiotherapy. Show ing recurrent T u m our (as PET volum e) in skull with actual craniotom y bone flap erazed
IMAGE CORRELATION - Résulta 111
Case 9:
A 38 year old man presented to the casualty department after striking his right eye on an open car door. He also complained of recent poor memory. Examination revealed a right homonymous hemianopia.
Figure 44 Case 9: CT scan showing biopsy target Figure 45 Case 9: Stereotactic Angiogram (AP) showing planned trajectory
IMAGE CORRELATION - Results 112
A diagnostic CT head scan demonstrated an enhancing mid-brain lesion that extended to the suprasella region. Due to the close proximity of major cerebral vessels stereotactic biopsy was planned from the combined information from CT and angiography performed under stereotactic conditions. The biopsy target was selected from the contrast enhanced CT images (fig. 44) and an avascular trajectory from the stereotactic angiograms figs 45 and 46.
IMAGE CORRELATION - Results 113
Case 10:
A 35 year old man presented with Jacksonian epilepsy involving the left arm and leg. On examination he had pyramidal weakness in the left arm. A CT head scan revealed enhancing lesions in the corpus callosum, right frontal and posterior right frontal regions. Open excision biopsy was performed on the right frontal lesion which proved to be a primary cerebral lymphoma. He was referred for stereotactic radiotherapy and had stereotactic CT performed in the relocatable frame for planning. Due to the extent and multiplicity of the lesions stereotactic radiotherapy was felt to be inappropriate and he had a course of conventional radiotherapy. He was re imaged under the same stereotactic conditions 2 months later to monitor the effects of therapy. The matched images are shown in fig: 47.
PRE RADIOTHERAPY 14.10.88
POST RADIOTHERAPY 8 .12.88
Figure 47 Case 10: Correlated CT images (para-sagittal reconstructions) demonstrating the response of a primary lymphoma to radiotherapy.
IMAGE CORRELATION - Discussion 114