Neuro--

Neuroradiology (1992) 34:453-456

radiology 9 Springer-Verlag 1992

The value of intraoperative scans during CT-guided stereotactic procedures A. Patil, P. Kumar, L. Leibrock, B. Gelber, and B. Aarabi University of Nebraska Medical Center, Omaha, Nebraska, USA

Summary. T h e a c c u r a c y s t e r e o t a c t i c p r o c e d u r e s p e r formed during the pre-computed tomography (CT) era was c o n f i r m e d b y i n t r a o p e r a t i v e X - r a y pictures. W i t h t h e a v a i l a b i l i t y of C T it is n o w p o s s i b l e to c o n f i r m t h e p o s i t i o n of t h e p r o b e - t i p o n an i m a g e o f t h e target. F o r b i o p s y o f small lesions in critical a r e a s of t h e brain, p e r m a n e n t p l a c e m e n t o f r a d i o a c t i v e seeds, o r t h a l a m o t o m y , it w o u l d b e d e s i r a b l e to h a v e c o n f i r m a t i o n o f t h e site o f t h e p r o b e t i p p r i o r to p e r f o r m i n g t h e m a i n s t e p o f t h e p r o c e d u r e . Int r a o p e r a t i v e C T w a s p e r f o r m e d in 216 s t e r e o t a c t i c p r o c e d u r e s c a r r i e d o u t o n t h e s c a n n e r t a b l e including biopsies, a s p i r a t i o n o f cysts, b r a c h y t h e r a p y , a s p i r a t i o n o f abscesses, thalamotomy, and evacuation of intracerebral hematoma. I n 6 cases, i n a c c u r a c i e s w e r e d e t e c t e d , w h i c h it was p o s s i b l e to c o r r e c t so as to p l a c e t h e p r o b e w h e r e desired.

Key words: C o m p u t e d t o m o g r a p h y - S t e r e o t a x i s - B i o p s y P r i o r to t h e i n t r o d u c t i o n of c o m p u t e d t o m o g r a p h y (CT), m o s t s t e r e o t a c t i c p r o c e d u r e s w e r e p e r f o r m e d using p n e u m o e n c e p h a l o g r a p h y o r v e n t r i c u l o g r a p h y . T a r g e t positions w e r e d e t e r m i n e d in r e l a t i o n s h i p to s t r u c t u r e s r e n d e r e d visible b y t h e c o n t r a s t m e d i u m . A f t e r p r o b e insertion, X - r a y films w e r e o b t a i n e d to c o n f i r m a c c u r a t e p l a c e m e n t . W i t h t h e a v a i l a b i l i t y of CT, it is n o w p o s s i b l e to o b t a i n i m a g e s of t h e target, m a k i n g s t e r e o t a c t i c p r o c e dures easier and more accurate. Furthermore, by reviewing p o s t o p e r a t i v e C T a n d o v s e r v i n g c h a n g e s a r o u n d t h e lesion, it is p o s s i b l e t o a s c e r t a i n t h e a c c u r a c y of t h e p r o c e d u r e [1, 2]. H o w e v e r , p o s t o p e r a t i v e i m a g e s d o n o t always s h o w c h a n g e s at t h e t a r g e t a n d e v e n if t h e y do, t h e evid e n c e is p o s t hoc. C T d u r i n g t h e p r o c e d u r e allows d e t e c tion o f inaccuracy, a n d p e r m i t s t h e s u r g e o n to m a k e necessary corrections to obtain accurate probe placem e n t . W e r e p o r t e x p e r i e n c e w i t h d e t e r m i n a t i o n of t h e acc u r a c y o f s t e r e o t a c t i c p r o c e d u r e s using i n t r a o p e r a t i v e CT.

Materials and methods During the last 8 years the accuracy of stereotactic probe placement was determined intraoperatively in 216 procedures performed with the patient on the CT table. The patients' ages varied from 5 to

80 years. The indications for the procedures included biopsy, brachytherapy, aspiration of cysts, abscesses, intracerebral hematomas and thalamotomy. The procedures were performed using the Patil stereotactic frame, described in previous publications [3, 4]. The system (Fig. 1) consists of a base plate with two side stanchions, each carrying a pivot, on which a yoke with an arc is mounted. The probe holder is attached to the arc and can slide along it. During a procedure the side stanchions, pivots and arc are adjusted to bring the center of the arc on to the target. The Z coordinate (candocephalad) is adjusted by first moving the CT table to the scan position on which the target is located (Fig. 2) and then moving the side stanchion along the base plate until the center of the pivot aligns with the vertical laser positioning light. The scan is always obtained with the gantry vertical. The Y coordinate (anterioposterior) is measured by drawing a line from the target to the top surface of the base plate by means of the scanner cursor and using the measure distance mode (Fig.2). This coordinate is adjusted by moving the pivots on the side stanchions to the required height. The X coordinate (peripendicular distance from the target to the midline marker on the base plate or the lateral distance) is measured using the cursor and measure distance mode (Fig. 2). This coordinate is adjusted by moving the arc to the required distance on its yoke. These adjustments bring the center of the arc on to the target. The length of the probe used for the procedure is equal to the radius of the arc so that when the full length of the probe is inserted through the holder its tip is at the target. The probe holder can be moved to any position along the arc and the arc moved to any angle around its pivot th reach the target along any desired trajectory. The movement of components during the coordinate adjustment is by a rack and pinion system and Vernier scales are incorporated in the system to allow movements of as little as 0.1 mm. Phantom targets were reached with an accuracy of within 0.5 mm for the X and Y coordinates and 0.7 mm for the Z coordinate when the CT section thickness was 1.5 mm. The system is compact; although most of the parts are constructed from anodized aluminum, those that will be in the plane of scanning (probe holder, pivots and base plate) are non-metalfic. During a procedure the patient's head is fixed in the head holder of the stereotactic frame which, in turn, is fixed to the top of the scanning table. After injection of contrast medium with the gantry always vertical, a limited number of scans, 1.5-5 mm thick is obtained in the area of the target. The most central CT image on which the target is visible is used for coordinate measurement. The coordinates are measured and adjusted. The probe holder is adjusted on the arc at the desired position. With the arc vertical, a CT image in the plane of the target is obtained. Accuracy of the coordinate adjustment is confirmed by observing the lesion in line with the barrel of the probe holder (Fig.3a, b). The arc is then rotated around its pivot to the

454

~

F~" ARC

BASE

Fig.1. Diagram of the Patil stereotactic system. Arrow, Midline marker Fig.2. c r showing measurement of the coordinates. The base plate (open arrow) and its midline marker (solid arrow) are visible. The scan position (30.5 mm) is in the left upper corner.

The vertical line (distance 1) measures the Y coordinate, and the horizontal (distance 2) the X coordinate Fig.3a. CT showing a lesion in the right basal ganglia. The target point is marked by a cursor. b CT showing the barrel of the probe holder in line with the target, marked by a line drawn using the cursor of the scanner, e patient being scanned with the needle in the target. d CT showing the needle tip at the target, which is marked by the cursor

455

Fig.4a. CT sh~

a small enhancing lesi~ in the left parietal l~ b Intraoperative CI" showing accurate placement of the needle tip in the lesion

Figo5a. CT showing a diffuse right parietal tumor. The target is marked by the cursor, b Intraoperative CT showing the needle tip at the target

desired angle. The positions of the probe holder on the arc and the arc on its pivot are chosen, based on preoperative CT and MRI coronal and sagittal images, so that a trajectory is utilized. The skull is entered via a drill hole, using a power drill and a Steinman pin, which is guided through the probe holder of the stereotactic frame. The probe is then inserted through the holder to its full length to reach the target. CT scans are then obtained in the area of the target (Fig.3c). The image showing the probe deepest (Fig.3d) is considered to show its tip. If the tip is seen on an image in the same scanning plane as the chosen for the target, and at the point chosen as the target, probe placement is considered accurate. The target point for smaller lesions is the center of the lesion (Fig. 4). For larger, and more diffuse lesions the target point is marked by the cursor on the scanner screen. The cursor is left on the screen at that point until the target is probed. On the image obtained after the target is probed the cursor marks the target point (Fig. 5). During a brachytherapy procedure for permanent placement of radioactive seeds, CT images are obtained after placement of the probe tip at the target of the seed. (Fig. 6).

Results T h e a v e r a g e t i m e for an e n t i r e p r o c e d u r e is 75 min. T a b l e i shows t h e p r o c e d u r e s p e r f o r m e d . T a b l e 2 shows the l o c a t i o n o f t h e targets. I n 6 cases, i n a c c u r a c i e s w e r e

Fig.6a. CT shows a recurrent meningioma at the left petrous apex. b CT obtained after the deposition of a lz~Iseed (small white doO at the target. The white line is the image of the needle through which the seed was deposited; it appears interrupted as the stylet had been withdrawn by 1.5 cm. c C]7 showing the 125Iseed at the target, after the needle had been withdrawn

d e t e c t e d o n i n t r a o p e r a t i v e scans; in 2, i n a c c u r a c y r e s u l t e d f r o m a d e f e c t in t h e scanner's l a s e r p o s i t i o n i n g light; in ano t h e r 2 the c o o r d i n a t e a d j u s t m e n t was i n a c c u r a t e , while in t h e r e m a i n i n g 2 cases, e r r o r was i n t r o d u c e d d u e t o a b e n d in t h e n e e d l e . T h e i n a c c u r a c i e s w e r e d e t e c t e d , c o r r e c t e d , a n d t h e p r o b e was t h e n p l a c e d accurately.

Discussion B i o p s y o f i n t r a c r a n i a l lesions is n o w b e i n g p e r f o r m e d u n d e r C T g u i d a n c e [5-10]. To i m p r o v e t h e a c c u r a c y a n d ease of t h e p r o c e d u r e s , special g u i d a n c e a n d stabilizing s y s t e m s w e r e d e v e l o p e d [11-14]. T h e p r e s e n t s y s t e m is a f u r t h e r m o d i f i c a t i o n o f such t e c h n i q u e s . Its c o m p a c t n e s s allows it t o fit into t h e s c a n n e r a n d t h e m a t e r i a l s u s e d in its c o n s t r u c t i o n allow t h e o p e r a t o r to o b t a i n i n t r a o p e r a t i v e scans with m i n i m a l artifact. I n c o r p o r a t i o n o f t h e V e r n i e r scale a n d t h e r a c k a n d p i n i o n s y s t e m i m p r o v e s its accuracy. T h e c o o r d i n a t e s y s t e m allows direct m e a s u r e m e n t s of t h e c o o r d i n a t e s using the c u r s o r o f t h e s c a n n e r a n d t h e i r a d j u s t m e n t on t h e system. U s e o f t h e c e n t e r of the arc p r i n c i p l e allows p l a c e m e n t o f a f i x e d l e n g h t of p r o b e di-

456 Table 1. Procedures performed

Procedures Biopsy Brachytherapy Aspiration of cysts Thalamotomy Aspiration of abscess Evacuation of intracerebral clot

Number 161 30 9 8 5 3

Total

216

Table 2. Location of the targets

Target site

Obviously, not all procedures can be performed on the CT table: stereotactic craniotomy is impossible unless the scanner itself is in the operating room [15-17] and a proper sterile environment can be maintained. Stereotactic linear accelerator radiation or brachytherapy using a high intensity cobalt source would have to be performed in the radiotherapy department because of the requirement for special equipment and shielding.

References

Deep parietal Deep frontal Deep occipital Intraventricular Deep temporal Basal ganglion Cerebellum Hypothalamus Thalamus Brain stem Pineal area Skull base Cerebellopontine angle Cervical vertebral body Falx

Number of procedures 56 51 10 2 5 13 9 5 12 8 3 23 15 2 2

Total

216

rectly at the target without the need for intermediate steps, and the choice of any desired trajectory. Intraoperative scans enable the operator to detect inaccuracy during the procedure so that corrections can be made prior to completing it, In addition, this technique allows the operator to perform an internal check on the technique and instrumentation during the procedure. As the entire procedure is performed on the CT table, the patient does not have to be moved from one r o o m to another. This shortens total operating time and reduces patient discomfort. Intraoperative confirmation of the accuracy of probe placement during biopsy would be desirable for smaller lesions located in critical areas of the brain where the surgeon would like to avoid multiple passes and be assured of the site of biopsy. Confidence in the accuracy of the sample site could also reduce the need for routine frozen sections. During aspiration of abscesses, cysts and hematomas, intraoperative scans provide information regarding the residual lesion which helps to achieve satisfactory decompression. During placement of permanent radioactive seeds, it is critical to obtain intraoperative CT scans prior to deposition, because once the seeds are deposited they can not be retrieved. During ablative procedures, intraoperative CT scans assure the surgeon that the probe is within the anatomical limits of the target.

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