In 1977, Theodore Roberts and a third-year medical student, Russel Brown, were responsible for developing the Brown-Roberts-Wells System (BRW) at the University of Utah . This originally CT-based system consists of a skull base ring with carbon epoxy head posts that offers minimal CT interference. The ring is attached to the patient with screws that are tightened into the skull. The localizer unit is secured to the ring with three ball-and-socket interlocks and consists of six vertical posts and three diagonal posts, creating an "N" shaped appearance . It is this latter ''N'' construct that establishes the axial CT plane relative to the skull base by calculating the relative distance of the oblique to the vertical rods. Target coordinates are established by first identifying the axial slice that best features the lesion. The x and y coordinates for each of the nine fiducial rods are identified on the CT or MRI monitor, as are the x and y coordinates for the target. All coordinates are entered into a laptop computer (the SCSI), which computes the target coordinates in BRW stereotactic space. The BRW system then further includes a movable arc and a probe holder. The arc guidance frame has four motions that create infinite different probe pathways, but for any trajectory, the computation must include entry coordinates . Additionally, this system included a phantom base onto which the stereotactic frame including the arc could be placed to test the accuracy of the settings.
In the 1980s, Wells and Cosman simplified and improved the BRW by designing an arc guidance frame similar to the Leksell frame. The arc system directs a stereotactic probe isocentrically around the designated target, thus obviating a fixed entry point. The Cosman-Roberts-Wells (CRW) system included some of the same design elements as the BRW system, including a phantom frame, the same CT localizer, and the same probe depth fixed at 16 cm (Fig. 3A-C). New innovations included the introduction of MRI-compatible frames and localizers (Fig. 3A), and versatility in arc-to-frame applications that enabled inferior trajectories into the posterior fossa or lateral routes into the temporal lobe. For institutions with the Radionics OTS frameless image guidance system, target and trajectory calculations can now be performed with the OTS intraoperative workstation, which provides more flexibility than the SCSI laptop.
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