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A controlled experiment was conducted to compare head-tracked and non-head-tracked steering modes in the performance of an abstract beamtargeting task. Collected datarevealed a wide variety of mode preferences among the subjects. Subject performance, as measured by final score, task completion time and subject confidence, differed very little between the head-tracked steering modes taken as a group and the collective non-head-tracked modes. Some significant differences were observed between individual steering modes, both within and between the head-tracked and non-headtracked groups. INTRODUCTION Current research at the University of North Carolina at Chapel Hill is investigating the possible benefits to be gained by applying headmounted display (HMD) technology to radiotherapy treatmentplanning (RTP). Use of a head-mounted display for targeting of treatment beams suggests several possible steering modes for exploring the virtual world of the patient’s anatomy. To determine which steering mode is best suited to our application, auser study was conducted to investigate the relative merits of the different steering modes. Seven steering modes were used in an abstract beam-targeting task. Four modes used head-tracking information, while the other three modes did not. It was anticipated that head-tracking would provide an advantage in beam targeting through more natural steering and navigation that makes use of proprioceptive and vestibular infotmation, which are absent in non-head-tracked methods. Related workinmovementthroughavirtual worldisdescribed in [ 1. 2,3], but these studies do not deal with HMD’s and head-tracking. BEAM TARGETING The key to successful beam targeting in radiation therapy treatment ldanning is to orient and shape the beams so that the entire tumor is covered by eachbeam while as li ttleof the healthy surrounding tissue as possible is hit by the beams. Given the complex spatial srrangement of a patient’s anatomy (tumors may be draped around healthy organs or have tendrils snaking out into the healthy tissue), this is usually not an easy task. To evaluate the different steering modes, subjects were presented with an abstract anatomy model, consisting of a multi-colored spherical target (tumor analog) embedded in acollectionofuniquely Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the ACM copyright notice and the title of the publication and its date appear, and notice is given that copying is by permission of the Association for Computing Machinery. To copy otherwise, or to republish, requires a fee and/or specific permission. s 1992 ACM 0-89791-471-6/92/0003/0193...$1.50 colored monochromatic spherical dodges (organ analogs). (See Color Plate 1.) The subjects used each of the seven steering modes to manipulate the direction in which a conical virtual beam passed through the model. The beam was defied such that its source (cone vertex) was always a fixed distance from the target, its central ray (coneaxis)passedthroughtthecenterofthetarget,anditsdivergence was just large enough to encompass the target. The subject was instructed to fiid the beam direction that afforded the smallest volume of intersection between the conical beam and the dodges, a task analogous to a radiotherapist trying to avoid radiosensitive organs with a treatment beam. STEERING MODES The term “steering mode” refers to the method used to change one’s position or orientation in the virtual world. This is distinguished from navigation, which refers to understanding one’s current position and orientation relative to other objects in the virtual world. Head-Tracked These modes are linked to movement of the subject’s head and enable thesubjecttomakeuseofvestibular(innerearbalance) andproprioceptive (muscles, tendons, joints) senses for navigation, Walkaround (WLK). In Walkaround mode, the subject physically walks about in thevirtual worldcontaining the target/dodges model. The direction of the beam is defied by the vector from the subject’s eyes to the center of the target. To better examine the model and targetthebeamfromaboveandbelow.the subjectisgiventheability to vertically translate the model using a 6-D mouse. No other manipulation of the model is possible. Wa/karound/Rotafion(WKR). This is the same as Walkaroundmode, with the exception that the subject is able to also rotate the model about any axis in 3-space through its center by grabbing with the 6D mouse. (See 6-D Mouse section below.) Orbital (ORB). In Orbital mode the subject is constrained to always be looking at the center of the model from the beam source. Beam direction coincides with gaze direction. Unlike the Walkaround modes. Orbital mode uses only head orientation and ignores head position. As the subject’s head turns, the model is observed to translate about the subject’s head at a constant distance. (Hence the name Orbital.) Because the model undergoes no rotation, it can be viewed from any direction with a turn of the subject’s head. immersion (/MM). In Immersion mode the subject views the model looking outward from the center of the target. Like Orbital mode, Immersion mode makes use of head orientation only and ignores ‘CB#3175. Sitterson Hall, Chapel Hill, NC, 27599-3175. chung@cs.unc.edu (919) 962-1889

A comparison of head-tracked and non-head-tracked steering modes in the targeting of radiotherapy treatment beams