Pergamon ?? Physics Original Contribution Int. J. Radiation Oncology Biol. Phys., Vol. 28, No. 5, pp. 1229-1234, 1994 Copyright 0 1994 Elsevier Science Ltd Printed in the USA. All rights reserved 0360-3016/94 $6.00 + .OO 0360-3016(93)E0065-E zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGF IMAGE FUSION FOR STEREOTACTIC RADIOTHERAPY AND RADIOSURGERY TREATMENT PLANNING HANNE M. KOOY, PH.D.,* MARCEL VAN HERK, PH.D.,* PATRICK D. BARNES, M.D.,+ EBEN ALEXANDER III, M.D., *,# SUSAN F. DUNBAR, M.D.,* NANCY J. TARBELL, M.D.,* ROBERT V. MULKERN, PH.D.,+ EDWARD J. HOLUPKA, PH.D.* AND JAY zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQ S. LOEFFLER, M.D.* *Joint Center for Radiation Therapy, and the Stereotactic Radiosurgery and Radiotherapy Program, Departments of Radiation Oncology, +Radiology, and ‘Surgery, Brigham and Women’s Hospital, The Children’s Hospital, Harvard Medical School, Boston MA Purpose: We describe an image fusion application that addresses two basic problems that previously limited the use of magnetic resonance imaging (MRI) for geometric localization in stereotactic radiosurgery (SRS) and ste- reotactic radiotherapy (SRT). The first limitation is imposed by the use of a relocatable, MRI-incompatible, ste- reotactic frame for stereotactic radiotherapy. The second limitation is an inherent lack of geometric fidelity in current MRI scanners that invalidates the use of MRI for stereotactic localization. Methods and Materials: We recently developed and implemented a novel automated method for fusing computerized tomography (CT) and MRI volumetric image studies. The method is based on a chamfer matching algorithm, and provides a quality assurance procedure to verify the accuracy of the fused image set. The image fusion protocol removes the need for stereotactic fixation of the patient for the MRI study. Results: The image fusion protocol significantly improves on the spatial accuracy of the MRI study. We demonstrate the effect of distortion and the effectiveness of the fusion with a phantom study. We present two case studies, an acoustic neurinoma treated with SRS. and a pilocytic astrocytoma treated with SRT. Conclusion: The image fusion protocol significantly improves our logistical management of treating patients with radiosurgery and makes conformal therapy practical for treating patients with SRT. The image fusion protocol demonstrates both the superior diagnostic quality and the poor geometric fidelity of MRI. MRI is a required imaging modality in stereotactic therapy. Image fusion combines the superior MRI diagnostic quality with the superior CT geometric definition, and makes the use of MRI in stereotactic therapy possible and practical. Stereotactic radiosurgery, Stereotactic radiotherapy, Image fusion, Diagnostic imaging, Treatment planning, MRI distortion. INTRODUCTION Image fusion is a technique that combines the often com- plementary information from separate imaging studies into a single coherent study. The combination of MRI and SPECT studies, for example, allows a direct com- parison of anatomy and metabolic activity. Popular image fusion techniques are based on fiducial markers (18) or on surface matching between volumes identified on each modality ( 10, 13, 17). The fiducial marker technique re- quires markers visible on all modalities and permanent implantation for follow-up studies. The rather small number of such markers reduces the precision of the fu- sion. The surface matching technique, in practice, requires manual input to define analogous surfaces in each mo- dality. A more general fusion technique is based on the chamfer match technique (8, 11, 22). The chamfer tech- nique does not require the identification of individual volumes. Instead, individual, but not identical, sets of points belonging to the same anatomical structures on each study are used. These points can be automatically segmented from each study, thus making the chamfer technique automatic. We recently (spring 1992) commenced SRT treatments based on the Gill-Thomas-Cosman (GTC) stereotactic frame’ (9). Our treatments combine standard fraction- ation schemes with the precision and treatment planning of SRS (15). The localization accuracy is 0.4 mm in our Reprint requests to: Hanne M. Kooy, Ph.D., Joint Center for Radiation Therapy, 50 Binney St., Boston MA 02 I 15. Acknowledgments-We gratefully acknowledge the assistance of and discussions with Zach Leber and Bob Ledoux from RSA Inc. Accepted for publication 12 November 1993. ’ Radionics Inc., Burlington, MA. 1229