136 Australasian Physical & Engineering Sciences in Medicine Volume 27 Number 3, 2004 Stereotactic radiosurgery planning with ictal SPECT images T. Ackerly 1,2 , M. Geso 2 , G. O’Keefe 3 and R. Smith 4 1 Department of Physical Sciences, Peter MacCallum Cancer Centre, East Melbourne, Australia 2 School of Medical Sciences, RMIT University, Bundoora, Australia 3 Centre for PET, Austin & Repatriation Medical Centre, Heidelburg, Australia. 4 William Buckland Radiotherapy Centre, Alfred Group of Hospitals, Prahran, Australia Abstract This pap er is m otivated b y a clinical requ irement to uti lise ictal SPECT images for targ et loc alisation in stereotactic radiosurgery treatment planning using the xknife system which only supports CT and MRI images. To achieve this, the SPECT images wer e converted from r aw (pix el d ata on ly) fo rmat into a par t 10 complian t DICOM CT fileset. The minimum requirements for the recasting of a raw format image as DICOM CT or MRI data set are described in detail. The method can b e applied to the importation of raw format images into any radiotherapy treatment planning system that supports CT or MRI import. It is demonstrated th at the combination of th e low spatial resolution SPECT images, depicting fun ctional informat ion, with high spatial r esolution MRI images , which show th e stru ctural information , is suitable for stereotactic radiosurgery treatment planning. Key words Stereotactic Radiosurgery, SPECT, DICOM Introduction Stereotactic radiosurgery is a tec hnique coupling im age guided neurosurgical in tracranial localisation techniques with si ngle dose e xternal be am irradiation t o achieve non invasive treatm ent of small, easily visualised lesi ons. The XKnife RT system allows the routine use of CT and MRI images for stereotactic treatment planning of medical linear accelerator based radiosurgery. Ictal (during seizure) Single Photon Em ission Com puted Tom ography (SPEC T) with Technetium-99m is the preferred im aging m odality for epilepsy 1 . Ictal SPECT im ages acquire d at the Nuclear Medicine department of t he Austin and Repatriation Medical Centre (ARMC) have bee n incorporated into the stereotactic radiosurgical treatm ent planning at the William Buckland Radiot herapy Centre (WBRC) of t he Alfred Hospital. The ori ginal ictal SPECT images consisted of 65 slices separated by 2.3 mm covering a n axial field of view (FOV) of 149 mm, each slice made up of 128 by 128 square pixels of side length 1.8 mm covering a tra nsaxial FOV of side length 230 mm . The spatial resolution of t he SPECT imaging system was a nom inal 8 m m full widt h at half maximum. In consultation with the WBRC, the volumetric Corresponding author: T. Ackerly, Dep t o f Ph ysical Scien ces, Peter MacCallum Cancer Centre, Locked Bag 1, A’Beckett Street, East Melbourne, Victoria, 8006 , Australia. T el: 61-3-9656-1260, Fax: 61-3-9650 -4870, Email: Trevor.Ackerly@petermac.org Received: 24 November 2003; Accepted: 30 August 2004 Copyright © 2004 ACPSEM/EA SPECT data was co-registered at the ARMC wit h a volumetric Magnetic Resonance Imaging (MRI) data set, a technique that has been validated by O’Brien et. al. 2 . Both the original MRI a nd the re -sampled co-re gistered SPECT image data sets were then saved in raw pixel only format, 256 by 256 pixels per slice, 12 4 slices, 16 bits pe r pixel, little endian integer format. Unfortunately these im ages could not be imported directly into the WBRC xknife stereotactic treatment planning system. Although the xknife system supported t he importation and use of DIC OM 3 format CT and MRI , it did not support the importation or display of SPECT images 4 . Additionally, Radionics 4 did not have a raw to Dicom converter available 5 . At this point the Peter MacCallum Cancer Centre (PMCC) undertook to create a tra nslator from raw to DICOM format, so that the raw SPECT im age was e ffectively em bedded i n the pixel data section of multiple DICOM CT files form ing part of a DICOM PART 10 conform ant image set. In this way t he SPECT data would be split up and reassembled insi de t he xknife com puter under t he alias of a CT image. The first treatment went ahead as pla nned at the WBRC. This paper describes the implementation of the RAW to CT and RAW to MRI conversion. Method Clinical procedure The MR and SPECT scans were initially performed six weeks prior to the treatm ent of the patient. The two scans were co – re gistered and the SPECT scan was sampled and transformed to create a new SPECT scan that had the same