CAMERA-GUIDED MRI-MEG COREGISTRATION

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1 CAMERA-GUIDED MRI-MEG COREGISTRATION Chou-Ming Cheng 1,2 ( 鄭州閔 ), Li-Fen Chen 3,1 ( 陳麗芬 ), Yu-Te Wu 2,1 ( 吳育德 ), Jen-Chuen Hsieh 1,4,5,6 ( 謝仁俊 ), and Yong-Sheng Chen 7,* ( 陳永昇 ) 1 Laboratory of Integrated Brain Research, Taipei Veterans General Hospital, Taipei, Taian 2 Institute of Radiological Sciences, National Yang-Ming University, Taipei, Taian 3 Center for Neuroscience, National Yang-Ming University, Taipei, Taian 4 Institute of Health Inforation and Decision Making, National Yang-Ming University, Taipei, Taian 5 Faculty of Medicine, School of Medicine, National Yang-Ming University, Taipei, Taian 6 Institute of Neuroscience, School of Life Science, National Yang-Ming University, Taipei, Taian 7 Dept. of Coputer Science and Inforation Engineering, National Chiao Tung University, Hsinchu, Taian ABSTRACT Magnetic Resonance Iaging (MRI) and Magnetoencephalography (MEG) are noninvasive ethods for structural and functional brain research pursuit. To correlate the obtained activity inforation ith anatoical structure of the brain, e need to coregister the coordinate systes of both odalities. Conventionally, an operator is required to anually deterine at least three landark positions for the coordinate syste coregistration. Major drabacks of this kind of anual ethod include inter-rater variability and subjectivity, poor repeatability, and intensive labor. In this paper e propose an autoatic coregistration ethod that can align MRI iages and MEG data accurately and efficiently. Surface points of the face can be extracted fro the MRI volue and then be projected to iages captured by a calibrated trinocular caera rig. Coordinate syste transforation beteen the MRI and the caera rig can be deterined by iniizing the intensity differences aong the projected face surface points on the iages. Furtherore, the coordinate syste transforation beteen the caera rig and MEG can be deterined by aligning a set of face surface points hose 3D coordinates in both coordinate systes are obtained by using stereo vision technique and 3D digitizer, respectively. Concatenation of these to transforation results in the MRI-MEG coregistration. Another advantage of the proposed ethod is its superiority in longitudinal studies. Only one MRI scan is required for each subject and the MRI volue can be coregistered ith the MEG data of consecutive MEG studies by using the proposed ethod. Our experients have deonstrated the effectiveness of this ethod. * Corresponding author. This ork is partially supported by the National Science Council under grant NSC E and by Taipei Veterans General Hospital under grant VGH INTRODUCTION Magnetoencephalography (MEG) is coonly used to investigate neuronal activity of huan brain noninvasively. It provides excellent teporal resolution about 1 s but lacks for anatoical inforation. Therefore, coregistration of MRI and MEG data in one iage is necessary for localization and observation of neuronal activity over the subject s cortex[1-3]. Coregistration includes several procedures to transfor the coordinates of one odality into the coordinates of another odality[4]. In this study, e proposed a coputer vision based technique to iprove the coregistration result ith the present ethods. In principle, the MRI coordinate of MRI volue iage is transfored into the MEG coordinate of MEG achine through the orld coordinate syste defined by external digitization syste. First, transforation beteen the MEG and orld coordinate is perfored autoatically using head position indicator (HPI). Because HPI is a coil and eits current to induce agnetic field hich can be detected in MEG achine, it is identified in the MEG coordinate and also digitized in the orld coordinate. Second, the orld coordinate is transfored into the MRI coordinate once the transforation beteen these to coordinate systes is knon. In the literature, coregistration ethods can be classified into three ajor categories. 1) The first kind of ethod uses anatoical landarks to guide the coregistration (Fig. 1). In this ethod, ore than three recognizable and intrinsic landarks[5] ust be found in the MRI and orld coordinate so the alignent error of both odalities resulted ainly fro the difficulty for detecting the sae points on the head (orld coordinate) and in MR iage (MR coordinate). This approach does not need extra external arkers but it produces uncertain error fro anual label of the landarks in MR iage. 2) The second kind of coregistration ethod uses external fiducial arkers (such as oil filled arkers, asks, and dental bite bars). These arkers are visible

Fig. 3 (a) Original (b) orientation aligned (c) optiized digitization points and MRI surface (Kozinska et al. 2001) Fig. 1 Manual selection of anatoical landark a b c Fig.

The external arkers are identified uch easily in the MRI and orld coordinate [6,7] but the subject ay feel uncofortable ith arkers stuck to his/her face thus ay introduce confounding.

Besides, it faces errors fro detection of relatively large arkers (about 3) in MR iage. 3) The third kind of ethod uses geoetrical features of a subject to perfor the alignent.

3a) and the point set is rotated to the sae orientation as the head surface on the MRI coordinate (Fig. 3b).

This approach does not require fiducial arkers and is suitable for longitudinal studies.

In this study, the caera-guided coregistration ethod is adopted to iprove the disadvantage of the conventional ethods.

2 Fig. 3 (a) Original (b) orientation aligned (c) optiized digitization points and MRI surface (Kozinska et al. 2001) Fig. 1 Manual selection of anatoical landark a b c Fig. 2 Visible arkers in MRI iage ith three orientations. (Yoo et al. 1997) in MR iage (Fig. 2) and can be digitized on a subject s head. The external arkers are identified uch easily in the MRI and orld coordinate [6,7] but the subject ay feel uncofortable ith arkers stuck to his/her face thus ay introduce confounding. This approach is not suitable for longitudinal studies because one MRI scan of a subject ith extra arkers is needed for each MEG easureent. Besides, it faces errors fro detection of relatively large arkers (about 3) in MR iage. 3) The third kind of ethod uses geoetrical features of a subject to perfor the alignent. The ost coon ethod is distance based alignent [1,8,9]. More than 500 points fro a subject s head is digitized anually (Fig. 3a) and the point set is rotated to the sae orientation as the head surface on the MRI coordinate (Fig. 3b). Then, the distance beteen the point set and MRI head surface is iniized to coregister the orld and MRI coordinates (Fig. 3c). This approach does not require fiducial arkers and is suitable for longitudinal studies. Hoever, anual digitization of any points is tie consuing and alignent accuracy depends on unifor distribution of these points. In this study, the caera-guided coregistration ethod is adopted to iprove the disadvantage of the conventional ethods. Using the proposed ethod, the coregistration is perfored autoatically ithout external arkers and uch anual labor. Then functional inforation ith high teporal and spatial resolution is obtained. 2. MATERIALS AND METHODS Fig. 4 (a) The ROI in caera iage is the sae as (b) the ROI in MRI iage to extract (c) surface points in MRI 2.1. Overvie In MRI iages, the point set of head surface (Fig. 4c) is extracted fro the region of interest (ROI) (Fig. 4b) hich also can be selected in caera iages (Fig. 4a). Using initial guess of transforation atrix ( T ), the surface point set in the MRI coordinate (C M ) is transfored into the orld coordinate (C W ) (Fig. 5). Then, the point set in the orld coordinate is projected into three caera coordinates (C C ) of different orientation [10]. The iage intensity of the point set is obtained fro three caeras iages. The difference of iage intensity beteen caera iages is iniized for the optial transforation atrix ( T ). Using the transforation atrix, the points of head surface in the MRI coordinate are transfored into the orld coordinate. Therefore MRI-MEG coregistration is perfored through orld coordinate syste ith the proposed technique Instruentation The MRI scan of the sae subject as conducted using a Bruker 3T MR syste (MedSpec 30/100, Gerany). The 128 slices of T1-eighted iage ere obtained ith Repetition Tie (TR) 1800 sec, Echo Tie (TE) 35 sec, in-plane atrix 256*256 pixels, field of vie 230*230 2 and slice thickness 1.5. In MRI volue, the ROI is selected interactively to extract surface points ith the Matlab progra. Three SONY caeras (DFW-700) fixed on a rod ith suitable angle ere separated fro the sae distance (Fig. 6). The hole caera set is put on the adjustable caera holder for the optial height to take a picture of the subject s face. The axiu spatial resolution of each caera is 1024 pixels * 768 pixels. Caera iages are delivered to a coputer ith IEEE 1394 interface card.

MRI volue C M Surface point W T M segentation M T W C W C C Caera iage World coordinate Fig. 5 The flochart of transforation beteen the MRI and orld coordinate Fig. 7 Calibration board Fig.

There are several hite circles of ell-knon size ith black background on the calibrated board. The board ith echanic otor controlled by orkstation can reach the desirable distance.

Hoever, extrinsic caera paraeters are affected by of caera oveent and incorrect paraeters result in coregistration error.

3 MRI volue C M Surface point W T M segentation M T W C W C C Caera iage World coordinate Fig. 5 The flochart of transforation beteen the MRI and orld coordinate Fig. 7 Calibration board Fig. 6 Three caeras on the caera holder 2.3. Caera calibration Three caeras are ell calibrated in Acadeic Sinica ith the calibrated board (Fig. 7). There are several hite circles of ell-knon size ith black background on the calibrated board. The board ith echanic otor controlled by orkstation can reach the desirable distance. Iages of board are then captured for intrinsic and extrinsic paraeters calculation for all caeras [11]. Hoever, extrinsic caera paraeters are affected by of caera oveent and incorrect paraeters result in coregistration error. Therefore, on-site calibration can assure the accuracy of caera before each experient. On-site calibration is perfored ith one siple pattern (Fig. 8). There are 611 big circles ith a diaeter of 8 and 14 sall circles ith a diaeter of 5.6 in the picture so these circles of knon position can be used for calibration of extrinsic caera paraeters. In our study, on-site calibration for extrinsic caera paraeters is perfored before each experient and the calibration error is less than Optiization In the beginning, the good initial guess is used to check the perforance of hole procedure. Three obvious positions are selected and each position should be on 2 [ Ii ( Pi ) I j ( Pj )] = T arg a x i, j 2 2 [ Ii ( Pi )] [ I j ( Pj )] the sae location for MRI iage and three caera iages. With caera calibration paraeters, three specific points on each caera iage is transfored to the orld coordinate. With three specific points, the transforation atrix beteen the MRI and the orld coordinate is calculated as initial guess ( W T M ). Therefore, the surface points ( M ) fro the selected ROI in MRI iages can be transfored into the orld coordinate ith the initial guess. Under Labertian assuption, the iage intensity (I) of caera iage points hich are projected (P) fro the orld coordinate should be the sae for each caera (i,j). Then, the cross correlation coefficient of iage intensity for these points beteen all three caeras is axiized to extract the optial transforation atrix ( W T M ) beteen the MRI and orld coordinate. Using the optial transforation atrix, the MRI- MEG coregistration can be perfored through the orld coordinate. The optiization procedure is iterative calculation using Nelder-Mead siplex (direct search) ethod [12] on Matlab. 3. EPERIMENTAL RESULTS Fig. 8 There are 611 big circles ith a diaeter of 8 and 14 sall circles ith a diaeter of 5.6 in the picture. Therefore, on site calibration is perfored ith these circles of knon position. There are fe intrinsic features in our caera pictures so a gradient color light is projected on a subject s face to increase iage texture. When the good initial guess is applied, the surface points fro MRI iages atch the caera iages. If the initial guess is

4 shifted, the alignent error beteen the surface points and caera iages is increased after several iteration. The alignent error results fro poor difference of iage intensity beteen neighbor pixels so the rando color light is applied to increase iage features. Hoever, the rando extent of light pattern affects coregistration result. The color light of optial randoness is used to avoid uch local iniu and keep iage texture. After that, the result is close to the optial one ith shifted initial guess only along x axis (Fig. 10). Even ith shifted initial guess both along x and y axis (Fig. 11), the coregistration result is still good using optiization tice. The optiization ethod ith rando color light is less sensitive to initial guess than gradient color light. In our preliinary result, the autoatic coregistration beteen the MRI and orld coordinate is perfored ell. Furtherore, the hole coregistration procedures are perfored ith hoe-ade progras of Matlab and C code. These progras are integrated into Matlab code to iprove its efficiency. Based on this coregitsration result, autoatic MRI-MEG coregistration can be perfored using the proposed technique. 4. DISCUSSIONS a b Fig. 9 The MRI surface points (red points) ith (a) initial guess transforation atrix (shifted only on x-axis) (b) the optial transforation atrix on three caera iages ith suitable gradient color light a b There are four issues to be discussed as folloing: 1. Accuracy of caera calibration: The relationship beteen caera and orld coordinate is defined by caera calibration so it is iportant to assure the accuracy of caera calibration. If the caera calibration is correct, points on the center of circles in orld coordinate transfored fro to caera iages should be projected on the center of circle in the other caera iage (Fig. 8). Before experient is conducted, the accuracy of caera calibration is checked ith the ethod entioned above. Besides, on-site caera calibration is applied in this study to avoid calibration error during the transportation [13]. 2. Selection of the suitable optiization algorith: According to our preliinary data, the optiization ethod e used is fast but it is sensitive to initial guess. If orse initial guess is appiled, the alignent error is larger. Therefore a optiization ethod ith fe constraints on local iniu is better for our data [14, 15]. Furtherore, selection of objective function in optiization also affects coregistration result. If the objective function is tolerant to global difference of iage intensity beteen three different caeras, the coregistration is perfored ith little disturbance. 3. Verification of the co-registration result: The phanto study ill be applied for verification. The MEG phanto fro Neuroag copany can send electric current fro knon position. The anatoy Fig. 10 The MRI surface points (red points) ith (a) initial guess transforation atrix (b) the optial transforation atrix on three caera iages ith rando color light iage of MEG phanto is obtained by Coputed Toography (CT) to avoid daaging the electrical structure of the phanto by MRI. The position of the current source obtained ith the proposed ethod is copared ith knon position of the sae current source. The coparison is to verify accuracy of the proposed ethod [16, 17]. 4. Integration of progras: The core progra for optiization is ritten in C code to increase processing speed but the other part is ritten in Matlab to easily build up graphic user interface(gui). Matlab progras have ability to access C code through ME functions so all progras are integrated into Matlab platfor. The integrated progra is not only user-friendly but the optiization process of the progra still aintains fast. Hence, the hole syste is perfored on the sae platfor to iprove its efficiency. REFERENCES [1] Wang, B., et al., Head surface digitization and registration: a ethod for apping positions on the head onto agnetic resonance iages. Brain Topography, (3):p

5 [2] Carducci, F., et al., Multiodal integration of highresolution EEG and functional agnetic resonance iaging data: a siulation study. Cortex, (2):p [3] Kober H, G.P., Vieth J, Precise fusion of MEG and MRI toography using a surface fit. Bioedical Engineering, (suppl):p [4] Besl, P.J.M., H.D., A ethod for registration of 3-D shapes. IEEE Transactions on Pattern Analysis and Machine Intelligence, (2):p [16] Dale A., S.I., Iproved localization of cortical activity by cobining EEG and MEG ith MRI cortical surface reconstruction: a linear approach. Journal of Cognitive Neuroscience, (5):p [17] Huppertz, H.J., et al., Estiation of the accuracy of a surface atching technique for registration of EEG and MRI data. Electroencephalography & Clinical Neurophysiology, (5):p [5] Singh, K.D., et al., Evaluation of MRI-MEG/EEG coregistration strategies using Monte Carlo siulation. Electroencephalography & Clinical Neurophysiology, (2):p [6] Sipson, G.V., et al., Dynaic neuroiaging of brain function. Journal of Clinical Neurophysiology, (5):p [7] Yoo, S.-S., Guttann, Charles R.G.a, Ives, John R.b, Panych, Larence P.a, Kikinis, Rona, Schoer, Donald L.b, Jolesz, Ferenc A., 3D Localization of surface EEG electrodes on high resolution anatoical MR iages. Electroencephalography and Clinical Neurophysiology, (4):p [8] Kozinska, D., F. Carducci, and K. Noinski, Autoatic alignent of EEG/MEG and MRI data sets. Clinical Neurophysiology, (8):p [9] Kozinska, D.T., Oleh J.; Nissanov, Jonathan; Ozturk, Cengizhan, Multidiensional Alignent Using the Euclidean Distance Transfor. Graphical Models and Iage Processing, (6):p [10] Chen, Y.-S., Liou, Lin-Goa, Hung, Yi-Pinga, Fuh, Chiou-Shannb, Three-diensional ego-otion estiation fro otion fields observed ith ultiple caeras. Pattern Recognition, (8):p [11] Chen, Y.-S., Shih, Sheng-Wenb,Hung, Yi-Pinga,Fuh, Chiou-Shanna, Hung, Yi-Pinga, Siple and efficient ethod of calibrating a otorized zoo lens. Iage and Vision Coputing, (14):p [12] DW, M., An algorith for least squares estiation of nonlinear paraeters. Journal of the Society for Industrial and Applied Matheatics, (2):p [13] Tsai, R., A versatile caera calibration technique for high-accuracy 3D achine vision etrology using off-theshelf TV caeras and lenses. IEEE Journal of Robotics and Autoation, (4):p [14] Paul Viola, W.M.W.I., Alignent by Maxiization of Mutual Inforation. International Journal of Coputer Vision, (2):p [15] Weng, J.C., P. Herniou, M., Caera calibration ith distortion odels and accuracy evaluation. IEEE Transactions on Pattern Analysis and Machine Intelligence, (10):p

Image Processing for fmri John Ashburner. Wellcome Trust Centre for Neuroimaging, 12 Queen Square, London, UK.

Image Processing for fmri John Ashburner. Wellcome Trust Centre for Neuroimaging, 12 Queen Square, London, UK. Iage Processing for fmri John Ashburner Wellcoe Trust Centre for Neuroiaging, 12 Queen Square, London, UK. Contents * Preliinaries * Rigid-Body and Affine Transforations * Optiisation and Objective Functions