Do different MRI scanners produce different fMRI results (2001)

Introduction If fMRI is to become a clinical tool for investigating brain activity it should prove to be reproducible and independent of the machine and operator. Although in most experiments a single fMRI session is often assumed to give “the” functional map, evidence has been given of variability in both intra and inter subject experiments [1,2,3,4]. Whereas reproducibility in motor and visual tasks has already been investigated, reproducibility studies between different (but comparable) scanners have not yet been reported. In the present study we aim at comparing the intra subject reproducibility of fMRI performed on two different (but comparable) clinical MR scanners. Methods Scanners: Two 1.5 T clinical scanners (Siemens Vision and GE Signa) with high gradient capability were used for this study. Three healthy subjects underwent fMRI with visual stimulation on the two scanners (A and B) on the same day, and on either A or B a few days later. Within each session the same paradigm was repeated (without repositioning), to produce a “standard” of reproducibility. (SameSession reproducibility). MR Sequences: Axial single shot EPI sequences using high gradient performance were performed on both scanners. They were set to have the same imaging parameters (TE=66, matrix 128x128, TR=4.0 sec, slice thickness 4.5 mm, no gap, comparable bandwidth), and positioned on a sagittal plane. A 3D T1 weighted sequence was used as anatomical reference on both machines. Paradigm: A radial flickering checkerboard (8 Hz) within a threeblock design (left/right/rest) was presented. The visual stimuli were presented with an LCD fiber optic display positioned above the head coil (IFIS by Psychology Software Tools). The same presentation hardware was used in both scanners. Image analysis Images were motion corrected, smoothed spatially and temporally using a gaussian filter (FWHM= 2 pixels and 3 scans respectively) and analyzed using a two-sample t-test. For each session two statistical maps were created: left versus rest and right versus rest. We used a zscore threshold of 7.0. From the slice-based statistical data 3D statistical maps were created by mapping them onto the 3D anatomical image. All pre-processing and analysis were performed using Brain Voyager by Brain Innovation. One 3D anatomical image for each subject was chosen as reference. For each subject all 3D statistical maps were mapped onto this reference by applying the rigid body transformation which maps the corresponding 3D anatomical images onto the (anatomical) reference 3D image. This procedure rendered the 3D statistical maps aligned to each other. By means of this procedure, and because we only performed intra-subject comparisons, the use of Talairach space becomes redundant. The persons who did the analysis were not blinded to which scanner was used. Reproducibility was measured using three quantities: CM defined as the center of mass of the activated area, Rsizeij=2*Vsmallest/(Vi+Vj), and Roverlapij=2*Voverlap/(Vi+Vj), where Vi and Vj are the activated volumes, Voverlap is the overlap of Vi and Vj, and Vsmallest is the smallest of Vi and Vj [2,3]. As visual stimuli were presented we restricted our analysis to the occipital lobe. This reduces the number of spurious activated voxels. Results The EPI images on both scanners were of comparable quality, showed both a good temporal signal stability and comparable SNR and CNR. Strong occipital activation (primary visual cortex) was present in all subjects in all sessions. Values of the above mentioned measures of reproducibility (averaged) for the same-session are shown in table 1. Ratios of these quantities for same session vs. intersession and vs intermachine are presented in table 2. Moreover, we found that, on average, scanner B produced larger activated volumes, the ratio between B and A being 1.3 (not shown in the tables). Discussion As it was expected, reproducibility of intersession (and same machine) fMRI is smaller than reproducibility of same session fMRI. Reproducibility is further reduced when comparing different scanners. This might be due to the different technical implementation of the EPI imaging technique in the two scanners. The volume of the activated area in scanner B can be made smaller to match the volume of the activated area of scanner A by increasing the z-score threshold of the statistical maps for scanner B. This might improve reproducibility. We conclude that the influence of the scanner on the fMRI result should not be underestimated. References [1] W.C. Machielsen, S.A.R.B. Rombouts, F. Barkhof, P. Scheltens, M.P. Witter. Hum Brain Mapp 9, 156-164 (2000). [2] S.A.R.B. Rombouts, F. Barkhof, F.G.C. Hoogenraad, M.Sprenger, P. Scheltens. MRI 16 105-113 (1998). [3] A.Miki, J.Raz, T.G.M. van Erp, C-S. J. Liu, J.C. Haselgrove and G.T. Liu. AJNR 21 910-915 (2000). [4] D.J. McGonigle, A.M. Howseman, B.S. Athwal, K.J. Friston, R.S. Frackowiack, A.P. Holmes. Neuroimage 11, 708-734 (2000).