Measurement of reliability in functional MRI using intra-class-correlation coefficient and scatter-plots

NemoImage

11, Number

5, 2000, Part 2 of 2 Parts 10

E kL@

METHODS

- ANALYSIS

Measurement of Reliability in functional MRI using Intra-class-correlation coefficient and scatter-plots Karsten Specht*t,

N. Jon Shah*, Karl Zilles*, Lutz Jiincke?

*Institute of Medicine, Research Centre Jiilich tlnstitute of General Psychology, Otto-von-Guericke University, Magdeburg, Germany Clinical application of fMRI strongly depends on the validity and reliability of hemodynamic responses. The present project specifically was designed in order to measure reliability of hemodynamic responses in the context of visual and auditory stimulation conditions while the subjects were engaged in different attentional conditions.. For this we applied well known statistical methods, like intra-class correlation coefficients [l] and scatter-plots [2]. All functional images were acquired at a 1.5 T Siemens VISION scanner using a T2* weighted EPI sequence (TE 66ms, TR 2.2s, voxel size 3.125 x 3.125 x 4.4mm. 16 axial slices orientated parallel to the AC-PC line covering the auditory and visual cortex). To investigate the reliability of R measurements, the intra-class correlation coefficient (ICC), which is a relationship of the within subject (MSW) and between subject variance (MSB) related to the sum of both variances. Using SPM99 a voxelwise calculation of the ICC was performed and displayed. ICC=(MSB-MSW)/((MSB+(R-l)*MSW) The following thresholds are used: 0.3 < ICC < 0.5 = low, 0.5 < ICC < 0.8 = medium and ICC>O.8 strong Figure 1 shows an example obtained during a simple language task, where the subject had to handle the visual presented syllables under different attentional conditions (ignore, attend, or detect). In the figure the results obtained for the ignore and detect condition are presented for the three ICC threshold categories using the standard spm99 glasbrain. It can lx seen, that several areas in the vicinity of the primary and secondary visual cortex displayed considerable strong ICCs. However, there are also strong ICCs outside these areas in the inferior frontal gyri and the superior temporal gyrus area. Most interestingly, ICCs considerably increase when the subjects were required to detect a specfic target. Using the scatter-plot, plotting the t-values for each voxel of measurement 1 against measurement 2, one can analyse and display single subject reliability and calculate the correlation coefficients r and r*r Figure 2 shows a single subject example for two types of attentional conditions (left panel for the ignore right panel for the detect condition). It can be seen, that the task with more attentional effort leads to a stronger reliability in contrast to the ignoring task where no attentional afford is necessary. Ibis result is in good agreement with the ICC-group-result in figure 1. The differences are only caused by differences in the attentional effort, while the stimulus presentation was exactly the same at both tasks. Taken together, in order to receive reliable hemodynamic responses it is necessary to use a well-defined cognitive task for each subject. Passive stimulation with no explicit instructions on the other hand enlarges the degrees of freedom with which the subjects are able to cope with the stimulation and thus increases variability in neural responses and hemodynamic reactions. References [l] Specht [2] Bartko

et al., NeuroImage (1999); 9(6): S15; et al, J Nervous and Mental Disease (1976):163(5):307-317

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Figure ignore

2: Scatter-Plot for two (left), discriminate (right)

Figure 1: Display of three tions for two different tasks

ICC

Figure 1: Gender specific effects on the volume on the hippocampus

conditions:

classifica-

of aging