Brief breath holding may confound functional magnetic resonance imaging studies

Abstract

We demonstrate that breath holding of short durations may confound functional magnetic resonance imaging (fMRI) studies. Some subjects may hold their breath for a short time during task performance, especially if the task is challenging. Breath holding may therefore need to be considered specifically when interpreting fMRI experiments. We studied the temporal and spatial characteristics of cerebral T2*-weighted signal during short periods of breath holding by seven individuals in a 3-tesla MR scanner. We demonstrate that breath-holds as short as 3 s can result in regions of significant cerebral activation. More interestingly, we show that focal activation remains present when the data is analysed in a number of different ways, including analyses that correct for motion and model the task epoch as if it were 10 times longer than the actual breath-hold length. These findings have potential relevance for many researchers carrying out fMRI studies.

Figure 1

a: Activation (P _uncorrected < 0.0001) present in the 6‐s breath‐hold of Subject 3. b, c: Analysis of the first 6‐s and 18‐s (respectively) epoch of the same subject's 30‐s study. d: An enlarged labeled slice of the analysis of the first 18‐s epoch of the 39‐s breath‐hold study of Subject 1. e: The position of the slice acquisition is shown by the thick white line on an anatomical scout image. f: Number of voxels above statistical significance, plotted against the length of the breath‐hold epoch extracted from each subject's longest breath‐hold study (as per analysis 2a, see text). g: Mean % signal change in those voxels that were always significant in progressive 6‐s epochs extracted from 30 s of breath holding, plotted against the time to middle of the extracted epoch (as per analysis 2b, see text). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com. Bright white replaces all color in print.]

Figure 2

The 6‐s breath‐hold study of Subject 3 analysed using a Student's t‐test as if it were an 18‐s (a) and 30‐s (b) task. Considerable regions of significant activation (P _uncorrected < 0.0001) remain evident. c, d: Analysis of the complete time‐course of the same 6‐s breath‐hold study using the general linear model as implemented in SPM99, with motion‐correction parameters included in the model as confounds, and the task modelled with a canonical HRF convolved with a boxcar of task length 18 s (c) and 30 s (d). e: Significant activity was also seen in the short breath‐hold studies of other subjects, as demonstrated here for example in the 6‐s breath‐hold study of Subject 6, modelled in SPM99 (as described above) with a task length of 30 s. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com. Bright white replaces all color in print.]