Influence of fMRI data sampling on the temporal characterization of the hemodynamic response.

Experimental and modeling studies were used to estimate the effect of different sampling rates (repetition times, TR) and different sampling positions on the estimates of the temporal properties of the hemodynamic response function (HRF) derived from fMRI studies. Data were acquired at a TR of 250 ms and then subjected to various degrees of undersampling. Using a gaussian fitting function it is demonstrated that the accuracy of HRF peak time determination decreases with lower sampling rate (higher TR). The decrease in accuracy amounts to about 50 ms per second of TR increase. In addition, temporal shifts of the HRF peak time are found when reducing the influence of the more variable descending part of HRF curve by using a temporal cut-off after HRF peak time. The shift scales with TR, amounts up to 100 ms for a TR of 1500 ms and a cut-off of 3-4 s and depends on the sampling position. The use of the full HRF function does not lead to a shift but increases the influence of potential confounding factors as large veins and poststimulus undershoot. Since both accuracy and potential shifts of HRF peak determination scale with TR, it is important that temporal fMRI studies are carried out with high sampling rates.