Abstract
To determine the sample size needed to provide adequate statistical power in studies of brain volume by MR imaging, we examined the precision and variability of measurements in healthy controls. A cohort of 52 people (mean age, 25.1 years) was examined at weeks 0 and 12 at 1.5 T. We used an axial multisection T1-weighted sequence and a contiguous proton-attenuation/T2-weighted sequence. Data were registered to a probabilistic brain atlas, and an automated atlas-based program was used to segment brain tissue by type and by lobe. We assumed that there were no changes in volume because there were no intervening neurologic events. Sample sizes required to yield 80% statistical power in detecting a significant difference in volume were calculated for various experimental designs, assuming a patient-control volume difference of 5% or 2%. The precision of most measurements was excellent, but required sample sizes were larger than anticipated. If the goal was to detect a 5% difference in whole brain volume in a 2-sample cross-sectional study, the required sample was 73 patients and 73 controls because brain volume varies between individuals in a way that is not informative about disease effects. For a similar 2-sample longitudinal study, the required sample size was just 5 patients and 5 controls. Our results argue strongly for longitudinal studies in preference to cross-sectional studies, especially as research budgets decline. Our findings also suggest that there may be more uncertainty than expected in published MR imaging brain volume studies.
Highlights
AND PURPOSE: To determine the sample size needed to provide adequate statistical power in studies of brain volume by MR imaging, we examined the precision and variability of measurements in healthy controls
If the goal was to detect a 5% difference in whole brain volume in a 2-sample cross-sectional study, the required sample was 73 patients and 73 controls because brain volume varies between individuals in a way that is not informative about disease effects
Our findings suggest that there may be more uncertainty than expected in published MR imaging brain volume studies
Summary
A cohort of 52 people (mean age, 25.1 years) was examined at weeks 0 and 12 at 1.5T. Subjects Data for this study were collected as part of a 2-year randomized double-blind clinical trial that compared the efficacy and safety of olanzapine with that of haloperidol in patients experiencing firstepisode SZ.[4,5,6] Patient data from that trial were not used here; instead, we focused on brain volume data from 52 healthy controls, which were not reported in detail before.[6]. Controls were imaged at enrollment, again 12 weeks later on the same scanners, by using the same imaging protocol described for patients.[6] We used data only from subjects who were imaged at both time points and who had no health complaints at either time point. Controls were a mean age of 25.1 Ϯ 4.0 years at first scanning, with 67.3% being male, and the ethnic composition was 59.6% white, 28.9% African-American, and 11.5% other ethnicity
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