Comparison of magnetic resonance imaging in live vs. post mortem rat brains

Abstract

Magnetic Resonance Imaging (MRI) is an increasingly popular technique for examining neurobiology in rodents because it is both noninvasive and nondestructive. MRI scans can be acquired from either live or post mortem specimens. In vivo scans have a key advantage in that subjects can be scanned at multiple time-points in longitudinal studies. However, repeated exposure to anesthesia and stress may confound studies. In contrast, post mortem scans offer improved image quality and increased signal-to-noise ratio (SNR) due to several key advantages: First, the images are not disrupted by motion and pulsation artifacts. Second, they allow the brain tissue to be perfused with contrast agents, enhancing tissue contrast. Third, they allow longer image acquisition times, yielding higher resolution and/or improved SNR. Fourth, they allow assessment of groups of animals at the same age without scheduling complications. Despite these advantages, researchers are often skeptical of post mortem MRI scans because of uncertainty about whether the fixation process alters the MRI measurements. To address these concerns, we present a thorough comparative study of in vivo and post mortem MRI scans in healthy male Wistar rats at three age points throughout adolescence (postnatal days 28 through 80). For each subject, an in vivo scan was acquired, followed by perfusion and two post mortem scans at two different MRI facilities. The goal was to assess robustness of measurements, to detect any changes in volumetric measurements after fixation, and to investigate any differential bias that may exist between image acquisition techniques. We present this volumetric analysis for comparison of 22 anatomical structures between in vivo and post mortem scans. No significant changes in volumetric measurements were detected; however, as hypothesized, the image quality is dramatically improved in post mortem scans. These findings illustrate the validity and utility of using post mortem scans in volumetric neurobiological studies.

Figure 1. Timeline of imaging protocol.

Figure 2. Total brain volume for each age group/scan type.

Figure 3. Pearson’s correlation between brain volume, brain weight and body weight.

Figure 4. Volume of selected regions as a function of age and scan type.

Figure 5. Ventricles in Live and Postmortem brains.

Sagittal sections that highlight the lateral ventricles are shown for a live and Postmortem-2 brain scan of a P80 young adult animal. Ventricular spaces are highlighted in red. Note the post mortem image shows clear brain regional morphology, whereas the live image shows discretization artifacts in the highlights. These differences in image quality would be expected to improve segmentation accuracy and quantification of ventricles similar to other brain regions. However, the ventricles are fluid-filled spaces that are more likely to be altered by post mortem procedures. Although our ventricular volumes were not statistically different in live vs post mortem comparisons, at each age studied there is an apparent trend of reduced volumes from live to Postmortem-1 and Postmortem-2, which suggests post mortem ventricular values should be interpreted with particular caution compared to the other brain regions of interest.

Figure 6. Image quality.

(A) visual assessment. (B) intensity profile across the corpus callosum.

Figure 7. Signal-to-noise ratio.