Presented at the Neonatal Society 2004 Spring Meeting.
Bainbridge A1, Iwata O2, West DA2, Cheong JL2, Priest AN1, Cady EB1, Shanmugalingam S2, Raivich G2, Ordidge RJ3, Wyatt JS2, Robertson NJ2
1 Medical Physics and Bioengineering, University College Hospitals NHS Trust, London, UK
2 Paediatrics and Child Health, University College London, London, UK
3 Medical Physics and Bioengineering, University College London, London, UK
Background: Apparent diffusion coefficient (ADC) of water measured using MRI is sensitive to acute changes in the brain following hypoxia-ischemia (HI). The purpose of this study was to correlate changes in ADC values in the brain to histological data in piglet model of cerebral HI.
Methods: Five newborn piglets were studied before and after an acute HI insult (carotid occlusion and reduction of FiO2 to 12%). Two insult lengths were used, (i) 20 min (n=2; 18 and 30 hr survival); (ii) 50 min (n=3; one 22 hr and two 48 hr survival), yielding a wide spectrum of tissue damage over the whole group. A single axial imaging slice was selected so as to intersect the thalamus and the lateral ventricles. Single shot spin-echo diffusion weighted EPI (TE = 76ms; acquisition bandwidth = 200kHz; b = 0 and 817 s mm-2) was used and parameter maps of trace ADC were reconstructed. Brains were perfusion-fixed and slices were hematoxylin and eosin stained. The percentages of viable and necrotic neurons were counted in 10 cortical regions that were initially defined on the endpoint trace ADC maps.
Results: An example of the ADC changes in representative cortical grey matter regions during SEF is shown in figure 1. The percentages of viable and dead neurons were plotted against the final ADC, expressed as a percentage of the baseline value, for all cortical grey matter regions from all animals (figures 2 & 3). Reduced ADC was seen to correlate to a small percentage of neurones having a normal appearance and to a large percentage of dead neurones (p < 0.001; Pearson product moment correlation).
Discussion: This model produces a heterogeneous pattern of trace ADC changes following HI. A steep decline is observed in the sagital and para-sagital regions of the parietal lobe whereas a more gradual reduction is observed in the temporal lobe. Reduced ADC following HI correlates to large-scale neuronal death. However, the proportion of normal neurons varied greatly in areas where the measured ADC was unchanged (90 – 110% of baseline, figure 2). This suggests that whilst severe histological changes are observable using ADC, mild-moderate changes may be undetectable. These quantitative data support the findings of a previous qualitative study where hypo-intensity on ADC maps acquired from neonatal stroke patients correlated to areas of cytotoxic edema and neuronal death on histology (1).
References
1. Roelants-van Rijn AM et al, Neuropediatrics 2001; 32; 386-294.