In contrast to elaborate volumetric assessment, the measurement of T2 relaxometry is a quick technique and can be implemented in large studies and measured routinely by the radiographer or reporting radiologist (319). The measured value of both hippocampal volume and the T2 time are inversely correlated with each other (218, 247, 318, 320). The inverse correlation between T2 signal and hippocampal volumes is present in the ipsilateral hippocampus but not in the contralateral hippocampus (318). This indicates that a marked volume loss is associated with a significant increase in T2 relaxation time, or in other words that an atrophic hippocampus corresponds to a brighter signal in an individual T2-weighted image.
Whereas it is true in general that volume loss mirrors signal increase, there are exceptions to this rule. Some refractory TLE patients have only minimal volume loss but T2 signal increase (see, for example Fig. 4.28) (232, 321), whereas others have normal T2-relaxometry values but hippocampal volume loss (130). Histopathologic examination showed marked gliosis but no neuron cell loss. In unselected patients with refractory TLE, up to 15% do not have detectable hippocampal atrophy on MRI. A recent study evaluated whether T2 relaxometry could identify hippocampal pathology in patients with normal hippocampal volumes on volumetry. It compared 11 TLE patients without atrophy with 14 TLE patients with unilateral atrophy (232). All TLE patients with hippocampal atrophy and 9/11 (82%) patients with normal MRI had abnormally high hippocampal T2 values ipsilateral to the epileptic focus, which was defined based on history, video-EEG, and surgical response. It was concluded that hippocampal T2 mapping provided evidence of hippocampal damage in the majority of patients with intractable TLE without hippocampal volume loss on MRI (232).
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