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12 clinical and research news in CA1, while PTSD is characterized by neuron loss in CA3 and dentate gyrus. The ability to distinguish among different patterns of hippocampal atrophy in vivo on a MRI could provide valuable information regarding the etiology of hippocampal atrophy. Progress of Structural Imaging of the Hippocampus On conventional whole brain T1 or FLAIR images at 1.5 T or 3T, the resolution (typically around 1×1×1 mm) and contrast are usually not sufficient to appreciate subtle differences within the hippocampus. Consequently, it appears to be globally shrunken, often accompanied by an increased FLAIR signal, regardless of the underlying disease process. If discernible at all, the internal structure often seems to be blurred or even lost. In comparison, the appearance of an atrophied hippocampus on a dedicated, high-resolution T2 or PD-weighted fast-spin echo image obtained at 3 T or higher is strikingly different, depicting details of its internal structure (Figure 1). Even though the resolution is far from that of a histological preparation, a hypointense line representing the myelinated fibers in the stratum lacunare, moleculare, and vestiges of the hippocampus sulcus are easily and reliably recognized. The distance between this hypointense line and the outer boundary of the hippocam- Figure 2 Upper panel: Histological preparation of the hippocampus showing the hippocampal subfields. Lower panel: Manual labeling scheme used for subfield volumetry. The manual parcellation shows a reasonable correspondence with the histological image. Some deviations from the histological scheme, e.g., part of the subiculum included in CA1, allow for more consistent marking between raters.
Figure 3 Side-by-side comparison (T1 ((MPRAGE) TR/TE/TI = 2300/3/950 ms, 7° flip angle, 1.0 × 1.0 × 1.0 mm 3 resolution, acquisition time 5.17 min) with high resolution image) of the four atrophy types of MST seen in TLE. pus provides an excellent estimate of the thickness of the hippocampal cortex at this point. This permits identification of circumscribed regions of hippocampal atrophy, particularly if they are combined with patchy hypointensities. This hypointense line, along with external hippocampal landmarks, can also be used to subdivide the hippocampus into subsections. These subsections correspond to the histological subfields (Figure 2), so the volumes obtained by this procedure can be used as surrogate subfield volumes. Clinical Applications of Hippocampal High-Resolution Imaging In some diseases the hippocampal volume losses are so characteristic that their regional selectivity is easily detected on visual inspection. A typical example is temporal lobe epilepsy (TLE) with mesial temporal sclerosis (TLE-MTS). In TLE-MTS the seizures arise from the hippocampus and neighboring mesial temporal structures, e.g., parahippocampus. Mesial temporal sclerosis (MTS), a specific form of hippocampal atrophy, is the hallmark of TLE-MTS. Large histopathological studies of surgical specimens have shown that MTS is not homogenous, but has four major subtypes: isolated DG atrophy, isolated CA1 atrophy, CA1 and DG atrophy with sparing of CA2, and global atrophy. CA1 and DG is the most common and is often referred to as “classical type” MTS. Figure 3 shows a side-by-side comparison of the hippocampus in a standard 4T T1 whole brain image and a dedicated high-resolution image. While all four atrophy types are easily identifiable by visual inspection in the highresolution image, this is difficult to do on the conventional T1 image, despite the fact that the hippocampal atrophy is obvious to the experienced reader. A quantitative assessment using statistical thresholds to distinguish atrophied subfields from those in the normal range confirmed the existence of atrophy patterns in a larger population of TLE- MTS patients. The ability to distinguish among different MTS patterns on a MRI is not solely of academic interest. Large histopathological series have shown that the different atrophy patterns may represent different clinicopathological TLE subtypes and may even have predictive value for the 13
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62 department update
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