Practical microscopic neuroanatomy of the limbic system and basal forebrain identifiable on clinical 3T MRI

Abstract

Microscopic neuroanatomy of limbic system and basal forebrain on MRI is complex and is a terra incognita for many radiologists, clinicians, and neuroscientists. Interestingly, most of the important structures/at least anatomical regions containing these structures demonstrable on cadaveric and surgical dissections can be identified on clinical MRI, with 3T being much better than 1.5T. This article teaches the practical MRI identification of these structures which will greatly help in evaluating complex ailments like temporal lobe epilepsy, Alzheimer dementia, and other neuropsychiatric disorders. This knowledge will also aid in accurate reporting of tumor spread along the white matter fasciculi in the temporal stem/basal forebrain region. Limbic system includes the mesial temporal structures and their connections, piriform cortex including “area tempestas,” and the septal area comprising of subcallosal area and paraterminal gyrus. Basal forebrain includes structures like substantia innominata with basal nucleus of Meynert, diagonal gyrus/diagonal band of Broca, and nucleus accumbens lying in between the anterior perforated substance inferiorly and the anterior commissure superiorly.

Injury to the circuit of Papez: An overlooked cause of recurrent seizures

“…Using the convulsant toxin pentylenetetrazole in animal models was helpful in recognizing the role of MTT and MB as an important pathway in seizure propagation to the thalamic anterior nucleus and frontal cortex. 1,2 On MRI, the MTT can be identified on axial and coronal T2-weighted sequences as thin craniocaudally…”

“…The Papez' circuit is a closed cerebral neural circuit, starting from the hippocampus, then coursing through the fornix, mammillary body (MB), MTT, anterior nucleus of the thalamus (ATN), cingulum, entorhinal cortex/parahippocampal gyrus then back to hippocampus 1 . It was reported that the circuit is associated with learning, memory, and emotion functions as well as seizure activity.…”

“…It was reported that the circuit is associated with learning, memory, and emotion functions as well as seizure activity. Using the convulsant toxin pentylenetetrazole in animal models was helpful in recognizing the role of MTT and MB as an important pathway in seizure propagation to the thalamic anterior nucleus and frontal cortex 1,2 . On MRI, the MTT can be identified on axial and coronal T2‐weighted sequences as thin craniocaudally oriented low‐signal white‐matter tracts on either side of the midline, posterior to the slightly larger hypointense columns of fornix 1 .…”

“…The Papez' circuit is a closed cerebral neural circuit, starting from the hippocampus, then coursing through the fornix, mammillary body (MB), MTT, anterior nucleus of the thalamus (ATN), cingulum, entorhinal cortex/parahippocampal gyrus then back to hippocampus. 1 It was reported that the circuit is associated with learning, memory, and emotion functions as well as seizure activity. Using the convulsant toxin pentylenetetrazole in animal models was helpful in recognizing the role of MTT and MB as an important pathway in seizure propagation to the thalamic anterior nucleus and frontal cortex.…”

“…Using the convulsant toxin pentylenetetrazole in animal models was helpful in recognizing the role of MTT and MB as an important pathway in seizure propagation to the thalamic anterior nucleus and frontal cortex. 1 , 2 On MRI, the MTT can be identified on axial and coronal T2‐weighted sequences as thin craniocaudally oriented low‐signal white‐matter tracts on either side of the midline, posterior to the slightly larger hypointense columns of fornix. 1 In our case, the seizure activity was attributed to an abnormality in the Papez' circuit based on MRI features of chronic infarction in the left ATN and MTT, atrophic left mamillary body, and fornix, along with the diffusion tractography findings of decreased tracts of left Papez' circuit.…”