We report on the selective thinning of the CA1 apical neuropil layer relative to the CA1 cell body layer in subjects with mild AD, visualized in vivo using 7-T MRI. By permitting analysis of the microscopic hippocampal structures that are most sensitive to early pathology in AD, ultra-high-field 7-T MRI may be able to contribute to AD diagnosis and tracking.
The CA1 apical neuropil is an early target of AD pathology. Neurofibrillary tangles appear first outside the hippocampus, in the entorhinal cortex.2
The perforant pathway axons, projecting from the entorhinal cortex to the hippocampus, target the outer molecular layer of the dentate gyrus and the distal apical dendrites of CA1 neurons in the stratum lacunosum-moleculare. It is of interest, then, that these 2 neuropil areas of the hippocampus are among the next sites for tau pathology to appear, still quite early in AD pathogenesis.4,6
Some have hypothesized that tau pathology spreads in a network-dependent fashion,4,5,20
perhaps passively by deafferentation,21
or more actively by a prion-based mechanism.22
In the stratum lacunosum-moleculare, the varicose, tau-laden CA1 dendrites go on to disappear as clinical signs of AD become more pronounced, possibly corresponding to the thinning of the CA1-SRLM observed in vivo in this study and in prior studies of postmortem tissue.7,8
It was not surprising that CA1-SRLM thickness was better than overall hippocampal volume at distinguishing subjects with AD from NCs. While global hippocampal atrophy in AD is now a well-accepted phenomenon,23
cross-sectional studies often use larger sample sizes or more severely affected subjects with AD to detect group differences. In our small study of subjects with mild AD, there was no significant loss of hippocampal volume. Our work supports the notion that areas of the hippocampus less affected by AD may dilute changes in heavily, but focally affected areas. Importantly, as sensitive as CA1-SRLM thickness was at distinguishing subjects with AD from NCs at the group level in this dataset, there was still considerable overlap between the groups (). Therefore, like many other neuroimaging markers investigated to date, it is not clear whether measuring CA1-SRLM thickness alone will be able to contribute to a diagnosis in an individual patient.
A focal, early hit to the CA1-SRLM is consistent with the fact that short-term episodic memory impairment is the most frequent first symptom in AD. The synapses in the CA1 apical neuropil are well-known for their robust propensity to undergo NMDA receptor-dependent long-term potentiation, a form of synaptic plasticity associated with synapse unsilencing and memory formation,24
and it is conceivable that loss of those synapses may contribute to short-term memory loss in AD. A prior study revealed a tight correlation between neuropsychological measures and synaptic density in this region at autopsy,25
supporting the notion that there may be phenotypic relevance to atrophy of the apical neuropil in AD. Besides AD, there are other disease processes that cause damage to CA1 and result in memory impairment. As an example, CA1 pyramidal neurons are selectively vulnerable to global cerebral ischemia, as experienced during cardiac arrest,26
and it would be of interest to investigate whether any changes observed in the CA1-SP and CA1-SRLM at 7 T correspond to the degree of episodic memory deficits.
Our work adds to growing efforts to visualize the microscopic features of the human hippocampus using MRI technology. At lower field strength, several groups have traced approximate hippocampal subfield boundaries onto 3-dimensional hippocampal projection maps and shown that CA1 and subiculum atrophy occurs in AD and predicts cognitive decline in normal or mildly impaired individuals.27–30
This technique, while powerful in using advanced computational methods to extract detailed information from low-resolution raw data, does not permit analysis of the hippocampal strata visible only in cross-section.
Several other groups have reported AD-associated changes in hippocampal cross-sectional anatomy using MRI.31–34
One group noted thinning of CA1 but not other subfields in subjects with dementia relative to controls using 1.5-T MRI.31
Also noting CA1 thinning in AD, another group used 4-T MRI to produce better visualization of anatomic landmarks, and thus improved reliability in marking the dimensions of individual subfields.32,33
Both groups considered the CA1 subfield as a whole unit, without differentiating CA1-SP from CA1-SRLM. In this study of patients with mild AD, no difference in overall CA1 thickness was observed. Aside from one 7-T MRI study of young subjects,9
the use of MRI to visualize hippocampal strata has been limited to autopsy specimens.17–19
We reliably distinguished CA1-SRLM as the hypointense band between the more intense DG and CA1-SP cell layers.9
While 7-T GRE MRI afforded excellent in-plane resolution and gray-white differentiation, it was only good enough to separate neuropil from cell body layers, and significantly better resolution (~60 μm) would be necessary for differentiation of stratum radiatum from stratum lacunosum-moleculare.35
Likewise, borders between DG, CA4, and CA3 remained obscure in our study, and the stratum oriens was only barely visible as a hypointense band superficial to the SP measuring about 1 pixel (200 μm) in thickness (). In this study, the SRLM was only approximately 2-3 voxels (400-600 μm) wide, and the values for subjects with AD and NC differed on average by only about 1 voxel. Thus, the thinning of the SRLM associated with mild AD in this study was already at the detection limit of our 7-T GRE protocol, and we could not detect any difference in SRLM thickness between the groups on our 3-T images (not shown). This point highlights an important limitation of our technique and of our study, that the absolute measurements that we recorded, as interpolations on raw, pixelated images, are only rough approximations of the true tissue dimensions. Although there is no reason for the presence of AD to bias the measurements one way or the other (raters were blinded to the diagnosis), and thus no reason to discount the presence of some difference between the groups, it is not possible to draw any firm conclusion about the absolute magnitude of this difference. None of the subjects in this study has come to autopsy, and it was not possible to validate our imaging measurements with postmortem histopathology. Amyloid plaques may also be at or beyond the limit of ultra-high-field MRI resolution. Others have observed amyloid plaques as areas of increased susceptibility in mice36
and in human brains in vitro,37
possibly even in vivo.38
Although we did not explicitly attempt strong susceptibility weighting, we observed no plaque-like formations in our images.
This study was powered to demonstrate the feasibility of performing 7-T MRI in elderly cognitively impaired patients and to detect differences in hippocampal metrics between 2 groups. To determine the sensitivity and specificity of CA1-SRLM measurements for detecting mild AD, more work would be required on larger numbers of subjects, including subjects with other clinical diagnoses. To establish CA1-SRLM thinning as an objective imaging biomarker of AD, it would additionally be important to establish intrarater and interrater reliability of the manual measurements used in this study. Importantly, the use of 7-T MRI as a clinical tool is limited by its current availability in only a few academic centers, and by its greater sensitivity to motion and other artifacts compared to lower field MRI.
Our data support the use of 7-T MRI as a tool for detecting submillimeter changes in hippocampal structure, as well as the growing notion that AD is more a disease of synaptic loss than of neuronal loss.1
Use of CA1-SRLM thickness as a potential biomarker of AD is a rational extension from pathology and is now feasible in vivo, using a 7-T MRI sequence easily tolerated by patients. In addition to addressing the limitations discussed above, longitudinal analysis of NCs and patients with mild cognitive impairment will also be important to determine whether CA1-SRLM thinning predicts future cognitive decline.