Noninvasive measurement of the cerebral blood flow response in human lateral geniculate nucleus with arterial spin labeling fMRI

Short-term apparent brain tissue changes are contributed by cerebral blood flow alterations

Structural MRI (sMRI)-identified tissue "growth" after neuropsychological training has been reported in many studies but the origins of those apparent tissue changes (ATC) still remain elusive. One possible contributor to ATC is brain perfusion since T1-weighted MRI, the tool used to identify ATC, is sensitive to perfusion-change induced tissue T1 alterations. To test the hypothetical perfusion contribution to ATC, sMRI data were acquired before and after short-term global and regional perfusion manipulations via intaking a 200 mg caffeine pill and performing a sensorimotor task. Caffeine intake caused a global CBF reduction and apparent tissue density reduction in temporal cortex, anterior cingulate cortex, and the limbic area; sensorimotor task induced CBF increase and apparent tissue increase in spatially overlapped brain regions. After compensating CBF alterations through a voxel-wise regression, the ATC patterns demonstrated in both experiments were substantially suppressed. These data clearly proved existence of the perfusion contribution to short-term ATC, and suggested a need for correcting perfusion changes in longitudinal T1-weighted structural MRI analysis if a short-term design is used.

Automatic segmentation of the lateral geniculate nucleus: Application to control and glaucoma patients

BACKGROUND

The lateral geniculate nucleus (LGN) is a key relay center of the visual system. Because the LGN morphology is affected by different diseases, it is of interest to analyze its morphology by segmentation. However, existing LGN segmentation methods are non-automatic, inefficient and prone to experimenters' bias.

NEW METHOD

To address these problems, we proposed an automatic LGN segmentation algorithm based on T1-weighted imaging. First, the prior information of LGN was used to create a prior mask. Then region growing was applied to delineate LGN. We evaluated this automatic LGN segmentation method by (1) comparison with manually segmented LGN, (2) anatomically locating LGN in the visual system via LGN-based tractography, (3) application to control and glaucoma patients.

RESULTS

The similarity coefficients of automatic segmented LGN and manually segmented one are 0.72 (0.06) for the left LGN and 0.77 (0.07) for the right LGN. LGN-based tractography shows the subcortical pathway seeding from LGN passes the optic tract and also reaches V1 through the optic radiation, which is consistent with the LGN location in the visual system. In addition, LGN asymmetry as well as LGN atrophy along with age is observed in normal controls. The investigation of glaucoma effects on LGN volumes demonstrates that the bilateral LGN volumes shrink in patients.

COMPARISON WITH EXISTING METHODS

The automatic LGN segmentation is objective, efficient, valid and applicable.

CONCLUSIONS

Experiment results proved the validity and applicability of the algorithm. Our method will speed up the research on visual system and greatly enhance studies of different vision-related diseases.

Arterial spin labeling fMRI measurements of decreased blood flow in primary visual cortex correlates with decreased visual function in human glaucoma

PURPOSE

Altered metabolic activity has been identified as a potential contributing factor to the neurodegeneration associated with primary open angle glaucoma (POAG). Consequently, we sought to determine whether there is a relationship between the loss of visual function in human glaucoma and resting blood perfusion within primary visual cortex (V1).

METHODS

Arterial spin labeling (ASL) functional magnetic resonance imaging (fMRI) was conducted in 10 participants with POAG. Resting cerebral blood flow (CBF) was measured from dorsal and ventral V1. Behavioral measurements of visual function were obtained using standard automated perimetry (SAP), short-wavelength automated perimetry (SWAP), and frequency-doubling technology perimetry (FDT). Measurements of CBF were compared to differences in visual function for the superior and inferior hemifield.

RESULTS

Differences in CBF between ventral and dorsal V1 were correlated with differences in visual function for the superior versus inferior visual field. A statistical bootstrapping analysis indicated that the observed correlations between fMRI responses and measurements of visual function for SAP (r=0.49), SWAP (r=0.63), and FDT (r=0.43) were statistically significant (all p<0.05).

CONCLUSIONS

Resting blood perfusion in human V1 is correlated with the loss of visual function in POAG. Altered CBF may be a contributing factor to glaucomatous optic neuropathy, or it may be an indication of post-retinal glaucomatous neurodegeneration caused by damage to the retinal ganglion cells.

Quantification of the human lateral geniculate nucleus in vivo using MR imaging based on morphometry: volume loss with age

BACKGROUND AND PURPOSE

Because it is a small subcortical structure, the precise measurement of the human LGN is still a technical challenge. In this article, we identify the LGN in vivo, measure its volume based on high-resolution MR imaging, and then relate its volume to subject age to evaluate the potential clinical application.

MATERIALS AND METHODS

A semiautomatic LGN isolation method was developed on scans obtained with 1.5T MR imaging, which involves highlighting the surrounding landmarks, obtaining candidate LGN voxels with a region-growing algorithm, and isolating the LGN from the ventral diencephalon. The method was accessed with a test-retest reliability on the results from 55 healthy subjects at different ages.

RESULTS

This method showed high test-retest within-subject reliability (ICC, 0.950 and 0.948 in left and right hemispheres, respectively) among 3 independent measurements in each subject. The unilateral volume was highly variable, ranging from 52 to 102 mm(3) in the left and 66 to 105 mm(3) in the right hemisphere, with significantly larger volumes on the right (86 mm(3)) than on the left (77 mm(3)). The combined bilateral volumes (controlled for ICV) significantly decreased in size with progressing age from 20 to 65 years (r = -0.512, P = .000). There was no sex difference in bilateral LGN volumes (male/female: 163.1 ± 18.2/162.2 ± 21.4 mm(3)).

CONCLUSIONS

Using our new technique, we were able to reliably determine the human LGN volume in vivo, which was found to decline with age. The volumes obtained by our method corresponded well with previously reported postmortem values, so our method may be considered to be superior for investigating the pathology of LGN.

Functional assessment of magno, parvo and konio-cellular pathways; current state and future clinical applications

The information generated by cone photoreceptors in the retina is compressed and transferred to higher processing centers through three distinct types of ganglion cells known as magno, parvo and konio cells. These ganglion cells, which travel from the retina to the lateral geniculate nucleus (LGN) and then to the primary visual cortex, have different structural and functional characteristics, and are organized in distinct layers in the LGN and the primary visual cortex. Magno cells are large, have thick axons and usually collect input from many retinal cells. Parvo cells are smaller, with fine axons and less myelin than mango cells. Konio cells are diverse small cells with wide fields of input consisting of different cells types. The three cellular pathways also differ in function. Magno cells respond rapidly to changing stimuli, while parvo cells need time to respond. The distinct patterns of structure and function in these cells have provided an opportunity for clinical assessment of their function. Functional assessment of these cells is currently used in the field of ophthalmology where frequency-doubling technology perimetry selectively assesses the function of magno cells. Evidence has accrued that the three pathways show characteristic patterns of malfunctions in multiple sclerosis, schizophrenia, Parkinson's and Alzheimer's diseases, and several other disorders. The combination of behavioral assessment with other techniques, such as event related potentials and functional magnetic resonance imaging, seems to bear promising future clinical applications.

High-resolution fMRI mapping of ocular dominance layers in cat lateral geniculate nucleus

In this work, we exploited the superior capability of high-resolution functional magnetic resonance imaging (fMRI) for functional mapping of ocular dominance layer (ODL) in the cat lateral geniculate nucleus (LGN). The stimulus-evoked neuronal activities in the LGN ODLs associated with contralateral- and ipsilateral-eye visual inputs were successfully differentiated and mapped using both blood-oxygenation-level dependent (BOLD)-weighted and cerebral blood volume (CBV)-weighted fMRI methods. The morphology of mapped LGN ODLs was in remarkable consistency with histology findings in terms of ODL shape, orientation, thickness and eye-dominance. Compared with the BOLD signal, the CBV signal provides higher reproducibility and better spatial resolvability for function mapping of LGN because of improved contrast-to-noise ratio and point-spread function. The capability of fMRI for non-invasively imaging the functional sub-units of ODL in a small LGN overcomes the limitation of conventional neural-recording approach, and it opens a new opportunity for studying critical roles of LGN in brain function and dysfunction at the fine scale of ocular dominance layer.