Chia-Shang J. Liu

Pediatric Perfusion Imaging Using Pulsed Arterial Spin Labeling

To test the feasibility of pediatric perfusion imaging using a pulsed arterial spin labeling (ASL) technique at 1.5 T.

Decreased Activation of the Lateral Geniculate Nucleus in a Patient with Anisometropic Amblyopia Demonstrated by Functional Magnetic Resonance Imaging

Although postmortem morphological changes in the lateral geniculate nucleus (LGN) have been reported in human amblyopia, LGN function during monocular viewing by amblyopic eyes has not been documented in humans. We used functional magnetic resonance imaging (fMRI) to study monocular visual activation of the LGN in a patient with anisometropic amblyopia. Four normal subjects, a patient with optic neuritis and a patient with anisometropic amblyopia were studied with fMRI at 1.5 T during monocular checkerboard stimulation. Activated areas in the LGN and visual cortex were identified after data processing (motion correction and spatial normalization) with SPM99. In the 4 normal subjects, comparable activation of the LGN and visual cortex was obtained by stimulation of either the right or left eye. In the patient with unilateral optic neuritis, activation of the LGN and visual cortex was markedly decreased when the affected eye was stimulated. Similarly, decreased activation of the LGN as well as the visual cortex by the affected eye was demonstrated in the patient with anisometropic amblyopia. Our preliminary results suggest that activation of the LGN is diminished during monocular viewing by affected eyes in anisometropic amblyopia. fMRI appears to be a feasible method to study LGN activity in human amblyopia.

Functional magnetic resonance imaging of the visual system.

Functional magnetic resonance imaging (fMRI), which is a technique useful for non-invasive mapping of brain function, is well suited for studying the visual system. This review highlights current clinical applications and research studies involving patients with visual deficits. Relevant reports regarding the investigation of the brains role in visual processing and some newer fMRI techniques are also reviewed. Functional magnetic resonance imaging has been used for presurgical mapping of visual cortex in patients with brain lesions and for studying patients with amblyopia, optic neuritis, and residual vision in homonymous hemianopia. Retinotopic borders, motion processing, and visual attention have been the topics of several fMRI studies. These reports suggest that fMRI can be useful in clinical and research studies in patients with visual deficits.

Functional Magnetic Resonance Imaging of Eye Dominance at 4 Tesla

We studied eye dominance in visual cortex and lateral geniculate nucleus (LGN) using functional magnetic resonance imaging (fMRI) at a very high magnetic field (4 tesla). Eight normal volunteers were studied with fMRI at 4 tesla during alternating monocular visual stimulation. The acquisition was repeated twice in 4 subjects to confirm reproducibility. In addition, magnetic resonance signal intensities during three conditions (right eye stimulation, left eye stimulation, and control condition) were compared to determine whether the observed area was truly or relatively monocular in 2 subjects. In both the individual and group analyses, the anterior striate cortex was consistently activated by the contralateral eye more than the ipsilateral eye. Additionally, we found evidence that there were areas in the bilateral LGN which were more active during the stimulation of the contralateral eye than during the stimulation of the ipsilateral eye. The activated areas were reproducible, and the mean ratio of the overlapping area was 0.71 for the repeated scans. The additional experiment revealed that the area in the anterior visual cortex could be divided into two parts, one truly monocular and the other relatively monocular. Our finding confirmed previous fMRI results at 1.5 tesla showing that eye dominance was observed in the contralateral anterior visual cortex. However, the eye dominance in the visual cortex was found not only in the most anterior area corresponding to the monocular temporal crescent but also in the more posterior area, presumably showing the greater sensitivity of the temporal visual field (nasal retina) as compared with the nasal visual field (temporal retina) in the peripheral visual field (peripheral retina). In addition, it is suggested that the nasotemporal asymmetry of the retina and the visual fields is represented in the LGN as well as in the visual cortex.

Decreased Lateral Geniculate Nucleus Activation in Retrogeniculate Hemianopia Demonstrated by Functional Magnetic Resonance Imaging at 4 Tesla

Functional magnetic resonance imaging (fMRI) can detect lateral geniculate nucleus (LGN) activation. We studied LGN function in 5 patients with retrogeniculate homonymous hemianopia using fMRI at 4.0 Tesla during binocular visual stimulation. Decreased activation of visual cortex and LGN on the side of the lesion was observed in all 5 patients. These findings suggest that retrogeniculate lesions are associated with decreased activation of the LGN, due to retrograde degeneration or a functional decrease caused by decreased feedback from ipsilateral visual cortex.

Magno- and Parvocellular Visual Cortex Activation in Anisometropic Amblyopia, as Studied with Functional Magnetic Resonance Imaging

Purpose: We studied the functional magnetic resonance imaging (fMRI) of the visual cortex in order to activate preferentially the parvocellular (PC) or the magnocellular (MC) visual system by manipulation of the spatiotemporal characteristics of the stimuli. Then we applied this technique to a patient with amblyopia to see how these two visual systems are affected in amblyopia. Methods: We acquired the fMRI at 1.5 T in 8 normal subjects and in one patient with anisometropic amblyopia, each receiving their best refractive correction. Each subject underwent experimentation under five conditions. The MC stimulus had a low contrast, a large check size, and a high temporal frequency. The PC stimulus had a high contrast, a small check size, and a low temporal frequency. After the activation in each condition had been determined by contrasting the visual stimulation conditions with the condition at rest, the inter-eye difference was determined. Results: In normal subjects, the activation map showed different visual areas as well as some overlap between the MC and PC stimuli, and similar visual cortex activations were obtained under the right-eye and the left-eye stimulation conditions. In the patient with amblyopia, the inter-eye difference was within a 95% confidence interval for the MC stimulation, but not for the PC stimulation. Conclusions: Our fMRI results show a good localization of the activity in the MC and PC pathways, and they are consistent with the findings in the literature, suggesting that the PC visual pathway is primarily defective in amblyopia.

Contralateral monocular dominance in anterior visual cortex confirmed by functional magnetic resonance imaging

PURPOSE Although it is known that the damage to anterior striate cortex results in temporal peripheral visual field loss of the contralateral eye in patients with cerebral visual disturbance, the monocularity of anterior striate cortex has not been demonstrated in normal living humans. The aim of this study was to investigate whether this could be shown noninvasively using functional magnetic resonance imaging of the human visual cortex. METHODS Eleven normal volunteers were studied with functional magnetic resonance imaging during alternating monocular visual stimulation using a 1.5 Tesla scanner. The data were motion corrected and spatially normalized to the standard brain. The monocular activation of the visual cortex was compared with the activation by the other eye. RESULTS In the individual data analysis, contralateral eye dominance was always observed in the anterior striate cortex. In the group analysis from 11 subjects, the area with contralateral eye dominance was found in the most anterior part of primary visual cortex where the calcarine fissure merged with the parieto-occipital sulcus. CONCLUSIONS This study shows that the contralateral eye dominance of anterior striate cortex can be detected noninvasively with functional magnetic resonance imaging during monocular visual stimulation. The finding confirms that the anterior striate cortex, where the monocular temporal crescent is represented, is primarily monocular, but the fact that greatest density of retinal ganglion cells and photoreceptors is in the nasal hemiretina must also be taken into account when interpreting these results.

Recognition of objects in non-canonical views: a functional MRI study.

Background: The neural correlate of object recognition in non-canonical views is uncertain, but there is evidence for involvement of neural pathways, possibly separate from those used for object recognition in canonical views. Methods: Boxcar functional MRI (fMRI) techniques were used to detect neural activity while eight normal subjects were instructed to identify digital photographs of objects in non-canonical and canonical orientations. Results: The right angular gyrus, the left inferior temporal gyrus, and the right cerebellum showed significant fMRI activity during non-canonical as opposed to canonical viewing. Conclusions: Subjects recognizing objects in non-canonical orientations engage in a process separate from, or in addition to, the process used in recognizing objects in canonical orientations.

Functional magnetic resonance imaging of lateral geniculate nucleus and visual cortex at 4 Tesla in a patient with homonymous hemianopia

We present the functional magnetic resonance imaging findings at 4 Tesla in a 49-year-old male patient with a right thalamic tumor and left homonymous hemianopia. Markedly asymmetric activation of the lateral geniculate nucleus and visual cortex was found, and a pregeniculate or geniculate lesion was suspected from these findings. Functional magnetic resonance imaging at a high magnetic field can be useful in localizing a lesion responsible for visual loss.

Functional magnetic resonance imaging of lateral geniculate nucleus at 1.5 tesla.

Although activation of the lateral geniculate nucleus has been detected by functional magnetic resonance imaging with magnetic field strengths higher than 2.0 Tesla, there have been no reports of functional magnetic resonance imaging of the lateral geniculate nucleus with the more widely available 1.5 Tesla scanner. The authors used functional magnetic resonance imaging techniques at 1.5 Tesla to detect lateral geniculate nucleus activation in five of seven healthy subjects. This study shows that visual activation of the lateral geniculate nucleus can be obtained with functional magnetic resonance imaging using conventional 1.5 Tesla scanners.