Beyond BOLD: Expanding the role of fMRI in low vision rehabilitation

Abstract

Purpose: Retinitis Pigmentosa (RP) is a common cause of peripheral first and then central visual field loss [1]. Virtually all fMRI studies of visual field loss utilize BOLD contrast despite longitudinal and inter-subject variability [2]. However, recent studies in animals and healthy humans demonstrate cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO₂) measures are more consistent over time within subjects and also between subjects [2–4]. The current study provides the first MRI application of CBF and CMRO₂ estimation in the low vision population providing a platform for future rehabilitation and cortical reorganization studies.

Methods: Two RP patients (age 34,45, visual acuity 20/90,110, central visual field 15, 5 deg.) participated in a monocular study. Four flashing stimuli (rings at two eccentricities and checkerboards at two resolutions) were used as subjects fixated on a central target. Anatomical and functional (simultaneous CBF and BOLD images) were acquired with a Siemens 3T Trio [5]. Data were analyzed using Matlab with SPM2 functionality, estimating CMRO₂ using a highly respected model [6].

Results: Activation maps demonstrate CBF's improved spatial specificity relative to BOLD. Furthermore, % ΔCMRO2 estimates, % ΔBF and absolute CBF results agree with previous studies that showed closer association with neuronal activity than BOLD alone.

Conclusions: The current work demonstrates for the first time that fMRI can be used to measure absolute and relative CBF and estimate relative CMRO2 in people with visual field loss. Integration of these techniques into future studies will demonstrate their benefit in rehabilitation intervention and cortical reorganization studies.

References:

[1]. National Advisory Eye Council. Bethesda: Inst. Health, MD; DHHS Publ. No. 93-186.[2]. Tjandra TI. NeuroImage.2005. 27:393-401. [3]. Wong EC. Functional MRI. 1999. 63-69. [4]. Sicard KM. NeuroImage. 2005. 25:850-858. [5]. Alsop DC. Radiology. 1998. 208:410-416. [6]. Davis TL. Proc Natl Acad Sci USA. 1998. 95:1834-1839.

Footnotes

 Supported by VA Rehabilitation R&D Service and NIH RO1 EB002009.