Methods and apparatus for monitoring oxygen saturation levels in tissue are disclosed. According to one aspect of the present invention, a sensor arrangement for use in an optical imaging system includes a first source structure, a second source structure, and a detector arrangement. The first source structure provides a first beam of light and the second source structure provides a second beam of light. The detector arrangement includes detector structures that have centerpoints, and receives the first and second beams of light after the first and second beams of light are reflected off of an external surface. The detector arrangement is arranged to define a first axis that passes through the centerpoint of each detector structure, and a distance from a centerpoint of the first source structure to the first axis is not equal to a distance from a centerpoint of the second source structure to the first axis.
