A noncontact optical detector for in vivo imaging has been developed that is compatible with magnetic resonance imaging (MRI). The optical detector employs microlens arrays and might be classified as a plenoptic camera. As a resulting of its design, the detector possesses a slim thickness and is self-shielding against radio frequency (RF) pulses. For experimental investigation, a total of six optical detectors were arranged in a cylindrical fashion, with the imaged object positioned in the center of this assembly. A purposely designed RF volume resonator coil has been developed and is incorporated within the optical imaging system. The whole assembly was placed into the bore of a 1.5 T patient-sized MRI scanner. Simple-geometry phantom studies were performed to assess compatibility and performance characteristics regarding both optical and MR imaging systems. A bimodal ex vivo nude mouse measurement was conducted. From the MRI data, the subject surface was extracted. Optical images were projected on this surface by means of an inverse mapping algorithm. Simultaneous measurements did not reveal influences from the magnetic field and RF pulses onto optical detector performance (spatial resolution, sensitivity). No significant influence of the optical imaging system onto MRI performance was detectable.
In order to validate and to optimize the imaging capabilities of a micro-lens-array (MLA) based optical detector dedicated for preclinical in-vivo small animal imaging applications a numeric investigation framework is developed. The framework is laid-out to study the following MLA detector parameters: micro-lens diameter (D) and focal length (f), as well as sensor pixel size (A). Two mathematical models are implemented for light modeling: line-based and cone-based ray projections. Since the MLA detector requires mathematical postprocessing, specifically inverse mapping for image formation, the framework is fully integrated into such approach. MLA detector designs have been studied within valid parameter ranges yielding sub-millimeter spatial resolution for in vivo imaging of mice for detector-object-distances (t) up to 50 mm. In summary, there is a non-linear dependency of the detector's spatial resolution, scaling with D and f, for any respective t. On the other hand, detector efficiency is strongly dependent on f. Regardless of mathematical postprocessing the following set of intrinsic detector parameters had been found optimal for the intended application: D = 0.336 mm, f = 4.0 mm, A = 0.048 mm.
When mathematical postprocessing is involved, particularly three-dimensional surface recognition, increasing f (cf. decreasing D) yields solid angles of the incoming rays closer to 90° and, thus, will decrease spatial depth information from the elementary images. Hence, a setup with D not larger than 0.5 mm and f between 2.0 mm and 3.0 mm is recommended.