The likely role of MR microscopy is not to supersede the utility of light microscopy, but to add intriguing capabilities beyond and complementary to those of light microscopy. In terms of spatial and temporal resolution, light microscopy is plainly superior to MR for imaging transparent and dissected or vivisected specimens. Furthermore the application of high resolution light microscopic methods to biology is ubiquitous, with new techniques such as ultra thin tissue sectioning and
in vivo two-photon microscopy pushing new bounds in this area
[26],
[27]. However MR does not require light transparency and is also non-deleterious, fitting an alternative role for full-volume,
in vivo imaging of individual specimens, with spectral capabilities not possible by light microscopy.
Drosophila pupae are particularly apt subjects for MR analyses, undergoing development of the adult body within an opaque cocoon; impossible to image live in toto by conventional light microscopy. Pre-pupae naturally affix themselves to glass or plastic surfaces in culture, and are viable in perfluorocarbon oil immersion, used to reduce magnetic susceptibility effects and improving magnetic field homogeneity. The pupal stages are ripe for biological study and, as we have demonstrated, well suited to MR preparation.
MR contrast agents are analogous to optical dye molecules of conventional microscopy. An agent like gadopentate dimeglumine alters relaxivity of resonating nuclei, thereby improving signal with shorter relaxation times (TR). The relaxivities of magnetic resonance contrast agents and the T
1 relaxation time values of tissues are strongly field dependent, with relaxation times being the dominant portion of total acquisition time; thus contrast agents at high field improve definition of a tissue with much shorter acquisition time. Conventional clinical contrast agents like these are used to improve signal-to-noise ratio generally, and in some cases highlight specific tissues or lesions. More exciting are recent advances in contrast agents that include a calcium ion concentration indicator, a UAS/Gal4 gene expression reporter, and an expressible protein contrast agent, analogous in potential utility to the Green Fluorescent Protein type of reporters pervasive in molecular/cellular techniques using light microscopy
[5],
[6].
On the issue of potential applications of MR techniques to
Drosophila, there are some conceivable directions to pursue. For example, the ability to image and quantify a neurotransmitter such as GABA, and couple this ability with existing techniques such as high-throughput (microarray) gene expression data, mutant studies, and RNA interference techniques, would yield a new totality of information with potential for improving and rapidly integrating human disease models. Previously, many molecules like GABA have been found to be difficult or impossible to detect amongst the complex milieu of chemical resonance signatures
in vivo, but development of spectral editing methods show that GABA, and other previously undetectable molecules, are quantifiable in living cells
[12]. Another speculative possibility to consider is the use of MR contrast agent-labelled insecticide compounds since insecticidal compounds have been intensely studied and bind to known ion channels in cell membranes. The use of these compounds as targeted labels of fly homologues to human receptors might comprise an intriguing tool for research, particularly when coupled with the fly model's existing strength in genomic and molecular approaches. Embryonic-stage flies (eggs) provide a rich area for further research due to being famously well studied by other methods, and are also viable in halocarbon oil. Embryos present a more challenging starting point for MR microscopy than
Drosophila pupae or adults due to their very small dimensions. However, at about 500×200 µm, fly embryos approach the size range of cells imaged in at least one prior study
[11], indicating that embryos may yet be feasible candidates for MR imaging and spectroscopy. Notably, the fly embryo undergoes a pre-cellularized, multinucleate (syncitial) stage of development, which is extremely advantageous for transfection techniques introducing artificial constructs into cells of the fly. While there have been advances, the utility of MR contrast agent indicators of cellular physiology and gene expression has been limited by the administration of contrast agents to the interior of cells. It remains to be seen whether the fly's syncitial development can be used to similar advantage in overcoming this bottleneck.
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