Nuclear magnetic resonance imaging (NMRI), usually shortened to magnetic resonance imaging, (MRI), is a non-invasive internal imaging technique used extensively in clinical diagnosis and medical research (Wehrli et al., 1988; Callaghan, 1991). As well as its ability to capture high quality in vitro images, MRI can be performed in vivo without harmful effects (Callaghan, 1991). Here we introduce the principles of magnetic resonance imaging and review its use in entomology. We illustrate the types of images that can be obtained from entomological material both in vitro and in vivo, and we discuss the advantages and disadvantages of MRI over conventional imaging techniques.
Magnetic resonance imaging generally utilizes interactions between protons (1H) in liquid phase molecules (e.g. water and lipids) and magnetic fields. These arise because the protons have a weak magnetic moment, and so behave like tiny magnets. The sample is placed within the bore of a powerful, but biologically harmless, magnet (typically producing a field of 0.5 - 2 Tesla, although magnets up to 17 Tesla strength may be used; as a reference, the Earth's magnetic field is ca. 5 x 10-5 Tesla) and radio-frequency pulses are applied to it. The radio-frequency pulses that must have the appropriate resonant frequency known as the Larmor frequency, interact with protons in the sample, causing them to absorb energy and jump to an excited state. As a result the protons in the sample coherently precess about the applied magnetic field and generate radio waves at the Larmor frequency. This radio-frequency emission is picked up by a receiver coil and the resulting NMR signal is used as the basis for imaging. The NMR signal changes over time depending on the protons' local microenvironment. For example, protons in fats have a different microenvironment than those in water, and thus produce a signal of different frequency. The differences in NMR signals produced by different tissues provide contrast in the image produced. Contrast can also be manipulated by changing the radio-frequency pulse sequence parameters, principally the repetition time and the echo time. In addition to protons, nuclei such as 13C and 31P, also exhibit magnetic resonance and may be used as the basis for imaging.
By applying three orthogonal magnetic field gradients, it is possible to determine the resonance signal from individual volume elements, known as voxels, within the sample. Computer integration and transformation of the signals received from the sample allows a two-dimensional map of proton density to be constructed, which can be visualized as a virtual "slice" through the sample. The resolution of the image is determined by the size of the voxels. Multiple two-dimensional slices through a sample can also be acquireed in different ways to provide a three-dimensional image.