Magnetic resonance imaging in the assessment of inflammation in the rat air pouch model of antigen-induced chronic inflammation

Magnetic resonance imaging (MRI) is now an established tool in clinical imaging and compares favourably with conventional x-ray, computerized tomography scanning. Already the ability of MRI to monitor pathological changes in soft and connective tissues has proved valuable in determining the extent and severity of connective tissue inflammation where the presence of the oedema and vascular changes cause a radical alteration of the appearance of tissues in MRI images. However, it remains a new technique, and the areas in which it will be most useful are still being assessed. We have performed a pilot study to determine the ability of MRI to assess the degree and extent of antigen induced inflammation established in the subcutaneous rat air pouch model.Six rats bearing air pouch is were challenged with bovine serum albumin (BSA) (2 mg) in 2% carboxymethylcellulose (CMC) or with CMC alone (controls). One pair of rat had been pre-sensitized with BSA 13 days prior to this. Six days after the final challenge the animals were anaesthetized with phenobarbital and multi slice proton magnetic resonance imaging was carried out on an Oxford Research Systems BIOSPEC imaging spectrometer with a 400 mm horizontal bore superconducting magnet operating at a field strength of 2.3 Tesla. The radio frequency signals from the hydrogen nuclei were detected using a cylindrical coil with an internal diameter of 15 cm specifically designed for rat studies and tuned to 100 MHz. For each animal, a set of 16 transverse images taken, covering the region from the forelegs down to the mid abdomen. Total scan time was less than eight minutes. After imaging the animals were killed by ether overdose, and the complete pouch was removed for histological examination.Delineation of the air pouch morphology in the MRI images was clear, even in control animals, because dermal layers, subcutaneous fat and muscle gave rise to regions of differing signal intensity. In the pre-immunized animals the viscous exudate within the pouch gave rise to very high signal intensities while this was absent in controls. In the BSA challenged groups fibrous tissue was easily visible, its distribution and quantity assessed from the MRI corresponding well with what was subsequently seen in the histology.These results indicate that proton MRI is capable of delineating regions of inflammatory activity and distinguishing between moderate and severe inflammation noninvasively in animal models. MRI may therefore prove to be of value for in vivo monitoring of the effect of different therapies on pain in the inflammatory processes in model systems and also has obvious clinical applications.