Magnetic Resonance Imaging of the Harderian Gland in Piglets

Abstract

The main purpose of the present study was to investigate the suitability of functional and morphological magnetic resonance imaging (MRI) to assess the extent of brain injury in a hypoxia‐ischemia piglet model and further to validate that the ischemic injury was successfully induced. MR imaging was performed at 1.5 T (Siemens Vision, Siemens Medical, Erlangen, Germany) in piglets (n = 10; age 12–36 h) under general anaesthesia. Hypoxia was induced by ventilation of a gas mixture containing 8% O2 in N2 (AGA, Oslo, Norway). Ischemia was induced by bilateral clamping of the common carotid arteries (CCA). HI was maintained for 30 min, followed by re‐oxygenation with ambient air (21% O2) for 2.5 h. Relative cerebral flow (rCF) was estimated using a single shot SE‐EPI sequence (TE = 85 ms, voxel size 1 × 1 × 5 mm3), measuring the first‐pass response following bolus injection of an intravascular iron oxide nanoparticulate contrast agent (NC100150 Injection; Amersham Health, Oslo, Norway) at a dose of 4.5 mg Fe/kg. rBF maps were generated on a pixel‐by‐pixel basis from the peak height of a gamma‐variate function fitted to the first‐pass contrast agent response. The presence of total carotid occlusion was assessed by MR angiography (MRA). MRA images (3D‐GRE, TR/TE 4.4 ms/1.8 ms, voxel size 0.9 × 0.9 × 1.5 mm3) were acquired after contrast administration; utilizing the T1‐effect of NC100150 injection. Mean blood pressure and heart rate were measured continuously during the experiment via a catheter placed in the femoral artery. Blood gasses and rectal temperature were measured at regular intervals. At the end of the experiment, the piglets were given an overdose of 150 mg/kg Pentobarbital i.v., and a dissection was performed in order to identify structures identified on the MRI. MR angio (MRA) and volume rendering identified a prominent, highly vascularized, defined structure close to the eyeball, medioventrally. At dissection, when comparing with the MR images, the deep gland of the third eyelid, the Harderian gland, corresponded to this structure both by topography and size. It was thick, brownish and finely lobulated, and extended from the cartilage of the third eyelid towards the origin of the extrinsic eye muscles at the apex of the orbit. In contrast, the lacrimal gland presented as a small, soft and pale structure that was difficult to distinguish from the surrounding connective tissue. At histological examination, the Harderian gland consisted mostly of compact areas of alveolar glands with abundant eosinophilic granules. Wide thin‐walled vessels were conspicuous in the interlobular connective tissue and the capsule. The present MRI demonstration of the Harderian gland was an accidental finding during an investigation to assess the extent of brain injury in a hypoxia‐ischemia piglet model. In pig, this gland has received little attention in the literature. However, taking its size and blood supply into consideration, it must be of significant importance also in the pig. It is mostly studied in rodents and lower vertebrates and is found to possess various endocrine functions as well as exocrine.