Diagnostic imaging of the pituitary gland in dogs

Abstract

Pituitary-dependent hyperadrenocorticism (PDH) is a frequently encountered endocrinopathy in dogs. Transsphenoidal hypophysectomy is an effective treatment of PDH in dogs. However, for pituitary surgery to be successful information about the size of the pituitary gland and the exact location of the pituitary gland in relation to the surgical (bony) landmarks is essential. Because CT dimensions of the pituitary gland have been used frequently, but no information was available on the reproducibility and the accuracy of these values, this has been determined in healthy dogs. CT measurements of the pituitary gland had a low intra- and interobserver variability. The CT dimensions were, however, systematically underestimated when compared with true dimensions in a phantom study. For accurate measurements different window settings have to be used to identify the different tissue interfaces between pituitary gland, brain and bone. Microadenomas, that do not affect the size and shape of the pituitary gland, may not be detected because of isoattenuation of the adenoma and the surrounding unaffected pituitary tissue. Dynamic contrast-enhanced CT, which includes a series of scans of identical slice thickness, at the same slice position through the center of the pituitary gland, during and after IV injection of a bolus of contrast medium, has been described in healthy dogs and dogs with PDH, which subsequently underwent transsphenoidal hypophysectomy. In healthy dogs, dynamic CT of the pituitary gland reveals the difference in enhancement between the neurohypophysis, seen as the pituitary flush, and the adenohypophysis. In dogs with PDH, it has been found that displacement, distortion and disappearance of this pituitary flush is diagnostic for an abnormal pituitary gland. Displacement and distortion of the pituitary flush reveals the location, size and shape of the pituitary adenoma. For more accurate pre-surgical localisation of the adenoma, a dynamic CT examination of the entire pituitary gland, rather than a single thin slice, should be performed. Therefore, dynamic CT examination of the entire pituitary gland has been described in healthy dogs, using helical CT. It has been found that dynamic helical CT can be used for visualisation of the enhancement pattern of the entire pituitary gland and also gives information about the surrounding vasculature of the pituitary gland. Magnetic resonance imaging (MRI) is the imaging modality of choice for the evaluation of the pituitary gland in humans. A protocol for pituitary gland imaging with an 0.2 T open MRI scanner has been developed. Thin slice MRI imaging of the pituitary gland in dogs using an open 0.2 Tesla MRI scanner is best performed using 3D gradient echo sequences in transverse and/or dorsal imaging planes, both before and after contrast medium injection. Both dynamic helical CT and MRI of the pituitary gland may give valuable information on the pituitary gland in dogs. A comparative study between helical CT and MRI should be performed in dogs with PDH to assess which of the techniques gives the most valuable information, with special reference to the bony surgical landmarks.