Relationship between Arginine Vasopressin and High Signal Intensity in the Pituitary Posterior Lobe on T1-Weighted MR Images in Dogs

Abstract

ABSTRACT. The posterior lobe of the pituitary gland (PL) normally shows characteristic high signal intensity (SI) on T1-weighted MR images (T1WI) in humans. The high SI is thought to represent storage of arginine vasopressin (AVP) in the PL. Normal dogs also show a high SI on T1WIs, but the origin is unclear. In the present study, we investigated whether the high SI in the PL on T1WIs in normal dogs is caused by AVP. We examined the SI in the PL on T1WIs, plasma AVP concentrations and plasma osmolality in normal dogs after excessive AVP secretion was induced by hypertonic saline overload. In addition, functional changes in the supraoptic nucleus and paraventricular nucleus of the hypothalamus under AVP secretion-stimulated conditions were examined immunohistologically. Under hypertonic saline overload, plasma osmolality and plasma AVP concentrations gradually increased, while the SI of the PL gradually decreased. This suggests that AVP secretion was stimulated by elevated osmolality. Moreover, there was a significant negative correlation between plasma AVP concentrations and the SI ratio of the PL. An immunohistochemical study of the hypothalamus nucleus revealed that AVP-immunopositive cells significantly increased in the hypertonic saline loaded dogs. We concluded that the high SI in the PL in T1WIs in normal dogs was caused by AVP stored at the site, and examination of the SI in the PL using MRI is useful for diagnosis of abnormal pituitary glands. KEY WORDS: arginine vasopressin, canine, MRI, pituitary, signal intensity.J. Vet. Med. Sci. 70 (7): [693][694][695][696][697][698][699] 2008 Magnetic Resonance Imaging (MRI) provides detailed anatomic information and is a useful diagnostic modality for a variety of diseases in human and veterinary clinical medicine. In particular, the utility of the MRI has been recognized in examinations of the central nervous system. It has been used not only to examine morphological changes in the hypothalamic-neurohypophyseal (HNS) system, but also to evaluate function by examining signal intensity in images [3]. For these reasons, the MRI is the first-choice modality for evaluation of HNS disease.The posterior lobe of the normal pituitary shows characteristically high signal intensity (SI) in T1-weighted images (T1WI) at two months after birth or later in humans, which differs from the anterior lobe of the pituitary [2,6,20]. In addition, the characteristically high SI is markedly decreased by depletion of arginine vasopressin (AVP) due to disorders such as central diabetes insipidus [8,[17][18][19]. On the basis of this change in T1WIs, the MRI is used for diagnosis of diabetes insipidus [16], ectopic posterior lobe [9,24] and depleted posterior lobe of the pituitary by continuous excessive AVP secretion due to diabetes mellitus [5]. Moreover, the presence and localization of the characteristically high SI (so-called "pituitary flush") can be helpful in diagnosing microadenoma in dogs with pituitary-dependent hyperadrenocorticism without obviously enlarged pituitaries [23]...

Pituitary lacks sexual dimorphism and displays reduced signal intensity on T1-weighted MRI in adolescents with histories of heavy prenatal alcohol exposure

“…Additionally, MRI measurements of the pituitary can provide insight into vasopressin levels stored in its posterior lobe. On T1-weighted MRI the neurohypophysis (i.e., posterior lobe of the pituitary) is hyperintense (i.e., a bright spot), but the signal intensity diminishes after water deprivation (Lee et al, 2001) or infusion of hypertonic saline into the blood stream (Teshima et al, 2008). The neurohypophyseal signal intensity change corresponds to blood plasma levels of vasopressin (Lee et al, 2001; Teshima et al, 2008).…”

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“…In addition, this characteristically high signal intensity is markedly decreased by the depletion of AVP in disorders such as CDI [10,33,34]. In a previous study, we concluded that the high signal intensity in the posterior lobe in T1-weighted images in normal dogs was caused by AVP stored at the site, and there was a significant correlation between the plasma AVP concentrations and the SI ratio in the posterior lobe [37]. In this study, the SI ratio was significantly lower in the CA group than in the control group.…”

“…Plasma arginine vasopressin (AVP) concentrations were assayed using the radioimmunoassay (RIA) double antibody technique (AVP RIA Mitsubishi, Mitsubishi Kagaku Iatron, Tokyo, Japan), as described previously [37]. Before TSS, the blood samples for the assay were collected prior to MRI of the pituitary.…”

“…The transverse images with the largest areas of the posterior lobe were used for measurement of the signal intensity (SI) ratio, as previously described [37]. SI was measured three times each at the posterior lobe and cerebral cortex from the transverse image, and the SI ratio was calculated from the transverse image as follows: SI ratio=average SI of the posterior lobe divided by average SI of the cerebral cortex.…”

Magnetic Resonance Imaging Assessment of Pituitary Posterior Lobe Displacement in Dogs with Pituitary-Dependent Hyperadrenocorticism

“…In the PDH and control groups, the SI ratios of the posterior lobe were measured, as previously described [22]. The SI value was measured at the posterior lobe and the cerebral cortex by MRI computer software on the T1-weighted transverse MR images.…”