Page 254 - Atlas of Small Animal CT and MRI

2.9

Sella and parasellar region

Normal pituitary gland of the neurohypophysis, followed by a slightly delayed
peripheral enhancement of the adenohypophysis with
The sella turcica is the osseous boundary of the pituitary diminished central enhancement.
gland. It is comprised of the pituitary fossa ventrally and On MR images, the normal pituitary gland usually
the rostral and caudal clinoid processes dorsally. The has a focal T1 hyperintensity, which is thought to
pituitary gland is located in the pituitary fossa of the represent either vasopressin‐containing neurosecretory
basisphenoid bone. The pituitary gland is formed of two granules or glial cell lipid droplets in the neurohypophy­
parts: the vascular, glandular adenohypophysis and the sis. T2 intensity is similar to cortical gray matter. The
2
neurohypophysis. It is suspended from the hypothala­ lipid‐rich marrow of the basisphenoid bone is hyperin­
mus by the infundibulum, which courses through an tense on both T1 and T2 images (Figure 2.9.2).
incomplete dural septum covering the dorsal aspect of Normal canine pituitary size on CT images is approxi­
the fossa. The ventral aspect of the third ventricle mately 4.5 mm in height and 6 mm in width. On MR
extends centrally through the infundibulum to the prox­ images, it is reported to be 5.1 ± .9 mm in height and
imal neurohypophysis. 1 6.4 ± 1.0 mm in width, with little correlation to brain
The pituitary gland is perfused by branches of the measurements or body weight. Normal feline pituitary
3,4
internal carotid and communicating arteries of the circle size has been estimated to be approximately 5 mm
of Willis, with venous drainage into the cavernous and in height and 3.5 mm in width on both CT and MR
intercavernous sinuses. The optic chiasm is located imaging. While these dimensions may be useful as
5,6
immediately rostral to the origin of the infundibulum, general guidelines, they are not particularly useful for
and the third cranial nerves arise caudal to, and course diagnosis of microadenomas. With both imaging
lateral to, the pituitary fossa. 1 modalities, the presence of a prominent convexity to the
On unenhanced CT images, the pituitary gland is dorsal pituitary margin and elevation of the dorsal
isoattenuating to deep gray matter and contiguous with margin above the sella turcica on a sagittal plane image
the adjacent hypothalamus, with ventral margins well are additional qualitative imaging features that are
delineated by the basisphenoid bone. When the third suggestive of a pituitary disorder.
ventricle is prominent, a relatively hypoattenuating
infundibular recess may be visible extending to the Dynamic CT and MR protocols
proximal neurohypophysis. On contrast‐enhanced CT
and MR images, the pituitary gland markedly contrast Dynamic contrast CT protocols have been used to detect
enhances compared to brain tissue because of a rich small (micro) hypophyseal masses. However, the current
vascular supply (Figure 2.9.1). Dynamic contrast imag­ trend in human and veterinary medicine has been
ing studies of the normal pituitary describe an initial toward the use of MR for diagnosis and characterization
central contrast blush, attributable to early enhancement of pituitary disorders. Dynamic CT and MR procedures

Atlas of Small Animal CT and MRI, First Edition. Erik R. Wisner and Allison L. Zwingenberger.
© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.
244