HEAD AND NECK 39
iodine content. The surrounding structures of the neck may also be imaged. Imaging with short scan times during dynamic infusion of contrast gives the best images and definition of the gland and surrounding structures. Magnetic resonance imaging MRI can image the gland in any plane along with the surrounding structures. The gland is of higher signal intensity than surrounding muscles on T2-weighted images. The use of surface coils improves detail. The parathyroid glands These endocrine glands are small lentiform structures measuring approximately 6 mm in length, 3-4 mm in transverse diameter and 1-2 mm in AP diameter. They usually number four - two superior and two inferior - but any number from two to six is possible. The glands lie posterior to the thyroid gland within its fascial sheath in 90% of cases. The superior glands lie on the posterior border of the middle third of the thyroid, and the inferior glands lie near the lower pole of the thyroid. The superior gland develops from the fourth pharyngeal pouch and does not migrate. The inferior parathyroid gland develops from the third pharyngeal pouch with the thymus and migrates inferiorly Maldescent may cause the inferior parathyroid gland to be found in ectopic sites, and this may be of clinical importance in the search for a parathyroid adenoma. The most common ectopic site is just below the inferior pole of the thyroid. Occasionally the gland descends into the superior mediastinum with the thymus. Less commonly it does not descend at all and remains above the superior parathyroid, or it may be found behind the oesophagus or in the posterior mediastinum. Most of the blood supply of the parathyroid glands is derived from the inferior thyroid gland. Radiology of the parathyroid glands Cross-sectional imaging The normal parathyroids are not seen radiologically. They may be imaged when enlarged. On ultrasound they are seen as hypoechoic structures lying posterior to the thyroid gland. On CT the parathyroids appear as structures of lower attenuation than thyroid tissue. Nuclear medicine studies The parathyroids may be imaged using a subtraction technique. Both the thyroid and the parathyroid glands take 201 up thallium-201 ( Tl) chloride. The thyroid (but not the 99m parathyroids) takes up Tc pertechnetate. By computerized subtraction of the technetium image from the thallium image, the parathyroids may be demonstrated.
THE NECK VESSELS The carotid arteries in the neck (Figs 1. 45-1. 52) The left common carotid artery arises from the aortic arch in front of the trachea and passes across this to lie on its left side in the root of the neck. The right common carotid arises from the brachiocephalic trunk behind the right sternoclavicular joint. From this point the vessels have a similar course. The common carotid artery passes upwards and slightly laterally. It is accompanied by the internal jugular vein on its lateral aspect, with the vagus nerve lying posteriorly between the two. All three structures are invested in the carotid sheath. The common carotid artery bifurcates into internal and external branches at the level of C4. The external carotid artery passes anteriorly and curves slightly posteriorly as it ascends to enter the substance of the parotid gland, where it terminates by dividing into maxillary and superficial temporal arteries. The internal carotid artery continues superiorly from its origin to the base of the skull, maintaining the relationship of the common carotid artery with the internal jugular vein and vagus nerve in the carotid sheath. It has a localized dilatation at its origin called the carotid sinus. It has no branches in the neck. Anatomical relations of the common carotid artery within the carotid sheath The anatomical relations are as follows: • Posteriorly is the sympathetic trunk. It is separated from the transverse processes of C4 to C6 by the prevertebral muscles.
