PARATHYROID AND THYROID IMAGING - Neuroradiology

PARATHYROID AND THYROID IMAGING 437Parathyroid imaging is controversial not onlyfrom the standpoint of the indications for imagingbut also from the issue of which modality (if any)to choose. In most institutions, preoperative localizationof the parathyroid glands by imaging is notperformed before the first surgery (for previouslyoperated patients, see subsequent discussion). Thisstems from the early surgical literature that suggeststhat operative time, morbidity, and mortality is notsignificantly influenced by preoperative localizationof parathyroid adenomas for hyperparathyroidism.7'a6,62 The surgical exploration entails bilateraldissection of the perithyroidal region, emphasizingthe inferior poles, where most parathyroid adenomasoccur. In experienced hands, this surgical procedurecan be performed quickly and accuratelywith success rates of over 90Vo.12'5e,62,65,84 This hasled Doppman84 to state that the best localizationprocedure a patient can obtain for parathyroid adenomasis to locate "an experienced parathyroidsurgeon."Proponents of preoperative localization of parathyroidadenomas even in unoperated cases cite(1) the need for only unilateral dissections when anadenoma is evident on imaging; (2) the identificationof ectopic adenomas preoperatively, allowingbetter planning and patient education; (3) detectionof other head and neck masses that mav reouiretreatment at the same time (e.g., thyroid masies);and (4) the reduction in operating room time, recurrentlaryngeal nerve paralysis, and postoperativehypoparathyroidism when preoperative imaging isperformed.38'43'63'6'86 In two studies bv Russell andtasas and their colleagues, the differ6nce betweenmean operating times with (71 minutes and 135minutes) and without (97 minutes and 180 minutes)preoperative imaging justified the cost of the imagingtest.8'63 The operative success rate also improvedfrom 90% to 1,00Vo with preoperative imaging.8Uden also noted that the time for surgeryand anesthesia decreased with preoperative imaging;however, when a cost-benefit analysis wasperformed, he found that the cost of the imagingprocedure outweighed its benefit.8T A reduction of28 minutes of operating room time in the study byRoe and colleagues did not justify the $901 meancost of localization.62 Other surgeons take a centristposition regarding bilateral or unilateral neck explorations.They perform unilateral neck dissectionsif imaging studies are definitive but convert to bilateralsurgery if (1) imaging is equivocal or showsmultifocal abnormality, (2) more than one enlargedgland is identified at surgery, (3) the patient hasa multiple endocrine neoplasia (MEN) syndrome(often associated with parathyroid hyperplasia), or(4) a unilateral exploration is unrevealing.63When a parathyroid adenoma is not identified ina stereotypical perithyroidal location, the surgeonmay empirically explore the anterior mediastinum,deep cervical space, periesophageal grooves, or carotidsheath region. The yield of surgery in thisscenario is much lower (907osuccess rate), and the surgiial complicationrate increases with such blind exoorations.l2The intrathvroidal parathvroid adenomi. which accountsfor a small'percentage of cases, cannot bereadily distinguished from thyroid adenomas andposes a particularly difficult problem.a5'84 To confusematters further, thyroidal abnormalities occur in asmany as 40% to 48Vo of patients with hyperparathyroidism.la5e. e5 These factors have led lessexperiencedsurgeons and those who have had aIess successful track record to choose preoperativelocalization of parathyroid adenomas.Parathyroid AdenomasExactly how accurate are the imaging studiesfor detecting parathyroid adenomas? On ultrasonography,parathyroid adenomas appear as oval,oblong, or bulbous lesions with echogenicity lessthan that of the thyroid gland (Fig. t;.s0, u Usinghigh-resolution ultrasound in a study of more than150 patients, a sensitivity of 64Vo and specificityof 94Vo for adenomas and hyperplastic glands wasachieved.5e Of those glands greater than 1 g in size,ultrasonography had a detection rate of 95%.se Otherinvestigators also quote sonographic sensitivities ofFigure 1. Ultrasonogram of parathyroid adenoma. Deepto the thyroid tissue, one can see a hypoechoic mass(arrows) representing a parathyroid adenoma.