**1. Introduction**

Noninvasive parathyroid imaging studies include technetium (99mTc) sestamibi scintigraphy, ultrasonography (US), computed tomography (CT) scanning, magnetic resonance imaging (MRI) and positron emission tomography (PET). Parathyroid glands need to be examined in case of a diagnosed hyperparathyroidism as a part of preoperative localization of the abnormal glands. Hyperparathyroidism is characterized by elevated parathyroid hormone (PTH) levels in the blood. Due to the underlying cause, it can be divided into primary and secondary. The primary hyperparathyroidism (PHPT) is due to excessive production of PTH from one or more abnormal parathyroid glands. Secondary hyperparathyroidism (SHPT) is a result of hypocalcemia caused by other concomitant diseases (end stage kidney renal disease, etc.). In SHPT usually more than one parathyroid glands are affected. Considered rare disease in the past, the incidence of PHPT has changed dramatically during the last 30 years with the introduction of routine calcium measurements in clinical practice, and is now considered to be approximately 42 per 100,000 persons. Women are affected more frequently than

men, in a ratio of approximately 3:1. PHPT occurs predominantly in individuals in their middle years with a peak incidence between ages 50 and 60 years and can reach 4 cases per 1000 persons in women after their 60s. At the time of diagnosis, most patients with PHPT do not have classic symptoms like osteitis fibrosa cystica, nephrocalcinosis, nephrolithiasis or other signs associated with the disease. Symptomatic PHPT is now exception rather than the rule, with more than threefourths of patients having no symptoms making detected changes of the blood values of calcium, phosphorus and parathyroid hormone (PTH) to be the only reason for diagnosis [1, 2]. By far, the most common lesion found in patients with PHPT is the solitary parathyroid adenoma, occurring in 85–90% of patients, while in the rest 10–15% primary hyperplasia of the parathyroid glands is present [3]. In the past the standard surgical approach for PHPT was the bilateral four-gland parathyroid exploration with the removal of each gland which showed changes macroscopically. While in most of the patients with PHPT only one parathyroid gland is being affected, the above mentioned surgical approach is inappropriate in all cases. Unilateral approaches are appealing in a disease in which only a single gland is involved. So nowadays, the currently most widely used surgical approach is the minimally invasive parathyroidectomy which is connected with less postsurgical complications and shortens the time of operation [4]. To be successful this procedure needs to rely on a precise preoperative localization of the abnormal parathyroid glands. That is, why preoperative parathyroid imaging gained so large importance. The rationale for locating abnormal parathyroid glands prior to surgery is that they can be notoriously unpredictable in their location.
