# Detection and Localisation of Abnormal Parathyroid Glands: An Explainable Deep Learning Approach
## 1 Introduction  
Parathyroid adenoma belongs to parathyroid proliferative disorders, including parathyroid hyperplasia, parathyroid adenoma, and parathyroid carcinoma [1]. Approximately 85 per cent of primary hyperparathyroidismicism (HPPT) is caused by a parathyroid adenoma, followed by parathyroid hyperplasia with a percentage of 15. Parathyroid carcinoma is rare [2]. However, recent evidence no longer supports the entity of hyperplasia in primary HPPT with multiple abnormal glands and, in this setting, suggests the presence of two or more parathyroid adenomas. Therefore, according to the WHO 2022 classification, the term \"hyperplasia\" should be confined to secondary HPPT, while primary HPPT should be replaced by \"primary HPPT-related multiglandular parathyroid disease\" [3]. Severe secondary HPPT is caused primarily by end-stage renal failure. In this situation, all parathyroid glands (PGs) are enlarged, each to a different degree. Tertiary HPTT denotes the persistence of HPPT after successful renal transplantation. Despite the current use of calcimimetic drugs, which succeed in lowering serum calcium and parathyroid hormone levels, the definitive cure of HPPT is the surgical excision of abnormal PGs. The surgical approach relies heavily on the imaging modalities' pre-operative localisation of enlarged glands. Preoperative localising methods include neckultrasound (U/S), parathyroid scintigraphy, dynamic contrast-enhanced computerised tomography, 4-D CT, and magnetic resonance imaging (MRI). Depending on local experience and expertise, U/S and scintigraphy are used first, while 4-D CT and MRI are usually reserved for negative or ambiguous cases. The radiotracer \({}^{99\text{m}}\)Tc-Sestamibi (MIBI) is injected to perform the dual-phase method in parathyroid scintigraphy. This method involves early acquisition and late acquisition image of the neck and the mediastinum. The early image is acquired 10 min after the MIBI administration and the late image is acquired 2 h post-injection. MIBI uptake by the thyroid gland in the early image may impede the detection of an adenoma. In many abnormal PG cases, prolonged tracer retention is observed. False negative scans may appear due to false clearance of MIBI from particular adenomas or hyperplastic glands. In some cases, the thyroid subtraction method is followed. The latter involves the administration of a second radioactive tracer \({}^{123}\)I or \({}^{99\text{m}}\)Tc-pertechnate for alienating the thyroid gland. The digital subtraction of the thyroid image from the early MIBI and the late MIBI images produces the result. Besides planar images, SPECT or SPECT/CT can increase the method's sensitivity and offer a more precise localisation of findings in the 3-D space [4]. Computer-aided diagnostic (CAD) assistance in parathyroid adenoma identification could alleviate human tiredness and routine in everyday clinical practice, allowing medical staff to concentrate on other tasks. Still, human expertise is indispensable to evaluating computer suggestions, which is a much simpler task. Deep learning (DL) methods have showed great performance in classification and object detection challenges involving medical images. This is due to the encapsulation of myriad potentially essential image features. An intuitive example of the DL implementation is cancer detection frameworks [5; 6; 7]. Recent clinical studies report novel optical technologies that enhance PGs' localisation or viability assessment. These technologies could complement the surgeon's eyes and improve surgical outcomes in thyroidectomy and parathyroidectomy [8]. [PERSON] et al. [9] developed an intraoperative technique with near-infrared (NIR) fluorescence for in vivo, real-time detection of the parathyroid glands. Forty-five patients participated in the experiment where NIR fluorescence was measured intraoperatively from patients undergoing parathyroidectomy and thyroidectomy. NIR fluorescence detected the parathyroid in 100% of patients. [PERSON] et al.