 Hi friends, so today we will talk about localization of parathyroid lesions, anatomical considerations and imaging techniques. So mainly we are going to talk about 4D CT or four-dimensional CT scan for parathyroid adenoma detection. So what is 4D CT? It sounds fancy, but it's only multi-phase CT scan dedicated for the diagnosis and localization of a parathyroid adenoma. When a radiologist is presented with a study for parathyroid adenoma localization, it is generally in a patient who is biochemically diagnosed with hyperparathyroidism. So the question is usually not that the patient, like whether the patient has hyperparathyroidism but it's usually where is the pathology or where is the parathyroid adenoma. And 4D CT has a lot of advantages, which is anatomical localization and detection of multi-glandular disease, also detection of other relevant pathology in the neck. So what the surgeon wants to know from a radiologist is the number, size and specific location of parathyroid lesions with respect to relevant surgical landmarks and the radiologist's opinion and confidence about these particular lesions. The other relevant information that the surgeon is looking for is ectopic or supernumeric parathyroid tissue, concurrent thyroid pathology and arterial as well as recurrent laryngeal nerve anomalies like anatomical variations in these structures. So let us go over the parathyroid glands. They include two superior and two inferior glands and there are less commonly there can be some anatomical variations in their location and knowledge of embryology will help us aid in detection of additional lesions and also anatomical localization since not uncommonly there are numerous parathyroid adenomas or hyperplasia in a person with hyperparathyroidism. The superior parathyroid glands are derived from the fourth pharyngeal pouch and the inferior parathyroid glands are derived from the third pharyngeal pouch and the migration pathways explain the variable positions of the gland. Another phenomenon is called acquired ectopia. So what that means is the relative superior or inferior position of parathyroid gland does not denote its embryological origin. So sometimes an enlarged superior parathyroid gland will descend into the superior mediasnum. So this phenomenon is called acquired ectopia and this is not very accurate in saying whether this is the superior or inferior parathyroid gland. What helps us more is the tractorisophageal groove. So if you see in the image on the right this enlarged superior parathyroid gland has migrated inferiorly but it is still posterior to the tractorisophageal groove and hence it is the superior parathyroid gland. So generally the superior glands are dorsal to the tractorisophageal groove and the inferior parathyroid glands are positioned anteriorly. One more thing I want to add is why do we use the tractorisophageal groove as an anatomical landmark because it closely mimics the plane of the ritter and laryngeal nerve which we usually cannot see. So tractorisophageal groove is the proxy for the RLN. So these four images and four different patients are examples of ectopic parathyroid tissue. Top left image demonstrates retrofaryngeal parathyroid gland and top right demonstrates an intra-thyroidal parathyroid adenoma. The bottom left demonstrates a mediasthenal adenoma and bottom right demonstrates adenoma within the carotid shape near the angle of mandible. Working in parathyroid adenoma is very helpful to the surgeons and radiologists need a certain level of expertise to reliably detect and localize parathyroid adenomas. So it is imperative that we take our time and effort to get better as there is a little bit of a learning curve here and it is also doubly important in recent like prior surgical candidates like patients who have already had surgery before. So what are the two types of surgeries there is minimally invasive parathyroidectomy that is MIP and the other one is bilateral neck exploration for people with multi glandular disease or in patients on home adenomas or hyperplasia was not diagnosed on imaging. So the surgeon has to do a BNE or bilateral neck exploration and try to search for the pathological gland imaging in that localization and not diagnosis because it is generally biochemically diagnosed and various techniques just aid and support the diagnosis like ultrasound nuclear imaging and four dimensional CT and what helps a radiologist is detailed knowledge of embryology and anatomy for accurate interpretation. Most of us may have performed ultrasound for detection of parathyroid adenoma during our training and it's as you know it's fairly simple straight forward parathyroid adenomas appear hypoechoic on ultrasound and thyroid is relatively hyperechoic but ultrasound also has other advantages it will help diagnose concurrent thyroid pathologies and the need for finatal aspiration cytology which it can aid and assist. So this is a diagrammatic representation of the locations of superior parathyroid glands and inferior parathyroid gland. So the superior parathyroid glands are usually within the red outline over here and the inferior within the blue outline. So as you see on the frontal projection they seem to overlap but on the lateral projection it is clear that the superior parathyroid lies posterior to the recurrent laryngeal nerve or the tracheoesophageal groove which we use as a proxy on CAT scan and the inferior parathyroid gland lies anterior to it and hence that helps us determine if it is the superior or inferior parathyroid gland. So the goal of HODCT for parathyroid adenoma detection is accurate localization and creating a roadmap for surgical planning and using the appropriate surgical landmarks. So as radiologists we tend to use cervical vertebral levels as anatomic landmarks which are not helpful to the surgeon. The surgical landmarks that are helpful to the surgeon include the cricoid cartilage, the tracheoesophageal groove which is a proxy to the recurrent laryngeal nerve. Some other surgical landmarks include the thyroid gland like the upper pole, lower pole and isthmus and the suprasternal notch and we should refrain from using cervical vertebral levels as anatomical landmarks. Also additional information such as thyroid, pathology, nodules have to be described and variant anatomy of the subclavin artery or recurrent laryngeal nerve and arch of aorta have to be mentioned because those help the surgeon in determining the surgical approach and preventing nerve damage. Also while interpreting HODCT we have to mention certain words which convey the level of confidence we have in this particular scenario. For example if we are fairly certain about the pathology being parathyroid edinoma or a hyperplastic gland we have to say something like this lesion posterior to the thyroid gland is consistent with, consistent being the key word consistent with parathyroid edinoma or parathyroid hyperplasia so on and so forth and then if you are less confident you can use words like hyperatenuating lesion on post contrast images posterior to the thyroid gland may be a parathyroid edinoma or parathyroid hyperplasia so may or probably being the words that are used to convey the level of confidence. So generally parathyroid edinomas are lower attenuation than the thyroid gland except in patients with chronic hypothyroidism who have less iodine in the thyroid gland so they may be like isoatenuating the parathyroid edinoma may be isoatenuating to the thyroid gland this is like a special circumstance. And the classic description of a parathyroid edinoma is that it is hyper enhancing on the arterial phase and demonstrates a washout on the venous phase but this is only seen in like 20% patients rest of them are type B and C which we will talk about in the upcoming slides also if there is some other pathology you have to describe it as either being associated with the thyroid gland, it being a lymph node or a blood vessel and extreme caution has to be taken in detecting parathyroid edinomas because not uncommonly they are like they are quite numerous so we have to be careful and not be satisfied with the search so if you find one edinoma we have to like sort of keep that in the back of the mind and keep searching for others. This is a typical protocol that can be used so point of note is it is a multi phase CT scan using a slightly higher tube voltage of 140 kVp and arterial phase imaging at 30 seconds and venous phase at 60 seconds with a slightly higher amount of contrast of 100 cc and from maxilla to the carina. What also helps in proper imaging is like shoulder lowering techniques using traction straps and like or improvised bed sheet placement. The third and final thing is to use the right arm as much as possible for contrast administration to reduce the streak and scatter artifacts from like the veins on the left side of the neck so we want to reduce that so try to use the right arm so this is the classic description of a parathyroid edinoma on post contrast imaging so the top left image demonstrates non-contrast axial scan through the neck demonstrating the left parathyroid edinoma which is hypo attenuating like hypodense and the center image demonstrates arterial phase with hyper enhancing edinoma and the top right image demonstrates venous phase with washout. This is type A enhancement and it is only seen in 20% of patients. Type B enhancement is different in the sense that the lesion is not hyper enhancing on the arterial phase so it appears hypodense on the pre-contrast, isodense with the thyroid on the arterial phase and demonstrates washout on the venous phase. Type C enhancement demonstrates the adenoma as hypodense compared to the thyroid as usual but it is iso attenuating meaning iso dense to the thyroid on both arterial and venous phase. As we talked earlier we have to be very careful in detecting multiple gland disease because that is not uncommon so once you find an adenoma like you have to keep searching for additional lesions which are separate from the lymph nodes, arteries and thyroid glands. So also we have to remember that sometimes when you don't see any pathology it is more likely to be multi glandular disease like parathyroid hyperplasia and an adenoma of size less than 7mm is sort of 88% specific for multiple gland disease. So differentials to parathyroid adenomas we know intuitively those are exophytic thyroid nodules or even large thyroid nodules and lymph nodes. So what helps us be more confident is the non-contrast images and correlation with ultrasound findings. Dual energy CT can also help us in this situation by negating the need for multi-phase imaging. So this is an example of multi gland disease as you can see both superior and inferior parathyroid glands are mildly enlarged. Variations in parathyroid lesions are as above so top left demonstrates a cystic appearing parathyroid adenoma. Top center and top right demonstrate nodules with intralesional fat and bottom right demonstrates parathyroid adenoma with calcification which is concerning for parathyroid carcinoma. So whenever you see calcification the possibility of carcinoma has to be raised and the referring physician has to be informed about this. So there is a recommendation on how to approach a 4D CT of the neck and to minimize missing any parathyroid lesions. So in searching the superior parathyroid glands you have to start along the posterior aspect of the upper thyroid lobes and then focus on the retro pharyngeal and retro esophageal positions. So much like less commonly the superior glands will be located within the carotid sheath or will be intra thyroid or in the scalene fat. In the search for inferior glands you have to start adjacent to the lower thyroid lobe and then look along the thyrothymic ligament and within the mediastinum. And less commonly the inferior glands will be within the carotid sheath and intra thyroidally. So in conclusion what the radiologist can provide to the surgeon and help them plan a great and accurate surgery is the number of lesions, the size of lesions, precise localization of the lesion and the radiologist opinion and confidence level like using particular words like this lesion is consistent with or using words like this lesion may represent or probably represents if you are less confident and then describing associated pathologies of the thyroid gland if present also mention ectopic or supernumeric parathyroid tissue.