 Okay, so before we start looking at any cases of 4DCT and learning how to interpret a 4DCT, we first need to know the basic anatomy. And that's what I had to do because I didn't do any 4DCT as a resident, I only did it as I mentioned when I was a first year attending. So I went back to the books and this is what I found. So the whole reason we're doing 4DCT is to look for the cause for primary hyperparathyroidism. This is the oversecretion of parathyroid hormone leading to hypercalcemia and resulting in some pretty nasty systemic effects to the GI system, the kidneys, the bones and also to the patient's mental health. And the cause, it's typically a single parathyroid adenoma, other causes hyperplasia, double adenoma and rarely carcinoma. Now the good news for the patient is that they can be cured of all these horrible systemic effects and that's by resecting that parathyroid adenoma. And so our job is to help that surgeon know which gland to resect. Now in the past, we didn't do a lot of parathyroid imaging because the treatment was simply a bilateral neck exploration. This is a transverse incision across the neck and looking at all four glands and this was 95% successful. So there was no role for imaging. In fact, this is what a radiologist said, the only localizing study indicated for primary hyperparathyroidism is to locate an experienced parathyroid surgeon. That was in 1969. Now that is certainly not the case now. So what changed? Well, it's the approach to surgery. The surgery now more commonly performed is focused or minimally invasive parathyroidectomy. And here's an example of the type of incision we're seeing now. This is a small incision in the quadrant where the parathyroid adenoma is located. This is a much shorter surgery, could be done as an outpatient, could even be done with local anesthetic and fewer complications. Now in order to do this surgery and to make the decision to do this surgery, we do need our trusty radiologist to localize that adenoma. We need to locate the quadrant and locate it with confidence and to exclude the chance of multi-glangular disease for which they would need to do the bilateral neck exploration. So the anatomy, there are four glands that could potentially be the culprit. There are two superior glands that are located posterior to the superior or middle one third of the thyroid. And two inferior glands that are inferior or posterior or lateral to the lower one third of the thyroid. That's the utopic locations, the typical locations. And a normal parathyroid is about the size of a grain of rice, typically not seen on imaging. However, when an adenoma, they are enlarged and so they are visible on imaging and on 4D CT. The difference between a superior and inferior gland is not the cranial cordal location, but actually the anterior posterior location. The superior glands typically are posterior to the tracheoesophageal groove and the inferior glands are anterior to the tracheosophageal groove. Here are two examples. When we've got ectopic locations in the thyroid, that's typically going to be an inferior gland anterior in the superior mediastinum, typically an inferior gland. And then posterior in the neck, posterior to the tracheosophageal groove, which is located here, the esophagus is here. That's typically a superior gland. And then here's another mediastinal case, but the difference is this is posterior to the trachea. This is a pair of superior glands that have descended as far as the superior mediastinum. Now what makes parathyroid imaging and surgery interesting is that we are dealing with cases where there are not always four glands and they're not always posterior to the thyroid or inferior lateral to the thyroid. There can be more than four glands, less than four glands. And then there are a fair number of cases that get to 40 CT where we're dealing with ectopic locations and we'll learn about those ectopic locations in the future sections. Now the reason behind those ectopic locations is the embryology. The superior glands arise from the fourth bronchial or pharyngeal pouch and that descends with the thyroid. The inferior parathyroid glands arise from the third bronchial pouch and they descend with the thymus. So which gland is more likely to be ectopic? Well it's the inferior gland because it needs to know when to get off the bus and stop with the thyroid before heading down into the mediastinum. So it doesn't head down to the mediastinum. So where do we look for our parathyroid adenoma? Do we look for them up to the level of the hyoid where the pharyngeal or bronchial pouches were all the way down to the mediastinum where the thymus is. And they're typically medial to the lateral aspects of the carotid sheath and of course anterior to the prevetable muscles and then finally in the thyroid itself. So hyoid to mediastinum, medial to the carotid sheath and in the thyroid itself. Okay to end this section I wanted to emphasise what the surgeon wants to know from us. They are saying I want to do minimally invasive surgery, friendly radiologists. So tell me where do I need to operate and tell me how long this case is going to take me. So how long it's going to take them and what type of surgery they do. We need to tell them the number of glands. If there's just one they can do minimally invasive surgery. We need to tell them the location, which part of the neck to do that minimally invasive surgery and approximately what size the lesion will be. And then finally it is really important to tell them how confident you are because if you've got low confidence then there are other options for parathyroid imaging other than 4D CT and they may do additional tests. Okay so next section is on my interpretation process.