Technical Details on Sestamibi Scanning for Parathyroid Disease
Hemant Chheda, MD
To detect parathyroid adenomas in patients with sporadic primary hyperparathyroidism. To discriminate between single and multi-gland disease. To assist the surgeon in localizing a parathyroid adenoma in a three dimensional setting, allowing a more directed operative approach. The use of the scan for "confirmation" of hyperparathyroidism is discouraged. This is a localizing study not a diagnostic study. It is also a functional study. As many as 10 to 15 percent of all patients with primary hyperparathyroidism will not localize, but they still have the disease! It has been suggested by others that a localizing rate below 80% suggests that the technique used should be revised. We agree! Our adenoma localizing rate is approximately 88% of ALL patients, which is nearly all of those with adenomas (our sensitivity in detecting all adenomas within the past 200 patients is 97.2%...remember, this scan is to differentiate adenomas from non-adenomas). Also note that another 9 % or so will have 4 gland hyperplasia...they almost never light up. This high rate has come with increasing experience and has increased yearly. This test is NOT for patients with secondary hyperparathyroidism or a patient with a MEN syndrome since they will rarely demonstrate a gland, and, they almost always demand a full bilateral standard parathyroidectomy.
Two to 2.5 hours (less if being performed immediately prior to minimal parathyroidectomy using intraoperative nuclear mapping...see below).
Some examiners prefer to give lemon juice within 20 minutes of initial imaging (20% juice/80% water). This is thought to decrease the uptake by salivary glands. We do NOT routinely do this, but have found it to be helpful on some cases.
Equipment and Energy Windows:
- Camera: Seimens ZLC 7500 Orbitor
- Collimator: Low energy, high resolution
- Window: 20%
- Peak: Tc99m Auto Peak
- Magnification: 1.6 on computer only
- Computer: Picker Odyssey 750
Radiopharmaceutical, Dose, and Technique of Administration
- Tc-99m sestamibi
- 20 mCi (740 Mbq) (+/- depending on extremes of body weight) [this is very important]
- Standard IV injection
Patient Position and Image Field:
Patient position is critical if the surgeon is to use this information to guide the operation. The patient should be positioned in front of the camera just as he would be on the operating table...with a roll under the shoulders and the neck extended. The neck is kept midline for all studies. LAO and RAO are obtained by moving the camera, not the patient's head. This point is extremely important so that all scans on each patient are obtained with the camera the same distance from the patient's neck, therefore, there is uniformity in the magnification of each view. Additionally, this will provide uniformity from patient to patient making these scans easier to interpret. Extend the neck as far as possible (to mimic the position on the operating room table) while still comfortable, so as to decrease chances of movement.
This is the most important information on this page! We perform nearly 400 sestamibi scans per year, and our volume of cases dictates that we review hundreds of scans performed at outside institutions. The following list is very dear to our hearts and we feel very strongly that these simple changes will make dramatic changes in both the sensitivity and specificity of these scans.
Lateral views are worthless. We have dozens of examples where a beautiful AP scan shows nothing on lateral views, yet a number of hospitals obtain them in order to get an idea of depth. It doesn't work!
Posterior views are worthless. Yes, we see this done from time to time.
LAO and RAO are the most important views. This is what everybody misses. By viewing from an oblique angle, many parathyroids which are "hidden" behind a thyroid lobe become obvious. The most common case is one where you cannot determine if a hot spot "on" the thyroid is a thyroid nodule or a posterior parathyroid. When comparing the LAO and RAO to the AP it becomes incredibly obvious what it is. We have numerous cases where poor quality AP views (not really positive and not really negative) became suddenly positive after LAO and RAO films were obtained. These views also give the three-dimensional orientation that is desired: superficial adenomas appear to "move" from one side of the neck to the other when comparing LAO and RAO, while deep adenomas (tracheo-esophageal groove) will stay near the midline while the more anterior thyroid seems to move from side to side. This is also how intra-thyroid parathyroid adenomas are detected. Trust me, this is a no brainer. This paragraph is the most important information on this page (other than too little of a dose being used).
We obtain 5 early and 4 delayed views: Ant neck, Ant neck with marker (early only), Ant mediastinum, LAO, and RAO. Each image is obtained at 8cm. Early views are obtained about 15 minutes after injection. Delayed views are obtained at 1.25 to 1.75 hours. The mediastinal view must show at least the top half of the heart. Without mediastinal imaging, 4 to 5% of adenomas will be missed. The mediastinum is not included in LAO and RAO views. If performing immediately prior to performing a minimal parathyroidectomy, the timing is more critical -- see below. If the adenoma shows within the neck on the early view (most common scenario) then we do not get a delayed mediastinal view to save time and effort. Also note, the delay protocol changes according to how the early films look. If they look good, we speed things up. Better for the nuclear medicine department, better for the patient, and better for the surgeon if the patient is going directly to the OR (this is the ideal situation...the nuclear medicine department takes part in the treatment of this disease rather than playing just a diagnostic role.
We acquire each view for a fixed time rather than a fixed number of counts. This way we find more uniformity with all images (early and delayed) which makes comparisons easier and subtle findings more apparent.
Early Images: Anterior, ant + mediastinum, LAO, and RAO views at 15 minutes, (one anterior with markers and one without). Markers are placed on the sternal notch, and 2 laterally along the lateral border of the SCM muscle 4 cm apart (distance guide).
Delayed Images: Timing discussed below. Anterior, ant + mediastinum, LAO, and RAO views are obtained. The lateral oblique views are at 31 degrees with the patient's head midline. Note: often we do the delayed films earlier (see below) if the adenoma shows up on the initial scan. Why 31 degrees? This is a frequently asked question. We have found that rotating the camera any further than this means that the patient's shoulder gets in the way, necessitating moving the camera further away from the patient's neck. This means that all the scans will not be obtained at the same distance from the patient's neck (as noted above, we aim to maintain the same distance for all views). We have done enough to know that 31 degrees is about all you can rotate the camera without pushing the patient's shoulder.
Lateral views are NEVER required. The importance of the LAO and RAO views is that they allow the parathyroid adenoma to be localized in three dimensions in relationship to the thyroid gland. If the adenoma is located at the level of the thyroid (in depth from the skin) then it will appear to "move" in the neck when comparing the right and left views. If the adenoma is located deep to the thyroid, it is almost always in the tracheoesophageal groove. In this case, the position of the adenoma will appear to be the same on the LAO and RAO views while the thyroid "rotates" from side to side. This 3-dimensional localization will help the surgeon by giving a good estimate of the adenoma depth.
Delayed Images and SPECT Imaging
There are very few indications for delayed images after 2.5 hours. Occasionally (rarely) thyroid activity can be a bit hot and re-scanning at 3 hours may be helpful. We do NOT think this is a common occurrence...in fact, it is extremely rare.
We had been using SPECT imaging for all patients in which there is a questionable adenoma (about one in 20). We used to think that SPECT analysis could increase sensitivity and specificity several percent, and therefore, used it selectively. If the standard views suggest single gland disease but cannot definitively say yes or no, then we would (in the past) perform a SPECT immediately after the delayed films. We no longer believe that SPECT adds much that the LAO/RAO views don't already give us. We also believe that if the standard Sestamibi does not show a single gland (obvious single gland) then that patient is NOT a candidate for minimal radioguided parathyroidectomy and needs a standard 4 gland exploration and a SPECT scan will NOT change that. That patient has a high likelihood of having 4 gland disease. Some centers perform a SPECT on all patients. We think this is overkill and unnecessary almost all of the time. Furthermore, if the patient is being taken directly to the OR, this wastes valuable time. We do feel that many re-do's might benefit from SPECT for the added 3-dimensional localization it provides, although we do NOT feel as strongly about this as we did a year ago. (Intraoperative nuclear mapping for these patients is rapidly changing our minds, however, and we do not feel so strongly about it any more!)
If Performing Prior to Intra-operative Nuclear Mapping for Minimal Parathyroidectomy
We have found that the ideal time to operate is about 2.0 to 2.5 hours after injection. This is our goal on every patient. We have operated on patients as long as 4 to 4.5 hours after injection and the radioactivity is too washed out to be of much use.For these patients, our goal is to begin the delayed scans 1.25 hours after injection (as early as 1.0 hours, and never later than 1.5 hours) and then take them directly to the operating room once these scans have been completed. If the early films show a nice gland, then we will perform our delayed scans at 1 hour and get the patient in the OR 2 hours after injection. Re-stated ... if we have nice gland on the early films we proceed faster. If we can see it on the scan, then the probe will find it very easy.
Note: if the early LAO and RAO views fail to show that a separation between the thyroid lobes and the radioactive parathyroid, we will mark the skin and then pass an ultrasound probe over the area quickly to determine if this is an intra-thyroid parathyroid. Note that this is done after the first set of images so that the operating room is not delayed. This situation occurs about 4 percent of the time (1 in 25 scans).
Remember, the idea is to be in the operating room at a time when there is a high degree of differential radioactivity between the thyroid and parathyroid...that is to say, after the thyroid washes out but before the parathyroid washes out. If you wait too long, the probe won't help much.
Dr Chheda is a Clinical Assistant Professor of Radiology at the University of South Florida and Medical Director of Nuclear Medicine at Tampa General Hospital where he performs more than 250 sestamibi scans each year. His expertise also allows him to review numerous scans performed at outside institutions thereby increasing his expertise and knowledge of this rapidly developing technology.