The Neck Phantom: The Significance of Its Use in Thyroid Uptake Studies

Tiffany Magill, Senior NMT Student,

Department of Radiologic Sciences, Medical College of Georgia

The thyroid uptake study is a key factor in the assessment of hyperthyroidism and for thyroid ablation dosimetry due to its evaluation of the thyroid feedback complex. This study allows for overall assessment of thyroid function since iodine is both trapped and organified by the thyroid (Gemmell 266). The basic procedure involved in the thyroid uptake study includes the administration of a radioiodine capsule that has been counted in a neck phantom, as well as 6- and 24-hour patient neck and thigh counts. The patient counts are then used along with the pre-administration capsule count to calculate the proportion of total radioiodine taken up by the thyroid tissue (Shackett 236). For this reason, it is essential that all counts be as accurate as possible, especially the pre-administration capsule count. To achieve an accurate capsule count, the capsule must be counted while inside a neck phantom. If not, the uptake values will be inaccurate, which will in turn affect thyroid uptake values.

In order to demonstrate the differences in uptake values obtained without the use of a neck phantom, we observed the results of several studies in which we calculated the patient’s thyroid uptake based on counts that we obtained with a phantom and also with counts obtained without a phantom. This paper will discuss the uptake study procedure, the neck phantom, normal uptake values, possible patient outcomes, and our findings as to how the use of the neck phantom affects thyroid uptake values.

The procedure for completing the thyroid uptake study is not very time intensive, but has many factors affecting it’s accuracy. Some common indications for the study include determination of the amount of I-131 to be administered for ablation therapy due to Grave’s disease or toxic nodular goiter, to differentiate Grave’s disease from painless thyroiditis or factitious hyperthyroidism, and to assist in diagnosing and confirming diagnosis of hyperthyroidism. Uptake measurements are of limited value in diagnosing hypothyroidism due to poor counting statistics at low count rates. Before the study, thyroid medication must be discontinued, and a low-iodine diet can be followed for 3-10 days prior to the administration of the capsule if required. Also, several questions must be asked of the patient concerning iodinated contrast, ingestion of iron-rich foods, pregnancy or nursing status, recent administration of radionuclides, pertinent laboratory data including thyroid function tests, and prior thyroid imaging or uptake tests. A precaution for this study is the prolonged discontinuation of thyroid medications, which can be hazardous to some patients. Radioiodine is the generally preferred radiopharmaceutical for the study. Data is acquired using a probe with a 2-inch thick sodium iodide crystal at least 2 inches in diameter with suitable shielding and a flat-field collimator providing a 10 cm diameter at the surface of the patient’s neck. Measurement of uptake is usually performed 24 hours after administration of the radioiodine, and sometimes at 4 to 6 hours after administration. The uptake is usually measured with 20-30 cm between the face of the crystal and the anterior of the neck or phantom. Neck counts, lower thigh counts, capsule counts and lab background counts are preferably obtained at each counting session. Capsule counts are corrected for decay at each patient counting session. The radioiodine uptake (RAIU) is calculated using the following relationship:

RAUI = [Neck cpm – Thigh cpm)]/[Administered cpm – Background cpm] x 100%

Customary normal ranges are between 10 and 35% for 24-hour uptakes and between 6 and 18% for 4-hour uptakes. These values vary by site and population. Uptake values are considered along with the patient’s history, laboratory data, and physical examination when interpreted. There are several sources for error including variations in neck to detector distance, improper centering of the probe over the patient’s neck, electronic instability, background variation, interfering food/medications, recent administration of other radionuclides, radioactivity in adjacent area, inappropriate neck phantom, and contamination of the neck phantom (Society… n. pag.). The last two sources for error pertaining to the neck phantom help to illustrate the significance of the neck phantom in this study.

There are several different types and characteristics of neck phantoms. There are some types of phantoms that are shaped to resemble the neck and have such features as additives that simulate portions of adipose and muscle tissue. Some phantoms also include a simulated spinal column, but comparisons between these and phantoms with no spinal column showed no difference in observed results (Thyroid Intercomparison… n.pag.). The standard phantom is a solid cylinder composed of acrylic and contains a cutout with a removable cylinder used for holding the capsule or a vial. The placement and size of the cutout simulates the placement and the 20.5g Reference Man mass of the thyroid and the body of the phantom simulates the overall size, shape, density, and attenuation of the human neck (Thyroid Calibration… n. pag.). Lines on the phantom indicate proper placement of the uptake probe over the phantom during capsule counting procedures.

For the uptake studies observed for this presentation, the standard neck phantom was used. The procedure for use of this type of phantom is very simple. The insert at the front of the phantom is removed and the capsule is placed inside it, then the insert is placed back inside the phantom. The uptake probe is then placed perpendicular to the phantom at the end of the probe guide arm to simulate the geometry used when counting the patient’s neck. The capsule count is then obtained. To obtain a background count, simply remove the capsule from the phantom and count again (Shackett 236).

Normal thyroid uptake ranges vary by facility due to differences in equipment and geometry, and by local population due to environmental and dietary factors (Shackett 237). At Doctor’s Hospital in Augusta, Georgia, where most of these studies were conducted, the normal values are as follows: 5-12% for 6-hour uptake and 10-30% for 24-hour uptake. These values are similar to the values suggested by the Society of Nuclear Medicine, which are between 10 and 35% for 24-hour uptakes and between 6 and 18% for 4-hour uptakes, and up to 12% at 6 hours and between 7 and 30% and 24 hours suggested by Early and Sodee (629). Patient uptakes higher than the normal range can indicate hyperthyroidism or aid in the diagnosis of hyperthyroidism. The uptake values are also used to calculate how much I-131 is to be administered for thyroid ablation therapy due to Grave’s disease or toxic nodular goiter, and therefore must be very accurate. To ensure accuracy at each site, the same geometry and neck phantom should be used for each count in each study.

There are several options for treating diseases related to the thyroid uptake study. In the case of hyperthyroidism due to Grave’s disease or toxic nodular goiter, the patient can undergo I-131 ablation therapy or surgery to remove some of the thyroid tissue. Ablation therapy doses are based on previously obtained radioiodine uptake values. The thyroid uptake should be performed as close as possible to the treatment (Society… n.pag). It is very important that the thyroid uptake be as accurate as possible, because if too much I-131 is given, too much thyroid tissue can be ablated. Conversely, if too little I-131 is given, not enough tissue will be ablated, and the patient may have to have the therapy again, resulting in excess exposure to the patient. In the case of surgery, it is also imperative that the thyroid uptake be as accurate as possible, because it would always be preferable for the right amount of thyroid tissue to be excised on the first try, rather than taking too little and having to start the process over, resulting in excess patient exposure, or taking too much thyroid tissue, which would result in the need for life-long pharmacologic intervention for hypothyroidism for the patient.

It was our finding that the use of the neck phantom indeed played a pivotal role in obtaining accurate thyroid uptake values. The uptake values that we obtained with no use of the neck phantom were an average of 3.85 percentage points lower than those obtained with proper neck phantom use. All studies observed did demonstrate a significant drop in uptake values when the phantom was not used, which would certainly affect thyroid ablation dose amounts, and in some cases would be considered a false negative for hyperthyroidism.

Our findings support the need for neck phantom use in thyroid uptake studies in order to obtain accurate capsule counts, and thus to obtain accurate thyroid uptake values. By simulating the attenuation of an actual human neck, the phantom provides a means for obtaining a capsule count suitable for use in calculating thyroid uptake values, which are an important part of aiding in diagnoses of several thyroid pathologies such as Grave’s disease and toxic nodular goiter, as well as for the calculation of thyroid ablation therapy doses.

Works Cited

Early, Paul J. & Sodee, Bruce D. (1995). Principles and Practice of Nuclear Medicine

(2nd Edition). St. Louis, MO: Mosby-Year Book, Inc.

Gemmell, H.G. & Smith F.W. (1989). Practical Nuclear Medicine. Engsham, Oxford

England: Oxford Univ. Press.

Shackett, Pete. (1995). Nuclear Medicine Technology: Procedures and Quick Reference.

Lippincott Williams and Wilkins.

Society of Nuclear Medicine Procedure Guideline for Thyroid Uptake Measurement 2.0

[WWW page]. (1999). URL http://www.snm.org/pdf/chapter6.pdf.

Thyroid Calibration Phantoms [WWW page]. (n.d.). URL http://www.pnl.gov/phantom/

thyroid.htm.

Thyroid Intercomparison Neck Phantom [WWW page]. (1999). URL http://www.hc-sc.

gc.ca/ehp/ehd/rpb/environ/ncrc/neck.htm.