The
Neck Phantom:
Magill, Tiffany, Owen, Mary Anne, Ingram, Juli. Medical College of Georgia, 2002
The neck phantom is an essential part of the process for obtaining accurate thyroid uptake values. We will illustrate its significance using the following methods:
A review of available literature was conducted to evaluate characteristics and protocols for use of neck phantoms. Also, thyroid uptake values obtained via proper phantom use were compared to uptake values obtained via no phantom use. The capsules for each patient were counted twice: once with the phantom and once without, and the patient thyroid and thigh counts were used to calculate both uptake values.
For the uptake studies observed for this presentation, the standard neck phantom was used. The standard phantom is a solid cylinder composed of acrylic and contains a cutout with a removable cylinder used for holding the capsule or 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. Lines on the phantom indicate proper placement of the uptake probe over the phantom during capsule counting procedures.
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.
Other Phantom Types
Capsule
Placement
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.
Probe
Placement
To count a capsule:
The uptake probe is placed perpendicular to the phantom at the end of
the probe guide arm (C) at a distance of 25 cm
(the arrow (A) and the line on the probe guide
arm (B) are aligned at 25 cm) at the point where
the two phantom guide lines intersect to simulate the geometry used when
counting the patient’s neck. The
capsule count is then obtained (10 min.). To
obtain a background count, simply remove the capsule from the phantom and count
again (also 10 min.).
To Count a Patient’s Neck:
The uptake probe is placed perpendicular to the patient’s neck over
where the thyroid gland lies beneath the skin.
The tip of the probe guide arm should touch the center of the patient’s
neck (A). Once
again, the probe should be set at 25 cm away from the patient’s neck by making
sure that the arrow (A) and the line on the
probe guide arm (B) are aligned at 25 cm.
This ensures that the same geometry used for the capsule count.
Position the probe over the patient’s lower thigh for background.
Both counts are 10 min. duration.
Thyroid function is a delicate balance regulated by the thyroid feedback axis. Nuclear medicine thyroid uptake values reflect the relative imbalance of blood serum levels by biochemical ratio and clearance of I-131 from the thyroid gland. Normal thyroid uptake ranges vary by facility due to differences in equipment and geometry, and by local population due to environmental and dietary factors. 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% and 6 hours and between 7% and 30% at 24 hours suggested by Early and Sodee in Principles and Practice of Nuclear Medicine.
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, toxic nodular goiter, or cancer, 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.
In the case of hyperthyroidism, the patient can undergo I-131 ablation therapy or surgery. Ablation doses are based on radioiodine uptake vales. It is very important that the uptake be accurate, because if too much I-131 is given, too much 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 (excess exposure). In the case of surgery, it is also important that the uptake be accurate, because the right amount of tissue should be excised on the first try, rather than taking too little, then starting over with another uptake study and ablation therapy (excess exposure), or taking too much thyroid tissue, resulting in the need for pharmacological intervention.
It
was our finding that the use of the neck phantom indeed played a pivotal role in
obtaining accurate thyroid uptake values. The
uptakes 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. 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.
Table:
Findings (MS Excel format)
