Parathyroid gland examination (image subtraction technique)

Unfortunately, no radioactive product is available to accumulate in parathyroid gland but does not in thyroid. Therefore, parathyroid gland cannot be examined directly by means of nuclear medicine without the disturbing effect of thyroid. The following technique is used to address this issue. One radiopharmaceutical accumulates in both parathyroid and thyroid, while the other accumulates only in thyroid. Two acquisitions are made from the patient by both pharmaceuticals and a dedicated subtraction technique may remove the disturbing effect of thyroid from the result.

First, a 201Tl image has to be aquired. In this case, the activity accumulates in both organs (parathyroid gland and thyroid). In the next step, avoiding any patient movement 99mTc radiopharmaceutical (Pertechnetate) need to administer for the patient. After Tc accumulation in the thyroid, another image has to be acquired. Since the Tl and Tc radiates significantly different γ energies, the energy window must be set from Tl to Tc in order to acquire the right Tc image.

Let us introduce the following denotes:

F₁: Tl uptake (activity in the thyroid and in parathyroid gland),
F₂: Tc uptake (activity in the thyroid)
p₁, p₂: The values of the same pixel (x, y) in the two images.
The difference pixel by pixel is: p = A (p₁ – B p₂) , where A and B properly chosen constants.

Figure 6. Tl image, Tc image, difference image

 
Let p⁺ denote positive part of p (i. e. p⁺ = p, ha p > 0, otherwise 0), adn
p^– the negative part of p (i.e. p^– = –p, if p < 0, otherwise 0).

The choice of A and B is appropriate if both F⁺ and F^– images contain only noise i.e. the thyroid’s projections of both images (F₁ and F₂) show small, but approximately the same intensity. The role of A is simply to make the less active regions more visible.

ROI (Region Of Interest)

 
Let T be such an area where the parathyroid gland isn’t projected, only the thyroid is there -e.g. middle the part of thyroid projection -.
S₁ and S₂ are the total activity of region T on F₁ and F₂ . ThenB = S₁ / S₂ is a good value for the examination of parathyroid gland without the disturbing effect of thyroid. Let’s do the following remark: the above-mentioned evaluation procedure may apply for ^99mTc - ^99mTcMIBI acquired parathyroid studies too. Instead of ²⁰¹Tl isotope ^99mTcMIBI radiopharmaceutical is administered providing similar biochemical processes, i.e. ^99mTcMIBI will accumulate in the parathyroid and thyroid like²⁰¹Tl did. In case of ^99mTc and ^99mTcMIBI application the same image evaluation and display procedure has to use as in Tl – Tc-s case (see Figure 7.).

Figure 7. Subtraction image of {SUP()}99m{SUP}TcMIBI and {SUP()}99m{SUP}Tc-Pertechnetate

“Weight Factored” Subtraction technique

Application of this procedure is suggested to use mainly for Tc – TcMIBI parathyroid studies, but may be applied for above mentioned Tl - Tc studies too. The main point is to detect the parathyroid more efficiently. The applied method differs from above-mentioned, because all the pixel value of the image to be subtracted are corrected by a 0<<1.5 multiplication factor i.e. the following operation is performed.
Similar way as above has been mentioned pixel by pixel is performed the subtractions by predefined , , ….. values. =0.6, =0.8, =1, =1.2, =1.4 value series have been applied for clinical evaluation to produce the various subtracting images. A clinical implementation of that method is presented on Figure 8.