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3.9. Evaluation of quantitative thyroid scintigraphy
The examination serves for qualitative and quantitative evaluation of the thyroid scintigraphic study using 99mTc, both native and after suppression or stimulation. Global accumulation of the radioindicator in the whole thyroid gland as well as regional accumulation in defined structures is calculated.
We equip the
scintillation camera with a high-resolution collimator. Prepare
about 10 MBq of 99m Tc pertechnetate in the syringe
. Before application, we insert this radioactivity into the
opening of the cylindrical absorption phantom of the neck and
place it in the middle of the field of view of the camera. With
the window on the 99m Tc photopeak, we measure the
activity to the application - on the camera panel, we read the
number of pulses accumulated in 1 minute. After iv application,
we measure the remaining activity in the syringe (including the
needle) in the same configuration. The difference of these
values, ie the actually applied activity in [imp./min.], Is
recorded and then used in the quantification of the scintigraphic
study *) ._______________________________________
*) With the THYR program, we can also use another way of entering the value of the applied activity - storing the image of a full and empty syringe as a separate scintigraphic study. From these scintigraphic images, the program then calculates the value of the applied activity in imp./min. Before application, the image of the syringe is stored under the camera for about 1 minute as a scintigraphic study. We store the syringe with the rest after application in the same way.
The actual thyroid scintigraphy is performed about 15-20 minutes after application.
Recommended storage mode:
matrix 64x64, 16 bit, zoom 2 x
1 image, storage time approx. 5 min.
We store large goiter without zoom. A thyroid scintigraphic study after suppression or stimulation is performed in the same manner. For better orientation behind the study identifier, we recommend adding the designation "NAT", "SUP" or "TSH". We write down the value of the applied activity and the time interval between the application and scintigraphy.
In the basic menu "PROCESSING" we call up the study of the respective patient (or with the help of "Next" both studies) and start the complex program THYR - Quantitative scintigraphy of the thyroid gland . When asked by the program, we specify which of them is native, after stimulation or after suppression. It is possible to process either only one study (both native and after stimulation or suppression), or two studies simultaneously - usually the first is native, the second after suppression or stimulation. However, a study after suppression and after stimulation can also be compared.
Quantification of image accumulation
The display shows a scintigraphic image from the first selected study (usually native) and a question about the applied activity and the time interval between application and scintigraphy *). After entering these values, the program performs a correction for dead time (default value DT = 6 microsec., Stored in the file CAMERA.PAR, can be replaced by another value if necessary) and for radioactive decay, after which it quantifies the scintigraphic image so that in each cell is directly the local value of accumulation in% of applied activity. At this stage, it is also possible to achieve the optimal brightness modulation of the image by setting a suitable upper and lower threshold. The same procedure is performed for the second study (usually after suppression or stimulation), if pre-selected.
Image translation and interpolation background subtraction
Follow the instructions on the screen to mark the ROI of the entire thyroid gland. The program translates the image and the ROI so that the center of gravity of the marked thyroid ROI is exactly in the middle of the image. Around this basic ROI, the program defines a concentric outer curve for background interpolation. This background can be a curve BC s Pade needs be modified - make sure that the curve passes through the points in which the activity is representative of the background areas of the thyroid, and avoided non-standard structures. After approval of the curve followed by the background interpolation subtraction nehomo gbackground. For each point within the thyroid ROI, the background value is calculated as a weighted average of the activity values ??of all points of the background curve, the weight of each point being inversely proportional to its distance from that point inside the thyroid - see Figure 3.3 . 1. In this way, the maximum possible objectivity of the subtraction of the inhomogeneous background in the thyroid gland is ensured. The modified image of a program stored in a SAVE AREA 10. If there is a second study (usually after stimulation or suppression) is performed for n and the same acts quantification and image interpolation background subtraction, the resulting image is stored in a SAVE AREA 11th
Since both images of the thyroid gland have been shifted so that their center of gravity is in the middle of the image matrix, their approximate positional coverage is ensured. The fine-tuning of the relative position of the corresponding images of the two studies is then performed by means of a two-dimensional cross-correlation so that the respective corresponding structures overlap as much as possible. The optimal matching of both images can also be done manually in the program by marking as an auxiliary ROI some suitable significant structure (eg autonomous adenoma) simultaneously on both images and then gently shifting the right image using appropriate commands so that the structures overlap. This operation is necessary for vwhen the two images differ significantly - for example, after suppression, the lobes disappear and only an autonomic adenoma remains. In such a situation, cross-correlation may partially fail.
Quantitative evaluation of accumulation
This is followed by a quantitative assessment of the accumulation of radioactivity in the thyroid gland. First, the ROI of the entire thyroid gland appears, which arises from the (set) unification of the ROI of the thyroid gland marked before the interpolation subtraction of the background in both images. We can modify this ROI as needed. Calculate the global í accumulation throughout the thyroid gland in% injected activity. According to the empirical formula, the approximate size (weight) of the thyroid gland is also determined on the basis of the ROI area (the values of the imaging scale [mm./cell] in the CAMERA.PAR file are used for the calculation). Then we can p odegree to draw any areas of interest (eg individual lobes, adenoma area, paranodular tissues, etc.), we insert their names and the program calculates for them the size and values ??of regional accumulation both absolutely (in% of applied activity) and relatively as a percentage of whole thyroid accumulation glands.
If we process both a native study and a study after suppression or stimulation, both images are displayed simultaneously, on which all ROIs are marked simultaneously and the respective accumulation values are calculated both for the native study and after suppression (or stimulation).
Then the accumulation images are offered for visual evaluation, the text of which can be inserted into the box at the bottom of the screen. We can photograph images, including evaluation text. Finally, the display shows a table of global and regional accumulation along with a verbal evaluation of the images. We can edit the text as needed and also insert the final evaluation. The material prepared in this way is then printed on the printer as the final report - Fig.3 ....
*) If we have recorded a scintigraphic study of the syringe before application and the rest after application in the manner described in the section "Data storage", we do not enter the measured values, but the program determines them from these scintigraphic images.
Cast of SAVE AREA after the end of the THYR program:
SA 10 and SA 11 - accumulation picture of the first and second study
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