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3.7. Evaluation of static scintigraphy of the kidneys

Evaluation of static renal scintigraphy using the 99m Tc-DMSA radioindicator consists of three stages:

Data storage

Scintigraphic examination is performed 2 to 4 hours after intravenous administration of 99m Tc-DMSA.

Recommended storage mode:

matrix 128 ´ 128, 16 bits, preselection min. 200,000 imp.

Image 1: Posterior
Image 2: Anterior

Additional projections as needed:
3rd image: DX, 4th image: SIN,
5th image: RPO, 6th image: LPO

This order of projections corresponds to the default preset in the RENSTAT program , so there is no need to permute the order of projections when evaluating it. If for some reason the front projection is not performed, the program respects this fact and performs the calculation of the separated function only from the rear projection (of course be from the correction for absorption).

Study evaluation

After calling up the required scintigraphic study in the basic menu PROCESSING, we will start the complex program RENSTAT - static scintigraphy of the kidneys .

Sequence of projections and visual evaluation

The display shows the stored images of the kidneys and describes the names of the projections in the default order: Posterior - Anterior - DX - SIN - RPO - LPO (only as many projections as the images stored have been displayed). If the projection order agrees and we want to keep all the images, we will answer the program question "Projection order okay?" positive. Otherwise (negative answer) we specify gradually on the instructions of the program the actual assignment of images to the names of projections. This way you can not just toss the sequence of projections, but also select specific images, and other marks the end h Nout for further display, processing and documentation. If we have displayed images with the corresponding assignment of projections, a visual evaluation begins, and we can use both the implicit standard formulation of the normal evaluation, e.g.

After intravenous administration of the radioindicator, the kidneys of the
usual shape, size and placement are displayed. The distribution of the radio indicator
is homogeneous, without bearing changes.

N ormal scintigraphy of the kidneys
with symmetric distribution of the function.

Signature: MUDr. xxxx",

or a non-standard free text of a pathological finding.

Designation of areas of interest

If we want to determine a separate function, the display first shows the kidneys in the rear projection with the left and right kidneys. Follow the instructions on the screen to mark the areas of interest :

Left kidney ........... ROI 1

Right kidney .......... ROI 2

Tissue background ...... ROI 3

The background is best located in the area between the two kidneys or below them. We implicitly use irregular regions of interest (IR), but we can also use quadrangular (RI). After completing the definition of ROI on the rear projection, we mark the ROI of the kidneys and the background in the image of the front projection in the same way. Pay attention to the order - in the front projection we start the ROI 1 of the left kidney on the right side of the image!

Quantification of a separated function

After completing the ROI marking, the calculation of the relative function is performed . For both images are integrated ROI left and right kidneys, subtract the background, using the geometric mean of the integrals corresponding to the kidneys themselves in the front and back projection is performed correction for differences in absorption of gamma radiation, and calculating the relative function of each kidney in the percentage of the global function of both kidneys.

Radiation absorption correction g

If the depth of placement of the left and right kidneys in the body is different, the g- rays from each of these kidneys must pass through a slightly different layer of tissue before being detected in the camera and will be absorbed differently. This may affect the result of determining the separated function of the left and right kidneys. The absorption of g radiation in tissue is given by the exponential law

         I   =   Io . e - m . d     ,

where I o is the original radiation intensity (without absorption), I is the intensity after passing through a tissue layer of thickness d, m is the linear attenuation factor (for radiation g 99m Tc with energy 140 keV is the absorption factor m @ 0.15 cm -1 ). However, to correct for absorption using this exponential law, we would need to know the exact depths d of both kidneys, which we do not know and which may depend on the patient's position (sitting, lying on his back or abdomen). We scanned, but if renal Scintigraphic images in two opposite P r ojekcích (front and back) can be measured by comparing the number of pulses of the opposed projections to make a correction for different radiation absorption g from kidneys stored at different depths. It follows from the exponential law of absorption that this correction will be achieved when we use the geometric averages of the accumulated number of pulses in the ROI of the left and right kidneys taken from the image of the posterior and anterior projection to calculate the separated function :

                      Ö(LPost . LAnt )    .    100 [ %]                            Ö(RPost . RAnt )    .    100 [ %]
      L = -----------------------  ,       R = ------------------------     ,
               Ö(LPost . LAnt ) + Ö(RPost . RAnt )                      Ö(LPost . LAnt ) + Ö(RPost . RAnt )

where L and R are the resulting separated function of the left and right kidneys as a percentage of the global function, LPost and Lant are the numbers of pulses (integrals) in ROI 1 of the left kidney from the images of posterior (Post.) and anterior (Ant.) projection, RPost and RAnt are the same quantities calculated for ROI 2 of the right kidney.

Both kidney projections appear on the screen with a number of intermediate results - kidney size, relative function of right and left kidney for individual projections and after correction for differential absorption of gamma radiation from left and right kidney, backward calculation of left and right kidney depth, recalculation of separated function in relation to the size of the left and right kidneys - Fig.3.7.1:

Here we have the option to either agree the calculation or repeat it for the same or different projections, with or without correction for absorption, or. explicitly specify the difference between the depths of kidney placement
*) If the acquisition program has only accumulated, for example, a rear projection, then the program recognizes that it is a single-image study and calculates the relative function of the kidneys immediately after completing the ROI definition in this projection. If we want to calculate the relative function from only one projection in a complete study, we mark the ROI only on this projection and not on the other. This ignores the second projection and the calculation is performed from only one projection, ie without correction. Such a situation on a camel when it is obvious that the front projection is of poor quality (eg. Sometimes with obesity) and the correction would not be correct. The final report states whether the calculation was performed for both projections, or only for the rear or front projection. However, band point out that correction for differential absorption of gamma radiation from the left and right kidneys can significantly refine the result. If the difference in kidney position exceeds 1/4 of the kidney height, the correction may be clinically significant; with a shift of 1/2 or more, a correction is necessary to obtain the correct result.

Final protocol

The display shows all preset kidney projections along with the result of the determination with the epared function.
In the box below we can edit the text of the verbal description of the scintigraphic images and the conclusion. Finally, we can print a report containing (in addition to basic data such as the patient's name) the relevant images, the result of determining the relative function of the kidneys and the text of the verbal evaluation, conclusion and signature - Fig.3.7.2.

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