European Nuclear Medicine Guide
Renal dynamic scintigraphy (radionuclide renography or nephrogram) is performed by serial imaging after intravenous administration of the selected radiopharmaceutical, to investigate perfusion, functional uptake, cortical transit and excretion [141].
Dynamic renal scintigraphy requires the drawing of regions of interests (ROIs) around the kidneys. Prior to the generation of curves from each ROI, subtraction of area-normalized background ROIs is necessary. The most accurate background ROIs are C-shaped surrounding the lower, lateral and upper part of the kidney [141].
Attenuation correction for kidney analysis is necessary to calculate the relative function, early-uptake and the clearance [142]. The tracer uptake of the kidney measured at body surface A0 can be calculated by
using the attenuation coefficient µ for 99mTc and the distance centre of the kidney – body surface. The attenuation coefficient for 99mTc is 0.14/cm.
There are several possibilities to correct attenuation. The Tonnesen formula [143] is a mathematical formula applying body weight and height.
Alternatively we can use an additional lateral measurement to calculate the kidney depth from the centre of the kidney to the body surface accurately [142]. (Fig. 1).
Fig. 1: Determination of the left and right kidney depth using additional acquisitions
The attenuation corrected dynamic sequence is used to perform a basic analysis of the functional behaviour of the kidneys. Here, ROIs over the kidneys are used to calculate
The relative uptake measurement is usually made by positioning a ROI over each kidney followed by measuring the integral of the counts in the renal ROI (between 1-3 min after injection) or using a Rutland-Patlak-Plot.
The determination of the relative function 
apply the area of the nephrogramm (after depth- and background correction) using the following formula:
Left and right functional values are valued as symmetric if both sides do not differ 50±7.5% [144].
The function of the parenchyma can be obtained by dynamic scintigraphy using only a ROI that is positioned over parenchyma. This is done in many commercially available systems by the automatic drawing of parenchyma ROIs. A better and more accurate visual interpretation is possible by parametric imaging (Tixel image or Mean-Transit-Time) or factor analysis. The tixel image is the simplest and easiest to implement. Here, for the value of each pixel of the parametric image, the time-point of the maximum of the corresponding pixel-time-activity curve is chosen [145]. In Fig. 2 normal patient is presented (linke Niere zeigt eine falsche ROI), The tixel image calculation can easily distinguish between parenchyma and pyelon. The time-activity curve for the parenchyma and the whole kidney is shown in Fig. 3. For a better comparison of functional behaviour the parenchyma curve is normalized to the maximum of the whole kidney curve.
Fig. 2: Parenchyma-pyelon separation:
Fig. 3: Time Activity curves for the left and right kidney (whole kidney and parenchyma)
The global function of the kidneys is measured by the clearance, which can be calculated by
The minimal transit time of a tracer is assumed between 2-3 minutes. During this time period, the kidneys serve as pure tracer depot and the clearance can be obtained.
As a parameter for the kidney function, the “early uptake” of A. Mostbeck. (measured in % of the applies activity) between the 2nd and 3rd minutes after injection can be chosen [142].
The following steps are necessary for this measurement:
Applying the input func
tion , the transfer function 
and a mathematical formalism called convolution 
the amount of the substance 
can be calculated.
Using the formula above for kidney analysis 
is known from the time-activity curves of the kidney whereas the input function 
can be obtained from TAC of the heart (or a region over the liver). To get 
a mathematical procedure called deconvolution has to be used.
The calculation procedure is an iterative process, thus small (statistical) errors of the parameters can lead to large deviations in the calculations. Therefore, it is necessary to filter the input and kidney curves (e.g. 9-point filter).
For the input ROI several possibilities exist, whereas a region that covers the heart is preferred. Another option would be the aortic region between the kidneys. The calculation procedure starts at the maximum of the input function.
The result of the deconvolution is shown in Fig. 4. After cutting off the background the min. transit time, the mean transit time, the transit time index and the max transit time can be determined if the value of the calculated curve falls below 0. Afterwards, the calculation process is stopped. The min. transfer time is the time point when the horizontal part of the curve is left. The max. transit time is the time point when the value of the transfer function reaches zero (Fig. 4).
Fig. 4: Schematic presentation of the transfer function.
The mean transfer time (MTT) is calculated by
In Fig. 5, the organ response for the same input function but different MTT values (5.4, 10.5 and 14) is presented. As seen in the below diagram, enhancing values for the MTT results in normal to delayed organ response.
Fig. 5: Organ response for the same input function but different MTT values
The transit time index (TTI) is the difference between the mean transit time and the min. transit time. Britton et al. [6] used the PTTI to differentiate between normal, dilated and obstructed pelvis (Fig. 6).
Fig.6:Transit time index: Reference values for normal, dilated and obstructed pelvis[146]
The reference values for the TTI found by Britton depend heavily on the preprocessing on the data such as filtering [146]. Unfortunately, the preprocessing steps are not published in detail. Therefore, the reference values which were performed at our department for renal studies (with 9-point filtering of the input and renal curves) are different (Tab. 1).
|
|
Britton |
WSP data (n=26) |
|
Parenchymal TTI |
10-156 |
24-63 |
|
Parenchymal meanTT |
100-240 |
133-194 |
|
total kidney TTI |
20-170 |
38-88 |
|
total kidney meanTT |
- |
146-216 |
Table 1: Reference values for mean transit time and transit time index