[Calculation of 99mTc-DTPA transit times in normal kidney]

Renal dynamic study is a well-established and popular test in routine practice of nuclear medicine. The test is non-invasive, rapid and unique in evaluation of the kidney function. In despite of its clinical values, the parameters derived from renogram are physiologically meaningless. That is due to the fact that renogram is not the true kidney function. From mathematical points of view, renogram is the convolution of kidney function and kidney input function. The real kidney function, which is usually called Impulse Retention Function, could be derived using deconvolution technique. The main parameters that can be derived from IRF are transit times. Methods and Materials: The data collected from 30 normal healthy individual volunteers were used for calculation of transit times. Due to sensitivity of the deconvolution technique to the error and to minimize the error in the raw data, an investigation was launched to determine the most suitable method of noise reduction, background subtraction and numerical method of deconvolution. Based on our finding, a software package was developed on the system and evaluated using software phantom. Our results shows that among different transit time parameters for different parts of the kidney, mean transit time of the parenchymal is statistically the most valuable parameter. Assuming a Gaussian distribution for the parameter and accepting a 95% confidence interval, the normal rage of mean parenchymal transit time was calculated to be “205±70 Sec”. Using the same logic, the abnormal rage of mean parenchymal transit time was calculated to be more than 300 sec. During the last decade, skeptical questions have raised about the suitability of renogram for deconvolution and validity of transit time parameters. Our experiences have proved that kidney is approximately a linear and stationary system, therefore suitable for deconvolution. The validity of transit time parameters is extremely depended on the validity of software being used. Commercial software usually don’t have sufficient accuracy for calculation of transit times.