Evaluation of Methods for Obtaining an Image Derived Input Function from Dynamic PET-images

Sammanfattning: Dynamic PET is a technique to follow the uptake kinetics of radioactive labelled molecules in the human body. The kinetic behaviour may be analysed to acquire parameters, such as perfusion of blood to tissue, with the knowledge of the blood activity time curve (also called input function). This is usually measured by continuous sampling by letting the blood flow through a detector but this is both burdensome and not without risk to the patient \cite{Feng2012}. Instead, an alternative method would be to determine the input function from the PET-images and thus get an image derived input function (IDIF). In this master thesis evaluation of analytical models, tested on both experimental sampled data of a phantom and on data from actual patients, were used to determine the IDIF from small blood vessels. A phantom was built from plastic tubes and plexiglass to test and evaluate different methods. In order to get a correct IDIF one needs to correct for partial volume effect (PVE) which in small volumes of interest (VOI) gives apparent lower activity than reality. The correction can be done in a few different ways but this paper focuses on multi-target correction (MTC) which uses two or more VOIs to obtain the true activity value \cite{PVE_corrections}. The method was evaluated using data from phantom measurements where the activity was known and could be used as a reference. The phantom was constructed using ten tubes of different dimensions, a plexiglass holder and a plastic box. The result from the PVE correction turned out to be highly dependent on accurately knowing the diameter. However, when the diameter of the VOI matched the diameter of the tube the error of activity was, on average, less than 6.1 \% (less than 4.9 \% for tubes larger than 6 mm in diameter) when evaluating the measured phantom data without added background. Also, varying backgrounds were added creating different contrasts between the tubes and background. When adding background the noise in the image is increased and the results from the PVCs, when using the most accurate diameter, were less accurate with a total average activity error of 17.9 \% (11.1 \% for diameters larger than 6 mm and 22.4 \% for diameters smaller than 6 mm). As a conclusion, the size of the blood vessel needs to be accurately known in order for the PVC to give the most accurate result. Also using vessels larger than 6 mm is beneficial.