Abstract
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Objectives: Kinetic modelling of dynamic PET data requires knowledge of the time-course of the arterial blood activity concentration. This is normally acquired by invasive means [1] and is called the arterial input function (AIF). Kinetic modelling of PET data can predict disease progression more accurately than conventional static PET [2]. The purpose of this study was to validate a previously tested [3] dual readout scintillating fiber-based non-invasive positron detector, hereinafter celled NID, developed to determine the AIF for dynamic PET against a previously validated microfluidic beta detector [4]. The NID will allow for non-invasive collection of the AIF, which will enable kinetic modelling of dynamic PET data to be performed in a greater number of clinics.
Methods: The wrist detector system consisted of a 1 mm diameter and 3 m long plastic scintillating fiber. Each end of the fiber was coupled to a 5 m long optical fiber cable for scintillation light transmission. The optical fiber cables were connected to analog photomultiplier tubes, which were read out by an oscilloscope. The scintillating fiber is enclosed inside the grooves of a 3D printed plastic cylindrical shell, where the patient will place their arm. The radial artery depth is 1.99 mm ± 0.99 mm. A cylindrical polyethylene phantom with a 3.2 mm hole drilled at a depth of 2.0 mm was used to simulate the human radial artery. The phantom was placed in the shell housed in a light-tight box and covered with opaque black cloth. A closed loop system was created using a polyethylene tube with 0.53 mm inner diameter and 0.965 mm outer diameter through the NID, the microfluidic beta detector, a pump and liquid reservoir. Each scan started with 30 seconds of water circulation (6 mL total) followed by an almost instant tracer injection, then mimicking the clearance of the radiotracer, 6 mL of water over one minute was added to water tank 2 minutes after the radiotracer injection. Total scan duration was 10 minutes. Six scans were performed, one with clinically relevant concentrations and one below clinically relevant concentrations for 18F, 11C and 68Ga. The clinically relevant concentrations were 5.6, 13.4 and 10.3 MBq/mL respectively. The clinically non-relevant concentrations were 2.0, 1.9 and 1.5 MBq/mL respectively. The data was normalized to the output of the microfluidic detector.
Results: The curves acquired with the clinically relevant concentrations show a good agreement between the two detectors. The signal to noise ratio (SNR) for 18F, 11C and 68Ga were 5.16, 22.16 and 15.38 respectively. At clinically relevant concentrations, the NID is capable of accurately detecting activity concentration changes in the system. The curves from scans below clinically relevant concentrations agreed roughly and have much lower SNRs. The NID will be used to measure the AIF from the human radial artery which has an average diameter of 2.2 mm [5], but this study used tubing with an inner diameter of 0.53 mm. Due to the increased diameter, there would be 17 times the volume in a human artery diluting the activity concentration by 17. Taking this into account, the highest concentrations used in this study, corresponding to ~0.33 MBq/mL of 18F in the human artery, are clinically relevant [6].
Conclusions: At clinically relevant concentration levels and using a phantom model, the NID provided a good SNR and agreed with the microfluidic detector using three radionuclides showing that it could be suitable for clinical use.