Application of PEPT to track a single labelled red blood cell in vitro

Abstract

Positron Emission Particle Tracking (PEPT) was invented and developed at Birmingham. This technique is a variant of the medical imaging technique of positron emission tomography (PET). The tracking technique enables a single positron-emitting tracer particle to be tracked accurately inside an opaque vessel, which is within the field of view of the PET camera. The basis of PEPT is to detect coincidentally a pair of opposite annihilation photons. This will define a line on which the annihilation is assumed to occur. The tracer moving at 1 m/s can be located to within 0.5 mm 250 times per second in 3D by triangulation of a number of detected annihilation events. PEPT has proved to be a very powerful tool for studying the fundamental physics of particulate systems or systems of industrial interest. Apart from these applications, we explored the possibility that PEPT could also be applied to the tracking of a red blood cell. As the progress of this feasibility study remained only at the preliminary stage, this thesis seeks to describe the main factors which would be involved in producing the 11CO gas and labelling the red blood cells. Some preliminary results and figures are also revealed. Using the Birmingham MC40 cyclotron, we produced 11C (t1/2=20.385 mins) and acquired 11CO gas through the chemical processing system. This 11CO gas was used as the positron emission source which combined with the red blood cells. Red blood cells have a high affinity for CO. In the human body, when CO is absorbed by a red blood cell, it combines with the oxygen carrying haemoglobin to form carboxyhaemoglobin. There are approximately 200~300×106 haemoglobin molecules in a single red blood cell. Ideally, if all the oxygen positions in a red blood cell could be replaced by 11CO, enough activity might be achieved for the purpose of tracking. In fact, we found that the possibility of tracking a single labelled red cell in vitro is low, due to the low specific activity of produced 11CO and the poor labelling technique. However using the PEPT technique, with some adjustments of energy windows and the distance of the detectors, a tracer with an activity much less than 0.15 μCi should be still possible to be tracked despite the maximum velocity of the tracer one can usefully track at is much lower