Standardisation of 11C
Introduction
11C is used to label molecules for a wide range of physiological studies and its radioactivity is typically measured using a well-type ionisation chamber. It is well known that, in the calibration of positron emitters using external detectors such as Germanium and NaI(Tl) systems, the location of the positron annihilation, which leads to the production of 511 keV quanta, does not necessarily coincide with the location of the initial disintegration. Calibration figures for the 511 keV quanta may then vary depending on the end-point energy of the originating positron. For ionisation chamber systems, there is the added complication that they may also respond to the bremsstrahlung radiation arising from the positron interaction with matter, with the response being dependent on the positron end-point energy.
Because of the short half-lifes of some positron-emitting radionuclides (11C τ1/2=20 min, 13N τ1/2=10 min and 15O τ1/2=2 min), nuclides such as 18F and 68Ga have been proposed as mock standards. In order to validate these proposals, and to determine whether there are any problems arising from the location of the annihilation and brehmsstrahlung detection, it is necessary to determine the 11C calibration figure using an absolutely standardised material. A collaborative project between the National Physical Laboratory (NPL) and Imaging Research Solutions Ltd. (IRSL—formerly, the Cyclotron Unit of the Medical Research Council (MRC)) in the UK was organised for this purpose. The half-lifes and positron energies of commonly used positron emitters are shown in Table 1. The decay scheme of 11C is shown in Fig. 1.
Section snippets
Source material
The 11C was produced as 11CO2 using the 14N(p,α)11C reaction. The target gas containing the 11C species was swept from the target and obtained as carbon dioxide. This was then cryogenically trapped (liquid argon) before being evaporated from the trap at room temperature into a solution (about 8 ml) of sodium hydroxide containing 500 μg g−1 NaHCO3 (this concentration of sodium bicarbonate assured the stability of the solution). On arrival at NPL, the solution was accurately diluted using a solution
Standardisation
11C decays by both positron emission (99.77%) and electron capture (0.23%) directly to the ground state. The standardisations were carried out by two different methods, namely the conventional 4π(PC)–γ-coincidence counting technique and the 4π(LS)–γ-coincidence technique using computer discrimination (Smith (1975), Smith (1987)). The 4π(PC)–γ-coincidence system was operated at atmospheric pressure using Ar/CH4 (90%:10%) as the counting gas. In both cases, a NaI(Tl) detector was used for γ-ray
Half-life measurements
The half-life for 11C was also measured and a value of 0.3389 (±0.0004) h was found. This half-life was used for all measurements in the work.
γ-spectrometry measurements
An intrinsic germanium detector was used to assay 2 ml flame-sealed ampoules containing 1 ml of solution for both 11C and 18F. In both cases, the solutions used were supplied by the IRSL and were shown to contain no other positron emitters. The “pulser” method was used to correct for dead-time and random-summing effects (Debertin and Helmer, 1988). These measurements were undertaken with the ampoule placed both at 10 and 55 cm from the detector face, with and without an annihilator surrounding the
Discussion
There are several results, which need to be noted for any discussion.
For the ionization chambers, the calibration figures in Table 2 represent the responses/106 positron emissions (pA/106β+) rather than the responses/106 disintegration.
If the responses are only due to the 511 keV photons and there are no positional effects, which depend on the positron energy, the calibration figures (pA/106β+) should be the same for 11C and 18F. In practice, the 11C figure appears to be about 0.8% higher than
Acknowledgements
This work was supported by the National Measurement System Policy Unit of the UK Department of Trade and Industry.
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