Dose resolution in radiotherapy polymer gel dosimetry: effect of echo spacing in MRI pulse sequence

In polymer gel dosimetry using magnetic resonance imaging, the uncertainty in absorbed dose is dependent on the experimental determination of T₂. The concept of dose resolution (DΔp) of polymer gel dosimeters is developed and applied to the uncertainty in dose related to the uncertainty in T₂ from a range of T₂ encountered in polymer gel dosimetry. DΔp is defined as the minimal separation between two absorbed doses such that they may be distinguished with a given level of confidence, p. The minimum detectable dose (MDD) is DΔp as the dose approaches zero. DΔp and the minimum detectable dose both give a quantifiable indication of the likely practical limitations and usefulness of the dosimeter. DΔp of a polyacrylamide polymer gel dosimeter is presented for customized 32-echo and standard multiple-spin-echo sequences on a clinical MRI scanner. In evaluating uncertainties in T₂, a parameter of particular significance in the pulse sequence is the echo spacing (ES). For optimal results, ES should be selected to minimize DΔp over a range of doses of interest in polymer gel dosimetry.