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

Abstract

In polymer gel dosimetry using magnetic resonance imaging, theuncertainty in absorbed dose is dependent on the experimental determination ofT2. The concept of dose resolution (DΔp) of polymer gel dosimeters isdeveloped and applied to the uncertainty in dose related to theuncertainty in T2 from a range of T2 encountered inpolymer geldosimetry. DΔp is defined as the minimal separation between twoabsorbed doses such that they may be distinguished with a given level ofconfidence, p. The minimum detectable dose (MDD) is DΔp as thedose approaches zero. DΔp and the minimum detectable dose both givea quantifiable indication of the likely practical limitations and usefulnessof the dosimeter. DΔp of a polyacrylamide polymer gel dosimeter is presented for customized32-echo and standard multiple-spin-echo sequences on a clinical MRI scanner.In evaluating uncertainties in T2, a parameter of particular significancein the pulse sequence is the echo spacing (ES). For optimal results, ES shouldbe selected to minimize DΔp over a range of doses of interest inpolymer gel dosimetry.