Optimization and deconvolution of lithium fluoride TLD-100 in diagnostic radiology

Lithium fluoride (LiF) TLD-100 is one of the most commonly used thermoluminescent (TL) materials for the measurement of entrance surface dose (ESD) in diagnostic radiology. However, the minimum detectable dose (MDD) achieved, as derived from measurements of the random uncertainty present in the background signal, is usually quoted as being 50-100 microGy. A more appropriate definition of MDD for use in the clinical setting is the dose at which measurements exhibit a specified level of random uncertainty. This definition will give rise to a higher value for the MDD. An MDD of 50-100 microGy precludes accurate measurement of ESD in high tube potential (kVp) chest or neonatal radiography. Techniques described in the specialist literature for the reduction of the MDD of LiF were assessed both in the laboratory, and during a patient dose survey of high kVp chest radiography. Optimization of the pre-irradiation annealing and post-irradiation TL read heating cycles in terms of sensitivity and precision resulted in an MDD of 5/80 microGy (derived from background signal variation and 20% random uncertainty at 95% confidence limits, respectively). Deconvolution of the glowcurve was found to result in an MDD of approximately 10 microGy. Clinical measurements were contrasted with calculated values derived from ionization chamber measurements of tube output. The results support the hypothesis that glowcurve deconvolution permits the measurement of ESDs from low dose examinations using basic TL dosimetry equipment available to virtually all medical physics departments.