Measurement of half-value layer in x-ray CT: A comparison of two noninvasive techniques

The purpose of this paper is to determine the accuracy and reproducibility of two noninvasive methods of measuring half-value layer (HVL), ring and lo calization, compared with an invasive technique (suspending tube rotation). The ring method uses concentric aluminum rings about a CTDI ionization cha mber at isocenter. Data were acquired using axial CT protocols (rotating x- ray tube, stationary patient table). The localization technique uses square aluminum sheets secured to the gantry shroud to filter the radiation beam, and a CTDI chamber suspended externally at isocenter. Data were acquired u sing localization image protocols (stationary x-ray tube, moving patient ta ble). The invasive technique was similar to the localization technique exce pt that the ion chamber was placed on the patient table and the tube rotati on disabled using service software. Data for all techniques were collected on the same CT system. Independent data sr:ts were collected to determine r eproducibility. Sensitivity to ionization chamber lateral displacement from isocenter was investigated. Measured HVLs (mm aluminum, mean+/-std, n=4) w ere 7.19+/-0.03 (ring); 7.17 +/-0.04 (localization); and 7.24+/-0.02 (servi ce mode), which were not significantly different (p = 0.05). Displacing the chamber from isocenter changes the HVL, depending on the bow-tie filter, b y as much as 5 mm aluminum. Aluminum filter to ion chamber distances of 25- 35 cm provided accurate results. Both noninvasive techniques were accurate and reproducible at isocenter. However, the measured HVL was dependent upon the bow-tie filter and the lateral displacement of the ionization chamber with respect to isocenter. Greater than 2 cm off of isocenter, the ring tec hnique did not provide accurate HVL measurements. (C) 2000 American Associa tion of Physicists in Medicine. [S0094-2405(00)03108-4].