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].