Keywords:
Radiation physics, Radioprotection / Radiation dose, CT, CT-Quantitative, Cone beam CT, Physics, Radiation safety, Education, Quality assurance, Education and training
Authors:
A. Albngali, A. Shearer, W. van der Putten, B. Tuohy, N. Colgan; Galway/IE
DOI:
10.1594/ecr2018/C-1445
Methods and materials
•With the small ion chamber method ,
the accumulated dose was directly measured by integrating the current from an ion chamber located at a fixed point in the phantom at the mid-point of the scanned length (Figure 6) ,
while the water phantom central axis was aligned with the CT rotation axis.
The Farmer chamber was placed in the phantom in order to centre the charge collection volume.
The charge collected by the ion chamber,
qh (nC),
was converted to accumulated dose using the equation D(0)=Nk qh (2)
•where q_h (nC) is the total charge collected by the ionization chamber during scanning over the length, and N_k (mGy/nC) is the ionization chamber calibration factor supplied by the National standards of the German National Laboratory.
• The equilibrium dose was determined for both the centre and peripheral axes.
The planar average equilibrium dose (3) was measured and compared with CTDI volume using
D(eq.t)=½ D(eq.center )+½ D(ep.eripheral) (3)
•The accumulative dose and CTDI100 at the centre and periphery were measured for three protocols (head,
chest and abdomen).
•CTDI was measured using a 100 mm long ionization chamber.
The CTDI100 was determined for central and peripheral axes using the perspex phantom.
To make a comparison between the CTDI approach and Dixon method,
we utilised the approach of Dixon et al; by taking the dose from the pencil chamber for one slice and multiplied by 100 then dividing by number of slices and the thickness of the beam (CTDI100=DxL/NxT) (4) determining the dose profile over the whole chamber.
•This allows the assessment of CTDI volume and comparison it with the planar average equilibrium dose (table 2)