|
|
WIDE BEAM CT DOSIMETRY: IMPLEMENTATION OF DIFFERENT METHODS ON A PHILIPS ICT DEVICE Host Publication: Abstract book of BHPA 2016 Authors: B. Keelson, G. Van Gompel and N. Buls Publication Year: 2016
Abstract: I.Introduction and purposeCurrently, the CTDI100 is the standard metric to obtain radiation dose of a CT scan. However, current CT systems often employ wider collimated X-ray beams (> 4cm), which can result in inaccuracies of the reported values1,2. This is because, by definition, this metric does not include radiation beam (both primary and scatter) outside its 100 mm integration range. A number of approaches towards redefining the CTDI100 for wider CT beams, as can be found in (IEC 3.0 2009 and IEC 3.1 2010)1, have been recommended. In this study, we implemented measurements of CTDI100 following (IEC 3.0 2009 and IEC 3.1 2010) on a Philips iCT 256 system. Calculated weighted CTDI (CTDIw) results were compared to the device’s indicated values as well as to CTDI obtained in two phantoms stacked together with a combined length of 35cm (CTDIfull). The CTDIfull served as an approximation of CTDI measured with an extended integration length. The purpose of this study was to assess the recommended methods for different scan collimations and tube voltage settings on an 8 cm wide beam Philips iCT system.II.Material and methodsA Philips iCT 256 scanner (Philips Electronics N.V) was used for all measurements. The body phantom used was a PMMA 32 cm diameter phantom of 20 cm in length for the CTDI100 measurements. A second body phantom also of 32 cm diameter of 15 cm in length was also used by attaching it to the first phantom for the CTDIfull measurements. Dose measurements were accomplished using a CT pencil ionization chamber of 10 cm active length connected via an RTI chamber adapter to the Piranha CB2� (RTI Electronics AB, Sweden). A fixed mAs of 75 and tube voltages of 80, 100,120 and 140 kVp were used for collimations (N X T) of 0.25,1,2,4,6 and 8 cm. CTDI measurements in the 32 cm body phantom were performed following the positioning of the phantom and the ionization chamber as per IAEA1 recommendations. CTDI100 following IEC 3.0 was implemented by dividing the measured dose in mGy.cm by the corresponding beam width in cm.Free-in-air measurements were determined in the isocenter. For collimations over 6 cm the chamber was translated in the longitudal (z) direction by the active length. IEC 3.1 was calculated by using the CTDI weighted measurement of a reference beam width (2cm) and scaling it by the ratio of the intended CTDI in air (CTDIfree-in-air NxT ) to the air measurements of the reference beam ( CTDIfree-in-air reference ). Measurements were also obtained using two phantoms by translating the chamber for three positions through the double phantom. This allowed to measure the dose profile as complete as possible. III.Results and discussionAll the implemented methods as well as the device’s reported CTDI underestimated the dose relative to CTDIfull. The dose difference between IEC 3.0 and IEC 3.1 was prominent with the 8 cm beam with IEC 3.0 underestimating by 14% at 120 kV. The choice of reference beam for implementing IEC 3.1 influenced the obtained result by 5% relative to the recommended 2 cm reference beam width. All the methods showed an increased deviation from CTDIfull as the KVp increased with the IEC 3.0 exhibiting a 30% underestimation at 140 KVp for the 8cm beam.IV.ConclusionsThe IEC 3.1 provides a potential improvement to the challenges of CTDI100 as applied in wide beam CT dosimetry. A gradual progression to the implementation of this standard in wide beam CT will enable more accurate dose comparison studies based on a single standard implementation. The method also shows a consistent bias relative to CTDIfull across collimations. REFERENCES1.IAEA human health report No. 5, 2011 2.Status of computed tomography dosimetry for wide cone beam scanners," Available: www-pub.iaea.org/ MTCD/Publications/PDF/Pub1528 web.pdf
|
|
