Correction for CT number changes caused by off-centre patient positioning: A phantom study

Xiaoming Zheng, Yazan Al-Hayek, Chris Cummins

Research output: Other contribution to conferencePresentation onlypeer-review


Objectives: To present a correction scheme for X-ray CT number variations caused by off-centre patient positioning and correction results of tissue equivalent materials within an electron density phantom.
Materials and Methods: A Somatom Emotion CT scanner (Siemens, Germany) was employed to acquire images of model 062M electronic density phantom from CIRS (Computerized Imaging Reference System). The phantom’s 17 tissue filling holes were first filled with water balloons. Images of water filled phantom were acquired at various positions of 0 (iso-centre), 2, 4, 6 and 8 cm off-centre. One water balloon at 450 of the outer ring of the phantom was then replaced by tissue equivalent materials. CT numbers were calculated by averaging 6 central slices of the phantom using ImagJ. The measured water CT numbers were used to correct for the CT numbers of tissue equivalent materials at various off-centre locations.
Results: Both water and tissue equivalent material’s CT numbers were found to be decreased as phantom position is shifted from the iso-centre. For an 8cm shift from the iso-centre and at the 450 outer ring location, the water’s CT number is decrease by 5.4 HU whilst a liver tissue is decreased by 6.1 HU. Employing measured water attenuation co-efficient in the Hounsfield formula for CT number calculations, the liver’s CT numbers at various off-centre shifts were calculated which is constant of 55 HU for all off-centre positions.
Conclusion: Local water’s attenuation co-efficient can be used to correct for tissues’ CT number variations caused by off-centre patient positioning.
Original languageEnglish
Publication statusPublished - 01 Jul 2021
Event19th Asian Oceanian Congress of Radiology - Virtual, Kuala Lumpur, Malaysia
Duration: 01 Jul 202104 Jul 2021 (Conference website) (Abstracts) (Program)


Conference19th Asian Oceanian Congress of Radiology
Abbreviated titleFrom Pixel to Clarity
CityKuala Lumpur
OtherAOCR is a congress for the Asian Oceanian Society of Radiology. The aim is to promote scientific collaborations, professional rapport, exchanges of knowledge and friendship amongst the Asian Oceanian region. The theme for the meeting is ‘From Pixel to Clarity’.
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