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Parameterizing Size-Based Variations in CT Number

S Rose*, J Ruyle, T Szczykutowicz, University Wisconsin-Madison, Madison, WI

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Much attention is given to the reduction in contrast observed with increases in tube potential in the design of CT protocols, however, it is well known that contrast also varies with patient size at fixed kV due to beam hardening. The purpose of this study is to parameterize this effect in terms of water equivalent diameter for a variety of materials and tube potentials, providing a simple means for predicting its impact on image quality.


Methods: Helical scans of the Mercury phantom (Sun Nuclear, Middleton, WI) were acquired at 80, 100, 120, and 140kV on a GE Revolution HD CT. The phantom consists of 5 modules of varying diameter, each having a polystyrene background and cylindrical inserts of polyethylene, bone, and iodine (10mg/mL). Reconstructions were performed using the Standard kernel with 5mm slice thickness and interval. Mean CT numbers were measured on the central three slices of each module. WEDs for each module were calculated per AAPM Report 220. Linear fits of CT number to WED were performed for each material and kV.


Results: At 80kV, the iodine insert’s CT number decreased by 6.0HU per centimeter increase in WED. The corresponding measurement at 140kV was 4.8HU/cm, demonstrating the contrast reduction from beam hardening is relatively consistent across kVs. A 5cm increase in WED at 100kV decreased the iodine insert’s CT number by 27HU. For comparison, increasing tube potential from 120kV to 140kV caused a 44HU reduction. Bone CT number was similarly affected with decreases of 13.9HU/cm and 10.6HU/cm at 80kV and 140kV, respectively. CT numbers of polyethylene and polystyrene both varied by less than 1HU/cm across tube potentials.


Conclusion: Patient size has substantial impact on the CT number of high Z materials (e.g. iodinated contrast agent) and should be considered in the design of CT protocols and AEC systems.

Download ePoster [PDF]

Funding Support, Disclosures, and Conflict of Interest: T. Szczykutowicz supplies CT protocols to GE Healthcare under a licensing agreement. He also has an equipment grant with GE HealthCare, is a consultant for GE, Takeda Pharmaceuticals and iMALOGIX, is on the medical advisory board of iMALOGIX, and is the founder of protocolshare.org.

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