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Estimating iodine concentration from CT number enhancement

R Eaton, A Shah, J Shekhdar

Mount Vernon Hospital


Background: We have undertaken preparatory phantom work for a project that will use dynamic contrast enhanced CT to assess the malignancy or otherwise of solitary pulmonary nodules. Iodine contrast agent will be injected intravenously. The CT number in a region of interest over the lung nodule will be measured pre-contrast and at 1 minute intervals post-contrast for several minutes. The maximum measured CT number will be used as an indicator for malignancy. Previous trials have shown that enhancement of > 15 HU at 120 kV is correlated with malignancy.

This paper will outline our investigations into using a lower kV in order to achieve greater CT number enhancement for the same iodine concentration, and thus better noise statistics.

Method: The correlation between CT number and iodine concentration was measured at 80, 100 and 120 kV. Measurements were made in air and surrounding a water phantom.

Lung nodules were simulated by introducing small volumes of low concentration iodine into an anthropomorphic thorax phantom. CT number enhancement over baseline (water) was measured for three nodule locations (centre of lung, heart border, near rib) at 80, 100 and 120 kV. Measurements were repeated for a 'large' phantom, obtained by surrounding the thorax phantom with saline bags.

Results: Greater enhancement in CT number was measured at lower kV. The gradient of CT number against iodine concentration decreased by around 20% when measured next to a water phantom relative to that measured in air.

Anthropomorphic phantom measurements showed that different CT numbers were measured for the same iodine concentration, depending on the location of the region of interest within the thorax. This was particularly acute for the phantom representing a small patient, and at low kV (80 kV).

Conclusions: CT number enhancement over baseline can potentially be used to determine iodine concentration in vivo. However, beam hardening effects mean that calculated concentrations are likely to have poor accuracy for small patients, particularly if low kV is used. The CT number to iodine concentration coefficient must be measured in an appropriate phantom to mitigate these effects as far as possible.

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