Iodine Quantification Using Dual Energy Computed Tomography and applications in Brain Imaging

Abstract: Purpose/Background Dual Energy Computed Tomography (DECT) have been clinically available for over ten years. Two such systems are currently used at Skane University Hospital (SUS) in Lund. By utilizing materials different attenuating properties at different energies, material decomposition and iodine quantification is possible. The aim of this master thesis was to compare the two DECT systems and determine how the accuracy of iodine quantification is dependent on iodine concentration, size of measured structure and radiation dose, in terms of CTDIvol. Furthermore the aim was to develop a method for quantitative evaluation of Multiphase CT Angiography (MP-CTA) using iodine quantification. When there is suspicion of stroke, MP-CTA and CT Perfusion (CTP) are performed. The goal was to find a correlation between MP-CTA and CTP and in extension see if MP-CTA can potentially replace CTP. Materials and methods A phantom study was performed on two DECT systems; IQon Spectral CT (Philips Healthcare) and Somatom Definition Flash (Siemens Healthcare) using a cylindrical Jaszczak phantom with syringes of different inner diameters (17, 7 and 3 mm) inserted. Syringes were filled with dilutions of contrast material solution (Iomeron® 400 mg I/ml, Bracco Diagnostic Inc.) with iodine concentrations in the interval 0.05 - 14 mg/ml. Scans with radiation doses corresponding to CTDIvol of 2.7, 9.0 and 18.1 mGy were performed to test for potential dose dependence. The deviation in measured iodine concentrations from the true value was studied considering the investigated parameters. For the quantitative evaluation of MP-CTA, on the IQon Spectral CT, a total of seven patients were studied who had CTP and MP-CTA performed. Iodine density as well as HU (corresponding to 70 kVp and mono-energetic 40 keV) was measured as a function of time in vasculatory regions of the brain. The regions were compared between left and right hemisphere and the results were compared to CTP findings. Results Measurements of iodine concentration showed that the accuracy of the iodine quantification decreased for lower concentrations. Both DECT systems had deviations smaller than 50% for concentrations of 0.88 mg/ml and higher. The root mean square deviation (RMSD) showed less accuracy for smaller syringe diameters; for 17, 7 and 3 mm the RMSD was 0.14, 0.43 and 1.00 for Flash and 0.15, 0.23 and 0.51 for IQon. The IQon was generally more accurate than the Flash. The Flash also had larger variations between successive measurements. RMSD was also higher for CTDIvol of 2.7 mGy as compared to the higher dose levels. Quantitative analysis on MP-CTA showed delayed arrival and excretion of contrast material in the same regions which presented with perfusion deficits for CTP. Difference between healthy tissue and tissue with perfusion deficits was larger for iodine density than for HU. Conclusion The results of the phantom study was in line with previously performed studies. Iodine quantification is above all highly dependent on the measured concentration. A trend for larger errors with smaller syringe diameter and lower dose level was also found. IQon had a higher accuracy than the Flash, especially for lower iodine concentrations and for smaller syringes. The patient study results showed good correlation between MP-CTA analysis and CTP findings. Results indicate that the MP-CTA can be used to detect perfusion deficits using iodine quantification. The method needs to be further developed and tested in order to see if it can replace CTP.