Relationship Between Contrast Media Volume and Tube Voltage in CT for Optimal Liver Enhancement, Based on Body Weight.

Overview

Computed Tomography (CT) is widely used in abdominal imaging for a variety of indications. Contrast media (CM) is used to enhance vascular structures and organ parenchyma. Attenuation of the liver makes it possible to recognize hypo- and hypervascular lesions, which are often invisible on unenhanced CT images. Lesions can only be detected in case they are large enough and the contrast with the background is high enough. Heiken et al. showed already in 1995 that a difference in Hounsfield Units (HU) of at least 50 HU is needed to be able to recognize liver lesions \[1\]. On the other hand, patients should not receive more CM than necessarily, because of possible underlying physiological effects \[2-4\]. Although there has been some controversy about this recently, there is no need to give patients more CM than needed, because of increased costs, no quality improvement and their might still be a relationship with contrast induced nephropathy (CIN) \[5\]. Recent publications suggested individualization of injection protocols that can be based on either total body weight (TBW) or lean body weight (LBW) \[6-9\]. In the investigators department an injection protocol based on TBW is currently used. Besides the CM injection parameters, scanner parameters are of influence on the attenuation as well. Because of recent technical developments it became possible to reach a good image quality (IQ) at lower tube voltages \[10\]. When the x-ray output comes closer to the 33 keV k-edge of Iodine, attenuation increases. In short, decreasing the tube voltage increases the attenuation of iodine. Scanning at a lower tube voltage therefore gives rise to even lower CM volumes. Lastly, it would be revolutionary to accomplish a liver enhancement that is homogenous, sufficient for lesion detection and comparable between patients and in the same patients, regardless of weight and scanner settings used.

Computed Tomography (CT) is a non-invasive imaging tool, used for a great variety of indications. Contrast media (CM) is used to enhance vascular structures and organ parenchyma. The visibility of liver lesions depends mainly on the ratio between the size and the difference of the lesion to the background. A large lesion might be visible without administration of CM, whilst a smaller lesion needs the addition of CM to become visible. Additionally, CM can be useful in the characterization of liver lesions. Heiken et al. (1995) found that an attenuation of the parenchyma after CM administration of at least ∆ 50 Hounsfield units (HU) compared to an unenhanced scan (in the same patient) is necessary to recognize liver lesions. This study proposed a dosing factor of 0.521 g I/kg to be necessary to reach such attenuation at a tube voltage of 120 kV \[1\]. The parenchymal enhancement depends on patient, CT scanner and CM factors. Weight, height, cardiac output, age, gender, venous access, breath-holding, renal function and comorbidity all fall under patient factors \[8\]. Recently much research showed preferable outcomes for individualized CM injection protocols, in which the contrast bolus is adapted to patient TBW, LBW or body surface area (BSA) \[6, 7, 9, 13-15\]. In a recent feasibility study in the department of the investigators, the attenuation of the liver parenchyma was evaluated. Results showed that a body weight adapted CM injection protocol resulted in more homogeneous liver enhancement compared to a fixed CM dose (not published yet). With recent technological developments in X-ray tube technology it became possible to use lower tube voltages. As a result making it possible to perform scans with a sufficient image quality (IQ) and a low tube voltage and therefore a lower radiation dose \[10\]. Another advantage lies in the fact that reducing the tube voltage, approaching 33 keV k-edge of iodine, results in an increase in attenuation of the iodine. The new technological developments make it possible to reduce the radiation dose and CM volume at the same time. So reducing the tube voltage, makes it also possible to reduce the CM volume. As recommended by the supplier, it is possible to calculate the total iodine load (TIL) that can be spared with the use of lower kV settings \[16\]. A reduction of 10 kV should result in a 10% reduction in CM volume. Reducing the tube voltage from 120 to 90 kV should therefore lead to a 30% reduction in CM volume. As mentioned before it is preferred to use an individualized CM injection protocol based on TBW or LBW. For this study, this theory is adapted to the concept of TBW. The following indicates which dosing factors should be used for each kV setting, based on the recommendations mentioned in the above. 120 kV -\> 0.521 g I/kg 110 kV -\> 0.469 g I/kg 100 kV -\> 0.417 g I/kg 90 kV -\> 0.365 g I/kg 80 kV -\> 0.313 g I/kg 70 kV -\> 0.261 g I/kg The aim of present study is to investigate if adapting the dosing factor based on TBW and therefore the CM volume to the tube voltage used, results in a more homogeneous liver enhancement. The hypothesis is to find a more homogeneous enhancement between patients and in the same patient, regardless of body composition and tube voltage used.