Dual Energy: Virtual Non-Contrast
The latest spiral dual energy scanning techniques facilitate easy derivation of “virtual non-enhanced” scans. Put simply, it gives us the possibility to derive non-contrast images from contrast scans without scanning the patient twice.
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Case 1: Scanning of a Small Polyp in Gallbladder Case 2: Exclusion of Urolithiasis in the Presence of Contrast Media The latest spiral dual energy scanning techniques facilitate easy derivation of “virtual non-enhanced” scans. Put simply, it gives us the possibility to derive non-contrast images from contrast scans without scanning the patient twice. Background The introduction of multi-detector row CT scanners has substantially broadened the spectrum of clinical applications and examination techniques in recent years. Thinner slices enable improved spatial resolution with a current minimal resolution of 0.3 mm. The true isotropic voxel has been realized since the 64-slice scanners and now the image quality of arbitrarily reconstructed planes is the same as the image quality of the scan plane. Faster scanning not only allows for the examination of long scan ranges such as the thoracoabdominal aorta or even the entire vascular tree, but also for studies in distinctively different contrast phases. This is very helpful for better discrimination of benign or malignant lesions especially in the highly vascularized organs such as the kidney or liver.1, 2 In kidney exams, the different phases of contrast uptake can be differentiated using either the nonenhanced, arterial, cortico-medullary, nephrographic or excretory phase. In liver imaging, these phases include non-enhanced, early arterial, late arterial, portal venous and late. Even vascular studies such as the assessment of the aorta may require several phases to depict all possible pathologies.3 Despite these technical advances, the basic principle of image acquisition has remained virtually unchanged since the introduction of Computed Tomography in the 1970s. The x-ray attenuation of any scanned object or organ is measured and expressed in Hounsfield Units.4 The scale was defined as follows: -1000 HU is air and 0 HU is water; the positive range of the scale has no limit. The variability of HU values found in a human body generally do not vary tremendously with the exception of bone (high HU values) and gas (air)-filled spaces. Soft organ tissue is generally iso-dense compared with the surrounding tissue, making the injection of contrast media necessary to better delineate individual organs or specific structures within the organ (e.g. focal liver lesions). However, sometimes the sought after pathology is only very subtly delineated, such as intramural hematomas when scanning on suspicion of aortic dissection. In these cases, contrast media should not be applied
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Dual Energy: Virtual Non-Contrast
in order to avoid masking the pathology with highly dense contrast material inside the aorta. Another example is kidney stones. Since the kidneys actively secrete i.v. administered contrast agents, small calculi within the ureter or the renal pelvis may be masked by the surrounding contrast medium. Clinical diagnostics not only involves detecting a given structure, but also its characterization. The detection of a “focal liver lesion” is clinically insufficient as the sp
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