Artifact Pulsation

Another kind of wall-effect was proposed by El perin (1967). He suggested that an adsorbed layer of polymer molecules could exist at the pipe wall during flow and this could lower the viscosity, create a slip, dampen turbulence pulsations, and prevent any initiation of vortices at the wall. Later work (Little 1969), however, with a transparent pipe and dyed polymer, showed that the adsorption could in be fact an experimental artifact (a quantity of polymer solution, trapped in pressure gage piping, slowly diffused back into the solvent flow). Although polymer molecules do more or less adhere to clean surfaces in thin films, there is no interaction with the bulk of the solution which could alter the flow properties (Gyr, 1974). Thus, it is evident that adsorption of the additives on surfaces is not the reason for the drag reducing effect. 

Voxels that represent the AIF must meet the following conditions (a) The maximum signal drop is larger, (b) the TTA is shorter, and (c) the rMTT is lower than that of normal brain tissue (Fig. 6.6). Furthermore only voxels in the vicinity of the feeding vessels are considered, whereas voxels within the feeding vessels suffer from pulsation artifacts and therefore cannot contribute to the calculation of AIF. 

Pulsation of the aorta causes artifacts, especially in the aortic root and ascending aorta. ECG-triggered data acquisition helps to significantly reduce these artifacts and therefore plays an important role in the examination of unclear chest pain or thoracic aortic dissection (Fig. 23.1). Furthermore, this technique enables the evaluation of coronary arteries and can replace invasive clinical diagnostics in some cases. Flowever, detailed protocols of the thoracic aorta are normally based on protocols of coronary artery CT. Due to their specifications, they lead to longer acquisition times than those of standard protocols and do not properly visualize the abdominal aorta. This can be overcome by an ECG-gated acquisition of the thoracic aorta and a change to the standard protocol for the abdominal aorta. In order to achieve a sufficient contrast in the abdominal aorta, the time delay to modify the examination protocol should be minimized (Fig. 23.2). 

Fig. 23.1. Axial images of the aortic root and the left coronary ostimn with (left) and without ECG gating (right). The images without ECG gating show pulsation artifacts, which could be misinterpreted as a dissection membrane...

An early diagnosis and operative treatment before hemodynamic aggravation or instability are most important for the prognosis of the patient (Nienaber and Eagle 2003). In CTA, pulsation artifacts of the ascending aorta can imitate or mask an intimal tear. An exact evaluation of the aortic root and of the coronary arteries should therefore be achieved by CT scanners with a fast acquisition and ECG-gated algorithms (see Fig. 23.1). 

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