Magnetic Resonance Angiography - Acquisition - 2D and 3D Acquisitions

2D and 3D Acquisitions

For the acquisition of the images two different approaches exist. In general, 2D and 3D images can be acquired. If 3D data is acquired, cross sections at arbitrary view angles can be calculated. Three-dimensional data can also be generated by combining 2D data from different slices, but this approach results in lower quality images at view angles different from the original data acquisition. Furthermore, the 3D data can not only be used to create cross sectional images, but also projections can be calculated from the data. Three dimensional data acquisition might also be helpful when dealing with complex vessel geometries where blood is flowing in all spatial directions (unfortunately, this case also requires three different flow encodings, on in each spatial direction). Both PC-MRA and TOF-MRA have advantages and disadvantages. PC-MRA has fewer difficulties with slow flow than TOF-MRA and also allows quantitative measurements of flow. PC-MRA shows low sensitivity when imaging pulsating and non-uniform flow. In general, slow blood flow is a major challenge in flow dependent MRA. It causes the differences between the blood signal and the static tissue signal to be small. This either applies to PC-MRA where the phase difference between blood and static tissue is reduced compared to faster flow and to TOF-MRA where the transverse blood magnetization and thus the blood signal are reduced. Contrast agents may be used to increase blood signal – this is especially important for very small vessels and vessels with very small flow velocities that normally show accordingly weak signal. Unfortunately, the use of contrast agents, typically containing Gadolinium, can be dangerous if patients suffer from particular diseases. Especially in cases of kidney malfunctions they might lead to severe poisonings or even death. For this reason non-contrast-enhanced methods have been researched recently.