Contrast Improvement, Artifacts, and Artifact Reduction
As discussed in preceding chapters, the basic rationale underlying time-of-flight (TOF) MRA is the utilization of inflow enhancement while compensating for flow-induced phase shifts. For high-resolution studies with isotropic voxels a three-dimensional, or volume acquisition technique is recommended, with the major flow along the slab select direction (Masaryk et al. 1989). While inflow enhancement and resulting vessel contrast is good enough on the side where the vessels enter the volume, the blood signal becomes progressively smaller towards the exit side of the volume. Therefore, the visibility of blood vessels decreases as blood penetrates the volume, as shown in Fig. 4.1. Ultimately, at some point there is no longer any contrast between blood and surrounding stationary tissue. The slab thickness and corresponding vessel coverage cannot be selected on an anatomical basis but rather are determined by contrast issues. The reason for the loss of blood signal is related to the decrease in the spins’ magnetization as they enter the imaging volume and experience the rf pulses in the sequence. This effect is demonstrated in Fig. 4.2 in the form of a numerical simulation. The graph shows the transverse magnetization Mx which corresponds to the signal from the blood as a function of the number of rf pulses. For spins moving at a constant velocity v the number of rf pulses directly corresponds to the spins’ position in the imaging volume when multiplying the velocity G. LAUB, PhD, Siemens AG, Medizinische Technik, Henkestraße 127, 91052 Erlangen, FRG by the pulse repetition time TR and pulse number. Three different curves are shown for different values of the rf excitation pulse.
KeywordsFlip Angle Magnetization Transfer Artifact Reduction Tone Pulse Contrast Improvement
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