| Description |
This thesis project summarizes improvements that have been made to the multiple overlapping thin slab three-dimensional acquisition magnetic resonance angiography (MOTSA MRA) technique. The primary research hypothesis states that images of the cervical carotid artery obtained using the MOTSA MRA technique can be improved by 1) optimizing imaging parameters during acquisition, 2) processing overlapping slice information, and 3) reducing background signal from fat tissue. Three parameters that affect quality during acquisition are flip angle (FA), repetition time (TR), and the slab excitation fraction (SEF). Be increasing the slab excitation fraction from 07.75 to 1.10, the slab boundary artifact was reduced by 29-56% (p<0.0001). The contrast-to-noise ratio (CNR) of the cervical catorid artery can be increased approximately sixfold (p<0.0001) by increasing the flip angle from 10° to 50°. The effect of the repetition time on image quality was not statistically significant for the range studied (52-70ms). To determine the utility of processing overlapping slice information, a study measuring the slab boundary artifact (V/P SNR) both with and without utilization of overlapping slice information was performed. For the three imaging parameter combinations studies (FA=30°, TR=52ms, SEF=0.75; FA=30°, TR=52ms, SF=1.10; FA=50°, TR=52ms, SEF=1.10), the V/P SNRs without utilizing overlapping slice information was 0.74, 0.86, and 0.83 respectively. Following postprocessing of the overlapping information, the V/P SNRs improved to 0.86, 0.91, and 0.93 respectively. This represents an average improvement of 46% of the slab boundary artifact. To reduce the background signal from fat tissue surrounding the carotid artery bifurcation, the 1331 fat saturation pulse was implemented into the MOTSA MRA pulse sequence. The saturation pulse flip angel (SPFA) and interpulse delay (tau) were studied to identify the values that produce the best vessel visibility. The optimal values for the SPFA and tau were 60° (7.5°,-22.5°, 22.5°, -7.5°) and 2.0-3.0ms respectively. AT these values there was a 1.56-fold increase (SD=±0.12, p<0.01) in the CNR between the left internal carotid artery and fat compared to the study without fat saturation. A threefold increase was noted for the smaller left occipital artery. In can be concluded that the slab boundary artifact of the MOTSA MRA technique can be reduced significantly by optimizing imaging acquisition parameters and processing overlapping slice information. The cervical carotid artery visibility can be significantly improved by optimizing imaging parameters and reducing background signal from fat tissue. |
