Noise power spectrum in compressed sensing magnetic resonance imaging.

Compressed sensing magnetic resonance imaging (CS-MRI) uses random undersampling and nonlinear iterative reconstruction. This study was conducted to clarify the noise power spectrum (NPS) characteristics of CS-MRI. We measured two-dimensional (2D) NPS of CS-MRI with various acceleration factors (AF) and denoising factors (DF) and compared their appearance to those of conventional parallel MR images. Results showed that the 2D NPS of CS-MRI exhibited the following characteristics: (1) local decrease in the low-frequency region, (2) gradual decrease in the high-frequency region, and (3) a stripe pattern aligned at unequal intervals in the phase-encoding direction. Specifically, the 2D NPS of CS-MRI reflects the random undersampling pattern of k-space data. Additionally, 2D NPS allowed visualization of AF-dependent noise characteristics of CS-MRI. Furthermore, 1D NPS graph shapes clarified the CS-MRI noise characteristic dependence on AF and DF.

[Optimization of Echo Train Length in Non-contrast Enhanced MR Angiography for Clinical Examination of the Calf Arteries].

PURPOSE: Non-contrast magnetic resonanse angiography (MRA) using the three-dimensional electrocardiogram-synchronized fast spin echo method uses systolic and diastolic arterial signal differences. The method relies on the flow void signal of the arterial flow because of dephasing during systole. However, depiction of slow flow such as that in a calf artery was degraded because of insufficient dephasing during systole. In this study, we optimized echo train length (ETL) using a flow phantom and normal volunteers for clinical examination of the calf arteries. METHODS: Flow phantom and normal volunteer images were obtained with various ETLs (40, 50, 60, and 70). An averaged profile across the tube in the phantom was used for detailed investigation of flow dephasing. Visual evaluation was performed and signal intensity change along vessels was measured using normal volunteer images. Comparison with peak systolic velocity (PSV) measured using ultrasound equipment was also conducted. RESULTS: Results of the flow phantom and normal volunteer study indicated that the overall depictability was improved with ETL 60 and 70, which was higher than the standard value. Additionally, the visualization of the peroneal artery with low PSV of ETL 70 had better depictability than ETL 60. CONCLUSION: This study suggested that ETL 70 might be better for clinical examination of the calf arteries.