All image data were saved as raw echo intensities and reconstructed offline with custom software. Partial Fourier raw data was acquired at lines –m ≤ u ≤ N/2 (m = 6, N= 64). The low frequency phase map of each coil was estimated from the Fourier transformation of the image generated by Hanning filtering the center portion of the raw data at lines –m ≤ u ≤ m. The final image was combined from each coil image, weighted by the conjugate of the corresponding low resolution phase map. A Region of interest (ROI) for each subject was defined to contain the entire brain.
For the first study, the relative combined inversion efficiency was defined as the ratio of the PCASL average difference signal when control and label are set below the brain to the average CASL difference signal after compensating for the duty cycle difference (dividing the CASL difference signal by dcycle). dcycle is the duty cycle of continuous ASL, which is 53.33% in our study. Previous simulations of adiabatic fast passage demonstrate that the labeling efficiency may not have a linear relationship with the RF duty cycle in the pulsed-form CASL(32 (link)). Simulation was performed to calculate the mean labeling efficiency for the laminar flow across the different time when spin passes through the labeling plane. The simulation result shows that the relative efficiency is within 2% difference with the RF duty cycle. This linearity between the RF duty cycle and labeling efficiency is likely valid because of the long pulse period (375 ms) used in the CASL sequence. This improvement with longer labeling blocks (A pulse period of 100 ms was better approximated by a linear relationship than a pulse period of 20 ms) was suggested in previously reported simulations (32 (link)). The relative efficiency loss of the control pulse was defined as the ratio of the PCASL average difference signal when the control is set below the brain compared with above the brain to the average CASL difference signal after compensating for the duty cycle difference. Because of the very low SNR in the average difference signal between controls in PCASL, we used the low-resolution phase map from the average difference signal between control and label when applied below the brain to phase correct the images from each coil. The relative systematic error between control and label pulses was defined as the ratio of the average PCASL difference signal between control and label when applied above the brain to the average CASL difference signal after compensating for the duty cycle difference. The relative efficiency of labeling pulse was defined as the ratio of the PCASL average difference signal when the label is set below the brain compared with above the brain to the average CASL difference signal after compensating for the duty cycle difference. We used low-resolution phase maps to correct the phase of the image from each coil as in the relative efficiency calculation of control pulse.
For the second study the relative combined efficiency described for the first study was employed. For the third study, frequency-dependent off-resonance saturation effects were analyzed by averaging the difference images (between the control and label images) across the phase direction.
For the first study, the relative combined inversion efficiency was defined as the ratio of the PCASL average difference signal when control and label are set below the brain to the average CASL difference signal after compensating for the duty cycle difference (dividing the CASL difference signal by dcycle). dcycle is the duty cycle of continuous ASL, which is 53.33% in our study. Previous simulations of adiabatic fast passage demonstrate that the labeling efficiency may not have a linear relationship with the RF duty cycle in the pulsed-form CASL(32 (link)). Simulation was performed to calculate the mean labeling efficiency for the laminar flow across the different time when spin passes through the labeling plane. The simulation result shows that the relative efficiency is within 2% difference with the RF duty cycle. This linearity between the RF duty cycle and labeling efficiency is likely valid because of the long pulse period (375 ms) used in the CASL sequence. This improvement with longer labeling blocks (A pulse period of 100 ms was better approximated by a linear relationship than a pulse period of 20 ms) was suggested in previously reported simulations (32 (link)). The relative efficiency loss of the control pulse was defined as the ratio of the PCASL average difference signal when the control is set below the brain compared with above the brain to the average CASL difference signal after compensating for the duty cycle difference. Because of the very low SNR in the average difference signal between controls in PCASL, we used the low-resolution phase map from the average difference signal between control and label when applied below the brain to phase correct the images from each coil. The relative systematic error between control and label pulses was defined as the ratio of the average PCASL difference signal between control and label when applied above the brain to the average CASL difference signal after compensating for the duty cycle difference. The relative efficiency of labeling pulse was defined as the ratio of the PCASL average difference signal when the label is set below the brain compared with above the brain to the average CASL difference signal after compensating for the duty cycle difference. We used low-resolution phase maps to correct the phase of the image from each coil as in the relative efficiency calculation of control pulse.
For the second study the relative combined efficiency described for the first study was employed. For the third study, frequency-dependent off-resonance saturation effects were analyzed by averaging the difference images (between the control and label images) across the phase direction.