We evaluated pulsed-CEST MRI contrast as a function of RF pulse flip angle. shows four simulated Z-spectra, with the RF flip angle being 90°, 180°, 360° and 540°, assuming an RF pulse duration of 15 ms. These results show that whereas prominent CEST contrast can be observed with moderate flip angles (90° and 180°), the contrast decreases significantly at larger flip angles such as 360° and 540°, likely due to concomitant RF spillover effect. It is important to note that the water signal on resonance with RF saturation deviated from zero at the 360°flip angle because the RF pulse restores the bulk water magnetization along the Z-axis before the crusher gradient. shows that for three representative exchange rates of 25, 50 and 100 s−1. The pulsed-CEST MRI contrast peaks when the RF flip angle is about 180–220°, but it bottoms out when the flip angle is beyond 360°. We solved the optimal RF flip angle for a typical range of exchange rate from 10 to 100 s−1. shows that the optimal flip angle increases slightly with the exchange rate, and similarly, with chemical shift ().
We further evaluated how the pulsed-CEST MRI contrast varies with the RF pulse duration, for a representative RF flip angle of 180°. shows three Z-spectra for RF pulse durations of 5, 15 and 30 ms. It is worth pointing out that the RF saturation bandwidth is significantly broadened at short pulse duration, which results in greatly attenuated signal intensity around the bulk water resonance. shows pulsed-CEST contrast as a function of RF pulse duration for three representative exchange rates of 25, 50 and 100 s−1
. The optimal RF duration is prolonged at slower exchange rate, suggesting lower RF amplitude, similar to findings from CW-CEST MRI studies (19
). We also evaluated how the optimal RF duration varies with exchange rate and chemical shift. Our results showed that the optimal RF pulse duration shortens with both exchange rate () and chemical shift (). This is expected given that for the same RF flip angle (i.e., 180°), the equivalent RF power/amplitude is higher at shorter RF duration, and hence, is suitable for imaging CEST agents of higher exchange rate and larger chemical shift. Given that RF bandwidth is associated with the pulse duration, our results suggest that the optimal RF bandwidth also varies with exchange rate and chemical shift. Specifically, the bandwidth is 1610 Hz for a 1 ms Gaussian inversion pulse (RF Shape tool, Bruker Biospin, Billerica MA), which allows conversion of the optimal RF duration into RF bandwidth.
We also compared the pulsed-CEST contrast with the more commonly used CW-CEST MRI. shows that for a representative chemical exchange rate range from 10 to 100 s−1
, the CW-CEST contrast is approximately equal to or higher than the pulsed-CEST contrast. We plotted the difference between pulsed-CEST and CW-CEST contrast as a function of exchange rate, which showed that the pulsed-CEST MRI became less effective than CW-CEST MRI, particularly at higher exchange rate. In addition, the loss of contrast is greater at higher chemical shift (). shows the optimal RF amplitude for the CW-CEST MRI increases with exchange rate for three representative chemical shifts, consistent with the notion that larger RF power is needed to efficiently saturate faster exchange. In addition, the equivalent RF amplitude of pulsed-CEST MRI is calculated as 1
). compares the equivalent RF amplitude of pulsed-CEST MRI with that of CW-CEST MRI. Importantly, the RF amplitude for pulsed-CEST MRI displayed an excellent polynomial relationship with that of CW-CEST MRI, found to be
. This linear regression relationship allows estimation of the optimal pulsed-CEST MRI parameter based on the better-understood CW-CEST MRI.
We validated the simulation using a creatine-gel phantom with serially titrated pH, whose chemical exchange rate covers a representative range of diamagnetic CEST agents (). CEST MRI indeed detected the difference in pH among the compartments, as per the CW-CEST and pulsed-CEST MRI shown in , respectively. For CW-CEST MRI, the RF amplitude is 1.25 μT, while for the pulsed-CEST MRI, we used an RF flip angle of 257° (after RF field calibration) and pulse duration of 20 ms. It is important to note that pulsed-CEST MRI showed noticeably lower contrast than the CW-CEST MRI, particularly for higher exchange rates/pH. shows that pulsed-CEST contrast initially increased with RF flip angle, but decreased after peaking around an intermediate flip angle. Similarly, pulsed-CEST MRI contrast increased with pulse duration, but decreased when pulse duration became too long (). shows a 2-D histogram of optimal RF flip angle and pulse duration for all six pH compartments, and optimal flip angle and pulse duration were found to be approximately 257°(after RF field calibration) and 20 ms, respectively. The optimal flip angle was slightly larger than that estimated from simulation(), likely caused by the relatively coarse intervals of pulse flip angle and duration. In addition, the chemical exchange rate can be estimated using a previously calibrated formula for base-catalyzed creatine amine proton exchange () (14
). showed that CW-CEST contrast increases approximately linearly with exchange rate, suggesting excellent pH response. In comparison, whereas the pulsed-CEST MRI contrast was approximately equal to that obtained with CW-CEST MRI for exchange rates below 50 s−1
, it became significantly less than the CW-CEST contrast at higher exchange rates/pH (). The difference in CEST contrast between pulsed-CEST and CW-CEST MRI was evaluated as ΔCESTR= CESTRpulse
, which showed sizeable loss of contrast for exchange rates above 50 s−1
, as predicted in .
Fig. 5 Experimental evaluation of pulsed-CEST MRI. a) Illustration of multi-compartment creatine-gel pH phantom. b) pH-weighted CW-CEST MRI of the phantom (B1=1.25 μT). c) pH-weighted pulsed-CEST MRI of the phantom (ϕ=257°(after RF field (more ...)
Finally, we evaluated pH-weighted endogenous APT MRI with both pulsed- and CW-CEST MRI sequences, using acute ischemic stroke animals. Ischemic lesion can be appreciated as hypointensity in ADC (), CW- (, B1=0.75 μT) and pulsed-APT (, τ=15 ms and ϕ=180°) images, from a representative coronal slice. A region of interest (ROI) was manually selected in the striatum region based on ADC decrease, being 0.47 ± 0.08 μm2/ms. Its APT contrast was found to be −6.4 ± 1.4% (mean ± S.D.) and −5.9 ± 1.2%, for pulsed- and CW-CEST acquisitions, respectively. Whereas the pulsed-APT is slightly lower than that of CW-APT MRI, it provides reasonably clear depiction of ischemic lesion, and therefore serves as a viable alternative for MRI systems unable to operate CW RF irradiation.
Fig. 6 in vivo evaluation of pH-weighted APT MRI. MCAO induced ischemic tissue damage can be observed in ADC map (a), which also displays hypointensity in pH-weighted APT maps acquired with CW-CEST (b, B1=0.75 μT) and pulsed-CEST (c, τ=15 ms, (more ...)