Effect of the saturation pulse duration on chemical exchange saturation transfer in amide proton transfer MR imaging: a phantom study

Abstract

Amide proton transfer (APT) contrast imaging is based on the chemical exchange saturation transfer (CEST) of protons between the amide groups and bulk water. Here, we demonstrate the effect of the saturation pulse duration on CEST in APT imaging with use of a clinical MR scanner. Four samples were prepared from chicken egg white diluted with H2O. Experiments were performed on a 3T clinical MR scanner with use of a body coil for two-channel parallel radiofrequency transmission. APT images were acquired at six frequency offsets (± 3.0, ± 3.5, ± 4.0 ppm) with respect to the water resonance as well as one far off-resonant frequency (-160 ppm) for signal normalization. The CEST effect was defined as asymmetry of the magnetization transfer ratio at 3.5 ppm. We measured the CEST effects in the egg white samples with different concentrations at seven saturation pulse durations. The influence of the extension of repetition time (TR) on the CEST effect was also evaluated. The CEST effect was not influenced by the change in TR. The CEST effect was increased significantly with the concentration when the duration was ≥1.0 s (P < 0.01). The CEST effect was highly correlated with the concentration at all saturation pulse durations, and its increase ratio was higher at longer saturation pulse durations. In conclusion, a long saturation pulse duration is useful for the sensitive detection of mobile proteins and peptides in APT imaging.

Clinical applications of chemical exchange saturation transfer (CEST) MRI

“…The RF saturation pulse is typically applied in the clinic at a low saturation power of 0.5–6 μT to improve the selectivity of saturating only the MR signal of the intended proton of a specific agent or biomolecule, hopefully without also saturating the MR signals of other protons, including water protons. In addition, the RF saturation is typically applied for a long duration of ∼1–5 seconds to allow saturation to reach steady state . During this saturation period, the exchange of protons between water and the agent or biomolecule can occur at a chemical exchange rate of ∼30–1000 Hz, thereby resulting in ∼30–5000 saturated protons transferred to water from each agent or biomolecule (ignoring the effects of T 1 relaxation).…”

“…In addition, the RF saturation is typically applied for a long duration of 1-5 seconds to allow saturation to reach steady state. 3,4 During this saturation period, the exchange of protons between water and the agent or biomolecule can occur at a chemical exchange rate of 30-1000 Hz, thereby resulting in 30-5000 saturated protons transferred to water from each agent or biomolecule (ignoring the effects of T 1 relaxation). With sufficient saturation and chemical exchange rate, as little as 2 mM of exchangeable protons on an exogenous agent or endogenous biomolecule can generate a 2% decrease in water signal, which can be quantitatively detected with standard MR acquisition methods (this evaluation is based on an estimate of 72% water composition in tissues; an exchange rate of 1600 Hz; saturation for 1 sec; 55.5 M concentration of H 2 O molecules in pure water; and two protons per H 2 O molecule).…”

The pH sensitivity of APT‐CEST using phosphorus spectroscopy as a reference method

“…The choice of values for saturation time and irradiation power were based on the results of a study performed at 3 T to investigate the APT-CEST contrast. 22 In addition to the saturated images (S sat ), a reference image (S 0 ) without RF irradiation (B1 = 0 μT) was also acquired.…”

An overview of CEST MRI for non-MR physicists

“…In addition to the pulse shape, pulsed saturation brings additional factors to take into account for CEST contrast optimization: individual pulse length, inter-pulse gap, number of pulses, etc. For example, it has been reported that the inter-pulse gap may be utilized to improve the specificity of CEST agents [28, 29]. However in practice, the optimization of the saturation RF pattern is often performed experimentally, not only due to the high number of factors but also because the exchange rate in vivo cannot be exactly known [23, 30].…”