Quantifying exchange rates in chemical exchange saturation transfer agents using the saturation time and saturation power dependencies of the magnetization transfer effect on the magnetic resonance imaging signal (QUEST and QUESP): Ph calibration for poly‐L‐lysine and a starburst dendrimer

Abstract

The ability to measure proton exchange rates in tissue using MRI would be very useful for quantitative assessment of magnetization transfer properties, both in conventional MT imaging and in the more recent chemical exchange saturation transfer (CEST) approach. CEST is a new MR contrast mechanism that depends on several factors, including the exchange rate of labile protons in the agent in a pH-dependent manner. Two new methods to monitor local exchange rate based on CEST are introduced. The two MRI-compatible approaches to measure exchange are quantifying exchange using saturation time (QUEST) dependence and quantifying exchange using saturation power (QUESP) dependence. These techniques were applied to poly-L-lysine (PLL) and a generation-5 polyamidoamine dendrimer (SPD-5) to measure the pH dependence of amide proton exchange rates in the physiologic range. Data were fit both to an analytical expression and to numerical solutions to the Bloch equations. Results were validated by comparison with exchange rates determined by two established spectroscopic methods. The exchange rates determined using the four methods were pooled for the pH-calibration curve of the agents consisting of contributions from spontaneous (k 0 ) acid catalyzed (k a ), and base catalyzed (k b ) exchange rate constants. These constants were k 0 ‫؍‬ 68.9 Hz, k a ‫؍‬ 1 Chemical exchange saturation transfer (CEST) has recently been proposed as a new imaging contrast mechanism (1) in which the radiofrequency-induced saturation of labile protons in the agent is transferred to water protons. The resulting MRI signal intensity depends on a multitude of parameters, including agent concentration, number of exchangeable protons, proton exchange rate, T 1 , T 2 , saturation time, and saturation efficiency (1-8). Of these, the chemical exchange rate is often the parameter of interest that reflects tissue pH and the molecular environment, such as salt or metal content. During saturation, labile protons of the lowconcentration solute are saturated and exchange multiple times with unsaturated protons of the large water pool, resulting in a fractional reduction of the water line. If this exchange rate and the T 1 of water are sufficiently large, there is an amplification of the MR sensitivity with respect to the agent concentration (1). Recently, we showed that macromolecules with multiple amide groups (2) or imino groups (5) can give a CEST effect within the micromolar range. The exchange rates of these proton types have a strong pH dependence in the physiologic range (9,10) and may be useful as pH reporters. Measurement of these rates would be a powerful means for pH calibration (3,4,11-15), but existing technologies for this, such as the water exchange (WEX) sequence (16) and measurement of linewidths, are spectroscopy (MRS) based. As a consequence, they are time consuming for the lower rates because of the low contrast agent concentration and not suitable for measuring the faster rates at high pH because of signal loss due to line broadening. If such ...

MRI detection of glycogen in vivo by using chemical exchange saturation transfer imaging (glycoCEST)

“…Thus, for higher exchange rates, this maximum will be reached at lower concentrations, and the amount of saturation depends on the effect of both parameters. An exact quantitative description of the CEST effect is possible through use of the Bloch equations using a two-compartment model including exchange and RF saturation (29,34,35). Under the assumption of slow exchange on the NMR time scale and for OH single-proton exchange rates (k), the CEST effect can be described analytically by…”

“…Livers were freeze-clamped (liquid N 2 ), pulverized with a mortar and pestle, extracted with 30% KOH, and precipitated with ethanol as described elsewhere (29). Isolated glycogen was hydrolyzed to glucose by using amyloglucosidase (Sigma-Aldrich, St. Louis, MO), and the resulting solution was assayed for glucose as described above.…”

“…When irradiating for a period of a few seconds, a cumulative saturation of the water signal occurs. This simple sensitivity enhancement approach has previously been demonstrated for small molecules in phantoms (25), paramagnetic agents in vitro (25)(26)(27) and ex vivo (28), protein fragments in vitro (22,29) and in vivo (30,31), and nucleic acids in vitro (32). It is not obvious that CEST could be applied to detect hydroxyl groups because OH protons exchange quite rapidly and have only a small chemical shift difference with bulk water protons.…”

Physical Modeling of Chemical Exchange Saturation Transfer Imaging

“…The experiment is based on the fact that the CEST effect should reach a steady-state as the saturation duration increases 24) and that CEST asymmetry can be reached at a saturation duration of almost 5 s. However, for the experiment at 3T, we were only able to apply 1 s as the maximum saturation duration due to the limitations of the MRI system. It is difficult to say whether the result at 3T has enough confidence.…”

“…24) Saturation efficiency, known as α, which express the relationship, is increased with increasing B 1 power in a certain range and is reached a steady-state with a further increased B 1 power. The proton transfer rate (PTR) which represents the CEST asymmetry in the two-poll exchange model is proportional to the saturation efficiency 25) shown in the following equations:…”