Determination of proton magnetization transfer rate constants in heterogeneous biological systems

Abstract

Two procedures are currently in use for the determination of proton magnetization transfer rate constants between macromolecular tissue components and water. The first method assumes that there are only two spin baths (macromolecular plus solvent) and that during off-resonance irradiation complete saturation of the "immobile" proton spin bath occurs (S. H. Koenig, R. D. Brown, III, R. Ugolini, Magn. Reson. Med. 29, 311 (1993)). This approach neglects the possibility of incomplete saturation and polydispersity, and yields an apparent magnetization transfer rate constant, Kapp. The second approach utilizes a formalism which can account for polydispersity and incomplete saturation of the immobile spin bath (K. Kuwata, D. Brooks, H. Yang, T. Schleich, J. Magn. Reson., in press). In this work magnetization transfer rate constants derived by the use of both methods for two systems, ocular lens tissue and cross-linked bovine serum albumin (BSA) were compared. For both samples Kapp was dependent on B2 off-resonance irradiation frequency and power when the first method was used. The second method provided values of the magnetization transfer rate constant that were similar to the values obtained by the first method, as the limit of complete saturation was approached.

Dynamics of paramagnetic agents by off-resonance rotating frame technique in the presence of magnetization transfer effect

“…R for biological tissues has wide distributions [35][36]. R=2 is close to the value reported for cross-linked 10% BSA gels [14,29,34]; while R=100 is relevant to the in vivo systems [35][36]. The Lorentzian lineshape is used to calculate R rfB , which can provide accurate magnetization dependence of θ at small θ angle, as discussed in the section 2.3.…”

“…2 ) 1 / R a T 2a (14) This limit depends primarily on the T 2 of the semi-rigid pool. For T 2B in tens of s, Eq.14 is valid at θ as low as 2º.…”

“…The mechanism of the spin coupling is described traditionally by a two-pool model for the protons of different mobility: the free water is denoted as liquid pool and the water bound to the biopolymers is denoted as semi-rigid pool [12][13]. Several groups have considered a threepool model by further dividing the liquid pool into two pools: one for the free water and the other for the mobile biopolymers [14][15]. The comprehensive reviews on the methodology of magnetization transfer can be found elsewhere [12][13].…”

“…We have chosen crossed linked bovine albumin (BSA) as the gel, which has been extensively used as tissue models in MRI [30]. Its parameters for simulating the steady state magnetization M s can be found elsewhere [14,29]. This system has intense magnetization transfer effect with M s as low as 0.1 [14].…”

“…Its parameters for simulating the steady state magnetization M s can be found elsewhere [14,29]. This system has intense magnetization transfer effect with M s as low as 0.1 [14]. The typical M s for tissue/organs ranges from 0.95 to 0.2 [12], the smaller M s normally represents the larger magnetization transfer effect.…”