Cross-relaxation in methacrylic acid-based dosimetry gels

H. M. Whitney1, J. Joers1, and J. C. Gore1
1Institute of Imaging Science, Vanderbilt University, Nashville, TN, United States

Introduction
Polymer gel dosimeters consist of a complex mixture that contains vinylic monomers in an aqueous gelatin matrix, and other minor components. Upon
exposure to ionizing radiation, polymerization is induced, and various subsequent changes can be used to quantify dose distributions (Whitney et al.,
2007). Such dosimeters are of approximate tissue composition (~85% water) and tissue density while also allowing high resolution MR imaging of 3D
dose distributions. The dose-response mechanism responsible for the observed effect is believed to primarily involve magnetization exchange between
polymer protons and the bulk water. However, the relative contributions of chemical exchange and dipolar cross-relaxation have not been explored.

Theory
Polymer gels are considered to have two or more proton pools, coupled by exchange and/or cross-relaxation: one is the free, mobile solvent protons,
and a second population consists of protons at the surface of the macromolecules that result from the polymerization. Dosimeter formulations differ, but
the underlying phenomena are consistent: as dose level is increased, the size of the macromolecular pool increases, and R2 increases. This model
affords useful insights into the relevance of both particle size and rigidity and chemical exchange on relaxation in polymer gels. It is believed that
chemical exchange between solvent water and hydroxyl groups dominates relaxation, but the possibility of through-space dipolar cross-relaxation
between water and the methyl and methylene moieties has not been investigated. Such effects are have been shown to be important in biological
systems (Zhong et al., 1990). The work described here explores the contributions of these processes in methacrylic-type polymer gels through studies
of the relaxation properties of the monomers present in the gel dosimeter. To do this, we investigated possible nuclear Overhauser enhancements
(NOEs) of the non-aqueous protons when water protons were selectively inverted.

Materials and Methods
Distilled water and methacrylic acid (6% w/w) were combined and placed in 5mm NMR tubes. Proton spectra were acquired using a 500MHz Bruker
spectrometer. Chemical shift differences (F) between the resonances of water and the methylene and methyl groups (as displayed in Figure 1) were
found to be 430Hz (a), 627Hz (b), and 1478 Hz (c). The chemical shift interaction was exploited to perform an inversion transfer experiment for each
peak (Robinson et al., 1985), using a 90-t-90-tau-90 pulse sequence, where t = 1/2F, and tau represents a variable inversion mixing time. Additionally, a
standard, non-selective, inversion recovery experiment was performed. The amplitudes of the various resonances were monitored for different values of
tau, and were exponentially fit.

Results
Figure 2 plots simple inversion recovery data for each of the three peaks of interest. In each case a simple exponential characterizes the recovery. T1 for
the methylene peaks is approximately 2.5s and approximately 2.8s for the methyl peaks. Figure 3 shows the behaviors of the other peaks when the
water is selectively inverted. There is apparently no evidence of exchange between proton a and water, but a small effect is noticeable for protons b and
c.


Figure 1: Molecular
structure of methacrylic
acid with groups of interest
noted (methylene (a and b),
methyl (c)).


Figure 2: Nonselective inversion
recovery of methylene and methyl
groups.


Figure 3: Relative increase of magnetization over inversion times for
the selective inversion recovery experiment.

Conclusions
These results show that dipolar cross-relaxation between water and the methacrylic acid monomer may contribute to relaxation through magnetization
transfer between the methyl proton and the “b” methylene proton with water in the methacrylic acid-based polymer gel dosimeter, but is possibly
negligible for proton a. Subsequent studies will investigate the importance of this contribution in both the monomer and polymer form of methacrylic
acid.

References
Gore J C, Brown M S and Armitage I M (1989) An Analysis of Magnetic Cross-Relaxation between Water and Methylene Protons in a Model System.
Magn. Res. Med. 9, 333-342.
Robinson G, Kuchel P W, Chapman B E, Doddrell D M and Irving M G (1985) A Simple Procedure for Selective Inversion of NMR Resonances for Spin
Transfer Enzyme Kinetic Measurements. Journal of Magnetic Resonance 63, 314-319.
Whitney H M, Ding G X, Coffey C W and Gore J C (2007) Gel Dosimetry for Measuring a Complex 3D Dose Distribution in Stereotactic Radiosurgery.
Med. Phys. 34, 2393-2393.
Zhong J H, Gore J C and Armitage I M (1990) Quantitative Studies of Hydrodynamic Effects and Cross-Relaxation in Protein Solutions and Tissues with
Proton and Deuteron Longitudinal Relaxation-Times. Magn. Res. Med. 13, 192-203.


Proc. Intl. Soc. Mag. Reson. Med. 16 (2008) 1459