Decay of dipolar order in diamagnetic and paramagnetic proteins and protein gels.

Magnetic relaxation in solids may be complicated by the creation and loss of dipolar order at finite rates. In tissues the molecular and spin dynamics may be significantly different because of the relatively high concentration of water. We have applied a modified Jeneer-Broekaert pulse sequence to measure dipolar relaxation rates in both dry and hydrated protein systems that may serve as magnetic models for tissue. In lyophilized and dry serum albumin, the dipolar relaxation time, T(1D) is on the order of 1 ms and is consistent with earlier reports. When hydrated by deuterium oxide, the dipolar relaxation times measured were on the order of tens of microseconds. When paramagnetic centers are included in the protein, the Jeneer-Broekaert echo decay times became the order of the decay time for transverse magnetization, i.e., the order of 10 micros or less. In the hydrated or paramagnetic systems, the dipolar relaxation times are too short to require inclusion in the quantitative analysis of magnetization transfer experiments.

Magnetic relaxation contrast agents in magnetization transfer imaging.

RATIONALE AND OBJECTIVES: The effects of magnetic relaxation agents are explored in the context of magnetization transfer pulse sequences using cross-linked protein gels as modeled tissue systems. METHODS: Magnetization transfer pulse sequences were used to study contrast agents that are designed to bind to rotationally immobilized protein targets. RESULTS: The dynamic range available from contrast agents, used in conjunction with magnetization transfer pulse sequences, is comparable with or better than that based on spin-echo imaging sequences with short repetition times. Furthermore, useful changes in the intensity of water resonances may be achieved by using this combined approach even though the paramagnetic metal center may not have a free coordination position in the chelate complex for water molecule exchange. CONCLUSIONS: The inclusion of magnetization transfer acquisition protocols in the context of magnetic imaging with contrast agents presents new opportunities for control of the information content of the image and for new classes of contrast agent structure and delivery.