Hyperpolarized krypton-83 as a contrast agent for magnetic resonance imaging

Abstract

For the first time, magnetic resonance imaging (MRI) with hyperpolarized (hp) krypton-83 (83Kr) has become available. The relaxation of the nuclear spin of 83Kr atoms (I = 9/2) is driven by quadrupolar interactions during brief adsorption periods on surrounding material interfaces. Experiments in model systems reveal that the longitudinal relaxation of hp 83Kr gas strongly depends on the chemical composition of the materials. The relaxation-weighted contrast in hp 83Kr MRI allows for the distinction between hydrophobic and hydrophilic surfaces. The feasibility of hp 83Kr MRI of airways is tested in canine lung tissue by using krypton gas with natural abundance isotopic distribution. Additionally, the influence of magnetic field strength and the presence of a breathable concentration of molecular oxygen on longitudinal relaxation are investigated.

Fig. 1.

⁸³Kr MRI using continuous-flow mode optical pumping. (A) Hp ⁸³Kr MRI of gas flowing around glass structures. (Inset) A photograph of the phantom used to produce the MRI. Gas flow was held constant at 125 cm³/min. All figures are displayed after zero-filling. (B)Hp ⁸³Kr MRI of a porous polyethylene sample with 70-μm average pore size obtained under continuous flow (100 cm³/min) conditions. (Inset) Sketch of the phantom used for the MRI. The center of the sample (i) is a 1.65-mm void space surrounded by a 0.76-mm PFA wall and an 11-mm wide area of a porous polymer (ii). The measurement at 9.4 T took ≈2.1 h and led to a resolution of 650 × 650 μm (raw data).

Fig. 2.

Hp ⁸³Kr MRI of canine lung tissue using stopped-flow optical pumping. (A) Micrograph of lung tissue demonstrating that the drying process preserves the alveolar structure. That microscopic anatomic structure is maintained as evidenced by the intact alveolar septum walls (long arrows) and alveolar epithelium (short arrows). (B) ⁸³Kr MRI of canine lung tissue obtained by using krypton stopped-flow optical pumping. The time needed for each of the 16 individual measurements is 10 min for the optical pumping process, ≈3 s for the krypton transfer, and 103 ms for rf pulse, gradient pulse, and acquisition at 9.4-T field strength. The resolution is 480 × 655 μm (raw data) with no slice selection applied. (C) ⁸³Kr MRI of a lung sample using only one krypton stopped-flow optical pumping cycle. The measurement takes 15 min for the pumping process, 3 s for the gas transfer, and 0.46 s for the rf pulses, gradient pulses, and signal acquisition. The small flip angle excitation (FLASH) image was produced by using sixteen 12° flip angle pulses. The resolution is 1,080 × 655 μm (raw data) with no slice selection.

Fig. 3.

Hp ⁸³Kr MRI showing surface-sensitive contrast. (A) Photograph of the phantom used to obtain surface-dependent contrast. Surface-siliconized 1.0-mm closest packed glass beads (hydrophobic) are located in the center ring (i). Separated by an untreated glass tube (seen as a white ring in the photograph because of illumination from below) is the outer region (ii) that contains untreated 1.0-mm glass beads (hydrophilic). (B) ⁸³Kr MRI of the glass bead sample (A) reconstructed from 16 individual krypton stopped-flow optical pumping cycles. The MRI sequence is applied ≈3 s after filling the sample with hp krypton leading to no appreciable MRI contrast between the two regions (note the glass wall is not resolved because the resolution is 424 × 864 μm). (C) Same as in B except a 9-s waiting period has been added. A clear contrast between the hydrophobic inner sample region (T₁ = 9 s) and the hydrophilic outer region (T₁ = 35 s) appears. Increasing the waiting time will lead to increased contrast but an overall reduced signal intensity.