← Back to MRS Resources

129Xe Diffusion Imaging

Probing pulmonary microstructure, alveolar size, and gas restriction using Apparent Diffusion Coefficient (ADC) mapping.

Brownian Motion in the Alveoli

In free space, Xenon gas atoms move rapidly in random directions (Brownian motion). However, inside the human lung, this diffusion is heavily restricted by the walls of the microscopic alveoli. By measuring how far gas atoms can diffuse during a short period (typically a few milliseconds), we can indirectly measure the physical size of the alveoli themselves.

Similar to proton Diffusion Weighted Imaging (DWI) used to detect strokes in the brain, Xenon diffusion imaging applies powerful, short-duration magnetic field gradients. These gradients encode the spatial position of the gas atoms, wait a specific time (the diffusion time, Δ), and then attempt to decode the position. Atoms that have moved significantly will not fully refocus, leading to signal loss.

Because gas diffuses roughly 10,000 times faster than water in tissue, the gradients used for Xenon DWI are much weaker and applied for much shorter durations than those used for proton DWI.

Key Metrics & Advanced Models

ADC (Apparent Diffusion Coefficient): The most straightforward metric. Calculated by taking multiple images with different gradient strengths (b-values) and fitting the signal attenuation to an exponential decay. Higher ADC values indicate less restriction (larger alveoli).
Mean Acinar Airway Dimension (Lm) / Morphometry: Advanced multi-b-value models (e.g., the cylinder model) can translate diffusion data into absolute geometric measurements of the lung microstructure, providing an average radius of the terminal airways.

Clinical Relevance

Xenon Diffusion Imaging is exquisitely sensitive to destruction of the lung architecture, making it a powerful biomarker for specific diseases:

Emphysema / COPD: In emphysema, the thin alveolar walls break down, merging multiple small alveoli into larger, less efficient air sacs (bullae). Xenon diffusion imaging detects this as a dramatic increase in ADC and Lm, long before it is visible on standard CT scans.
Idiopathic Pulmonary Fibrosis (IPF): While IPF primarily affects the alveolar barrier membrane (best seen with dissolved phase spectroscopy), some diffusion changes can also be observed due to structural remodeling.
Aging: Normal human aging results in a gradual enlargement of the alveoli, sometimes referred to as "senile emphysema," which is readily quantified by Xenon ADC mapping.