FIGURE 5. Controlling diffusion to turn tags on or off.

Main panel, high tilt-angle SEM image showing a square array of R = 2.5 m particles. Except for a defined circular region, all particles have their interiors filled, blocking water diffusion. Top left inset, a higher magnification SEM image of the boundary between open and filled particles. Top right inset, the resulting background-subtracted chemical shift MRI showing transferred magnetization saturation from the particles' shifted resonance. Signal is visible from those particles that have water diffusing through their open interior region (labelled 'On') but not from those particles that have their interiors filled (labelled 'Off'). The bottom of the image shows a region that contains no particles (labelled 'No tags'), providing a null background signal comparison. A scratch (seen at the lower right corner) removed 100 particles (about 10–20 per voxel). Its visibility in the magnetic resonance image suggests the potential for high-resolution imaging to spectrally distinguish individual such particles.