Experimental demonstration of diffusion limitations on resolution and SNR in MR microscopy

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biomedical Engineering
• Cellular Microscopy

Background:

• MR microscopy offers cellular resolution (<10 µm) but is limited by signal-to-noise ratio (SNR) and spatial resolution.
• Spin dephasing due to diffusion in strong gradients degrades image quality.
• Phase encoding is proposed to mitigate diffusion effects, but its quantitative benefits and optimal conditions are not well-established.

Purpose of the Study:

• To quantify the conditions under which phase encoding outperforms frequency encoding in MR microscopy.
• To emphasize the detrimental effects of diffusion on SNR and resolution.
• To provide a practical guide for choosing between phase and frequency encoding.

Main Methods:

• Utilized a 15.2 T Bruker MRI scanner with micro solenoid RF coils (<1 mm diameter).
• Quantified diffusion effects on SNR and resolution for both frequency and phase encoded acquisitions.
• Measured spatial resolution and SNR per square root time, and calculated point spread functions for voxels 3-15 µm in dimension.

Main Results:

• Experimentally demonstrated diffusion's negative impact on SNR during readout gradients.
• Measured resolutions were lower than nominal, with phase encoding showing higher actual resolution and SNR.
• Acquired 10 µm × 10 µm in-plane resolution images of rat spinal cord, highlighting phase encoding's superior performance.

Conclusions:

• Phase encoding offers significant advantages in SNR and resolution compared to conventional frequency encoding.
• Provided guidelines for selecting phase or frequency encoding based on voxel size, sample, and hardware properties.
• Phase encoding is recommended for achieving higher quality MR microscopy images, especially in diffusion-limited scenarios.