Mis-estimation and bias of hyperpolarized apparent diffusion coefficient measurements due to slice profile effects

Purpose: The purpose of this work was to explore the impact of slice profile effects on apparent diffusion coefficient (ADC) mapping of hyperpolarized (HP) substrates.

Methods: Slice profile effects were simulated using a Gaussian radiofrequency (RF) pulse with a variety of flip angle schedules and b-value ordering schemes. A long T₁ water phantom was used to validate the simulation results, and ADC mapping of HP [¹³ C,¹⁵ N₂ ]urea was performed on the murine liver to assess these effects in vivo.

Results: Slice profile effects result in excess signal after repeated RF pulses, causing bias in HP measurements. The largest error occurs for metabolites with small ADCs, resulting in up to 10-fold overestimation for metabolites that are in more-restricted environments. A mixed b-value scheme substantially reduces this bias, whereas scaling the slice-select gradient can mitigate it completely. In vivo, the liver ADC of hyperpolarized [¹³ C,¹⁵ N₂ ]urea is nearly 70% lower (0.99 ± 0.22 vs 1.69 ± 0.21 × 10^-3 mm² /s) when slice-select gradient scaling is used.

Conclusion: Slice profile effects can lead to bias in HP ADC measurements. A mixed b-value ordering scheme can reduce this bias compared to sequential b-value ordering. Slice-select gradient scaling can also correct for this deviation, minimizing bias and providing more-precise ADC measurements of HP substrates. Magn Reson Med 78:1087-1092, 2017. © 2016 International Society for Magnetic Resonance in Medicine.