Author: Nicholas Carlson, Joel J. St-Aubin 👨🔬
Affiliation: University of Iowa Hospitals and Clinics, University of Iowa 🌍
Purpose: To establish baselines metrics and determine longitudinal accuracy and reproducibility of Diffusion-Weighted Imaging (DWI) and Apparent Diffusion Coefficient (ADC) values for a 1.5T Elekta Unity.
Methods: Using the CaliberMRI Quantitative Imaging Biomarker Alliance (QIBA) model 128 DWI phantom, Echo Planar Imaging (EPI) DWI sequences were taken on a 1.5T Elekta Unity at various temperatures including an ice bath (~0 °C), chilled (~17 °C), and room temperature (~22 °C). Four consecutive DWI scans with identical sequence parameters (b-values of 0, 500, 900, 1000, 1500, and 2000) were used to calculate the ADC, and T1-weighted 3D images were taken before and after the DWI scans to correct the temperature dependence of ADC values using liquid crystal thermometers imbedded within the phantom. Mean ADC values and other important metrics were determined using the vendor's software for water (3 vials) and polyvinylpyrrolidone (PVP) 50% w/v (2 vials). The bias of the measured ADC values was assessed using a quadratic fit of NIST traceable ADC values within the CaliberMRI software. Short-term repeatability was measured within the same scan session and long-term repeatability was measured over two months.
Results: The water at ~0 °C met all the QIBA requirements besides ADC coefficient of variation (CV) however this was comparable to published data. There was a significant effect on ADC bias and repeatability coefficients for water with increasing temperature even after temperature correction. PVP 50% w/v ADC bias met QIBA requirements at all temperatures but suffered from poor SNR at b = 0 and a high CV which worsened with increasing temperature.
Conclusion: This data establishes ADC baselines for our Unity system at various temperatures. The results also show that our Unity system is longitudinally accurate and reproducible at 0 °C for water and is in line with published data at other institutions.