ADC measurements on the Unity MR-linac - A recommendation on behalf of the Elekta Unity MR-linac consortium

Background and purpose: Diffusion-weighted imaging (DWI) for treatment response monitoring is feasibleon hybrid magnetic resonance linear accelerator (MR-linac) systems. The MRI scanner of the ElektaUnity system has an adjusted design compared to diagnostic scanners. We investigated its impact onmeasuring the DWI-derived apparent diffusion coefficient (ADC) regarding three aspects: the choice ofb-values, the spatial variation of the ADC, and scanning during radiation treatment. The aim of this studyis to give recommendations for accurate ADC measurements on Unity systems.Materials and methods: Signal-to-noise ratio (SNR) measurements with increasing b-values were done todetermine the highest bvalue that can be measured reliably. The spatial variation of the ADC wasassessed on six Unity systems with a cylindrical phantom of 40 cm diameter. The influence of gantry rotationand irradiation was investigated by acquiring DWI images before and during treatment of 11 prostatecancer patients.Results: On the Unity system, a maximum b-value of 500 s/mm2 should be used for ADC quantification, asa trade-off between SNR and diffusion weighting. Accurate ADC values were obtained within 7 cm fromthe iso-center, while outside this region ADC values deviated more than 5%. The ADC was not influencedby the rotating linac or irradiation during treatment.Conclusion: We provide Unity system specific recommendations for measuring the ADC. This willincrease the consistency of ADC values acquired in different centers on the Unity system, enabling largecohort studies for biomarker discovery and treatment response monitoring.