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

Kooreman, E.S.; Houdt, P.J. van; Keesman, R.; Pos, F.J.; Pelt, V.W.J. van; Nowee, M.E.; ... ; Heide, U.A. van der (2020)

Article / Letter to editor

Background and purpose: Diffusion-weighted imaging (DWI) for treatment response monitoring is feasible
on hybrid magnetic resonance linear accelerator (MR-linac) systems. The MRI scanner of the Elekta
Unity system has an adjusted design compared to diagnostic scanners. We investigated its impact on
measuring the DWI-derived apparent diffusion coefficient (ADC) regarding three aspects: the choice of
b-values, the spatial variation of the ADC, and scanning during radiation treatment. The aim of this study
is 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 to
determine the highest bvalue that can be measured reliably. The spatial variation of the ADC was
assessed on six Unity systems with a cylindrical phantom of 40 cm diameter. The influence of gantry rotation
and irradiation was investigated by acquiring DWI images before and during treatment of 11 prostate
cancer patients.
Results: On the Unity system, a maximum b-value of 500 s/mm2 should be used for ADC quantification, as
a trade-off between SNR and diffusion weighting. Accurate ADC values were obtained within 7 cm from
the iso-center, while outside this region ADC values deviated more than 5%. The ADC was not influenced
by the rotating linac or irradiation during treatment.
Conclusion: We provide Unity system specific recommendations for measuring the ADC. This will
increase the consistency of ADC values acquired in different centers on the Unity system, enabling large
cohort studies for biomarker discovery and treatment response monitoring.

• All authors: Kooreman, E.S.; Houdt, P.J. van; Keesman, R.; Pos, F.J.; Pelt, V.W.J. van; Nowee, M.E.; Wetscherek, A.; Tijssen, R.H.N.; Philippens, M.E.P.; Thorwarth, D.; Wang, J.H.; Shukla-Dave, A.; Hall, W.A.; Paulson, E.S.; Heide, U.A. van der
• Date: 2020-12-01
• Journal: Radiotherapy & Oncology
• Volume: 153
• Pages: 106 - 113
• DOI: doi:10.1016/j.radonc.2020.09.046
• Link: https://www.sciencedirect.com/science/article/pii/S0167814020308215?via%...