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... Show moreBackground 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. Show less
PurposeSystems for magnetic resonance (MR-) guided radiotherapy enable daily MR imaging of cancer patients during treatment, which is of interest for treatment response monitoring and biomarker... Show morePurposeSystems for magnetic resonance (MR-) guided radiotherapy enable daily MR imaging of cancer patients during treatment, which is of interest for treatment response monitoring and biomarker discovery using quantitative MRI (qMRI). Here, the performance of a 1.5 T MR-linac regarding qMRI was assessed on phantoms. Additionally, we show the feasibility of qMRI in a prostate cancer patient on this system for the first time.Materials and methodsFour 1.5 T MR-linac systems from four institutes were included in this study. T1 and T2 relaxation times, and apparent diffusion coefficient (ADC) maps, as well as dynamic contrast enhanced (DCE) images were acquired. Bland–Altman statistics were used, and accuracy, repeatability, and reproducibility were determined.ResultsMedian accuracy for T1 ranged over the four systems from 2.7 to 14.3%, for T2 from 10.4 to 14.1%, and for ADC from 1.9 to 2.7%. For DCE images, the accuracy ranged from 12.8 to 35.8% for a gadolinium concentration of 0.5 mM and deteriorated for higher concentrations. Median short-term repeatability for T1 ranged from 0.6 to 5.1%, for T2 from 0.4 to 1.2%, and for ADC from 1.3 to 2.2%. DCE acquisitions showed a coefficient of variation of 0.1–0.6% in the signal intensity. Long-term repeatability was 1.8% for T1, 1.4% for T2, 1.7% for ADC, and 17.9% for DCE. Reproducibility was 11.2% for T1, 2.9% for T2, 2.2% for ADC, and 18.4% for DCE.ConclusionThese results indicate that qMRI on the Unity MR-linac is feasible, accurate, and repeatable which is promising for treatment response monitoring and treatment plan adaptation based on daily qMRI. Show less
