Abstract
ADC is a potential post treatment imaging biomarker in colorectal liver metastasis however measurements are affected by respiratory motion. This is compounded by increased statistical uncertainty in ADC measurement with decreasing tumour volume. In this prospective study we applied a retrospective motion correction method to improve the image quality of 15 tumour data sets from 11 patients. We compared repeatability of ADC measurements corrected for motion artefact against non-motion corrected acquisition of the same data set. We then applied an error model that estimated the uncertainty in ADC repeatability measurements therefore taking into consideration tumour volume. Test-retest differences in ADC for each tumour, was scaled to their estimated measurement uncertainty, and 95% confidence limits were calculated, with a null hypothesis that there is no difference between the model distribution and the data. An early post treatment scan (within 7 days of starting treatment) was acquired for 12 tumours from 8 patients. When accounting for both motion artefact and statistical uncertainty due to tumour volumes, the threshold for detecting significant post treatment changes for an individual tumour in this data set, reduced from 30.3% to 1.7% (95% limits of agreement). Applying these constraints, a significant change in ADC (5th and 20th percentiles of the ADC histogram) was observed in 5 patients post treatment. For smaller studies, motion correcting data for small tumour volumes increased statistical efficiency to detect post treatment changes in ADC. Lower percentiles may be more sensitive than mean ADC for colorectal metastases.
Highlights
The apparent diffusion coefficient (ADC) is calculated from diffusion-weighted magnetic resonance imaging (DWI)[1]
ADC is calculated from multiple DWI acquisitions that assume perfect spatial registration between images significant misregistration from motion will affect ADC accuracy
The results of this study, comparing three alternative methods to detect post treatment changes in colorectal liver metastatic tumour ADC, demonstrates the importance of addressing misregistration caused by respiratory motion
Summary
The apparent diffusion coefficient (ADC) is calculated from diffusion-weighted magnetic resonance imaging (DWI)[1]. Previous studies in the liver have found post-treatment mean ADC changes in the range of 10 to 30%10,11. It is important to avoid misinterpretation of post treatment mean ADC changes by calculating repeatability/. Accurate estimation of other metrics from whole tumour 3D histograms could increase observed post treatment changes. The 25th percentile was most sensitive to post treatment ADC changes in peritoneal tumours[17]. In a study comparing whole liver ADC with and without colorectal metastatic tumours, the 5th percentile was significantly lower for the diseased group[18]. Respiratory triggering or use of navigator echo techniques can mitigate motion effects, improving image quality in terms of SNR, while maintaining stable ADC values, when compared to breath-hold sequences[19,20] and free breathing acquisitions[21]. There is conflicting evidence with other studies showing no advantage to navigator triggering[22] with decrease in reproducibility and ADC stability compared to free breathing[23,24]
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