Accelerating MRI Colon Volume Measurements and Reducing Inter-Observer Variation through Automatic Segmentation and Human-in-the-Loop Correction
Pith reviewed 2026-07-03 01:57 UTC · model grok-4.3
The pith
Automatic ML segmentation with human correction reduces MRI colon analysis time from 56 to 11 minutes while preserving measurement accuracy.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The central claim is that nnU-Net based automatic segmentation of the colon on mDIXON MRI, followed by manual correction of the generated masks, reduces the time for analysis from an average of 56 minutes to 11 minutes. The corrected masks agree closely with fully manual segmentations for whole colon volume with ICC of 0.96 and for regional volumes with ICC 0.80-0.95, while also improving inter-observer repeatability over manual methods.
What carries the argument
The nnU-Net model for automatic segmentation of colonic regions including ascending, transverse, descending and sigmoid-rectal, with subsequent human correction of the masks.
If this is right
- Whole colonic volumes from the ML method are suitable for use with only minimal checks.
- Regional colonic volumes show good to excellent agreement after correction.
- The corrected method improves repeatability between observers compared to full manual segmentation.
- The time reduction makes detailed colon content analysis feasible in clinical settings.
Where Pith is reading between the lines
- This human-in-the-loop method could be tested on scans from different MRI scanners to check if the time savings hold.
- Similar automatic approaches might speed up volume measurements in other parts of the body or with different imaging types.
- Larger studies could use this to track changes in colon function over time with more patients.
Load-bearing premise
The automatic model, trained on the authors' specific dataset and scanner, will produce masks that need only short corrections on new scans from the same setup.
What would settle it
Finding that new scans require more than 20 minutes of correction on average or show ICC below 0.85 for whole volume would show the time savings and accuracy claims do not hold.
read the original abstract
The movement distribution, and volume of both chyme and gas in the colon, are important metrics to understand colonic function in health, disease, and the effects of treatments and different foodstuffs. Current methods available for assessment of these colonic contents using MRI consist mainly of manual segmentation or semi-automatic segmentation. However, these methods of segmentation are very labour intensive and too slow for clinical applications, require expert knowledge and some semi-automatic methods require use of bowel preparation. MRI scans were acquired in 2 breath holds using mDIXON sequences. We used the 'No New U-Net' (nnU-Net) ML model to automatically segment the colon, including colonic regions (ascending, transverse, descending and sigmoid-rectal). The ML-generated masks were corrected manually and the time taken for correction was recorded. ML segmentations were compared to both manual segmentations and observer corrected ML (CorrML) segmentations. Observer repeatability was also evaluated for both manual and CorrML methods to create a benchmark for the allowable error in the automatic segmentations. Analysis time was significantly reduced (p<0.0001) from 56 mins (+-11 mins (SD)) for manual masks to 11 mins (+-5 mins (SD)) for CorrML masks. Both DICE and ICC values showed excellent agreement between manual, ML and CorrML segmentations for whole colonic volume (ICC = 0.96) whilst regional volumes were good-excellent (ICC = 0.80-0.95). Inter-observer repeatability was improved when using CorrML methods over manual segmentation (ICC manual > 0.89, CorrML > 0.93). Analysis time was reduced by over 80% when using CorrML methods and whole colonic volumes measured by ML would be suitable for use with minimal checks. Hence the methods proposed here would be clinically useful.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript applies the nnU-Net model to automatically segment the whole colon and its regions (ascending, transverse, descending, sigmoid-rectal) from 2-breath-hold mDIXON MRI scans. It compares fully manual segmentation, raw ML output, and human-corrected ML masks (CorrML), reporting a statistically significant reduction in analysis time from 56±11 min to 11±5 min (p<0.0001), excellent whole-colon agreement (ICC=0.96) and good-to-excellent regional agreement (ICC 0.80-0.95) between methods, plus improved inter-observer repeatability with CorrML (ICC>0.93) versus manual (ICC>0.89). The authors conclude that CorrML reduces effort by >80% and is clinically useful.
Significance. If the reported time savings and agreement metrics generalize, the approach would meaningfully lower the barrier to routine colonic volume and content quantification in clinical research and practice. The work directly addresses a known bottleneck (labor-intensive manual or semi-automatic segmentation) with a concrete, measurable improvement and supplies repeatability benchmarks that could serve as reference values for future studies.
major comments (2)
- [Abstract / Methods] Abstract and Methods: The central claim that CorrML reduces analysis time by >80% while preserving clinical utility rests on the untested assumption that nnU-Net masks require only brief corrections on future scans acquired under the same mDIXON protocol. No training-set size, cross-validation procedure, or external test set (different scanner, field strength, or minor protocol variation) is reported, so the generalizability of the time-reduction and ICC results cannot be assessed from the given data.
- [Results] Results: The reported inter-observer ICC improvement (manual >0.89 to CorrML >0.93) and the claim that ML volumes are 'suitable for use with minimal checks' are derived from the same internal cohort used to train and evaluate the model; without an independent test cohort these figures may overestimate performance on new subjects.
minor comments (1)
- [Abstract] Abstract: The phrase 'MRI scans were acquired in 2 breath holds using mDIXON sequences' should specify the number of subjects or scans used for training versus testing to allow immediate evaluation of sample size.
Simulated Author's Rebuttal
We thank the referee for the constructive comments highlighting the need for clearer reporting on model training and evaluation cohorts. We address each major comment below and will revise the manuscript accordingly.
read point-by-point responses
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Referee: [Abstract / Methods] Abstract and Methods: The central claim that CorrML reduces analysis time by >80% while preserving clinical utility rests on the untested assumption that nnU-Net masks require only brief corrections on future scans acquired under the same mDIXON protocol. No training-set size, cross-validation procedure, or external test set (different scanner, field strength, or minor protocol variation) is reported, so the generalizability of the time-reduction and ICC results cannot be assessed from the given data.
Authors: We agree that the manuscript should explicitly report the training details to allow assessment of generalizability. In the revised Methods section we will add: (i) the size of the training dataset (number of subjects and scans), (ii) the cross-validation procedure used by nnU-Net (default 5-fold), and (iii) confirmation that the time and ICC metrics were measured on a held-out test subset. We will also insert a limitations paragraph in the Discussion noting the absence of external validation on different scanners or protocols and that the reported time savings apply to the tested mDIXON protocol. revision: yes
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Referee: [Results] Results: The reported inter-observer ICC improvement (manual >0.89 to CorrML >0.93) and the claim that ML volumes are 'suitable for use with minimal checks' are derived from the same internal cohort used to train and evaluate the model; without an independent test cohort these figures may overestimate performance on new subjects.
Authors: The inter-observer repeatability experiments were performed on scans held out from model training to prevent leakage, yet they remain within the same single-site cohort. We will revise the Results and Discussion to (a) clarify the separation between training and repeatability sets, (b) replace the phrase 'suitable for use with minimal checks' with a more qualified statement, and (c) add an explicit caveat that the ICC improvements and time savings are internal benchmarks and may not fully generalize to new subjects or sites. This constitutes a partial revision because the underlying data remain internal. revision: partial
Circularity Check
No circularity; direct empirical validation of segmentation pipelines
full rationale
The paper conducts an empirical study comparing manual segmentation, nnU-Net ML masks, and human-corrected ML (CorrML) masks on the authors' mDIXON MRI dataset. It reports measured quantities (analysis time, Dice scores, ICC agreement, inter-observer repeatability) obtained by applying standard metrics to the outputs of each pipeline. No derivation chain, equations, or first-principles predictions exist that could reduce to fitted parameters or self-referential definitions. No self-citations are invoked to establish uniqueness or to smuggle in ansatzes. The central claims rest on observed performance differences within the study cohort rather than any tautological reduction.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption mDIXON MRI sequences provide adequate contrast for reliable colon boundary detection across subjects
Reference graph
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discussion (0)
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