Free water imaging is an advanced diffusion magnetic resonance imaging approach that attempts to separate the diffusion signal arising from brain tissue from the signal contributed by freely diffusing water in the extracellular space. In principle, the method quantifies a ‘free water fraction,’ a scalar measure that rises when extracellular fluid accumulates. Because neuroinflammation, cell loss, and tissue degeneration are all thought to expand the extracellular space, an elevated free water fraction has been interpreted by many groups as a proxy for neurodegenerative change. The nigrostriatal system, the midbrain circuitry that deteriorates in Parkinson’s disease, has been the principal target of these measurements, and numerous studies have reported elevated free water in the substantia nigra of patients compared with healthy controls.

The appeal of the technique is easy to understand. Parkinson’s disease remains a clinical diagnosis, supported by dopamine transporter imaging and response to levodopa, yet the field has long lacked a biomarker that tracks the underlying biology over time. Clinical rating scales are influenced by medication, symptom fluctuation, and rater variability. Structural atrophy measures change slowly and nonspecifically. Against this backdrop, a diffusion metric that appears sensitive to microstructural change in the substantia nigra, potentially within a single scanning session and without ionizing radiation or contrast agents, has generated considerable enthusiasm, including as a candidate progression biomarker in therapeutic trials.

That enthusiasm, however, has collided with a persistent methodological problem: the free water signal is extraordinarily sensitive to how the data are acquired and processed. The model underlying free water imaging fits a two-compartment representation to diffusion-weighted signals, and this fitting problem is ill-conditioned, meaning that small perturbations in image quality, noise, motion, or gradient performance can shift the estimated free water fraction by amounts comparable to the group differences reported in disease studies. Echo-planar imaging distortions, eddy currents, subject head motion, and even the choice of preprocessing pipeline can leave systematic fingerprints on the resulting maps. Critics have argued that some reported patient-control differences may reflect these technical confounds rather than genuine biology.

The critique that prompted the reply appears to press exactly this point, arguing that free water findings in Parkinson’s disease should not be treated as a binary verdict, for or against the method, but that the method itself matters decisively. The phrase ‘not a binary verdict’ suggests a call for nuance: the question is not simply whether free water imaging works, but under which acquisition protocols, preprocessing choices, and analysis pipelines it can be trusted, and where its limits lie. The reply, published in the same journal, represents the original authors’ defense of their approach and their response to the methodological objections raised.

Debates of this kind are not academic quibbles. If free water imaging is adopted as a secondary or exploratory endpoint in clinical trials, systematic measurement error could obscure true disease slowing, inflate apparent effect sizes, or generate spurious signals that misdirect therapeutic programs. Conversely, if genuine biological signal exists and is dismissed because of technical skepticism, the field may abandon a useful window into neuroinflammation and tissue integrity. The stakes are amplified by the broader push toward biomarker-based staging of Parkinson’s disease, exemplified by recent biological definitions of the disease that incorporate alpha-synuclein seed amplification assays and other molecular measures. Imaging markers that complement these fluid biomarkers would be valuable, but only if their measurement properties are rigorously characterized.

Methodological scrutiny of free water imaging has intensified in recent years. Studies have examined the test-retest reliability of the measure, the influence of scanner vendor and field strength, and the reproducibility of findings across independent cohorts. Some analyses have found that free water elevations in the substantia nigra are robust and correlate with clinical severity, while others have reported that apparent effects diminish or change direction when alternative preprocessing pipelines are applied. Multi-site harmonization efforts have highlighted the difficulty of pooling free water estimates across scanners, and work in other neurological conditions has shown that the metric can be confounded by factors as mundane as ventricular proximity and as consequential as prior imaging artifacts.

Within this contested landscape, the exchange in npj Parkinson’s Disease illustrates how the field is negotiating standards. Replies and counter-replies of this sort serve a function beyond the immediate dispute: they force researchers to articulate the assumptions of their models, the sensitivity analyses they performed, and the conditions under which their conclusions hold. For readers and clinicians, the practical takeaway is that a free water fraction reported in a paper is not a universal constant but the output of a specific acquisition and analysis chain. Comparing values across studies without accounting for those chains risks comparing apples to oranges, a caution that applies to many advanced diffusion techniques, including neurite orientation dispersion and density imaging and related microstructural models.

For patients and families, the debate may seem remote, but its consequences are concrete. Biomarkers determine who is enrolled in trials, when treatments are judged to work, and how quickly disease-modifying therapies reach the clinic. A reliable imaging marker of nigral degeneration could shrink trial sizes, shorten durations, and enable earlier intervention, which is why the National Institutes of Health and the Parkinson’s community have invested heavily in biomarker validation programs. The current exchange should be read as part of that validation process: an insistence that before free water imaging is elevated to a verdict on disease progression, the method must demonstrate that its signal is separable from its noise.

The publication of the reply, alongside the critique it addresses, gives the research community and interested readers an unusually transparent view of a scientific disagreement in progress. Both documents are openly accessible through the journal, allowing independent readers to weigh the arguments for themselves. Whatever the resolution, the episode underscores a principle that extends well beyond this single technique: in neuroimaging, the method matters, and the credibility of any biomarker rests on the reproducibility of the pipeline that produces it. As free water imaging continues to be tested in longitudinal cohorts and interventional studies, exchanges like this one will help determine whether it earns a durable place in the Parkinson’s disease toolkit or remains a promising but contested research measure.

Subject of Research: Free water diffusion imaging as a biomarker of neurodegeneration and disease progression in Parkinson's disease

Article Title: Reply to ‘The method matters: free water imaging in Parkinson’s disease is not a binary verdict’

Article References: Roh, Y. H., Youn, J., Kim, S.-Y., Heo, H., Song, S., & Sohn, B. (2026). Reply to ‘The method matters: free water imaging in Parkinson’s disease is not a binary verdict’. npj Parkinson's Disease, 12(1), Article 218. https://doi.org/10.1038/s41531-026-01490-w

Image Credits: AI Generated

DOI: 10.1038/s41531-026-01490-w

Keywords: Parkinson's disease, free water imaging, diffusion MRI, biomarkers, substantia nigra, neurodegeneration, npj Parkinson's Disease, neuroimaging, clinical trials, methodology, test-retest reliability, nigrostriatal degeneration