The one-line answer
If your MRI report contains the word "incidental", the most likely truth is that you are fine. Between 5 and 40 per cent of MRIs turn up a finding the scan was never asked to look for. The overwhelming majority of these are benign, need no treatment, and mean nothing for your day-to-day health. A minority need a short follow-up. A very small number need action. This piece is about telling the three apart without spending the next fortnight worrying.
Why they happen
MRI is extraordinarily sensitive. When a consultant orders an MRI of your knee, the scanner does not politely ignore everything else in the frame. It captures the surrounding soft tissue, bone marrow, blood vessels and any organ that happens to be in the imaged region. A lumbar spine MRI will show slices of the kidneys and lower abdomen. A brain MRI will show sinuses, orbits and the top of the neck.
Because MRI resolves such fine detail, it routinely shows small anatomical variants, tiny cysts, mild age-related changes and old healed injuries that nobody would ever have noticed otherwise. The scanner is doing its job. The report is honest. And the finding is, more often than not, medically silent.
The three categories of incidental findings
It helps to think of every incidental finding as sitting in one of three buckets. A good radiology report will make clear which bucket applies, though the language can be technical.
- Benign, no action. The commonest category. A small cyst, a developmental venous anomaly, a tiny disc bulge in someone with no back pain. The report may describe it, then say "of no clinical significance" or "no follow-up required".
- Needs follow-up. A finding that is probably benign but worth watching. A repeat scan at 3, 6 or 12 months confirms stability. Examples include certain renal cysts, small pulmonary nodules, small meningiomas and some adrenal lesions.
- Needs action now. Rare, but real. Unexpected tumours, aneurysms, acute strokes, new demyelinating lesions or spinal cord compression. In these cases the radiologist will usually phone the referring clinician directly.
Common benign findings by body area
Here are the incidental findings you are most likely to see on a UK MRI report, grouped by region. All are common. All are usually benign.
Brain and head. Small arachnoid cysts, developmental venous anomalies (DVAs), mucosal thickening in the paranasal sinuses, small pineal cysts and non-specific white matter changes in older patients. Sinus thickening is almost universal in people with any history of hay fever or colds. DVAs are simply anatomical variants of normal veins.
Spine. Schmorl's nodes (small vertebral endplate indentations), mild disc bulges, small Tarlov cysts around sacral nerve roots, and mild facet joint changes. A striking proportion of asymptomatic adults have visible disc bulges on lumbar MRI - they are the imaging equivalent of grey hair.
Pelvis and abdomen. Simple ovarian cysts, small uterine fibroids, small kidney cysts, mild fatty liver changes and small hepatic haemangiomas. Ovarian cysts smaller than 3cm in premenopausal women are almost always physiological.
| Body area | Common benign finding | What it usually means |
|---|---|---|
| Brain | Small pineal cyst, DVA, sinus thickening | Anatomical variant or minor inflammation |
| Spine | Schmorl's node, small disc bulge, Tarlov cyst | Age-related, no treatment needed |
| Kidney | Simple cortical cyst under 4cm | Benign, Bosniak I - no follow-up |
| Liver | Small haemangioma, simple cyst | Benign vascular or fluid-filled lesion |
| Ovary (premenopause) | Simple cyst under 3cm | Physiological, resolves on its own |
| Uterus | Small fibroid | Very common, monitor if symptomatic |
Findings that warrant follow-up
The middle category is the one that causes most anxiety, because the report will describe the finding but stop short of saying "you are fine". A short interval repeat scan is standard, cautious practice, not a hint that something is wrong. The usual candidates are:
- Renal cysts over 4cm or with septations. Graded on the Bosniak scale. Bosniak II or IIF cysts get a repeat ultrasound or MRI at 6 to 12 months.
- Adrenal nodules under 4cm. Small non-functioning adrenal adenomas are common, usually benign, and followed with a repeat scan and a blood test for hormonal activity.
- Pulmonary nodules seen on the edge of a spine or cardiac MRI. Typically referred for a dedicated low-dose CT rather than a repeat MRI.
- Meningiomas under 3cm without symptoms. Slow-growing benign brain tumours. Watch-and-wait with an MRI at 6 to 12 months is the standard UK approach.
- Thyroid nodules. Seen on cervical spine MRIs. Usually referred for a thyroid ultrasound rather than immediate action.
Findings that need action now
The urgent category is uncommon but the reason radiologists exist. When something serious is spotted incidentally, the reporting radiologist does not simply write it up and post the report. They ring the referring clinician the same day. Findings in this group include unexpected malignant tumours, unruptured aneurysms of concerning size, acute strokes and territory infarcts, new demyelinating lesions suggestive of MS, and spinal cord compression from disc, tumour or haematoma.
If a radiologist has been sufficiently worried to phone your consultant, you will usually know quickly. Your GP or specialist will be in touch, often the same day or the next morning. That is not bad news arriving badly - it is the system doing exactly what it is designed to do.
