Uterine Pathology
- The uterus has two layers that matter on imaging: the inner endometrium (the lining that sheds each month) and the surrounding myometrium (the muscle wall). Most uterine pathology is a problem in one of those two layers.
- Fibroids (leiomyomas) are benign knots of myometrial muscle — extremely common, well-defined, and graded by where they sit relative to the cavity.
- Adenomyosis is endometrial tissue burrowing into the muscle wall — a diffusely thickened, ill-defined junctional zone, best shown on MRI.
- The endometrium is the watch-this layer: how thick it is matters most in postmenopausal bleeding, where thickening raises concern for cancer.
- Ultrasound is the first look; MRI is the problem-solver when you need a map.
The uterus is basically a muscular pear that spends decades quietly remodeling itself on a monthly schedule, and almost everything that goes wrong with it is a story about one of its two layers misbehaving. Get those two layers straight and most "uterine pathology" stops being a scary list and becomes a tidy filing system.
The two layers you actually care about
Picture the uterus as a thick-walled rubber ball. The endometrium is the soft lining on the inside — the bit that builds up and sheds with each cycle, which is why its thickness changes depending on where someone is in their month (and whether they've gone through menopause). The myometrium is the muscular wall wrapped around it, the part that does the heavy lifting during labor.
On ultrasound — almost always the first test for the pelvis — the endometrium shows up as a bright stripe down the middle. On MRI there's a third structure worth naming: the junctional zone, the dark inner band of myometrium right against the lining. Think of it as the fence between the lawn (endometrium) and the field (outer muscle). When that fence gets fuzzy and overgrown, something is wrong.
Fibroids: the muscle knots
Fibroids — the textbook name is leiomyomas — are the most common thing you'll meet here. They're benign clumps of myometrial muscle, like a callus that grew a little too enthusiastically. They're typically well-defined and round, and on MRI they're classically dark on T2 because they're packed dense muscle and fibrous tissue. (Some get a little fancier when they degenerate, but the round, well-circumscribed shape is the constant.)
What actually changes management is location, so we file them by where they sit relative to the cavity:
| Type | Where it sits | Why it matters |
|---|---|---|
| Submucosal | Bulging into the endometrial cavity | Most likely to cause bleeding and fertility problems |
| Intramural | Entirely within the muscle wall | The most common; effect depends on size |
| Subserosal | Bulging out from the outer surface | Can press on neighbors (bladder, bowel) or hang off on a stalk |
A subserosal fibroid on a stalk (pedunculated) can wander far from the uterus and masquerade as an ovarian mass. Tracing its blood supply back to the myometrium — the "bridging vessel" — is what saves you from calling it the wrong organ entirely.
Adenomyosis: endometrium gone spelunking
If fibroids are a discrete knot, adenomyosis is the opposite — it's endometrial tissue that has tunneled into the muscle wall and set up camp where it doesn't belong. Instead of a ball you can point to, you get a wall that's diffusely thickened and ill-defined.
The MRI signature is a thickened, blurry junctional zone — that fence between lawn and field turns into an overgrown hedge. You may also spot tiny bright specks within the wall (little pockets of trapped tissue). This is the classic place MRI earns its keep, because the diffuse, mappy nature of adenomyosis can be genuinely hard to call on ultrasound alone.
Fibroids and adenomyosis love to coexist and can look similar on a quick scan, but they are managed differently — one is a discrete mass you might resect or embolize, the other is a diffuse process. The tell is the borders: fibroids are well-defined and round, adenomyosis is ill-defined and spread along the wall.
The endometrium: the layer everyone watches
The lining is where the highest-stakes question lives, and the right answer depends entirely on the patient's stage of life. In someone still cycling, endometrial thickness swings wildly through the month, so a thick stripe is often just normal timing — context is everything.
The scenario that snaps everyone to attention is postmenopausal bleeding. After menopause the lining should be thin and quiet, so any bleeding plus a thickened stripe raises concern for endometrial cancer and earns further workup. The flip side is reassuring: a reliably thin stripe in that setting makes cancer much less likely.
Postmenopausal bleeding is the symptom that drives the endometrial-thickness question. The measurement is a triage tool, not a diagnosis — it decides who needs tissue sampling, and the bleeding itself is what must be explained.
Not everything thickening the lining is sinister. Benign polyps (focal outgrowths of the lining) and submucosal fibroids both bulge into the cavity and cause bleeding too. A saline-infusion sonohysterogram — squirting a little sterile fluid into the cavity to outline it on ultrasound — is the elegant trick that separates a focal polyp from diffuse thickening.
How the tests fit together
Ultrasound asks "is something there?"; MRI answers "exactly what and where is it?" Reach for MRI when you need a precise map — surgical planning for fibroids, confirming adenomyosis, or characterizing a confusing mass.
The same logic that handles a confusing ovarian finding applies next door: when ultrasound leaves you uncertain, a focused pelvic MRI is the problem-solver. And because pelvic anatomy is crowded, always confirm a mass actually belongs to the uterus and not the ovary — the same care you'd take when working up an adnexal mass. Keep the two layers in mind, decide which one is misbehaving, and uterine pathology mostly organizes itself.