Scrotal Ultrasound
- Scrotal ultrasound is the first and usually the only test for almost any scrotal complaint — it's fast, painless, and uses no radiation.
- Know normal: a testis about 5 × 3 × 2 cm and homogeneously echogenic, an epididymal head 10–12 mm across, and 2–3 mL of fluid between the layers of the tunica vaginalis.
- The headline emergency is testicular torsion: absent flow on color Doppler and a twisted cord (the whirlpool sign, 92% sensitive and 99% specific) are the alarm bells. Pooled across studies, Doppler ultrasound is 0.86 sensitive and 0.95 specific for torsion in adults, better in recent series.
- Most palpable lumps sort into two camps: cystic and benign (hydrocele, epididymal cyst, varicocele) versus a solid mass inside the testis, which is cancer until proven otherwise.
- After blunt trauma, a heterogeneous testis with a broken contour is rupture: ultrasound was 100% sensitive and 93.5% specific for it in the landmark series.
Few imaging requests come with as much patient anxiety per square centimeter as a scrotal ultrasound. The good news is that the scrotum is, anatomically, a gift: two egg-shaped organs sitting right under thin skin, perfectly placed for a high-frequency probe. There's no bowel gas, no ribs, no fat to wade through. What you see is what you get — which is exactly why ultrasound owns this body part outright.
Why ultrasound, and why nothing else
Sound waves are great at telling fluid from solid and at watching blood move, and that's basically the whole job here. A scrotal study answers a small set of questions: Is there blood flow? Is the lump cystic or solid? Is it inside the testis or just hanging around it? That last distinction is the entire game.
The workhorse is the high-frequency linear probe, the same flat one used for superficial structures. Pair it with Doppler — the mode that turns moving blood into color — and you've got everything you need. Good technique matters: you always scan both sides, because the painless testis is your built-in control. When the right one looks weird, the left one tells you what "normal" was supposed to look like.
Findings by modality
Ultrasound: grayscale
Normal is the reference for everything else. The adult testis measures about 5 × 3 × 2 cm, is homogeneously echogenic, and slowly regresses with age. The epididymal head sits on the upper pole and is the largest part of the epididymis at 10–12 mm; the body and tail trail down behind. A small amount of fluid, 2–3 mL, normally lies between the layers of the tunica vaginalis, so a trace of anechoic fluid is not a hydrocele.
- Torsion: early on the grayscale image can be near-normal, which is the trap. As hours pass the testis swells, turns hypoechoic, then heterogeneous once it infarcts. The whirlpool sign — the spermatic cord twisted into a spiral just above the testis — is the grayscale finding that clinches it.
- Epididymo-orchitis: an enlarged, hypoechoic (sometimes heterogeneous) epididymis, with or without a swollen testis, plus a reactive hydrocele and thick scrotal skin.
- Hydrocele: anechoic fluid surrounding the testis, beyond the normal 2–3 mL.
- Epididymal cyst or spermatocele: a well-defined anechoic cyst outside the testis, in or beside the epididymis.
- Varicocele: a tangle of tubular, serpiginous veins in the cord, usually on the left. Some authors propose a pampiniform vein diameter cut-off of 2.5 mm at rest or 3.0 mm during Valsalva.
- Intratesticular mass: a solid lesion arising within the parenchyma, hypoechoic or mixed, with or without calcification.
- Rupture: heterogeneous testicular echotexture plus a break in the tunica albuginea contour, often with hematocele.
- Microlithiasis: tiny non-shadowing echogenic foci scattered through the testis.
Ultrasound: color and spectral Doppler
Doppler is the deciding vote in the painful scrotum, and the comparison with the other side is the technique. Absent or markedly reduced intratesticular flow on the symptomatic side, with normal color on the other, is torsion until proven otherwise. Increased flow in an enlarged epididymis or testis is epididymo-orchitis. In a varicocele, color fills the dilated veins and reverses on Valsalva. In a solid intratesticular mass, internal vascularity supports tumor over an avascular hematoma or infarct.
MRI
Rarely needed. MRI is the problem-solver when ultrasound cannot tell a segmental infarct from a tumor, cannot define a hematoma after trauma, or cannot locate an undescended testis. It is never the first test in the acute scrotum.
CT
CT has no role in the scrotum itself; its job with testicular cancer is the retroperitoneum, where these tumors send their nodes along the gonadal vein. If you find a solid intratesticular mass, the next picture is a CT of the abdomen and chest, not another scan of the scrotum.
The emergency: testicular torsion
Here's the one you cannot miss. The testis dangles from its cord like a yo-yo on a string. If that string twists, it pinches off the testis's own blood supply — first the veins, then the arteries. Tissue without blood dies, and this tissue dies on a schedule measured in hours, not days. Sudden severe pain, often in a younger patient, should make torsion the headline on your differential.
Torsion is a clinical-plus-imaging emergency, not a leisurely workup. If the clinical story screams torsion, the patient goes to the operating room — a reassuring-looking ultrasound should not be used to talk a surgeon out of saving a salvageable testis. Time is testicle.
A twist can be partial or come and go. Intermittent or incomplete torsion may leave some flow, so "I see a little color" does not equal "all clear." Asymmetry compared with the normal side, a swollen abnormal-looking testis, and a whirlpool in the cord still count as red flags. This is exactly why the pooled sensitivity is not 100%.
The numbers
| What | Threshold / value | Why it matters |
|---|---|---|
| Normal adult testis | About 5 × 3 × 2 cm; homogeneously echogenic; regresses with age | Anything heterogeneous, or a size mismatch between sides, needs an explanation |
| Normal epididymal head | 10–12 mm in diameter (the largest part of the epididymis) | Bigger and hypoechoic with hyperemia is epididymitis |
| Normal tunica vaginalis fluid | 2–3 mL | More than this is a hydrocele |
| Varicocele vein diameter (proposed) | 2.5 mm at rest; 3.0 mm with Valsalva | Proposed cut-offs, not universal; the reflux on Valsalva is the physiology |
| Microlithiasis, isolated, no risk factors | No follow-up (ESUR) | Spares a lot of men a lot of scans |
| Microlithiasis with risk factors | Annual ultrasound up to age 55 (personal or family history of germ cell tumor, maldescent, orchidopexy, testicular atrophy) | Risk factors, not the microliths, drive the surveillance |
| Microlithiasis with a testicular mass | Urgent specialist referral | The mass is the problem; the microliths are the bystander |
How good is the test
- Torsion (adults, meta-analysis): color Doppler ultrasound had an overall sensitivity of 0.86 (95% CI 0.79–0.91) and specificity of 0.95 (0.92–0.97); in studies published after 2010 the pooled figures rose to a sensitivity of 0.95 (0.84–0.99) and specificity of 0.98 (0.93–0.99).
- Whirlpool sign: as summarized, sensitivity 92% and specificity 99% for torsion — the single best sign, and the reason you image the cord, not just the testis.
- Epididymitis: as summarized, sensitivity 70% and specificity 88%, so a normal-looking epididymis does not exclude it.
- Varicocele: as summarized, about 97% sensitive and 94% specific.
- Testicular rupture after blunt trauma: sensitivity 100% and specificity 93.5% in a series of 65 patients, using heterogeneous testicular echotexture plus a contour abnormality; across series, sensitivity 64–100% and specificity 65–93.5%, which is why an equivocal post-traumatic testis is explored rather than watched.
- Intratesticular tumor: no robust pooled accuracy figure; the working rule is that ultrasound detects the mass reliably but cannot tell histology.
The great mimic: epididymo-orchitis
Torsion's evil twin is infection. Epididymo-orchitis — inflammation of the epididymis, the testis, or both — also causes a painful, swollen scrotum, so clinically it overlaps with torsion. But on Doppler it does the opposite thing: inflamed tissue gets more blood flow, not less. So the classic split is increased flow (infection) versus absent flow (torsion). Helpful, but never a substitute for the clinical picture and surgical judgment.
The painless lump: cyst versus cancer
When there's no pain, just a lump, the job becomes sorting benign plumbing from something sinister. Ultrasound's superpower is telling cystic from solid.
| Finding | What it is | Classic look | Worry level |
|---|---|---|---|
| Hydrocele | Fluid around the testis | Anechoic (black) fluid surrounding a normal testis, more than the normal 2–3 mL | Low |
| Epididymal cyst / spermatocele | Fluid pocket near the epididymis | Well-defined anechoic cyst, outside the testis | Low |
| Varicocele | Dilated cord veins | Tangle of tubular vessels that fill in with Valsalva | Low (but relevant to fertility) |
| Solid intratesticular mass | Tumor until proven otherwise | Solid lesion inside the testis, often with internal flow | High |
The location word does the heavy lifting. Fluid collections and most cysts sit around or next to the testis. A solid mass within the testis is the one that buys an urgent referral, because the overwhelming majority of solid intratesticular masses in the relevant age group are malignant.
"Inside the testis and solid" is the phrase that should make your pulse jump. Cystic and outside the testis is usually nothing; solid and inside is cancer until a urologist says otherwise.
Testicular microlithiasis, briefly
Those scattered bright specks worry everyone, and the ESUR guideline is refreshingly calm about them: isolated microlithiasis with no risk factors needs no follow-up. With risk factors — a personal or family history of germ cell tumor, maldescent, orchidopexy, or testicular atrophy — the recommendation is an annual ultrasound up to age 55. Microlithiasis plus a testicular mass is an urgent referral, because of the mass. The dedicated page on testicular tumors goes further.
Telling the look-alikes apart
| Mimic | Looks similar because | Tell them apart by |
|---|---|---|
| Epididymo-orchitis versus torsion | Both: acute pain, swollen hemiscrotum, hydrocele | Doppler: increased flow (infection) versus absent or reduced flow (torsion); whirlpool in the cord means torsion |
| Torsion of an appendage versus torsion | Acute pain in a child or young man | A small avascular nodule at the upper pole with surrounding hyperemia; the testis itself has normal flow |
| Segmental testicular infarct versus tumor | Both are a focal intratesticular lesion | Infarct is wedge-shaped, avascular, and shrinks over time; tumor is round with internal flow |
| Hematoma versus tumor after trauma | Focal heterogeneous intratesticular lesion | Hematoma is avascular and evolves on follow-up; a broken contour points to rupture |
| Focal orchitis versus tumor | Hypoechoic intratesticular area with flow | Orchitis is hyperemic with an inflamed epididymis and resolves; tumor persists |
| Inguinal hernia versus scrotal mass | Extratesticular "mass" | Bowel with peristalsis or fat, continuous with the inguinal canal, changes with Valsalva |
| Spermatocele versus hydrocele | Both anechoic | Spermatocele is a discrete cyst at the epididymal head; hydrocele surrounds the testis |
Trauma, the appendages, and odds and ends
After injury, ultrasound checks whether the testis itself is intact — a heterogeneous testis with a disrupted tunica albuginea contour is rupture (the two findings that produced the 100% sensitivity above), versus a simple contusion or hematocele. There are also tiny normal vestigial bits — the appendix testis and appendix epididymis — that can twist on their own and cause pain, usually a far less dramatic affair than true cord torsion but a classic mimic in kids. The dedicated pages on testicular torsion, epididymo-orchitis, and scrotal trauma go deeper on each.
Reporting
What the emergency physician or urologist needs:
- Both testes: size in three dimensions, echotexture, and whether the two match.
- Flow: present, reduced, or absent on the symptomatic side compared with the other side, and whether spectral Doppler shows arterial and venous waveforms.
- The cord: whirlpool present or absent. Say it explicitly in every acute scrotum.
- Epididymis: size and echogenicity of the head, hyperemia.
- Any lesion: intratesticular or extratesticular, cystic or solid, vascular or not, size, and location.
- Fluid: hydrocele beyond the normal trace, hematocele, scrotal wall thickening.
- Trauma: tunica albuginea intact or disrupted, parenchymal heterogeneity, hematocele.
- Microlithiasis: present or absent, and whether any risk factor or mass changes the follow-up.
So the scrotal ultrasound boils down to two reflexes worth burning in: when it hurts, interrogate the blood flow and the cord; when there's a lump, decide cyst-or-solid and inside-or-outside. Get those two reflexes right and you've handled the vast majority of what walks through the door.
References
- Lam Shin Cheung J, Bhaduri M. A sound approach to stay on the ball—a review of scrotal pathologies on ultrasound imaging. Br J Radiol 2026;99(1181):847–858. Used for: the normal testis size (about 5 × 3 × 2 cm, homogeneously echogenic, regressing with age), the 10–12 mm epididymal head, the 2–3 mL of normal tunica vaginalis fluid, the proposed varicocele vein cut-offs of 2.5 mm at rest and 3.0 mm with Valsalva and the summarized 97% sensitivity and 94% specificity for varicocele, the whirlpool sign sensitivity 92% and specificity 99%, the epididymitis sensitivity 70% and specificity 88%, and the across-series rupture ranges (sensitivity 64–100%, specificity 65–93.5%) in the Key Points, the Measurement chips, "Findings by modality," "The numbers," "How good is the test," and "The painless lump."
- Ota K, Fukui K, Oba K, et al. The role of ultrasound imaging in adult patients with testicular torsion: a systematic review and meta-analysis. J Med Ultrason 2019;46(3):325–334. Used for: the pooled color Doppler ultrasound sensitivity 0.86 (95% CI 0.79–0.91) and specificity 0.95 (0.92–0.97) for torsion in adults, and the post-2010 figures of sensitivity 0.95 (0.84–0.99) and specificity 0.98 (0.93–0.99), in the Key Points and "How good is the test."
- Buckley JC, McAninch JW. Use of ultrasonography for the diagnosis of testicular injuries in blunt scrotal trauma. J Urol 2006;175(1):175–178. Used for: the 100% sensitivity and 93.5% specificity of ultrasound for testicular rupture in 65 patients using heterogeneous echotexture plus contour abnormality, in the Key Points, "How good is the test," and "Trauma, the appendages, and odds and ends."
- Richenberg J, Belfield J, Ramchandani P, et al. Testicular microlithiasis imaging and follow-up: guidelines of the ESUR scrotal imaging subcommittee. Eur Radiol 2015;25(2):323–330. Used for: the microlithiasis follow-up rules (no follow-up without risk factors; annual ultrasound up to age 55 with risk factors of personal or family history of germ cell tumor, maldescent, orchidopexy, or testicular atrophy; urgent referral when a mass is present) in "The numbers" and "Testicular microlithiasis, briefly."