DMSA Cortical
- DMSA is a nuclear medicine scan that maps functioning kidney cortex — the part of the kidney that actually does the work — rather than how fast urine drains.
- The tracer (Tc-99m DMSA) gets soaked up and held by healthy proximal tubule cells, so on the picture, bright = working tissue and dark = not.
- Its two big jobs: spotting acute pyelonephritis (a cold defect during infection) and measuring scarring plus split function (how much of the total each kidney contributes).
- It is the go-to scan in children with febrile urinary tract infections and suspected scarring — not a drainage study and not for obstruction.
Most renal nuclear scans are obsessed with flow — how fast a tracer washes in and drains out, like watching dye race through a garden hose. DMSA is the calm one in the family that doesn't care about the plumbing at all. It only wants to know one thing: how much living, working kidney is actually there?
The one-sentence idea
DMSA stands for dimercaptosuccinic acid, which is a mouthful nobody enjoys, so just file it under "the cortex tracer." Tagged with technetium-99m, it travels to the kidneys and gets grabbed and held by the cells of the proximal tubules — the busy little factory workers packed into the kidney's outer rim (the cortex). Healthy cortex hoards the tracer and lights up. Damaged, infected, or scarred cortex can't take it up, so it stays dark.
Think of it like spray-painting a sponge that only sticks to the dry parts: wherever the sponge is soggy or torn, no paint. The map of where the paint stuck is your map of working kidney.
DMSA is a cortical function and anatomy tracer, not a drainage tracer. If the clinical question is "is it obstructed?" you want the dynamic renal scintigram (MAG3/DTPA renogram), not this scan.
What you're actually looking at
Because the tracer just sits in the cortex, DMSA gives gorgeous, high-resolution still pictures of kidney shape — two bean-shaped blobs glowing evenly from edge to center. You're not watching a movie; you're admiring a portrait. Images are typically taken a few hours after injection, once the tracer has settled into the cortex and the background has quieted down.
The reader hunts for two things: defects (dark gaps where cortex should be glowing) and the balance between the two kidneys.
Split function: the kidney popularity contest
One of DMSA's most useful tricks is differential (split) function — what percentage of total kidney work each side is doing. Because uptake tracks functioning tissue, you can draw a region around each kidney, count the signal, and report something like "55% right, 45% left."
A roughly even split (close to 50/50) is the reassuring normal. When one kidney is clearly slacking, that number tells the urologist how much that kidney is still worth — which matters a lot before anyone decides whether to repair it or remove it.
Acute pyelonephritis vs. scar — same dark spot, different story
Here's where DMSA shines and also where it gets tricky. Both an active kidney infection and an old scar show up as cold defects (areas that don't take up tracer). The trick is in the shape and the timeline.
| Finding | Acute pyelonephritis | Cortical scar |
|---|---|---|
| Appearance | Defect without loss of kidney volume; cortex looks dented but not shrunken | Defect with volume loss; the outline is notched or flattened |
| Edges | Often patchy, wedge-shaped, can be multiple | Sharp, focal, usually at the poles |
| Over time | Can resolve on a follow-up scan | Permanent — it's a literal divot |
This is why timing matters. A defect seen during a fever might be active infection that heals. The same-looking defect months later, after the infection cleared, is a scar — and scarring is the thing everyone is trying to prevent, because enough of it can nudge a kidney toward hypertension and chronic damage down the line.
A normal anatomic dent can masquerade as a scar. The kidney has a few legitimate quirks — a flattening between the upper and lower halves, or a bump on the left (the "splenic hump"/dromedary hump) — that show smooth, preserved cortex rather than a true volume-losing defect. Don't call every contour irregularity a scar.
Where it fits in real life
DMSA lives mostly in pediatric practice. A child with a febrile urinary tract infection — especially with suspected vesicoureteral reflux (urine sneaking backward from bladder to kidney) — is the classic candidate, both to confirm the kidneys are being affected during the acute illness and, later, to check whether permanent scars formed.
In adults it's used more selectively: confirming pyelonephritis when the picture is murky, sorting out split function before surgery, or characterizing an oddly-shaped kidney.
A bonus party trick: because DMSA marks functioning tissue, it's excellent at telling apart a true non-functioning kidney from one that's just hiding. A "pseudotumor" (a lump of normal kidney tissue in an unusual spot) lights up like normal cortex, while genuinely dead tissue stays dark.
The one thing to remember
If you forget everything else: DMSA paints the working cortex, holds still, and answers two questions — how much kidney is there? and how is it split between the two sides? Bright is good, dark is the problem, and whether that dark spot is a temporary insult or a permanent scar comes down to volume loss and time.