First and second year — the floor first, then every step
Grade the laceration by AAST 2018 on the nephrographic phase, then use the delayed excretory phase to prove or exclude a collecting system or ureteropelvic injury — the one question a single-phase CT cannot answer.
Orient first
The kidney sits inside Gerota fascia, which contains most bleeding; whether active bleeding is contained within Gerota or extends beyond it is a grade boundary.
Urine leak only shows when contrast is excreted — on the DELAYED (excretory) phase, several minutes after injection. A trauma CT without it cannot grade a deep laceration.
The ureteropelvic junction avulsion (deceleration, often in children) can leave a normally perfused kidney with contrast pooling medially and a non-opacified ureter.
Acquire the study
Trauma CT with an ARTERIAL phase (or a split-bolus single acquisition) and a PORTAL VENOUS phase; add a DELAYED phase (about 5–10 min) whenever there is solid organ injury or free fluid of uncertain source — it separates active bleeding from a contained vascular injury and shows urine leaks.
Thin slices with CORONAL and SAGITTAL reformats; read the arterial and portal venous phases side by side.
Delayed EXCRETORY phase (about 5–10 min) whenever there is a renal laceration, perirenal fluid, or haematuria with a deceleration mechanism.
The manoeuvre
Nephrographic/portal venous phase: each laceration — its depth from the cortex in cm and whether it reaches the collecting system.
Subcapsular haematoma and perirenal haematoma — confined within Gerota fascia or not.
Compare every focus of contrast outside the normal vessels across phases: ACTIVE EXTRAVASATION grows and changes shape from arterial to portal venous to delayed; a PSEUDOANEURYSM or arteriovenous fistula is a well-defined focus that follows the blood pool and washes out without growing.
Measure attenuation of free fluid: simple fluid about 0–20 HU, unclotted blood about 30–45 HU, clotted blood (the sentinel clot) about 45–70 HU — the highest-attenuation clot sits next to the injured organ.
Wedge-shaped or global absence of enhancement: segmental or main renal artery injury (occlusion or dissection); a cortical rim sign can appear after some hours.
Delayed excretory phase: contrast outside the collecting system (urinoma) and where it leaks from — the calyces, the pelvis, or the ureteropelvic junction; opacification of the ureter distal to the UPJ.
Main renal artery and vein on the arterial phase: intimal flap, occlusion, avulsion at the hilum.
What confirms it
Urinary extravasation is excreted contrast outside the collecting system on the delayed phase, not dense fluid on the portal venous phase.
What licenses you to exclude it
Without a delayed excretory phase you cannot exclude a collecting system injury in a deep laceration — say so and recommend it.
The classic misread
Calling excreted contrast in the normal collecting system extravasation, or blood in the perirenal space a urinoma.
Missing a UPJ avulsion because the parenchyma looks normal.
Missing a pre-existing abnormality (hydronephrosis, tumour, horseshoe kidney) that makes a trivial mechanism cause a major injury.
Reporting the injury
Classification to use
AAST Organ Injury Scale 2018 (kidney). CT criteria: I — subcapsular haematoma and/or parenchymal contusion without laceration. II — perirenal haematoma confined to Gerota fascia; laceration ≤ 1 cm deep without urinary extravasation. III — laceration > 1 cm without collecting system rupture or urinary extravasation; any vascular injury or active bleeding contained within Gerota fascia. IV — laceration into the collecting system with urinary extravasation; renal pelvis laceration and/or complete ureteropelvic disruption; segmental renal artery or vein injury; active bleeding beyond Gerota fascia; segmental or complete infarction due to vessel thrombosis without active bleeding. V — main renal artery or vein laceration or avulsion of the hilum; devascularised kidney with active bleeding; shattered kidney with loss of identifiable parenchymal anatomy.
Measurements — and how to take them
Laceration depth from the cortical surface, in cm (threshold 1 cm).
Perirenal haematoma thickness in cm; infarcted parenchyma as an estimated percentage.
What to report
Laceration depth and whether it reaches the collecting system; haematoma confined to Gerota or not.
Urinary extravasation (from where) on the delayed phase, or the absence of a delayed phase.
Vascular injury, active bleeding (within or beyond Gerota), infarction, main vessel status.
The contralateral kidney — present and functioning; pre-existing renal abnormality.
The AAST 2018 grade.
How to report it
CT: "Left renal laceration extending through the interpolar cortex into the collecting system, with excreted contrast leaking into the perirenal space on the 8-minute delayed phase. Perirenal haematoma confined within Gerota fascia. No active arterial bleeding. The ureter opacifies distally. Normal right kidney. AAST 2018 grade IV renal injury."
What not to report
Do not diagnose a urinoma without a delayed phase.
Do not grade a renal injury without stating whether a delayed phase was performed.
Associated injuries to look for
Lower rib and transverse process fractures, splenic (left) and hepatic/duodenal (right) injury, adrenal haemorrhage, lumbar spine fractures.
What changes management
Active bleeding or pseudoaneurysm — angioembolisation.
UPJ avulsion or complete ureteric disruption — surgical repair; a collecting system leak — often stent and observation.
Main artery occlusion — revascularisation is time-critical and often not possible; say so plainly.
Solitary kidney — every decision becomes renal-sparing.
Reference values
Each value carries the caveat that keeps it from being misused. Normal limits and diagnostic criteria are kept apart on purpose: a disease cut-off read as a normal range is the more dangerous mistake.
Normal limits
Gerota fascia / perirenal space · Perirenal stranding versus a page kidney
a named perirenal fat plane inside Gerota; stranding is a stone / pyelo / trauma look-for — companion to rest-page-kidney, not a fake fascia millimetre
A subcapsular haematoma (page kidney) is not simple stranding.
report the organ, laceration depth / subcapsular extent, and ANY vascular injury (active extravasation, pseudoaneurysm, AVF, or hilar disruption). Modern AAST grades upgrade on vascular injury — a blush is not “grade III because someone remembered 3 cm”. Read the current organ table; do not invent an edition year
A blush is a vascular injury, not a “grade III because I remembered 3 cm”. Name the organ and the edition. Versioned criterion — verify against the current edition before clinical use.
absent. Page kidney = a subcapsular haematoma or collection that flattens the cortex and can cause renin-mediated hypertension (after trauma or biopsy). Report collection thickness, how much parenchyma is flattened, and whether the collecting system is compressed. A thin physiologic subcapsular film without mass effect is not Page. Cortical thickness is the already-registered ≥7 mm entry
Mass effect plus hypertension is the conversation, not the word “subcapsular”. A perinephric (Gerota) haematoma is outside the capsule and is not Page.
CT · USG · MRI
See it on real cases
Direct links to Radiopaedia — the reference article and worked cases with their images. Each opens on Radiopaedia.