Renal Infarction / Renovascular Ischemic Kidney

00Overview

Renal infarction 是腎動脈供血中斷導致的腎實質缺血壞死，臨床表現常模擬 renal colic 或 pyelonephritis。影像任務核心是：(1) 在 contrast-enhanced CT 上辨識 wedge-shaped perfusion defect；(2) 確認病因（embolic vs thrombotic vs dissection vs vasculitis）；(3) 評估 salvageable kidney 以決定 revascularization 策略。最容易出錯的地方：把 acute renal infarction 的 flank pain + hematuria 誤診為 renal colic，初始只做 non-contrast CT（看不到 perfusion defect）而延遲診斷。

01Critical concepts

• Renal infarction 的 CT hallmark 是 cortical rim sign：peripheral cortex 由 capsular arteries 供血而保留 enhancement，central parenchyma 無 enhancement → "cortical rim sign" 出現在 subacute phase（24–72 hours 後）
• 最常見的病因是 cardiac embolism（atrial fibrillation、valvular disease、mural thrombus），其次是 renal artery thrombosis（atherosclerosis、dissection、FMD）、vasculitis（PAN）、與 cholesterol embolization
• Acute renal infarction 需要 contrast-enhanced CT 才能診斷：non-contrast CT（renal colic protocol）幾乎無法偵測 acute infarction，因此 clinical suspicion is key
• MRI / DWI 在 acute infarction 顯示 restricted diffusion（low ADC）+ T2 hyperintensity + post-contrast wedge-shaped non-enhancement，可用於 contrast 禁忌或 problem-solving
• Bilateral renal infarction 或 infarction with other organ infarction → 高度懷疑 systemic embolic source（cardiac / aortic）

01正常 anatomy / 常用 modality

每側腎臟由 single renal artery 供血（20-30% 有 accessory renal artery）。Renal artery 分為 anterior + posterior divisions → segmental arteries → interlobar → arcuate → interlobular。此為 end-artery system — 一條 segmental artery 阻塞即造成對應的 wedge-shaped infarction。Capsular、ureteric、pelvic、與 adrenal branches 可形成 peripheral collateral，這是 cortical rim sign 的解剖基礎。

常用 modality：

• CT with IV contrast（nephrographic phase）：首選，清楚顯示 perfusion defect；arterial phase 可見 renal artery thrombus / dissection flap
• CT angiography (CTA)：renal artery pathology 的 definitive evaluation（stenosis、dissection、aneurysm、FMD、PAN microaneurysms）
• Doppler ultrasound：bedside screening — absent or reduced flow in affected kidney；但 sensitivity 有限，尤其 segmental infarction
• Contrast-enhanced ultrasound (CEUS)：在 renal-impairment 或 iodinated contrast 禁忌時可即時顯示 wedge-shaped non-enhancing area；對 segmental infarction 敏感度優於 grayscale + Doppler，無 nephrotoxicity 且可 bedside 重複追蹤
• MRI / MRA：替代 CTA（腎功能不佳或 contrast allergy）；acute infarction 在 DWI 呈 restricted diffusion（high b-value high signal, low ADC），T2 略 hyper，post-Gd 為 wedge-shaped non-enhancement
• Nuclear medicine (DMSA scan)：quantify differential renal function + cortical scarring in chronic phase
• Catheter angiography：diagnostic + therapeutic（thrombectomy / thrombolysis / stenting）；PAN 可見 multiple small（1–5 mm）intrarenal microaneurysms

02常見 pattern 分類

Wedge-shaped cortical perfusion defect

• Definition：contrast-enhanced CT 上看到的 triangular / wedge-shaped area of non-enhancement，base 朝向 cortex，apex 指向 hilum
• Why it matters：最經典的 renal infarction pattern，直接對應 occluded segmental artery 的供血區域
• What it points toward：embolic infarction（most common）；renal artery branch thrombosis；FMD-related dissection
• Phase 時序：arterial phase 已可見 segmental enhancement asymmetry，但 nephrographic phase（80–100 sec）對比最強；delayed phase 可出現 flip-flop enhancement pattern（早期 non-enhancing 的 infarcted segment 在 delayed phase 因 contrast slow wash-in via collateral / interstitial leak 反而略 hyperdense，而周圍正常 parenchyma 已 wash-out）
• Common trap：早期 infarction（< 24 小時）可能 enhancement difference subtle → 需要 delayed nephrographic phase 才能清楚顯示；MRI DWI 在此 hyperacute window 比 CT 更敏感

Global non-enhancement of entire kidney

• Definition：整個腎臟在 contrast-enhanced CT 上幾乎無 enhancement，僅有最外層 cortical rim 的 faint enhancement
• Why it matters：代表 main renal artery occlusion → entire kidney at risk；需 emergent revascularization consideration if < 6-12 小時
• What it points toward：renal artery embolism at main trunk；renal artery thrombosis（atherosclerotic + superimposed thrombus）；aortic dissection extending into renal artery
• Common trap：chronic renal artery stenosis with atrophic kidney 也可 globally hypoenhancing → 但 kidney size 會 small（atrophic），而 acute occlusion 時 kidney size 正常或 slightly enlarged

Cortical rim sign

• Definition：thin peripheral rim of enhancement（1-3 mm）surrounding non-enhanced infarcted parenchyma，seen in subacute phase（24-72 hours 後出現）
• Why it matters：cortical rim sign 是 renal infarction 的 subacute marker，由 capsular artery collateral supply 解釋（capsular、perforating、ureteric branches 維持最外 1–3 mm cortex 的 viability）
• What it points toward：established infarction（beyond hyperacute window）；poor chance of salvage of deep parenchyma
• Common trap：cortical rim sign 不出現在 hyperacute phase — absence does not rule out infarction in first 24 hours；ATN 與 acute cortical necrosis 也可出現相似 rim，但 cortical necrosis 是 cortex 本身 non-enhancing 而 medulla 殘存 enhancement（pattern 相反）

Multifocal bilateral perfusion defects

• Definition：bilateral kidneys 多處 wedge-shaped defects，可能合併 other organ infarctions（spleen、brain、extremities）
• Why it matters：==bilateral multifocal infarctions = systemic embolization or vasculitis== → need cardiac source evaluation（TEE、TTE、Holter）+ 考慮 PAN / cholesterol emboli
• What it points toward：atrial fibrillation with mural thrombus → shower emboli；infective endocarditis（septic emboli）；cholesterol crystal embolization syndrome（post-catheterization）；polyarteritis nodosa（medium-vessel vasculitis with microaneurysms）
• Common trap：cholesterol emboli syndrome 在 CT 上不一定有 classic wedge pattern — 可呈 punctate / peripheral cortical non-enhancement + livedo reticularis + blue toe syndrome + eosinophilia + hypocomplementemia 等 systemic clues，影像 alone 易 underdiagnose

03Top common diagnoses

• Cardioembolic renal infarction：AF / valvular disease / LV thrombus → most common cause，通常 segmental
• Renal artery thrombosis：atherosclerotic stenosis + superimposed thrombus；or FMD-related dissection in young patients
• Aortic dissection with renal malperfusion：Stanford type B dissection → false lumen compressing renal artery → global ischemia
• Fibromuscular dysplasia (FMD) with dissection：young female，"string of beads" in mid-distal renal artery，spontaneous dissection → segmental infarction
• Polyarteritis nodosa (PAN)：medium-vessel necrotizing vasculitis；catheter / CTA 顯示 multiple small intrarenal microaneurysms（1–5 mm）+ wedge infarcts + perinephric hemorrhage（ruptured microaneurysm）；常合併 mesenteric / hepatic microaneurysms
• Cholesterol embolization syndrome：post-aortic intervention（cath、surgery）+ livedo reticularis + blue toe + eosinophilia；renal cortex 呈 punctate peripheral non-enhancement
• Renal artery aneurysm with thromboembolism：thrombosed renal artery aneurysm → distal embolization to segmental branches

04Cannot-miss diagnosis / emergency

05高頻 mimics 與 discriminators

Renal infarction vs acute pyelonephritis

• Why they get confused：both present with acute flank pain + fever（infarction can cause low-grade fever）+ elevated WBC
• Most useful discriminators：(1) pyelonephritis 有 striated nephrogram（alternating linear bands of hypo- and normal enhancement radiating from papilla to cortex，由 tubular obstruction + interstitial edema + vasospasm 造成 patchy tubular contrast transit delay）而非 sharply demarcated wedge defect；(2) UA — pyuria + bacteriuria → pyelonephritis；hematuria → infarction；(3) LDH markedly elevated in infarction（常 > 1000 U/L），less so in pyelonephritis；(4) pyelonephritis 常有 perinephric fat stranding + thickened renal pelvis + urothelial enhancement
• Common trap：focal bacterial nephritis（lobar nephronia）可呈 wedge-shaped，影像上與 infarction 相似 → 臨床區分（UA, blood culture, LDH）；striated nephrogram 並非完全 specific — ureteral obstruction、acute tubular necrosis、contrast nephropathy 也可出現

Renal infarction vs renal colic（ureteral stone）

• Why they get confused：acute flank pain + hematuria → initial clinical impression is renal colic → order non-contrast CT → miss infarction
• Most useful discriminators：(1) renal colic 在 NCCT 見 ureteral stone + hydronephrosis + perinephric stranding；(2) renal infarction 在 NCCT 幾乎 invisible →若 NCCT negative for stone + clinical suspicion remains → request contrast-enhanced CT；(3) LDH very elevated in infarction（> 1000 U/L common），not in colic
• Common trap：most missed renal infarctions occur because contrast CT was not performed after negative NCCT

Acute vs chronic renal infarction

• Why they get confused：both show perfusion defect on CT
• Most useful discriminators：(1) acute — normal kidney size, wedge defect without volume loss，DWI restricted diffusion；(2) chronic — cortical thinning, focal scar, overall kidney size reduced, no diffusion restriction；(3) cortical rim sign → subacute（transitional）；(4) DMSA scan — photopenic defect corresponding to scar in chronic phase
• Common trap：incidentally discovered chronic cortical scar 不需 acute intervention — 但 should prompt cardiac source evaluation if no known history

Embolic infarction vs PAN-related infarction

• Why they get confused：both present with multifocal wedge defects
• Most useful discriminators：(1) PAN 常伴 multiple intrarenal microaneurysms（CTA / angiography），cardioembolic 無；(2) PAN 常多器官受侵（mesenteric、hepatic microaneurysms、skin、peripheral nerve），cardioembolic source 在 cardiac chambers；(3) PAN 可出現 spontaneous perinephric / subcapsular hemorrhage（ruptured microaneurysm），embolic 罕見
• Common trap：早期 PAN microaneurysms 體積小（1–5 mm），CTA 可能漏診 → 高度懷疑時 catheter angiography 仍是 reference standard

06Next step / protocol / appropriateness

影像 protocol 選擇：

• Acute flank pain, negative NCCT, high clinical suspicion：CT with IV contrast（arterial + nephrographic + delayed phases）→ perfusion defect diagnosis；renal impairment 時改 MRI（DWI + post-Gd）或 CEUS
• Confirmed renal infarction：CTA from aortic arch to pelvis → evaluate entire aorta（dissection？）+ renal artery（thrombus？stenosis？microaneurysm？）+ cardiac chambers（thrombus？）
• Embolic source workup：echocardiography（TTE + TEE）+ ECG / Holter monitoring → AF, valve disease, mural thrombus
• Suspected vasculitis (PAN)：catheter angiography of renal + mesenteric + hepatic arteries → microaneurysm survey
• Renovascular hypertension / chronic ischemia / suspected Page kidney：CTA or MRA renal arteries → stenosis grading + 評估 subcapsular collection；若 > 70% stenosis + clinical criteria → consider revascularization
• Follow-up：DMSA scan at 3-6 months → quantify residual function → guide long-term management

07Pitfalls / normal variants

• Column of Bertin：prominent column of Bertin（hypertrophied cortical tissue between pyramids）與 normal cortex isoenhancing across all phases，不會造成 perfusion defect — 此為 mass mimic 而非 infarct mimic；若見「wedge-shaped non-enhancement」就不是 column of Bertin
• Fetal lobulation：persistent fetal lobulation of kidney surface 可模擬 cortical scarring → smooth undulating contour without parenchymal loss，cortex 厚度正常
• Junctional parenchymal defect（US finding）：US 上 triangular echogenic / fat-containing area at junction of upper and middle thirds of kidney → normal developmental variant，not infarct；CT 上對應為 small fat-density triangular indentation，不應被誤判為 wedge infarct
• Dual renal artery supply：accessory renal artery 可限制 infarction extent（collateral supply from second artery），perfusion defect 邊界可不對稱於預期 segmental territory
• Contrast-induced nephropathy concern：不應因擔心 contrast injury 而避免 contrast-enhanced CT in clinically suspected renal infarction → 延遲診斷的風險遠大於 contrast risk；腎功能極差者可改 MRI DWI 或 CEUS