Retained Soft-Tissue Foreign Body with Complication

00Overview

Retained soft-tissue foreign body (FB) 是急診與門診影像中常見且容易被忽略的問題。影像任務核心是：(1) 偵測 foreign body 的存在與位置；(2) 評估併發症（infection、granuloma、neurovascular injury）；(3) 提供 surgical extraction 所需的定位資訊。最容易出錯的地方：radiolucent foreign bodies（木頭、塑膠、部分小片玻璃）在 radiographs 上不可見，初次 X-ray 對 retained FB 的 miss rate 可高達 30–40%（隨 material 與 fragment size 而異）。

01Critical concepts

• 不是所有 foreign body 都是 radiopaque 的：金屬幾乎 100% 可見、玻璃 > 2 mm 多數可見（與是否含 lead 無絕對關係，但小片 < 2 mm non-leaded glass 仍可能 X-ray invisible）、木頭 / 植物性材料 / 大多數塑膠 radiolucent
• Ultrasound 是偵測 radiolucent FB 的首選工具：sensitivity > 90% for superficial soft tissue FB，可同時評估深度、與 neurovascular structure 的關係
• Foreign body 的慢性期反應：granuloma formation → 包裹性 mass → 可模擬 soft tissue tumor
• 及早移除 organic FB（木頭、植物刺）：organic materials 的感染率顯著高於 inorganic（玻璃、金屬），因為 organic material 可作為 bacterial culture medium
• US artifact 是材質判讀關鍵：clean posterior shadow（wood）、reverberation / comet-tail（metal、glass）為 material discriminator，必須在報告中描述

01正常 anatomy / 常用 modality

Soft tissue foreign bodies 最常見於 hands / feet（尤其 plantar surface），其次是 face、extremities。重要鄰近結構包括 tendons、neurovascular bundles、joint capsules。

常用 modality：

• Radiographs（X-ray）：first-line — metal + most glass（> 2 mm）readily visible；wood / plastic / organic usually invisible；建議 two orthogonal views ± soft-tissue technique（低 kVp）以提高小 FB detection
• Ultrasound with high-frequency linear probe：首選 for radiolucent FB — 一般 linear probe 12–15 MHz，淺表 / 手指 / 異物建議使用 hockey-stick probe 15–22 MHz；典型表現為 hyperechoic linear structure + posterior shadow / reverberation artifact + surrounding hypoechoic halo（inflammation / granulation）
  • Clean posterior acoustic shadow：典型見於 wood、thorn 等 organic material（強吸收、低反射）
  • Reverberation artifact（"ladder-like"）：典型見於 metal、smooth glass surface（平整反射界面 → 多次回波）
  • Comet-tail artifact：短而漸縮的 reverberation，常見於 small metallic fragment（如 needle tip）
• CT：metal / glass / bone FB well seen；wood density 變異大 — fresh / wet wood 約 0 至 +50 HU（接近 water / soft tissue），dry wood 可呈 −600 至 −800 HU（接近 air）→ pitfall：dry wood 可被誤判為 soft tissue gas / abscess，必須結合 linear morphology 與 penetrating wound history 判讀；best for deep or complex anatomic locations
• MRI：FB 本身偵測有限，但對 complication（abscess, tenosynovitis, osteomyelitis）評估佳
  • FB signal：wood / glass / plastic 多呈 T1 與 T2 hypointense linear structure，周圍 T2 hyperintense edema halo / rim enhancement
  • Metallic FB：susceptibility artifact / blooming on GRE / SWI，T1 / T2 signal void
  • MR safety：metallic FB 並非絕對 contraindicated，需依 ferromagnetic 性質、位置與固定程度判斷（MR conditional）。Orbital metallic FB 為高風險區（移位可致 globe injury / blindness），疑似時必須先做 orbital radiograph / CT screening；其他部位若 FB 已 fibrotic encapsulated 且非 ferromagnetic 通常可掃

02常見 pattern 分類

Hyperechoic linear structure with posterior acoustic shadow

• Definition：US 見 linear hyperechoic focus with clean posterior shadow，surrounded by hypoechoic halo
• Why it matters：最典型的 soft tissue FB 超音波表現，sensitivity > 90% for superficial FB
• What it points toward：wood splinter、glass shard、metallic fragment、thorn / spine
• Artifact 鑑別意義：clean shadow → organic（wood / thorn）；reverberation / comet-tail → metal / smooth glass；可協助術前預判材質
• Common trap：calcified tendon / calcific deposit 也可呈 hyperechoic with shadow → 需在兩個 orthogonal planes 確認 linear morphology + clinical history of penetrating wound

Radiopaque foreign body on radiograph

• Definition：radio-dense structure within soft tissues on X-ray，形態可辨識（needle tip, glass fragment, bullet fragment）
• Why it matters：radiograph 是 fastest + cheapest screening；metal 和 > 2 mm glass easily detected
• What it points toward：metallic FB（needles, pins, staples, wire）; glass fragments; gravel / stone
• Common trap：small glass fragments（< 2 mm，特別是 non-leaded thin glass）can be invisible even on radiograph → if clinical suspicion + negative X-ray → US is mandatory next step

Foreign body granuloma / chronic inflammatory mass

• Definition：chronic retained FB 被 granulation tissue 包裹形成 well-defined or ill-defined soft tissue mass，MRI 呈 T2 variable signal + rim enhancement；中心可見 T1/T2 hypointense linear focus 代表 residual FB
• Why it matters：可與 soft tissue neoplasm（sarcoma, nerve sheath tumor）混淆 → biopsy 前需考慮 FB history
• What it points toward：foreign body granuloma（wood especially）；chronic abscess；foreign body reaction to suture material post-surgery
• Common trap：FB granuloma 在 MRI 上的 differential diagnosis 包含 soft tissue sarcoma → 如果沒有 penetrating injury history，granuloma 不會被考慮 → 仔細詢問 remote trauma history essential

Infectious complication pattern

• Definition：retained FB site 出現 surrounding cellulitis（soft tissue edema + fat stranding）、abscess（rim-enhancing collection）、tenosynovitis（fluid + enhancement around tendon sheath）、或 osteomyelitis（cortical destruction + marrow edema）
• Why it matters：infected retained FB 不會 resolve with antibiotics alone — FB removal is mandatory
• What it points toward：bacterial infection from contaminated FB（especially organic materials）；mycetoma（chronic fungal infection from plant thorn）；actinomycosis
• Common trap：cellulitis without abscess may initially mask the retained FB → 若 cellulitis recurrent / refractory to antibiotics → search for retained FB with US

03Top common diagnoses

• Glass foreign body：second most common retained FB after metal；大多數 > 2 mm glass 在 radiograph 可見；含 lead glass radiopacity 較高，但 non-leaded glass 同樣多為 radiopaque，僅在 fragment < 2 mm 時 detection 顯著下降
• Wood / plant material foreign body：most commonly missed on radiograph；radiolucent；high infection rate；organic material → urgent removal
• Metallic foreign body：easily detected on all modalities；rust / corrosion may cause surrounding inflammatory reaction
• Sea urchin spine / fish bone / thorn：organic；may not be visible on X-ray；chronic granulomatous reaction if retained
• Gravel / road debris：embedded after road rash / MVA；usually radiopaque；can be multiple

04Cannot-miss diagnosis / emergency

05高頻 mimics 與 discriminators

Foreign body vs calcified structure (tendon / vessel)

• Why they get confused：both can appear as hyperechoic focus on US or radiopaque on X-ray
• Most useful discriminators：(1) morphology — FB typically linear / angular / irregular；calcification typically rounded / amorphous；(2) location — FB within soft tissue mass with inflammatory halo；calcification within tendon / vessel wall；(3) history — penetrating wound → FB；chronic tendinopathy → calcification；(4) two-plane confirmation on US（FB maintains linear structure in orthogonal view）；(5) artifact pattern — reverberation / comet-tail 強烈傾向 metal / glass FB，而 calcification 多為 clean shadow without reverberation
• Common trap：calcific tendinitis can be acutely painful and mimic acute FB → history of penetrating injury is the key differentiator

FB granuloma vs soft tissue tumor

• Why they get confused：chronic FB granuloma on MRI can appear as enhancing soft tissue mass indistinguishable from sarcoma
• Most useful discriminators：(1) history of penetrating injury or remote wound → granuloma；(2) US may identify residual FB within the mass（hyperechoic + shadow / reverberation）；(3) CT may show FB density if glass / metal；(4) granuloma typically has rim enhancement；sarcoma may have internal enhancement + necrosis；(5) central T1/T2 hypointense linear focus on MRI 強烈支持 retained FB；(6) if uncertain → needle biopsy before excision
• Common trap：remote trauma（years to decades earlier）may not be volunteered by patient → specific questioning essential

Retained FB infection vs cellulitis without FB

• Why they get confused：both present with erythema, swelling, pain → treated empirically with antibiotics
• Most useful discriminators：(1) recurrent cellulitis in same location → search for retained FB；(2) non-responding cellulitis after adequate antibiotics → US to look for FB / abscess；(3) gas in soft tissues without FB → necrotizing fasciitis consideration；(4) periosteal reaction adjacent to infection → FB-related osteomyelitis
• Common trap：first episode may be treated successfully with antibiotics only → FB remains → recurrence inevitable

06Next step / protocol / appropriateness

影像 protocol 選擇：

• Suspected FB after penetrating injury：X-ray（2 orthogonal views, soft-tissue technique）→ if positive → localize + plan extraction
• Negative X-ray but high clinical suspicion：high-frequency US（linear 12–15 MHz 或 hockey-stick 15–22 MHz）→ detect radiolucent FB + evaluate complications
• Deep or complex location（face, hand, foot plantar structures）：CT for precise localization before surgical extraction；判讀 dry wood 時注意 air-attenuation pitfall
• Chronic / recurrent soft tissue infection：US → search for retained FB + abscess → if FB found → surgical removal
• Soft tissue mass with suspected FB granuloma：MRI for extent mapping（含 GRE / SWI 偵測 metallic blooming）→ CT for FB detection → US-guided biopsy if diagnosis uncertain
• Pre-MRI screening for metallic FB：orbital / 高風險 occupational exposure 病史 → 先 orbital radiograph 或 CT；non-orbital encapsulated non-ferromagnetic FB 多為 MR conditional

07Pitfalls / normal variants

• Sesamoid bones in hand / foot：normal sesamoid bones can be mistaken for FB on X-ray → know normal sesamoid locations（MCP, IP joints）
• Phleboliths / vascular calcifications：round calcifications in soft tissues are usually phleboliths → differentiate from FB by shape（round vs angular/linear）
• Surgical clips / suture material：post-surgical patients may have retained sutures or clips → compare with surgical history
• Skin surface artifact：surface debris on skin can project over soft tissues on X-ray → repeat with clean field if doubtful
• Wood FB becomes less echogenic over time：fresh wood is hyperechoic with clear shadow；chronic retained wood absorbs water → becomes iso/hypoechoic → harder to detect → compare with surrounding tissue carefully
• Dry wood on CT mimics gas：dry wood 可呈 air-attenuation（−600 至 −800 HU），易被誤判為 soft tissue gas / abscess；以 linear morphology + 無 fluid collection + penetrating injury history 鑑別
• "All glass is radiopaque" 過於絕對：thin / small non-leaded glass < 2 mm 仍可能 X-ray invisible；臨床高度懷疑時必須 US 補做