亨氏單位

亨氏單位（英語：Hounsfield unit，縮寫：HU），為亨氏量表（Hounsfield scale）阻射率（英语：Radiodensity）的數量尺度單位，命名自X射線電腦斷層掃描的發明人高弗雷·豪斯費爾德（Godfrey Newbold Hounsfield）。亨氏單位常用於X射線電腦斷層掃描，因此又稱為CT值（CT number）。 亨氏單位為其所測量之介質的線性衰減係數的線性映射，其定義將蒸餾水與空氣在標準溫度與標準壓力下的阻射率（英语：Radiodensity）分別設為0 HU和-1,000 HU。因此，在線性衰減係數介於平均值間的體素中，其亨氏單位為： H U = 1000 × μ − μ water μ water − μ air {\displaystyle HU=1000\times {\frac {\mu -\mu _{\textrm {water}}}{\mu _{\textrm {water}}-\mu _{\textrm {air}}}}} 其中 μ water {\displaystyle \mu _{\textrm {water}}} 與 μ air {\displaystyle \mu _{\textrm {air}}} 分別為水與空氣的線性衰減係數。 由於空氣的線性衰減係數趨近於0，一個HU的變化相當於水線性衰減係數0.1%的變化。 由於X射線電腦斷層掃描的功能在於將生物（主要是陸上生物，如人類）的體內結構成像化，因此採納上述標準作為所有介質的參照依據。 亨氏單位僅適用於常見的雙能電腦斷層掃描（dual-energy CT scan），而不適用於錐狀射束電腦斷層掃描（cone beam CT scan）。 亨氏單位常用於辨識腫瘤，以腎上腺腺瘤（英语：Adrenal adenoma）為例，當該腫瘤的阻射率（英语：Radiodensity）小於10 HU，則其脂肪成分偏高，代表該腫瘤有較大機率為良性腫瘤。 Lev, Michael; Gonzalez, Rodolfo. CT Angiography and CT Perfusion Imaging. 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