【正文】 CT值的精度 ? Table Location 掃描床的定位 ? CT is a high dose procedure CT診斷是一個高劑量輻射的過程 It is important to know the dose for various studies and to select appropriate dose for optimal clinical performance 重要的是了解各種研究所需的劑量，從而選擇適當(dāng)?shù)牧恳垣@得最優(yōu)的臨床表現(xiàn) Clinical Technique 臨床技術(shù) ? Once the basic dose performance of the scanner is determined, the physicist should work with physician colleagues to optimize imagequality and dose 一旦確定了掃描儀的工作劑量，物理學(xué)家就要與臨床醫(yī)師合作優(yōu)化影像質(zhì)量 ? Each site should have techniques tailored to the local needs 每一個地區(qū)應(yīng)當(dāng)使用滿足當(dāng)?shù)匦枰募夹g(shù) ? Manufacturer supplied techniques may not be optimum 制造商提供的技術(shù)并不一定是最適宜的 Clinical Technique 臨床技術(shù) ? Technique should be adjusted so that small patients have reduced dose and large patients have increased dose 應(yīng)作適當(dāng)?shù)恼{(diào)整，根據(jù)病人的實際情況增減劑量 ? The reconstruction filter should be selected to trade noise vs resolution 應(yīng)對重建濾波器進行選擇以提高分辨率、減少噪聲 ? kV, pitch and slice thickness should be selected to optimize doseimage quality 應(yīng)對 kV值、螺距和層厚進行選擇以優(yōu)化劑量 影像關(guān)系 Image Uniformity 影像均一性 ? CT uses many detectors CT使用很多檢測器 ? The detectors may not be pletely uniform 檢測器并不一定完全一致 ? They may drift 它們可能漂移 ? Quality control is necessary to insure uniformity 對于均一性而言，質(zhì)控是必需的 Hounsfield Number Hounsfield值 Average attenuation value for Voxel 像素的平均衰減值 Tissue Density Xray Energy 組織密度 X線能量 utissue uw uw CT number = 1000 Uniformity Checked 均一性檢測 ? Scanning Air 掃描空氣 ? ―Air Cals‖ 空氣校準(zhǔn) ? Scanning Water 掃描水 ? Accuracy of Hounsfield Numbers Hounsfield值精度 ? Uniformity 均一性 ? Noise 噪聲 ? Absence of Artifacts 干擾偽影的消失 Phantoms 模體 ? Scanning Water Requires 需要掃描水 ? Water filled phantom 充滿水的模體 ? Carefully made ―solid water‖ 特制的“固體水” ? Solid water has the same density and atomic number as water 固體水具有與水相同的密度和原子數(shù) Phantoms 模體 ? Phantoms supplied by scanner manufacturer 模體由掃描儀制造商提供 ? Often designed to work with scanner specific software 通常是被設(shè)計用來與掃描儀軟件配套的 ? Software may check and adjust the scanner 軟件可以用來檢查和調(diào)整掃描儀 ? Some manufacturers use tighter specifications than others 一些制造商使用比較苛刻的技術(shù)指標(biāo) Standards 標(biāo)準(zhǔn) ? Hounsfield values Hounsfield值 ? Depend on scanner 取決于掃描儀 ? Should be no more than +/7 應(yīng)在 +/7 之內(nèi) ? Uniformity depends on scanner 均一性取決于掃描儀 ? Noise depends on technique used for scan 噪聲取決于所使用的掃描技術(shù) ? Data should be charted so trends can be seen 應(yīng)將有關(guān)數(shù)據(jù)作圖以觀察趨勢變化 8642024681 2 / 2 5 / 2 0 0 1 2 / 1 3 / 2 0 0 2 4 / 4 / 2 0 0 2 5 / 2 4 / 2 0 0 2 7 / 1 3 / 2 0 0 2 9 / 1 / 2 0 0 2 1 0 / 2 1 / 2 0 0 2CT Number Accuracy CT值精度 ? Check accuracy and constancy of known materials 檢查已知物質(zhì)的精度和穩(wěn)定性 CT Numbers CT值 ? Air 空氣 1000 970 ? Polyethylene 聚乙烯 107 87 ? Water 水 7 7 ? Acrylic 丙烯酸 110 130 You need to verify which material is in the phantom. If in doubt, check with the manufacturer. 需確認(rèn)模體中是何種物質(zhì)，如有疑問，與制造商溝通。 multiple scans 直接進行 TLD和多掃描 When the scans are separated by the total nominal scan thickness (I = nT) and ALL dose under the tails is measured 當(dāng)掃描被總標(biāo)稱掃描厚度分割以及所有尾跡下的劑量均被測量時 MSAD = CTDI When I ? nT MSAD = nT ? CTDI I Volume CTDIw (CTDIvol) 體積（空間） CTDIw(CTDIvol) ? New term recently agreed upon by the IEC IEC最新使用的術(shù)語 ? CTDI100 integrates over 100 mm CTDI100積分 100mm ? CTDIw averages over x (L/R) and y (A/P) CTDIw計算 x和 y軸的平均 ? CTDIvol averages over x, y and z (C/C) CTDIvol計算 x， y和 z軸的平均 ? CTDIvol estimates the MSADw and is operationally easier to measure and pute CTDIvol估算 MSADw而且使得測試和運算更簡便 When I = N