《CT的质量控制》PPT课件.ppt

Quality Control for Computed Tomography (CT), CT, 的质量控制 Melissa C. Martin, M.S., DABR Certified Medical Physicist Fellow- ACR, ACMP, AAPM NIM - China Meeting June 13, 2005 Qualifications and Professional Activities 专业资质和业务活动 American College of Radiology 美国放射学会 - Commission on Medical Physics 医学物理学委员会 - Chair of Government Relations Committee 政府关系委员会主席 - Mammography Accreditation Committee 乳腺X线照相术认证委员会 - Image Quality Committee for Digital Imaging 数字影像质量委员会 American Association of Physicists in Medicine (AAPM) 美国医学物理学家 协会 - Liaison (Advisor) to the Conference of Radiation Control Program Directors 辐射控制计划理事会大会顾问 CT Dose and Image Quality CT辐射剂量和影像质量 Quality Control and Quality Assurance Procedures and Issues 质量控制和质量保证步骤 CT and MRI imaging modalities were CT and MRI imaging modalities were named as the most important medical named as the most important medical advances in the last half of the 20advances in the last half of the 20th th centurycentury CTCT和和MRIMRI被认为是人类二十世纪下半叶最重被认为是人类二十世纪下半叶最重 要的进展要的进展 Developments in CT Technology CT技术的发展 Early CT detectors and scanner 早期的CT检测和扫 描装置 Modern Multi-Slice CT Scanner 现代多层CT扫描仪 Early CT Image (Siemens) 早期的CT影像 Multi-Slice CT Images (2004) 多层CT影像 Quality Assurance 质量保证 There are a variety of artifacts than can occur because of poor adjustment of the scanner 由于扫描仪调节不到位，产生很多干扰伪影 Loss of resolution 分辨率下降 Accuracy of CT numbers CT值的精度 Table Location 扫描床的定位 CT is a high dose procedure CT诊断是一个高剂量辐射的过程 - It is important to know the dose for various studies and to select appropriate dose for optimal clinical performance 重要的是了解各种研究所需的剂量，从而选择适当的量以获得最优的临床表现 Clinical Technique 临床技术 Once the basic dose performance of the scanner is determined, the physicist should work with physician colleagues to optimize image-quality and dose 一旦确定了扫描仪的工作剂量，物理学家就要与临床医师合 作优化影像质量 Each site should have techniques tailored to the local needs 每一个地区应当使用满足当地需要的技术 Manufacturer supplied techniques may not be optimum 制造商提供的技术并不一定是最适宜的 Clinical Technique 临床技术 Technique should be adjusted so that small patients have reduced dose and large patients have increased dose 应作适当的调整，根据病人的实际情况增减剂量 The reconstruction filter should be selected to trade noise vs resolution 应对重建滤波器进行选择以提高分辨率、减少噪声 kV, pitch and slice thickness should be selected to optimize dose-image quality 应对kV值、螺距和层厚进行选择以优化剂量-影像关系 Image Uniformity 影像均一性 CT uses many detectors CT使用很多检测器 The detectors may not be completely uniform 检测器并不一定完全一致 They may drift 它们可能漂移 Quality control is necessary to insure uniformity 对于均一性而言，质控是必需的 Hounsfield Number Hounsfield值 Average attenuation value for Voxel 像素的平均衰 减值 Tissue Density X-ray Energy 组织密度 X线能量 utissue -uw uw CT number = 1000 Uniformity Checked 均一性检测 Scanning Air 扫描空气 “Air Cals” 空气校准 Scanning Water 扫描水 Accuracy of Hounsfield Numbers Hounsfield值精 度 Uniformity 均一性 Noise 噪声 Absence of Artifacts 干扰伪影的消失 Phantoms 模体 Scanning Water Requires 需要扫描水 Water filled phantom 充满水的模体 Carefully made “solid water” 特制的“固体水” Solid water has the same density and atomic number as water 固体水具有与水相同的密度和原 子数 Phantoms 模体 Phantoms supplied by scanner manufacturer 模体由扫描仪制造商提供 Often designed to work with scanner specific software 通常是被设计用来与扫描仪软件配套的 Software may check and adjust the scanner 软件可以用来检查和调整扫描仪 Some manufacturers use tighter specifications than others 一些制造商使用比较苛刻的技术指标 Standards 标准 Hounsfield values Hounsfield值 Depend on scanner 取决于扫描仪 Should be no more than +/-7 应在+/-7 之内 Uniformity depends on scanner 均一性取决于扫描仪 Noise depends on technique used for scan 噪声取决于 所使用的扫描技术 Data should be charted so trends can be seen 应将有 关数据作图以观察趋势变化 CT Number Accuracy CT值精度 Check accuracy and constancy of known materials 检查已知物质的精度和稳定性 CT Numbers CT值 Air 空气-1000-970 Polyethylene 聚乙烯-107-87 Water 水-7-7 Acrylic 丙烯酸110130 You need to verify which material is in the phantom. If in doubt, check with the manufacturer. 需确认模体中是何种物质，如有疑问，与制造商沟通。 Resolution 分辨率 Resolution Test Object 受试目标分辨率 Commercial Phantoms 商业模体 Variety of phantoms available 可选的模体很多 Accreditation program in the U.S. requires use of phantom designed by American College of Radiology (ACR) which measures more than QC Phantoms from manufacturers 在美国，认证过程中需使用放射协会设计的模体，而不仅仅 是制造商提供的质控模体 - Costly 昂贵 - Doesnt work with manufacturers software 不能与制造商提供的 软件一起使用 - Requires annual physicist evaluation 每年需要评估 Monitor Display & Film Printing 监视器显示和底片打印 Monitor should be adjusted to have a proper gray scale 监视器应进行调节，以便获得适当的灰度 Printed films should match monitor display 底片的打印应与监视器显示状态相匹配 Printers and monitors must function in DICOM format 打印机和监视器应在DICOM格式状态下工作 SMPTE Test Pattern used for image quality evaluation of both monitors and printers 对于打印机和监视器，应用SMPTE图形进行影像质量的监 测 SMPTE Test Pattern Accuracy of Table Location 扫描床的定位精度 Accuracy of Positioning Lights and Table Motion should be within + 2 mm 定位光与扫描床位移之间的精度应在+ 2 mm之内 Return of table to the same location is especially important for interventional studies 扫描床的归位对于干涉分析特别重要 Protocol for Table Localization 扫描床定位步骤 Set slice width to thinnest available 将层厚设至最小值 Position radio-opaque wire on table and align with positioning lights 将线定位于扫描床并与定位光对齐 Scan wire should appear in image 扫描线应出现在影像中 Move table out to extreme and reposition to same location using numerical indicators 将扫描床移出顶端，使用数字指示重新将它定位 Scan wire should appear in image 扫描线应出现在影像中 Frequency of Testing 测试频度 Test frequency is not as well defined as for some other imaging methods 测试频度与其它的影像测试方法定义不同 Physicist should determine test frequency based on 其定义基于 Manufacturers recommendations 制造商的建议 Experience with scanner 对扫描仪的工作经验 Frequency Testing Recommendations 测试频度建议 Dose Evaluation 剂量评估 Annually 每年 Following major service 大修后 Clinical Technique Evaluation 临床技术评估 Annually 每年 When a new technique is developed 开发新技术时 Water Evaluation 水测试 Weekly 每周 Frequency Testing Recommendations 测试频度建议 Monitor & Film Printer Evaluation 监视器和打印 机评估 Monthly 每月 Table Motion and Alignment Lights 床位移和光 定位 Quarterly 每季 Weekly if used for therapy planning 对于治疗方案 ，可每周进行 Full Physics Evaluation 全部评估 Annually 每年 Following major service 大修后 Essential QC Activities 基本质控 Determine doses from scanner 确定扫描剂量 Working with physician colleagues determine appropriate imaging protocols 与临床人员协调 确定适当的工作方案 Insure that water has a appropriate CT number 确定水测试时获得适当的CT值 Accurate 精度 No artifacts 无伪影 Uniform 均一 Appropriate noise 噪声适当 Essential QC Activities 基本质控 Insure that the monitors and film printers work correctly 确认监视器和底片打印机工作正常 Insure that positioning lights and table motion are correct 确认定位光和扫描床定位正确 Determine doses from scanner 确定扫描剂量 CT Dose Fundamentals 基本原则 CT Dose Terminology 术语 CT Dose Variations Equipment 剂量变化设备 CT Dose Variations Exams 剂量变化测试 CT Dose Optimization 剂量优化 CT Dose Evaluation CT计量评估 X Z Y CT Coordinates CT 坐标 Ideal World . 理想状态 Radiation delivered only to the imaged volume 辐射仅涵 盖成像部分 - 5 mm 0 + 5 mm Peak dose = “dose” Real World . 现实情况 Radiation distribution crosses the imaged volume 辐射跨越 成像区域 - 5 mm 0 + 5 mm Peak dose “Tails” of dose distribution 剂量分布尾迹 Computed Tomography Dose Index ( CTDI ) CT剂量指数（CTDI） Integral under ONE radiation dose profile 在单一剂量曲线 下积分 Dose profile has very long “tails” 剂量曲线有很长的尾迹 FDA uses finite integration limits ( 7 T) FDA使用定积分 极限（ 7 T ） Typically measured with TLD (laborious) 典型测量使用TLD Underestimates tail dose for slices 生物伤害 Whats the dose from an abdominal CT scan? 腹部CT扫描的剂量？ Radiation Absorbed Dose (rad) Energy (100 ergs) Mass (1 gram) = Whats the dose from an abdominal CT scan? 腹部CT扫描的剂量？ Intuitively, we understand that the biologic risk from 20 CT slices is greater than from 10 CT slices. 直觉上，我们认为20层CT扫描的危害大于10层扫描 If the average dose is 2 rad for 10 slices, what is the dose for 20 slices? 如果10层CT扫描的剂量是2 rad，20层的剂量是多 少哪？ MSADw = 2 rad ten 1-cm slices MSADw STILL = 2 rad twenty 1-cm slices So how do we represent the greater biologic risk? 因此，我们如何描述较严重的生物风险哪？ Integral dose 累计计量 Average dose # slices = integral dose 平均计量乘以层数等于累计计量 But, dose and integral dose have same units (leads to confusion) and is not specific enough (doesnt reflect the difference between 1 or 10 mm slices) 但是，剂量和累计剂量具有相同的单位，而且并不 明确 Better concept . 更好的概念 Dose Length Product (DLP) 剂量长度积 Represents integrated dose in terms of total scan length (# slices slice width) 代表了总扫描路径中的累计剂量（层数乘以层厚） Related to energy imparted but more intuitive than using energy units (Joules) 涉及应用的能量，但与使用能量单位相比更直观 Required in Europe, migrating to US 欧洲要求使用，后被美国采用 DLP = CTDIvol (mGy) scan length (cm) So, DLP represent the greater biologic risk! 所以，DLP代表了更高的生物风险！ DLP = 20 radcm DLP = 40 radcm CTDIvol = 2 rad ten 1-cm slices CTDIvol STILL = 2 rad twenty 1-cm slices SI units (1 rad = 10 mGy) 国际单位(1 rad = 10 mGy) DLP: 20 mGy 20 cm = 400 mGycm DLP gives an indication of the overall “risk” DLP表明了总的风险 DLP can from scan length or CTDI (primarily mAs, kVp, detector mode) CTDIvol (scan overlap or gap, e.g. pitch) Need to pay attention to all three! 需特别注意这三个值（ scan length ， CTDI 和CTDIvol ）！ Whats the dose (risk)? 剂量（风险）是多少？ Radiation detriment better expressed by Effective Dose (E 辐射损害的较好表达为有效剂量 a single dose parameter which reflects the risk of a non- uniform exposure in terms of a whole-body exposure 就整个躯体的而言，单个剂量参数反映了非均匀暴露的风险 Published values of effective dose per DLP provides a quick estimate of effective dose 已出版的每DLP的有效剂量值提供了一个快速估算途径 ICRP 60 Weighting Values 国际辐射防护委员会的60加权值 Gonads0.20 RBM, colon, lung, stomach 0.12 Bladder, breast, liver0.05 Esophagus, thyroid 0.05 Skin, bone surface0.01 Remainder0.05 1.00 Typical effective dose values 典型的有效剂量 Head CT 头CT1 - 2 mSv (100 - 200 mrem) Chest CT 胸CT 5 - 7 mSv (500 - 700 mrem) Abdomen CT 腹部CT5 - 7 mSv (500 - 700 mrem) Pelvis CT 骨盆CT 3 - 4 mSv (300 - 400 mrem) Abdomen & pelvis CT8 - 11 mSv (800 - 1100 mrem) 腹部和骨盆CT Average U.S. background radiation 平均本底辐射 3.6 mSv (360 mrem) Effective dose values (cont.) 典型的有效剂量 Stereo chest x-ray0.1 mSv (10 mrem) 立体胸部X线 Low dose chest CT0.7 mSv (70 mrem) 低剂量胸部CT Mammogram1.8 mSv (180 mrem) 乳腺 CTC (5 mm)5.5 mSv (550 mrem) CTC (5 mm) Double contrast BE7.5 mSv (750 mrem) 增强对比BE CTC (2.5 mm) 9.8 mSv (980 mrem) CTC (2.5 mm) CT liver perfusion 15 mSv (1500 mrem) CT肝灌注 Why is Multi-Slice CT Radiation Dose of Interest? 多层CT辐射剂量的意义何在？ To evaluate different MS CT systems 评估不同的CT系统 To triage applications between MS and SS CT 在多层CT与单层CT之间进行应用比较 To design and optimize MS CT Protocols 设计和优化多层CT应用方案 Tradeoff: Thinner slices vs. radiation dose 权衡：薄层vs.剂量 Increased use of CT warrants increased attention to radiation dose 良好的CT应用需要提升对辐射剂量的关注 Why is MultiSlice dose dependent on total slice width (detector configuration)? 为何多层CT的剂量取决于检测器配置结构？ The radiation dose profiles are larger than active detector width 辐射剂量分布大于检测器宽度 Required in MultiSlice geometry to keep the penumbra off of the active detectors 就几何形状而言，半影区要避开检测器 Size of penumbral region relatively constant 半影区的尺寸要相对恒定 Different percentage “inefficiency” for different total scan width 对不同的扫描厚度具有不同的无效百分比 Good imaging, good physics . 好的成像，好的物理特性 Use CTDI, CTDIvol and DLP to help optimize procedures (ALARA) 运用CTDI， CTDIvol和DLP帮助优化操作程序 low dose (increased image noise) does not mean poor image quality 低剂量并不意味着损失影像质量 noise level should be appropriate for the specific diagnostic task - not just to get “pretty” pictures 对于特殊的诊断任务，噪声应控制在适当的水平不只是为得 到好的画面 narrow slice widths (narrow beam width) should be appropriate for the specific diagnostic tas