Review! Film, CR, DR! Evaluation parameters! DR applications! DSA 2

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1 Lecture VI CT introduction 1

2 Review! Film, CR, DR! Evaluation parameters! DR applications! DSA 2

3 Contents 1. Introduction 2. CT principle & History 3. CT construction and features 4. CT number & viewing 3

CT Introduction! Conventional radiography Subtle differences of less than about 5% in subject contrast are not visible.! Computed tomography Diff.

4 CT Introduction! Conventional radiography Subtle differences of less than about 5% in subject contrast are not visible.! Computed tomography Diff. of a few tenths of a percent in subject contrast are revealed.! Projection of 3D to 2D obscures subtle diff.! Large-area X-ray beam produce considerable scattered radiation 4! Cross-sectional view q Segmentation q Registration q Recognition q 3D reconstruction q Automatic label q CAD

再把该矩阵转变为图像像素的灰度分布, 从而实现人体某断层图像的成像 Tomography ( 断层 ),

5 CT Introduction X-CT 是运用一定的 X 线扫描方式和技术, 在得到某一确定断层的多个投影数据后, 采用一定的图像重建方法, 经计算机处理, 求解出衰减系数值在该人体断层上的二维分布矩阵, 再把该矩阵转变为图像像素的灰度分布, 从而实现人体某断层图像的成像 Tomography ( 断层 ), 指受检体上选定的一个欲成像的平面薄层传统厚度 3-10mm, 目前可达到 1mm 以内 Voxel/Pixel( 体素 / 像素 )and slice thickness. 5

6 CT imaging concept 1. X 射线光源代替可见光,Detector 代替眼睛,Computer 代替大脑, 就构成了 CT 系统 2. 从多个角度观测一个半透明的物体, 可以估计出物体的内部结构 3. 多视角检查和物体的半透明度, 两者同样重要 6

7 I = I e µx 0 Principle of CT imaging! The patient is assumed to be a homogeneous medium.! Actually, human body is not ( µ x +µ x ) I I = I 0 e i= = e 1 I 0 n 1 1 µ i x i Lambert-Beer theorem 2 2 One single transmission measurement could not determine the separated attenuation coefficients. p n I = ln = µ i Δx Δx 0 I p = ln = ( x)dx I 0 i= 1 I µ L 0 在 CT 中,p 为投影测量值上式说明,p 表示 X 射线路径上衰减系数的线积分! CT 重建问题表述如下 : 给定一个物体衰减系数的线积分, 如何确定或计算它的衰减分布 7

History of CT (4 generations) 1979 Hounsfield, Sir Godfrey Newbold

, from 1951 and was the head of medical systems sections between 1972

8 History of CT (4 generations) 1979 Hounsfield, Sir Godfrey Newbold ( ). British scientist. He served at EMI, Ltd., from 1951 and was the head of medical systems sections between 1972 and 1976, chief staff scientist in , and senior staff scientist after His research on radar and computer design led to the invention of computerized axial tomography, (CAT scan), also known as computed tomography 8

History of CT (4 generations) Type Spec.

5 o ~20 o 小扇形束 ;3~30 crystal detectors 30 o ~45 o

9 History of CT (4 generations) Type Spec. Performance Status First-gen. 笔形束扫描 ;1 detector 3-5 min/ Slice 淘汰 2nd-gen. Third-gen. 5 o ~20 o 小扇形束 ;3~30 crystal detectors 30 o ~45 o 广角扇形束 ;250~700 detectors sec 生理运动伪影 2-9 sec whole body 弧形探测器 ; 主流 Fourthgen. 600~2000 detectors 1-5 sec whole body 性价比低, 趋于淘汰 9

10 Ultra-fast or Electron-beam CT (the 5 th gen.) The UFCT is often referred to as electron-beam computed tomography [EBCT]. It was initially referred to as a cardiovascular CT scanner. The UFCT unit was developed in 1983 by D. Boyd and his colleagues at the University of California-San Francisco. 电子束扫描的最大特点是取消了 X 线管与探测器之间的同步扫描机械运动, 利用电子技术控制非机械运动式同步扫描 10

11 Multi-slice Spiral CT scanning ( 多层螺旋 CT) 11

12 Multi-slice Spiral CT scanning Advantage of spiral CT with Conventional CT Faster image acquisition Fewer motion artifacts Improved z-axis resolution Improved coronal, sagittal and 3D imaging Less partial volume artifact 64 slices spiral CT 1s organ; 5s heart; 10s whole body Spiral CT is based on the slip ring technique. 12

13 Multi-slice Spiral CT scanning Pitch( 螺距 )= Patient couch movement per rotation Slice thickness*slices If pitch = 1: 旋转一圈床的位移等于探测器宽度, 即扫描全覆盖 ; Low pitch: higher resolution; Higher dose; longer imaging time If pitch > 1: 扫描覆盖不全, 有遗漏数据 ; If pitch < 1: 重叠扫描 ( 精细组织 ) 13

14 Conventional CT construction ü X-ray tube ü High voltage generator ü Detector ü Collimator ü Gantry ( 机架 ) ü Slip ring ( 滑环 ) ü Table ü 医学成像技术及系统 P

15 Features of main components! X-ray source 1st, 2nd-gen: stationary-anode; focal spot 2*16 mm; air-oil cooled 3rd, 4th-gen: 10,000 rpm rotating-anode; ratings 6 million heat units; ma; 1 or 0.6 mm 2 apparent focal spot;! High voltage generator 80~140kV! X-ray detector Gas-filled or solid-state detectors DQE: 98%~99.5% 15

16 Features of main components! Collimator X-ray tube side; detector side Function: 1. Confine, define the thickness of CT slice 2. Reduce scattered radiation Voxel: Pixel*height of collimator (slice thickness) Musle Bone Partial-volume Artifact 部分容积伪影! Gantry & slip ring Keep precise angle and position <<1mm Mechanical -> electromagnetic Slip ring: connection between rotated gantry and other stable components. 16

17 17

CT numbers (Hounsfield number) Tissue u(cm -1 ) Bone 0.528 Blood 0.208 Gray matter 0.212 White matter 0.213 CSF( 脑脊液 ) 0.207 Water 0.206 Fat 0.185 Air 0.0004 Linera Atten. Coeff.

18 CT numbers (Hounsfield number) Tissue u(cm -1 ) Bone Blood Gray matter White matter CSF( 脑脊液 ) Water Fat Air Linera Atten. Coeff. For 60 kev CT number = 1000 ( µ - µ ) µ water water Higher electron density Higher Atten. Coeff. Higher CT number Brighter HU~1000 Hu 18 Tissue CT number Bone 1000 Blood 10 Gray matter 30 White matter 35 CSF 5 Water 0 Fat -100 Air CT numbers (HU)

19 CT viewing system 1000 HU 255 ROI W w L w 0, p(x, y) L w W w 2 p(x, y) (L w W w p w (x, y) = 2 ) I W max, L w W w w 2 p(x, y) L + W w w 2 I max, p(x, y) > L w + W w HU

20 CT viewing system Example 1 If window level = 100, and window width = 100, what s the CT number range will be displayed? 50 ~ 150 Example 2 The CT number of brain parenchyma is around 35Hu, if we want to see the structure of brain tissue, what should we set the window level and minimum width? Window level = 35; Window width =

21 DICOM medical image format Digital Imaging and Communications in Medicine ( 数字影像和通信标准 ) Header 21

22 Conclusion 1. Basic concepts of CT 2. Five generation CT features 3. Multi-slice spirial CT 4. CT number and viewing window 5. CT construction and features 22

23 Question??? 23

Microsoft PowerPoint - Sens-Tech WCNDT [兼容模式]

Microsoft PowerPoint - Sens-Tech WCNDT [兼容模式] X-ray data acquisition systems for NDT applications 技股份有限公司先锋科技股份有限公司科技股份有限公司先锋科技股份有限公司 www Sens-Tech Ltd UK based company 40 Staff Specialise in detection and data acquisition systems for light and