医学影像设备学课件：CT Princeples

1、CT（Computed Tomography)History、Structure、Princeple,Modern CT systems employ slip ring technology to allow for continuous data acquisition. a) Photograph of a measuring system during assembly with slip rings (left) and components (right) visible. b) Schematic representation of slip rings, which allow

2、 the transfer of electrical energy to the x-ray components and data from the detector system to the image reconstruction unit.,Multi-Source CT. a,b) Schematic drawings of a proposed three-source scanner and of a dual-source scanner that was actually built. c) Photo of the Siemens SOMATOM Definition

3、Flash dual source CT system installed in the West China Hospital,From a historical perspective, four scanner generations are known in CT. Head scanners, which scanned the patient by translation and rotation of the measurement system with a pencil beam (a) or a small fan beam (b), and fan beam system

4、s, in which all body sections can be scanned with a continuous 360 rotation. The 3rd generation, featuring a rotating detector (c), has clearly outdistanced the 4th generation, which utilizes stationary detector rings (d).,EBT : Electron Beam Tomography,X-Ray Discovery,X-ray was discovered by a Germ

5、an scientist Roentgen 100 years ago. This made people for the first time be able to view the anatomy structure of human body without operation,But its superimposed And we couldnt view soft tissue,Godfrey N. Hounsfield, the English engineer who developed the first CT scanner and received the Nobel Pr

6、ize in medicine in 1979 together with the physicist A.M. Cormack,Computed tomography - perception in slices. CT provides transverse section images of the human body in digital form. A coordinate system then results which basically conforms to the anatomical main axes and planes.,Analog or digital im

7、ages? Continuous imaging of anatomy or scanning of single slices? In early CT images, here a brain scan produced in 1974 with an 80 x 80 image matrix (a), the computed discrete picture elements are clearly visible, just as the steps in the secondary reconstructions generated from single scans taken

8、with a spacing of 13 mm (b). For volume scans obtained in spiral scanning mode and reconstruction of finer matrices (c) picture elements cannot be discerned from analog images anymore; they can also be viewed in the third dimension quasi-continuously (d).,Why does CT offer higher contrast? The high

9、contrast offered by CT images - here only illustrated by way of numbers - is due to the imaging principle and not due to radiation dose or scan parameters.,Overview of CT Scanner,Patient is placed in the center of the measurement field X-ray is passed through the patients slice from many direction a

10、long a 360 path The transmitted beams are captured by the detectors which digitizes these signals These digitized signals called raw data are sent to a computer which create the CT image,How does CT Work?,X-ray goes through collimator therefore penetrate only an axial layer of the object, called sli

11、ce,How does CT Work?,The object slice is divided into small volume elements called voxels. Each voxel is assigned a value which is dependent on the average amount of attenuation,How is CT Image generated?,The attenuation values are transferred to the computer where they are coded & used to create a

12、slice image,How is CT Image generated?,Concept of X-ray Attenuation,An X-ray beam passing through the body is attenuated (loses its energy) by : Absorption Scattering,Incident X-ray,Transmitted ray,SCATTERED X-RAYS,BODY TISSUE,Absorption by the tissue is proportional to the density,Less dense tissue

13、,More dense tissue,MORE ATTENUATION,LESS ATTENUATION,X-Ray Intensity Measurement,Io,I,I=Io e - d,Calculation in IRS,I = I0 e - d, = 1 / d ln I0 / I,Each element (= voxel) is assumed to be homogenous in our calculation., = 1 / d ( ln I0 ln I),What do we measure in CT? The intensity I of radiation. Th

14、e attenuation value, i.e. the projection value P, results and thus, in the simplest case, the attenuation coefficient (case 1). For inhomogeneous objects tomographic imaging is necessary to determine the distribution (x,y).,How do we measure an object in CT? In the simplest case x-ray intensities an

15、d therefore object attenuation will be measured with a pencil beam for many different angular positions.,How do we compute a CT image? Algebraic procedures are the most easily comprehensible approach to image reconstruction. The N unknown values of an N x N image matrix can be determined by solving

16、a system of linear equations. For larger matrices this has to be done iteratively.,Image reconstructions in CT by convolution and backprojection. Direct backprojection of attenuation profiles results in unsharp images. Convolution of attenuation profiles before backprojection, essentially a high pas

17、s filtering, counteracts this unsharpening.,Principles of Computed Tomography,Image characteristics can be influenced by the choice of the convolution kernel, whereby increasing spatial resolution or edge enhancement also means increasing image noise.,Attenuation Law in Body,How do we measure an obj

18、ect in CT? In the simplest case x-ray intensities and therefore object attenuation will be measured with a pencil beam for many different angular positions.,CT Number (i),All attenuation values are compared with attenuation of water, - water CT Number = X 1000 water Water has CT number 0 Tissues mor

19、e dense than water have a positive CT number Tissues less dense than water have a negative CT number. Water = 0 HU Bonemax = + 3071HU Air = - 1024HU,Water Scaling,CT Number (ii),The Hounsfield scale. CT values characterize the linear attenuation coefficient of the tissue in each volume element relat

20、ive to the -value of water. The CT values of different tissues are therefore defined to be relatively stable and to a high degree independent of the x-ray spectrum.,Displayed matrix,imaging,image matrix: 1024 2 pixel (interpolated),Windowing,Windowing: conversion of CT values to grey scale for image

21、 displaying on monitor, By varying Window width and center we can observe different tissues. By varying Window width and center, a small range of CT numbers can be spreaded over a large of grey scale values, so This it is easy to detect very small changes in CT number.,Windowing,Window Width,Window Center,Windowing procedures to display CT images. The diagnostically relevant range of CT values is selected by choosing the center and width (C/W) of the window. The complete gray value scale is assigned to this se