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Lecture VICT introduction

2

Review

!   Film, CR, DR

!   Evaluation parameters

!   DR applications

!   DSA

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Contents

1.  Introduction

2.  CT principle & History

3.  CT construction and features

4.  CT number & viewing

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CT Introduction !   Conventional radiography

!   Projection of 3D to 2D obscures subtle

diff.

!  Large-area X-ray beam produce

considerable scattered radiation

!   Computed tomography Subtle differences of less than about 5% in subject contrast are not visible.

!  Cross-sectional view q  Segmentation q  Registration q  Recognition

Diff. of a few tenths of a percent in subject contrast are revealed.

q  3D reconstruction q  Automatic label q  CAD

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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.

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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。

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Principle of CT imaging xe µ−= 0II

( )22110II xxe µµ +−=

Lambert-Beer theorem

!   The patient is assumed to be a homogeneous medium.

One single transmission measurement could not determine the separated attenuation coefficients.

∑=

Δ⋅=⎟⎟⎠

⎞⎜⎜⎝

⎛−=

n

ii x

IIp

10

ln µ ( )dxxIIp ∫=⎟⎟⎠⎞

⎜⎜⎝

⎛−=

L0

ln µ0x→Δ

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。

∑= =

−n

iii xµ

e 10II

!   Actually, human body is not…

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History of CT (4 generations)

Hounsfield, Sir Godfrey Newbold (1919–2004). 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 1976–77, and senior staff scientist after 1977. His research on radar and computer design led to the invention of computerized axial tomography, (CAT scan), also known as computed tomography

1979

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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. 5o~20o 小扇形束；3~30

crystal detectors

20-90 sec 生理运动伪影

Third-gen. 30o~45o 广角扇形束；250~700

detectors

2-9 sec whole body

弧形探测器；主流

Fourth-gen.

600~2000 detectors

1-5 sec whole body

性价比低，趋于淘汰

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Ultra-fast or Electron-beam CT (the 5th 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。

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

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Multi-slice Spiral CT scanning

•  Faster image acquisition •  Fewer motion artifacts •  Improved z-axis resolution •  Improved coronal, sagittal and 3D imaging •  Less partial volume artifact

Advantage of spiral CT with Conventional CT 64 slices spiral CT 1s organ; 5s heart;

10s whole body

Spiral CT is based on the “slip ring” technique.

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Multi-slice Spiral CT scanning

Pitch(螺距)= Patient couch movement

per rotation

Slice thickness*slices

Low pitch: higher resolution; Higher dose; longer imaging time

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If pitch = 1: 旋转一圈床的位移等于探测器宽度，即扫描全覆盖;

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）

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Conventional CT construction

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《医学成像技术及系统》P108-113

ü X-ray tube

ü High voltage generator

ü Detector

ü Collimator

ü Gantry (机架)

ü Slip ring (滑环)

ü Table

ü ……

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Features of main components !   X-ray source

80~140kV !   High voltage generator

!   X-ray detector Gas-filled or solid-state detectors DQE: 98%~99.5%

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; 100-600 mA; 1 or 0.6 mm2 apparent focal spot;

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Features of main components 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)

!   Collimator

Musle Bone

Partial-volume Artifact 部分容积伪影

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!   Gantry & slip ring Keep precise angle and position <<1mm Mechanical -> electromagnetic Slip ring: connection between rotated gantry and other stable components.

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CT numbers (Hounsfield number)

( )water

water-1000number CTµµµ=

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. For 60 keV

Tissue CT number Bone 1000 Blood 10

Gray matter 30 White matter 35

CSF 5 Water 0

Fat -100 Air -1000

CT numbers (HU)

Higher electron density Higher Atten. Coeff. Higher CT number

Brighter -1000 HU~1000 Hu

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CT viewing system

-1000 HU

1000 HU

0

255

Ww Lw ROI

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pw(x, y) =

0, p(x, y) ≤ Lw −Ww

2

p(x, y)− (Lw −Ww

2)

Ww

Imax , Lw −Ww

2≤ p(x, y) ≤ Lw +

Ww

2

Imax , p(x, y) > Lw +Ww

2

⎧

⎨

⎪⎪⎪⎪

⎩

⎪⎪⎪⎪

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•  Example 1 If window level = 100, and window width = 100, what’s the CT

number range will be displayed? •  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?

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CT viewing system

50 ~ 150

Window level = 35; Window width = 60.

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DICOM —— medical image format •  Digital Imaging and

Communications in Medicine

(数字影像和通信标准) •  Header

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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

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Question???

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