0 說明

本文主要內容是關于X-ray physics 的個人學習筆記，簡單入門介紹。不少內容是博主自己的學習總結和看法，有不對的地方歡迎一起交流學習。

remark: 由于內容大致是小組presentation的演講稿。所以大部分篇幅基本是英文，但是都是比較簡單的英文，當然沒有閱讀里故意難住考生的長難句，有興趣的jmm還有xdm希望能有耐心閱讀。

1 History

The history of medical technology has been shaped by groundbreaking innovations. 125 years ago wihelm conradrenkin discovered x-rays and changed the world. just a short time later, the engineers developed the first x-ray tubes specifically designed for medical use. their quality was recognized by renkin himself. it took fifteen minutes for renkin to capture the world famous first shot of his wife’s hand. With our innovations we were able to capture more and more details of the body continuously with lower radiation doses and shorten examination times to enable precision medicine and human care. In 2005, we launched the world’s first computed tomography scanner with two X-ray tubes.Today based on the image processing data, we can create three -dimensional photo realistic image from inside the body.

fig1. 第一張X-ray圖片

fig2. 第一個X-ray tube

2 Production

2.1 X-ray tube 的主要構件

從上面的圖片可以看到，X-ray tube主要包括的構件，下面對每一個構件的作用進行說明。

cathode filament:
There is filament of metal with negatively charged where electrons accumulate on it. once filament is heated, electrons are boiled off. This process is called thermionic emission.

high-voltage power supply:
在陰極和陽極之間提供高壓電源。Its purpose is to accelerate the electrons.

vacuum chamber ：
What I have to mention here is that the vacuum chamber can ensure that high-speed electrons hit the anode as straightly as possible without deflection or loss of energy due to scattering.

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中間打斷一下，這里面最重要的一步是，如何從電子的動能轉化為x-ray 光子。高速的電子從陰極射向陽極，在陽極上速度會急速下降，在這個過程中電子的動能會轉化成x-ray 光子和熱能。具體是怎么發生這個反應的是怎么進行轉換的，這個過程并不簡單，后面會單獨進行介紹。這里必須注意的一點的是：陽極是在不停的旋轉的，電子沒次轟擊anode的位置都是不一樣的。

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Lead shielding：
Contain stray electrons and x-ray photons.there may be some electrons scattered by residual air molecule and x-ray photons scattered by glass molecule.

filter
help to absorb low energy photons, Only high-energy photons emitted. the reason is that low energy photons get absorbed in soft tissue lead to higher dose but no useful contribution.

2.2 Interaction of incident electron

這里說明的就是上一部分我提到的，X-ray光子怎么產生的問題。即 incident electron在anode上是怎樣的interaction.

根據高速電子在anode上與金屬原子的不同部分的Interaction的不同，會產生兩種不同的X-ray光子，即characteristic X-ray 和bremsstrahlung.

2.2.1 characteristic X-ray

Generation of a characteristic x-ray in a target atom occurs in the following sequence: (1) The incident electron interacts with the K-shell electron via a repulsive electrical force. (2) The K-shell electron is removed (only if the energy of the incident electron is greater than the K-shell binding energy), leaving a vacancy in the K-shell. (3) An electron from the adjacent L-shell (or possibly a different shell) fills the vacancy. (4) A Ka characteristic x-ray photon is emitted with energy equal to the difference between the binding energies of the two shells.

2.2.2 bremsstrahlung

Bremsstrahlung radiation arises from energetic electron interactions with an atomic nucleus of the target material. In a “close” approach, the positive nucleus attracts the negative electron, causing deceleration and redirection, resulting in a loss of kinetic energy that is converted to an x-ray. The x-ray energy depends on the interaction distance between the electron and the nucleus; it decreases as the distance increases. This figure shows x-ray photons of different energy levels. If the photon stops directly inside the atom, the energy of the generated x-ray photon reaches the maximum. If the electron is far away from the nucleus, it will produce low-energy photons.

2.2.3 energy spectrum

The figure over there is about the energy spectrum of a beam emitted from an X-ray tube. The very low energies are absorbed by the tube itself. Characteristic lines are seen as sharp lines, superimposed upon a broad energy distribution from general radiation.

Characteristic lines的兩個峰值，是由于不同原子殼層之間的能量差是不一樣的，例如，L-shell補充K-shell的空位，和M-shell補充L-shell的空位，這兩種能量差就不同，形成了兩個峰值。

在X-ray最大值時，是Bremsstrahlung radiation，incident electron 直接停在了原子內，這時候將會有X-ray photon能量會達到最大。

3 Interaction with matter

X-ray穿透人體或者小動物時，在體內會發生多種Interaction。這里主要寫到了三種Interaction。

3.1 光電效應

A photoelectric interaction between an incident X-ray and tissue involves an inner electron being emitted (left), an electron from an outer shell filling the hole in the inner shell (middle), and the difference in the binding energies being transferred to a characteristic X-ray. This X-ray has very low energy and is absorbed after travelling ~1 millimeter in tissue.

3.2 Compton scattering

• 基本介紹
  As for the interaction with matter, Compton scattering occurs when the incoming photon interacts with an outer shell electron.Compton scattering can be simply explained as the collision of photons with electrons, and the electrons gain part of their energy and leave the original orbit to become free electrons, while the energy of the scattered photons decreases. and it has a different direction. That is why we call it scatter. A x-ray photon in with an electron and a scattered photon out.
  
• fomula
  發生康普頓散射是，能量會減少，相對應的波長會變長，以下是波長變化以及能量變化的公式。
  
  
• The characteristics and effects of Compton scattering:

Occurs at all energy levels

Predominate at high energies

Increase patient dose

Increase bystander dose

Decrease image contrast

Compton scattering is the most common, but the least desirable photon interaction. Remember that Compton scattering does results in ionization. So this free electron that gets created crashes through the surrounding tissues and cells that results in biological damage in the body. The scattered photo can also be absorbed in the patient’s tissues which causes even more harm and more dose. Compton scatter also has a negative effect on the bystander dose such as the radiographer. Moreover,Compton scattering ,of course scattering, which in turn decreases the image contrast.

The combination of photoelectric effect and transmission creates what would otherwise be a high contrast, high quality image, what we mean by this is there’s clearly visible differences in the shades of gray. Compton scattering and really any kind of scattering adds meaningless noise to the image.

4 detector

remark: 這一部分和下一部分的imaging application可以不看。因為這兩部分我也沒仔細研究，也沒啥內容，但是為了文章的完整性，我還是放上來了，可能有強迫癥叭，我是一個沒有感情的搬運工。【囧😳】

• Traditional X-ray film
• Digital detector
  • Advantages
    • Higher quality image
    • Store and transfer easily
  • Types
    • Computed Radiography
    • Digital Radiography

Protective layer – protection of the phosphor layer
Phosphor or active layer (熒光層) – image acquisition
Reflective layer – improve the detection efficiency
Support Layer – provide strength for the imaging plate
Backing Layer – protect back of the cassette

5 Imaging Application

• Clinical applications
  ? Projectional radiographs
  ? Computed tomography
  ? Fluoroscopy
  ? Radiotherapy
  

6 寫在后面的話

非常感謝各位jmm和xdm能看到這，本人研究生萌新，方向醫學圖像重建，歡迎有興趣的朋友們一起交流學習。

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