Lecture 5: Basics of Radiation Physics: X-Ray and Others PDF

Lecture 5 Basics of Radiation Physics: X-Ray and Others By Dr. Ziyad Shihab Ahmed Al-Sarraj College of Pharmacy Ashur University 2023-2024 1  Basics of X-...

Lecture 5 Basics of Radiation Physics: X-Ray and Others By Dr. Ziyad Shihab Ahmed Al-Sarraj College of Pharmacy Ashur University 2023-2024 1  Basics of X-ray Physics Key points  X-rays are produced by interaction of accelerated electrons with tungsten nuclei within the tube anode.  Two types of radiation are generated: 1- characteristic radiation and 2- bremsstrahlung (braking) radiation  Changing the X-ray machine or settings alters the properties of the X-ray beam X-ray production 2 -X-rays are produced within the X-ray machine, also known as an X-ray tube. -No external radioactive material is involved. -Radiographers can change the current and voltage settings on the X-ray machine in order to manipulate the properties of the X-ray beam produced. - Different X-ray beam spectra are applied to different body parts. The X-ray tube 3  A small increase in the filament voltage (1) results in a large increase in tube current (2), which accelerates high speed electrons from the very high temperature filament negative cathode (3) within a vacuum, towards a positive tungsten target anode (4). This anode rotates to dissipate heat generated. X-rays are generated within the tungsten anode and an X-ray beam (5) is directed towards the patient. 4 X-rays are generated via interactions of the accelerated electrons with electrons of tungsten nuclei within the tube anode. There are two types of X-ray generated: 1. characteristic radiation; and 2. bremsstrahlung radiation. Characteristic X-ray generation When a high energy electron (1) collides with an inner shell electron (2) both are ejected from the tungsten atom leaving a 'hole' in the inner layer. This is filled by an outer shell electron (3) with a loss of energy emitted as an X-ray photon (4). 5 Bremsstrahlung/Braking X-ray generation  When an electron passes near the nucleus it is slowed and its path is deflected. Energy lost is emitted as a bremsstrahlung X-ray photon.  Bremsstrahlung = Braking radiation  Approximately 80% of the population of X-rays within the X-ray beam consists of X-rays generated in this way. 6 The X-ray spectrum As a result of characteristic and bremsstrahlung radiation generation; a spectrum of X-ray energy is produced within the X-ray beam. This spectrum can be manipulated by; 1- changing the X-ray tube current or voltage settings, or 2- by adding filters to select out low energy X-rays. In these ways radiographers are able to apply different spectra of X-ray beams to different body parts. 7 The X-ray beam Key points  X-rays travel in straight lines  Body parts further away from the detector are magnified compared with those that are closer  Occasionally magnification can be helpful in localising abnormalities - X-rays travel in straight lines and a beam of X-rays diverges from its source. - Structures the beam hits first will be magnified in relation to those which are nearer the detector. - To reduce magnification, the X-ray source can be moved further away from the subject. - Structures (Objects) that need to be measured accurately should be placed closer to the detector. 8  - The X-ray beam Key points X-rays travel in straight lines Body parts further away from the detector are magnified compared with those that are closer Occasionally magnification can be helpful in localizing abnormalities. X-rays travel in straight lines and a beam of X-rays diverges from its source. Structures the beam hits first will be magnified in relation to those which are nearer the detector. To reduce magnification, the X-ray source can be moved further away from the subject. Structures that need to be measured accurately should be placed closer to the detector. 9 - Anterior-Posterior (AP) magnification 10  oBasics of X-ray Physics Tissue densities o Key points o An X-ray image is a map of X-ray attenuation o Attenuation of X-rays is variable depending on density and thickness of tissues o Describing X-ray abnormalities in terms of density may help in determining the tissue involved 11 o A radiographic image is composed of a 'map' of X-rays that have either passed freely through the body or have been variably attenuated (absorbed or scattered) by anatomical structures. o The denser the tissue, the more X-rays are attenuated. For example, X-rays are attenuated more by bone than by lung tissue. Describing densities o Contrast within the overall image depends on differences in both the density of structures in the body and the thickness of those structures. The greater the difference in either density or thickness of two adjacent structures leads to greater contrast between those structures within the image. o For descriptive purposes there are five different densities that can be useful to determine the nature of an abnormality. 12 o The 5 X-ray densities o Low density material such as air is represented as black on the final radiograph. Very dense material such as metal or contrast material is represented as white. Bodily tissues are varying degrees of grey, depending on density, and thickness. X-ray tissue densities o Here are the four natural tissue densities seen on a chest radiograph. Note there is a range of greyness, depending on the thickness of each tissue. 13 o Natural tissue densities o 1 - Air/Lung o 2 - Fat (layer between soft tissues) o 3 - Soft tissue o 4 - Bones 14 Nature of X-Ray: According to the quantum theory electromagnetic radiation can also be considered as a particles called photons. Each photon has associated with it an amount of energy: h = 6.63x10-34 J s Relationship between wavelength and frequency: λ = c/ѵ c – velocity of light (~3x108 m/s) Most of the kinetic energy of the electrons striking the target is converted into heat, less than 1% being transformed into x- rays. Properties of the Continuous Spectrum Smooth, monotonic function of intensity vs wavelength. The intensity is zero up to a certain wavelength – short wavelength limit (λ SWL). 15 The electrons transfer all their energy into photon energy: 16 To measure the un-attenuated (transmitted) beam intensity (I), we use, I = I0 e -µx ---------- (1) where I0 = un-attenuated (transmitted) beam intensity. I = initial beam intensity. μ = linear attenuation coefficient. e = 2.718 x = thickness of the attenuator such as (brain tumor, bone, aluminum) 17 18 What are the acute health effects of radiation exposure? At very high doses, radiation: can impair the functioning of tissues and organs and produce acute effects such as nausea and vomiting, skin redness, hair loss, acute radiation syndrome, local radiation injuries (also known as radiation burns), or even death 19