Physics of Diagnostic X-Rays 2024-2025 PDF

University of Babylon Dr.Entidar.J.Khamees College of medicine semester 1 \2024-2025 medical physics Part 1 Physics of diagnostic X-Rays Topics of the Lecture  X- ray Production and Types of X- ray...

University of Babylon Dr.Entidar.J.Khamees College of medicine semester 1 \2024-2025 medical physics Part 1 Physics of diagnostic X-Rays Topics of the Lecture  X- ray Production and Types of X- ray  Methods of X- ray interaction with matter  Radiographic image quality X–Rays: electromagnetic radiation (EMR) of very short wavelength (λ 1-0.1 A°) and very high penetrating power. It is very useful in diagnosis & radiotherapy X-rays belong to a group of radiation called electromagnetic radiation. Electromagnetic radiation is the transport of energy through space as a combination of electric and magnetic field. The field includes radio waves, radiant heat, visible light, and gamma radiation. The amount of energy carried by each photon depends on the frequency of radiation: E = hυ = h c / λ Where h = Plan's constant = 6.6*10−34 (joule. sec) c = velocity of light = 3* 108 m/sec υ = frequency of radiation. X- ray Production: X-rays are produced when highly energetic electrons interact with matter, converting some of their kinetic energy into electromagnetic radiation. Accordingly, the main components of the X-ray tube are: 1. A source of electrons (the cathode, filament); the number of electrons (as well as the number of produced X-ray photons) is controlled by the product of the tube current and time (mAs). 2. An evacuated space in which the electron is speed up (glass envelope). 3. A high positive potential to accelerate the negative electrons which control the energy of the electrons (as well as the energy of the X-ray photons) (kV). 4. A target which the electrons strike (the anode). 1 University of Babylon Dr.Entidar.J.Khamees College of medicine semester 1 \2024-2025 medical physics NOTE: In the X-ray tube, up to 99% of accelerated electrons energy is converted to heat and approximately 1% is converted to X-ray photons. In general, the higher the atomic number (Z) of the anode, the more intensity X-ray beam is produced (Z tungsten = 74). Increase the current in the cathode circuit = increase electrons. Increase kv = increase speed of electron = increase energy of photons = high resolution.  1. The anode material should have a high melting point (for tungsten 3400 C°) 2. Equipped the X-ray tube with two filaments which are used interchangeably to produce large or small focal spots, focal spot is an area on the target struck by electrons. The small focal spot produces less image blurring than the large focal spot but it concentrates the heat on small area. 3. Increasing the area struck by electrons (focal spot area) without increasing the image blurring by the angulations of the anode 10 ° to 20 °. This technique is called as line- focus principle 4. Using rotating anode X-ray tube (3600 rotations per minute). Types of X- ray 1. Bremsstrahlung Spectrum (Continuous X- ray): A small fraction of the accelerated electrons come near an atomic nucleus within the target and are influenced by its positive electric field. The electrons are decelerated and change its direction, causing a loss of kinetic energy, which is emitted as an X-ray photon of equal energy (bremsstrahlung radiation). For a given number of electrons, the amount of bremsstrahlung depends on: (1) The atomic number of the target (the more protons in the nucleus, the greater the acceleration of the electron towards the nucleus. (2) The kilovolt (kV peak) the faster the electron, the more likely it will penetrate into the region of the nucleus 2  University of Babylon Dr.Entidar.J.Khamees College of medicine semester 1 \2024-2025 medical physics 2. Characteristic X- ray Sometimes a fast electron strikes a K-electron in a target atom and knocks it out of its orbit and free of the atom. The vacancy in the K-shell is filled almost immediately when an electron from an outer shell of the atom falls into it, and in the process, a characteristic K X-ray photon is emitted. The difference in energy levels of the orbits in an atom is specifying that atom; hence the emitted radiation is called characteristic.  Diagnostic X-rays typically have energies of 15 to 150 keV, while visible light photons have energies of 2 to 4 eV.  The number of electrons accelerated toward the anode depends on the temperature of the filament, and the maximum energy of the x- ray photons produced is determined by the accelerating peak (kVp). voltage-kilovolt  The broad smooth curve in above figure is due to Bremsstrahlung, and the spikes represents the characteristic X-rays. Energy of X-rays One kilo electron-volt (keV) is the energy an electron gains or losses in going across a potential difference of 1000V. 1keV=1.6*10-9 erg = 1.6*10-16 J. The (kVp) used for an x-ray study depends on the thickness of the patient and the type of study.  For mammography: 25 to 50 kVp  For chest: ≈ 350 kVp Electron current: 100 ~ 500 or 1000 mA 3 University of Babylon Dr.Entidar.J.Khamees College of medicine semester 1 \2024-2025 medical physics  The x-ray energy produced is not monoenergetic; it is a spectrum of energies up to its maximum.  The power P (watt) = I (amp.) * V (volt) = 1 A and 100 kV = 100 kW => 99% appears as heat: damaged anodes. X- ray Absorption by Tissue The attenuation of an x-ray beam is its reduction due to the absorption and scattering of some of the photons out of the beam. the intensity I decreases approximately exponentiation as shown in the figure below I = I˳ e –μx where Io = initial beam intensity. I = un attenuated (transmitted) beam intensity. μ = linear attenuation Coefficient. e = 2.718 x = Thickness of the attenuator such as (brain tumor, bone, aluminum). Linear attenuation Coefficient (μ): measure the probability that photon interact (absorbed or scattered) per unit length it travels in specified material. It depends on: 1. energy of x-rays 2. atomic number (Z) 3. density (ρ) of material Half value thickness HVT (X1/2): is the thickness of material which reduce the intensity of the beam of radiation one – half of its value (50%). (HVT) X1/2 = 0.693 / μ 4 University of Babylon Dr.Entidar.J.Khamees College of medicine semester 1 \2024-2025 medical physics Methods o f X- ray interaction with matter Photoelectric effect (P.E): In this process the incoming X-ray photon transfers all of its energy to an electron which will use it to overcome the binding energy and get away from the nucleus (which will be +ve). This free photoelectron will use the remainder of the gained energy in ripping electrons off (ionizing) surrounding atoms. Photoelectric is more common in the high Z elements than in those with low Z, and more apt to occur at low energies. At 30 keV bone (as a heavy material) absorbs X-rays about 8 times better than tissue due to photoelectric effect. Compton Effect(C.E) occurs when the X-ray photon collide with loosely outer shell electron. In this case the electron will receives part of the photon energy and the remainder energy is given to Compton photon. Compton Effect is more likely to occur in material with low (Z) number. C.E. occur greatest at low Z material. e.g.:  In water or soft tissue C.E. is more probable occur than P.E effect at energy ≥ 30 KeV.  In bone C.E. is more probable occur than the P.E. effect at energy ≥ 100 KeV. Pair production is rarely occurring at diagnostic energy range as the minimum energy required for pair production is 1.02 Mev. In pair production the high energy photon enters the electric field of the nucleus and converted into two particles (electron and positron) which vanish and their mass- energy appears as two photons called annihilation radiation. 5 University of Babylon Dr.Entidar.J.Khamees College of medicine semester 1 \2024-2025 medical physics X -ray contrast media: This technique is made to make further use of the photoelectric effect Radiologists often inject high Z material into different part of the body (contrasting media). Barium compounds and iodine compounds are the most commonly used contrast agents. ( Z barium =56, Z iodine =53, Z soft tissue =7.42) e.g.: 1. Compounds containing iodine injected into the blood stream to show the arteries. 2. Oily mist containing iodine is sometimes sprayed into the lungs to make airways visible. 3. Barium compound is given orally to see parts of the gastrointestinal tract (upper GI). 4. Air is used to replace some of fluid ventricles of the brain, when a pneumoecephalgram is taken. 5. Barium enemas to view the other end of the digestive system (lower GI). 6. Air & barium are used separately to show the same organ in a double contrasting study. X- ray Image Formation The basic requirements for X-ray image production are X-ray source and image receptor. There are three types of image receptors or films: Types of films:  Double-sided radiographic film: emulsion layer of light-sensitive crystals coated onto both sides of a transparent base material. Used most in plain film imaging  Single-sided camera film: one emulsion layer only. Used in mammography  Non-screen film: x-ray photons used directly to expose film. Used in dental x-rays. 6 University of Babylon Dr.Entidar.J.Khamees College of medicine semester 1 \2024-2025 medical physics Part 2 Making an X-ray image: The radiographic image presents the information in a visual form, which is relatively easy for a trained observer to understand. Different parts of the body absorb the x-rays in varying degrees. Dense bone absorbs much of the radiation while soft tissue, such as muscle, fat and organs, allow more of the x-rays to pass through them. As a result, bones appear white on the x-ray film, soft tissue shows up in shades of gray and air appears black. How to increase the sharpness of an X-ray image? X-ray images are basically images of shadows cast on film of various structures in the body. So we are in need to make these shadows as sharp as possible. This can be done by: 1. blurring in the image can be reduced by using a small focal spot. 2. positioning the patient as close to the film as possible. 3. Increasing the distance between the X-ray tube and the film as much as possible. 4. Reducing the scattered radiation striking the film by using grids consisting of a series of lead and plastic strips. 5. Holding breath when having a chest X-rays to reduce motion which in turn reduces blurring. 7 University of Babylon Dr.Entidar.J.Khamees College of medicine semester 1 \2024-2025 medical physics Radiographic image quality: The main problem involved in obtaining good X-ray image is blurring. The blurred edge of an object in the X-ray image is called penumbra. The width of the penumbra can be calculated by: 𝑫 𝒑= ∗𝒍 ……… P = penumbra width, 𝑳 D = focal spot size L= focal-object distance 𝒍 = object-film distance To obtain a good x-ray image (without blurring) it should be 1. Small focal spot, small D, to reduce the penumbra, 2. Positioning the patient as close to the film as possible, small 𝒍 l, 3. Increasing the distance between the x-ray tube and the film as much as possible, large L, 4. Reducing the amount of scattered radiation striking the film as much as possible by using a grid consisting of a series of lead and plastic strips, 5. It's necessary to avoid motion during the exposure, since motion causes blurring. Grid In order to obtain good X-ray image, it is necessary to reduce scatter radiation reaching the film. In general, the thicker the body part (abdomen or pelvis) the more the scatter radiation is produced. The best way to reduce the scatter radiation is by using the grids. The grid is a series of lead and plastic strips aligned to absorb the scattered radiation by lead strips and the main beam photon will pass through the plastic strips. The main disadvantage of grids is that they increase the radiation dose to patient as they may absorb some of primary beam photons. 8 University of Babylon Dr.Entidar.J.Khamees College of medicine semester 1 \2024-2025 medical physics X-ray beam Filtration Low-energy x-rays will not penetrate the entire thickness of the body; thus they will increase the patient radiation dose without increasing the diagnostic quality of the image. Most of this low energy radiation will be removed with filters, thin plates of aluminum, copper, or other materials placed in the beam in front of X-ray tube window. Units of measure and exposure The measure of X-rays ionizing ability is called the exposure. The unit used for radiation exposure is the roentgen (R), a measure of the amount of electric charge produced by ionization in air, since, 1 R = 2.58×10-4 C/kg of air. Since an exposure to a large area is more hazardous than the same exposure to a small area, a useful quantity for describing radiation to the patient is the exposure-area product (EAP), which is given by: EAP (rap) = exposure (roentgen) × area (cm²) Where, 1 rap = 100 R cm² So, if one receives an exposure of 0.6 R to an area of 33 cm² (a typical dental exposure), he will receive 20 R cm² (or 0.2 rap). Typical exposures received by an adult are given in the following Table, Radiation Risk of X-ray Examinations The radiation risk refers to the damage produced by ionising radiation due to energy deposition in tissues. This energy may result in ionisation within the tissues. The radiation interactions with tissue are either:  Direct, wherein the radiation energy is directly transferred to the DNA causing structural changes in its molecules; or  Indirect interaction, where the radiation energy is absorbed by water molecules forming free radicals which in turn cause damage to DNA molecules. The adverse health effects of radiation can be classified into two groups: 1) Deterministic effects which follow high radiation doses and result in relatively immediate damage (within minutes, hours, days and even weeks). 2) Stochastic effects which follow low radiation doses and may result in cancer development. The lag period between irradiation and cancer development for stochastic effects is at least 5 years and may reach to 10 or 20 years. 9 University of Babylon Dr.Entidar.J.Khamees College of medicine semester 1 \2024-2025 medical physics Fluoroscopy Fluoroscopy refers to the continuous acquisition of a sequence of X-ray images over time (real-time X-ray movie of the patient). Fluoroscopic systems use X-ray detector systems capable of producing images in rapid sequence. Fluoroscopy is used for positioning catheters in arteries, visualising contrast agents and invasive therapeutic procedures (surgery). It is also used to make X-ray movies of anatomic motion, such as of the heart or the oesophagus. Computed tomography (CT scan) CT images are produced by passing X-rays through the body at a large number of angles, by rotating the X-ray tube around the body. A detector array, opposite the x-ray source, collects the transmission projection data. The numerous data points collected in this manner are synthesised by a computer into tomographic images of the patient. The term tomography refers to a picture (graph) of a slice (tomo). The advantage of CT over radiography is its ability to display three- dimensional (3D) slices of the body eliminating the superposition of anatomical structures. 10  University of Babylon Dr.Entidar.J.Khamees College of medicine semester 1 \2024-2025 medical physics Magnetic Resonance Imaging (MRI)  In MRI, the patient is placed in strong magnetic field to utilise magnetic resonance properties of the proton in hydrogen nuclei of water molecules within the body.  Then a pulse of radio waves is generated by coils positioned around the patient.  The protons in the patient absorb the radio waves, and subsequently re-emit this radio wave energy after a period of time.  The radio waves emitted by the protons in the patient are detected by the coils that surround the patient. The returning radio waves signal carry What is the difference between X-Ray, CT Scan and MRI? X-rays: Used to detect fractures and deformities in bones. The rays pass through the body and take pictures of bones, but fine details of soft tissues may not be clear. - CT scan: Provides detailed cross-sectional images of bones and soft tissues by combining several X-ray images from different angles. Helps diagnose complex fractures and conditions that require fine details. - Magnetic resonance imaging (MRI): Uses magnetic fields and radio waves to accurately image soft tissues such as tendons and ligaments. It does not show the details of bones as well as X-rays or CT scans, but it is excellent for diagnosing soft tissue problems 11