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Physics Medical
Physics Spring Y13
| Question | Answer |
|---|---|
| When waves change speed | Speed and wavelength change not frequency |
| Ultrasound: A scans | Give intensity/voltage at different times (distances). First peak is just the transmission. Small peaks are where it reflects partially at boundaries. A scans are just a single beam? of sound. |
| Ultrasound: B scans | Multiple partial reflections to build 2d image (i.e. non-baby parts transmitted for longer before reflecting) Is a bunch of A scans where each plotted on screen with brightness of each dot proportional to power of reflected wave |
| Piezoelectric materials | Piezoelectric material generates an EMF when it changes shape, and changes shape when an EMF is applied across it. Ideal for both generating and detecting ultrasound. (works because ionic, so charges move relative to each other under pd) |
| Piezoelectric transducer | Both sends and receives ultrasound waves. High frequency (ab 5 MHz) AC current makes crystals vibrate at that frequency: longitudinal waves. Returning echoes cause crystals to expand/contract: electrical signal. Pulses to allow reflected to be received |
| Why such a high frequency ultrasound | Making wavelength smaller means less diffraction in the size of spaces inside the human body, so less blurry. |
| What is acoustic impedance | How much ultrasound reflected at boundary. Z = pc (density x speed of sound in material) |
| Ratio of reflected intensity to *incident* intensity | Ir/Io = (Z2 - Z1)^2/(Z2 + Z1)^2 [the equation that looks like that] If slightly different, reflection small. Denser materials both higher p and often c, so more echo *Only when angle of incidence zero* |
| Impedance matching (and why we use gel for ultrasounds) | Impedance matching = similar Z Gel and skin have very similar Z, but air is way way lower than anything in the human body, so would be huge reflection if air gap instead of gel |
| Using doppler for ultrasound | Speed of blood flow (e.g. for clots, heart function, umbilical cord) deltaf/f = 2vcostheta/c c = wave speed in the blood v = blood speed Only the movement parallel to the sound can be seen, so probe not perpendicular to skin. cos90 = 0 |
| Why X ray scans work | X rays absorbed at different rates depending on type of tissue: gives a shadow of these tissues. Bones have calcium which is dense enough to absorb some. Camera behind subject. |
| Main issue of X rays | High frequency so high energy EM radiation, so is ionising. |
| Electron guns | Metal filament heated in vacuum (so electrons dont collide with air before the target, and also to protect the filament, presumably from oxidation). By thermionic emission, emits electrons. Accel through pd towards anode with small hole to make beam |
| Thermionic emission | Energy to electrons by heating means release similar to photoelectrons (giving energy for ionising) |
| Using eV = 1/2mv^2 for electron guns assumption | Assuming electrons start with negligible KE |
| How we make X rays | X-ray tubes: evacuated tube. Filament, thermionic, accelerated into dense material 'target' (e.g. tungsten). On hit, rapid decelerate so transfer energy as X-ray photon (needs to be v fast for X rays) |
| Efficiency of x ray tubes | Very very low (like 0.6%) we just throw a ton of electrons even under very low current, so this still makes enough x rays |
| Braking radiation (aka bremsstrahlung) | Continuous spectrum produced by range of decelerations experienced by electrons. |
| Braking radiation graph | Graph of intensity of x rays on y and wavelength on x Some sections are smooth and low intensity, which are simply when electrons slow down and release energy as photons Some singular line spikes called K-lines |
| K-lines (aka characteristic radiation) | Bound electrons in inner shells full ionised by bombarding electrons, outer-most electrons sometimes 'transition down' to fill gaps: v high energy deexcitation so can be x rays. K lines unique to target metals. |
| What is attenuation | The decrease in *intensity* of x-rays. Intensity falls exponentially as x rays pass through a material so that I = I[subscript 0] * e^-(mu*x) mu = linear attenuation coefficient x = material thickness |
| Linear attenuation coefficient | The higher the number, the faster it decays In m^-1 If attenuation is 0.21 cm^-1, that equals 21 m^-1. Not weird exponentials |
| X rays and gamma | Dont reflect/refract much |
| Equivalent of half-life for attenuation | Half thickness. x subscript 1/2 (same thing for half life too) |
| Attenuation mechanisms | Happens as photons interact with atoms: Simple scattering Photoelectric effect Compton scattering Pair production Energy ranges overlap: variety will happen for each wavelength of photon |
| Simple scattering as an attenuation mechanism | Photon absorbed by electron so reemitted upon deexcitation in all directions rather than being transmitted Lowest energy range - photon just sent in new direction. Too low for hospital x rays. |
| Photoelectric effect as an attenuation mechanism | Photon absorbed by electron. Has enough energy to ionise so electron removed from atom. Goes to KE of electron ((Higher electron may transition down, so low energy photon)) 2nd lowest energy, in range of hospital x rays so main mechanism of attenuation |
| Compton scattering as an attenuation mechanism | Electron ionised with some energy going to also producing a scattered low energy photon Can be lower energy than pair, but has upper bound. Is one mechanism for disrupting cancer cells. |
| Pair production | High energy photon spontaneously turns into a matter/antimatter particle pair (normally electron positron). Antimatter almost immediately annihilates, producing 2 photons in opposite directions (to conserve p) |
| Pair production as an attenuation mechanism | X-ray photon changes into an electron-positron pair due to nucleus (electric field) interaction. Has a high lower bound (E = mc^2 for e+ e- pair) of energy and no upper bound. Disrupts cancer cells in therapeutic x-rays |
| Pair production example: calculate max wavelength of an x-ray photon responsible for pair production | e- mass = e+ mass. Antimatter not literal. hc/lambda = 2m[subscript e] * c^2. This is the max wavelength - the energy that is normally left over would otherwise be KE. |
| Contrast media | Used in x rays and CT scans. Injecting a contrast medium to soft tissues (which have low attenuation coefficients) helps show up on x rays. Iodine and barium compounds dense + nontoxic so good |
| CT (Computerised tomography) scans compared to X rays | Give better view of soft tissue (i.e. better distinguishes different soft tissues) so don't always need contrast media. More detailed, clearer. Potential for 3D models. Slower, expensive, more ionising radiation exposure, more stressful to patient |
| CT scans how they work | Source and detector array rotate around patient as patient passes through scanner, recording multiple x ray images in fan-shaped beam per 360 rotation. Images from each rotation processed into cross-section slices. Can be combined for 3D. |
| Nuclear imaging overview | Putting radioactive material in body and detect emissions. Detects function not structures, so often combined with CT for structure. SPECT and PET. Dependent on radionuclide availability and sophisticated tech for gamma detection. |
| Gamma camera components | Collimator, then scintillator, then photomultiplier tubes, then computer. |
| Collimator in gamma camera | Hexagonal (so they tessolate) lead tubes. Rays at an angle hit the tubes and so are absorbed. Only allows rays travelling along tubes axis to reach scintillator, so we know where they came from. |
| Scintillator in gamma camera | Sodium iodide. About 1/10 gamma photons interact with it, each producing thousands of visible photons (much lower energy each). |
| Photomultiplier tubes in gamma camera | Converts visible photons into cascades of electrons, so each photon generates an electrical pulse. Photon onto photocathode for an electron. Then series of high pd dynodes so cascades build up from each to the next |
| Computer in gamma camera | Uses electrical pulses to determine where gamma photons hit scintillator. Hence determines origin of gamma photons and constructs image. |
| SPECT | Single photon emission CT. Gamma emitting element (normally Technetium) in body, emits single photons which are detected. |
| PET basic mechanism | Positron Emission Tomography Put a positron (B+) emitter into body. Emits B+: immediately annihilated by electron, producing a PAIR of gamma photons in opposite directions. If scanner detects opposite photons with same energy, traces position using lag |
| Why PET so good | We can make FDG (radioactive version of glucose with Fluorine 18) which emits B+. Shows hot spots of early stage cancer. Specificity of molecules very important based on intent. |
| PET scanner technicality | Not the same thing as a gamma camera but uses similar mechanisms, with lots of detectors in a ring. |
| F18 production | Cyclotron. Bombards O18 with protons. Nuclear transformation. |
| PET vs SPECT | PET very good for activity, but more expensive and needs radionuclides to be made on site. |
| Why short half life for radionuclides in medicine | Need a certain amount of activity to be detected properly. A longer half-life produces slower, so more would be needed, leading to more long-term exposure. Also need to use less of the source |
| Cyclotron structure | 2 hollow D-shaped electrodes called Dees, with a high voltage source Oscillator across them. Gap between them. This cylindrical space for the particles to move then has a magnet on either side of it to make constant field. |
| Cyclotron function | Charged particles accel between Dees by electric field. Steered by Dees at constant time period of revolution by magnetic field. Direction of electric field changes every half revolution. Spiral around 1000s of times, getting faster, leaving to impact. |
| Proof that time period of rotation in cyclotron constant | F = Bqv = mv^2/r so v = Bqr/m T = distance/speed = 2pi*r/v so v = 2pi*r/T 2pi*r/T = Bqr/m T = 2pim/Bq = constant for given particle in uniform magnetic field |
| Radionuclide generation via decay | Slow decaying 'parent' radionuclide (from core of nuclear reactor) transported easily. Decays into 'daughter' radionuclide, which is removed regularly with an eluting solvent |
| Graph for Radionuclide generation via decay | Activity on y, time on x. Parent is gradual curve downwards. Daughter has high gradient upwards curves which decrease in gradient as they approach the parent curve. Once they hit curve, line down as they are extracted, labelled 1st elution. |
| Graph for Radionuclide generation via decay why daughter activity curve shape | The daughter is also decaying, so is a curve towards equilibrium where creation = decay |
| Pictures 27/03/26 for the equations (get used to them but not memorise). We use the one that decays in 6 hours. | |
| Difference between gamma and x rays | They overlap in energy, so defined by their source. If by annihilation or decay, gamma. If by braking, X-rays (sometimes called 'soft' x rays if lower energy than gamma can be - normally the type for medical scans). |
| To prevent the anode from overheating in an electron gun | rotated so a new area is constantly exposed to the electron beam or it is cooled with circulating water supply |
| After x-ray tube | Often through a narrow 'window' for just the area needed, then through a collimator so only perpendicular ones used. Very important that its collimated so that the intensity will only reduce from attenuation not spreading out |
| Smooth part of braking radiation graph | Hump-shaped, going down gradually on the right to the maximum photoenergy |
| Textbook keywords for attenuation mechanisms | Simple scattering 'elastic scattering' Compton 'inelastic interaction) |
| Why heavier elements (like for contrast media) have higher attenuation coefficients | Lots of electrons |
| In italics on PMT - whats the relationship between attenuation coefficient and proton number | Proton number cubed proportional to coefficient Def don't need to know, but prob useful to remember such high effect of proton number |
| Technetium-99m | The ‘m’ stands for metastable: remains in a high energy state for prolonged period. When it decays via gamma emission (half-life 6hrs), forms Tc-99. Used for many organs e.g. can make compound for only brain cells |
| Technetium-99m photon energy | PMT says photon always has same energy so easy to detect |
| Frequency chosen for ultrasound to increase intensity | Resonant frequency to crystal |
| Ir/Io for ultrasound other name | Reflection coefficient |
| Watch out for acoustic impedance equation to see if looking for amount reflected or transmitted | |
| X-ray tube | MOST ***kinetic*** energy from electrons decelerating goes to thermal (hence why we spin the anode rotated). Only some electrons make x-rays. Mention x-rays characteristic to target |
| Reducing X-ray intensity by 40% | To 60% |
| Finding linear absorption coefficient | Just need x and I/I0, which are both given by previous Q parts where we found the half-thickness |
| Two substances for medical contrast mediums | Barium (meal) for digestive system and iodine (injected) for liquids i.e. veins Say specifically intestines rather than organs, and also veins |
| Necessity of medical contrast mediums | Mention x-rays dont show soft tissues well Mention eaten/injected High attenuation coefficient so when build up in organs, absorb more x-rays, so 'shadow' on x-ray scan |
| Describe how CT scan image produced | Mention patient moved small distance and repeat. Mention attenuation coefficient, and is by photoelectric. Mention possible contrast medium. Mention better than x-rays. Say computer makes 3D model from it. |
| If Q asking for attenuation coefficient in cm^-1 | Don't convert from cm earlier in Q. Keep track of if m^-1 or cm^-1 then convert as normal by dividing by 100 |
Created by:
Pyrogearos2