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Medical physics
| Question | Answer |
|---|---|
| X-ray tube | HIGH-voltage cathode, heated for thermionic emission. Must also be an anode. In a vacuum chamber. Target metal, often tungsten, angled towards a collimator. |
| How do accelerate electrons produce x-rays. | Electrons kinetic energy converted to photons. Electrons are captured, releasing photon, or excite other electrons, which fall back to original energy level and release a photon. Electron deceleration produces photons. |
| 4 attenuation mechanisms | Simple scattering. Pair production Photoelectric effect Compton effect |
| Simple scattering | X-ray photon is deflected/scattered by an electron, changing its direction. |
| Pair production | High energy photon interacts with a nucleus/ is close to it. Energy transformed into matter and antimatter - an electron and positron. Photon disappears |
| Photoelectric effect | An x-ray photon is absorbed by an electron, freeing it from the atom. The absorption of a photon decreases beam intensity |
| Compton effect | High energy photon is absorbed by an electron. The electron is emitted and releases a lower energy photon in a random direction (SCATTERED). Photon lower energy, different direction so less intensity. |
| Units of mu | Attenuation coefficient. Measured in distance^-1. Avoid converting this figure if possible, but if not 1cm^-1 is 100m^-1, not 0.01m^-1 etc. |
| Contrast media | Barium and Iodine, might have to know specifically. Can make area injected into, like artery, visible on x-ray due to high mu. This means blood clots can be visualised, for example. |
| CAT scan | Computerised axial tomography. Rotating x-ray tube and camera opposite produces thing FAN SHAPED images, on rotation producing a thin slice. Patient incremented down to make many slices, creating 3d. |
| Single x ray vs CAT scan. | Single x ray produces single 2d image which provides less detail. Multiple angles of each scan show internal 3d structure in cat scan, and higher RESOLUTION. Much higher radiation dose/exposure in a cat scan. |
| Medical tracers, not PET. | technetium–99m - gamma emitter. Half life 6hrs, minimise long term dose but used for a while with consistent output during scan. Put in 3D scanner of gamma cameras . Higher reading along a line means blood accumulate. |
| PET | Uses Flourine-18, beta+ emitter. Soon after production, will collide with an electron. Annihilation occurs, producing a pair of gamma photons. Difference in time between two photons being detected shows position |
| PET continued | (Ring of gamma cameras). Software can, in real time, analyse position of of pair production. More production means more blood - higher brain activity maybe due to brain use or a tumour. |
| Gamma camera components in order. | First collimator - only allows parallel photons and absorbs the rest. Means detector can know along which line the photon originates. Then scintillator converts gamma into many visible photons with same total energy. |
| Gamma camera pt2. | Photocathode - each visible photon is absorbed and for each, one electron is emitted, which goes into a photomultiplier tube. Electrons are accelerated towards a series of dynodes, which in turn release more electrons -> electrical signal. |
| Define ultrasound | Mechanical longitudinal wave with frequency greater than 20kHz |
| Define piezoelectric effect | The material expands or contracts when a p.d. is applied across its opposite faces. Also induces an e.m.f. when contracted or expanded manually. |
| Structure of a transducer. | Electrodes on either face of one (or many) piezoelectric crystal(s). Electrodes connected to a signal generator and oscilloscope. When a.c. applied, generates a sound wave of the same frequency, and generates an e.m.f. on reflection. |
| Ultrasound A-scan. | One crystal sends out a pulse. It will receive a series of reflected wave peaks, It can use these to determine distances directly in front of the transducer. Remember 2x distance. |
| B-scan | Many transducers used and software compiles the boundaries detected at their distances to produce a live 2d slice. |
| What is z in z=rho*c | Acoustic impedance - density * speed of sound in medium. This dictates the reflection of a soundwave. The bigger the difference in this quantity, the higher proportion will be reflected |
| Trap for ultrasound reflection questions | You will calculate proportion reflected, but they may ask for proportion transmitted. |
| Why gel in ultrasound | Similar z to skin, so boundary will not cause a great reflection. If in air, nearly entire wave would be instantly reflected. |
| Measuring speed of blood | PULSES of ultrasound aimed at blood cells in artery. Probe at an angle. Detected frequency has changed due to component of velocity of arteries in plane of sound waves, doppler shift. Ratio of delta f to f enables calculation of v. |
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pemmb