Medical Physics Flashcards
Principles of radiation safety
Time
Distance
Shielding
Precision vs accuracy
ACcurate is Correct - how close to correct answer
PRecise is Repeating - ability to reproduce results
Radiotherapy
Treatment of cancer. Use Linacs - linear accelerators. Maximise dose to tumour and avoid healthy tissue. Damages genetic material.
How does LINAC work
Electrons acelerated, collide with heavy metal targer. Produce high energy x-rays.
Shaped to conform to shape of tumour - multileaf collimator, beam comes out of gantry.
Medical physicist determines dose and amount of time to give dose
Nuclear medicine
Use radioactive substance for diagnosis
Radiolabelled pharmaceuticals. Usually looking for function - thyroid/bones/brain. Use pharmaceutical used in that area to image using gamma camera.
Can be used in treatment. Radio-iodine - taken up by thyroid
Scintillation
Property of luminescence when excited by ionising radiation
Gamma camera components
Collimator
Scintillator
Photomultiplier
Gamma camera function
COLLIMATOR ensures photons are from directly below (lead absorbs)
Gamma photon disturbs CRYSTAL (SCINTILLATION LAYER) - knocks off electron and flash of light happens. Detected by PHOTOMULTIPLIER TUBE and computer sums counts to create image
PET
Positron emission tomography
Positron emitting tracer, positrons annhilate with electrons giving off gamma rays at 180 degrees - cameras use these to place initial burst - 3D
Gamma source
Technetium 99m chosen for relatively long half life and ability to be incorporated into molecules to target parts of body
Clinical engineering
Provide scientific support for deliveryof physiologyical services.
Spinal chord monitoring
Intercranial pressure monitoring
Home aximetry measurement - sleep apnea
Diagnostic radiology
CT - computed tomogrophy - x-rays - 3D
CT scan
Rotating x-ray tube goes around body with detectors opposite. Creates 2D images or slices which are stacked to create 3D image - used for diagnosis or guiding treatment
Energy of x-rays determined by voltage
Attenuation
Loss of intensity through a medium. Bone is radiodense - absorbs more. Essentially CT is map of attenuation coefficients. Why metals spike.
Good images (and considering patients)
Spatial resolution - ability to differentiate between two objects with how close they are - and contrast - ability to differentiate between intensities
Images are better with higher dose but this isn’t better for patient, compromise
Proton beam therapy
Uses beam of protons instead of radiation to kill cells. Particle accelerator speeds up protons. Unlike ordinary radiotherapy, protons ‘stop’ once they hit cancerous cells - less damage.
Non-ionising imaging
Ultrasound, MRI, optical imaging
Ultrasound
High frequency soundwaves - 2-18MHz.
Travel in body and bounce of boundaries between tissues - some get reflected back, some go through and hit next boundary. Reflected rays picked up and relayed to machine - distance calculated using time of pulse and speed.
MRI
Uses H protons. These spins line up in B field.Radio frequency produced, which flips spins. Turn field off. protons return to initial state, giving off RWs as they do. Takes different amount of time depending on tissue - imaging can happen. RWs detected at different times and this creates a map.
Optical imaging
Measuring properties of light to make diagnosis - shining light of certain wavelength onto cancerous tissues to see if exhibit different absorption, light scattering or fluorescence to normal skin.
Ionising radiation
Radiation with enough energy to detach electrons from atoms/molecules
Can be subatomic particles, ions at high speed or EM radiatoin (UV/x-ray/gamma)
Directly vs indirectly ionising
Directly - charged massive particle ionises through fundamental interaction through Coulomb.
Indirectly - electrically neutral, does not strongly interact with matter. Photons - photoelectric effect and compton effect - cause ejection of electrons at relativistic speed which ionise.
Why is ionising radiation dangerous, what is most at risk
Damages genetic material, when ionise cell will either die, fix itself or mutate - can become cancerous.
Cells that reproduce most and are least specialised are most likely to be affected by radiation - fetus.
Radiation Sickness
Health effects visible within 24 hours of exposure to high level of radioactivity. Depends on dose.
Smaller dose: gastrointestinal effects - nausea, vomitting. Larger: neurological effects - seizures, tremours, lethargy.
Radiation safety physics
Measure & calculate doses of raditation for staff and patients.Survey environment to monitor performance and ensure it complies.
Contrast
Low contrast: more shades of grey, harder to tell difference.
High: less shades, easy to tell difference
Determined by attenuation of tissues
Contrast eq.
c prop. (mu_1 - mu_2) x t
mu is attenuation coeff, t is thickness
Contrast higher with bigger difference in mu’s and bigger t
Factors affecting contrast
Linear attenuation coefficient (LAC), overlying tissue, scatter
LAC depends on compton and photoelectric LAC
Increase contrast
decrease E
increasing difference in Z
increase density
Spatial resolution
How far apart two things must be before they can be seen as separate details. Measured by MTF, 1 is same as image.
Nyquist sampling: sample at double object spatial freq.
Noise
Random variation in no. of photons forming each part of image.
QN prop 1/root N
SNR = N/root N
Reducing proportion of noise improves quality.
Lasers
Emit light through optical amplification based on stimulated emission. coherant light. Can be used in cancer treatment to shrink/destroy tumours, usually superficial cancers.
Classes of laser range from 1 (contained laser_) to 4 which can burn skin, use protection which blocks out certain wavelength.
Danger of lasers
Coherance and low divergence means it can be focussed onto small spot on retina and burn.
Dosimetry
Measurement, calculation and assessment of ionising radiation dose absorbed by an object.
Good scientific practice
Professional practice scientific practice clinical practice reserach and development and innovation clinical leadership
Fluroscopy
‘video’ version of x-ray, continuous source of x-rays, higher dose. 20-50mGy/min. Can see internal function+structure. Can be used in digestive system with substance opaque to x-rays that is swallowed.
Gray
one joule of radiation energy per kg of material. Dose in radiotherapy. 1 J/kg. Whole body exposure to 5Gy lead sto death in 2 weeks. Treatment typically 45-60Gy in 2Gy chunks.
Pulse oximeter
Measures pulse and oxygenation. Red + IR light. LED opposite sensor, as light goes through body it is absorbed by amount dependent on wavelength and average amount of O atoms attached to each Haemoglobin. Amount fluctuates as arteries expand and contract too - pulse.
Electrocardiogram
Record activity of heart through electrodes on patients skin. Heart beats bcecause of changes in electrical potential, activity that body conducts to its surface. Altered by tissue but still reflects cardiological cycle.
Compton scattering
Decrease in energy of photon, part of energy transferred to electron. Inelastic scattering.
Photoelectric effect
Emission of electrons when light falls onto a material. Electrons dislodged when photons have energy greater than the threshold energy.
Geiger counter
Uses townsend avalanche phenomenon to produce easily detectable electronic pulse from as little as single ionising event. Detects alpha, beta, gamma, x-rays and can be adapted to detect neutrons.
Radiosensitivity
Susceptibility of cells, tissues, organs to harmful effects of ionising radiation. Cells that are undifferentiated, well nourished, dividing quickly are most at risk.
Proportional to rate of cell division, inversely proportional to degree of cell differentiation
High and low radiosensitive organs
High: lymphoid organs (tonisils, lymph nodes, thymo gland, spleen), bone marrow, testes, ovaries, intestines
Low: spinal chord, brain, muscle
Radon
Dangerous because one isotope of radon decays to another, if it is in your lungs it will decay with alpha radiation, as will daughter nuclei.
Power eq.
Power = delta W / delta t (work done per unit time)
W = f.x (force x distance moved in direction of force) W = delta KE W = delta PE
Radioactivity eq.
A = delta N/delta t A = lamda N N = N0 e^-lamda t Thalf = ln2 / lamda
NHS constitution
Set of principles and values of the NHS, as well as rights of patients staff and public, responsibilities and pledges by staff
Principles that guide constitution
Service for all, regardless of gender, race etc
Access based on need not ability to pay
Patient at heart of everything
Aspires to highest standards
Constitution values
Work together for patients Respect and dignity Compassion Improving lives Commitment to quality of care Everyone counts
Research development and innovation in GSP
Search and critically appraise scientific literature. Identify innovative ways to practice healthcare. Collaberative research, develop new techniques, evaluate research, interpret data, present data to peers, support wider healthcare team
