Basic Physics and Radiation Safety

Basic Physics

EMR = Electromagnetic Radiation

c = speed of light in vacuum = 3.0 x 108 m/s = λv; λ = wavelength (m); v = frequency (Hz)

transverse wave with electric and magnetic vectors oscillating perpendicular to each other and direction of propagation; self propagating due to changing electric magnetic fields

quanta/photons of energy with E = hv; h = Planck’s constant = 6.63 x 10-34 Js

wave-particle duality – all matter and energy exhibits wave and particle-like properties (quantum mechanics)

  • wave properties: diffraction, refraction, polarization, interference, λ and f
  • particle properties: photoelectric effect dependent on f not I, travels in straight line, photons

polarisation – orientation of electric oscillations which can be linear or circular/elliptical (rotating around direction of propagation)

eV = electron volt = kinetic energy an electron would receive if accelerated through 1V = 1.6 x 10-19 J

from ↑v, ↓λ, ↑E , EM spectrum includes electric power, radio (radiofrequency = RF), TV, radar, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays, γ-rays; ionising radiation includes UV and above

Atomic Structure

Z = atomic number = number of protons

A = mass number = protons + neutrons

_{1}H(hydrogen), _6^{12}C (carbon), _7^{14}N (nitrogen), _8^{16}O (oxygen), Mg (magnesium), _{13}Al (aluminium), Ti (titanium), Mn (manganese), _{29}Cu (copper), Ga (gallium), Sr (strontium) Y (yttrium), _{42}Mo (molybdenum), _{43}Tc (technetium), _{45}Rh (rhodium), Cd(cadmium), In (indium), Sb (antimony), _{53}I (iodine), Cs (caesium), Ba (barium), La (lanthanum), Sm (samarium), Dy (dysprosium), Er (erbium), _{74}W (tungsten), Tl (thallium), _{82}Pb (lead), _{86}^{222}Rn (radon), _{88}^{226}Ra (radium), _{96}^{238}Ur (uranium), _{94}Pu (plutonium)

atomic diameter ≈ 100,000 times nuclear diameter

electron cloud in shells with 2 K, 8 L, 18 M, 32 N and 50 O potential electrons; placed in lowest energy levels first

ground state = lowest energy state of an atom or nucleus

energy-level diagram = number of electrons shown in each shell, with binding energy on the right

valence electrons = outer shell

binding energy = energy required for ionisation, extra energy is kept as electron kinetic energy

quantum mechanics prohibits electrons from receiving/loosing small amounts of energy, and can only jump between energy shells (absorbing/releasing a photon) equal to the difference in binding energies

Radiation Safety


D = absorbed dose = de/dm = energy imparted/mass; gray = Gy = J/kg = 100 rad

integral dose = energy imparted = total energy absorbed by body

K = air kerma (kinetic energy release in the medium) = dE(tr)/dm = initial energy/mass; Gy

HT = equivalent dose = ωR.DT

E = effective dose = ΣTT x HT); sievert (Sv) = 100 rem

X = exposure; C (coulomb).kg-1; roentgen (R)

collective dose = total E to population (for planning purposes only)

ESD = entrance surface dose = skin entrance absorbed doses

BERT = background equivalent radiation time

Radiation Biology

ionization = loss of orbital elections > ion pair

excitation = of outer valence electron, altering chemical force

free radicals = single unpaired election

Background Radiation

natural background radiation (2.4mSv/y)

  • internal sources/diet (0.3mSv/y)
  • cosmic rays (0.4mSv/y)
  • terrestrial (0.5mSv/y)
  • radon gas (1.2mSv/y; doesn’t contribute to GSD)


spontaneous mutation rate = 10-5 per gene per generation

population mutation burden (as mutations enter gene pool at same rate deleterious one leave)

GSD = genetically significant dose = exposure in gonads of persons who later produce children

doubling dose = E to induce mutations equal to natural level = 2Sv acute, 4Sv prolonged

most sensitive cells are actively proliferating, undifferentiated, length of mitotic phase

spermatids and spermatozoa are sensitive to mutation but radio-resistant to death

chromosomal damage

restitution in 90%; arm breakage otherwise lethal for cell

translocation (w another chromosome), inversion (of fragment), ring (ends join), dicentric (chromosomes combine)

non-disjunction = imbalance in number of chromosomes, leading to polysomy > solid cancers, Down’s, late abortions

Carcinogenesis Models

cancer from XR indistinguishable from natural causes

mutations in growth regulating genes

multi-event concept = initiationpromotion (may/may not be carcinogens)progression

latent period leukaemia 7-10y, bone 10-15y, thyroid 20y, breast 22y, solid tumours 20-30y

absolute risk model = additive model = constant risk of cancer through life

relative risk model = multiplicative mode = risk increases in proportion to natural rate of cancer

data from studies include

  • LSS (live span studies) of Japanese survivors of atomic bomb; DDREF = dose and dose rate effectiveness factor, reduction of risk factor by 2x from Japanese data
  • treatment with radiation for ankylosing spondylitis, tuberculosis
  • post-partum mastitis, chronic breast disease vs breast and lung Ca
  • chest fluoro in Tb therapy in Massachusetts and Nova Scotia vs breast Ca
  • 131I treatment for hyperthyroidism and thyroid cancer vs thyroid, leukaemia, colon Ca
  • inhalation/ingestion of 131I in Marshall Islanders exposed after US nuclear weapons testing vs thyroid Ca
  • Chernobyl children and ingestion or inhalation of substantial amounts of 131I vs thyroid and leukaemia
  • Chernobyl liquidators vs solid tumours
  • Uranium miners in Canada, Czech, USA vs lung Ca
  • radium dial painters vs lip mouth and throat Ca (ingestion from licking)
  • indoor radon gas concentrations in Sweden and USA vs positive and negative correlations w lung cancer
  • occupationally exposed workers slight increase in leukaemia

Stochastic Effects

radiation hormesis = adaptive response effect theory = low doses stimulate repair mechanisms

assumed no threshold

linear-quadratic model for low LET radiation; cancer induction = a.D + b.D2 = curvilinear; drops at high D due to cell killing

OER = oxygen enhancement ratio = D(hypoxia)/D(normal) to produce end point = 2.5-3, < in high LET

population fatal carcinogenesis at high dose rate and dose = 10%/Sv

nominal probability coefficient = population fatal carcinogenesis at low dose rate = 5-6%/Sv

risk for children 2-3x higher, >50yo 5-10x lower (except leukaemia peaks again in late life)

5mSv/y for 40y increases fatal cancer risk of 0.8%

1/million chance of death from one CXR (cancer) = 100km by car = 1 cigarette = living 2/7 in NY

XRs cause ≈ 1% of all cancers

Deterministic Effect Acute (Gy) Chronic (Sv/y)
testis sterility – temporary
– permanent
ovarian sterility – temporary
– permanent
cataracts 5 0.15
early transient erythema 2
temporary epilation 3
main erythema 6
permanent epilation 7
fibrosis, telangiectasia, dry desquamation 14
necrosis, secondary ulceration 20

Deterministic Effects

cell killing by mitotic death

increased risk with dose, cannot tell definitely what effect will be for given dose

Radiation in Pregnancy

see table for typical doses

in diagnostic dose levels, the only real risk is increased risk of cancer induction (3%/Gy)

effects on foetal development highest during 2nd&3rd months

<4/40 – failure to implant; 50-100mGy

4-10/40 – malformation, most sensitive for child cancer induction; 100mSv (consider termination if higher)

8-25/40 – severe mental retardation worst 16-25/40; 100mGy, 25 IQ points/Gy

>4/40 – growth retardation, cancer in life

guideline for abdominal region exposure – ask if pregnant, if unsure proceed if LMP <28 days ago (when risk starts if actually pregnant)

old ’10-day rule’ – examinations booked in 1st 10 days of menstrual cycle for women of childbearing age; can used for high dose procedures (tens of mGy to foetus)

Acute doses

LD50 = median lethal dose for uniform body irradiation = 3 Sv without medical support, 4.5 Sv with

1Sv radiation sickness; LD50 prodromal syndrome (anorexia, n+v, lethargy); 3-8Sv haemopoietic syndrome w death at 1-2/12; 10 Sv gastrointestinal death 1-2/52; 100 Sv cerebrovascular death 1-2/7


detriment includes weighted risks of fatal and non-fatal cancer, severe hereditary effects, length of life lost

detriment for adult workers 5.6%/Sv, whole pop 7.3%/Sv

Measures of Detriment

ωR = radiation weighting factor, for stochastic effects (photons 1, electrons 1, protons 2, neutrons 2.5-20, alpha 20)

ωT = tissue weighting factor (red marrow, colon, lung, stomach, breast, remainder tissues 0.12; gonads 0.08; bladder, oesophagus, liver, thyroid 0.04; bone, brain, salivary glands, skin 0.01)

LET = linear energy transfer = energy released per length; keV/μm

Bragg peak = increase in LET before particle rest

RBE = relative biological effectiveness

lineal energy = energy released per average dia of volume keV/μm

Dose Limits/Constraints


ESD (mGy)

E (mSv)

Uterus D

skull PA/lat



chest PA/lat




T-spine AP/lat



L-spine, abdo, pelvis, hip AP




fluoro normal


fluoro hi


Ba meal

DAP 25 Gycm2



Ba enema

DAP 60 Gycm2



CT head



CT chest



CT abdo



CT pelvis



CT fluoro




1.4 MGD





99mTc bone scan





exposure from natural sources excluded, except when direct consequence of practice

occupational exposure = 20mSv/y averaged over 5y w max 50mSv in one year (risk of induced fatal and weighted non-fatal cancer plus hereditary defects of 5%); lens 150mSv/y; skin 500mSv/y (averaged over 1cm2); hands & feet 500mSv/y

ALI = annual limit in Bq on intake for ingestion that would give equivalent dose limit in mSv

DAC = derived air concentrations (Bq/m3) for inhalation

pregnancy = 2mSv to abdomen for remainder of pregnancy (~1mSv to embryo/foetus); 1/20 of ALI

medical exposure = no limit; diagnostic reference/guidance/action levels to trigger investigation if exceeded for average sized patient (see table)

public exposure = 1mSv/y; lens 15mSv/y; skin 50mSv/y

difference between occupational and public exposures due to inclusion of children (more radiosensitive, longer to live), aware of risks and can take precautions, accept risk as conditions of employment

exposure to comforters and carers (result of support that is other than part of their job) = 1mSv/yr, 5mSv/episode

exposure to volunteers in research = 5mSv/y or 2mSv/y averaged over 5y and cumulative for <18yo 5mSv; exposure trivial when <0.1mSv, minor-intermediate 0.1-10, moderate >10

controls done to sources, environment and individuals

emergencies have reference levels; in immediate remedial work = 0.5Sv except for life-saving actions; after immediate emergency limit = occupational

controlled area = require works to follow procedures to control radiation; supervised area = kept under review, but no procedures normally needed; areas outside the workplace cannot exceed limits for public

Radiation Protection

ICRP (international commission on radiological protection) framework

use factor = time radiation present; occupancy factor = time receiver of radiation is present

source = origin of a pathway that may lead to exposure

practices = activities that increase exposure; justification (needs to produce sufficient benefit), optimisation (ALARA = as low as reasonably achievable), dose and risk limits

interventions = activities than can decrease exposure or potential exposure; more good than harm, benefit less detriment should be maximised

Reduction of Exposure to the Patient

XR: limit area exposed (also reduced scatter), filtration, max kVp, largest SSD=skin-source distance (mobile >30cm, fixed >45cm), AEC, avoid mobile equipment, reduce grids when not required, minimise absorption before receptor, latest screens, optimised film processing, gonadal shielding, compression, reduce views

II: largest FOV and most collimation, plain XR instead of fluoroscopy, best geometry (far from focus, high SID = source-image distance), pulsed fluoro

CT: lowest mA and highest pitch

NM: appropriate radiopharmaceutical with lowest activity and multi-person checking, thyroid blocking agents (Lugols solution, KI, KClO4), hydration and frequent voiding or laxatives, diuretics/cholecystokinin to reduce kidney and GB retention

QA programs

foetuses and growing children tissues more radiosensitive, and greater potential for manifestation of deleterious effects due to greater life expectancy

radiation to abdo/pelvis of reproductive capable women should be minimum

pregnancy: delay until 25/40 or <8/40 (except XR away from foetus or CT head); estimate potential D to foetus, substitute for non-radiological procedure, discuss w physician, NM best avoided (may be concentrated in foetal organs) unless life-threatening and consideration for termination

breastfeeding stop completely for treatment 131I, category I stop for 3/52 (131I, 125I, 67Ga, 201Tl [thallium]), II for 12/24 (131I/125I/123I-hipurate, 99mTc), III for 4/24 (99mTc-RBC/phosphonates/DTPA), IV no restrictions (51Cr-EDTA)

children (high retake risk): sedation in NM and occ CT, mechanical immobilisation (compression bands, sand bags, tapes), short exp times, no grids, rapport

Reduction of Exposure to the Worker

from primary beam, tube leakages and scatter

lead gloves (if hands in 1y beam), hands on exit beam side, needle holders, full wrap around lead aprons (min 0.25mm, preferably 0.5mm Pb; 0.3mm lead block 90% @ 100kVp), stand back (scatter @ 1m 0.5%, 2m 0.1%), thyroid shield and lead glasses (for cardiologists and interventional radiologists), II on staff side

nuclear medicine

record, contamination check outer package, shielded stores and waste (Pb pots), patients regarded unsealed sources, ventilated, impervious smooth benches with splash-backs, Pb syringe shields, protocols for contamination (incl survey monitors), cater for excretion (faeces, urine, saliva, blood, breath, skin; short-lived waste can be flushed in sewers), protective clothing, prohibit eating/smoking/cosmetics, care w punctures/cuts/abrasions, audits, training & education, QA, minimise activity used and ensure net benefit

hazards: milking 99mTc/99Mo generator, delivering to, positioning, removing and storing patient, disposal, cleaning and contamination

external hazard = radiation from patient; contamination = blood, urine, saliva, breath, handling objects w contaminated gloves

XR after bone scan OK with minimal degradation to image quality and dose to radiographer

HVL (half value layer) for 140 keV γ rays is 0.25mm Pb

β emitters 1st shielded by low Z (Perspex) to minimise bremsstrahlung


Personal Dosimetry

worn under apron for XR/fluoro, also outside apron if always worn (DSA)

typical occupational mSv/annum: radiographer 0.25, radiologist 0.75, NM tech 1.83, cardiologist 2.5

film badge

unexposed film with selective filters (Pb, Al, plastic, Cd [cadmium]; for conversion of film D to tissue D)

worn for 2-8/52, unexposed control compared and optical density read with densitometry

TLD = thermoluminescent dosimeter

energymetastable states until heated to cause electrons to return to normal state with emission of light (energy absorbed), measured with photomultiplier

LiF:Mg/Ti, CaF2:Mn, CaSO4:Dy, Li2B4O7:Mn (LTB = lithium teraborate)

can be made small and any shape, reused; influenced by humidity and temperature, need individual calibration

direct reading ion chambers

small charged capacitor connected to glass fibre electroscope in pen-type housing; radiation  loss of charge (K)deflection of fibre or digital readout (with alarms)

moisture > high readings, not rugged, expensive

other dosimeters

optically stimulated luminescent dosimeters

electronic dosimeters

Dosimetry of Patients in Radiology

see table for typical values

for head CT, lens receives 50-100mGy (therefore need gantry angulation)

BERT of CXR 3d, CT abdo 7y

phantom measurements = tissue-equivalent models in beam

Rando Phantom = 25mm transverse slices with TLD capsules/pellets

skin D with TLDs (TLDs don’t intrude on radiograph) taped to pt skin; calibration with exposure to known K

ionization chambers coupled to electrometers display K of reproduced exposure setting

D at depth = ESD.PDD

PDD = percentage depth dose tables allow calculation of D at depth in tissue; don’t account for tissue inhomogeneity

ESD = entrance surface dose = ESAK x BSF

ESAK = entrance surface air kerma (calculated using inverse square law) = K.mAs.(d/SSD)2

mass energy absorption coefficient = correction for ESAK to tissue D (muscle = 1.06)

BSF = backscatter factor (depends on kVp, filtration, FOV)

foetal D ≈ 1/3 maternal ESD

iso-dose curves used to estimate D off the central axis

Monte Carlo calculation

individual photons traced with statistical variations of interactions and angles through a body simulating mathematical phantom

Dosimetry of Patients in Nuclear Medicine

most estimates depend of Monte Carlo calculation, assume activity accumulated and distributed homogeneously through an organ and vary with anatomical diff and physiological function

Dt = absorbed dose to target organ = Σss.S(t,s) (Σs = sum of D from all source organs)

s = total integrated/cumulated activity (MBq.h)= sum of all instantaneous activity over time = = A0 x 1.44 x T1/2

S(t,s) = S-factor (mGy.MBq-1.h-1) = D from source (s) to target (t) organ = Δ.φ(t,s)/m; Δ (J/Mbq.h) = total energy in that radiation type; φ = absorbed fraction of radiation emitted by s that is absorbed by t; m = mass of t

for β rays it can be assumed that all energy will be deposited in source organ

E in mSv; GIT RBC 5.2, DMSA 2.6, MAG3 3.9, V/Q 0.3+0.8, bone scan 4.2, FDG-PET 3.4

QA = Quality Assurance

system of act ivies to assure quality control job is being done effectively

primary goal to ensure effective and timely diagnosis; secondary goal to reduce radiation exposure

includes: education (retake analysis, biological effects, radiologist critical of technical quality), specification of equipment, acceptance testing, calibration, routine testing, evaluation of new products, record keeping (to observe trends)

benefits: reduction in retakes (reduces costs, dose, workload), improved quality of images, reduced dose, consistency between images, reduced non-scheduled down-time, reduced legal implications from failures

QC = quality control = performance of tests on equipment, should detect changes before they affect image quality or dose

QC in X-Ray

acceptance testing by medical physicist; complete testing by physicist every 1-3y

processing tests weekly (or daily for mammography): sensitometry = film exposed under light source with optical step wedge; densitometry measures base+fog, speed index (density 1.0 above base+fog), and contrast index (difference between density 2.0 above base+fog); trends are observed with upper and lower tolerances of the measurements

in CPH, weekly exposure of 2mm sheet of copper onto each CR plate for calibration

film-screen contact with fine copper mesh on cassette and checking for blurring

check of relative sensitivity of screens with a 10cm Perspex phantom and checking density on image

phantom images for total system quality measurement (breast and CT phantoms)

QC in Nuclear Medicine

need to ensure inference from contamination, other radioactive patients, hot laboratory is minimised

radiopharmaceutical preparation: correct isotope species (check for 99Mo breakthrough), low impurities, sterility, cell viability, absence of pyrogens

dose calibrator: ion chamber calibrated from few kBq to GBq (weekly checks with 137Cs to check temporal stability; monthly linearity and reproducibility checks)

imaging device: daily field uniformity check using flood field

computer processing: check of ADCs, for high count rates, dead-time losses and pulse pile-up, centre of rotation (computer centre must coincide within 1mm of true centre of rotation)

Electrical Safety

the heart is more sensitive to electrical currents at 50Hz (electrical f used in NZ); the high f cauterizes are not of any great risk

macroshock – many electrical paths through the body to the ground, tolerable up to 10mA @50Hz

microshock – direct transmission of current to the heart, tolerable up to 100μA

MEN = multiple earth neutral – earth used as a return path for electrons back to the source generator as well as the neutral wire (to halve power lost and increase efficiency); neutral wire connected to ground at both source generator and end-user and therefore electrons will pass through a person touching ground and active wire

Precautions to Prevent/Reduce Electrical Shock

isolating/separating transformer – separates connection with ground so fuse will blow if there is high ground current; circuit past this generator is not connected to ground (robust system)

RCD = residual current detector – measures current between active and neutral wires, so A-N >10mA (trip current; in home = 30mA) indicates loss through to ground (ie electrical shock) and power is disconnected

similar to RCD, but alarms are sounded instead of disconnecting power, to keep essential equipment running