BME 311 Lecture 4

absorption of energy and radiation dose

• fluence Φ = photons/area (cm^-2) passing through a surface

• fluence rate = flux = (photons/area)/time

• energy fluence ψ = energy/area = (photons/area) x (energy/photon) = ΦE (keV/cm²)

exposure - amount of electrical charge (ΔQ) produced by ionizing radiation per mass of air exposure ; X = ΔQ/ Δm (C/kg) – historical unit roentgen (R)

• 1 R = 2.58 × 10^-4 C/kg

• defined because it’s easy to measure for a beam, used in calibrating instruments

• not as meaningful for understanding biological effects

equivalent dose

• dose modified to reflect the relative “effectiveness” of the type of radiation in producing biologic damage

• radiation weighting factor (w_R) established by International Commission on Radiological Protection (ICRP)

• H - W w_R

• SI unit = sievert (Sv)

effective dose

• may vary across body

• want single number to characterize risk to whole body

• not all tissues equally sensitive to effects of ionizing radiation

• tissue weighing factor (w_T) established to assign a particular organ or tissue (T) its vulnerability to radiation

effective dose = sum of products of equivalent dose to each organ or tissue irradiated and corresponding weighting factor

• most related to increase in cancer risk = ~0.004%/mSv

• typical radiology exam 1-10 mSv, max worker = 50 mSv annually

• E(Sv) = ΣwT x HT (Sv)

risk estimates assume most conservative model

• cumulative, linear, no-threshold

• data from A-bomb in Japan

• small doses (<50mSv) not proven to be harmful

• assumptions are to be on safe side

radiation risks in perspective

• effective dose is most closely related to increase of overall lifetime cancer risk = ~0.004%/mSv

• radiological procedures ~10 mSv means increase from ~20% normal lifetime risk of cancer to 20.04% (1 in 2500 increase)

• radiation workers allowed 50 mSv max/year

x-rays are generated in an x-ray tube, which consists of

• a vacuum tube with a cathode and anode;

  • cathode current for thermionic emission (~5A)

  • tube voltage for electron acceleration (20 kV-150kV)

  • kinetic electron energy gained = qV so in units of keV (20 keV to 150 keV);

  • resulting “tube current” from cathode to anode (~100mA)» what produces x-rays

heat dissipation electron-anode interaction

• a high speed electron can collide with an outer shell electron

  • secondary electron is the finally dissipated into heat

• most electron beam energy is lost this way

• cooling of anode is essential

bremsstrahlung (braking) radiation electron-anode interaction

• interaction with a nucleus (much larger than an electron)

• covers entire spectrum, and energy of bremsstrahlung photons is bounded by E≤ Emax = qV, e.g., for V=90 kV, Emax=90 keV

• continuous spectrum, keV is convenient unit

characteristic radiation electron-anode interaction

• an electron can collide with an inner shell electron (eg. rom the K shell) and eject it, leaving a hole

• this hole is refilled when an electron of higher energy (eg. from L shell) drops into the hole while emitting a photon of a very specific energy

• the energy of this photon is the difference between the two electron state energies

• such transition yields characteristic peaks in the x-ray spectrum

• K series, L series

characteristic x-rays can be useful too

x-ray tubes

tube current vs. filament current

• filament current determines temperature - rate of thermionic electron emission

• small change in filament current can produce relatively learge change in tube current

• tube current determines x-ray intensity

• saturates/plateaus

  • electrons usually considered independent of each other

  • but at high cathode currents, electron cloud builds around the filament, called “space charge”, repelling electrons from each other

fixed anode

• copper supports tungsten target and removes heat

• small target area limits heat dissipation rate and thus, limits maximum tube current, x-ray flux

• dental x-ray units, portable x-ray units use fixed anode x-ray tube

rotating anode

• allow greater heat loading and consequently higher x-ray output

• rotation speed - 3000 - 10000 rpm

• still gets hot - lots of thermal engineering here

• CT scanners use these

x-ray tube housing

• supports, insulates and protects x-ray tube insert from the environment

• lead shielding inside the housing attenuates x-ray emitted in all directions

• hole in the shield at x-ray tube port

collimators

• adjust size and shape of x-ray field emerging from the tube port

• bea, of light reflected by the mirror mimics the x-ray beam - identifies the collimation from the shadow

x-ray generator components

x-ray generator, x-ray tube

power rating of an x-ray tube

• is the maximal power that an x-ray tube focal spot can accept or the generator can deliver

  • power = tube voltage x tube current

  • so for 100 mA = 0.1 A tube current at 100 kV

  • power = 100 kV * 0.1 A = 10kW but for short bursts

factors affecting x-ray fluence

• target material (anode)

  • mainly proton number as Z²

• tube current (mA) which depends on

  • cathode current

  • tube voltage as kVp²

• exposure time (sec)

  • the duration of x-ray production (sec)

  • often combined with tube current as mAs = milliamp-seconds ~ number of electrons used

  • mAs most useful for operator/technologist since they control it

• beam filtration

x-ray quantity = fluence

depends on Z² x mAs (which is a function of kVp² and cathode current and exposure time)

filtration

• when the x-ray beam is produced, many energies of photons exist. many are of such low energies that they will offer nothing to the production of the radiograph

  • metals such as aluminum will absorb the soft low energy rays

• the purpose of the filtration is to reduce the patient exposure and “beam hardening” which causes artifacts in CT imaging

inherent filtration

thickness (1-2 nm) of glass at x-ray tube port

additional filtration

• added glass and aluminum - effectively attenuate x-rays below 15 keV

• other common filter materials - copper and plastic (acrylic)

beam hardening

• low energu (soft) x-ray will not penetrate most tissues

• soft photons preferentially removed, leaving more “hard” ones

• shift to higher average enrgy as the beam traverses matter - beam hardening

• beam intensity drops, but effective energy increases

• filtering results in x-ray beam with higher effective energy/HVL

• when it occurs in the body, can cause artifacts in CT imaging

beam hardening vs beam filtering

changing shape of energy spectrum by preferential absorption of low energy photons in the beam

hardening

• occurs in patient

• not desirable in CT because image reconstruction algorithms assume that beam energy does not change as it goes through the body

• mitigate by narrowing spectrum of x-rays going into patient

filtering

• purposely added material in front of beam to change shape of spectrum

• removes lower energies which add dose to patient but don’t contribute to image (most would be absorbed by body)

• can also remove high energies that have poor contrast for the imaging situation (like mammography)

• reduces beam hardening effect in body - like pre-hardening the beam