Medical Imaging X-Rays I

Principle of X-ray A source of radiation

Principle of X-ray A source of radiation A patient of non uniform substance

Principle of X-ray A source of radiation A shadow A patient of non uniform substance

Principle of X-ray A source of radiation

X-ray tube Working Principle: Accelerated charge causes EM radiation: Cathode filament C is electrically heated (VC = ~10V / If = ~4 A) to boil off electrons Electrons are accelerated toward the anode target (A) by applied high-voltage (Vtube = 40 – 150 kV); Deceleration of electrons on target creates "Bremsstrahlung" evacuated gas envelope filament VC, If + A - C - +

X-ray tube Cathode Filament (-) Tube (vacuum) Coil of tungsten wire High resistance in coil ->temperature rise to > 2200oC Thermionic emission of electrons Tube (vacuum) Typical: Vtube = 40 – 150 kVp, Itube = 1-1000mA evacuated gas envelope filament VC, If - + - - - - - - A - - - C space charge stops further emission kVp, Itube - +

X-ray tube Anode Tungsten (high atomic number Z=74) Electrons striking the anode generate HEAT and X-Rays In mammography ->Molybdenium (Z=42) and Rhodium (Z=45) Stationary anode-> tungsten embedded in copper Rotating anode (3000 to 10,000rpm) -> increase heat capacity, target area evacuated gas envelope filament VC, If - + - - - - - - A - - - C space charge kVp, Itube - +

XRAY PRODUCTION

X-RAY production X-ray tube produces two forms of radiation Bremsstrahlung radiation (white radiation) Characteristic radiation

White radiation, Bremsstrahlung (Brake) Inelastic interaction with atoms nuclei Loss of kinetic energy Xray (E) = lost kinetic E X-Ray High kinetic energy Forward radiation Emission  Z2 electron Coulombic interaction (Atomic number) # of protons

White radiation, Bremsstrahlung -Smaller L produce larger X-ray -Broad range of emitted wavelengths X-Ray L

How many wavelength will be emitted by a beam of electrons underegoing “Bremsstrahlung ”

White radiation, Bremsstrahlung -Smaller L produce larger X-ray -Broad range of emitted wavelengths X-Ray L impact with nucleus maximum energy

X-ray intensity -QUANTITY Overall Bremsstrahlung intensity I : 90% of electrical energy supplied goes to heat, 10% to X-ray production X-ray production increases with increasing voltage V

Bremsstrahlung spectrum relative output Theoretically, bremsstrahlung from a thick target creates a continuous spectrum from E = 0 to Emax Actual spectrum deviates from ideal form due to Absorption in window / gas envelope material and absorption in anode Multienergetic electron beam Peak voltage kVp

Characteristic radiation relative output Energy must be > binding energy Discrete energy peaks due to electrons transitions Ka transition L->K Kb transition M,N,O->K Peak voltage kVp

Characteristic radiation Incident electron

Characteristic radiation l2 Incident electron Occurs only at discrete levels There is a possibility of forming Auger electrons

Characteristic radiation In Tungsten characteristic X-ray are formed only if V>69.5 kV because K shell binding energy is 69.5 keV Molybdenum K-shell can be obtained at V> 20kV L shell radiation is also produced but it’s low energy and often absorbed by glass enclosure

X-ray intensity -QUALITY Effective photon energy produced Effective = ability to penetrate the patient Effective photon energy ~ 1/3 to ½ of energy produced Higher energy better penetration Beam filtration – beam hardening

Beam Hardening Polyenergetic beam ------------------------------->monoenergetic beam

X-ray tube construction

Anode Most of the energy deposited on the anode transfers into heat

Reduction of anode heating Made of Tungsten, high melting point high atomic number Z = 74 Kinetic energy of incident electrons 100keV electron 6 MeV electron

Anode the target angle, 7 to 20 (average 12) Seffective = Sactual*sin() -----------> Line focusing principle

Anode filament balance General radiography

Heel effect - SID source to image distance - Heel effect is smaller at smaller SID Reduction of intensity on the anode side SID The reduction in intensity can be used to reduce patient exposure

Beam collimation Size and shape of the beam Lead shutters Dose reduction

Reduction of anode heating Anode angle of 7º…15º results in apparent or effective spot size Seffective much smaller than the actual focal spot of the electron beam (by factor ~10) Rotation speed ~ 3000 rpm Decreases surface area for heat dissipation from by a factor of 18-35.

Limitations of anode angle Restricting target coverage for given source-to-image distance (SID) "Heel effect" causes inhomogeneous x-ray exposure

X-ray tube - space charge Space charge cloud forms at low tube voltage At low filament current a saturation voltage is achieved, rising tube voltage will not generate higher electron flow At high filament current and low tube voltage, space charge limits tube current->space-charge limit

Space charge limited At high filament current and low tube voltage, space charge limits tube current->space-charge limit

Generator Single phase Three phase Single phase input (220V, 50A) Single pulse or double pulse->rectifier Min exposure time 1/120 sec Xray tube current non linear below 40kV Three phase Three phase wave, out of phase 120 deg More efficient higher voltage Better control on exposure

Rectifier Protects cathode from anode thermionic emission

Rectifier 1 phase 3 phase

BREAK!

Principle of X-ray A source of radiation A patient of non uniform substance

Attenuation N = Noe-mL N True for Loss of photons by monoenergetic x-ray Loss of photons by scattering or absorption L1 L No N m -> linear attenuation coefficient L1

m linear attenuation coeff. m = mr+ mph+ mc+ mp [cm-1] rayleigh photoelectric Compton pair

m linear attenuation coeff. m = mr+ mph+ mc+ mp [cm-1] depends on tissue soft tissue, hard tissue, metals m decreases when energy increase soft tissue: m = 0.35  0.16 cm-1 for E = 30  100keV m depends on density of material mwat > mice> mvapor

Mass attenuation coeff.

Mass attenuation coeff. N = Noe- r(m/r)L rL = mass thickness

Mass attenuation coeff. N = Noe- r(m/r)L rL = mass thickness I x

Poly-energetic beam Mass attenuation coefficient and linear attenuation coefficient are for mono-energetic beam Half-value layer is for quantifying poly-energetic beams

HVL half value layer Thickness of material attenuating the beam of 50% - narrow beam geometry HVL for soft tissue is 2.5  3.0 cm at diagnostic energies

HVL half value layer Transmission of primary beam: 10% chest radiography 1% scull radiography 0.5% abdomen radiography Mammography (low energy HVL 1 cm)

Mean free path 1/m Average distance traveled before interaction MFP=1/m HVL mfp

Principle of X-ray A source of radiation A shadow A patient of non uniform substance