Resident Physics Lectures (Year 1) Christensen, Chapter 5 Attenuation George David Associate Professor Medical College of Georgia Department of Radiology

Beam Characteristics Characteristic Radiation Bremsstrahlung Discrete energies Characteristic of target material Bremsstrahlung Energy range 0 – kVp selected

Beam Characteristics 1, 2, 3, ... ~ ~ ~ ~ ~ Quantity number of photons in beam 1, 2, 3, ... ~ ~ ~ ~ ~

Beam Characteristics 1 @ 27 keV, 2 @ 32 keV, 2 at 39 keV, ... ~ ~ ~ ~ Quality energy distribution of photons in beam 1 @ 27 keV, 2 @ 32 keV, 2 at 39 keV, ... ~ ~ ~ ~

Beam Characteristics ~ ~ ~ ~ ~ ~ ~ ~ Intensity 324 mR Intensity weighted product of # & energy of photons depends on quantity Quality Beam intensity can be quantified by ionizations Meter reading in milliRoentgens (mR) ~ ~ ~ ~ ~ ~ ~ ~

So what’s a Roentgen? Unit of measurement for amount of ionizing radiation that produces 2.58 x 10-4 Coulomb/kg of air @ STP 1 C ~ 6.241509324×1018 electrons

Beam Intensity Can be measured in terms of # of ions created in air by beam Valid for monochromatic or for polychromatic beam 324 mR

Monochromatic Radiation (Mono-energetic) Radioisotope Not x-ray beam all photons in beam have same energy attenuation results in Change in beam quantity no change in beam quality # of photons & total energy of beam changes by same fraction

Monochromatic radiation beam Attenuation Formula N = No e -mx where No = number of incident photons N = number of transmitted photons e = base of natural logarithm (2.718…) m = linear attenuation coefficient (1/cm); property of energy material x = absorber thickness (cm) No N x Monochromatic radiation beam

Monochromatic radiation beam If x=0 (no absorber) N = No e -mx where No = number of incident photons N = number of transmitted photons e = base of natural logarithm (2.718…) m = linear attenuation coefficient (1/cm); property of energy material x = absorber thickness (cm) No N X=0 Monochromatic radiation beam

Monochromatic Radiation Let’s graph the attenuation of a monochromatic x-ray beam vs. attenuator thickness 60% removed 40% remain Monochromatic radiation beam

Monochromatic Radiation Yields straight line on semi-log graph 1 .1 .01 .001 Fraction (also fraction of energy) Remaining or Transmitted N / No = e -mx 1 2 3 4 5 Attenuator Thickness Monochromatic radiation beam

Polychromatic Radiation (Poly-energetic) X-Ray beam contains spectrum of photon energies highest energy = peak kilovoltage applied to tube mean energy 1/3 - 1/2 of peak depends on filtration

X-Ray Beam Attenuation reduction in beam intensity by absorption (photoelectric) deflection (scattering) Attenuation alters beam quantity quality higher fraction of low energy photons removed Beam Hardening Higher Energy Lower

Monochromatic radiation beam Half Value Layer (HVL) absorber thickness that reduces beam intensity by exactly half Units of thickness value of “x” which makes N equal to No / 2 HVL = .693 / m N = No e -mx Monochromatic radiation beam

Half Value Layer (HVL) Indication of beam quality Valid concept for all beam types Mono-energetic Poly-energetic Higher HVL means more penetrating beam lower attenuation coefficient

Factors Affecting Attenuation Energy of radiation / beam quality higher energy more penetration less attenuation Matter density atomic number electrons per gram higher density, atomic number, or electrons per gram increases attenuation

Polychromatic Attenuation Yields curved line on semi-log graph line straightens with increasing attenuation slope approaches that of monochromatic beam at peak energy mean energy increases with attenuation beam hardening 1 .1 Polychromatic Fraction Transmitted .01 Monochromatic .001 Attenuator Thickness

Applications As photon energy increases subject (and image) contrast decreases differential absorption decreases at 20 keV bone’s linear attenuation coefficient 6 X water’s at 100 keV bone’s linear attenuation coefficient 1.4 X water’s

Scatter Radiation NO Socially Redeeming Qualities no useful information on image detracts from film quality exposes personnel, public represents 50-90% of photons exiting patient

Scatter Factors An increase in any of above increases scatter. Factors affecting scatter field size thickness of body part kVp An increase in any of above increases scatter.

Scatter & Field Size Reducing field size causes significant reduction in scatter radiation II Tube X-Ray II Tube X-Ray One of the most effective ways of minimizing operator exposure is to reduce field size through collimation. Even a relatively small reduction in field size can often result in a substantial reduction in operator exposure. This occurs for two reasons. The first is that a smaller beam irradiates a less volume of tissue so that there is less tissue to act as a scatter radiation source. Secondly reducing beam size means that scatter radiation must travel further through the patient before exiting. The increased travel distance means a less intense scatter field for the operator. A fluoroscopist should always collimate the x-ray beam to a size no larger than is required clinically.

Field Size & Scatter Field Size & thickness determine volume of irradiated tissue Scatter increase with increasing field size initially large increase in scatter with increasing field size saturation reached (at ~ 12 X 12 inch field) further field size increase does not increase scatter reaching film scatter shielded within patient

Thickness & Scatter Increasing patient thickness leads to increased scatter but saturation point reached scatter photons produced far from film shielded within body

Scatter Management Reduce scatter by minimizing field size thickness within limits of exam thickness mammography compression

Scatter Control Techniques: Grid directional filter for photons Increases patient dose

Grid Structure

Purpose Directional filter for photons Ideal grid passes all primary photons photons coming from focal spot blocks all secondary photons photons not coming from focal spot Focal Spot “Good” photon Patient “Bad” photon X Grid Film

Scatter Control Techniques: Air Gap Gap intentionally left between patient & image receptor Grid not used Grid Air Gap Patient Patient Air Gap Grid Image Receptor