X-rays discovered in 1895 by Wilhem Roentgen

Roentgenology – Branch of medicine dealing with x-ray or gamma rays in diagnosis and treatment.

Radiology – Includes x-rays, but also:- Natural & artificial nuclides- CT- MRI- Ultrasound

Radiography – The art and science ofrecording x-ray images on a medium

Fluoroscopy – Observation of x-rays ona fluorescent screen to show anatomicalfunction and structure

The Electromagnetic Spectrum

All electromagnetic waves (including x-ray)travels at the speed of light 186,000 mi/sec(3 X 108 cm)

An X-Ray Tube Consists of:

1) Degassed Tube

2) Hot Filament (Negative)

3) Target (Positive)

4) High Voltage Across the Electrodes

5) Oil

Conditions Necessary for X-Ray Production

1) Separation of electronsAccomplished through:- Thermionic emission - Production of a space charge

2) Production of high speed electrons- Produced by applying high voltage from secondary of high voltage transformer- Current between electrodes measured in mA- Total current measured in mAS (mA X time)

3) Focusing of electron

4) Deceleration of electrons at the anode- 99.4% of electron energy given off as heat, .6% converted to x-ray

X-Ray Production

Electron Interaction at the TargetBremsstrahlung Radiation

Electron

Electron Interaction at the TargetCharacteristic Radiation

Target Material

Must meet 2 criteria:

1) High melting point

2) High atomic number

- Higher atomic number produces more Brems radiation- Higher atomic number also produces more penetrating characteristic radiation

Target material is primarily composed oftungsten (AN 74)- May be rotating or stationary- Molybdenum added in mammography targets & to dissipate heat in other targets- Rhenium added to increase elasticity of target

Target efficiency = K X Z X kVpFor tungsten – (1 X 10-4) X 74 X 80 - .6%

Properties of X-Rays

1) Highly penetrating and invisible

2) Electrically neutral

3) Polyenergetic

4) Liberates small amount of heat when passing through matter

5) Travel in straight lines

6) Ionize gases indirectly

7) Cause certain crystals to fluoresce

8) Cannot be focused by a lens

Properties of X-Rays

9) Travels at the speed of light

10) Affect photographic film

11) Produce chemical and biological changes

12) Produce scatter and secondary radiation

Specifications of the X-Ray BeamQuantity (Intensity)

• Amount of radiant energy flowing per second per unit area of surface

- Affected by both mA and time

• Roentgen (R) – A given amount of ionization of air particles produced by radiation

• Quantity (output) is measured in R/min

• Output = Exposure in R (R/Min) Time in Min.

• Total Exposure = R/Min (output) X Time

The Five Factors Controlling Output (Intensity)of the X-ray Beam

1. Target Material

2. Tube Current

3. Tube Potential

4. Distance

5. Filtration

Quality (Penetration) of the X-Ray Beam

Planck’s Quantum (Corpuscular) Theory – X-ray consists of packets of energy called photons that are electrically neutral & travelat the speed of light.E = hfE = Photon energyh = Planck’s constant (4.15 X 10-15 eV-s)f = Photon frequency in Hz

• Photon energy is directly proportional to frequency and inversely proportional to wavelength.

• Highly penetrating x-rays have high frequency and short wavelengths.

• High kVp produces x-ray with higher frequency, shorter wavelengths and provides increased penetration for large or high density anatomy.

- However, this is not a direct relationship.

Polyenergetic Vs Monoenergetic Radiation

• Polyenergetic radiation – - Stated in kVp

• Monoenergetic radiation – Stated in electron volts (eV) eV – The energy acquired by an electron when it is accelerated through a potential difference of one volt.100 kVp is approximately 30 – 50 kEv

Causes of polyenergetic x-rays

1. Fluctuating kVp

2. Process of x-ray production

3. Numerous interactions with target atoms

4. Off-focus radiation

Spectral Distribution Curves – Graphic representations showing the range of intensitiesof x-ray photons at given kVp’s

- Photon energy and intensity range increase with increasing kVp

Methods to Specify X-Ray Quality

Can be done by:1. Half-value layer – That thickness of material that will decrease radiation output by 1/2.

2. Applied kVp

Hard Vs Soft X-Rays

Hard – High energy, very Penetrating- Produced with high kVp & high atomic number filtration

Soft – Low energy, low penetration- Produced with low kVp and low atomic number filtration- Grenz Rays – 10 -20 kVp (very damaging to patient)

Interactions of Ionizing Radiation and Matter

Attenuation – A decrease in the number ofradiation photons per unit of travel in matter.

Attenuation occurs due to:1) Absorption

2) Emission of scatter and secondary

Scatter – Photons undergoing a change ofdirection

Secondary – Emission of characteristicradiation after an x-ray is absorbed byan atom.

X-Ray Interactions With Matter

Some Basics:• X-rays are photons

• Travel at the speed of light

• Are electrically neutral

• X-rays interact with electrons in atoms

• The binding energy is greatest for inner shell electrons

• Outer shell electrons have lower binding energies and can be removed more easily

Photoelectric Effect

•1.An x-ray gives up all of its energy to free an inner shell electron (usually k shell)2. Inner shell hole is filled by a higher level electron with emission of characteristic radiation3. Holes in successively higher shells are filled by electrons (cascading)

Key Points:• Occurs in absorbers of high atomic number• The major contributor to patient dose• The major contributor to image contrast• Characteristic radiation from photoelectric is a form of secondary radiation• Predominates below 150 kVp

Coherent (Unmodified/Thomson) Scatter

1. A very low energy photon strikes an outer shell electron, but does not dislodge it.

2. The incident photon undergoes a change of direction without change in wavelength, frequency or energy.

Key Points:• Occurs below the useful range of x-ray• Produces scatter radiation

Compton Interaction (Modified Scatter)

1. X-ray photon dislodges an outer shell electron- This electron is now known as a Compton (recoil) electron

2. X-ray photon is scattered- Has decreased energy- Energy of photon depends on angle of scatter (greater angle = less energy)

Key Points:• The major contributor to film fog & occupational personnel• Predominates above 150 kVp

Pair Production

1) A megavoltage photon interacts with the nucleus of an atom.2) This gives rise to a negatron (electron) and positron (positive electron).3) The negatron combines with other atoms in need of an electron.4) The positron combines with and annihilates an electron. The energy of this annihilation effect is carried off by two, .51MeV photons.

Points to Remember

• Occurs above the diagnostic range of x-rays• Becomes the predominant reaction above 24 MeV• Is used in PET to construct images• Positron decay from isotope causes pair production which is detected by PET detectors to form an image

Pair Production

Scatter and Secondary RadiationFrom Tissue Interactions

Primary electrons -Electrons that comes from inside the atom after contact with a radiation photon.1) Photoelectrons2) Compton (recoil) Electrons3) Positrons/Negatrons• These account for the absorption of radiation.

Secondary Radiation – Radiation that occurssecondary to an interaction of a radiation photonand atom (i.e., characteristic radiation)• Exception - Characteristic produced at target is considered primary radiation

Tissue Interaction Resulting Radiation

1) Characteristic (non-target) Secondary 2) Coherent Primary Scatter3) Modified Scatter Primary Scatter4) Annihilation Effect Secondary

Scatter Radiation – Occurs as a result of initial contact between radiation photon & atom. Includes

Summary

Photoelectric

• Predominates in the 25 – 150 kVp range•Is the primary contributor to image contrast and patient dose• Occurs in greater amounts in tissues with higher atomic number (i.e., bone Vs soft tissue)

Compton Effect

• Degrades image quality through the production of fog• Is the primary contributor to occupational personnel exposure• Predominates above 150 kVp• Comprises 80 – 90% of the beam in soft tissue

Pair Production

• Predominates at 24 mEv and above

Measuring Radiation

Roentgen (R) – Quantity of x-ray or gamma ray causing a certain amount of ionization in air. - 1 R = 2.56 X 10-4 coulomb/kg air - Applies only to x-ray or gamma rays

Linear Energy Transfer (LET) – Amount of ionizing energy transferred to tissue per unit of travel by radiation.

RAD (Gy) – Amount of ionizing energy absorbed per gram of irradiated tissue.- Equivalent to 100 ergs per gram- 1 Gy = 100 RAD- Applies to measuring dose from any type of radiation

Both LET and RAD effected by:1) Quantity of radiation (effected by mA & time)2) Quality (penetration)3) Nature of tissue4) Distance

Modification of the X-Ray BeamUsing Filters

1) Inherent Filtration – Filtration in the tube housing.

A) glass envelopeB) oilC) Window (port)

2) Added Filtration – Filtration outside the tube housing

A) Al/Cu filtersB) MirrorC) Collimators

Filtration Affects:1) Exposure rate (quantity) – Increased filtration will decrease exposure and image density and vice versa

- This is an inverse relationship2) Quality (penetration) Increased filtration (HVL) or increased atomic number of filter will increase the average penetration of the beam & vice versa

- Direct relationship- Increased filtration decreases contrast