Factors affecting the X-Ray output Dr. Nahla Nagy

What is intensity? Quantity Quality Intensity: Total energy contained in the beam (product of quality and quantity of x-ray photons) per unit area per unit time Quantity is the number Quality is the energy Together = the intensity of the X-ray

X-Ray Quantity X-ray quantity: measure of the number of x-ray photons in the useful beam. AKA x-ray output, intensity, or exposure Unit Roentgen (R) Directly affected by: Miliamperage-second (mAs) Kilovoltage (kVp) Distance (d)

X-Ray Quality X-ray quality: : measurement of the penetrating ability of the x-ray beam. Describes the distance an x-ray beam travels in matter. High energy x-ray photons travel farther in matter- more penetrating Numerically represented by the Half-value layer (HVL).

Factors Affecting Intensity Factors affecting intensity of x-ray beam actually affect no. of photons or penetration power of photon in x-ray beam. Target Material X-ray Tube Current (Miliamperage- mA) Tube voltage (kVp) Exposure time (s) Filtration Collimation Source to film distance

Target Material Determines efficiency of X-ray production Higher the atomic number- greater will be the efficiency of the production of x rays

X-ray Tube Current (mA) Produced by flow of electrons from their point of origin (filament) to the anode (target) of x-ray tube A change in filament voltage of about 5% result in 20- 30% change in x-ray tube current (mA) Hotter filament emits more electrons and adds electrons to the current electrons (mA) across the x-ray tube Increased mA means increase in quantity of electrons available to hit the target -that results an increase in the quantity of x-ray photons

mA is directly proportional to quantity of x-ray beam y axis = number of X-ray photons (quantity) x axis = energy of X-ray photons (quality) Area under the curve = intensity (product of quantity and quality)

Exposure Time (s) 1) Increasing time (s) heats filament for a longer time. 2) Longer the filament is heated more electrons are produced. 3) Increase in quantity of electrons

mAs (miliamperage seconds) mA multiplied by s mA and exposure time have a direct effect on the quantity of X-ray photons produced mAs is the product of mA (current) and exposure time (seconds); determine total number of x-ray photons produced The miliamperage required for a given exposure is inversely proportional to the exposure time mAs Rule: Original mA x Original time = New mA x New time

(original mA x original time = new mA x new time) Example In an examination controls are set for 10 mA and time is 30 sec, what is the mAs? 10mA x 30s = 300 mAs If mA is increased to 15, how much time is required to produce same mAs? (original mA x original time = new mA x new time) 10mA x 30s = 15mA x Xs            X = 20 sec.

Tube Voltage (kVp) Increasing kVp increases potential difference between the cathode and anode. kVp controls the speed of the electrons, increasing kVp will increase the number of x ray photons produced per unit time Shorter wavelength and more penetrating photons of x-ray radiation are produced (i.e., increases speed) Increase in quantity and quality of x-rays. Quantity = number (speed) Quality = energy

Increasing kVp, increases both number and energy of x-rays Drastic increase in intensity (area under the graph)

Tube Voltage (kVp) Intensity is proportional to (kVp)2 In dentistry to maintain same density :rule of thumb Increase of 15 kVp requires halving the exposure time Decrease of 15 kVp requires doubling the exposure time If kvp doubled from 50 to 100-x ray intensity increases by factor of four

Filtration Filtration is the removal of low energy, less penetrating photons from x-ray beam by placing filter in the path of x- ray beam. Filters are aluminum sheets placed at the end of the tube where x-rays exit

Filtration Types of Filtration Inherent-X-ray tube and its housing Added-Sheets of metal placed in the path of the beam The patient

Filtration No. of photons decreas Mean (average) energy of photons increase Maximum energy is unchanged Effects quality and quantity of x-ray beam

Half value Layer Definition: The thickness of an absorber required to reduce the number of x-ray photons passing through it by one half of its original value In dentistry : 69 kVp/ below         HVL of 1.5mm Al 70-90 kVp               HVL of 2.5 mm Al

Collimation A collimator is a metallic barrier with an aperture in the middle used to reduce the size of the x-ray beam, thus, the volume of irradiated tissue within patient is also reduced Main Functions Protect the patient Decrease the scatter radiation

Collimation Reduces patient exposure Reduces scatter radiation Increases film quality

Round beam limiting device, 20 cm (8”) focus-film distance, 6 cm (2,36”) output field diameter.  Rectangular beam limiting device as an option to reduce patient’s exposure to radiation. www.jzimaging.com/SigmaBiomedics_Intraos_70.htm

Collimation Number of photons decreases Mean energy of photons is unchanged Maximum energy of the photons is unchanged

Source –film distance Inverse Square Law The x-rays of primary beam emerge from the protective tube housing not as parallel rays but as divergent rays Intensity of the x-ray beam decreases as the distance from the source increases Inverse Square Law The intensity of the X ray beam is inversely proportional to square of the source-film distance I1/I2 = (D2)2/(D1)2

Source –film distance Doubling the SFD distance ,exposure time will have to be increased by the factor of 4 to maintain the same density on the film

Inverse Square Law No of photons decreases Mean energy of photons is unchanged Maximum energy of the photons is unchanged

Review Factors affecting Quality kVp Factors affecting intensity Filtration Factors affecting intensity Tube voltage (kVp) Current (mA) exposure time (s) Filtration Collimation Source to film distance Factors affecting Quantity kVp mA Exposure time Filtration Collimation Distance

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