Exposure Factors Chapter 4

Exposure Factors Are adjustable variables that can influence the diagnostic qualities of a radiograph. Proper selection of the exposure factors will lead to a decreased number of retakes = safer and more cost effective.

Exposure Factors Include X-ray Machine mA Kvp mAs FFD Collimation Technique Chart Patient Thickness of patient Pathological changes Movement Appliances Cassette Dark Room / Film Processing

Producing x-rays Electrical currents must be applied to both cathode filament and field between the cathode and the anode. What are these two applied currents called?

Quality and Quantity Quality of an x-ray beam is determined by its penetrating power. Higher frequency=greater penetrating power=shorter wavelength. Quantity (intensity) of x-ray beam is defined as the amount of energy flowing per second through a unit area perpendicular to the direction of the beam. Basically number of x-rays traveling from x-ray tube to image receptor in a period of time.

What affects Quality and Quantity? mA Time kV or kVp Distance

Milliamperes (mA) 1/1000 of an ampere Source of heat that is applied to filament causing electrons to start forming electron cloud. This process is called thermionic emission Once electrons are pulled away from atom, they start to travel to target spot on anode. Amount of electrons available is directly dependent on the heat applied to the filament.

Quantity continued Number of x-rays produced at the anode depends on the size of electron cloud. mA affects intensity of x-ray beam and is measure of quantity of x-radiation produced.

Time Time- total quantity of x-rays produced during a given exposure depends on the length of the exposure. There exists a direct relationship between the mA and the length of exposure (time). Exposure time- period during which the x-rays are permitted to leave the x-ray tube. Measured in fractions of seconds (1/60, 1/40, etc).

How many x-rays are available? Determined by the number of electrons and the period of time set for their release. Available quantity of x-rays is expressed as the product of the mA and the time. mA x time= mAs s = seconds

Advantages of high mA Allows for shorter time setting with the same number of x-rays produced. Shorter time, possibility of motion is decreased. Decreases the exposure of restraining personnel. Greater amount of x-rays produced. Allows examination of thicker anatomic areas.

mA continued Suitable mA settings depends on the thickness and type of tissue being radiographed. Machines with higher mA potential are more powerful and have increased diversity of use in practice. Smaller x-ray machines have a constant mA capability with no provision for alteration.

mAs mA & time are inversely related – but often combined settings on the x-ray machine As mAs increase the x-ray becomes blacker in color and as mAs decrease they become whiter

Kilovoltage Describes the electrical potential (or difference) between the cathode and anode. Responsible for accelerating the electrons from the cathode to the anode. Relates to the penetrating power of x-rays. Measured in kilovolts. kVp= maximum energy available at that kV.

Affects of changing kilovoltage. Changes penetrating power Increase kVp= Increase in penetrating power Increase kVp allows for lower mAs settings which call for shorter exposure time. Sante’s Rule- a method of estimating needed kilovoltage in relation to area thickness. 2 x thickness + 40= kVp

Examples of Sante’s Rule If measured thickness is 10 cm. (2 x 10) + 40 =60 kVp If measured thickness is 12 cm. (2 x 12) + 40 = 64 kVp Sante’s rule is for table top x-ray but can be modified to fit other conditions.

Why do we use 40? Represents the distance from the x-ray tube focal spot to image receptor (film) in inches. This distance can be referred to as the Focal Film Distance (FFD) or Source-image Distance (SID).

Distance Distance between x-ray source and image receptor affects the intensity of the image produced. Decrease in distance (SID or FFD)= Increase in intensity of x-rays. Increase in distance= decrease in intensity of x-rays. Decrease in distance= decrease in image sharpness

SID/FFD continued Should be kept constant. In vet practices, most likely will be 36-40 inches. Where can I find this measurement? The use of this knowledge aids with the preparation of a technique chart.