Prime Factors Flashcards
What are 3 principle factors that affect xray quality and quantity?
Miliamperage per second (mAs)
Kilovoltage
Distance
They are under the direct control of the radiographer
What is quantity?
-Measure of the number of xray photons in the useful beam
-aka output, intensity or exposure
What is quantity prime controller ?
mAs and also affected by kVp,distance, and filration
What is quality?
Measure the penetrating ability of the xray beam or distance it travels in matter
High energy photons travel farther in…
tissue
What is the prime controller of quality?
Kvp and also directly affected by filtration
what does milliamperage per second (mAs) measure?
The xray tube current
What is the measure of the xray tube current?
(3)
-number of electrons crossing the tube from cathode to anode per second
-as mA increases, so does the number of electrons
-As time increases so does the number of electrons
mA and time are both directly proportional to the number of electrons crossing the tube and…
directly proportional to the number of xray photons created
mAs x s = or
tube current x exposure time
mAs
can adjust mA and time separately to result in
same xray exposure
IR exposure is assessed by reviewing numeric exposure value known as the…
Exposure Index (EI)
Exposure index does not indicate…
Patient dose
Index range and values vary from vendor to vendor
ex) S#
EX#
EI#
EI value must fall within a specified range
-any number over or under the range is an over or underexposed image
- May vary depending on the body part
-Some systems may be the inverse
mAs is the prime controller of…
image receptor exposure
There’s a direct proportional relationship between …
As long as mAs is constant, any combo of mA and time will create…
the same IR exposure
Increasing the kVp will cause an increase in…
the speed and energy of the electrons applied across the tube
As kVp increases so does…
the number of electrons interactions at the target (quantity)
15% increase in kVp=
double number of photons
Double kVp=
number of photons increases about 4 times
Greater photon energy=
more penetrability
kVp affects both…
quality and quantity of xray beam
An increase in kVp causes an increase in penetrability and decreases…
in image contrast
Contrast is..
A characteristic of image quality; visibility of detail or different between IR exposure levels
Contrast helps with the ability…
to distinguish the differences between tissues or structure
A primary beam with higher kV will result in an overall…
rise in penetration through tissues and will result in lower contrast radiograph or longer scale of contrast (more shades of grey)
What is the 15% rule?
an increase or decrease in kVp by 15% which will cause a doubling or halving mAs
kVp should not be used to control….
the IR exposure
At a lower kVp…
more radiation is absorbed by the patient
Higher kVp and lower mAs will reduce..
patient dose but at the expense of contrast
The intensity of the x-rays vary….
greatly with distance
What is inverse square law?
doubling of distance increases the area 4x’s which decreases the intensity by 1/4
Intensity is measure in?
Roentgen (R)
Increasing distance causes…
a decrease of exposure
When do we use Direct Sqaure Law?
to compensate for IR exposure changes when distance changes must be made
Setting the appropriate mAs, kvp, and distance is the responsibility of..
the radiographer
The radiographer should select the kvp based on…
the desired contrast
The radiographer should adjust…
mAs to provide the appropriate total exposure to the receptor
Distance is generally set based on the desired beam geometry
A distance is selected based on the procedure
