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CH-9 X-Ray Emission

Half-Value Layer (HVL) The thickness of absorber or homogenous filter that will reduce an X-ray beam to half its original value or intensity
Kinetic Energy Energy of motion
Quality (X-ray) - The penetrability or penetrating power of an X-ray beam - X-rays with high penetrability are termed high-quality or hard X-rays, and those with low penetrability are of low quality and are called soft X-rays.
Energy And Beam Quality As the energy of an X-ray beam is increased, the penetrability or quality of the X-ray beam is also increased
Kilovoltage - As kVp is increased, so is X-ray beam quality as well as the half-value layer. - Increased X-ray beam quality results in a more penetrating X-ray beam -kVp is the primary control of x-ray beam quality and therefore beam penetrability.
High kVp = lower subject contrast Long scale contrast (less difference between areas receiving varying amounts of radiation)
Low kVp = high subject contrast Short scale contrast (more black
white; more difference between areas receiving varying amounts of radiation) increases patient dose
Filtration Also expressed in terms of half-value layer As filtration is increased, beam quality increases but quantity decreases
Primary purpose of Flitration To selectively remove low-energy X-rays that have no chance of getting to the film Causes the X-ray beam to have nearly the same energy or be mono-energetic
Materials used as X-ray filter
Advantages of a FilterResults in X-ray beam with Results in X-ray beam with higher energy Greater penetrability Higher beam quality Increased in half-value layer
Inherent Filtration Filtration that is a result of the composition of the tube and housing
Added Filtration Placed between the protective tube housing and the external housing or collimator\ Has two sources that totals 2 and 3mm sheets of aluminum equivalent
Compensating filter • filters are used to compensate for differences in subject radiopacity used to even out widely differing tissue densities
wedge filter used to radiograph body parts that varies considerably, such as
Trough filter bilateral wedge filter) used mainly for chest radiography
Bow-tie-shaped filters used in some CT scanners to compensate for the shaped of the head or body
• Cone filters concave or convex shaped, and find application in digital fluoroscopy, where image intensifier tube and receptor are round
Step-wedge filter Used in some special procedures, where long sections of the anatomy are radiographed with two or three separate films. Used in rapid film changers for translumbar and femoral arteriography and venography.
Aperture diaphragm Flat sheet of metal, usually lead with a hole cut in the center and attached to the x-ray tube port.
Change in mA A change in mA or mAs means a change in the amplitude or quantity (exposure rate) of the X-ray emission spectrum at all energies No effect on beam quality (penetrating capability)
Increased kVp means higher energy X-rays Shorter wavelengths and higher frequency
Increased kVp means increased X-ray beam quality or penetrating capability, hence less patient absorption
Target Material Increasing target atomic number enhances the efficiency of x-ray production and the energy of characteristic and bremsstrahlung x-rays
Gray scale of contrast refers to the range of OD's from the whitest to the blackest part of the radiograph
high contrast short scale low kVp
low contrast long scale high kVp
kVp Controlling factor of contrast Change of approximately 4% in kVp is required to usually affect the scale of contrast in the 50 to 90 kVp range
15% rule • used to change scale of contrast from long to short or vice versa. • change the kVp by 15 % while changing the mAs by one half or double to produce the same OD
5% rule • a smaller technique compensation for a change in contrast scale • an increase of 5% in kVp may be accompanied by a 30% reduction in mAs to produce the same OD
mAs although mAs affects x-ray quantity, it still influences contrast if the mAs is too high or too low, the predominant OD will fall on the shoulder or toe of the characteristic curve
Intensifying screens results in shorter scale of contrast compared with non-screen
Collimation removes some scatter radiation producing radiographs of shorter scale of contrast
Anti-scatter grids also reduces scatter
Radiographic Detail the sharpness of appearance of small structures the sharpness of image detail refers to the structural lines or borders of tissues in the image and the amount of blur
Penumbra is the imperfect, unsharp shadow surrounding the distinctly sharp are of a shadow called the umbra
As the FS size decreases the penumbra decreases, thus increasing resolution
Source Image Distance (SID) Resolution is improved when SID increases and degraded when it decreases
Object Image Distance (OID) The minimum OID should be used to improve detail Minimum OID is obtained with non-Bucky procedures that place part directly on the cassette
collimation reduces scatter that degrades radiographic image
grids reduces scatter
Types of motion • voluntary - motion under the direct control of the patient • involuntary - not under the conscious control of the patient equipment motion - reciprocating grids can cause vibrations of the cassette, overhead tube suspension system can drift
Methods of reducing motion Communication Exposure time reduction Immobilization
SID • the greater the SID, the smaller the magnification • 40" (100cm) is the current routine SID
OID • critical distance in both magnification and resolution • OID must be minimized to decrease magnification
Calculating size distortion • magnification or size distortion can be assessed by calculation of the magnification factor • M = SID/SOD where M = magnification factor
Shape distortion The misrepresentation by unequal magnification of the actual shape of the structure being examined
elongation the anatomical part appears bigger than normal. Due to poor alignment of the IR or the x-ray tube
foreshortening the anatomical part appears smaller than normal due to poor alignment of the anatomical part • changes in the tube angle cause elongation but never foreshortening
What term refers to the removal of low-energy x-rays from the useful beam? Filtration
What term refers to the thickness of the absorber needed to cut the original intensity of an x-ray beam to half its original value or intensity? Half-value layer
Increased x-ray beam quality results in what type of x-ray beam? More penetrating x-ray beam
What is the main controlling factor of x-ray beam quality? kVp
What is used to selectively remove low-energy x-rays from the x-ray beam? Filters
What is the main controlling factor of x-ray beam quantity? mAs
Filtration Removal of low-energy X-rays from the useful beam with aluminum or other metal
Created by: nastronomical