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Exposure = Exposure rate x Time
EXAMPLE: If the exposure rate = 225 milliroentgen (mR)/h, whatwould be the total exposure after 36 minutes?
225 mR/h x 36/60 = 225 x 0.6 hours= 135 mR
Inverse Square Law
New Intensity = Old Distance^2
Old Intensity New Distance^2
what will it be at 90 inches?
If the intensity at 30 inches is 1.3 roentgen (R /min),
x = 30^2 ; x=1.3(1/9) ; x = 0.14R/min
(1) at least .5 mm lead (Pb)
(2) 1.0 mm Pb recommended
b. Tenth-value layer (TVL) - ≅ 3.3 Half-value layers (HVLs)
affect the offspring of exposed individuals. Examples: leukemia alteredsex ratios, congenital effects, etc.
affect only exposed individuals. Examples: whole-body syndromes,carcinogenesis, etc.
the risk of contracting effects increases as the dose is increased anddoes not exhibit a threshold
the severity of the effect increases as the dose isincreased, also known as non-stochastic effect. Examples: hematologic deficiencies,cataracts, skin erythema, etc.
Radiosensitive tissues are very easily damaged by radiation.
Radioresistant tissues are not easily damaged by radiation.
LET is the rate at which energy (radiation) is deposited per unit length of travel.LET is measured in units of kiloelectron volt (keV)/micrometer (um).
As the LET of radiation increases, the RBE increases. Its relationship is
Direct damage to cells...
accounts for 95% of damage to the cell by radiation. Photons interactwith smaller molecules, such as H2 O, and create free radicals, which in turn damage thesensitive macromolecules (DNA).
Acute radiation syndrome – conditions for experiencing:
Entire body irradiated
Radiation is delivered within minutes to hours
Radiation dose is high and penetrating
Aprons – should be a minimum of .50 mm Pb equivalent
Gloves, bucky slot cover, curtain – should be a minimum of .25 mm Pb equivalent
Primary shield – any wall or floor exposed to the primary beam, 1/16 in. Pb equivalent,
extending 7 1/2 ft up the wall.
Secondary shield – any wall exposed to scatter radiation (ceiling), 1/32 in. Pb equivalent.
Compton Scatter radiation
The number of electrons in each shell follows this formula: 2n2 (n = the
shell in relation to its place in the atom).
K-shell (inner shell) is closest to the nucleus and holds a maximum of two
electrons. It is given the number 1: 2 (1)2 = 2.
L-shell (#2 shell, second in relation to the nucleus) holds a maximum of 8
M-shell holds a maximum of 18 electrons.
N-shell holds a maximum of 32 electrons.
An atom becomes chemically stable when there are
n a neutral atom, the total number of orbital electrons equals the exact.
An atom of a given element maintains its identity only if its nuclear charge
(number of protons) is
A proton is about 2000 times heavier than an electron. The total number
of protons and neutrons in the nucleus determines its mass number; knownas the
Ionization is the addition or removal of orbital electrons.
• Cations – positive ions (missing an electron)
• Anions – negative ions (gain an extra electron)
Direct Current (DC)
is a current that fluctuates between themaximum potential difference of positive and negative. Transformers operate only on AC.
Rectification is the conversion of AC to pulsating DC, which is essential for x-rayproduction. X-ray circuits use AC circuits in order to generate the high voltage neededto produce x-radiation. However, x-ray tubes should never allow AC to flow through thetube because it may cause permanent damage.
Therefore, a rectification circuit isrequired between the production of the high voltage in the circuit and the x-ray tube.Usually made of semi-conducting materials, such as solid-state diodes (silicon orgermanium).
Ohm’s Law states that the potential difference in a circuit is equal to the current times the
Series circuit is
Parallel circuit is
Motor is a device that
Synchronous motor – must be synchronous with the speed of the generator.
Induction motor – has a stator, along with a rotating part called a rotor in the center. Used in the x-ray tube to rotate the anode target for better heat dissipat
Transformer is a device used to
The high tension or step-up transformer receives its input voltagefrom the
The filament transformer is a
Capacitor is a device used to
Rheostat is a
Produces a thermionic cloud of electrons.
Conducts the high voltage to the gap between the cathode and the anode.
3. Focuses the electron stream to the anode.
Filament is a
Filament is a small coil of thoriated tungsten wire (1% to 2% thorium, whichhelps increase efficiency and prolong tube life). Tungsten’s high melting point(33700 C) makes vaporization (turning into a gas) difficult.
Focusing cup is a
1. Serves as a target surface for the high voltage electrons.
2. Source of x-ray photons (production).
3. Conducts (path for) the high voltage from the cathode back into the x-raygenerator circuitry.
4. Serves as the primary thermal conductor.
Stationary anodes (dental units) are
made of rhenium-alloyed tungstenembedded in a 45o-angled end of a copper rod.
Rotating anodes have a
Rotating anode disks range from
Target area is the area on the anode
Focal track describes
Focal spot refers to the area of the focal track that is struck by theelectron beam at one particular time.
Line-focus principle -
Located outside the glass vacuum, the stator consists of the
induction-motor electromagnets that are responsible for turning the anode.Whenever the rotor switch is depressed, a current is sent to the stator to turnthe anode.
The rotor is located inside the stator and inside the glass envelope. It is made up of a hollow copper cylinder or cuff that is attached to the anode disk by amolybdenum shaft. This cuff, or stator, is the device affected by theelectromagnets and is responsible for spinning the anode. Anodes commonlyrevolve approximately 3000 to 3600 rpm. High-speed anodes (10,000 to 12,000rpm) dissipate heat faster.
Made of Pyrex glass approximately 10 inches in length, 6 inches in diameter atthe center, and approximately 3 inches at the ends. The window at the bottom of the envelope where the radiation exits, is usually a thinner section of glass to allow for lessabsorption or scatter of photons.
A vacuum environment helps to increase the efficiency of the tube so there is nointeraction with gas atoms.
Pre-reading voltmeter indicates
the voltage that is selected and is placedbetween the autotransformer and the high-voltage transformer
Automatic exposure control (AEC) is
High-voltage transformer (step-up transformer) operates on
alternating currentand the principal of mutual induction. Consists of primary and secondary coils.
Four-diode unit is
Six or twelve diodes are used
High-frequency generators result in
mA meter measures
Quantity is a measure of the number of x-ray photons in the useful beam, output, intensity, orexposure. Factors affecting quantity are mA, time, mAs, kVp, distance, and filtration.
Quality is the measurement of the penetrating ability of the beam and the distance the beam travelsin matter. High energy is produced by hard x rays that travel farther and are more penetrating.Low energy is produced by soft x rays that travel less and are less penetrating. Factors affecting thequality are kVp and filtration.
Milliamperage-seconds (mAs) is directly proportional to quantity. It is a measurement of the x-raytube current, or the number of electrons crossing the tube. Increasing mA increases the number ofelectrons. mA is directly proportional to the tube current.
2 mA = 2 x the number of electrons.....
The length of time is also directly proportional. X-ray current is measured in milliamperes (1milliampere = 1/1000 of an ampere). mA ranges are from 25 to 1500. The time of exposure rangesfrom .001 second (1 ms) to 8 to 10 seconds. (1 millisecond = 1/1000 of a second, 1000 milliseconds= 1 second).
A larger collimation field size producesmore scatter and therefore more density
Developer temperature and density aredirectly proportional
Density relationship to kVp (15% kVp Rule)
An increase of 15% in kVp is equivalent to doubling the mAs in terms of radiographic density.
A decrease of 15% in kVp is equivalent to halving the mAs in terms of radiographic density.
Direct square law – to compensate for loss of radiation intensity when increasing distance from asource, increase the mAs as follows:
when distance from a source to the image receptor is doubled,the factor of mAs will need to be quadrupled in order to compesate for the loss of radiographic density. 2 x D = 4 x mAs
Beam restrictors include aperture diaphragms, cones, cylinders,and collimators. The larger thebeam size, the higher the radiographic density on the image.
Additive effects tend to decrease radiographic density and require an increase in normal technicalfactors. They are associated with:
pneumonia ,pleural effusion , Paget’s diseaseosteopetrosis, ascites, sclerosis, atelectasis, cirrhosis, abscess, acromegaly
Destructive effects tend to increase radiographic density and require a decrease in normal technicalfactors. They are associated with:
osteoporosis, pneumothorax, osteogenesis, imperfectagangrene, aseptic necrosis, atrophyemphysema, bowel obstruction, gout
6) The smallest area of collimation equals the least density
7) The greatest development time, temperature, and replenishment rate of the processor equals the greatest radiographic density.
8) Anode heel effect. Be sure that the cathode is always placed at the thickest body part to take advantage of the cathode end of the tube producing the greatest radiation intensity.
Radiographic contrast refers to the extreme differences between the blacks and whites of an image,appearing as a series of shadows of different densities
Long-scale contrast (low contrast) has many subtle shades of gray and occurs with higher kVpranges.
Short-scale contrast (high contrast) is black and white and occurs with lower kVp ranges.
kVp controls the primary differences in radiographic densities known as contrast. Increase in kVp =increase in penetrability = image with less contrast.
Diagnostic ranges are from 40 to 150 kVp in one to two increments. As the kVp increases, the x-raywavelength becomes shorter and more penetrating. Because kVp is the penetrating power, there must be enoughpenetration to move the x-ray beam through the object to carry the information across to the film.
3-phase generator – 2 x cm thickness + 25 = kVp
1-phase generator – 2 x cm thickness + 30 = kVp This formula can be used for all radiographs, except chest radiography, which typically requires higher kVp because of the high inherent subject contrast of the chest.
Whenever kVp is increased, more remnant radiation, as well as scattered and secondary radiation,reaches the film. Developed film with more grays appears flat or dull, providing better visibility of manystructures of varying tissue density and thickness. At first glance, high contrast, black and white film, is morepleasing to the eye.
Longer-scale contrast is recommended because it provides better visualization and thepatient receives less radiation exposure because of the use of lower mAs at higher kVp.
kVp is the penetrating power. When kVp is increased, more remnant radiation, as well as scatteredradiation and secondary radiation, reaches the film.
An increase in filtration produces a beamwith more energy, which producesmore scatter
Developer temperature changes usually reduce contrast
Formula to lengthen scale of contrast
Increase kVp by 15% and decrease mAs by 50% in order to lengthen scale.
Original 80 kVp, 30 mAs
80 kVp x 15% = 12; 12 + 80 = 92 kVp50% of 30 mAs = 15 mAs
New technical factors = 92 kVp, 15 mAs
Formula to shorten scale of contrast
Need to decrease the kVp by 15%, increase mAs by 2 times
Example: Original 94 kVp, 15 mAs
94 x 15% = 14.1; 94 -14 = 80 15 mAs x 2 = 30 mAs
New technical factors = 80 kVp @ 30 mAs
Original Technique: 200 ma, 1/10 s. 66 kVp
Now lengthen the scale of contrast:
Determine mAs first 200 x 1/10 = 20 mAs,
Then reduce mAs by 50%: 20 x 0.5 = 1066kVpx15%=9.9; 66+10=76kVp;
New technical factors = 76 kVp @ 10 mAs
Original Technique: 400 ma, 0.02 s , 72 kVp
Shorten the scale of contrast and use 200 ma:
15% of 72 = 10.8 (11); 72 - 11 = 61 kVp
Original mAs: 400 x .02 = 8;
Then increase mAs by 100% or by a factor of 2; new mAs: 8 x 2 = 16;16 divided by 200 ma = .08 s;
New technical factors = 61 kVp, 200 mA @0.8 seconds
Scattered radiation is not part of the useful beam because it places an added density on the film,impairing image quality. Minimizing the amount of scatter reaching the film can best done by restricting the sizeof the beam or by using a grid.
When the size of the beam is restricted properly, the total amount of tissue irradiated is minimal, resultingin improved image quality and decreased patient dose. Beam restrictors include aperture diaphragms, cones,cylinders, and collimators. The smaller the beam size, the higher the contrast (shorter scale).
1) The lowest kVp = greatest contrast (short scale).
2) The highest grid ratio = greatest contrast (short scale).
3) The smallest area of collimation = greatest contrast (short scale).
4) The fastest film/screen speed combination = greatest contrast (shortest scale).
5) Film processing – the greater the developer time, temperature, or replenishment rate, the lower the contrast (long scale).
6) Filtration – the greater the total filtration, the lower the contrast (long scale).
(a) SID – effect on contrast is negligible and is therefore, not a consideration.
(b) OID – the greater the object distance, the higher the contrast because of
the “air-gap” technique.
8) Anatomical part – the greater the tissue density and thickness, the more scatter radiation produced,
thereby reducing contrast (long scale). Also, the lower the atomic number of the tissue irradiated (e.g.,fat, muscle, H2O), the greater the scatter due to the Compton effect (long scale). A compression devicemay be used to reduce tissue thickness, thus increasing contrast (short scale).
9) Pathology – destructive pathology tends to increase contrast (short scale), whileadditive pathology tends to decrease contrast (long scale).
Detail is controlled primarily by focal spot size. The smaller the focal spot size, the greater the recordeddetail. Other factors affecting recorded detail are those affecting the sharpness of the image. One of the twogeometric properties of radiographic-image quality, recorded detail is the degree of geometric sharpness oraccuracy of the structural lines actually recorded in the image. It is also referred to as definition, sharpness,resolution, or detail.
Resolution is the quantified term. A unit of resolution is the line pairs per millimeter(lp/mm) or cycles per minute. A radiographic-resolution test tool consists of pairs of lines a specified distancefrom each other. Human visual acuity is approximately 5 lp/mm.
Some of the factors affecting recorded detail are referred to as radiographic unsharpness.
Forms Of Radiographic Unsharpness:
Motion unsharpness occurs when an image shows evidence of motion (patient breathed or moved),requiring it to be repeated. This results in more or additional radiation-exposure for the patient and a wasteof time and film. This most common and severe form can be controlled by using the shortest possibleexposure time, immobilizing devices, and proper communication and instruction to the patient.
(GU) occurs because x rays are generated on an area (focal spot) and, oncegenerated, they diverge from the source. Therefore, geometric unsharpness is determined by the focal spotsize (FSS) and the distance from a source (both the SID and the OID affect geometric unsharpness). Tubesusually have two focal spots, which are determined by the size of the cathode filaments.
Sizes vary from 0.1to 2.0 mm. The larger the FSS, the more unsharpness. The original term for geometric unsharpness waspenumbra. The mathematical representation of geometric unsharpness is:
GU = FSS x OID (object-image receptor distance) /SOD (source-object distance)
Inherent unsharpness refers to an already-existing factor that usually cannot be changed. This type dealswith the film and intensifying-screen speed used. Film usually has resolving capabilities in the range of 100lp/mm. This is far beyond that of the human eye. Direct exposure film has the highest resolution, but requires20 to 100 times more mAs.
Quantum mottle is a phenomenon that may dramatically affect recorded detail when high-speed screens areused with extremely low mAs. These screens are so sensitive that a sufficient density can be achieved at verylow mAs. If the total number of incident photons reaching the film is insufficient to activate enough phosphor toemit a light to cover the entire surface of the film, quantum mottle occurs. It can be corrected by increasing themAs.
Film/screen contact. The film is sandwiched between two screens by pressure pads that keep them in directcontact with one another. When this contact is broken, in even just a small area of the cassette, the image detailis not sharp because of increased geometric unsharpness. In such cases, a wire-mesh test should be performedand the cassette should be repaired or discarded.
1) Smallest focal spot = greatest detail.
2) Shortest object-image receptor distance (OID) = greatest detail.
3) Longest source-image receptor distance (SID) = greatest detail.
4) Shortest exposure time = greatest detail if motion is considered.
5) Slowest film/screen combination = greatest detail.
6) Better film/screen contact = increased detail.
All radiographic images have a certain amount of distortion. Radiographing a three- dimensional objectand putting the image on to a two-dimensional film will generally result in shape distortion. There are two majortypews of distortion: size and shape.
Size distortion can be calculated or assessed by the use of the magnification factor (MF).
SID/SOD = MF ; Image Size/Object Size = SID/SOD
40/30 = MF = 1.33
The magnification will be 33% or the image will be 133% of the object size.
Answer: Since the SOD is not supplied, find it by SID = SOD + OID; or SID – OID =SOD.
40 = SOD + 2
SOD = 40 – 2; SOD = 38 in.
MF = SID/SOD; 40/38 = 1.05
The MF permits calculation of the actual size of an object that is projected as an image by usingthe formula:
S= IM (S = actual size, I = image size, M = Magnification Factor)
Example: If a projected image measures 5 inches and the magnification factor is 1.02, what is the size ofthe actual object?
image size SID
------- = ----
object size SOD
Shape distortion is the unequal magnification of the actual shape of the structure being radiographed. Itresults because structures lie at different levels in the body, also because the divergence of the beam displacesthe projected image of an object from its actual position. It can be classified as either elongation orforeshortening.
Elongation projects an object so it appears to be longer than it really is and occurs when the tube or the imagereceptor is improperly aligned with the anatomical part.
Foreshortening projects an object to appear shorter than it really is and occurs only when the anatomical part isimproperly aligned.
Alignment of the central ray with the anatomical area and the film determines whether or not shape distortionoccurs. Proper positioning is achieved when the central ray is at right angles to the anatomical part and the film,which must be parallel. When this can not be achieved because of the human anatomy, creative positioningneeds to occur to minimize distortion.
1) Object-image receptor distance (OID) = directly proportional
2) Source-image receptor distance (SID) = inversely proportional
1) X-ray tube – part alignment
2) X-ray Tube – film (image receptor) alignment
3) Anatomical part – film (image receptor) alignment
The relationship between radiation exposure and radiographic density on the image receptor, as measuredby the densitometer, is known as the characteristic curve. The densitometer is basically measuring how muchlight is able to penetrate the radiograph at a given point. The darker the film is, the less light that is emittedthrough the film at that point, thus the higher the density reading.
The densitometer reads logarithmically to base10. Thus, if 100% of the light is able to penetrate the film, there is no density whatsoever on the film and thelog-to-base 10 reading would be 0. If 10% of the light were able to penetrate, then the logarithmic reading givenby the densitometer would be 1.0, and so on. You can summarize this in a table for your students as follows:
Log 10density % light transmitted Fraction of
light transmitted 0 100% 0
1.0 10% 1/10
2.0 1% 1/100
3.0 0.1% 1/1000
Note that the number of zeroes in the fraction equals the density reading.
he useful range of densities in diagnostic radiography ranges from 0.25 to 2.5.
An increase in density of 0.3 is equal to a doubling in radiographic density. For example, an area on the
film that measures 0.6 is twice as dark as an area that measures 0.3.
Dmax is the area at the very top of the curve, which is the point of maximum density on the film and usually
measures greater than 3.0 on the densitometer.
Horizontal axis is the log relative exposure.
Vertical axis is the radiographic density reading. When comparing several types of film on a characteristic
curve, the curve that is closest to the vertical axis at 1.0 density represents the film with the highest speed. ....
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