kettering image production (MG)

kVp (kilovoltage peak)

A) is an influecing factor of radiographic receptor exposure. increases in kVp will increase receptor exposure until 100 percent of the beam is successful in reaching the image receptor

b) increases in kVp increases the percentage of beam penetration through a given body part that will increase the amount of radiation reaching the image receptor

15 percent rule

a 15 % increase or decrease in kVp, keeping all other factors constant, will result in a doubline or halving of radiographic receptor exposure.

example: it is determined that an underexposed abdominal radiograph needs to be repeated with exposure factors doubled. if the original exposure factors were 70 kvp at 32 mas, what new kVp should be used to improve this radiograph?

70 x 0.15=10.5

70 + 10.5= 80.5 kVp

Grid ratio formula

Grid ratio = h/D

if no adjustment in techneique is made, as grid ratio increases radiographic receptor exposure will_____

decrease

grid ratio bucky factor

grids

-are beam attenuators

-if no adjustment in technique is made, as grid ratio increases receptor exposure will decreases

-to maintain a desired amount of radiographic receptor exposure, changes in mAs are required to compensate for the presence of a grid between a patient and a image receptor

filtration (photographic factor)

a) filters are beam attenuators; decrease skin exposure to the patient

b) as the amount of filtration in the path of the beam increases the amound of radiation available to expose the image receptor will decrease, resulting in a decrease in radiographic receptor exposure

factors that contribute to the absorbing ability of a given body part include

1) thickness

2) atomic number

3) specific gravity

thickness

as thickness increases beam attenuation increases and receptor exposure will decrease

atomic number

as the atomic number of an object increases attenuation will increase yielding a decrease in radiographic exposure

specific gravity

as specific gravity (comparison of the receptor exposure of an object to water) increases attenuation will increase yielding a decrease in radiographic exposure

additive diseases will increase the amount of the beam attenuation of the objects involved, and will result in a corresponding

decrease in radiographic receptor exposure

destructive diseases will decrease the amount of beam attenuation yielding an

increase in radiographic exposure

beam restriction

increasing collimation or beam restriction results in decreasing field size, decreasing radiographic receptor exposure and also decreasing the amount of scatter reaching the IR

destructive diseases

decreased subject receptor exposure, increased radiographic receptor exposure

additive diseases

increased subject receptor exposure, decreased radiographic receptor exposure

annode heel effect

-variation in x-ray beam intensity with an increase in beam intensity toward the cathode end of the beam and a decrease intensity toward the anode end of the beam

-AP T-SPINE, AP FEMUR

FAT-CAT

when needed, place the thickest portion of the anatomy (FAT) toward the cathode (CAT) end of the beam

kVp

a) is the primary controlling factor of SUBJECT contrast

-rate of attenuation of different body parts

umbra

area of image sharpness

blur or penubra

are of unsharpness surrounding the image

*aka geometric unsharpness

*geometric unsharpness is always brighter on what end of the beam?

cathode side of the beam

*straight OG

OID: obect ot image reciever distance (geometric and photographic factor)

a) effects recorded detail and magnification (size distortion) by allowing an increase in divergance of the remnant beam prior to reaching the IR

b) as OID Increases recorded detail decreases

SID: source to image reciever distance (geometric and photographic factor)

a) effects recorded detail and magnification (size distortion)

b) as SID increases , recorded detail increases, while magnification decreases

actual focal spot size

the size of the area on the anode target that is exposed to electrons from the tube current

effective focal spot size

refers to the focal spot size as measured directly under the anode target

line focus principle

Describes the relationship between the actual and effective focal spots in the x-ray tube. A smaller target angle produces a smaller effective focal spot.

distortion

1) deals with the defgree of pervasion or "untrueness" of the image recorded on the image receptor

2) types of distortion include size distortion(magnification) & shape distortion (elongation and foreshortening

shape distortion

-angulation of the tube, angulation of the part, angulation of the image receptor, and motion

size distortion

-limited to changes in Object to image reciever and source to image reciever distances

-OID AND SID

technique charts

1) pre programmed techniques- anatomically programmed radiography (APR)

a) technique charts are programedd into the control unit

b) the technologists selects an anatomic display of the part and the microprocessor automatically selects the appropriate kVp and mAs

fixed kVp chart

a) also known as a variable mAs chart

b) pre established kVp value is used for each body part

variable kVp chart

a) also known as a fixed mAs chart

b) pre-established mAs is used for each body part

c) kVp is determined by using calipers to accurately measure thickness of the body part being evaluated

d) typically, a 2 kC change is made ofr each cm in part thickness when operating at 80 kvp

wet plaster casts

usually recquire a doubline of exposure factors: either a 100% increase in mAs or an 8-10 increase in kVp

dry plaster cast

usually requires either a 50%-60% increase in mAs or a 5-7 increase in kVp

fiberglass

usually requires a 25%-30% increase in mAs or a 3-4 increase in kVp

part thickness

as a general rule, parts measuring greater than 10-13 cenetimeters require use of a grid to offset the production of secondary and scattered radiation in the volume of tissue as the x-ray beam passes through the tissue

Automatic Exposure Control (AEC)

Device that terminates the exposure when a specific quantity of radiation has reached the image receptor (IR).

effects of changing exposure factors on radiogrpahic quality

a) higher kVp techniques will result in a reduction of radiographic contrast

b) higher mAs rechniques will result in a longer exposure time and possibility of motion on the resultant radiograph

c) quantum mottle may occur with AEC techniques if the recquired mAs is to low-this can be prevented by decreasing kVp which will result in an increase in mAs

detector selection

-phototiming cell or field that is selected depends on the part under examination

-if the central ray is directed to the part under examination ex: an ap projection of the hip, the centr cell should be selected

-an exception to this rule is a PA projection of the chest: selecting the center cell will place tit over the heart and may result in over penetration of the lung fields

-bc of this, an outside cell(s) may be selected for a PA chest radiograph

anatomic alignment

-since the amount of radiation required to produce a quality, diagnostic radiograph depends on the cell that is selected, careful positioning and central ray centering is required when using AEC techniques

Exposure adjustments (density +1 or -1)

a) the receptor exosure of a radiograph produced using AEC techniques may be influenced by adjusting the receptor exposure control

b) for very thing patients or pediatrics a negative receptor exposure contral may be selected to help prevent overesposure

c) for very large patients a positve receptor exposure control may be selected to help prevent underexposure

how does a technologist adjust the rate of exposure when using an AEC device

*adjust the density control

spatial resolution

-is the ability of the system to record adjacent small structures

-the sharpness of the structural edges record in the image

-measured in line pairs per mm (lp/mm) <-straight player

pixel (picture element)

1) smalest area depicted in an image

2) two-dimensional sqaure that contains discrete grey shade

3) size-measured end to end

*as pixel size increases, spatial resolution??

decreases

-vice verse if pixel size decrease SR increases

pitch

-measured center to center

as pixel pitch increases, spatial resolution??

decreases

pixel density

-number of pixels per millimeter

-determined by the pixel size and pixel pitch

-as pixel size decreases, pixel density increases, thus SR increases

DEL (detector element) size

-size used with direct capture radiography (casette less)

-uses a flat panel detector(IR used in DR)

-spatial resolution is determined by the detctor element size (DEL)

-DR uses detector element size

-pitch

As DEL increases, spatial resolution ____?

decreases

fill factor

a ratio of a pixels light sensitive area versus a pixels total area

as fill factor increases, spatial resolution ___

increases

matrix

-two dimensional array of pixels ("x" and "y")

-matrix size is the total number of pixels

-matrix size is dependent upon FOV and pixel density

a)example 35 cm x v43 cm at 5 pixels/mm

--350mm x5 =1750

--430mm x5=2150

--2150x1750=3,762,500 pixels

increase image recpeptor size results in increase in ??

matrix size

Decreasing pixel size results in ___ matrix size and ___ spatial resolution

-increased

-increased

sampling frequency

-the number of pizels sampled per millimeter as the laser scans each line of the imaging plate* main controlling factor of spatial res. in CR

1) the more pixels sampled per millimeter the greater the sampling frequency

2) the greater the sampling frequency, the longer it takes to process the plate due to the amount of information being collected

3) in some systems, using a smaller IR permits more pixels per MM to be scanned yielding improved image quality

4) increasing the sampling frequency results in the laser moving a smaller distance and there is an increase in spatial resolution

5) CR uses sampling frequencey

nyquist frequency

- The relationship between the sampling frequency (number of pixels/mm scanned by the laser, at a rate of 2x the highest frequency present) and the spatial resolution (resulting image detail, sharpness, etc...

what is the min. rate at which signal can be sampled w/o introducing samples?

2x the highest frequency

contrast resolution

-ability to detect subtle changes in the gray scale

a) bit depth

b) Detective Quantum efficiency (DQE)

c) beam restriction

Signal

-results from x-ray deposition of energy in a detector (image data)

noise

results from extaneous information (interferance, limits the ability to visualize objects

contrast resolution/dynamic range

range of values over which a digital image receptor will respond; greater dynamic range will yield greayer contrast resolution

dynamic range

-series of exposure values used to produce acceptal image.

-the lightest lights to the darkest darks that can be seen

exposure latitude

-ability of a system to both over and under expose yet still produce an acceptable image

-exposure latitude as a range within the dynamic range

quantum noise (quantum mottle)

results when to few x-rays reach the IR- underexposure, the IR is photon starved and is related to the IR photon intensity

-increased intensity results in decreased noise, and decreased intensity results in increased noise

scatter noise

A refraction of X-ray beams away from the original beam path; if this refraction reaches the detector, it can result in unwanted radiation exposure on the image, which degrades the image & makes it more difficult to interpret.

electronic noise

Random signal fluctuations in an electrical signal; characteristic of all electronic circuits.

Signal to noise ratio (SNR)

-ratio between "signal" or meaninful information and "noise" or background information

SNR

-as noise increases it is more difficult to visualize small objects

a) it is desirable to have a high SNR

b) NOISE IMPACTS THE PRECEPTIBILITY OF SPACIAL RESOLUTION

e) noise decreases are ability to see all spatial resolution and contrast resolution on an image

basic information that must be present on the radiograph

-patient data: name & identification number

-examination data: including postural and side markers

-examination date: date exam was performed

-institutional data: hospital/clinic name where exam was performed

what gives the best SNR?

a) 121: 11

b) 8 :1

c) 49: 7

d) 5: 2.5

a) 121: 11->11:1

b) 8 :1

c) 49: 7-> 7:1

d) 5: 2.5->2:1

a minium mAs change of ___% is neccesary to yield a noticable receptor exxposure change within the radiographic image

30 %

Grids

used to control scatter reaching the IR

Grid focus distance decnetering

using a grid outsid ethe established focal range will exhibit uniform cutoff along the lateral edges of the image

lateral decentering

Lateral off-centering of the central ray to a focused grid angulation of the central ray across the transverse axis of a focused grid will exhibit the presence of lead strips of more frequency on one side of the image than the other

moire pattern

-occurs when two linear grids are placed on top of one another to make upa crosshatched grid, but the lead lines are not aligned at right angles to one another

recorded detail/spatial resolution: motion

a) recorded detail decreases as patient motion increases

b) voluntary motion must be controlled when the radiographic procedures demands it

c) short exposure time will help minimze the effects of voluntary and involuntary patient motion

size distortion

a) image magnification is increased by a long OID and/or a short SID

b) a short OID is the best way to minimize the effects of voluntary and involuntary patient motion

shape distorion

affected by angulation of the bubel angulation of the part, angulation of the image receptor and motion

Ghost image

Caused when there is insufficient erasure of an image and a ghost image is seen on the new image

** CR ONLY **

xray tube construction: KNOW IT DRY

EVERYTHING IS IN THE GLASS ENVELOPE EXCEPT

THE STATOR

cathode

-negative electrode

-composed of the filament wires (thoriated tungste) and focusing cup (molybdenum or nickel)

focusing cup

-Negatively charged

-Provides electrostatic field to control the path of electrons from cathode to anode at exposure time (- to +)

-Aids in maintaining line focus principle

Selecting mA determines the number of electrons free around the filament

-Effectiveness depends on size and shape, and the charge, size, shape and position of the filament in the focusing cup

effective focal spot size

projected toward the patient

actual focal spot size

where electrons actually strike the target

line focus principle

-the size of the focal spot directed toward the image receptor (the effective focal spot) is smaller than the size of the focal spot (actual focal spot) measured on the surfuce of the annode

what is bigger the actual or effective focal spot

actual

smaller effective focal spot

better spatial resolution

the smaller the annode angle

smaller the effective focal spot-> better spatial resolution

induction motor

-stator wiindings and rotor

-operates the rotor that spins and and the anode during a radiogrpahic exposure

-engaging the rotor swithc fires the stator in a circular direction around the x-rau tube resulting in a circular movement of the rotor within the tube

x-ray beam

* xrays are produced isotropically, which means they are produced with equal intensity in all directions--> SCP

leakeage radiation

-radiation that is emitted through the metal protective housing

-should not exceed 1mGy/hr at 1 meter

Principles of Automatic Exposure Control (AEC)

a) AEC units are designed to reproduce a desired amount of receptor exposure on images taken of selected body parts regardless of changes within the patients being examined

The importance of the AEC is

- radiographs taken on mixed population will fall within an acceptable or diagnostic range

- Images WILL NOT necessarily be identical to another

-images will vary due to changes in subject contrast within the patient population

AEC is designed to

reduced the number of repeated radiographs taken on a mixed populatiion of patients due to over and under exposure

AEC places a premium on

effective & accurate positioning & phototimer cell selection

biggest disadvantage of using the AEC

innadequate positiong and phot timer cell collection