Radiology Quality of Xrays Topic 2 Chp 4, 5, 6,

Rules of Casting Shadows

1. Small focal spot 2. Long target- object distance 3. Short object- image receptor distance 4. Parallel relationship between object and image receptor 5. Perpendicular relationship between central ray of the xray beam and both the object and the image receptor

Identify advantages and disadvantages of digital radiography.

Advantage- reduction in radiation exposure 0-50% claims up to 80% when compared to lower speed D film more efficient Disadvantage- higher retake percentage

Density

Degree of darkness or image blackening increase mAs will darken the image by allowing a higher quantity of xrays to penetrate

Digital sensor contrast relationship

Digital sensors have a built in contrast set by the manufacturer

List and describe the types of digital image receptors.

Direct - CCD and CMOS Indirect- PSP

Long scale contrast

Less difference between grays High kVp 80-100 better periodontal disease detection

Short scale contrast

More difference between grays Low kVp 60-70 better cavity detection

Exposure Contrast Relationship

Over and Under exposed film will cause poor contrast because the film will either have too much density or not enough density

Processing and Contrast Relationship

Processing improperly as in time or temp will result in poor contrast

Define radiopaque and radiolucent and give examples of dental structures for each.

Radiolucent are darker areas- shows little or no resistance to penetration Radiopaque are lighter show areas that are resistant to xrays and absorb or resist the xray

How do pixels capture an image?

The xrays hit the pixels and they become excited and the gray value increases or decreases according to the number of xrays that strike - transmits info via radiowaves to computer

Define penumbra and umbra formation.

amount of shadow around an object more penumbra = more shadow

Sharpness

detail or clarity (geometric factor) measured by the detail of the outline of structures affected by : movement Digital- may be affected by the pixel size

kVp variations

determine quality of the xray by increasing or decreasing the wavelength and produce better or worse penetrating power increase kVp- Density increase (darker) less contrast Decrease kVp- Density decreases (lighter) more contrast Increase kVp by 15 decrease mAs by dividing by 2 Decrease kVp increase time by multiplying by 2

analog vs. digital

digital - made up of pixels analog- smooth blend

Pixel is equivalent to what in film based radiography

equivalent to silver halide crystal

Magnification

increase in size to reduce keep target (PID) object (tooth) distance long and object (tooth) image (film or sensor) short to reduce magnification and increase sharpness

Exposure time Density Relationship

interval that the xray machine is fully activated and xrays are produced increased time - increases the density (darkens) decreased time- decreases density (lighter)

kVp Contrast relationship

inverse relationship higher kVp produces lower contrast - blacks are grayer and whites are grayer lower kVp produces higher contrast- blacks are blacker and whites whiter fewer shades of gray in between

Small focal spot

less penumbra results in sharper image with less magnification (within the tube head)

Milliamperage (mA) Density Relationship

mA increase= increases density mA decrease= decreases density

List and describe five software features use to enhance digital image interpretation.

magnification - density and contrast- measurement tools- charting- draw on image digital subtraction - comparison of xrays

Milliampere/seconds mAs

milliampere/seconds may be combined because they always work together- easiest factor to alter When mA is increased amount of xrays increase when time in increased amount of xrays increase Formula mA x seconds = mAs moving patient = increase mA so that time can be shortened with the same amount of xrays produced example: mA x s = mAs 10 mA x 0.6 sec = 6 mAs 15 mA x ? sec= 6 mAs Divide 6 mAs 15 mAs equals 0.4 sec

High Contrast

more shades of gray appears too light less defining differences between structures

spatial resolution

number and size of the pixels measured in line pairs greatest number of paired lines visible in millimeter of an image more lines per mm - sharper image

Gray scale- digital

number of shades of gray visible most important characterisit diagnosis and detection rely on this produce 65,500 shades of gray- monitor can only pick up 256 and human eye can only detect 32

PSP

photostimuable phospor plates also called storage phosphor system parallel film in the look and the way the picture is captured as analog data and then processed. The polyester plates capture the image just as silver halide crystal and then are placed in a laser scanning device that trasfers the image to an electrical signal

Parallel relationship between object and image receptor will result in? (long axis of tooth )

prevents distortion

Perpendicular relationship between the the central ray of the xray beam and both the object and the image receptor will result in? (PID to tooth/ film )

prevents distortion

Short object image receptor distance will result in? (Film to tooth)

reduce penumbra and magnification

Long target-object distance results in? (PID to tooth)

reduces penumbra and magnification influenced by length of PID

Explain the fundamental concepts of digital radiography.

replaces film with a solid state (no moving parts) image receptor called a sensor acquire an image vs. taking a radiograph

Subject Contrast Relationship

result of difference in absorption of xrays. Humans have contrast because human tissues vary in size, thickness, and density

State the purpose and uses for digital imaging.

same as film

Scatter radiation and contrast relationship

scattered xrays add a uniform exposure and decrease contrast

List the equipment used in digital imaging.

sensor

Contrast

shades of gray that separate the dark and light areas Radiographic definition- visible difference between densities

State the rule regarding film speed and sharpness.

slow speed film- sharper the image

State the rule regarding sensor pixel size and sharpness of digital images.

smaller the pixel size- sharper the image

Pixel arrangement

structured order in rows and columns x-coordinate and a y-coordinate and gray value

List the three radiographic qualities.

structures should be as close to their natural shape and size Distortion and superimposition should be kept to a minimum area should be imaged completely with enough surrounding tissues free of errors and proper density, contrast and definition

Target image receptor distance

sum of target object and the object image distance PID to tooth plus the tooth to film increasing this improves quality position the film as far away from tooth keeping it parallel to increase this

State the purposes of the computer used for digital imaging.

to view and manipulate the image

Define distortion

unequal magnification of different parts of the same object TROUBLESHOOT- plane of the image receptor must be parallel with long axis and central ray must be perpendicular