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40 Cards in this Set

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processing done by the computer to the raw imagedata. typically done to make corrections to the imagedue to imperfections inherent in the equipment and the beam.

Automatic Rescaling

is when the computer adjusts the brightnessvalues to display consistent image brightness regardless of overexposure orunderexposure.

Exposure field recognition

This is when the computer identifies the collimatedborders from the image, and then identifies the anatomy (VOI) within thecollimation.

VOI (values of interest)-

a. These are the values contained in the histogram that represent the patient’s anatomicalinformation (ie. Subject contrast).There will be a minimum through maximum value identified. It is these valuesthat the computer will manipulate to display the final image.


A graphical representation of exposure values extracted from an imagereceptor. It is a bar graph that represents the varying shades of gray (called luminescenceor brightness values) in an image


(look up table). Is a table containingalgorithms that will adjust the raw image gray scale (contrast). The steeper the curve the higher the contrast. Thishappens after histogram analysis.

Exposureindicator numbers

1. areused by digital systems to reflect the amount of radiation the image receptorwas exposed to. EI numbers are not an accurate measure for patient dose.

S number

Used by Konica, and Fuji. Uses 200 as baseline. The S number isinversely related to exposure. A lower number indicates overexposure and ahigher number indicates underexposure

EI number

used by carestream. Ideal EI number is 2000. based on a logarithmic system, using increments of 300. So, an EI of2300 indicates the IR was overexposed by a factor of 2.

LgM number

used by AgFa.The center of the scale is set at 2.5. This system uses a logarithmicscale, with every 0.3 change in the LgM number indicating over orunderexposure. An LgM of 2.8 = over exposure


when the IR does not receive enough radiation. This presents in digitalimaging as quantum mottle.


1. Thiscauses low contrast a. Overexposurecauses an increase in transmission. This increase in transmission provides lessdifference between digital values. This displays as a decrease in contrast.


describes a systems sensitivity to radiation. In digital imaging thespeed of the system is set by the manufacturer. A speed class of 200 requires 1mR of exposure. Speed class of 400 requires 0.5mR exposure. This shows thatsystem speed in inverse to exposure. As speed increase, exposure required toproduce an image decreases.


1. whenexposures in digital imaging increase because there is no visual negative cueto let you know that you over exposed your image. This is b/c the computercompensates for overexposure with processing to display an image withconsistent brightness. Often, overexposed images look better and techs areafraid of quantum mottle, so higher techniques are used which increase patientdose.

Dose area product

1. DAP.Is a meter used mostly in cassetteless systems. It is used to provide a measureof entrance skin dose. This is thoughtto give a more adequate measure of patient dose. The meter is inside the unit,just under the collimators. The DAP value is dependent on exposure and fieldsize (expressed in m2) and is expressed in cGy/m2. cGy isa unit to express at patient’s absorbed dose. So, this describes the amount ofdose an exposed area absorbs.

The horizontal axis (x axis) represents

the brightness value

vertical axis (y axis)

represents the frequency, or the number of timesthat value occurs in the image

where is barium located on the histogram?

far left

where is bone located on the histogram?


where is soft tissue located on the histogram?


where is air located on the histogram?

far right

when are the VOI determined?

From the exposure field recognition process

Pixels that received a lot of radiation (indicating high transmission)are assigned

higher values (low brightness)

Areas that receive low exposure (indicatingabsorption) receive

low values (high brightness)

Type 1 histograms

have a spike on the far right side of thehistogra. (a) This is typically used for your extremities. The firstvalue of interest would be bone and the 2nd value of interest wouldbe the skin line.

Type 2 histograms

(a) are typically used for exams where there is notexpected to be a spike from raw radiation striking the IP. The minimum value ofinterest would be bone, whereas the 2nd VOI would be a structurethat allowed for the most transmission (ie. Air).

Type 3 histograms

(a) expect a spike on the left side of the histogram. Thiswould be anything that is highly attenuating (ie. Contrast, metal).

Neutral histogram analysis

This type of histogram has several storedhistograms for each examination. The “scanned” histogram is compared to a“stored” histogram that most closely matches its appearance.

Increasing kVp will create

a narrowhistogram, This will give the image receptor a more uniformexposure resulting in less difference between signal values

Decreasing kVp will create

a wide histogram, more absorption of the beam so bigger difference in signal values

Histogram analysis errors can result from

a. Irrelevant exposure information included in theanatomy/collimation

b. Exposure field recognition error- this is when the computer cannot identify thecollimated borders of the exposure field.

c. You did not select the correct part from the workstation menu

d. Your centralray is not centered to the anatomy

e. IP not erased

f. grossly over or underexposed the image

EI numbers used by Fuji and Konica

uses the “S” number. Uses 200 as baseline. The S number is inversely related to exposure

EI number used by Phillips

(1) Valuesare rounded to whole numbers and each number change reflects a 25% change inexposure. For instance, if the EI number is 220 and 110 is the base line, thatmeans the IR received twice the amount of exposure necessary.

EI used by Hologic

DAP. Is a meter used in some DR units and digital fluoro in place of or inaddition to the exposure index. This is thought to give a more adequate measureof patient dose. The meter is inside the unit, just under the collimators. TheDAP value is dependent on exposure and field size (expressed in m2)and is expressed in cGy/m2. cGy is a unit to express at patient’sabsorbed dose. So, this describes the amount of dose an exposed area absorbs.

EI used by carestream

Use an EI. 2000 is the center point. 2300 means you’ve overexposed by a factor of2, 1700 means you’ve underexposed by a factor of 2.

EI used by AgFa

uses the LgM. The center of the scale is 2.5. An LgM of 2.8 would meanyou’ve over exposed by a factor of 2.An LgM of 2.2means you’ve underexposed bya factor of 2.

Speed is ____________ to exposure


100 speed=

2 mR

200 speed=

1 mR

400 speed=

0.5 mR