The DigitalThe DigitalRadiographic Radiographic

ImageImageGroup One : CuTe_RaDsGroup One : CuTe_RaDs

Source: Bushong and GoogleSource: Bushong and Google

SPATIAL RESOLUTIONSPATIAL RESOLUTION Resolution in spaceResolution in space Is the ability of an imaging system to Is the ability of an imaging system to

resolve and render on the image a resolve and render on the image a small high-scale contrast object.small high-scale contrast object.

Usually described as the size of the Usually described as the size of the object that can be viewed.object that can be viewed.

In medical medicine, it is described In medical medicine, it is described by the quantity “spatial frequency”.by the quantity “spatial frequency”.

Spatial FrequencySpatial Frequency Is expressed in line pairs per Is expressed in line pairs per

millimeter.millimeter.– lp/mmlp/mm

Line PairsLine Pairs – black line on a white – black line on a white background; one bar and its background; one bar and its interspace of equal widthinterspace of equal width

QuestionQuestion A digital radiographic imaging A digital radiographic imaging

system has a spatial resolution of 3.5 system has a spatial resolution of 3.5 lp/mm. How small an object can it lp/mm. How small an object can it resolve?resolve?

Answer : 143 Answer : 143 μμmm

SolutionSolution 3.5 lp/mm3.5 lp/mm Line PAIR; which means there are 7 Line PAIR; which means there are 7

objects in 1 line pair, or simply objects in 1 line pair, or simply 7/mm.7/mm.

And the reciprocal of 7/1 is 1/7 which And the reciprocal of 7/1 is 1/7 which is equal to 143 is equal to 143 μμm.m.

QuestionQuestion A screen-film mammography imaging A screen-film mammography imaging

system operating in the system operating in the magnification mode can image high-magnification mode can image high-contrast microcalcifications as small contrast microcalcifications as small as 50 as 50 μμm. What spatial frequency m. What spatial frequency does this represent?does this represent?

Answer : 10 lp/mmAnswer : 10 lp/mm

SolutionSolution It takes TWO 50-It takes TWO 50-μμm objects to form a m objects to form a

single line PAIR.single line PAIR. Therefore, 1 lp = 100 Therefore, 1 lp = 100 μμm, or m, or

1lp/1001lp/100μμmm 1 lp/100 1 lp/100 μμm = 1 lp/0.1mmm = 1 lp/0.1mm 1/0.1 = 101/0.1 = 10 10 lp/mm10 lp/mm

Spatial FrequencySpatial Frequency Most people can see objects as small Most people can see objects as small

as 200 as 200 μμm; therefore the spatial m; therefore the spatial resolution of the eye is described as resolution of the eye is described as 200 200 μμm.m.

Clearly, as the spatial frequency Clearly, as the spatial frequency becomes larger, the objects become becomes larger, the objects become smaller.smaller.

Spatial FrequencySpatial Frequency Large soft tissues such as the liver, Large soft tissues such as the liver,

kidneys, and brain have low spatial kidneys, and brain have low spatial frequency and therefore are easy to frequency and therefore are easy to image.image.

Bone trabeculae, breast Bone trabeculae, breast microcalcifications, and contrast-filled microcalcifications, and contrast-filled vessels are high-frequency objects; vessels are high-frequency objects; therefore, they are more difficult to therefore, they are more difficult to image.image.

Spatial FrequencySpatial Frequency Spatial resolution in digital imaging is Spatial resolution in digital imaging is

limited by pixel size.limited by pixel size. No digital imaging system can image No digital imaging system can image

an object smaller than 1 pixel.an object smaller than 1 pixel.

QuestionQuestion What is the spatial resolution of a What is the spatial resolution of a

512 * 512 CT image that has a field 512 * 512 CT image that has a field view of 30cm? What spatial view of 30cm? What spatial frequency does that represent?frequency does that represent?

Answer : Answer : 0.59mm or0.59mm or 0.83 lp/mm or 8.3 lp/cm0.83 lp/mm or 8.3 lp/cm

Modulation Transfer Modulation Transfer FunctionFunction

Also known as MTF.Also known as MTF. It is a term borrowed from radio It is a term borrowed from radio

electronics that has been applied to the electronics that has been applied to the description of the ability of an imaging description of the ability of an imaging system to render objects of different system to render objects of different sizes onto an image.sizes onto an image.

Can be viewed as the ratio of an image Can be viewed as the ratio of an image to object as a function of spatial to object as a function of spatial frequency.frequency.

QuestionQuestion There is a cut-off spatial frequency of There is a cut-off spatial frequency of

4 lp/mm for DR. What is the pixel 4 lp/mm for DR. What is the pixel size?size?

Answer : 125 Answer : 125 μμmm

SolutionSolution1.1. 4 lp/mm4 lp/mm2.2. 8 objects/mm8 objects/mm3.3. 8 pixels/mm8 pixels/mm4.4. RECIPROCALRECIPROCAL5.5. 1/81/86.6. 0.125 mm0.125 mm7.7. 125 125 μμmm

CONTRAST RESOLUTIONCONTRAST RESOLUTION Is the ability to distinguish many Is the ability to distinguish many

shades of gray from black to white.shades of gray from black to white. The principal descriptor for contrast The principal descriptor for contrast

resolution is gray-scale, also called resolution is gray-scale, also called dynamic rangedynamic range..

Dynamic RangeDynamic Range The dynamic range of a screen-film The dynamic range of a screen-film

radiograph is essentially three orders radiograph is essentially three orders of magnitude, from an OD of near 0 to of magnitude, from an OD of near 0 to 3.03.0

The viewer can visualize only about The viewer can visualize only about 30 30 shades of grayshades of gray..

No more than 30 shades of gray can be No more than 30 shades of gray can be viewed because of the limitations of viewed because of the limitations of the human visual system.the human visual system.

A principal advantage of digital A principal advantage of digital imaging is the ability to preprocess imaging is the ability to preprocess and postprocess the image for the and postprocess the image for the purpose of extracting even more purpose of extracting even more information.information.

With screen-film radiographic With screen-film radiographic images, what you see is what you images, what you see is what you get.get.

PostprocessingPostprocessing Postprocessing allows visualization of all Postprocessing allows visualization of all

shades of gray.shades of gray. One postprocessing activity is One postprocessing activity is window window

and leveland level.. With the use of window and level tool, With the use of window and level tool,

any region of the grayscale can be any region of the grayscale can be expanded into a white-to-black expanded into a white-to-black grayscale.grayscale.

This is especially helpful when soft This is especially helpful when soft tissue images are evaluated.tissue images are evaluated.

Signal-to-Noise RatioSignal-to-Noise Ratio SignalSignal – portion of the image-forming – portion of the image-forming

x-rays that represents anatomy.x-rays that represents anatomy. It represents the difference between It represents the difference between

those x-ray transmitted to the IR and those x-ray transmitted to the IR and those absorbed photoelectrically.those absorbed photoelectrically.

Image noise limits contrast resolution.Image noise limits contrast resolution. Practitioners strive for high an SNR as Practitioners strive for high an SNR as

possible.possible.

Signal-to-Noise RatioSignal-to-Noise Ratio SignalSignal increases in proportion to the increases in proportion to the

number of imagesnumber of images NoiseNoise increases in proportion to the increases in proportion to the

square root of the number of images.square root of the number of images. If there are four images then the SNR is If there are four images then the SNR is

equal to 2.equal to 2. SolutionSolution Signal = 4Signal = 4 Noise = √4 = 2Noise = √4 = 2 4/2 = 2; therefore the SNR = 24/2 = 2; therefore the SNR = 2

CONTRAST-DETAIL CURVECONTRAST-DETAIL CURVE Contrast-detail curve shows that Contrast-detail curve shows that

when object contrast is high, small when object contrast is high, small objects can be imaged.objects can be imaged.

Mammography has the best spatial Mammography has the best spatial resolution.resolution.

Computed tomography has the best Computed tomography has the best contrast resolution of all x-ray contrast resolution of all x-ray imaging systems.imaging systems.

PATIENT DOSE PATIENT DOSE CONSIDERATIONSCONSIDERATIONS

With the acceleration to all-digital With the acceleration to all-digital imaging, practitioners have the imaging, practitioners have the opportunity to reduce patient dose opportunity to reduce patient dose by 20% to 50%, depending on the by 20% to 50%, depending on the examination. However, quite the examination. However, quite the opposite has occurred-something opposite has occurred-something that many call “dose creep”.that many call “dose creep”.

Image Receptor ResponseImage Receptor Response Because digital image receptor response is Because digital image receptor response is

linearly related to radiation dose, image linearly related to radiation dose, image contrast does not change with dose.contrast does not change with dose.

One cannot overexpose or underexpose a One cannot overexpose or underexpose a digital image.digital image.

Therefore, a digital image should never Therefore, a digital image should never require repeating because of exposure require repeating because of exposure factors.factors.

Technique creep should replace dose creep.Technique creep should replace dose creep.

Detective Quantum Detective Quantum EfficiencyEfficiency

DQE = #X-rays absorbed/#incident DQE = #X-rays absorbed/#incident X-rays multiplied by 100X-rays multiplied by 100

Related to the absorption coefficient Related to the absorption coefficient to the spatial frequency of the signal.to the spatial frequency of the signal.

DQE is the measure of x-ray DQE is the measure of x-ray absorption effficiency.absorption effficiency.

Patient dose in DR should be low Patient dose in DR should be low because of high DQE.because of high DQE.

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Babadilla, Geraldine Datu, Eunice Babadilla, Geraldine Datu, Eunice LorenzLorenzCunanan, KellyCunanan, Kelly Julabar, Marona Julabar, Marona YsabelYsabelMallari, KevinMallari, Kevin Viray, Anna ThereseViray, Anna Therese