Radio I Lec 5 (6-15)

Given an acceptable O.D. level, contrast adjustments are made by varying kVp

15% rule

5% rule for minor adjustments Generally -30% change in mAs

5/30 rule

absorbtion of x-rays can increase penumbra size.

Absorption blur

Muscular. Skinny Fat frail

Body types: Sthenic: Hyposthenic: Hypersthenic:' Asthenic:

anything denser will be lighter. Less dense --> black.

Centering gray scale around a tissue:

1/2= brighter X= normal 2X= darker

Changes in OD w mAs 1/2X: X: 2X:

A: window level 40 --> 400 B: window width 100 --> 3500

Changing windowing

Magnification Shape Distortion

Components of every image: Other

sharpness/unsharpness

Components of every image: Sharpness of Recorded Detail:

Dependent on intensity, contrast, and noise. (on screen) Even if visible, image is worthless if not recognizable (geometric integrity) (data coming in)

Components of every image: Visibility of Recorded Detail Recognizability:

into a visual scale from black to white.

Computer can resolve any range of gray tones

The ability to distinguish anatomic structures of similar subject contrast

Contrast resolution

regardless of dose. gamma/OD slope remains constant. windowing and post processing matter more. Pt safety is most important concern.

Contrast resolution is preserved in digital imaging

mAs -direct proportionality SID -via inverse square law Problematic when varied Usually fixed @ 90 cm for mobile, 100 cm -table, 180 cm -chest studies kVp -disproportionateOD effect Beam intensity (B.I.) at patient = kVp2 B.I. at image receptor = kVp4 or 5

Controlling optical density mAs SID kVp

Pixel size

Detail (digital)

focal spot size. SID OID motion

Detail (film)

noise (mottle) w decreased techniques.

Digital images much more susceptible to?

enhance contrast resolution. Major advantage over film.

Digital imaging has the ability to?

adjacent structures as low with as 1% differences.

Digital imaging software allows for perception of?

positioning

Distortion

perpendicular angles. but greater distance can increase magnification and decrease sharpness..

Distortion minimized by what angle?

Number of gray shades an imaging system can produce. bid depth/capacity per pixel. n bit = 2^n 8 bit = 256 16 bit = 16,384. Human eye can only do 32 shades of gray.

Dynamic range Digital imaging.

number of gray shades that can be appreciated DRangeof film essentially 3 orders of logarithmic magnitude -from 0 to 3.0 Reflects approx. 1000 gray tones

Dynamic range of image receptors DRange:

only approx. 30 shades of gray (2.2) related to bit capacity per pixel (e.g. 16 bit = 216= 65,536 shades)

Dynamic range of image receptors Viewer/visual system can nonetheless visualize? Digital systems?

focusing image receptors on desired range. Image brightness can then be adjusted (post-processed) virtually independent of exposure level Auto-adjust histograms correct over-exposure -computer can bring densities into visible range

Dynamic range of image receptors Windowing

The primary control of optical density is mAs changes are proportional The mAs value must be changed by approx.30% to produce a visible change in O.D. The kVp setting must be changed only by approx. 4% to produce a visible change in O.D.

Exposure technique film imaging important points:

Patient Factors Image-Quality Factors Optical density Contrast Image detail & distortion Exposure-Technique Factors mAs, kVp, SID are principle factors

Factors influencing technique:

Thickness differences Density differences Atomic no. differences Quality of radiation (kVp)

Factors that alter subject contrast (4):

Maximum slope of curve

Film gamma

requires subject contrast difference of at least 10% to enable perception of adjacent structures.

Film has poor contrast resolution

Minimizing object-film distance (OFD) Maximizing source-image distance (SID) Decreasing focal spot size

Focal spot (Geometric) blurring: Factors which image magnification decrease sharpness Sharpness optimized by beam penumbra:

background radiation --> darkening with out x ray. Less of a problem w digital, but image receptors can get fog from x ray machine during exposure.

Fog

THE ULTIMATE EFFECT OF FOG AND SCATTER IS TO REDUCE RADIOGRAPHIC CONTRAST

Fog and scatter the enemy of contrast

Sharpness of recorded. Magnificaiton. Shape distortion.

Geometric integrity (recognizability) 3 components:

Greater slope = Increased inherent film contrast & vice versa High contrast film = "Short gray scale"film & vice versa

Image contrast Inherent property of a film/image receptors, determined by the slope of the straight line portionof the characteristic curve

Increases penumbra. Increases umbra. Magnifies image. Decreases edge sharpness.

Increased Source-Image distance and Decreased Object-Image distance:

Decreases penumbra. Increased sharpness. Loss of magnification.

Increased Source-Image distance and object image distance:

Stepper slope Has higher contrast due to fewer grey (color) tone. Black and white (2 colors) are the highest possible contrast.

Increased contrast?

match energy of x ray to binding energy of k shell. Increase in kVp will decrease radiation penetration.

K edge (limits amount kVp increases will increase punching power)

greater magnification.

Larger umbra =

range of exposures over which the image receptor responds with ODs in the diagnostically useful range

Latitude

FOR EVERY 0.3 CHANGE IN OPTICAL DENSITY, THE % T INVERSELY CHANGES BY A FACTOR OF 2.0 %T= ( I_t/ I_0) I_0is light intensity incident on film It is light transmitted through film

Light transmittance (percent transmission, how much is transmitted compared to blocked)

pt moves, image blurs. shorter exposure = less blur.

Motion blur

"static" noise. variation more pronounced with fewer photons due to decreased exposure. Graininess in image.

Mottle?

If object plane and image plane are not parallel,distortion occurs

Object distortion Object position

Thick objects distorted more than thin objects

Object distortion Object thickness

OPTICAL DENSITY (OD) IS USED TO MEASURE FILM BLACKENING OD = log_10( I_0/ It) I0is light intensity incident on film It is light transmitted through film

Optical film density

balance between scatter radiation and pt dose.

Optimum kVp well above minimum required for penetration =

Thickness of Part Should be measured Body Composition Primarily soft tissue, bone, or both? Patient Habitus Sthenic, hypo-or hyperstenic, asthenic Pathology -Radiolucent or opaque?

Pt factors influencing technique

Most important source of mottle/noise! Due to short time of exposure.

Quantum mottle

increased quantum mottle.

Quantum mottle Faster exposure time -->

smaller Grass seed analogy +/-Square root of average number of photons = % fluctuation (Stan. Dev.)

Quantum mottle OCCURS WITH VERY SHORT EXPOSURE TIMES Result of statistical fluctuation in # of photons per unit area of the X-ray beam. % Fluctuation per square mm. increases as the average # becomes?

Defined as the random fluctuation or unwanted variation of film density following a uniform exposure Also known as "Noise" decreases sharpness.

Radiographic mottle

Fidelity w which anatomic structure is imaged on radiograph.

Radiographic quality

DR exposure shouldn't be repeated due to brightness or contrast concerns.

Resolution in digital is pixel size. Contrast resolution preserved so

The ability to image two separate objects as being distinct. Contrast and spatial.

Resolution:

Increasing part thickness Increased field size Increased energy (kVp) of X-rays

Scatter radiation increases with:

distance from anode to image receptor. Distance from anode to object.

Source image (film focal) distance: Object image distance:

The smallest absolute object size that can be reproduced is inversely proportional to one-half the spatial frequency (lp/mm)

Spatial resolution

minimum object size (mm) = 1/2 [1/SF]

Spatial resolution formula

small high contrast objects. Line pairs per mm lp/mm

Spatial resolution is ability to resolve ?

OS= 1/2 (1/3) = 1/6 = 0.17 mm OS= 1/2 (1/5) = 1/10 = 0.10 mm

Spatial resolution math What are the smallest absolute object sizes that can be resolvedby units with spatial frequencies of 3.0 & 5.0 lp/mm?

The ability to image small objects that have high subject contrast (e.g. bone-soft tissue interface)

Spatial resolution:

HOW MANY PHOTONS ARE NEEDED TO YIELD THE SAME OPTICAL DENSITY? (CONVERSION EFFICIENCY?)

Speed vs noise.

fewer gray tones with lower kVp

Step wedge

Lower kVp = Higher contrast Higher kVp = Lower contrast

Subject contrast Changes to kVp

do not affect subject contrast

Subject contrast Changes to mAs

Mostly soft tissues of similar mass density. Use high contrast. Low subject contrast Use low kVpand high mAscombos

Subject contrast Abdomen: Low subject contrast

Differing thickness of same material will attenuate X-ray beam differently. Thicker parts = increased attenuation Thinner parts = decreased attenuation Magnitude of difference is proportional to degree of contrast

Subject contrast Anatomic part thickness

attenuate more X-rays Primarily results from photoelectric attenuating absorptions in patient Especially true at lowerkVp's

Subject contrast Atomic number Higher atomic numbered tissues

Lung = low mass density Bone = high mass density Mediastinalstructures = intermediate High subject contrast Can use high kVp& low mAscombos Safer, lowers image contrast

Subject contrast Chest:High subject contrast

(mass per unit volume) of a tissue results in greater beam attenuation.

Subject contrast Greater density

Water is 9% more dense than ice even though it is all H2O !

Subject contrast Greater density Water & Ice Analogy:

Lower kVp = Higher contrast Higher kVp = Lower contrast Changing mA x time (mAs) does not affect subject contrast!

Subject contrast Radiation quality (kVp) Increasing kVp decreases subject contrast& vice versa

Increased kVp --> decreases contrast.

Subject contrast change in kVp?

Intermediate to high subject contrast. Intermediate to high subject contrast -but kVp must be lowered due to part thickness

Subject control: Extremities:

Combination of setting on control panel.

Technique machine settings

Sharp part of image, center. Blurred part of image, edges.

Umbra: Penumbra:

speeds and inherent contrast.

Visibility of data films (image receptors) are designed w differing?

2 fewer grey tones, lower kVp. Outlines better. Note medulary canal is visible in image 1. so it depends on what your focus is.

Which one is higher contrast?

The value of each pixel is changed by addition or subtraction increase window level proportionally increased image density & vice versa (direct relationship)

Window Level(density):

Each pixel value changed by multiplication/division increased window width lengthens gray scale proportionally decrease image contrast & vice versa a.k.a. gray scale expansion or compression

Window Width(controls contrast):

Range changes without moving center.

Window?

Window level changes scale by moving center.

Window?

An extremelynarrow window width requires the compute ignore a large amount of data outside the chosen range Window width therefore can be said to controlvisibility of detail

Windowing An extremely wide window width requires the computer to ignore fine contrast differences in order to display the entire range of data

mAs can vary more and still result in diagnostic image Wider latitude, greater margin for error Inversely related to film contrast

With wider latitude

patient exposure.

contrast resolution is preserved in digital regardless of?

1000 shades. many more. Image noise is an issue.

dynamic range of screen film about 3 orders of magnitude: Digital is 4-5 orders

penetration Contrast.

kVp

brightness/ optical density (generally time)

mAs

increased in DR, spatial resolution stays the same, contrast resolution is improved.

mAs doesn't affect film contrast if mAs is?

A: edges grayer, less sharp. B:less gray edes, less blur. C: no absorption blur. D: edges grayer, less sharp.

object shape

darker lighter

optical denisty high numbers: low numbers

random darkening due to x ray process. Scattered radiation that reaches film produces unwanted density

scatter

digital optical density chart vs film (curve)

slope for digital is constant

Changing kVp affects: Penetration (average photon energy) Scatter radiation Patient dose Contrast Conversion efficiency of screens The simplest method to increase or decrease OD is via mAs

variable kVp (not tested)

Usesfixed mAs& variable kVp kVpgenerally varies with thickness of anatomy by 2 kVp/cm Typically uses lower kVp, therefore short scale contrast & higher pt. dose Fixed-kVptechnique chart is best! For each anatomic part there is an optimum kVp

variable kVp (not tested)

grey tones based on attenuation coefficients. less dense --> darker.

voxel 3D pixels.

ranges we can distinguish

windowing puts gray tones to