Resident Physics Lectures

Resident Physics Lectures

The The RadiographRadiographic Image & ic Image & GeometryGeometry

George DavidAssociate ProfessorDepartment of RadiologyMedical College of Georgia

Contrastdifference in density between areas on the

radiograph

Contrast depends onsubject contrastreceptor contrastscatter

Subject Contrastdifference in x-ray

intensity transmitted through various parts of subject

Depends onthickness differencedensity differenceatomic number

differenceradiation quality (kVp,

HVL)

I

IS IL

Subject Contrast = IS / IL

*

Subject Contrast & Radiation Quality

high kVp = lower subject contrast long scale contrast (less difference

between areas receiving varying amounts of radiation)

low kVp = high subject contrast short scale contrast (more black &

white; more difference between areas receiving varying amounts of radiation)

low kVp increases patient dose

*

ScatterReduces contrastProduces unwanted densityMostly a result of Compton

interactionsIncreases with

kVp part thickness field size

collimation reduces scatter

kVp & Exposure LatitudekVp affects latitudeIncreasing kVp

decreases contrastincreases latitude

kVp must match latitude requirements of exam

*

Exposure Latitude With Filmrange of

incident radiation intensities which produce desired film density

Latitude & contrast vary inversely high contrast = low

latitude low contrast = high

latitude

log rel. exp.

OpticalDensity

.25

2.0

Latitude

Speed & ContrastContrast controls slope of

characteristic curve

Lower ContrastHigh Latitude

log relative exposure

OpticalDensity

Higher ContrastLow Latitude

log relative exposure

OpticalDensity

*

Whites whiter, blacks blacker

Exposure Latitude

For low contrast filmshallow slopegreater exposure latitude

wider mAs range produces proper film density

increasing kVp causesdecreased contrast (slope)increased latitude

log relative exposure

OpticalDensity

log relative exposure

OpticalDensity

Low ContrastHigh LatitudeHigher kVp

High ContrastLow LatitudeLower kVp

Film/Screen Limited Latitude

Film required proper radiation exposure

High Digital Latitude

Image Qualityability of image receptor to record

each point of image as point on the display

Influenced byradiographic mottle

also called noisenoisesharpnessresolution

Image Quality: What is it?Depends only on intrinsic, objective

physical characteristics of imaging systemCan be measured independent of observerQuantitative

Whatever observer says it isSubjective perception of image

Defined by observer’s ability to achieve an acceptable level of performance for a specified task.

Courtesy Ralph Schaetzing, Carestream Health

Quantum MottleAppearance

irregular density variations in mid-density areas exposed to uniform x-ray fields

Causerandom x-ray emissionstatistical fluctuations in # of

quanta / unit area absorbed by receptor

Mathrelated to square root of total

number of photons interacting with receptor

Quantum MottleMath (cont.)

fractional fluctuation greatest when # of photons is smallest 10 100

---- > --------- (.1 > .01) 100 10,000

throw a dice 12 times or 12,000 times; variation from expected 1/6 for each face will probably be more for 12 throws!

Numerator is square root of denominator

Quantum MottleBest visualized on good-quality high

contrast radiographPoor detail (blurring) may mask quantum

mottle Raising kilovoltage while maintaining the

same receptor exposure results in:

lower patient exposurelower mAs forfewer x-ray photons

higher quantum mottle

Speed

Film Measure of sensitivity to light Faster speed means

less light (or radiation) required to achieve same image density (darkness)

Image produced with less radiation Increased quantum mottle (noise) at same density

Digital No “fixed” speed (Sprawls) Can produce images with good contrast over wide

range of receptor exposure Receptor exposure dictates image noise

Noise & SpeedCause of noise (quantum mottle)

statistical fluctuation in # of x-ray photons forming image

Ability to see high contrast objects limited by image sharpness

High noise reduces visibility of low contrast objectsmost important diagnostic information here

Similar Triangle Review

FocalSpot

Object

Receptor

a b c h---- = --- = --- = --- A B C H

Receptor

B A

H

CObject

b a

h

c

size of image--------------------

size of object

Magnification DefinedFocalSpot

ObjectFilm

(image)

focus to film distance HMagnification = ---------------------- -------- = --- focus to object distance h

Using Similar TrianglesFocalSpot

ObjectFilm

(image)

h

H

size of imageMagnification = --------------------

size of object

focus to receptor distance Hmagnification = -------------------------------- = --- focus to object distance h

Using Similar TrianglesFocalSpot

ObjectFilm

(image)

h

H

size of image

Magnification = --------------------size of

object

size of image = size of object X Magnification

focus to receptor dist.size of image = size of object X ---------------------------------

focus to object dist

Optimizing Image Quality

Minimize magnificationMinimize object-receptor distanceMaximize focal-receptor distance

FocalSpot

Object

Receptor(image)

h

H

focus to receptor distance Hmagnification = --------------------------------------- = --- focus to object distance h

*

Automatic ArtifactOccurs whenever we image a 3D object

in 2DWork-around

Multiple views

?? ??

SharpnessAbility of receptor to

define an edgeSharpness and Contrast

unsharp edge easier to detect under conditions of high contrast

sharp edge are less visible under conditions of low contrast

One cause of unsharpness PenumbraPenumbraShadow caused by finite size of focal

spot

Minimizing Geometric Unsharpness

minimize focal spot size

maximize source to image distance

minimize object to image distance

minimize

maximize

Minimize

Sources of Unsharpness

GeometryMotion

minimized by short exposure timesAbsorption

absorber may not have sharp edges round or oval objects

Total Unsharpness combination of all the above

BUTnot the sum!

larger than largest componentlargest component controls unsharpness

improvement in smaller components don’t help much

Sharpness & Resolution

Sharpnessability of imaging system to record sharply

defined margins or abrupt edgesResolving Power (Resolution)

ability to record separate images of small objects very close together

Distortion TypesX-RayTube

Image

Shape Distortion

X-RayTube

Image

Relative Position Distortion

minimal distortion when object near central beam & close to receptor

PenumbraLatin for “almost

shadow”also called edge gradientedge gradient

region of partial illumination

caused by finite size of focal spotsmears edges on imagezone of unsharpness called

geometric unsharpness penumbra edge gradient Image

Line sourcefocal spot

Penumbra Calculation

Line sourcefocal spot

Object

F

P

SOD

OID OIDP = F x ------- SOD

SID

Minimizing Penumbra

•Minimize object-receptor distance (OID)

•Maximize source-object distance (SOD)

•Makes focal spot appear smaller

•Minimize focal spot size

Motion UnsharpnessCaused by motion during exposure of

patientTubeReceptor

Effectsimilar to penumbra

Minimize byimmobilizing patientshort exposure times

Absorption UnsharpnessCause

gradual change in x-ray absorption across an object’s edge or boundary thickness of absorber presented to beam

changesEffect

produces poorly defined margin of solid objectsX-Ray

TubeX-RayTube

X-RayTube

Inverse Square Lawintensity of light falling on flat

surface from point source is inversely proportional to square of distance from point sourceif distance 2X, intensity drops by 4X

Assumptionspoint sourceno attenuation

Causeincrease in exposure area with

distance

Intensity 1/d2

d

Trade-offGeometry vs. Intensity

maximize SID to minimize geometric unsharpness

butdoubling SID increases mAs by X4

increased tube loadinglonger exposure time

possible motiongoing from 36 to 40 inch SID requires

23% mAs increase

F

P

SOD

OID

SID

Off-Axis Variationfocal spot measurements

normally made on central rayapparent focal spot size

changes in anode-cathode directionsmaller toward anode sidelarger toward cathode sideless effect in cross-axis

direction

Focal Spot SizeTrade-off

heat vs. resolving powerexposure time vs. resolving power

Focal Spot Size most critical formagnificationmammography

Resolution

Unitslines or line pairs per

distance such as lead bars

separated by equally wide spaces

Expresses limiting resolution Limiting resolution

implies high contrast situation

does not indicate how well system preserves contrast

1 mm

4 lines (line pairs) per mm

Modulation Transfer Function (MTF)

value between 0 and 1¤ MTF = 1 indicates all information

reproduced at this frequency¤ MTF = 0 indicates no information

reproduced at this frequency

MTFIf MTF = 1

all contrast reproduced at this frequency

RecordedContrast

Contrast providedto film

MTFIf MTF = 0.5

half of contrast reproduced at this frequency

RecordedContrast

Contrast providedto film

MTFIf MTF = 0

no contrast reproduced at this frequency

RecordedContrast

Contrast providedto film

MTFas sharpness decreases so does contrast

less sharp system blurs dark & light areas together maximum density decreases minimum density increases

at very high line pairs per mm film will be uniform gray

Modulation Transfer Function (MTF)

Fraction of contrast reproduced decreases at increasing frequency because lines and spaces blur into one another

Lowest Frequency

Highest Frequency

MTFCombines concepts

sharpnessresolutioncontrast

1

MTF

Frequency0