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WK 3Lecture

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PRINCIPLES OF TECHNIQUE AND EXPOSURE

2Factors Affecting DETAILMAGNIFICATIONDISTORTIONTECHNIQUE SELECTION (TIME)FOCAL SPOT SIZEIR Speed (f/s) OID / SOD / SID

Factors Affecting DENSITYPATIENT THICKNESS,PATHOLOGYMAS & KVPSID

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OID

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Part to IR position

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Detail

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OID

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Part to IR position

PRIME FACTORSSID BEFORE YOU LEAVE THE ROOMKVP CONTAST RANGE FOR PARTMAS CHANGES WITH PT SIZE , Pathology

11TECH NICAL FACTORSAFFECTING QUALITY

Producing optimal radiographs

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SID & SOD

SID –Source to Image Receptor Distance TUBE TO CASSETTE 40” (44) AND 72 “ STANDARD

SOD - Source to Object Distance NOT LESS THAN 12 “ SHOULD BE 15”

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SID CHEST 72 “ = 6’ (WING SPAN)

NOT 10’ (120”)

TUBE LOAD – Heat load on tube – the longer the SID the more Technique required to produce the image

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IMAGES DENISITY = THE AMOUNT OF BLACKENING

“DARKNESS” ON THE RADIOGRAPH

CONTRAST – THE DIFFERENCES BETWEEN THE BLACKS TO THE WHITES

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TECHNICAL FACTORS KVP / MAS / SID DEPENDING ON PART & SIZE of PART

Use FASTEST TIME POSSIBLE – TO REDUCE MOTION ↑ kVp & ↓ Mas = lower Patient dose

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How does the technique

Influence the image

Film screen / CR / DR ?????

CONTRAST & DENSITYPRIME FACTORS

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Radiographic Prime factors The factors principally responsible for x-ray quality and

quantity. These are mAs, kVp, distance (SID).

19X-ray quantity (mAs)is a measure of the number of x-ray photons in the beam. Also called x-ray output, intensity or exposure.

X-ray quality (kVp)is a measure of the penetrating ability of the x-ray beam

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IMAGES DENISITY = THE AMOUNT OF BLACKENING

“DARKNESS” ON THE RADIOGRAPH

CONTRAST – THE DIFFERENCES BETWEEN THE BLACKS TO THE WHITES

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Digital Imaging AMOUNT OF ‘BRIGHTNESS”

Can be controlled and changed with computer****

*** proper technique and exposure index still important

Overexposed

Referring to a radiograph that is too dark because tooUnderexposed

Referring to a radiograph that is too light because too x-radiation reached the image receptor

Film Screen

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Good techniques – importantfor GOOD images + lower PT doses

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EXPOSURE FACTORS STILL IMPORTANT!

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80 Ma x.04s vs 160 ma X .02 ?mas

3.2

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MAS DOUBLED – DENSITY

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Kvp & contrast (F/S)

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“SHORT” VS “LONG” SCALE low kVp Higher kVp

More on this in Ch 11, 25 & 26 Next week !

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How Are X-rays Made?

X-rays are produced when electrons strike a metal target.

The electrons are liberated from the heated filament and accelerated by a high voltage towards the metal target.

The X-rays are produced when the electrons collide with the atoms and nuclei of the metal target.

mAs – amount

kVp - energy

Kilovoltage Peak - kVp

One kilovolt is = to 1000 volts The amount of voltage selected

for the

x-ray tube Range 45 to 120 kVp

(diagnostic range) kVp controls contrast

mA X s = mAs

34MilliamperagemAs

•One milliampere is equal to one thousandth of an ampere.•The amount of current supplied to the x-ray tube•Range 10 to 1200 mA

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Time In seconds How long x-rays will be produced 0.001 to 6 seconds

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MAS Changes -at least 20 - 30 % mas change needed to see a visible change in density

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+ 25% + 50 % mas

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+ 15% kvp = 15% kvp

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Influencing factors: kVp 15% rule:

15% kVp = doubling of exposure to the film

15% kVp = halving of exposure to the film

15% rule will always change the contrast of the image because kV is the primary method of changing image contrast.

Remember : 15% change ( ) KVP has the same effect as

doubling or ½ the MAS on density

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CHEST RADIOGRAPHY UPRIGHT - WITH GRID

KVP RANGE (90 – 120 )

MAS ( 1 – 3 PA) ( 5 – 20 LAT)

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“AVE” CHEST EXPOSURE - 2 IMAGES (Ave – Pt meas 23cm pa 40cm lat)

PA 110 KVP 5 MAS 16:1 GRID 400RS 72”LAT 110 KVP 20 MAS 16:1 GRID 400 RS 72”

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FOR CHEST – TO GO FROM PA TO LAT:

↑ Technique by 4 x = Double the MAS and go up 10 kvp

INCREASE DENSITY BY 4 X you could: Example: PA CHEST: 2 mas - 110 kvp

FOR LATERAL:

Go up 4 X in mas 8 mas - 110 kvp

Go up 2 X MAS & ↑ 15% Kvp 4 mas - 125 kvp

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MAINTAIN SAME ODFOR CHEST FILM

80 KVP , 5 MAS, 94 ?

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CHEST RADIOGRAPH: too light& want longer scale contrast

4 mAs 85 KVP ____ mAs ____ KVP

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Shorten TIME 400 MA, 1/20s, 70 KVP 100 MA, _____ , 80 KVP