Download - RT 123 WK 3 Lecture #3
1 RT 123
WK 3Lecture
#3
PRINCIPLES OF TECHNIQUE AND EXPOSURE
2Factors Affecting DETAILMAGNIFICATIONDISTORTIONTECHNIQUE SELECTION (TIME)FOCAL SPOT SIZEIR Speed (f/s) OID / SOD / SIDFactors 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 techniqueInfluence the imageFilm 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
OverexposedReferring to a radiograph that is too dark because tooUnderexposedReferring 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