TECHNIQUE CONVERSIONS - Rules of “Thumb”
ONE METHOD TO CALCULATE NEW MAS for Grids & Screens
GRIDS Bushong’s Table ( Carlton’s Table is different)*
Non Grid = 1X * These numbers change slightly from Author to Author
5:1 = 2X
8:1 = 4 X
12:1 = 5X
16:1 = 6X GRID CONVERSIONS
This formula is used to determine the conversion rate from no grid to grid:
GCF -grid conversion factor :
GCF= mAs without the grid
mAs with the grid
This formula is used when changing from one grid ratio to another:
mAs 1 = GCF 1 mAs 1 is the original mas, GCF 1 is the original Grid
mAs 2 GCF 2 mAs 2 is the new mas, GCF 2 is the new Grid used
(Note they are directlyproportional)
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FILM/ SCREEN COMBINATIONS
This formula is used when changing from one film/screen speed to another:
RS = SPEED OF SCREENS IN THE FILM/SCREEN CASSETTE
This may also be known as F/S for film/screen combination
mAs 1 = RS 2 Can be otherwise written as:
mAs 2 RS 1
RS 1 X mAs 1
(Note they are inverse) mAs 2 = ------
RS 2
mAs 1 is the original mas, RS 1 is the original screen speed used
mAs 2 is the new mas, RS 2 is the new screen speed used
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RULES OF ORDER FOR CONVERSION (Suggested – any order is okay)
1. MAS
2. GCF
3. KVP
4. RS (F/S) FILM/SCREEN SPEED
5. SID
TECHNICAL “RULES OF THUMB”
TISSUE THICKNESS -
For every 1cm of change in thickness, add or subtract 2 kvp
For every 5 cm of change in thickness, either double or half the mAs.
CENTRAL RAY ANGLE -
Decrease SID by 1" for every 5 tube
MAS Changes -at least 20 - 30 % mas change needed to see a visible change in density
Maintaining Density LAW L when changing SID - mAs Changes
Above 40 “ = increase mAs 5% for every 1" increase in SID
mAs 1 = D 12 mAs 1 is the original mas, D 1 is the original distance
mAs 2 D 22 mAs 2 is the new mas, D 2 is the new distance
(Note they are directly proportional- distance increases – mas should increase
** When changing SID from 40" to 72" = increase mAs X 3.24 (ave 3 MAS)
When changing SID from 72" to 40" =( divide) mAs 3.24
( example : KUB - 40mas at 40 “ = 129.6 mas at 72")
Use mas/distance formula to verify
GENERAL RULES FOR DISTANCE CHANGE:
When changing from 40" SID to:
New Distance Technique Change (MAS)
30"½
40"1 (standard)
60"2 X
72"3 X
80"4 X
96"6 X
Remember : 15% change ( ) KVP has the same effect as doubling or ½ the MAS
INCREASE DENSITY BY 2 X you could: Example: 20 mas - 70 kvp
Go up 2 X in mas 40 mas - 70 kvp
or Go up 15% Kvp 20 mas - 81 kvp
INCREASE DENSITY BY 4 X you could: Example: 20 mas - 70 kvp
Go up 4 X in mas 80 mas - 70 kvp
or Go up 2 X MAS & 15% Kvp 40 mas - 81 kvp
or Go up 30% in KVP (15% x 2) 20 mas - 91 kvp
Maintaining Density when changing OID -
- Increase mAs 10% for every 1" increase in OID
FOR CHEST AND THORAX AREA - TO GO FROM AP/PA TO LAT:
* Double the MAS and go up 10 kvp