2. Identifies reliable exposure factors (mA,
kVp and exposure time) for a known tissue
thicknesses
► A radiograph exposure chart can be
constructed to suit any X-ray machine.
►
► Exposure charts may be kVp-variable or
mAs-variable
3. ► A kVp-variable chart uses constant settings
for mA and exposure time (seconds) and
indicates the appropriate kVp for specific
tissue thicknesses and types
► An mAs-variable chart, recommends
various mAs settings for specific tissue
thicknesses and types, allowing the
operator to keep the kVp constant for all
exposures.
4. ► Two basic charts –
one to list exposures to use in conjunction with a
grid
►
the other to indicate the settings for gridless
radiography.
►
►
A grid is recommended for radiographing
thoraxes deeper than 14cm and any other
body part thicker than 10cm.
6. Selecting Patient:
► Average
size animal (say 40-50 lbs dog)
with clean coat, either anesthetized or a
cooperative patient.
► The initial technique chart is constructed for
abdominal settings.
► Other anatomical regions can be
extrapolated from this chart.
7. To Begin
With patient in lateral recumbency, measure
the thickness of the desired part (abdomen at
approximatety twelth rib)
► If the measurement is an intermediate value
(such as 13.4cm=14) record the next highest
whole number.
► Measure FFD( generally 40“ for large stationary
systems)
► Measure Grid ratio (8:1)
►
8. lnitial kVp setting:
►Sante's rule:
(2x measured thickness) + FFD+ (grid factor if > 10 cm) =initial kVp
► Grid Factor Ratio
5:1
8:1
12.1
16:1
Added kVp to sum of Sante's Ru
6-8
8-10
10-15
15-20
9. lnitial mAs setting:
►
Screen Type
Fast (High Speed)
Medium( Par Speed)
Slow (Ultra Detail)
►
Recommended mAs based on a par (medium) speed screen
►
►
►
►
►
mAs
2.5 - 10
5 -12.5
30-40
Table top (no grid) extremity: 2.5mAs
Thorax: 5mAs
Abdomen: 7.5mAs
Spine: 10mAs
. Cats- halve the mAs from dog
High kVp for soft tissue exams( longer gray scale)
Lower kVP for bone studies( shorter gray scale)
10. Evaluate the exposure
technique
► by
the following parameters :
Density - the degree of blackness
Contrast- density difference between two
areas of a finished radiograph.
Goal = radiograph with a "perfect" exposure
Perfect exposure means:
► For abdominal imaging a longer gray scale preferred
to differentiate between soft tissue structures.
► Bone
studies shorter gray scale
11. Technique modification if required
►Film
too Dark
too Light contrast
Sufficient
between bone and soft
tissue
YES
NO
mAs 30-50%
kVp10-15%
Film
Outline of abdominal
structures visible
mAs 30-50%
kVp10-15%
12. Retake radiograph until a perfect film exposure
is obtained.
► Extrapolate values from "perfect exposure” film
for abdominal technique chart as follows:
►
Add 2 kVp for each cm increase from the original
measurement upto 80 kVp.
Add 3 kVp for each cm increase between 80-100
kVp
Add 4 kVp for each cm increase that places the kVp
above 100
Subtract 2kVp for each cm decrease from the
original measurement
13. Example
Dog
Thickness at 12 th rib=12”
FFD = 32
Grid ratio=6:1
kVP (2xmeasured thickness) + ffd+ (grid factor if > 10 cm) = kVp
► (2x12) +32+8=64 kVP
► A good starting point for mAs setting for our
medium sized dog for abd. is 7.5 mAs.
15. "Tweaking" the technique chart
►It is preferable to use high kVp for soft
tissue exams due to the longer scale of
contrast
►To modify the extrapolated techniques
to allow for higher kVp settings use the
following rule:
Add 10 kVp then halve the mAs