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# Technique chart construction

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### Technique chart construction

1. 1. 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
2. 2. ► 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.
3. 3. ► 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.
4. 4. Construction technique of
5. 5. 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.
6. 6. 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) ►
7. 7. 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
8. 8. 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)
9. 9. 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
10. 10. 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%
11. 11. 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
12. 12. 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.
13. 13. ABDOMEN THICKNESS (cm) 8 9 10 11 12 13 14 Grid Ratio 6 :1 kV p 56 58 60 62 64 66 68 mAs 7.5 7.5 7.5 7.5 7.5 7.5 7.5 16 72 7.5 17 74 7.5 18 76 7.5 20 78 7.5 21 80 7.5 22 83 7.5 23 86 7.5 24 89 7.5
14. 14. "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
15. 15. ABDOMEN THICKNESS kvp (cm) 8 66 9 68 10 70 11 72 12 74 13 66 Grid Ratio 6:1 mAs 14 15 16 3.75 17 3.75 18 3.75 19 3.75 20 3.75 21 22 7.5 23 68 70 72 74 76 78 80 83 86 89 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5
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