Class Notes 09.15.08

Class notes 09.15.08 Visibility of detail Visibility of detail is the ability to see the detail or the sharpness of the radiographic image. Sharpness of the image detail refers to the structural lines or borders of tissue in the radiographic image. KVp can influence the radiographic density, such as: • Low kVp = high contrast = decrease density – short scale of gray. • High kVp = low contrast = increase density – long scale of gray. 15% rule controls radiographic density. • + 15% of kVp = 2 X density • - 15% of kVp = ½ X density Key word is to maintain the same density. Using 70 kVp the radiologist a higher contrast radiograph? Decrease the kVp 15%, increase mAs 2 times to compensate. Question: 70 kVp, 10 mAs, higher contrast radiograph, two times the density. 70 kVp x 15% ≈ 60 kVp, there for the mAs will automatically be cut in half, but since you need to compensate for density you need to multiply 10 mAs x 2 = 20 mAs to bring it back to normal, but since the question ask for two times, you will need to double it again which will give 40 mAs. If you want a lower contrast radiograph with ½ of the original density, increase kVp 15% = 80 kVp, automatically it will double the density, you’ll want to cut the mAs in half to compensate for the automatic adjustment, but since the question want ½ of the original you would have to cut the mAs cut in half again. Inverse square law The closer the source of x-ray photon is to grid. Conversion Factors: From non-grid Direct square law mA – distance Time – distance mAs – distance Screen Speed Grid conversion factors in mAs Grids mAs 5:1 2 X mAs 6:1 3 X mAs 8:1 4 X mAs 10:1 4 X mAs 12:1 5 X mAs 16:1 6 X mAs

Class notes 09.15.08

Grid conversion factors in kVp Grids kVp 5:1 +8 6:1 + 12 8:1 + 15 10:1 + 15 12:1 + 20 – 25 16:1 + 20 – 25 Correction Factors for Age Age Correction Factor Birth – 3 months 0.2 4 months – 2 years 0.3 2 years – 2 years 0.4 5 years – 5 years 0.5 7 years – 12 years 0.6 a. Multiply the correction factor time the mAs employed for the adult anatomic part. b. Employ the kVp for centimeter thickness. c. Do not use for chest correction.

Cast Radiography a. Measure the part and add 10 kVp for a DRY Cast. b. Measure the part and add 10 kVp for a WET Cast. Reciprocating Bucky conversion a. A Bucky technique requires, at least, 3 times the exposure from non-Bucky. b. A Bucky technique requires an increase of approximately 20 kVp from nonBucky. Conversion Factor – field size 14 X 17 – in field to 10 X 12 – in field, increase mAs 40% 14 X 17 – in field to 8 X 10 – in field, increase mAs 60% Beam Restrictions Devices • Item used to shape the x-ray beam to fit the part of the body • Device types – Collimator – Cone – Diaphragm Body Habitus Sthenic shape – average, most radiographic technique charts are based on this shape. Hyposthenic shape – thin shape