Water's View of Skull

Water’s view of the skull. We add 4-6 to the KVP of a PA Skull. Therefore, we must first find an optimal technique for a PA Skull. 

Using the Technique by Proportional Anatomy table, we know PA skull is 2X the mAs of a AP C-spine technique. Therefore, we multiplied the mAs from the AP C-spine 6.3 by 2 to get to 12.6mAs (real world 12.5mAs) for a PA skull. Which made our technique 74 kVp 12.5 mAs. These factors resulted in a DI value of -5.1. 

To fix the DI value we increased the kVp 15% resulting in a kVp of 85. We also increased the mAs by 50% (2x25%) resulting in a mAs of 19.5 (real world 20mAs). Making our technical factors of 85 kVp and 20 mAs with a DI value of 0. 

Now that we have the optimal technique for a PA skull we add 5 to the kVp resulting in a kVp of 90 for a water’s view of the skull. 

Using 90 kVp and 20 mAs technical factors resulted in a DI value of 2.7. To decrease the exposure, we reduced the mAs by a total of 40% (2x20%) resulting in a mAs of 12.8 (real world 12.5mAs).  When using 90 kVp and 12.5 mAs it resulted in a DI value of 0.5. 

FINAL EDIT:

Through further review, we noticed that the Water’s projection was not positioned correctly. Therefore, on week 3 of our experiment we repositioned the phantom with a SID of 36 since we decided to use a 20˚ caudal angle. We shot the image with the same technical factors from week two of 90 kVp and 12.5 mAs. Resulting in a DI of -1.2. 

To fix the underexposures we increased the mAs by 25% making the mAs 15.6 (real world 16 mAs). When we shot the projection at 90 kVp and 16 mAs with 36” SID and 20˚ caudal the DI value was -0.2.