Materials and Methods

1) Brainstorm an approach for developing a proper fixed kVp technique chart on a phantom using the Thales Flat Panel Detector system.

2) Measure the phantom parts for each projection/position assigned using calipers.

3) Document each measurement of the phantom’s anatomic parts in centimeters depending on the position the phantom’s parts are in for each projection and the anatomic path required.

4) Using both the provided lab technique chart and the comparative anatomy approach, determine the baseline kVp range needed for each anatomical region and projection that will be sufficient enough to penetrate the part.

5) For each projection, position the phantom using the proper CR centering, part/tube/IR alignment, grid, SID, tube angles, FOV, OID, patient position, markers, etc.

6) Enter phantom patient information, proper part and projection, and technical factors into the Thales system for each exposure being taken.

7) Once a sufficient kVp is determined based on the thickness of the part for whichever projection (AP/PA, Oblique, or Lateral) being imaged, test out varying mAs values on the console that will generate a DI value in the -0.5/+0.5 range for each separate exposure needed.

8) To determine sufficient mAs, make calculations using the 30% mAs rule and 4 cm rule and increase/decrease mAs values as needed. Remember, no amount of mAs can make up for insufficient kVp, so ensure the part is being properly penetrated by evaluating the subject contrast of each exposure. Use the 15% rule to adjust kVp as needed.

9) Note the technical factors, DI and EI values, contrast, spatial resolution, etc. for each acceptable exposure and save/send the image to PACS.

10) Use the data obtained to draft a fixed-kVp chart and make sure all projections are accounted for.

11) Complete assignments for the project as a group before stated deadlines.

During the experimentation phase of the project, our group brainstormed and contemplated the approach we would take when developing a fixed-kVp chart on a full-body phantom from scratch while using the Thales Flat Panel Detector system. We decided a good start to the development of this chart would be to take some calipers and measure the thickness of each assigned part based on the necessary position the part would be in for each projection. After gathering the data and measurements in centimeters, we figured we would use the comparative anatomy approach to get an idea of what the minimum kVp would be needed to penetrate each anatomic part. The concept of comparative anatomy states that different parts of the same size or thickness can be radiographed using similar technical factors as long as the minimum kVp needed to penetrate the part is used for each projection. It was crucial for us to remember that no amount of mAs set on the console can make up for insufficient kVp.

After obtaining the measurements, we decided to group our anatomic parts by similar thicknesses while using the standard technique chart provided in the lab as a baseline for our kVp value starting point. Since the full-body phantom presents internal issues such as the presence of atypical hardware, lack of flexibility, lack of biological variability, simplified tissue representation, etc., we understood that while the phantom is meant to represent an average, sthenic person, it may present limitations that alter our technique and images taken. However, we proceeded with the experiment by positioning the phantom using the proper CR centering, part/tube/IR alignment, grid, SID, FOV, OID, patient position, markers, etc., for each projection. We also entered phantom patient information, proper part and projection, and technical factors into the Thales system for each exposure being taken. We decided to take each exposure in order of the thinnest part to the thickest part in terms of the measurements we gathered earlier. 

Following some contemplation and trial and error, we decided that 60 kVp would be enough to penetrate a PA hand and lateral ankle. Along with this, 75 kVp would be enough to penetrate AP knee, AP shoulder, PA axial clavicle, AP hip, AP oblique ribs, and PA Caldwell skull. And, 80 kVp would be enough to penetrate AP lumbar spine, while 110 kVp would be enough to penetrate a lateral chest. These kVp values were determined on the habitus of the full-body phantom and the specific thickness measurements of its parts, although we believe the kVp values would be enough to penetrate parts of small, medium, and large thicknesses as well, which is one of the goals of a fixed-kVp chart. 

Once we determined the kVp values that would sufficiently penetrate the part and provide optimal subject contrast, mAs adjustments were made for each projection. This was based on whether or not the DI value for the exposure was in the positive range (overexposure) or negative range (underexposure). Since 30% mAs adjustments are needed to see a visual difference in the radiograph, we tried to adjust mAs value by at least 30% increments until an optimal DI value in the +/- 0.5 range was achieved for each exposure. The kVp values determined earlier, in tandem with the mAs adjustments we made for each exposure, provided optimal subject contrast, exposure, and quality for all 10 projections needed.

Finally, we noted the technical factors, DI and EI values, contrast, spatial resolution, etc. for each acceptable exposure and saved/sent the image to PACS. We then used the data obtained to draft a fixed-kVp chart while making sure all projections were accounted for.