Question
I am used to performing spot analysis with a Lynx and now am using the myQA Phoenix and Sphinx Compact. What do I look at to know if my image is a good image?
Answer
For the Phoenix and the Sphinx Compact, the focus for a good image is not on the image intensity but in the count limit. Below defines the count limit for the devices.
Historically for the Lynx, in myQA FastTrack a user would acquire an image with the Lynx and look at the image in what is called Lynx Range, which will display the image scaled according to its original intensity which gives you the error bars of keeping the signal above 30 percent for under irradiation and below 90 percent for over irradiation. Below is an image showing this range on a Lynx being used with a Sphinx V1 phantom, not to be confused with the Sphinx Compact. So, for the Lynx to adjust for saturation or low signal a user would adjust the iris or MU per spot to correct for this.
For the Phoenix and Sphinx Compact, there is a change to that process. Both have a larger range original intensity range than the Lynx, because of the availability of the ability to change the capacitor using the gain values.
For the Phoenix and Sphinx Compact the image is displayed in Image Max and Lynx range is not available, which means the intensity of the spot is relative to the highest intensity signal in your image. Making the 90 percent and 30 percent thresholds less relevant to determining if you have a good image.
For the Phoenix and the Sphinx Compact it is important more to focus on the counts rather than the intensity to determine if your image is saturated or in the low SNR and the fix to adjust for that is to just adjust the gain and remeasure the same spot pattern.
Below is a chart for determining the gain based of the energies and MU/Spot that lead to saturation or low SNR.
For the Phoenix and the Sphinx Compact, for a quick check if after you take an image the software should denote if the image is saturated by having a yellow triangle on the measurement (see snippet below).
To determine how close you are to Saturation, in myQA FastTrack, you can look at total counts in two places. First is by clicking on the profile tab next to analysis and changing normalization to Raw Data (cnt). This will display the profiles of the spots of where your cursors lines intersect with.
Second, is to use Multiple spot analysis, which will display the maximum counts of each spot. One using a 1-D Gaussian fit and the other using a 2-D Gaussian fit.
Keep in mind these count values are not per frame and our limit for saturation is 65536 counts per frame. It is good to keep in mind what measurement setting you used to capture your spots to figure out how close you are to approaching a saturated image. After measurement those settings can be found in the bottom right corner of myQA FastTrack.
For more reading on the relationship of spot size and position as they relate to the gain value and the linearity of the gain, see the articles below.