Answer to Question #6078 Submitted to "Ask the Experts"

Q

I am currently running some quality-control tests on a mammography unit. I have been measuring kVp and comparing it with nominal kVp. The unit allows selection of two different target anodes (molybdenum [Mo] and rhodium [Rh]) and two different filtrations (Mo and Rh). I know that Rh filtration is used to produce harder beams for bigger breasts. When using the combination of the Mo target and Rh filtration at low kVps, there is a considerable difference between the measurements and the nominal value on the order of 10 percent-20 percent. This happens when I am operating the machine in manual mode. The results are far better when using the auto mode. My opinion is that when in manual mode (constant mAs) at low nominal kVp, the Rh attenuates considerably more photons with kVp energy and therefore the meter measures a higher kVp. When using higher nominal kVp, more photons with kVp energy manage to get through the filtration and the meter detects more precisely the kVp peak and the right nominal value. Should I use the Mo/Mo combination?

A

For mammography, the kVp accuracy test is done with the Mo/Mo combination. That said, here is the physics:

1. An Rh filter attenuates more of the lower-energy photons in proportion to the higher energy when compared with the Mo filter, that is, beam hardening is greater with the Rh filter for a given thickness.
   
   
2. It does not matter if the x-ray unit is operated in manual or auto mode, since the transmitted x-ray spectrum shape is the same for a given target/filter combination.
   
   
3. There is a finite rise time required by your kVp meter once the exposure starts that you must wait for it to register the correct "measured kVp." If this exposure is terminated during the rise time (that is, small mAs), then there will be a large discrepancy in your reading, that is, this error is inversely proportional to the mAs during this period. Assuming your meter is calibrated and reading correctly, the measured kVp converges to the set kVp during this rise time.
   
   
4. Now returning to your stated observation, it seems that you might be operating at a lower mAs setting under manual mode and still in the early part of the rise time, whereas under AEC mode, the system sets the mAs (under auto-time) which might be large enough to set the exposure time beyond the rise time.
   

Kishor M. Patel, PhD, DABR