X-ray field must coincide with the light field of the variable aperture light localizing collimator. any kind of misalignment may cause cutoff of pertinent anatomy. These days most machines use Positive Beam Limitation (PBL) collimators or automatic collimation.
Collimation misalignment must not exceed:
+/- 2% of SID
Focal Spot Size
Focal spot is part of the target from which x-rays are emitted. The smaller the focal spot; the better the spatial resolution of the image.
Small Focal Spot
small anatomy (ex. Extremities)
Large Focal Spot
large anatomy (ex. Abdomen)
Focal Spot Size can be measured by using:
pinhole camera-excessive exposure
the star pattern-limited to focal spot < 0.3mm
slit camera-commonly used
Effective focal spot size should be within _____% of size stated in equipment specifications provided by the vendor
as tech we all set up kVp for every examination (unless pre-programmed due to AEC). Tests are done to ensure the accuracy of kVp selected and that the x-ray generator is accurately calibrated.
The measured kVP should be within +/- ___% of the indicated kVp.
Exposure Time Accuracy
mostly responsible for patient dose. Controlled either by tech or AEC. Medical physicists are using many commercial devices to assess the accuracy of the exposure timer.
Exposure timer accuracy should be within +/- ___% of the indicated timer for exposure times greater than 10 ms. AEC must also be checked for consistency on providing optical density regardless of patient thickness.
test to check whether a radiographic unit is producing constant radiation output for various combinations of mA and exposure time. Tested with a digital dosimeter-how much exposure that's absorbed is the same.
The variation of linearity cannot exceed +/- ____%.
Also tested with a digital dosimeter, basically involves kVp, mA and exposure time that is used to produce an optimum image. If any of these factors are changed and then reset again to the same original values, radiation exposure should be precisely the same. Want to be able to reproduce the same image every time.
Sequential radiation exposures should be reproducible to within +/- ____%.
Radiographic Intensifying Screens
Should be cleaned depending on the workload of the department to avoid any appearance of artifact on the final image. Screen cleaner and soft, lint free cloth should be used.
Screen-film contact should be evaluated once a year by using a wore mesh pattern test. make sure screen contact is proper.
Blurry areas on the film indicates foam or felt pad under screen should be replaced.