Timer Accuracy

While the quality of the X-ray beam is affected predominantly by kilovoltage peak (kVp) selection, the quantity of radiation present in the X-ray beam is primarily affected by selection of both milliamperage and exposure time; this selection is collectively referred to as mAs. Appropriate selection of mAs is one of the most influential ways that a technologist can reduce patient dose. It is critical then, that the exposure timer is properly calibrated and accurately describes for how long radiation is emitted from the X-ray tube.

To test this accuracy, this lab was performed by exposing a digital timer, such that the digital timer would display how long an exposure was made as measured to the millisecond. This procedure was performed on two different X-ray tubes. Exposures were made at 80 kVp, large focal spot, 200 mA, and starting from 5 mAs (i.e exposure time = 25 ms), exposure time was doubled after each of five exposures.

Table 1: Exposure factors

Exposure	kVp	mA	mAs	Exposure Time
1	80 kVp	200 mA	5 mAs	25 ms
2	80 kVp	200 mA	10 mAs	50 ms
3	80 kVp	200 mA	20 mAs	100 ms
4	80 kVp	200 mA	40 mAs	200 ms
5	80 kVp	200 mA	80 mAs	400 ms

Image 1: Digital X-Ray Timer

After each exposure, the digital timer would read out how long the actual exposure was, as seen in image 1 above. The evaluation of the accuracy of exposure time was therefore a qualitative one. Table 2 below shows three types of values: the selected exposure time at the control console, the digital timer readouts for both rooms, and the acceptable limits stated by Safety Code 35 and by the HARP Act.

Table 2: Selected Exposure Time, Digital Timer Readouts, and Government Standards

Exposure	Selected Exposure Time	Readout for Room 3	Readout for Room 6	SC 35 Standard (within ± 5%)	HARP Act Standard (within 1/30 of a second or 10%, whichever is greater)
1	25 ms	26 ms	26 ms	23.75 to 26.25 ms	23.75 to 26.25 ms
2	50 ms	51 ms	51 ms	47.5 to 52.5 ms	47.5 to 52.5 ms
3	100 ms	101 ms	101 ms	95 to 100 ms	95 to 100 ms
4	200 ms	201 ms	201 ms	190 to 210 ms	190 to 210 ms
5	400 ms	401 ms	401 ms	380 to 420 ms	380 to 420 ms

As Table 2 shows, the results are well within the acceptable limits enforced by Safety Code 35 and the HARP Act. Note that the standards for the HARP Act are those stipulated for a non-mechanical timer. Consequently it appears that no corrective action is needed.

This lab outlines an interesting point to make concerning quality control tests in general. It would be irresponsible to assume that because of the results of this test, corrective action is necessarily not required. However the test relies on a digital timer which itself must be properly functioning and calibrated for the results of this test to be valid. Hence, before the accuracy of the exposure timer can be evaluated it must be known for certain that the digital timer used to measure is also properly functioning. This concept of using properly calibrated test equipment extends to various other tests, including some of the labs discussed in Cluster 1.

As mentioned above, the accuracy of an exposure timer is crucial to an X-ray technologist because it helps reduce patient dose. If the exposure duration is longer than expected then the patient becomes exposed to ionizing radiation for longer than necessary. Furthermore, if the image should become overexposed while in a film-screen environment it may be diagnostically unusable, requiring a repeat image. Conversely, if the exposure duration is too short in either the film-screen or digital radiography environments, the resulting image will also be diagnostically unusable. In cases of under or overexposure, a repeat image is required and patient dose increases. Ensuring a properly functioning exposure timer allows the technologist to uphold the ALARA principle and provide the best patient care possible.