DR Flatfield QC

               Although pixels look small, they make big impacts when it comes to diagnostic imaging. A pixel’s duty is to receive the beam electrons and then later transfer the data and form an image or a radiograph.  If there are a lot of non-functioning pixels in a detector, it would greatly decrease the quality of the image.   Each pixel contains a unique data that other pixels cannot be replaced and that is why pixel’s condition is so important.  It is important to check every 3 to 6 months to see if all the pixels are still functioning normally. Otherwise, dead pixels would show up as artifacts or white dots and other technologists might think it is an actual artifact and might lead to an unnecessary repeat.

             Since pixels play such a big role, I think it is important to maintain it quality.  A Quality Assurance Procedures, or formerly known as QAP is the only way to test if there are any dead pixels in a detector. As mentioned above, too many dead pixels will decrease the quality of the image because the dead pixels will appear white and shown as artifacts.  Having such artifacts will affect the result of diagnosis because the artifacts might be on the exact same spot of pathology.  Each manufacturer has a different acceptance range of bad pixels. According to GE, the manufacturer of our machine, the pass or fail depends on the number of dead pixels. Here is our test result.

                  As seen above, there was only one dead pixel and we passed the image quality test. We should keep track of each month’s QAP to see if there are any more dead pixels, because eventually, the dead pixels would accumulate and fail the quality assurance test.  If we do not perform any QAP, the dead pixels will gives us troubles during our labs because of the numbers of the dead pixels. This is another reason why quality control should be carried, so that the equipments are kept at high quality and utilize itself.

kVp Accuracy

In every day’s use of radiographic equipments, technologists must select optimistic kVp for all examinations according to patient size, diagnostic purpose, and etc. In diagnostic radiography, kVp is very powerful. A variation of approximately 4% is necessary to influence image optical density and image contrast.  kVp can significantly affect image quality, special resolution, tube load, and most importantly, the patient dose. Therefore, x ray generator must be calibrated periodical and scheduledly by quality control technologists.

This quality control test can be performed in rooms 3, 4, 5, or 6. My group decided to do it in room 5. First of all, we need to change the mAs from 15 into 16 in our lab manual, because the x ray console does not allow us to select 15 mAs. Secondly, when we collimate the x-ray field to the kVp meter during the test, we need to make sure we collimate to include the sensor (田 showed in the middle of meter), but not the whole meter.

We change the kVp setting into 60 kVp (5mAs), 81kVp (5mAs), and 102kVp (5mAs), then for the same kVp settings we use 16 mAs. For each setting, we expose 3 times. Then record the reading from kVp meter for each setting. All the collected data is in the following chart:

Nominal kVp	mAs	Measured kVp  1	Measured kVp 2	Measured kVp 3	Average kVp
60	5	59.6	59.6	59.5	59.57
60	16	59.6	59.6	59.7	59.63
81	5	82.3	82.4	82.3	82.33
81	16	81.9	81.9	81.9	81.9
102	5	102.9	102.9	102.9	102.9
102	16	102.4	102.5	102.4	102.43

According to SC35, the loading factors must not deviated from the selected value, for any combination of loading factors, by more than 10% for x-ray tube voltage.

According to HARP standards, the average peak kilovoltage corresponds to the selected value to within + 8%

Therefore, the x-ray equipment in room 5 is working properly.

By looking at the chart, we will find that the reading from each measurement at the same kVp and mAs setting is slightly different sometimes. Is the reading a true reading? The answer is yes. They are all correct in their own ways. The reason why there is a difference is because there is an inherent, random variation in all measurements.

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.

Conclusion of Cluster 2

Conclusion of Cluster 2

The three lab tests that are coveredin this week are kVp accuracy, timer accuracy and Dr flatfield QC. The kvP accuracy test is to check if theselected kVp from the control console is actually the same as the measured kVpfrom a kVp meter. From the kVp accuracy table, the averaged measured kVp inroom 5 falls under the acceptance range according to HARP and SC35. TimerAccuracy is to test the measured the actual exposure time. Results show bothroom 3 and 6 are functioning properly and the timers are well calibrated as theresults collected are within the acceptable range enforced by SC 35 and HARP. ForDR flatfield QC, by performing a QAP, we can tell how many dead pixels arepresent in the detector. The result showed there was only one dead pixel androom 6 passed the quality control test. Although, all the rooms are functioning properly, it does not mean therooms do not require any more quality control tests. In fact, QC tests shouldbe carried out as the same way as they are to make sure the equipments aremaintain at a high level performance.

Cluster 1 Conclusion

The purpose of a quality control program is for technologists to examine and maintain acceptable quality of the X-ray equipment. This kind of program has been operating for decades in the radiographic field and playing an increasingly more prominent role, as technology advances.

The three topics mentioned in our ePortfolio briefly introduced three critical components of a radiographic quality control system which can identify changes in radiographic equipment. These changes may impair the accuracy and efficiency of radiologic diagnosis. Through periodical quality control testing, technologists can request repair or calibration services before harmful consequences can occur.

The EI calibration test consisted of three trials to see if the digital radiography equipment was properly calibrated to give a consistent EI within an acceptable range. The CR erasure was performed to test whether erasure of the CR cassette was sufficient to be reusable. The half value layer test measured the quality of an X-ray beam to evaluate if filtration was adequate. These tests ensure high image quality produced by X-ray equipment. Moreover, they help ensure low repeat rates, which consequently keep the patient dose and adhere to the principles of ALARA.

EI Calibration

      Justin Shek     0842536

 

      An exposure index, as known as EI, gives us an idea whether our image has got enough signals or not.  It is mostly used in film screen and computated radiography (CR) since the cassettes have to be processed. Just by looking at the EI, we can tell if we have gotten a reasonable image.  Each manufacturer of x-ray machines has a different EI acceptable range. For instance, the acceptable range of EI is from 1400 to 1700.  Any EI falls in between the range is considered reasonable meaning the image has captured enough photons and is diagnostic. Therefore, an accurate EI is crucial to information of an image. The purpose of this lab is to carry out three trials to see if CR is properly calibrated and would it give us a consistent EI within an acceptable range in the three trials.

       The reason why I chose putting EI calibration into my e-portfolio is because it was not just fun and easy to perform, but it also required a little bit of creativity of doing it in the first time.  As the procedures requested a 172cm SID between the x-ray tube and the sensor of a dosimeter, we actually had to think how to get the SID because the procedure could only be performed vertically.  However, the largest distance between the x-ray tube and detector was not greater than 140cm, so we had to figure out a solution to this problem. After a second, we recalled a trick from previous years, which was, we set a hand protocol on the control panel so the detector did not have to be used. Then we could put the dosimeter or the cassette on the floor with the required distance. 

 This is the bottom of the dosimeter that we used

       

      However, we made a mistake in our trials because we collimated too tight, and the x-ray machine’s collimator was a bit old so it was not as precise as before. As a result, we got such a small EI which was around 800 and the images were too dark and not acceptable.  However, once we opened up our coning, the exposure index of the three trials are 1600, 1690 and 1700 respectively and were reasonable and acceptable meaning there were enough photons captured per image. If the EI was not calibrated correctly, it would affect the image quality which would increase the rate of repeats and patient dose. However, since the technology is getting more advanced, we can always change the contrast and brightness afterwards if the EI is not too far off.