Cluster 3 Introduction

In cluster 3 of 3H03 course, we are considering the following quality control features:
 

1. Linearity
2. Reproducibility
3. Dark noise
4. Leeds test tool
5. CR technique chart

In this e-portfolio, we will talk about three of labs more in detail. The three labs are Dark Noise, Linearity, and Reproducibility which are presented by Justin, Arthur, and Jun respectively.

Linearity

It is imperative that a properly functioning general X-ray unit produces proportional changes in exposure as milliamperage is changed. The test performed in this lab tests the accuracy of linear output regarding two factors: changes in mAs (with fixed kVp) and reciprocity between mA and exposure time (in seconds).

For the first component of the lab, exposure output (in milliroentgens) was recorded using a radiation meter. Using a fixed kV (70 kV) and starting at 10 mAs, two exposures were taken after approximately doubling the selected mAs until 64 mAs. Exposures were taken at 80 mA on a small focal spot setting and at 160 mA at a large focal spot setting. The lab was conducted in rooms 3 and 6.

Since theoretically, exposure should change proportionately to mAs, the ratio of exposure to mAs (mR/mAs) should be the same for any two sets of exposures at different mAs settings. Both Safety Code 35 and the HARP Act stipulate that the difference in the average mR/mAs values for any two adjacent mA settings must be less than or equal to 10% of the sum of the averages:

|x1 - x2| ≤ 0.10(x1 + x2)

where x1 and x2 are the average mR/mAs (milliroentgens divided by milliampere-seconds) values obtained at the two selected settings of mA (milliamperes)

Tables 1A-D below summarize the results for the first half of the lab in rooms 3 and 6:

Table 1A: Exposure readouts at 80 mA, small focal spot in room 3

80 mA small focal spot	10 mAs	16 mAs	32 mAs	64 mAs
Exposure 1 [mR]	0.083	0.133	0.265	0.529
Exposure 2 [mR]	0.082	0.133	0.265	0.529
Average mR	0.083	0.133	0.265	0.529
mR/mAs (xn)	8.25 x 10-3	8.31 x 10-3	8.28 x 10-3	8.27 x 10-3

Table 1B: Exposure readouts at 160 mA, large focal spot in room 3

160 mA large focal spot	10 mAs	16 mAs	32 mAs	64 mAs
Exposure 1 [mR]	0.086	0.133	0.263	0.523
Exposure 2 [mR]	0.086	0.133	0.263	0.523
Average mR	0.086	0.133	0.263	0.523
mR/mAs (xn)	8.60 x 10-3	8.31 x 10-3	8.22 x 10-3	8.17 x 10-3

Table 1C: Exposure readouts at 80 mA, small focal spot in room 6

80 mA small focal spot	10 mAs	16 mAs	32 mAs	64 mAs
Exposure 1 [mR]	0.098	0.154	0.307	0.605
Exposure 2 [mR]	0.096	0.154	0.306	0.602
Average mR	0.097	0.154	0.307	0.604
mR/mAs (xn)	9.70 x 10-3	9.63 x 10-3	9.58 x 10-3	9.43 x 10-3

Table 1D: Exposure readouts at 160 mA, large focal spot in room 6

160 mA large focal spot	10 mAs	16 mAs	32 mAs	64 mAs
Exposure 1 [mR]	0.095	0.153	0.308	0.608
Exposure 2 [mR]	0.095	0.152	0.308	0.61
Average mR	0.095	0.153	0.308	0.609
mR/mAs (xn)	9.50 x 10-3	9.53 x 10-3	9.63 x 10-3	9.52 x 10-3

Although they show slight variances in mR/mAs, none of the above data for any n demonstrate deviance outside of the accepted limits stated by SC 35 or the HARP Act; hence corrective action is not needed.

The second part of this lab tests for linearity between the reciprocity of mA and exposure time. At a fixed kV (70 kV), mA and exposure time were adjusted so that 16 mAs was always selected for exposure. Two exposures were made for three different mA stations, and the mR/mAs value was calculated. Tables 2A and B below summarize the results for the second half of the lab in rooms 3 and 6:

Table 2A: Exposure readouts for room 3

	80 mA x 0.2 s	160 mA x 0.1 s	320 mA x 0.05 s
Exposure 1 [mR]	0.133	0.133	0.131
Exposure 2 [mR]	0.133	0.133	0.13
Average mR	0.133	0.133	0.131
mR/mAs (xn)	8.31 x 10-3	8.31 x 10-3	8.22 x 10-3

Table 2B: Exposure readouts for room 6

	80 mA x 0.2 s	160 mA x 0.1 s	320 mA x 0.05 s
Exposure 1 [mR]	0.154	0.155	0.148
Exposure 2 [mR]	0.151	0.152	0.149
Average mR	0.153	0.154	0.149
mR/mAs (xn)	9.53 x 10-3	9.59 x 10-3	9.28 x 10-3

These data show even more variance but are still within the accepted limits and hence, corrective actions is also not needed regarding the linearity of mA and exposure time reciprocity.

While control charts are the cornerstone of radiologic quality control, I feel that a different tool can be used to effectively evaluate the linearity of mAs and exposure output. Below is a plot of average exposure [mR] as a function of mAs for the data shown in table 1A:

The line shows the ideal, proportionate change in exposure as mAs is changed, assuming a baseline level at the first exposure. The points around this line are those found in table 1A. Error bars that reflect the acceptable limits in SC 35 and the HARP Act are included. So long as the line is within the error bars of the data points, corrective action is not needed. This method of plotting values against an ideal function can be used as a rapid tool for visually evaluating linearity between exposure and mAs.

Reproducibility

Normally, when x-ray technologists input a given radiographic technique, the radiation intensity produced by the x-ray generator should be same from one exposure to another when using the same technique. In order to check the reproducibility of the x-ray unit and mammo unit, we need to take 10 consecutive exposures and record each reading from Rad Check. For each exposure, we need to change the factor settings to provide different combinations of mAs.


Before each exposure, we need to put an apron on table then put Rad Check on it. It is showed in the picture below:

Reproducibility for mammo unit:

Because the mammo unit in our department does not allow us to change exposure time or mA, the combination of mAs cannot be changed. In this lab, we keep mAs same for each exposure.

5 mAs @ 25 kVp, 100 cm SID:

	Exposure #1	#2	#3	#4	#5	#6	#7	#8	#9	#10	Average
Rad Check Reading	0.096	0.097	0.097	0.096	0.097	0.097	0.096	0.097	0.096	0.097	0.0966

X_i = ith exposure measurement

 = mean value of exposure measurements

n = number of exposure measurements

C= 1/0.0966 * {[(0.096-0.0966)² + (0.097-0.0966)² + (0.097-0.0966)² + (0.096-0.0966)² + (0.097-0.0966)² + (0.097-0.0966)² + (0.096-0.0966)² + (0.097-0.0966)² + (0.096-0.0966)2 + (0.097-0.0966)2] / 9}1/2 = 0.0053457

For two extreme values 0.096 and 0.097:
(0.096-0.0966)/0.0966=-0.62%
(0.097-0.0966)/0.0966=0.41%

According to SC33, Coefficient of radiation for any 10 consecutive radiation should not be greater than 0.05. Mean value of 10 measurements should be within 15%. Therefore, the reproducibility of mammo unit in our department is within acceptable limits.

Reproducibility for a x-ray unit in room 4:

5 mAs @ 60 kVp, 100 cm SID:

X_i = ith exposure measurement

 = mean value of exposure measurements

n = number of exposure measurements

C = 1/0.0128{[(0.013-0.0128)² * 8 + (0.012-0.0128)² * 2]/9}^1/2 = 0.03294


For the two extreme values 0.012 and 0.013:
(0.012-0.0128)/0.0128=-6.25%
(0.013-0.0128)/0.0128=1.56%

According to HARP, Coefficient of radiation for any 10 consecutive radiation should not be greater than 0.08. Mean value of 10 measurements should be within 20%.

According to SC35, Coefficient of radiation for any 10 consecutive radiation should not be greater than 0.05. Mean value of 10 measurements should be within 15%.

No matter which reference we choose, the x-ray unit in room 4 is within acceptable limits.

Dark Noise

               As technology is getting more and more advanced, more beneficial equipments are being developed in diagnostic imaging.  Examples like DR and CR are huge improvements compare to the film screen in the past.  However, having good quality equipments does not mean they do not need to do any testings or maintenance.  In fact, in order to keep up the performance of these advanced equipments, regular checkups are required and that is the purpose of quality control. Dark Noise is a quality control method for CR readers. It is a simple and fast control test because it only requires one to erase a cassette and then process it without exposure.  A cassette and a CR processing station are the materials that one needs to perform this test.              

The purpose of a Dark Noise test is to give indication of system noise.  This is extremely important to diagnostic imaging because if a CR reader is not working properly, there would be artifacts showing up on the image and these artifacts would lead to inaccurate diagnosis and increase patient dose.  Therefore, a dark noise test should be done once in a while to ensure the CR reader is performing properly.  Another reason that this test is important is because it also ensures the uniformity of the CR reader.  Although there are other quality control test that would test the uniformity of the CR reader, this test can also ensure the reader is working in a consistent manner. 

The image above is a picture that i took during the lab. It is the result of the dark noise test. As one can see, there are no artifacts at all and the exposure index is 10. According to the guideline for safety, the exposure index of GE should not be exceeding 80.Thus, our CR reader passed the Dark noise test.

Cluster 3 conclusion

Cluster 3 Conclusion

              

          The three quality control tests our group did are linearity, reproducibility and dark noise.  Linearity is to test the accuracy of linear output according to changes in mAs with a fixed kVp and reciprocity between mA and exposure time.  For the first part of the linearity test, although the results show there was a variance in mR/mAs, all the results were well within the acceptable range suggested by SC 35 and HARP act. For the second part of the linearity test, a different combination of mA and time were used to test the linearity of mR/mAs.  Again, the results showed a slightly variation of mR/mAs but they were all within the acceptable range of SC 35 and HARP act. The second quality control test is reproducibility, the test is to check whether the mammo unit and an x-ray unit produce a consistent exposures.  Since the mammo unit did not allow us to alter mAs settings, we used the same technique in all 10 exposures.  The coefficient of radiation from the mammo unit was 0.0053457 and the coefficient of radiation in the x-ray unit is 0.03294. According to SC 35 and HARP act, the coefficient of radiation should not be greater than 0.05 and 0.08 respectively. In a result, our mammo and x-ray units both passed the reproducibility control test. Note that the x-ray unit has a higher coefficient than the mammo unit and that is because the mammo unit was using the same mAs and kVp throughout the ten exposures while a different combination of mAs and kVp was used in the x-ray unit. The last quality control test was dark noise and the purpose is to test the uniformity of the CR processor. The CR reader did not show any artifacts while having a low EI value, which was 10, on the image. The acceptable range of the dark noise test was anything less than 80. Thus, the CR processor passed the test.  Though all the units are functioning properly, quality control tests should be done frequently to ensure the equipments keep up the same level of performance.

ePortfolio: Conclusion

The conclusion of our cluster 3 labs brings our ePortfolio to a close. The labs, modules, assignments, and assessments upon which we have reflected here have helped us to better appreciate the importance of quality control practices in a radiology department. Perhaps most importantly though, the work accomplished over the span of this term is practical and directly applicable to future careers in the radiological clinical setting.

Though this ePortfolio is ended, it is important to note that as hopeful competent QC technologists, it is our responsibility to remain up to date on constantly changing technological advances. Only by remaining vigilant in quality control techniques can an X-ray technologist provide exceptional patient care.

Introduction to Cluster 2

Cluster 2 labs of this course considered the following quality control aspects:

• Beam congruency and perpendicularity
• kVp accuracy
• Timer accuracy
• CR Processing Uniformity
• Limiting spatial resolution in both CR and DR
• DR Flatfield

The three labs discussed in this ePortfolio will include kVp accuracy, timer accuracy, and DR flatfield, as presented by Jun, Arthur, and Justin respectively.

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.