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ECR 2019 / C-2411
Radiographer Award
An investigation on the appropriateness of Exposure Index (EI) calibration and Exposure Index target (EIT) values as a method of ensuring diagnostic image quality and optimum patient exposures in Digital Radiography.
Congress: ECR 2019
Poster No.: C-2411
Type: Educational Exhibit
Keywords: Radiographers, Digital radiography, Education, Quality assurance
Authors: S. Mc Fadden1, G. Forsythe2, P. Doyle1; 1BELFAST/UK, 2Newry, CO. Down/UK

Findings and procedure details

Ethical and Research Governance Approval was sought and obtained. Eight DR rooms using equipment from three different manufacturers were investigated in one UK Trust. This included three Siemens Ysio units, two Carestream Evolution and three hybrid systems consisting of Shimadzu consoles and Canon detectors.

 

The methodology was divided into six phases.

1. Initially the EI used by a variety of DR manufacturers was investigated to check if it was calibrated according the IEC standard. Each manufacturer was contacted by email to ascertain how their EI was calibrated and identify their EIT per body part.

 

2. A patient dose audit was undertaken for common patient examinations to document current EI and EIvalues and benchmark exposure levels against local DRLs to assess the scope for patient dose optimisation. This included 30 examinations per room for five body parts (n=1200).The exposure factors of kVp, mAs, EI and Dose Area Product (DAP) values were retrieved from the Dicom elements for each examination.

 

Inclusion criteria:

-X-ray examinations of Chest Postero-anterior (PA), Antero-posterior (AP) Abdomen, AP Pelvis, AP knee and Lateral lumbar spine.

-Only PA chest projections were included to ensure the source to image detector distance (SID) was standardised at 180cm.

-Both in-hours and out-of-hours examinations

 

Exclusion criteria:

-Paediatrics under 16 years of age to reduce significant variation in size.

 

3. Retake analysis was performed over three months prior to EI calibration checks to benchmark current repetition rates and categorise reasons for repeat exposures.

 

4. Experimental studies were performed with the aid of Medical Physics to determine if the EI calibration was according to IEC standard. ;

 

 

5. Where required, the findings from the EI investigation in phase 4 were    used to

i. Calibrate the EI in the relevant equipment to IEC standard

ii. Adjust EIT values as recommended by each manufacturer

iii.Adjust patient exposure factors to optimise clinical examinations to within local DRLs.

 

 

6. After the corrective action was undertaken, a repeat patient dose audit for each of the five body parts and a further retake analysis was undertaken to allow for comparison with baseline levels.

 

Results

The EIT from each manufacturer is illustrated in Table 1, which is applied across all body parts.

 

 

Table 1. Target Exposure (EIT) indices for each manufacturer.

 

Manufacturer 1

Manufacturer 2

Manufacturer 3

226.22

312

200

 

The exposure data from the dose audit are illustrated in Table 2. These values are the average obtained over 30 examinations for each of the five body parts per manufacturer.

 

Table 2. Mean kVp, mAs, DAP EI, EI T, and DI values per five selected body parts for each manufacturer.

 

Manufacturer 1

 

 

 

 

 

 

 

 

kVp

mAs

DAP

 dGycm²

 DRL (dGycm²)

EI

EIT

DI Value

Abdomen AP

73.78

19.83

11.44

25

205.51

226.22

0

Chest PA

106.17

1.17

0.64

1

225.3

226.22

0

Lateral Lumbar spine

91.35

25.98

11.92

25

216.42

226.22

0

Pelvis AP

75.03

9.85

6.35

22

302.04

226.22

+1

Knee AP

64.23

2.1

0.42

none

277.72

226.22

+1

 

 

 

 

 

 

 

 

Manufacturer 2

 

 

 

 

 

 

 

 

kVp

mAs

DAP

dGycm²

 DRL (dGycm²)

EI

EIT

DI Value

Abdomen AP

75.01

17.95

10.64

25

226.18

312

-1

Chest PA

125.34

1.11

0.51

1

128.42

312

-4

Lateral Lumbar spine

85.05

32.04

13.57

25

191.36

312

-2

Pelvis AP

71.85

14.38

8.87

22

313.49

312

0

Knee AP

59.8

2.03

0.5

none

165.33

312

-3

 

 

 

 

 

 

 

 

Manufacturer 3

 

 

 

 

 

 

 

 

kVp

mAs

DAP

 dGycm²

 DRL (dGycm²)

EI

EIT

DI Value

Abdomen AP

78.6

19.54

21.54

25

219.2

200

0

Chest PA

125

1.19

0.96

1

49.97

200

-6

Lateral Lumbar spine

84.98

71.58

41.17

25

245.56

200

+1

Pelvis AP

74.31

29.01

26.21

22

320.64

200

+2

Knee AP

68.02

2.19

0.52

none

350.71

200

+2

 

 

The average DI value was obtained by the following equation;

DI=10 log10(EI/EIT)

Dose levels from manufacturer 1 were below the DRLs, the EI values achieved were close to the EIT thus DI values were acceptable (0 and +1).

Dose levels from manufacturer 2 were below DRLs but as EI values were significantly below EIT, DI values were negative and indicating underexposure. Only pelvis examinations had a DI of 0.

Dose levels from manufacturer 3 showed that the average DAP of 41.17 dGycm²  for lateral lumbar spines (range 6-129 dGycm 2) and 26.21 dGycm²  for Pelvis examinations (range 3-98dGycm 2) exceeded the DRL with 63% and 55% respectively of the sample receiving a dose above the DRL. There was a great variation noted in DI with no direct correlation between DI and DAP.

 

Retake analysis identified retake rates of 5.9%, 6.2% and 12.1% for Manufacturers 1- 3 respectively. On further analysis, less than 1% of images from each manufacturer were exposure related. The majority of reject reasons given by the radiographer were classed as “radiographer issues” i.e. positioning and anatomy cut off.

 

Experimental studies were performed and EI values were recalibrated with the aid of medical physics. As Manufacturer 1 was calibrated optimally, no further data was collected on that system, the DAPs were below the DRLs and retake rate was the lowest of the 3 manufacturers. DI appeared to correlate with DAP.

 

A repeat data collection (n= 900) and a further retake analysis was undertaken for Manufacturers 2 and 3 to establish if the corrective measures/recalibration had led to improvements in dose and image quality. Further results are illustrated in table 3.

 

 

Table 3. Mean kVp, mAs, DAP and EI values per five selected body parts for Manufacturer 2 and Manufacturer 3 post implementation of optimised calibration technique.

Manufacturer 2

 

 

 

 

 

 

 

 

kVp

mAs

DAP

dGycm²

 DRL (dGycm²)

EI

EIT

DI Value

Abdomen AP

75.27

18.71

11.72

25

236.53

312

-1

Chest PA

124.9

1.188

0.622

1

95.24

312

-5

Lateral Lumbar spine

85.186

33.94

15.16

25

199.98

312

-2

PelvisAP

75.33

20.87

27.5

22

358.27

312

+1

Knee AP

60.03

2.11

0.5242

none

198.71

312

-2

 

 

 

 

 

 

 

 

Manufacturer 3

 

 

 

 

 

 

 

 

kVp

mAs

DAP  dGycm²

 DRL (dGycm²)

EI

EIT

DI Value

Abdomen AP

78.66

22.3

24.406

25

266.13

200

+1

Chest PA

125

1.077

1.056

1

52.28

200

-6

Lateral Lumbar spine

95

47.67

36.04

25

300.32

200

+2

PelvisAP

83.37

15.72

19.72

22

322.4

200

+2

Knee AP

67.93

2.04

0.629

none

371.22

200

+3

 

 

The second retake analysis identified retake rates of 7.98% and 10.12% on Manufacturer 2 and 3 respectively. Previously Manufacturer 2 was 6.2% whereas Manufacturer 3 was 12.1%. Again the number of images repeated due to inappropriate radiation exposure, were very low with 0.22% and 0.35% for Manufacturers 2 and 3 respectively.

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