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Section 5: Automatic Exposure Control

Introduction

This chapter provides information on the requirements and procedures for quality assurance testing of x-ray machines which are equipped with automatic exposure control (AEC). This applies to all systems with AEC, such as cameras, cine, digital, etc.

Mandatory HARP Tests

Recommended Technical Tests

Although the HARP Act does not specifically state which tests should be performed for machines equipped with AEC, common sense dictates that any parameter which can be changed, should be tested. The above table lists these parameters.

Ultimately the best method to check automatic exposure control would involve exposing a film for each test. The optical densities of the films would then be measured and evaluated for proper AEC response. Over time, this method would be expensive. As an alternative, we recommend that the output be measured with a dosemeter and these dose measurements be assessed to check for proper AEC response. If any unexpected measurements are recorded, further tests could then be performed using film.

Please note that Patient Entrance Exposure (PEE) measurements should be recorded for the following views: AP Abdomen, Chest, Mammography and Adbomen for spot films, camera, cine and digital.

For information on AEC testing for mammography, spot film, camera and digital refer to either the fluoroscopy or mammographic sections.

 

Reproducibility

Standard

Section 13 (2) of the HARP Regulation states:

"Every diagnostic x-ray machine shall be so constructed that,

(a) over the normal range of use of the machine for any given combination of x-ray tube potential (in kilovolts peak), tube current (in milliamperes), exposure time (in seconds) or for selected radiation exposure to image receptor (in milliroentgens),

(i) the estimated coefficient of variation of any ten consecutive radiation exposure measurements taken at the same source-to-detector distance within a time period of one hour is no greater than O. 08, and

(ii) each of the ten radiation exposures referred to in subclause (i) is within 20 percent of the mean value of the ten measurements";

Equipment

Radiographic, Spot Film and Photospot

Dedicated Chest Unit and Upright Buckies Used for Phototimed Chest

Phantoms for Patient Entrance Exposure measurements

Procedure

Radiographic - Abdomen

1. Place the 20 cm water phantom on the x-ray table between the x-ray tube and the image receptor.

2. Position the x-ray tube at the target-image receptor distance normally used and centre it over the phantom.

3. Collimate the useful beam to the phantom. Ensure that all photocells are covered. If all the photocells cannot be covered by phantom then select the mid cell only.

4. Position the dosemeter detector probe approximately 21 cm above the table top. This must be centred over the water. Ensure that no part of the detector or probe is directly over or covering any portion of a photocell.

5. Select the technique used for a 23 cm AP Abdomen patient. If the x-ray machine is equipped with automatic exposure programming, select the program for a 23 cm AP Abdomen.

Chest Unit-Upright Bucky

1. If testing a chest unit, position the LucAL phantom in front of and touching the chest stand ensuring that the phantom covers all photocells. The dosemeter detector will be attached to the arm of the phantom (please refer to LucA1 phantom diagram). Select an appropriate technique for a 23 cm PA Chest.

2. Make three exposures and record the output (and time if possible) for each exposure.

Evaluate as follows:

If the first three output measurements are similar, (for example within 20% of the mean value) then the output is acceptable.

If the measurements are not similar, repeat the test. Take ten exposures over a period of one hour. Calculate the coefficient of variation. See General Radiography - Section 4 for an example. If reproducibility is still unacceptable, stop using the x-ray machine until it is serviced and this test repeated.

 

Patient Entrance Exposure Measurements

Radiographic Exposures

1. Position the 20 cm water phantom over the photocells. Ensure that the water phantom is wide enough to cover all activated photocells. Collimate to the water phantom.

2. Position the dosemeter probe over the centre of the phantom and in close proximity ensuring that no part of the detector covers a photocell.

3. Set the technique normally used for a 23 cm AP Abdomen projection.

4. Place a loaded cassette in the bucky.

5. Make an exposure and record the output. Repeat twice to verify the result and reproducibility.

6. Apply the backscatter factor to determine the PEE. (See Backscatter Factors on next page).

kVp

Half-Value Layer

Size of X-ray Field at Entrance Surface to Patient

5 x 5

10 x 10

15 x 15

30 x 30

60

1.1

0.88

0.85

0.83

0.82

1.5

0.86

0.83

0.81

0.79

1.9

0.85

0.80

0.79

0.78

2.3

0.83

0.79

0.78

0.76

75

1.4

0.86

0.82

0.81

0.81

1.8

0.83

0.79

0.79

0.78

2.2

0.82

0.78

0.76

0.76

2.7

0.81

0.76

0.75

0.74

90

1.7

0.85

0.80

0.80

0.79

2.2

0.84

0.78

0.76

0.75

2.6

0.83

0.76

0.74

0.73

3.3

0.81

0.75

0.72

0.71

100

2.0

0.83

0.79

0.76

0.76

2.6

0.83

0.77

0.75

0.73

3.1

0.81

0.75

0.72

0.70

3.8

0.80

0.74

0.70

0.68

Higher kVp

  

0.80

0.74

0.70

0.68

Chest Exposures

1. Position the phantom in front of and touching the chest stand and ensure that the phantom is centred to the photocells.

2. Attach the dosemeter to the arm of the phantom.

3. Set the technique normally used for a 23 cm PA Chest projection.

4. Make an exposure and record the output. Repeat twice to verify the result and reproducibility.

5. Using the inverse square law, calculate the actual PEE at 23 cm in from of the face of the chest stand.

 

Response to mA Change

Procedure

1. If it is possible to change the milliamperage selected, choose a mA station lower than the one which is used routinely.

2. Make an exposure and record the output and measured time.

3. If possible, select a mA station higher than the one which is used routinely.

4. Make an exposure and record the output and measured time.

5. Return to the original mA station.

Evaluate as follows:

The output should remain constant. With all other factors constant, the mAs for a given kilovoltage should not change for the same size phantom or patient. For example, doubling the milliamperagc should result in approximately the same output, but at half of the measured time. A +20 per cent tolerance is recommended. If using film, ensure that the optical density remains the same on all films.

 

Response to kVp Change

Procedure

1. Select a kilovoltage at least 10 kVp lower than the original setting. In mammo, a kVp change of 2 is sufficient.

2. Make an exposure and record the output and measured time.

3. Select a kilovoltage at least 10 kVp higher than the original setting. In mammo, a kVp change of 2 is sufficient.

4. Make an exposure and record the output and measured time.

5. Return the kVp to its original setting.

Evaluate its follows:

As kilovoltage increases, the output (and time) should decrease when all other factors remain the same. An important consideration, when increasing kilovoltage for this test is the minimum response time of the x-ray machine. If the output and time increase (with the increased kilovoltage), the minimum response time has probably been reached. To verify whether or not this is the case, perform this test at a lower kilovoltage. Conversely, as kilovoltage decreases, the output and time will increase. If using film, the density should remain the same.

Response to Density Change

Procedure

The density selector allows operators to control the overall density on the film. Operators should know by what percentage the overall density on the film would be changed at each density selection available.

1. Leaving all other technique factors constant, pick a density selection one step higher than the original density selection.

2. Make an exposure and record the output and measured time.

3. Pick a density selection one step lower than the original density selection.

4. Make an exposure and record the output and measured time.

5. Return the density selector to its original position.

Evaluate as follows:

As the density setting is changed, the radiation output (and time) should change accordingly (i.e., as higher densities are selected, the radiation output and density should increase) and vice versa. The exposure should change by a consistent value. Commonly, this value is about 20 per cent for each selection.

If the measured exposure does not change by a consistent value, the density selector may be malfunctioning and must be repaired.

Each facility should identify the changes expected for density selection when the unit is installed and establish specific acceptance limits.

 

Response to Change in Patient Thickness

Procedure

1. Select the technique factors as specified for the reproducibility or PEE measurements.

2. Increase phantom thickness by placing a copper sheet (or other material) between the phantom and dosemeter. (Use aluminum only when testing mammography).

3. Make an exposure and record the output and time measured.

Evaluate as follows:

There should be an increase in both output and time. If using film, the density should remain the same.

 

Back-up System

Standard

Section 13 (1) of the HARP Regulation states:

"(1)

Every diagnostic x-ray machine that is equipped with an automatic exposure control shall be equipped with,

(a) an indicator that shows when the automatic exposure control mode of operation has been selected,'

(b) a means of terminating the exposure,

(i) of an x-ray tube with a potential of less than fifty kilovolts peak, when the product of the x-ray tube current and the exposure time is 2000 milliampere-seconds per exposure, or

(ii) of an x-ray tube with a potential of fifty kilovolts peak or more, when,

(A) the product of the x-ray tube current and the exposure time is 600 milliampere-seconds, or

(B) the product of the peak x-ray tube potential, current and exposure time is sixty kilowatt-seconds per exposure; and,

(c) an indicator that warns the operator that a condition set out in subclause (b)(i) or (ii) has been reached."

Procedure

1. Select a low kVp technique.

2. Place the copper sheets and lead between dosemeter and the phantom.

3. If the back-up is set manually, select a time lower than the original time measured during the reproducibility or PEE tests.

4. Make an exposure and record the output and measured time. If the unit does not go to backup, add additional lead. If it is necessary to repeat the test, allow the tube to cool for 5 minutes. Document if back-up has been reached.

5. In a manual system the back-up may be determined by multiplying the mA used by the facility by the back-up time selected (or by calculating the kilowatt-seconds).

Note:

With manual back-up systems there must be a label indicating the maximum backup time to be used with each mA selection normally used. With manual systems, it is important to note that the time selected is not an indicator that back-up has been reached. There must be a separate indicator (audible, visible or machine shut-off) that back-up has been reached.

Evaluate as follows:

If the unit operates at a potential less than 50 kVp, calculate the mAs by multiplying the mA used by the time measured. It must not exceed 2000 mAs.

If the unit has a potential of 50 kVp or more calculate:

(i) the mAs by multiplying the mA used by the time measured at back-up. It must not exceed 600 mAs, or

(ii) back-up limited to 60 kilowatts seconds. Using the following formula (for single phase machines) the kilowatt-seconds can be calculated.

(5) kilowatt-seconds kw = (a*v)/1000

where:

kw is kilowatts

a is milliamperes divided by 1000 (or tube current in Amperes)

v is voltage and is equal to kilovoltage x 1000

(or tube voltage in volts)

To obtain kilowatt-seconds multiply the kilowatts by the time measured in seconds at back-up. For three phase units multiply this product by 0.785.

Note: This formula is particularly useful for falling-load generators.

Acceptable back-up indicators may include: a light indicator, an audible indicator or machine shut down.

 

Photo-Cell Variation

When changing from one cell to another or from one combination of cells to another, make exposures using the same technique factors and phantom, the radiation output should be the same for each cell(s) selection. It is recommended that each facility discuss the amount of variation programmed intemally with their service company.

Procedure

1. Test each photo-cell independently (if possible).

2. Make an exposure and record the output and measured time.

3. Change the cell selection and repeat step 2.

4. Select the third cell and repeat step 2.

5. Return the cell selection to its original position.

Evaluate as follows:

The exposure should remain the same among all cells.

Note: If a variation in output is measured between cells, discuss with the facility to confirm if the equipment has been purposefully set up that way. This is more common with dedicated chest units.