COLLIMATION SYSTEM

7.4. COLLIMATION SYSTEM

7.4.1. Collimation system

7.4.1.1. Scope

— Objective: To evaluate the coincidence between the radiation field and the image receptor; to evaluate the

coincidence between the light field and the radiation field; to evaluate the alignment of the compression paddle with the margin of the breast support;

— References [3, 20, 24]; — Minimum frequency: At acceptance, annually, and after changes.

7.4.1.2. Instrumentation (1) Six radio-opaque objects (e.g. coins or clips): five of the same size and one larger.

(2) Two loaded mammography cassettes: one that fits the Bucky and another, larger one, whenever possible. (3) Ruler.

7.4.1.3. Methodology (1) Place the larger cassette on the breast support, so that the cassette extends 20 mm beyond the margin of

the chest wall. 21 (2) Place the other cassette in the Bucky. (3) Temporarily remove the compression paddle and turn out the room lights to make it easier to see the light

field margins. (4) Turn on the collimation lights and place four radio-opaque objects on the margins of the light field, but just within the light field. The single large radio-opaque object should be on the margin of the light field on the chest wall.

(5) Replace the compression paddle and locate it approximately as it would be used clinically (40–60 mm above the breast support). (6) Place the sixth radio-opaque object on the lower surface of the compression device, tangent to the inner surface of the lip of the compression device, adjacent to the chest wall. Place it sufficiently off the centre line such that it does not interfere with the sensor of the AEC.

(7) Make an exposure with the AEC. In the event that the X ray unit does not permit irradiation without compression force, place an object of sufficient thickness under the support arm of the compression device and operate the compression paddle.

(8) Process the films. (9) Determine the coincidence of the light field with the radiation field by measuring the distance between the

margins of the X ray field and those of the radio-opaque objects. These measurements should be done on the film that was in the cassette placed on the breast support. Note the result on the data collection sheet.

(10) Overlap the films of the two cassettes on the viewbox so that the most external margins of the radio- opaque objects are coincident. For each of the four edges, determine the deviation between the radiation field (as determined from the film exposed on the breast support) and the image field (as determined from the film exposed in the Bucky). Note the result on the data collection sheet.

(11) Determine the coincidence between the edge of the compression paddle and the edge of the image receptor corresponding to the chest wall by measuring the extent to which the radio-opaque object placed on the compression paddle is only partly visible in the image of the film located in the Bucky. Note the result on the data collection sheet.

21 If necessary, to avoid overexposing the film, place a developed film between the intensifying screen and the film, or place the film with the emulsion in reverse. If large sized cassettes are not available, two smaller ones placed side by side may

be used to ensure complete coverage of the irradiated field.

(12) Repeat the procedure for all cassette sizes and collimators that are commonly used. (13) Repeat the procedure for all X ray tube targets and for magnification mode.

7.4.1.4. Interpretation of results and conclusions Tolerances: (1) Coincidence between light field and radiation field: achievable; 1% of focus-film distance (FFD) on any

side. (2) Coincidence between film and radiation field: achievable; the beam completely irradiates the image receptor, but does not extend beyond the breast support except at the chest wall where it can extend beyond the breast support to a maximum of 5 mm acceptable; may be inside the breast support by 2% of the FFD for the three sides other than at the chest wall. For the chest wall, it can extend beyond the breast support to a maximum of 5 mm. 22

(3) Compression paddle alignment: acceptable; the chest wall edge of the compression paddle is not visible in the image and should not extend beyond the image receptor edge by more than 1% of the FFD.

7.4.1.5. Recommendations and corrective actions

If measurements are outside of tolerance, contact the person in charge of the maintenance service.

22 It is preferred that the entire film be exposed to radiation as any unexposed area of film can result in glare when viewing the processed film.