Collimation and Light Field Brightness
Physics and Reasoning
Principle 1: Primary X Ray beam is blocked by breast support table. Principle 2: Need to properly imageas as much breast tissue as possible. Note: Try to get as much chest wall tissue as possible. X rays must extend to edge. Detection as close to edge as possible. Pi Principle i 3: Rd Reduce extraneous light in image. Note: Exposing entire detection area will not increase dose or scatter.
Physics and Reasoning
Principle 4: Light field should indicate what area is being imaged. Note: Paddle can interfere with chest wall edge and can push patient away.
Test Procedure For Film Screen and CR Systems Cassette in holder and on top of breast support table, with scales placed as indicated, zeros at light field boundary. 18x24 cassette in holder and 24x30 on top. 24x30 cassette in holder and 24x30 on top at angle.
Test Procedure For DR (and CR) Digital Systems Please ensure that the free QuickTime Player from Apple is installed in order to play to movie: leadscalesetup_18x24.mov Note that current procedure is to support the compression paddle at 45mmand use manual control at 28kVp and 100mAs.
Test Procedure Lead scales used to mark light field edges and measure extent of x ray field. Phosphorstrips strips are for digital testing. Light field edge Phosphor strip placed at 13mm outside light field (2% SID) Chest wall A hold over from film screen which should be changed to 10mm Phosphor strip placed at 5mm from breast support edge for testing x ray field extent past chest wall.
Test Procedure Zero of four scales placed on light field edges. Note light field shining on breast support table. Fifth scale placed with zero at chest wall edge and phosphor at 5mm beyond. Scales may need to be taped in position. Scales establish light fieldposition relative to detector. Phosphors on scales establish tolerance of x ray field position relative to light field.
Test Procedure Compression paddle at standard height (45mm) should be supported. Scale taped to bottom of paddle with zero at chest wall edge. Phosphor on paddle scale is in x ray field as control test for glow.
Test Procedure Physics DPMA phantom in addition to an MTF tool, also contains a missing tissue tool. This phantom will already be imaged in large and mag views by a different test.
Test Procedure The light meter in collimation tests. The field light needs to be tested for brightness. The light used in small and large fields of view is measured. Background light (field light off) is also measured. The measurements are in lux.
Test Procedure As described in Digital Mammography Quality Control for the Mammographic Physicist, Revision 3.1 Tape four phosphor strips around breast support and cover with foil as in diagram 4. Add 45mm PMMA with scale at chest wall edge. Tape four scales with zero at light field edge Tape scale to compression paddle bottom, zeroed at chest wall edge. Lower paddle to make light contact. Measure background lightand field lightin in luxfor both small and large detector sizes.
Test Procedure Diagram 4
Example Test Image Digital Collimation Image (GE Essential 18x24 central) Zero at Edge of Paddle Zero at Chest Wall Light Field
Example Test Images Zero at Edge of Paddle Collimation test of 24x30 size on GE Essential
Example Test Image Collimation test of mag stand (1.4x) for Siemens Inspiration. i Zero at Edge of Paddle
Example Test Image IMSGiotto (non mag) DMPA phantom contains missingtissue scale which should show less than 7mm tissue pushed out.
Example Test Image GE Essential (non mag) Pushed out tissue is at border line condition.
Example Test Image DMPA phantom on GE Essential Mag Stand (1.5x)
Pitfalls of Test Be sure to avoid overlaps or ensure properly visible image. Tape scales to avoid shifts when compression paddle reattached. hd Take care to properly establish the light field edge.
Pitfalls of Test All image receptor sizes need to be tested. Be sure light field brightness and background have been measured. Be sure to observe phosphors in digital testing in darkened room.
Machine Specific Tips and Tricks GE Essential will reset light field to central position on paddle connection. Must position light field just before image. Please ensure you have the free QuickTime Player installed from Apple to play the included movie: GE_Essential_18x24_autoshift.movEssential autoshift
Machine Specific Tips and Tricks Note: Simple webcams require no additional software. They can be taped in position observing phosphors p while the physicist stays protected. As an option, webcams can capture video of phosphor. Wb Webcam taped dbeside tube port See QuickTime movie of central phosphor lighting: phosphorglow.mov.
Pass/Fail Criteria Detector and light field edges within 10mm Missing tissue at the chest wall: Achievable: 5 mm Acceptable: 7 mm Radiation field covers detector with extending past breast support, except possibly at chest wall by less than 5 mm Paddle extends no more than 5mm beyond chest wall. Light field intensity more than 160 lux above background. Must be corrected within 30 days.
Film Screen Spread Sheet Pass/ Fail criteria Onlyone smallrelevant section to complete. Sum of misalignments must be less than 13 mm Paddle edge not beyond chest wall by more than 7 mm.
Digital Spread Sheet (GE Essential) Acquisition sheet documents images. Pass/Fail Criteria Note Light Field Intensity
Pass/Fail Criteria DigitalTestingRev3.1: Collimation7 Light Field Intensity (GE Essential)
Pass/Fail Criteria DigitalTestingRev3.1: Collimation7 Light Field Dt Detector t Congruence (GE Essential)
Pass/Fail Criteria DigitalTestingRev3.1: Collimation7 X Ray Field Exceeds Limits (GE Essential) Thistest result isonly observable fromthe reaction This test result is only observable from the reaction of the phosphor strips during x ray exposure.
Pass/Fail Criteria DigitalTestingRev3.1: g Collimation7 Missing Tissue, Coverage and Compression Paddle (GE Essential)
For more information please refer to Digital Mammography Qualtity Control for the Mammographic Physicist, Revision 3.1