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U.S. Department of Health and Human Services

Radiation-Emitting Products

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Most non-invasive kVp meters are only calibrated for Mo/Mo and Mo/Rh beams and therefore give inaccurate kVp readings when used with FFDM units that only have W/Ag or W/Rh target/filter combinations. How should I compensate for this error when doing surveys and MEEs on such units?

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The kVp value needs to be measured under one target/filter combination for which the kVp meter is calibrated (e.g., Mo/Mo, Mo/Rh, etc.). However, if the mammography unit has only a W/Ag or W/Rh target/filter combination, the measurement results must be appropriately corrected. FDA believes that the issue of inaccurate non-invasive kVp measuring instruments is one that ultimately is best resolved by the kVp instrument manufacturers and the medical physicists. Until this matter is resolved, FDA has the following suggestions for the equipment scenarios listed below:

 
kVp Measurements
 
Scenario 1.  If you are using a kVp meter that has been manufacturer-certified for W/Ag and W/Rh beams and a mammography-calibrated ion chamber, you may accept the measurement results and record them in the medical physicist survey or mammography equipment evaluation (MEE). 
 
Scenario 2. If your kVp meter has not been manufacturer-certified for W/Ag and W/Rh beams and you are using a mammography-calibrated ion chamber, you should include a correction factor in your kVp measurements. This correction factor may be obtained from the kVp meter manufacturer, or by working with the mammography unit’s service engineer (by comparing the engineer’s manufacturer-approved measurements during machine installation with your non-invasive meter results). You may also use this correction factor when performing MEEs or surveys using this kVp meter/ion chamber combination on other FFDM units by the same manufacturer that also have W/Ag or W/Rh target filter combinations.
 
Scenario 3. If your kVp meter has not been manufacturer-certified for W/Ag and W/Rh beams and you are using a separate solid-state probe, then you should follow Scenario 2 for kVp Measurements.
 
The table below gives a summary of the action needed under each of the three Scenarios discussed above.
 
kVp Measurement of W/Ag and W/Rh Beams
Scenario
kVp Meter
Air Kerma Instrument
Action
Correction Factor
1
Certified by manufacturer for W/Ag and W/Rh
Ion chamber calibrated for mammography
Accept measurement
N/A
2
Not certified by manufacturer for W/Ag and W/Rh
Ion chamber calibrated for mammography
Apply correction factor to kVp measurements
Obtain from kVp meter manufacturer or develop by comparing engineer’s manufacturer-approved measurements during machine installation with your kVp meter
3
Not certified by manufacturer for W/Ag and W/Rh
Solid state probe
Apply correction factor to kVp measurements
Obtain from kVp meter manufacturer or develop by comparing engineer’s manufacturer-approved measurements during machine installation with your kVp meter

 

HVL Assessments 
 
Scenario 1. If you are using an integrated solid state instrument (one that automatically measures kVp, HVL, and dose) and the instrument is manufacturer-certified for W/Ag and W/Rh, you may accept the measurement results and record them in the medical physicist survey or MEE.  
 
Scenario 2. If you are using an integrated solid state instrument that is not manufacturer-certified for W/Ag or W/Rh, you should determine the HVL using exposure measurements in the standard manner. You may then use this HVL value with the appropriate dose conversion table that is provided in the FFDM manufacturer’s QC manual to estimate the dose. Determine from the manufacturer of the dosimetry device if the exposure readings need to be corrected as aluminum is added to the beam. If so, then apply the correction factors applicable to each reading. It may be necessary to determine a typical correction factor by intercomparison with a mammography-calibrated ion chamber; this factor may then be used in the future. If the estimated dose is above 2.7 milligray (270 mrad), you should determine the actual HVL with a mammography-calibrated ion chamber and then use the actual HVL value to estimate the dose using the same conversion table provided in the QC manual.
 
Scenario 3. If your kVp meter has not been manufacturer-certified for W/Ag and W/Rh beams and you are using a separate solid-state probe, you should follow Scenario 2 for HVL Assessments.
 
The table below gives a summary of the action needed under each of the three scenarios discussed above.
 
HVL Assessments of W/Ag and W/Rh Beams
Scenario
kVp Meter
Air Kerma Instrument
Action
Correction Factor
1
Integrated solid state instrument (measuring kVp, HVL, and dose) certified by manufacturer for W/Ag and W/Rh
Accept measurement
N/A
2
Integrated solid state instrument (measuring kVp, HVL, and dose) NOT certified by manufacturer for W/Ag and W/Rh
· Apply correction factor to exposure
· Calculate HVL in standard manner
· Use corrected kVp with estimated HVL from dose table (in FFDM manufacturer’s QC manual) to see estimated dose (see scenario 2 for more info)
Obtain from kVp meter manufacturer or develop by comparing engineer’s manufacturer-approved measurements during machine installation with your kVp meter
3
Not certified by manufacturer for W/Ag and W/Rh
Separate solid state probe
· Apply correction factor to exposure
· Calculate HVL in standard manner
Use corrected kVp with estimated HVL from dose table (in FFDM manufacturer’s QC manual) to see estimated dose (see scenario 2 for more info)
Obtain from kVp meter manufacturer or develop by comparing engineer’s manufacturer-approved measurements during machine installation with your kVp meter

 

 

Modification to the table in Quality Assurance/Equipment/Annual Physics Survey, Quality Assurance/Equipment/Mammography Equipment Evaluations, and Quality Assurance/Equipment/Use of Test Results
 
For any adjustment, change, or repair not listed in the table below, or if the facility is unsure as to whether the adjustment, change, or repair qualifies as a major repair, the facility should consult its medical physicist to determine the proper extent of his or her involvement in evaluating the item.

 

Item
Major Repair
Medical Physicist Involvement
Automatic Exposure Control
 
 
AEC replacement
Y
MP conducts evaluation in person
Thickness compensation internal* adjustment
N
MP oversight
AEC sensor replacement
Y
MP conducts evaluation in person
AEC circuit board replacement
Y
MP conducts evaluation in person
Density control – internal* adjustment
 
N
MP oversight
Bucky (New to Facility) Replacement
 
 
AEC sensor also replaced
Y
MP conducts evaluation in person
AEC sensor not replaced
 
N
MP oversight
FFDM detector also replaced
Y
MP conducts evaluation in person
FFDM detector not replaced
N
MP oversight
Cassette Replacement
 
 
Same screen speed
N
MP oversight
Faster screen speed
 
N
MP oversight
Slower screen speed where the dose increase may exceed 3.0 mGy for the standard breast
Y
MP conducts evaluation in person
Collimator
 
 
Replacement
Y
MP conducts evaluation in person
Reassembly with blade replacement
Y
MP conducts evaluation in person
Adjustment
 
N
MP oversight
Compression Device
 
 
Pressure adjustment
N
MP involvement optional
Thickness scale accuracy adjustment but only if it affects AEC performance
N
MP oversight
Repair of auto decompression
 
N
MP involvement optional
Compression Paddle
 
 
Paddle (new to facility) replacement
N
MP oversight
Deflection adjustment
N
MP oversight
Adjustment due to extension beyond allowable limit, or visibility on images
 
N
MP oversight
Darkroom
 
 
Repair of light leaks
N
MP involvement optional
Safe light change
 
N
MP involvement optional
Film Type/Speed Change
 
N
MP oversight
Processor
 
 
Chemistry type change
N
MP involvement optional
Fixer/Developer replacement
N
MP involvement optional
Installation
Y
MP conducts evaluation in person
Reassembly
Y
MP conducts evaluation in person
Replenishment rate adjustment
N
MP involvement optional
Roller replacement
 
N
MP involvement optional
X-ray Unit
 
 
kVp, mA or time internal* adjustments
N
MP oversight
High voltage generator replacement
Y
MP conducts evaluation in person
X-ray tube replacement
Y
MP conducts evaluation in person
Filter replacement
Y
MP conducts evaluation in person
Installation
Y
MP conducts evaluation in person
Reassembly
Y
MP conducts evaluation in person
Manufacturer's software modifications (see approved alternative standard)
Y
MP conducts evaluation in person
FFDM detector replacement or repair
Y
MP conducts evaluation in person
FFDM Display (monitor)/Printer Replacement
Check FFDM manufacturer’s QC manual
Follow FFDM manufacturer’s QC manual

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