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MAMMOGRAPHY PHYSICS
MEASUREMENTS:
EXPECTATIONS,
INSTRUMENTATION,
PERFORMANCE,
REQUIREMENTS
Libby Brateman, Ph.D., University of Florida
Philip H. Heintz, Ph.D., University of New Mexico
No Conflicts of Interest to Disclose
FLAAPM / FLHPS
2015
--For regulatory requirements (FDA / MQSA)
--For accreditation guidelines (ACR)
--For dose estimates for individual patients
Mammography Measurements
Measure Air Kerma
Exposure Accuracy
Appropriate Beam Energy and Rate Characteristics
Meet Instrument Specifications
Determine whether equipment meets MQSA requirements
EXPECTATIONS
Requirements for Regulations and Accreditation
kVp accuracy and reproducibility to meet standards
Exposure or Air Kerma (AK)
Reproducibility and minimum radiation output to meet standards
For use in average glandular dose calculations
Half-value layer (HVL)
To meet minimum HVL standards for all available target/filter combinations
For use in average glandular dose calculations
Average glandular dose associated with a specific phantom to meet maximum AGD standards
Noninvasive measurements
Early kVp Measurements
RMI Mammography kVp Cassette (1980)
The Wisconsin Mammographic
kVp Cassette (RMI):
Average: ±2 kVp (1ø)
Range: -6 to -2.5 kVp, W/Al
-3.5 to +3.0, Mo/Mo
Cruty MR and Ghilardi-Netto T. Med. Phys. 7:151-6 (1980)
Measuring kVp Non-Invasively:
Safer than using a voltage divider!
Based on the fact that a
heavily-attenuated spectrum is
approximately linear with kV,
and a ratio can predict the kVp.
Original research used filters to
attenuate spectra to 2% and
8% of the original intensity with
Cu filtration.
kVp
Graphic from Barnes GT in Screen
Film Mammography, Barnes GT and
Frey GD, eds., Medical Physics
Publishing (1991) (See Joseph PM. Med. Phys.
2(4) 1975, 201-207, 208-212)
RMI Model 232 kVp Meter (1991)
Mo/Mo and W/Al
1ø, 3ø, Constant Potential
If 3ø, accuracy is +0.8 to + 1.0 kV
If constant potential, accuracy is – 0.3 to + 0.1 kV
Compression plate decreased kVp measurement
by 0.22 kV at 25 kV
Law J. Phys. Med. Biol. 36: 1133-9 (1991)
Solid-State kVp: Radcal 9095 (1998)
40X5-MO
Mammography kV Sensor
Designed for Mo/Mo
Sensor photo from P. Sunde
LiF Thermoluminescent Dosimeters (TLD’s)
Ionization Chamber with Electrometer
Early Dosimetry (1970’s)
Thermoluminescent Dosimeters:
Harshaw LiF TLD-100
TLD dosimetry has been used for reference
measurements in many mammography dosimetry
papers
7.5% 6Li, 92.5% 7Li
3mm x 3mm x 9 mm
Calibrated individually
Morgan TJ and Brateman L. Health Phys.
33(4) 339-3314 (1977)
NCRP Report No. 66: Mammography (1980)
Individually measured with
Eberline TLR-5 TLD Reader
Victoreen 555 R-Meter with 2.5 R (533) chamber
Analog Charger/Reader
Graphic from Hendee WR.
Medical Radiation Physics, 2nd ed.
Year Book Publishers (1984)
Medium Energy Ionization Chambers for
Victoreen Model 570 Condenser R-Meter
1 R 2.5 R
Morgan TJ, Brateman L and Dirkse J.
Med. Phys. 4(5) 448-450 (1977)
Common 1970’s Diagnostic Survey
Meters and Ionization Chambers
Lowest HVL evaluated was 0.4 mm Al
Victoreen 666-10
Diagnostic Chamber
Keithley 96030
Diagnostic Chamber
1977
1978
Morgan TJ, Brateman L and Dirkse J.
Med. Phys. 4(5) 446-447 (1977)
Morgan TJ, Brateman L and Dirkse J.
Med. Phys. 5(2) 162-`63 (1978)
The First Ionization Chamber Designed
for Mammography (1975)
Holt JG, Perry DJ and Reinstein LE. The Design of an Ionization Chamber to Measure Exposure in Air for Mammographic Techniques. Med. Phys. 2,172 (1975) (abstract for AAPM meeting presentation)
“Since ionization chambers which are used over the normal range of diagnostic x-ray beam qualities are generally not suitable for low energy x-ray exposure measurements, an ionization chamber has been developed with a reasonably flat energy response (± 2.5%) with respect to air over the limited range of HVL’s 0.3 to 1.5 mm Al.
The ionization chamber is pill-box shaped with a thin front window of thickness not exceeding 7 mg/cm2, with 3 mm spacing between electrodes and a collection volume of 3 cm3.”
Photo from Hendee WR. Medical
Radiation Physics, 2nd ed. Year
Book Publishers (1984)
• Digital Readouts for Ionization Chambers
• Commercial Mammography Ionization Chambers
Systems Integrating kVp with Dosimeters
• Solid-State Dosimeters (SStDs) with data transfer
to computer
MODERN INSTRUMENTATION
Radcal 10X5–6M (1978)
6 cm3 active volume, Mylar window
Sunde designed the 6M chamber in early 1978:
FDA started using the 6M chamber in the early 1980’s
Prior to that, FDA used the -6 diagnostic chamber with the MDH
instrument and applied correction factors to the digitized readout.
Reference chamber
Radcal 10X5–6M Energy Response
Designed for Mammography
Contrast-enhanced mammography uses Cu filtration with HVLs >2 mm Al.
First All-in-One Instrument for Mammography (1994)
Gambaccini M, Marziani M, Rimondi O.
A fast non-invasive beam check for
mammography x-ray units. Phys. Med.
Biol. 39 (1994) 1423-1435
A single x-ray exposure measurement of
high-voltage waveform, kVp, HVL,
exposure time and exposure for Mo-
anode, Mo-filter equipment
New Solid-State Detectors (SStDs)
SStDs can give a measurement of kVp, HVL and
exposure or air kerma with one x-ray exposure
and report it digitally.
All SStD’s have to be corrected for energy
response because they are not air ionization
chambers.
Rationale for Study
Using new instruments with solid-state dosimeters (SStDs)
on new technology full-field digital mammography
(FFDM) units, we found unexplained differences in
average glandular dose (AGD) that required reporting
to FDA and ACR.
AGD depends on kVp, HVL and exposure/AK.
SStDs measure kVp and HVL in addition to
exposure/AK.
Reference ionization chambers (ICs) for mammography
exposure or are essentially energy-independent, but
SStDs are not.
This study evaluated factors relating to average glandular dose (AGD) as measured with solid-state dosimeters (SStDs) and ionization chambers (ICs) for two major vendors of full-field digital mammography (FFDM) units.
• GE Essential and Hologic Dimensions
• Mammography only, not tomosynthesis
• Not contrast-enhanced mammography
Research Initiative
Measurement Instrumentation
Radcal 9095 (UF Health)
10X9-6M mammography ionization chamber (IC)
Accu-kV 40X9-MO kVp detector
Radcal Accu-Gold system (UNM)
10X6-6M mammography IC with adaptor
Model AGMS-M Solid-State Multi-Sensor (SStD)
Unfors RaySafe Xi SStD (UF Health)
RTI Piranha SStD Model 657 v.3.1(borrowed from vendor)
PTW NOMEX Multimeter SStD v.1.1(borrowed from vendor)
Radcal Accu-Gold
Very small solid-
state sensor
AGMS-M sensor
for mammography
Ion chamber
connection
Readout by
computer
Unfors RaySafe Xi
Portion of sensor used for mammography measurements
Target/filter manually selected on unit
Requires 2 mm Al filter (provided) for Mo/Rh measurements
kVp calibrated for measurements at 6 cm from the chest wall
RTI Piranha
“Fits in your hand”
Precise alignment is required.
Selection of target/filter on
computer
Diagnostic software for data
input: Error messages
Frequent software updates
with embedded correction
factors by T/F
PTW NOMEX Multimeter
Large sensor is directionally independent
Data transfer to computer
Measurement Instrumentation
All SStD’s and one IC had been recently calibrated by their
manufacturers.
The Radcal 9095 10X9-6M IC was calibrated by an ADCL.
All solid-state detectors (SStDs) were calibrated by vendors:
Air Kerma
kVp
HVL
There is no NIST reference standard for kVp or HVL.
Instrument Vendor Specifications and
Maximum Calculated Errors for AGD
Instrument kVp Air Kerma
(%)
Half-Value
Layer
Max. AGD Error (%)
28-30 kVp, All Filters
PTW NOMEX ± 0.5 kV ± 2.5 ± 0.01 mm Al ± 4.3
Radcal AccuGold
AGMS-M
± 2% or
± 0.7 kV*
± 5 ± 10% or
± 0.05 mm Al*
± 13.2
RTI Piranha ± 2% or
± 1 kV*
± 5 ± 10% ± 13.4
Unfors RaySafe Xi ± 2% or
± 0.7 kV*
± 2.5 ± 5% ± 6.9
*Whichever is greater
Two Mammography Units Used in the Study
General Electric Essential Mammography Unit
Hologic Dimensions 5000 Mammography Unit
Target Filter and Thickness kVp Range Max. Paddle Height
Mo Mo (0.03 mm) 22-32 kVp 21.5 cm
Mo Rh (0.025 mm) 22-40 kVp 21.5 cm
Rh Rh (0.025 mm) 25-49 kVp 21.5 cm
Target Filter and Thickness kVp Range Max. Paddle Height
W Rh (0.05 mm) 20-39 kVp 33.9 cm
W Ag (0.05 mm) 20-39 kVp 33.9 cm
W Al (0.7 mm) 26-49 kVp 24.0 cm
The Rh filter thicknesses are different for GE and Hologic.
Study Protocol and Experimental Setup
The study protocol was developed with the intention to follow manufacturers’ recommendations as specified by MQSA, but many refer to the ACR Mammography Quality Control Manual, published in 1999 when exposure (air kerma) and HVL were measured with ionization chambers.
Foam supports were constructed for each sensor to set the measurement location reproducibly at 4.2 cm above the breast support.
Instruments and their supports were secured onto a beam-blocking plate to allow transfer between mammography units for accurate and reproducible placement on the breast support.
Measurement Geometry: kVp & HVL
Pictures here
Hologic Dimensions with Radcal
Accu-Gold 10X5-6M IC
GE Essential with Unfors
RaySafe Xi SStD
Measured vs. Set kVp Measurements
• NIST has no reference kV calibration standard.
• Not all instruments measured kV for all T/Fs.
• Not all instruments measured the full range of kVs
kVp Measurements
Reference Measurements: kVp and HVL
Manufacturers’ procedures refer to ACR manual.
ACR quality control procedures were followed as much as possible.
The smallest field sizes were used: GE: 10 cm x 11 cm
Hologic: 7 cm x 8.5 cm
The paddle was raised to 21 cm for consistency between measurements on mammography units, instead of maximum paddle height.
The set kVp was used as the reference kVp for each mammography unit, as both mammography units had been installed and calibrated within 1 month of the beginning of the study.
kVp: GE
Essential
20
25
30
35
40
20 25 30 35 40
Measu
red
kV
p
Set kVp
Rh/Rh
20
25
30
35
40
20 25 30 35 40
Measu
red
kV
p
Set kVp
Mo/Rh
20
25
30
35
20 25 30 35
Measu
red
kV
p
Set kVp
Mo/Mo
Radcal 9095
Unfors RaySafe Xi
Radcal AGMS-M
RTI Piranha
PTW NOMEX
Identity
kVp: Hologic
Dimensions 20
25
30
35
40
20 25 30 35 40
Measu
red
kV
p
Set kVp
W/Rh
20
25
30
35
40
20 25 30 35 40
Measu
red
kV
p
Set kVp
W/Al
Radcal 9095
Unfors RaySafe Xi
Radcal AGMS-M
RTI Piranha
PTW NOMEX
Identity20.0
25.0
30.0
35.0
40.0
20 25 30 35 40
Measu
red
kV
p
Set kVp
W/Ag
Entire Range of kVp Measurements
PTW
NOMEX
Radcal
Accu-Gold
AGMS-M
Radcal
9095
40X9-MO*
RTI
Piranha
Unfors
RaySafe
Xi
Minimum
Difference
(kV) -1.0 -0.9 -0.5 -1.1 -0.1
Maximum
Difference
(kV) 1.5 0.7 1.1 0.6 1.2
Differences between Measured and Set kVp by Instrument
•Some instruments had problems with a particular target-filter
combination. We removed Radcal 9095 W/Ag and RTI Piranha
W/Al because of known deficiencies by manufacturer.
*Discontinued
“Clinical” kVp Measurements
PTW
NOMEX
Radcal
Accu-
Gold
AGMS-M
Radcal
9095
40X9-
MO*
RTI
Piranha
Unfors
Raysafe
Xi
Minimum
Difference (kV) -0.5 -0.6 -0.4 -0.9 -0.1
Maximum
Difference (kV) 0.2 0.3 0.0 0.5 0.1
Differences between Measured and Set kVp by Instrument
Four “Clinical” kVp/Target/Filter Combinations:
1. 28 kVp / Mo target / Mo filter
2. 29 kVp / Rh target / Rh filter
3. 28 kVp / W target / Rh filter
4. 29 kVp / W target / Al filter (except RTI Piranha)
*Discontinued
Single-Point HVLs for SStDs vs. Reference HVLs
• NIST has no reference standard for HVL
• Not all instruments measured HVL for all T/Fs
• Reference HVLs with ionization chamber and Al filters
• HVLs for entire range of kVp’s for Rh/Rh and W/Rh
• “Clinical Subset” for 4 kVp/target/filter combinations
• SStD HVLs with Al filters do not work well.
Half-Value Layer Measurements
Reference Measurements: HVL
The reference instrument for HVL measurements was the Radcal Accu-Gold with 10X6-6M mammography ionization chamber.
Filters for reference HVLs were hand-selected from type 1145 Al filters of nominal 0.1 mm thickness.
Filters used had attenuations within 0.3% for a Rh/Rh beam at 29 kVp.
Thicknesses of filters were measured with a high-precision micrometer (0.0001 inch) and converted to mm for calculations of HVL with interpolation.
Single-Point SStD HVL: Essential and Dimensions at 25, 28, 29, 30, 36 kVp
0.45
0.50
0.55
0.60
0.45 0.50 0.55 0.60
Measu
red
HV
L (m
m A
l)
Reference HVL (mm Al)
W/Rh
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.30 0.35 0.40 0.45 0.50 0.55 0.60
Measu
red
HV
L (m
m A
l)
Reference HVL (mm Al)
Rh/Rh Unfors RaySafe Xi
Radcal AGMS-M
RTI Piranha
PTW NOMEX
Identity
Single-Point HVLs for 3 kVp’s and All
Target/Filter Combinations: Essential
Measured at 28, 29, 30 kVp for Mo/Mo, Mo/Rh, Rh/Rh
Color for instrument, symbol for target/filter
0.35
0.40
0.45
0.35 0.37 0.39 0.41 0.43 0.45
SStD
HV
L (m
m A
l)
Reference HVL (mm Al)
Unfors RaySafe Xi Mo/Mo
Radcal AGMS-M Mo/Mo
RTI Piranha Mo/Mo
PTW NOMEX Mo/Mo
Unfors RaySafe Xi Mo/Rh
Radcal AGMS-M Mo/Rh
RTI Piranha Mo/Rh
PTW NOMEX Mo/Rh
Unfors RaySafe Xi Rh/Rh
Radcal AGMS-M Rh/Rh
RTI Piranha Rh/Rh
PTW NOMEX Rh/Rh
Identity
Single-Point HVLs for 3 kVp’s and All
Target/Filter Combinations: Dimensions
Measured at 28, 29, 30 kVp for W/Rh, W/Ag, W/Al
HVL for W/Al with small field created using diaphragm
More spread in data than for GE Essential
0.45
0.50
0.55
0.60
0.65
0.45 0.50 0.55 0.60 0.65
SStD
HV
L (m
m A
l)
Reference HVL (mm Al)
Unfors RaySafe Xi W/Rh
Radcal AGMS-M W/Rh
RTI Piranha W/Rh
PTW NOMEX W/Rh
Unfors RaySafe Xi W/Ag
Radcal AGMS-M W/Ag
RTI Piranha W/Ag
PTW NOMEX W/Ag
Unfors RaySafe Xi W/Al
Radcal AGMS-M W/Al
PTW NOMEX W/Al
Identity
“Clinical” Single-Point HVL Measurement Ratios
Compared to Reference HVL with Al filters
“Clinical”
kVp / T / F
Reference
HVL
(mm Al)
PTW
NOMEX
Radcal
AGMS-M
RTI
Piranha
Unfors
RaySafe
Xi
Min.
Max.
28/Mo/Mo 0.366 1.01 0.99 0.97 1.00 0.97 1.01
29/Rh/Rh 0.429 1.00 1.00 0.96 1.02 0.96 1.02
28/W/Rh 0.529 1.03 1.00 0.94 1.02 0.94 1.03
29/W/Al 0.512 1.03 0.99 --* 0.98 0.98 1.03
Min. 1.00 0.99 0.94 0.98
Max. 1.03 1.00 0.97 1.02
*Excluded: RTI Piranha required special software for the Al filter that was
not available. Without this software, errors were as large as 13%.
Exposure measurements were compared for each T/F/kV with the UF
Radcal 9095 10X9-6M ionization chamber, with its independent
calibration from an ADCL for Mo/Mo (±3%).
• All measurements were made with a beam-blocking plate
underneath the instrument. Therefore all measurements are relative.
Ratios are for each instrument as compared with the reference.
• For ICs and SStDs, the effects of this plate on AK were < 1%.
• All measurements were recorded in mR because of dose
conversion tables for DgN in (mrad/mR).
Air Kerma / Exposure Measurements
Reference Measurements: Exposure / AK
Instead, we centered the dosimeter (ionization chamber or SStD)
in the large field (R/L) without the phantom, at 4 cm from the
chest wall, supported on a constructed foam holder to the correct
height on a beam block, with the large paddle in contact with the
instrument.
The ACR procedure locates the instrument for
entrance exposure beside the phantom, with the
phantom centered in the field (R/L) and at the
chest wall, with the paddle in contact. The ion
chamber is 4 cm from the chest wall.
AK calibration was traceable to NIST for Mo/Mo only.
Instrument vendors employ additional internal calibration factors, and
calibration reports vary widely among vendors.
Air Kerma / Exposure Measurements
Target/filter, kVp and mAs were chosen
from phantom exposed in AEC mode (GE
AOP/Standard, Hologic Autofilter). Values
were similar to 50 mAs for Rh/Rh and 100
mAs for W/Rh.
Study measurements were made without
the phantom present at 28, 29 and 30 kVp
at constant mAs for all targets and filters
for each mammography unit
50 mAs for GE Essential
100 mAs for Hologic Dimensions,
except 50 mAs for W/Al
Setup for Hologic
Dimensions with Unfors
RaySafe Xi SStD
Exposure: GE Essential
28, 29, 30 kVp
Mo/Mo, Mo/Rh, Rh/Rh
Radcal 10X5-6M is an
ionization chamber with the
AccuGold system.
RTI Piranha has a 1.06
correction factor applied.
600
650
700
750
800
850
600 800
Measu
red
Exp
osu
re (
mR
)
Reference Exposure (mR)
Mo/Mo
500
550
600
650
700
500 600 700
Measu
red
Exp
osu
re (
mR
)
Reference Exposure (mR)
Mo/Rh
450
500
550
600
650
450 550 650
Measu
red
Exp
osu
re (
mR
)
Reference Exposure (mR)
Rh/Rh
Unfors RaySafe Xi
Radcal AGMS-M
RTI Piranha
PTW NOMEX
Radcal 10X5-6M
Identity
Exposure: Hologic Dimensions
350
400
450
500
550
350 450 550Measu
red
Exp
osu
re (
mR
)
Reference Exposure (mR)
W/Rh
450
500
550
600
650
700
450 550 650Measu
red
Exp
osu
re (
mR
)
Reference Exposure (mR)
W/Ag
28, 29, 30 kVp
Mo/Mo, Mo/Rh, Rh/Rh
Radcal 10X5-6M is an
ionization chamber with the
AccuGold system.
RTI Piranha has a 1.06
correction factor applied.
400
450
500
550
600
400 500 600
Measu
red
Ex
po
sure
(m
R)
Reference Exposure (mR)
W/Al Unfors RaySafe Xi
Radcal AGMS-M
RTI Piranha
PTW NOMEX
Radcal 10X6M
Identity
Radcal 10X5-6M
Summary of Exposure Ratios for 5
Instruments Compared with Reference
3 SStDs consistently underestimated exposure, compared with IC measurements
One SStD was inconsistent (sometimes high, sometimes low)
Partially explained by different effect of scatter on SStDs and ionization chambers
Perhaps also due to differences in calibration among vendors
Radcal
10X5-6M
Ionization
Chamber
PTW
NOMEX
Radcal
AGMS-M
RTI
Piranha
Unfors
RaySafe
Xi
Min. Max.
0.911 1.073
Min. 1.001 0.911 0.977 0.964 0.943
Max. 1.011 0.948 1.000 1.073 0.978
AVERAGE GLANDULAR DOSE
AGDs were calculated from exposure measurements and are relative values, compared with the average exposures from both ionization chambers for each T/F/kV.
• We used AGD conversion factors from tables in ACR and Hologic manuals.
Average Glandular Dose: Dg
Dg = (XESE) (DgN )
XESE is the corrected entrance exposure, “free-in-
air”, using a calibrated mammography probe.
XESE = Xinst (Correction Factor)
DgN is the particular conversion factor for the parameters of
interest (target, filter, kVp, HVL, breast thickness,
composition).
• The procedure does not specify Set kVp or Measured
kVp. We used Set kVp.
NCRP Report No. 149
Relative AGD: GE Essential (28, 29, 30 kVp)
Reference AGD from
Reference Exposure, Set kVp
and Measured HVL with Al
filters (Identity)
1.10
1.20
1.30
1.40
1.50
1.60
1.70
1.10 1.60
AG
D (
mG
y)
Reference AGD (mGy)
Mo/Mo
1.00
1.10
1.20
1.30
1.40
1.50
1.60
1.10 1.60
AG
D (
mG
y)
Reference AGD (mGy)
Rh/Rh
1.10
1.20
1.30
1.40
1.50
1.60
1.10 1.60
AG
D (
mG
y)
Reference AGD (mGy)
Mo/Rh Radcal 9095
Unfors Raysafe Xi
Radcal AGMS-M
RTI Piranha
PTW NOMEX
Radcal 10X5-6M
Identity
Relative AGD: Hologic Dimensions (28, 29, 30 kVp)
1.10
1.20
1.30
1.40
1.50
1.10 1.30 1.50
AG
D (
mG
y)
Reference AGD (mGy)
W/Rh
1.20
1.30
1.40
1.50
1.60
1.70
1.80
1.90
2.00
1.20 1.70
AG
D (
mG
y)
Reference AGD (mGy)
W/Ag
1.00
1.10
1.20
1.30
1.40
1.50
1.60
1.00 1.20 1.40 1.60
AG
D (
mG
y)
Reference AGD (mGy)
W/Al
Unfors Raysafe Xi
Radcal AGMS-M
RTI Piranha
PTW NOMEX
Radcal 10X6M
Identity
Reference AGD from
Reference Exposure, Set
kVp and Measured HVL
with Al filters (Identity)
Radcal
9095
Unfors
RaySafe
Xi
Radcal
AGMS-
M
RTI
Piranha
PTW
NOMEX
Radcal
10X5-6M
Ionization
Chamber
Min. Max.
0.910 1.011
Min. 1.000 0.943 0.982 0.964 0.910 1.001 Range =
0.101 Max. 1.000 0.977 0.998 1.008 0.962 1.011
Condition 1: Set kVp, Reference HVL, Measured Exposure
Condition 2: Measured kVp, Single-Point HVL, Measured Exposure
Radcal
9095
Unfors
RaySafe
Xi
Radcal
AGMS-
M
RTI
Piranha
PTW
NOMEX
Radcal
10X5-6M
Ionization
Chamber
Min. Max.
0.875 1.009
Min. 0.997 0.937 0.982 0.875 0.935 0.998 Range = 0.134
Max. 1.000 0.996 0.995 0.978 0.935 1.009
AGD Ratios:
28/Mo/Mo, 29/Rh/Rh, 28/W/Rh, 29/W/Al
Relative AGD Compared with Reference
for Average of 28, 29, 30 kVp
0.84
0.86
0.88
0.90
0.92
0.94
0.96
0.98
1.00
1.02
Mo/Mo Mo/Rh Rh/Rh W/Rh W/Ag W/Al
PTW NOMEX
Radcal AGMS-M
RTI Piranha
Unfors RaySafe Xi
Reference
Target/Filter
GE Essential Hologic Dimensions
Rela
tive
AG
D
Ionization Chambers vs. SStD’s
Scatter Measurements
Effects of Scatter: Phantom
Some SStDs have internal collimation to exclude the effects of
scatter on measurements, but ionization chambers respond to
scatter, e.g., from the phantom and the paddle.
Effect of Phantom Adjacent to Dosimeter:
• Ion chamber readings increased up to 1.2% when placed next to
the ACR phantom.
• Unfors Xi reading increased very little, if at all, with the phantom
adjacent
Effects of Scatter: Paddle
Effect of Paddle Height on Instruments, Normalized to “in Contact”,
Averaged over Target/Filter Combinations
RTI Piranha was not tested, but the manufacturer supplied a 1.06
factor to correct exposure values for effects of scatter.
Unit Paddle
Height
Unfors
RaySafe Xi
Radcal Accu-
Gold AGMS-M
PTW
NOMEX
Radcal Accu-Gold
10X5-6M
Ionization
Chamber
Dimensions 33 cm 1.000 0.983 0.989 0.957
Dimensions 21 cm 0.996 0.989 0.998 0.964
Essential 21 cm 0.993 0.972 0.999 0.968
Both Contact 1.000 1.000 1.000 1.000
Paddle Effect <0.01 % 1.1-2.8% 0.1-1.1% 3.2-4.3%
Study Conclusions
kVp
kVp accuracy < 1.5 kVp, except 1 instrument, 1 target/filter.
Not all instruments measured all T/F’s or all kV ranges.
Some vendors specify locations for measurements.
HVL
Single-point HVLs over all targets and filters ranged from
-2% to +6%, except for one instrument.
ACR Mammography Quality Control Manual does not include
single-point measurements of HVL; however, it is best not to use
Al filter measurements of HVL with SStDs.
MQSA requires verification with standard measurement
conditions.
Study Conclusions
SStDs’ insensitivity to scatter as compared with ionization chambers
underestimates geometry-dependent entrance exposure.
Over the range of 28-30 kVp for all target/filter combinations:
Entrance exposures varied from -9% to +7%.
Relative AGD was underestimated by 13% or overestimated by
1.4% for all instruments.
Sensitivity of AGD is:
Linear with entrance exposure and small changes (<10%) in HVL
Less sensitive to kVp and depends on target and filter
Study Findings & Vendor Specifications
PTW NOMEX failed HVL specifications for 2 T/F’s,
but their specifications were tighter than other
vendors and would have passed other vendor specs.
Unfors Xi failed AK specifications unless scatter
corrections were used and would have passed other
vendor specs. RTI Piranha failed specifications
W/Ag with or without scatter. PTW NOMEX failed
all specifications for AK and appeared to need a
different calibration factor.
Instrumentation Concerns: Vendors and ACR
Manufacturers should provide good training for users before a physicist starts using the equipment. A problem exists because physics personnel change. Therefore it is important for vendors to provide complete and updated documentation for users to keep physicists trained.
Users need to understand the proper measurement locations for their instruments.
Conflicts between manufacturers’ recommendations and ACR manual:
For single-point HVL
For instrument placement next to phantom
There are no recommendations for using Set kVp or Measured kVp in determining AGD.
Calibration vs. Clinical Measurement Conditions
Discrepancies in Dose Measurements
What is the True AGD?
NCRP: Mammography Dose Paradigm
NCRP Report No. 149: A Guide to Mammography and Other Breast Imaging Procedures (2004) p. 175
“Free in air at
the entrance
skin surface”
Concerns Regarding AK and AGD
NIST provides calibrations for T/Fs in addition to Mo/Mo, but
vendor calibrations may not. It is important for vendors to
provide calibration conditions to users.
There is no calibration standard for HVL or kVp.
Mammography clinical geometry is likely to be different from
the instrument calibration geometry.
What is the true AGD?
Should it be measured with or without scatter?
MQSA Requirements for SStDs
From FDA Policy Guidance Help System (PGHS)
• Updated September 9, 2014
MQSA Definition: Traceable to a
national standard
900.2(xx) Traceable to a national standard means an instrument is calibrated at either the National Institute of Standards and Technology (NIST) or at a calibration laboratory that participates in a proficiency program with NIST at least once every 2 years and the results of the proficiency test conducted within 24 months of calibration show agreement within +/- 3 percent of the national standard in the mammography energy range.
Calibration Observations
New NIST reference mammography beams are
available with a Mo target with Mo, Rh and Al filters,
and a Rh target with Rh and Al filters.*
Vendors and most ADCLs appear to use only Mo/Mo
NIST calibrations even though NIST has additional T/F
standard calibration beams.
Diagnostic physicists frequently use vendor calibrations
because of updates, repairs and internal calibration
adjustments.
*IRD-P-03:
PGHS: kVp Measurement Compensations for
Calibrations for Mo/Mo and Mo/Rh but W/Ag or
W/Rh is used Updated 08/13/2014
PGHS: HVL Assessments with Integrated Solid-State
Instrument Updated 08/13/2014
PGHS: HVL Assessments with Integrated Solid-State
Instruments
Our recommendation: Don’t use a SStD with Al filters
in HVL measurements.
Updated 08/13/2014