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