STEREOTACTIC BREAST BIOPSY
EQUIPMENT SURVEYS
JAMES A. TOMLINSON, M.S.
Diagnostic Radiological Physicist
American Board of Radiology Certified
Medical Physics Consultants, Inc.
Bio
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28 yrs experience
100% Dx Rad
>100 mammo systems/yr
15 are dedicated stereotactic units
SBB-Testing Requirements
• Currently exempt from MQSA certification and ACR
accreditation (voluntary).
• Some states require testing, all tests that are applicable.
• This focus is on dedicated units with digital IR.
Testing Procedures
• The ACR-SBB manual contains testing procedures for
technologists and physicists. There are options for filmscreen units and those with digital receptors.
SBB Units
• Dedicated prone table units are made by Fischer and
Lorad. Other re-salers have put their label on the Lorad
unit.
• Add-on stereotactic devices are also used.
• We will only discuss the prone table units.
FISCHER MAMMOTEST
Multi-Angle Biopsy
Image Receptor
9-10 lp/mm 10242
Min-R screen
5x5cm
Fiber-optic taper
CCD 1”x1”
1024x1024
MTF
LORAD STEREOGUIDE
Detector assembly
Breast support
Image Receptor
~5 lp/mm 5122
7-10 lp/mm 10242
Lens
Mirror
CCD 1”x1”
1024x1024
Lanex screen
EQUIPMENT TESTING
1.
2.
3.
4.
5.
6.
Unit assembly evaluation
Collimation assessment
Focal spot performance/system resolution
kVp performance
Beam quality (HVL)
AEC or manual exposure performance
EQUIPMENT TESTING
7.
8.
9.
10.
11.
Digital image uniformity
Breast radiation dosimetry
Image quality
Artifact evaluation
Localization accuracy
1. Unit Assembly Evaluation
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•
Free-standing dedicated unit is mechanically stable
All moving parts move smoothly
All locks and detents work properly
Image receptor holder assembly is free from vibrations
Image receptor is held securely by assembly
Compressed breast thickness scale accurate to 5 mm
Patient or operator not exposed to sharp or rough edges
or other hazards
2. Collimation Assessment
• Radiation to digital image alignment
• Radiation beam confinement to image receptor
Coin Method
Monitor
Film
Collimator Tool Method
Monitor
Film
Collimation Tolerance
• The radiation field should extend beyond the image
receptor
• But not more than 5 mm on any edge
Radiation Beam Confinement
• Often, the rad field will exceed the image receptor at the
chest wall
unnecessary radiation
breast
radiation field
image field
metal
compression
paddle
3. Focal Spot Performance
• The focal spot performance is tested with the res pattern
and film (no screen)
Focal Spot Performance
• ≥ 11 lp/mm in the direction perpendicular to the anode
cathode axis
• ≥ 13 lp/mm in the direction parallel to the anode
cathode axis
13 parallel
11 perp
System Resolution (Digital)
• The resolution test pattern is used with the digital
receptor to find limiting resolution of the 50x50 mm
image size.
• For 512 image, pixel size is = 0.098 mm = 5.1 lp/mm
• For 1024 image, pixel size is 0.049 mm = 10.2 lp/mm
• Typically see 7-10 lp/mm
Q1- The maximum limiting system resolution for a
dedicated SBB device in 1024 mode would be:
20%
20%
20%
20%
20%
1.
2.
3.
4.
5.
1 lp/mm
3 lp/mm
5 lp/mm
7 lp/mm
10 lp/mm
10
5.
10 lp/mm
Bushberg, Jerrold T, Ph.D., et al; The Essential Physics
of Medical Imaging, Second Edition; Lippincott,
Williams & Wilkins, 2002, p. 284
4. KVp Performance
kVp Accuracy and Reproducibility
• Mammography units should have clinically used kVp
accuracy to within 5% of the indicated kVp
• From four measurements at most commonly used kVp,
compute mean, s.d., and coefficient of variation
(sd/mean)
• C.V. should be within 0.02
Q2- Tube potential accuracy should be within:
20%
20%
20%
20%
20%
1.
2.
3.
4.
5.
1%
1 kVp
2%
2 kVp
5%
10
5.
5%
American College of Radiology; Stereotactic Breast
Biopsy – Quality Control Manual; 1999, p. 65
5. Beam Quality (HVL)
• The HVL requirements are slightly different from filmscreen mammo, because the image port is open (no
paddle attenuation)
• The HVL should be above HVL ≥ kVp/100 (mm Al)
• The ACR suggests an upper limit of HVL < kVp/100 +
0.12 (mm Al) for Mo/Mo
• At 28 kVp,
0.40 ≥ HVL ≥ 0.28
Half-Value Layer
High-purity
aluminum
Half-Value Layer
• Logarithmic interpolation can be are used to calculate
HVL
• Using the logarithmic method, where E0=0 mm,
Ea= first added filtration, Eb=second total added
filtration
t b ln (2E a / E o ) - t a ln (2E b / E o )
HVL =
ln ( E a / E b )
Q3- The half-value layer (HVL) at 28 kVp must be
greater than or equal to:
20%
20%
20%
20%
20%
1.
2.
3.
4.
5.
0.26 mm Al
0.28 mm Al
0.30 mm Al
0.32 mm Al
0.35 mm Al
10
2.
0.28 mm Al
American College of Radiology; Stereotactic Breast
Biopsy – Quality Control Manual; 1999, p. 68
6. AEC Performance (if used)
• For the performance test, 4, 6, and 8 cm of lucite is
evaluated with clinical technique factors
AEC System Performance
• For digital systems:
– read the signal value in the center of the image
– signal from 6 and 8 cm should be within ± 20% of the 4 cm
reading
– if not, a technique chart should be developed and posted
Technique Chart
• Posted techniques should be evaluated and adjusted to
keep image quality high, yet exposure times < 2 seconds
7. Digital Image Uniformity
• 4 cm lucite image
• Signal analysis in center and quadrants
– Signal
– Standard deviation (sd)
– SNR
• Calculate quadrant deviance from the center values
• Systems without ROI capability, use wire mesh pattern
Lorad Tolerances
• SNR in each corner ± 15% of center (early manuals
show 128x128 box). I’ve never found a Lorad unit that
will pass with this protocol.
• Later test protocols use a 32x32 box and specifies the
corner areas to sample. The specified tolerance was that
the SNR in each corner not exceed ± 20% of the central
SNR. Most units will pass with these parameters.
6% of total area
128x128 512 image
0.4% of total area
32x32
512 image
Newer Lorad Units
• On the latest units, the operator manual indicates to take
signal in the center of 1024 image with 32x32 box
(0.1% of total area). This also requires significantly
more radiation to keep signal at 4k, when quadrupling
image matrix.
• Signal in each quadrant should be within 15% of the
center. All of them appear to be able to pass this
protocol.
Fischer Tolerances
• SNR in each corner ± 15% of center (ACR manual
shows 128x128 box)
• Most units will pass this tolerance
Q4- The ACR digital receptor uniformity tolerance
for signal value is:
20%
20%
20%
20%
20%
1.
2.
3.
4.
5.
5%
10 %
15 %
20 %
25 %
10
3. 15%
American College of Radiology; Stereotactic Breast
Biopsy – Quality Control Manual; 1999, p. 75
8. Breast Radiation Dosimetry
• Find technique factors that give manufacturer
recommended signal values for the ACR phantom or the
mAs given from an AEC technique.
• Position ion chamber in port at 4.5 cm from breast
support. Measure manual exposure 4 times. For AEC
techniques, use closest mAs.
• Calculate average, s.d., and c.v. < 0.05
Breast Dose
Breast Dosimetry
• For AEC technique, calculate ESE from:
ESEaec = ESEman (mAsaec/mAsman)
• Calculate the average glandular dose from look-up table
in ACR manual (kVp vs HVL) for Mo/Mo. Typical
conversion factor is 0.18 at 28 kVp.
• Dose should be less than 3.0 mGy (300 mrad).
(Michigan requires 2.0 mGy for all mammography)
9. Image Quality
• Make an exposure of the ACR phantom using the kVp
and density normally used clinically.
• For the standard ACR phantom, image in each quadrant
to determine IQ score.
• For the ACR mini-phantom, only one image is needed.
ACR Phantom
• Screen-film:
• Digital:
4.0 fibers, 3.0 specks, 3.0 masses
5.0 fibers, 4.0 specks, 3.5 masses
Mini-Phantom
Mini-Phantom
• Screen-film:
• Digital:
2.0 fibers, 2.0 specks, 2.0 masses
3.0 fibers, 3.0 specks, 2.5 masses
ACR Phantom Scoring
• Fibers are scored down to the smallest full or half fiber
visible. A full point is given for full fiber, 0.5 for half.
• Specks are scored down to the smallest visible group. A
full point is given if specks 4, 5, & 6 are visible, 0.5 if
only 2 & 3 are visible, and 0 if only 1 speck is visible.
• Masses are scored down to the smallest visible. A full
point is given if round and in the right location, a 0.5
point if in the right location but not round.
Score Subtraction for Artifacts
• If a fiber-like artifact is as apparent as an actual fiber, it
is subtracted from the last half or full fiber.
• If speck-like artifacts are present, they are deducted on a
one for one basis from the last group counted.
• If a mass-like artifact is as apparent as an actual mass, it
is subtracted from the last real mass.
Mag Tool
Q5- The digital image quality required for the
dedicated mini-phantom:
20%
20%
20%
20%
20%
1.
2.
3.
4.
5.
4.0 fibers, 4.0 specks, 4.0 masses
4.0 fibers, 3.5 specks, 3.5 masses
3.5 fibers, 3.5 specks, 3.5 masses
3.0 fibers, 3.0 specks, 3.0 masses
3.0 fibers, 3.0 specks, 2.5 masses
10
5. 3.0 fibers, 3.0 specks, 2.5 masses
American College of Radiology; Stereotactic Breast
Biopsy – Quality Control Manual; 1999, p. 91
10. Artifact Evaluation
• For digital IR – image lucite sheet large enough to cover
IR
• Adjust the window width and level to look for defects in
the image
Very narrow window
11. Localization Accuracy
Mammotome
Scout Image (00)
Stereo Scouts (+150, -150)
Oh Baloney !
Thin metal pieces
Stereo Scouts
Pre-Fire
Post-Fire
Technologist’s QC
• Zero Alignment - before each patient
• Localization Accuracy (XYZ) - each day of use
• Phantom Imaging - each week of use
• Visual Checklist - monthly
• Hardcopy QC - monthly (if applicable)
• Compression - semiannual
• Repeat Analysis - semiannual
Coordinates Calibration (XYZ)
Hardcopy QC (monthly)
• Print SMPTE pattern from console
• Print SMPTE from printer
or
• Radiograph a “step-wedge” and print
Q6- The localization accuracy (in air) and phantom
image quality should be conducted by the
technologist:
20%
20%
20%
20%
20%
1.
2.
3.
4.
5.
Daily, daily
Daily, weekly
Each day of use, weekly
Each day of use, each week of use
Weekly, weekly
10
4. Each day of use, each week of use
American College of Radiology; Stereotactic Breast
Biopsy – Quality Control Manual; 1999, pp. 17&21
SUMMARY
• SBB-ACR accreditation is voluntary, for now…
• Even though exempt from MQSA testing regimen,
dedicated units should be tested routinely by the
Medical Physicist, and may be required by state
agencies.