Download Lecture 10 Mammography Thur - International Atomic Energy Agency

RTC on RADIATION PROTECTION OF PATIENTS FOR
RADIOGRAPHERS
Accra, Ghana, July 2011
Optimization of Protection in Mammography
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International Atomic Energy Agency
Topics
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Important physical parameters
The mammographic X-ray tube
Focal spot size
High voltage generator
Anti-scatter grid
Automatic Exposure Control
Dosimetry
Quality control
Digital mammography
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Mammograms are the most technically
demanding of all x-ray examinations
• Low contrast objects
• Normal tissues and cancers have similar density
• Means we need low kVp, which in turn means high
dose and need for compression
• Very small high contrast objects
• Means we need high spatial resolution
• Breast has high radiosensitivity
• Screening for cancer can start at age 45-50 – cumulative dose can be
high
• Even for diagnostic mammograms, dose must be considered
• Result is that QC is of great importance – best
image at lowest dose
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Film Characteristics – Tied to H&D Curve
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What do we want to see ?
Mass with
spiculated
margins
Architectural
distortion
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Clusters of
micro/macro
calcifications
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• IF WE CAN’T SEE THESE OBJECTS, WE
ARE WASTING PATIENT DOSE, AND
PUTTING THE PATIENT AT RISK FROM
UNDIAGNOSED DISEASE!
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…………and more than most x-ray imaging,
mammography demands attention to quality
control to optimise dose, and to produce
diagnostic images
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X-ray Equipment
• Dedicated x-ray
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equipment
Compression
paddle
Grid
Image receptor
(screen /film,
CR, DR)
AEC
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Main variables of the mammographic
imaging system
• Contrast: capability of the system to make
visible small differences in soft tissue density
• Sharpness: capability of the system to make
visible small details (calcifications down to 0.1 mm)
• Dose: the female breast is a very
radiosensitive organ and there is a risk of
carcinogenesis associated with the technique
• Noise: determines how far the dose can be
reduced given the task of identifying a
particular object against the background
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Contrast
• Linear attenuation coefficients for different
types of breast tissue are similar in
magnitude and the soft tissue contrast can
be quite small
• The contrast must be made as high as
possible by imaging with a low photon
energy (hence increasing breast dose)
• In practice, to avoid a high breast dose, a
compromise must be made between the
requirements of low dose and high contrast
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Variation of contrast with photon
energy
Contrast
1.0
Ca5 (PO4)3 OH
Calcification
of 0.1mm
0.1
0.01
•The glandular tissue
contrast falls below 0.1
for energies above 27 keV
Glandular tissue
of 1mm
0.001
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•The contrast decreases
by a factor of 6 between
15 and 30 keV
20
30
40
50 Energy (keV)
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Breast dosimetry in screen-film
mammography
• There exists a small risk of radiation induced
cancer associated with X-ray examination of
the breast
• Achieving an image quality at the lowest
possible dose is therefore required
• Hence breast dosimetry is crucial
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Radiation dose to the breast
• Dose decreases rapidly with depth in tissue
due to the low energy X-ray spectrum used
• Relevant quantity: The average glandular
dose (AGD) related to the tissues which are
believed to be the most sensitive to
radiation-induced carcinogenesis
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Radiation dose to the breast
• The breast dose is affected by:
 the breast composition and thickness
 the photon energy
 the sensitivity of the image receptor
• The breast composition has a significant
influence on the dose
• The area of the compressed breast has a small
influence on the dose
 the mean path of the photons < breast dimensions
 majority of the interactions are photoelectric
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Mean Glandular Dose (arb. Units)
Variation of AGD with photon energy
•The figure demonstrates
the rapid increase in dose
with decreasing photon energy
and increasing breast thickness
20
10
8 cm
2
1
•For the 8 cm thick breast there
is a dose increase of a factor of 30
between photon energies of 17.5
and 30 keV
2 cm
•At 20 keV there is a dose increase
of a factor of 17 between
thicknesses of 2 an 8 cm
0.2
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20
30
40
(keV)
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Focal spot size
• For the screening unit a single-focus X-ray
tube with a 0.3 focal spot is recommended
• For general mammography purposes, a dual
focus X-ray tube with an additional fine focus
(0.1) to be used for magnification techniques
exclusively is required
• The overall system resolution should be
measured yearly or when resolution decays
rapidly
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X-ray tube filtration
• The beam quality is defined by the HVL
• HVL with compression paddle in place at
28kV Mo/Mo to be over 0.30 mm Al
equivalent and typically < 0.4 mm Al (see
for example European Guidelines for QA
in mammography screening for more
exact numbers)
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Mammography x-ray generators
• A nearly constant potential waveform – usually
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medium or high frequency generaors
The tube voltage range should be 25 - 35 kV
The tube current should be at least 100 mA on
broad focus and 50 mA on fine focus.
The range of tube current exposure time
product (mAs) should be at least 5 - 800 mAs
It should be possible to repeat exposures at the
highest loadings at intervals < 30 seconds
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Why an anti-scatter grid ?
• Effects of scatter may significantly degrade
the contrast of the image and the need for
an efficient anti-scatter device is evident
• Mammography should ONLY be performed
with a (moving) grid
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Automatic exposure control (AEC)
• The system should produce a stable optical
density (OD variation of less than  0.2 ) in spite of
a wide range of mAs
• Hence the system should be fitted with an AEC
located after the film receptor to allow for quite
different breast characteristics
• The detector should be movable to cover different
anatomical sites on the breast and operate
effectively over the full possible range of kVp and
breast thicknesses
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Measurement of breast dose
• The AGD cannot be measured directly but it
is derived from measurements with the
standard phantom for the actual technique setup of the mammographic equipment
• The Entrance Surface Air Kerma (ESAK) freein-air (i.e. without backscatter) is the most
frequent used starting quantity for patient
dosimetry in mammography
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ACR phantom MGD – NSW screening
Mean 1.24 mGy
3rd quartile 1.28 mGy
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Mammography Quality Control
• BSS requires Quality Assurance for medical
exposures
• Guidelines prepared by EC, PAHO, ACR etc
• A Quality Control program should ensure:
• The best image quality
• With the least dose to the breast
• Hence regular check of important parameters
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Parameters to be considered by a QC
program (1)
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Focal Spot size (star pattern, slit camera)
OR System resolution (line pair gauge)
Tube voltage (reproducibility, accuracy, HVL)
AEC system (kVp and object thickness
compensation, OD control, short term
reproducibility)
 Compression paddle (compression force,
alignment to patient chest wall)
 Screen-Film (inter-cassette sensitivity,
screen/film contact)
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Parameters to be considered by a QC
program (2)

Film Processing
 Baseline (temperature, processing time, film
OD)
 Film and processor (screen cleanliness,
sensitometry)
 Darkroom (safelights, light leakage, film
storage)
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Film Viewing
 Viewing Box (brightness, homogeneity)
 Environment (room illumination)
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Parameters to be considered by a QC
program (3)
System Properties
 Reference Dose (average
glandular dose)
 Image Quality (image contrast,
threshold contrast visibility,
exposure time)
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Compression
Typical force used = 100 to 200 Newtons = 10-20 daN (~10-20 kg)
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• NO (OR INSUFFICIENT) COMPRESSION
MEANS:
• Too much tissue to be penetrated by the beam
ie. too much attenuation
• Higher dose
• Less visibility of soft tissue objects eg. Masses
• More overlay of objects in the beam
• Need typically >10kgf (daN) compression
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Towels to protect
compression plate
and breast support
Bathroom scales
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Distribution of actual compression force
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Image Quality
• Uses special phantom (eg. RMI 156)
• Tests visibility of “rods” (fibrosis), “specks”
(calcifications), and “masses” (tumours)
• Image taken with AEC
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Image Quality Phantom (ACR) - X-Ray
Should be able to see:
•4 thickest fibres
•3 largest specks
•3 largest masses
Chest Wall Edge
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Phantom Image Quality Evaluation
• Measure the optical
density at the
centre of the
phantom and in
and outside the disc
to calculate
difference in OD.
• Circle any artifacts
or grid lines on the
film. Investigate
the source of
artifacts.
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Phantom Image Viewing Conditions
•Phantom images should be read under
optimal viewing conditions
• Images should be masked to eliminate
extraneous light around the film
• Use a large field of view magnifying glass to
assist in the visualization of specks.
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Routine measurements
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All repeated measurements related
to the image receptor should be
performed with the same film
cassette (or CR plate) to rule out
AEC variations and differences
between screens and cassettes, or
CR plates
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Mark the cassette as “Test” for
example
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Routine measurements - OD
 The optical density (OD) of the
processed image taken under AEC with
a uniform PMMA phantom 45 mm thick,
is measured at a reference point, which
lies 40-60 mm from the chest wall side
and laterally centered.
 The reference optical density is ~1.6-1.8
including (base + fog) density (~0.2)
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Viewbox requirements
• Mammo films have much higher optical
density than conventional films
• CANNOT effectively use a conventional film
viewbox for mammo films
• You will not see subtle density differences
• You may miss seeing a tumour
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Measurement of Viewbox Brightness
or Luminance (cd.m-2)
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Measurement of Room Brightness
or Illuminance ( lux)
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Viewbox requirements
• Viewbox brightness (luminance) measured
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in Cd/m2
A conventional viewbox might be 800 – 1500
Cd/m2
A mammo viewbox should be >3000 Cd/m2
….and must be more uniform in brightness
….and (for the doctor) must have a mask to
exclude light at the edge of the film
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Other measures
• Correct patient positioning
• Retake analysis
• Record QC data
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Digital mammography
• Film/screen mammography rapidly being
replaced by computed radiography (CR) or
direct radiography (DR)
• These have special features compared to
film, and require some changes in QA
(requires a separate talk!!)
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Mammography Technologies
Screen/film
Analogue
CR Plates
Computed
radiography
X-Ray
Image
Capture
CCD
A-Si
(amorphous
silicon)
Indirect image
detection
(phosphor)
Digital
Integration
Digital
detector
systems
Direct image
detection
(no phosphor)
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Direct
photon
counting
Adapted from KCARE UK
A-Se
(amorphous
selenium)
Si wafer
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Schematic: CR Plate Readout
Rotating
Polygon 
Mirror

Stimulating Laser
Photomultiplier Tube
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ADC

Digitized
signal
Fibre Optical Coupling
CR Plate Moved Translationally
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Dual-sided readout also possible (Fuji)
CR plate erasure
• Required even after image readout due to
scattered and background radiation storage
(so-called ghost images)
• Secondary erasure of all plates daily
• Primary erasure of all plates weekly
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a-Si Indirect Detectors
CsI-scintillator with a-Si switching diodes or TFT-read out
100 m pixel
size giving
~5 lp/mm
X-rays
scintillator
Pixel matrix
Used by
GE and
others
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line driver
contacts
photo diode
switch
amplifier, multiplexer &
ADC
Direct Detection : a-Se
Used by Fuji and others
Top
electrode
Charge
collection
electrode
X-rays
IN
a-Se layer
Signal Out
TFT
Gate
pulse
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Charge
storage
capacitor
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Use “Roam and Zoom” - 50μ del example
18 cm 3600 pixels
Each image:
Typically 32 MB
of information !
24 cm
Display:
4800 pixels
2400x1700 pixels
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Display resolution = 12 lp/mm
Monitor and Image QC
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Monitor QC
Monitor cleanliness
Viewing conditions
Printer area cleanliness
Printer QC
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Monitor QC
• Monitors now all flat panel
• 1.3 (normal PC monitor), 3, and 5 (dedicated)
megapixel (MP)
• Use software test pattern (TG18),
appropriate resolution (eg. 2k version)
• Weekly test, evaluation preferably by same
person
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TG18-QC
Phantom
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Check for jitter
Check resolution
elements are resolved
Resolution elements
Confirm 20
discrete shades of
grey all present
95%
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Check number of visible
letters in “Quality Control”
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Monitor cleanliness
• Cleanliness of the monitor can have an
effect on the quality of the displayed
images
• Recommend WEEKLY cleaning of monitors
to ensure they are free of dust, fingerprints
and other marks that might interfere with
image interpretation
• The manufacturer’s specific instructions
should be adhered to when choosing
cleaning agents
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What the radiographer can do – digital
mammography
• CR plate erasure
• To remove “stored” image remnant
• Daily, before imaging – secondary erase (what
is done at each read cycle)
• Weekly (first working day) – primary erase
(deeper level of erasure)
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• Viewing conditions (CR and DR)
• Daily - Check monitor screens for
dirt/dust/fingerprints and clean
• Make sure there are no reflections on the
monitor visible to the reader
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• Printer check (CR and DR)
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At least weekly – print test pattern
Check that all features visible
No artifacts such as lines
If wet laser printer, also check densities as per
film if possible
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• Full-field artifact (CR and DR)
• Daily or before patient examination if infrequent
• Need uniform absorber (4 cm perspex/PMMA),
so may be difficult
• Simple AEC exposure, look for “dead” pixels,
streaks, other artifacts
• Use narrow window for viewing
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• Detector calibration (flat field test) – DR only
• As recommended by manufacturer, and using
their test materials
• Recalibrates detectors automatically
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• Phantom image quality (CR and DR)
• Weekly
• Use ACR or other phantom, expose with AEC
• Score as recommended for that phantom
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• CR plate cleaning – monthly or as
recommended (particularly in dusty areas)
• Follow manufacturer’s method
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• CR plate uniformity
• Expose perspex eg 4cm, under AEC
• Record each cassette’s exposure index (S#,
lgM, EI etc)
• Should be within manufacturer’s limits (these
differ from manufacturer to manufacturer)
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QC of the radiographer!
• Check images for correct positioning,
visibility of chest wall, nipple correctly
placed, no skin folds etc.
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Summary
• To achieve the best image quality while
keeping the breast dose as low as
reasonably achievable is the final goal to be
reached when consistently using
mammography equipment.
• Implementing a well defined QC protocol
can effectively contribute to the achievement
of such a goal.
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How good is
your mammo.
QC program?
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