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Transcript
FRCR: Physics Lectures
Diagnostic Radiology
Lecture 8
Mammography and tomosynthesis
Dr Tim Wood
Clinical Scientist
Overview
• Introduction to mammography
• The mammography X-ray set
• The physics of mammography
– X-ray spectra for mammo
• Film/screen vs digital mammography
• The NHS Breast Screening programme
• Tomosynthesis
An introduction to mammography
• The aims of mammography are:
– Demonstrate microcalcifications – features with high
inherent contrast, but 100 μm or less in diameter
– Demonstrate low contrast masses within the breast
tissue – these may be relatively large, but are often
very low contrast as glandular and adipose tissue
have very similar atomic number (7.4 and 6.5,
respectively)
• Hence, require good spatial and contrast
resolution
• Also used for screening so must be lowest dose
possible (more on this later)
• Presents a different technical challenge
compared with everything discussed so far
An introduction to mammography
The mammography X-ray set
• Highly specialised X-ray
systems required for
mammography
• Older sets based on a
film-screen system
• Modern sets utilise flat
panels (typically direct
conversion)
• Sectra (now Philips)
MicroDose uses a
scanning fan beam
geometry (see later)
The mammo unit
• Cathode-anode axis is
perpendicular to chest wall
(CW)
– Cathode is positioned at the CW
• Fall off in intensity due to
anode-heel effect is at the
nipple edge (NE)
– More tissue at the CW, so
requires more intense X-ray
beam
– NE tends to be thinner due to
shape of anatomy (despite
compression) so requires less
Farr’s Physics for Medical Imaging
2nd Edition
The mammo unit
• Focal spot is located directly
above the CW edge
• Dual focus tubes are used
– ~0.4 mm for general mammo
– ~0.1-0.15 mm for magnification
• Due to small focal spots, tube
current tends to be limited to
~100 mA
– Avoid melting the anode
• kVp tends to be in the range
24-35 kV
– Maximise radiographic contrast
Farr’s Physics for Medical Imaging
2nd Edition
The mammo unit
• Breast is positioned on the
support platform
• Compression is applied to;
– Hold the breast still during
exposure (exposure times can
be quite long due to low mA) –
reduce motion unsharpness
– Reduce the thickness of tissue
so lower exposure factors can
be used (lower dose)
– Spread the anatomy out to
minimise overlaying structures
(improve contrast)
Farr’s Physics for Medical Imaging
2nd Edition
The mammo unit
• Compression is applied to;
– Equalise thickness to ensure
homogenous density on the
radiograph
– Bring the structures closer to the
detector to reduce geometric
unshaprness
Farr’s Physics for Medical Imaging
2nd Edition
The mammo unit
• A moving anti-scatter grid is
incorporated in the support
platform (bucky) to improve
image contrast
• AEC detectors on film-screen
systems are positioned
behind the film cassette (as
opposed to in front, like in
general radiography), as they
will be seen on the resulting
image due to the low kV used
Farr’s Physics for Medical Imaging
2nd Edition
Figure 1. Typical process of x-ray mammography.
Mahesh M Radiographics 2004;24:1747-1760
©2004 by Radiological Society of North America
The mammo unit
• The film/screen AEC chamber
position can be moved to
accommodate different sized
women
– Slides back and forth along the
central axis
• Digital systems use the signal
incident on the detector as the
AEC, and can use quite
complex algorithms to
determine the region of the
image to use
Farr’s Physics for Medical Imaging
2nd Edition
The X-ray tube
• In mammography, to maximise contrast use the
lowest kV possible
• Typical compressed breast thickness is 40-50
mm, and rarely greater than 80 mm
• Ideal mono-energetic X-ray beam would be 1622 keV
• Hence, the use of the full range of the
Bremsstrahlung spectrum from a tungsten target
with aluminium filters is not ideal for mammo
• Use a range of different combinations of target
and filter for different breast thicknesses and/or
detectors
The mammography X-ray spectrum
• Traditional film-screen mammography relies on
the spectrum from a Molybdenum (Mo) target
with a Mo filter for small to average sized breast
(up to about 50-60 mm)
• Alternative filters such as Rhodium (Rh) can be
used for thicker breasts, and some (but not
many) systems use a Rh target as well
• Tungsten targets are also used for thicker
breasts, but in conjunction with the Rh filter (not
Al like general radiography)
• W/Rh is also primary target filter combination in
digital systems (plus some other filters such as
silver and very thin aluminium)
The mammography X-ray spectrum
• So why use these combinations for
mammography?...
• Mo
– K-edge at 20 keV
– Characteristic X-rays at 17.4 and 19.6 keV
• Rh
– K-edge at 23.2 keV
– Characteristic X-rays at 20.2 and 22.8 keV
K-edge (revision)
20 keV
Mo/Mo Spectrum
Intensity
Mo target - unfiltered
2.15E+05
2.10E+05
2.05E+05
2.00E+05
1.95E+05
1.90E+05
1.85E+05
1.80E+05
1.75E+05
1.70E+05
1.65E+05
1.60E+05
1.55E+05
1.50E+05
1.45E+05
1.40E+05
1.35E+05
1.30E+05
1.25E+05
1.20E+05
1.15E+05
1.10E+05
1.05E+05
1.00E+05
9.50E+04
9.00E+04
8.50E+04
8.00E+04
7.50E+04
7.00E+04
6.50E+04
6.00E+04
5.50E+04
5.00E+04
4.50E+04
4.00E+04
3.50E+04
3.00E+04
2.50E+04
2.00E+04
1.50E+04
1.00E+04
5.00E+03
0.00E+00
Mo no filter
0
5
10
15
kV
20
25
30
Intensity
Mo target – Mo filter
2.15E+05
2.10E+05
2.05E+05
2.00E+05
1.95E+05
1.90E+05
1.85E+05
1.80E+05
1.75E+05
1.70E+05
1.65E+05
1.60E+05
1.55E+05
1.50E+05
1.45E+05
1.40E+05
1.35E+05
1.30E+05
1.25E+05
1.20E+05
1.15E+05
1.10E+05
1.05E+05
1.00E+05
9.50E+04
9.00E+04
8.50E+04
8.00E+04
7.50E+04
7.00E+04
6.50E+04
6.00E+04
5.50E+04
5.00E+04
4.50E+04
4.00E+04
3.50E+04
3.00E+04
2.50E+04
2.00E+04
1.50E+04
1.00E+04
5.00E+03
0.00E+00
‘Soft’ X-rays absorbed
by filter (as for general
X-ray set)
Mo no filter
Mo/Mo
K-edge at 20
keV absorbs
most of ‘hard’ Xrays
0
5
10
15
kV
20
25
30
Mo/Rh Spectrum
Intensity
Mo target - unfiltered
2.15E+05
2.10E+05
2.05E+05
2.00E+05
1.95E+05
1.90E+05
1.85E+05
1.80E+05
1.75E+05
1.70E+05
1.65E+05
1.60E+05
1.55E+05
1.50E+05
1.45E+05
1.40E+05
1.35E+05
1.30E+05
1.25E+05
1.20E+05
1.15E+05
1.10E+05
1.05E+05
1.00E+05
9.50E+04
9.00E+04
8.50E+04
8.00E+04
7.50E+04
7.00E+04
6.50E+04
6.00E+04
5.50E+04
5.00E+04
4.50E+04
4.00E+04
3.50E+04
3.00E+04
2.50E+04
2.00E+04
1.50E+04
1.00E+04
5.00E+03
0.00E+00
Mo no filter
0
5
10
15
kV
20
25
30
Intensity
Mo target – Rh filter
2.15E+05
2.10E+05
2.05E+05
2.00E+05
1.95E+05
1.90E+05
1.85E+05
1.80E+05
1.75E+05
1.70E+05
1.65E+05
1.60E+05
1.55E+05
1.50E+05
1.45E+05
1.40E+05
1.35E+05
1.30E+05
1.25E+05
1.20E+05
1.15E+05
1.10E+05
1.05E+05
1.00E+05
9.50E+04
9.00E+04
8.50E+04
8.00E+04
7.50E+04
7.00E+04
6.50E+04
6.00E+04
5.50E+04
5.00E+04
4.50E+04
4.00E+04
3.50E+04
3.00E+04
2.50E+04
2.00E+04
1.50E+04
1.00E+04
5.00E+03
0.00E+00
Mo no filter
Mo/Rh
K-edge at 23.2 keV
absorbs less of the
‘hard X-rays’ than Mo
filter – hence, higher
mean energy
0
5
10
15
kV
20
25
30
Intensity
Mo target – both filters
2.15E+05
2.10E+05
2.05E+05
2.00E+05
1.95E+05
1.90E+05
1.85E+05
1.80E+05
1.75E+05
1.70E+05
1.65E+05
1.60E+05
1.55E+05
1.50E+05
1.45E+05
1.40E+05
1.35E+05
1.30E+05
1.25E+05
1.20E+05
1.15E+05
1.10E+05
1.05E+05
1.00E+05
9.50E+04
9.00E+04
8.50E+04
8.00E+04
7.50E+04
7.00E+04
6.50E+04
6.00E+04
5.50E+04
5.00E+04
4.50E+04
4.00E+04
3.50E+04
3.00E+04
2.50E+04
2.00E+04
1.50E+04
1.00E+04
5.00E+03
0.00E+00
Mo no filter
Mo/Mo
Mo/Rh
0
5
10
15
kV
20
25
30
Mammography X-ray spectra
• Traditionally, mammography relies on the
characteristic X-rays and K-edges of target/filter
materials to produce a low energy spectrum
around the ideal 16-22 keV
• Thicker breasts, however, require more
penetrating beams, so W or Rh target may be
used
• Result in higher mean energy of X-ray beam
• For thinner to thicker breasts, systems tend to
use:
– Mo/Mo then Mo/Rh then Rh/Rh or W/Rh
Rh/Rh Spectrum
Rh target – unfiltered
Intensity
8.50E+04
8.00E+04
7.50E+04
Rh no filter
7.00E+04
6.50E+04
6.00E+04
5.50E+04
5.00E+04
4.50E+04
4.00E+04
3.50E+04
3.00E+04
2.50E+04
2.00E+04
1.50E+04
1.00E+04
5.00E+03
0.00E+00
0
5
10
15
kV
20
25
30
Rh target – Rh filter
5.00E+04
4.50E+04
Rh/Rh
4.00E+04
Intensity
3.50E+04
3.00E+04
2.50E+04
2.00E+04
1.50E+04
1.00E+04
5.00E+03
0.00E+00
0
5
10
15
kV
20
25
30
Rh target – Rh filter
8.50E+04
8.00E+04
Rh no filter
Rh/Rh
7.50E+04
7.00E+04
Intensity
6.50E+04
6.00E+04
5.50E+04
5.00E+04
4.50E+04
4.00E+04
3.50E+04
3.00E+04
2.50E+04
2.00E+04
1.50E+04
1.00E+04
5.00E+03
0.00E+00
0
5
10
15
kV
20
25
30
Intensity
W target
1.08E+06
1.07E+06
1.06E+06
1.05E+06
1.04E+06
1.03E+06
1.02E+06
1.01E+06
1.00E+06
9.95E+05
9.90E+05
9.85E+05
9.80E+05
9.75E+05
9.70E+05
9.65E+05
9.60E+05
9.55E+05
9.50E+05
9.45E+05
9.40E+05
9.35E+05
9.30E+05
9.25E+05
9.20E+05
9.15E+05
9.10E+05
9.05E+05
9.00E+05
8.95E+05
8.90E+05
8.85E+05
8.80E+05
8.75E+05
8.70E+05
8.65E+05
8.60E+05
8.55E+05
8.50E+05
8.45E+05
8.40E+05
8.35E+05
8.30E+05
8.25E+05
8.20E+05
8.15E+05
8.10E+05
8.05E+05
8.00E+05
7.95E+05
7.90E+05
7.85E+05
7.80E+05
7.75E+05
7.70E+05
7.65E+05
7.60E+05
7.55E+05
7.50E+05
7.45E+05
7.40E+05
7.35E+05
7.30E+05
7.25E+05
7.20E+05
7.15E+05
7.10E+05
7.05E+05
7.00E+05
6.95E+05
6.90E+05
6.85E+05
6.80E+05
6.75E+05
6.70E+05
6.65E+05
6.60E+05
6.55E+05
6.50E+05
6.45E+05
6.40E+05
6.35E+05
6.30E+05
6.25E+05
6.20E+05
6.15E+05
6.10E+05
6.05E+05
6.00E+05
5.95E+05
5.90E+05
5.85E+05
5.80E+05
5.75E+05
5.70E+05
5.65E+05
5.60E+05
5.55E+05
5.50E+05
5.45E+05
5.40E+05
5.35E+05
5.30E+05
5.25E+05
5.20E+05
5.15E+05
5.10E+05
5.05E+05
5.00E+05
4.95E+05
4.90E+05
4.85E+05
4.80E+05
4.75E+05
4.70E+05
4.65E+05
4.60E+05
4.55E+05
4.50E+05
4.45E+05
4.40E+05
4.35E+05
4.30E+05
4.25E+05
4.20E+05
4.15E+05
4.10E+05
4.05E+05
4.00E+05
3.95E+05
3.90E+05
3.85E+05
3.80E+05
3.75E+05
3.70E+05
3.65E+05
3.60E+05
3.55E+05
3.50E+05
3.45E+05
3.40E+05
3.35E+05
3.30E+05
3.25E+05
3.20E+05
3.15E+05
3.10E+05
3.05E+05
3.00E+05
2.95E+05
2.90E+05
2.85E+05
2.80E+05
2.75E+05
2.70E+05
2.65E+05
2.60E+05
2.55E+05
2.50E+05
2.45E+05
2.40E+05
2.35E+05
2.30E+05
2.25E+05
2.20E+05
2.15E+05
2.10E+05
2.05E+05
2.00E+05
1.95E+05
1.90E+05
1.85E+05
1.80E+05
1.75E+05
1.70E+05
1.65E+05
1.60E+05
1.55E+05
1.50E+05
1.45E+05
1.40E+05
1.35E+05
1.30E+05
1.25E+05
1.20E+05
1.15E+05
1.10E+05
1.05E+05
1.00E+05
9.50E+04
9.00E+04
8.50E+04
8.00E+04
7.50E+04
7.00E+04
6.50E+04
6.00E+04
5.50E+04
5.00E+04
4.50E+04
4.00E+04
3.50E+04
3.00E+04
2.50E+04
2.00E+04
1.50E+04
1.00E+04
5.00E+03
0.00E+00
W no filter
0
5
10
15
kV
20
25
30
W target – Rh filter
1.50E+04
Intensity
W/Rh
1.00E+04
5.00E+03
0.00E+00
0
5
10
15
kV
20
25
30
Intensity
W target – Rh filter
1.08E+06
1.07E+06
1.06E+06
1.05E+06
1.04E+06
1.03E+06
1.02E+06
1.01E+06
1.00E+06
9.95E+05
9.90E+05
9.85E+05
9.80E+05
9.75E+05
9.70E+05
9.65E+05
9.60E+05
9.55E+05
9.50E+05
9.45E+05
9.40E+05
9.35E+05
9.30E+05
9.25E+05
9.20E+05
9.15E+05
9.10E+05
9.05E+05
9.00E+05
8.95E+05
8.90E+05
8.85E+05
8.80E+05
8.75E+05
8.70E+05
8.65E+05
8.60E+05
8.55E+05
8.50E+05
8.45E+05
8.40E+05
8.35E+05
8.30E+05
8.25E+05
8.20E+05
8.15E+05
8.10E+05
8.05E+05
8.00E+05
7.95E+05
7.90E+05
7.85E+05
7.80E+05
7.75E+05
7.70E+05
7.65E+05
7.60E+05
7.55E+05
7.50E+05
7.45E+05
7.40E+05
7.35E+05
7.30E+05
7.25E+05
7.20E+05
7.15E+05
7.10E+05
7.05E+05
7.00E+05
6.95E+05
6.90E+05
6.85E+05
6.80E+05
6.75E+05
6.70E+05
6.65E+05
6.60E+05
6.55E+05
6.50E+05
6.45E+05
6.40E+05
6.35E+05
6.30E+05
6.25E+05
6.20E+05
6.15E+05
6.10E+05
6.05E+05
6.00E+05
5.95E+05
5.90E+05
5.85E+05
5.80E+05
5.75E+05
5.70E+05
5.65E+05
5.60E+05
5.55E+05
5.50E+05
5.45E+05
5.40E+05
5.35E+05
5.30E+05
5.25E+05
5.20E+05
5.15E+05
5.10E+05
5.05E+05
5.00E+05
4.95E+05
4.90E+05
4.85E+05
4.80E+05
4.75E+05
4.70E+05
4.65E+05
4.60E+05
4.55E+05
4.50E+05
4.45E+05
4.40E+05
4.35E+05
4.30E+05
4.25E+05
4.20E+05
4.15E+05
4.10E+05
4.05E+05
4.00E+05
3.95E+05
3.90E+05
3.85E+05
3.80E+05
3.75E+05
3.70E+05
3.65E+05
3.60E+05
3.55E+05
3.50E+05
3.45E+05
3.40E+05
3.35E+05
3.30E+05
3.25E+05
3.20E+05
3.15E+05
3.10E+05
3.05E+05
3.00E+05
2.95E+05
2.90E+05
2.85E+05
2.80E+05
2.75E+05
2.70E+05
2.65E+05
2.60E+05
2.55E+05
2.50E+05
2.45E+05
2.40E+05
2.35E+05
2.30E+05
2.25E+05
2.20E+05
2.15E+05
2.10E+05
2.05E+05
2.00E+05
1.95E+05
1.90E+05
1.85E+05
1.80E+05
1.75E+05
1.70E+05
1.65E+05
1.60E+05
1.55E+05
1.50E+05
1.45E+05
1.40E+05
1.35E+05
1.30E+05
1.25E+05
1.20E+05
1.15E+05
1.10E+05
1.05E+05
1.00E+05
9.50E+04
9.00E+04
8.50E+04
8.00E+04
7.50E+04
7.00E+04
6.50E+04
6.00E+04
5.50E+04
5.00E+04
4.50E+04
4.00E+04
3.50E+04
3.00E+04
2.50E+04
2.00E+04
1.50E+04
1.00E+04
5.00E+03
0.00E+00
W no filter
W/Rh
0
5
10
15
kV
20
25
30
Mammography X-ray spectra
• Digital mammography tends to use W/Rh
combination as digital gives much better contrast
than the fixed dynamic range of film
– Hence, the poorer contrast due to the harder X-ray
beam is less significant than the gain due to the use
of digital (window and level, image processing, etc)
– Allows lower doses to be used for the same image
quality
– In practice, contrast resolution in digital is much
better than film
Film-screen
• Film-screen
mammography
requires high spatial
resolution
– As using lower kV,
thinner screens may
be used to improve
resolution
– Single screen and
emulsion layer to
minimise crossover
and parallax
Film-screen
• The screen is positioned on the distal side of the
film
– Hence, highest proportion of photon interactions will
be on the side of the screen in contact with the film
– This minimises the spread of light from the screen to
the film
• Film-screen mammo system is capable of ~15 lp
mm-1 or better
– Compared with ~7 lp mm-1 for general radiography
General radiography screenunsharpness
Object
Phosphor
Film
Mammography screenunsharpness
Object
Film
Phosphor
Film-screen
• Film-screen system is
still limited by the
fixed dynamic range
of film
• High contrast
(gamma) films are
used
• Critical to ensure
exposure factors are
matched to the film
– AEC setup and QA
very important
Figure 3. Limitations of SFM in imaging a breast composed of a wide range of tissues.
Mahesh M Radiographics 2004;24:1747-1760
©2004 by Radiological Society of North America
The automatic exposure control
• AECs are implemented in
different ways by different
manufacturers
– Manual selection of kV and
T/F based on exposure chart
– Automatic select everything
based on breast thickness
and/or pre-pulse of X-rays
Film-screen versus digital
• The advantaged and limitations of the filmscreen system:
Figure 2. Typical response curves for SFM and digital mammography.
Mahesh M Radiographics 2004;24:1747-1760
©2004 by Radiological Society of North America
Film-screen versus digital
• Digital mammography tends to use direct
conversion flat panel detectors
– Better spatial resolution than indirect detectors, but
still limited to ~7-10 lp mm-1
– Hence, limiting spatial resolution is poorer c.f. film
• BUT, image processing and window/level
controls result in better contrast resolution
• Lower dose as more efficient (better DQE) and
higher beam qualities used
• Higher throughput as instant image on screen
– Particularly valuable on ‘vans’
• Plus all the other advantages of digital (and
disadvantages, such as cost!)
Digital Mammography
• Directly acquire the data
in digital format
• Main technologies:
HKD2
(Room 2)
HKD1
(Centenary)
– Phosphor coupled to a
read out device – Indirect
conversion flat panels
– a-Se/TFT array – Direct
conversion flat panels
– Photon counting scanning
detector
HSU (Health Central), HKD3 (Room 3),
HKD4 (Cromwell Road), HMU1-3,
HGA1 & 2
(Grimsby)
Direct conversion flat panels
• Amorphous Selenium (a-Se)
is a photoconductor
– Converts X-rays directly to
electrons
• Deposited directly onto
amorphous silicon TFT array
• No phosphor, hence no light
spread
• Resolution governed by
effective pixel pitch
• Every mammography set
manufacturer (except GE)
use this technology
MicroDose ‘photon counting’
• Photon counting detector = no
electronic noise or analogue-to-digital
conversion
• Uses a scanning fan beam rather than
a full field digital flat panel
• Hence, much less scatter is
generated in the breast
– Do not need a moving anti-scatter grid,
– Improves image quality, and/or
– Reduces patient dose
• Doses tend to be lower for this type of
system than flat panel sets (~50%!)
• BUT, the tubes are worked much
harder
The MicroDose system
The output
from
‘Collimator 1’
Photon counting
• Detector counts photons
as they hit the detector
• Large number of
crystalline Si strips in
edge-on geometry
(ensure long enough to
absorb X-rays)
• X-ray interactions excite
electron-hole pairs
(several thousand)
• Bias voltage applied to
induce current
• Fast read-out electronics
to count the pulses (2 ms)
Photon counting
Photon counting
• Flat panels = integrating systems
– Sum the electrical charge from all X-ray photon interactions
– High energy photon = more electrical signal = greater weighting
in the image = poorer contrast
• Photon counting
–
–
–
–
–
High energy = larger pulse
Low energy = smaller pulse
Can separate ‘high’ and ‘low’ energy photons
Can apply equal weighting to improve contrast
Opens up the possibility of dual energy mammography (next
version of MicroDose has this functionality, with glandularity
calculation as standard)
Patient dose
• Patient dose optimisation particularly important
in mammography (see later)
• Systems designed to only expose the breast
• Hence, effective dose is not a particularly useful
quantity
• The mean glandular dose (in mGy) is used
instead
– Typical values fall in the range 1.5-3 mGy per film
– In the UK, the dose to the standard breast (4.5 cm
Perspex, equivalent to 5.3 cm breast) must be less
than 2.5 mGy
– Doses in digital will be much lower than this
– 2 mGy ~ 1 in 50,000 risk of fatal cancer (age 50-65)
Magnification
• Sometimes referred to as
macroradiography
• Mammo only modality that
routinely uses magnification
views
• Use a raised platform to
support the breast about
half way between focus and
film i.e. not in close(ish)
contact anymore
• Magnification is the result of
the diverging X-ray beams
travelling in straight lines
Magnification imaging
• Magnifies image by a factor of
between 1.5-1.8
• Magnification increases
geometric unsharpness so
small focal spot is used
Focal spot
– Increases exposure times due to
lower mA
Object
• Can remove grid to lower
exposure factors (and patient
dose) as scatter is reduced by
the air gap
• Overall patient dose is higher
– Magnification requires
individual justification
Film/
detector
Penumbra
The NHS Breast Screening Programme
• Based on the Forrest Report (1986), the first
breast screening programme in the world was
setup in England in 1988
• All women in the age range 50-70 invited every
3 years for mammogram
– Two views of each breast
– Age range recently extend to between late 40s-73
• However, the fundamental principle of screening
programme means that healthy women are
exposed to radiation
– What about justification?
Justification of the NHSBSP
• The vast majority of women X-rayed in the
NHSBSP show no signs of cancer, and have no
symptoms prior to attending
– Women who display symptoms should go to their GP
and be referred to the symptomatic unit at their local
hospital (outside the scope of the NHSBSP, but in Hull
these are the same)
• On an individual basis, justification is difficult
• Justification for the NHSBSP is based on the net
benefit to the population, not the individual
• However, this makes patient dose optmisation
and quality assurance (QA) critical to
effectiveness of the programme
– QA also important due to the basic principles of
mammography e.g. low kV, high spatial resolution, etc
QA and the NHSBSP
• QA is a fundamental principle of the Breast Screening
Programme
• You are X-raying ‘healthy’ women, with no symptoms, so
the JUSTIFICATION for the exposure (under IR(ME)R)
is based on the net benefit to the population (rather than
each individual), versus the risk of irradiating a large
number of healthy women
– This is a very current issue, given the recent publications about
risk-benefit and overdiagnosis in the NHSBSP…
• OPTIMISATION (under IR(ME)R) to ensure doses are
ALARP is therefore incredibly important, so must make
sure the X-ray equipment is fit for use!
The NHSBSP and QA
• The national NHSBSP is split into localities (e.g.
Humberside BSP) that then belong to regions (e.g.
North East, Yorkshire and Humberside)
• Quality Assurance Reference Centres (QARCs) are
regional bodies that ensure the local BSP is
fulfilling their obligations under national guidance
for the NHSBSP
• Inspect whole of local BSP (including surgery, etc)
• For mammography, there are national guidance
documents outlining the QA required, split into
daily, weekly and monthly Radiographer tests and
6 monthly Physics QA
Physics QA
• Test/measure (6 monthly):
–
–
–
–
–
–
–
–
–
–
–
kV accuracy
X-ray tube output for all T/F and range of kV/mAs
Half-value layers
AEC performance
Mean glandular dose to the standard breast
Image quality with phantoms
Beam alignments
Uniformity
Focal spot sizes
Compression force
Etc…
Radiographer QA…
• Daily
– AEC consistency (B1)
– Visual check of acquisition and reporting monitors (B2)
– Inspection of breast support table and associated equipment (B3)
• Weekly
– Contrast-to-noise ratio (CNR) (B5)
– Image quality (B6)
– Artefacts and uniformity (B7)
• Monthly
–
–
–
–
Image quality (B6)
AEC consistency with varying thickness including CNR (B10)
Mechanical safety and beam function (B11)
Compression force (B12)
• Other (weekly/before use)
– Stereo-tactic localising device (B8)
The QARC website
Tomosynthesis
• Digital tomosynthesis can be considered a halfway house between tomography and CT
• There are applications (and systems available)
in general radiography, but most common
implementation is in digital mammography
• The basic principle is that a number of
projections are acquired over a narrow range of
angles, and the images are processed to give a
series of ‘planes’ through the breast (strictly
speaking they are not slices like in CT)
Tomosynthesis equipment
Tomosynthesis
• Wider angle of projections = better 3D resolution
• More projections = slower to acquire
• Data processing is manufacturer specific;
– Back projection/shift and add for Hologic (see Ben
Johnson slides from UKMPG 2011)
– Filtered back projection (like CT) for Siemens
– Iterative reconstruction for IMS (like new CTs)
Ben Johnson, Barts and the London NHS Trust, UKMPG 2011
Ben Johnson, Barts and the London NHS Trust, UKMPG 2011
Ben Johnson, Barts and the London NHS Trust, UKMPG 2011
Ben Johnson, Barts and the London NHS Trust, UKMPG 2011
Ben Johnson, Barts and the London NHS Trust, UKMPG 2011
Ben Johnson, Barts and the London NHS Trust, UKMPG 2011
Tomosynthesis
• Like tomography, the
basic idea is that in
plane structures are
in focus, out of plane
structures blurred
• Unlike tomography,
reconstruct a number
of different projections
to get a series of
images through the
volume (like CT)