Transcript digital image

Advanced Biomedical
Imaging
Lecture 5
Advanced X ray machines
&
Mammogram
Dr. Azza Helal
A. Prof. of Medical Physics
Faculty of Medicine
Alexandria University
Advanced X ray machines
Computed Radiography (CR)
is revolutionizing medicine in the same way that digital
cameras changed photography.

CR uses instead of ordinary cassette, an
imaging plate coated with storage phosphor to
capture x-rays as they pass through the patient.

When irradiated, the enhanced phosphor
absorb and store x-ray energy.

This trapped energy comprises a latent image

CR cassette is then placed in a digitizer where
the phosphor plate is scanned causing release of
trapped energy into visible light.
• This light is captured and converted
into an electrical signal, which is
converted into digital image
• Digital image can be displayed on
laser-printed films
workstations.
or
softcopy
Advantages of computed Radiography (CR)

Lower radiation dose

Higher contrast.

Better image quality.

Rapid result

No need to repeat examination.

Post-processing capabilities…

Enables workstation display & archiving.
Digital Radiography (DR)
It is performed by a system consisting of:
•A digital image receptor
•A digital image processing unit
•An image management system
•Image and data storage devices
•A communications network
•A display device with viewer operated controls
Digital cassette
Mammogram imaging modalities

Include mammography, US & MRI.

Mammography is used to detect breast pathology
& cancer.

US is used to differentiate solid from cystic
lesions which have similar appearance on
mammography.

MRI is used for evaluation of silicon implants and
assessment of stage of breast cancer.
Mammography

Approximately 1 woman in 8 will develop breast cancer
over a lifetime.

Breast cancer screening programs rely on mammography
because it is a low-cost, low-radiation-dose procedure
with sensitivity to detect early-stage breast cancer.

It demonstrates both micro calcifications (high contrast)
& much low contrast areas of tissues on same film.

Breast is composed of fatty & glandular tissues.

There is a small x-ray attenuation differences
between them which decrease with high energy.

The best differentiation between tissues obtained
at low x-ray energy but this increases patient
dose and exposure time.

So breast imaging needs a special designed xray tube.
System design
 Because of the risks of ionizing radiation, techniques
that minimize dose and optimize image quality are
essential, and have led to:
– Specialized x-ray equipment
– Specialized x-ray tubes
– Compression devices
– Antiscatter grids
– Phototimers
Photo timer: Radiation sensor
used to control the exposure.
It sets the optimal kV from a
short test exposure.
Target
Breast is composed of fatty & glandular tissues.
There is a small x-ray attenuation differences
between them which decrease with high energy.
So the best differentiation between tissues
obtained at low x-ray energy
Target
Photoelectric
Conventional x ray tube
Tungsten (Z=74)
EK=70Kev
EL=12Kev
Ch. Rad.= 58kev
Mammography
Molybdenum (Z=42)
EK=20Kev
EL=2.5Kev
Ch. Rad.= 17.5kev
Focal spot

Small focal spot is used for best contrast (0.10.3mm)

The problem is heat dissipation ( tube cooling
problem).
Filter

Inherent filtration must be kept low; beryllium
(Z = 4) is used for the tube port as it has low z
so low U & less filtration.

Added tube filters of the same element as the
target reduce the low- and high-energy x-rays in
the
spectrum
and
allow
transmission
characteristic x-ray energies. (Mo/Mo).
of
Uniformity of x ray beam:
 Heal effect: x ray travels toward anode edge have
more target to cross and attenuated more than those
travel toward cathode edge so intensity is different.
 Anode heel effect: thickest part of breast at cathode
side end where beam is more intense.
• This decreases the equipment bulk near patient’s head
for easier positioning
Anode heel effect
Target
Molybdenum (Mo)
Filter
Mo ch, rad
Molybdenum
17.5-19.5
For large dense breast, implant
tungsten target , Rhodium filter
Focal spot, Film λ
0.1-0.3mm, tube cooling prob, 3
Beryllium window, Not glass
min. filtration (z=4) due to low U
Air gap
used
Grids, GR
Moving
SID
65cm
Tube voltage
25-35kv
Screen
single screen
Main source of contrast
Effective dose
photoelectric effect
0.5-1msv
Factors affecting doses in mammography:
Beam energy:

↑ Kv requires ↓ mAS & ↓ lower dose, ↓contrast

So low Tube voltage is used (25-35Kev)
Target & filter material:
– Mo is used → emit characteristic x-ray (18 - 20 kev)
– Rhodium is used for thick or dense breast → (23 kev)
– Tungsten →dense breast.
Grids:

Breast dose (MGD) increased by 2-3 but image
contrast improves by factor of 2.
Breast thickness & tissue composition:

Large & dense breast are more difficult to penetrate so
↑ energy x-ray beam but it ↑ average glandular dose.

Small breast & of more adipose tissue → ↓ AGD

Breast compression to reduce overlap tissues & scatter,
more contrast, less motion, lower tissue radiation dose.
Magnification:
 1.5 to 2 times in mammography
 image small breast but ↑ AGD, best achieved with
small focal spot.
 achieved by moving breast away from image and close
to tube
– decreased scatter
– Increased resolution,
– but ↑ dose to breast
Screen film comb & film processing conditions:

Film processing are important as image must detect
small object & object with ↓ contrast

Single-screen and single emulsion film, it has better
resolution (SR).

AGD is limited to 3 mGy or 300 mRad per film for a
compressed breast thickness of 4.2 cm.
Decrease dose in mammography

Small breast
Compression
Increase kv
Increase contrast

•
Decrease kv
Beryllium window
Grids / air gap
Film gamma 3,
low speed
Increase resolution
• Small focal spot
• Film processing
Single screen and single emulsion film
Questions
1.
Tabulate the differences between conventional
and mammographic x ray machine?
2.
Mention the main difference between
conventional, computerized and digital
radiography?
3.
Define heel effect & mention its importance?