physics behind mammography

Physics behind

Mammography

Asterios Ntais | Radia8on & Ma:er

Breast Cancer

•  is a malignant tumor which starts in the 8ssues of the breast

•  caused by abnormal and irregular prolifera8on of abnormal cells in body 8ssues

•  it is one of the most frequently cancers worldwide

•  first in incidence among the female popula8on •  more than one in eight women are diagnosed during their life 8me

The main types of Breast Cancer •  lobular carcinoma starts in the lobules, which produce milk

•  ductal carcinoma starts in the tubes (ducts) that deliver milk to the nipple

•  other areas of the breast, but it is very rare

1: Chest wall 2: Pectoralis muscles

3: Lobules 4: Nipple

5: Areola 6: Milk duct 7: Fa:y 8ssue

8: Skin

Tests used to diagnose breast cancer

•  Mammography: it is used to find tumors and determine if they are cancerous or non-‐cancerous

•  Breast Ultrasound: uses sound waves to iden8fy if the lump is solid or fluid-‐filled

•  CT Scan: to explore if the breast cancer has spread

Tests used to diagnose breast cancer

•  Molecular Breast Imaging -‐ MBI: a nuclear medicine technique cheaper than MRI

•  Breast MRI: to receive addi8onal informa8on and aWer mammography

•  Breast Biopsy •  Self Breast ExaminaAon

Mammography

•  is a type of radiographic examina8on used on the breasts

•  is a screening method which plays an important role in prognosis and in early diagnosis

•  is the most sensi8ve technique for detec8ng non-‐palpable lesions

•  uses low energy X-‐Rays (20-‐30 KeV)

Mammography

•  it is a challenging imaging task because the breast is composed completely of soW 8ssues

•  the connec8ve 8ssue, glandular 8ssue, fa:y 8ssue and skin have very similar a:enua8on coefficients (li:le subject contrast)

•  it tries to image blood vessels, ducts and micro-‐calcifica8ons (diameter = 100 μm)

•  it needs a reasonable radia8on dose

Mammography •  is a low-‐cost method •  low radia8on procedure

•  has the sensi8vity to detect breast cancer at early stage

*early detec8on of a

malignant tumor gives the best chance for

successful treatment

Screening Mammography

•  it is used to iden8fy cancer and it’s more simple. Requires craniocaudal (CC) and mediolateral oblique (MLO) views of each breast

Screening Mammography

CC view for leW breast

MLO view for leW breast

DiagnosAc Mammography

•  evaluate abnormali8es when there are symptoms

•  it is more complex •  it requires addi8onal views, addi8onal X-‐Rays projec8ons

•  needs magnifica8on views •  needs different angles and spot compression views

ADenuaAon of Breast Tissue

•  normal 8ssues and cancerous 8ssues have small a:enua8on differences

•  a:enua8on differences are highest at very low X-‐Ray energies (10 – 15keV)

•  a:enua8on differences are poor at higher energies (>35 keV)

•  low X-‐ray energies give the op8mal differen8al a:enua8on between the 8ssues

ADenuaAon of Breast Tissue

A:enua8on of breast 8ssues as a func8on of energy

The Essen8al Physics of Medical Imaging (2nd Edi8on)

A typical mammography

unit

* Film / Screen (Analog) * CCD – Flat Panel (Digital)

X-‐RadiaAon

•  X-‐rays are created by taking energy from electrons and conver8ng it to photons with appropriate energies

•  the quan8ty (exposure) and the quality (spectrum) can be controlled by adjus8ng the electrical quan88es (KeV, mA)

X-‐Ray Tube •  it consists of an electron vacuum tube with two electrodes

•  the cathode with -‐ usually dual – filaments •  the filaments are inside the focusing cup (nega8ve charge)

•  the anode with dual track (i.e. Mo/Rh) target •  when a high current is applied to the filaments, outer-‐shell (K-‐shell) electrons are ejected and accelerate from the cathode to anode

X-‐Ray Tube

•  the e-‐ will travel at half of the speed of light (≈1.5x108 m/s)

•  with very small resistance (vacuum) •  a li:le energy will be lost •  the focusing cup controls the size and the shape of the beam

X-‐Ray Tube

•  when the projec8le electrons strike the target, only 1% of their Kine8c Energy is converted to X-‐rays

•  the rest 99% produces heat at the anode •  modern mammography units have rota8ng anodes in order to allow higher tube currents in very short 8mes

•  finally the heat produced is not confined in a single area

Efficiency of X-‐Ray producAon Efficiency of X-‐Ray producAon

*in mammography is used low tube voltage (keV) – low efficiency of producJon

where: i: tube current V: Voltage Z: atomic number

k: constant

Pbeam = kZV2i

Pel =Vi

Efficiency = PbeamPel

⇒

Efficiency = kZV2i

Vi= kZV

X-‐Ray Tube

•  mammography units are manufactured by Molybdenum (Z=42), Rhodium (Z=45) or Tungsten (Z=74) targets matched with appropriate filters

•  these targets have a high mel8ng point

•  they have different atomic numbers (Z)

X-‐Ray Tube

•  these elements have different emission spectrum

•  Molybdenum (Mo)produces characteris8c X-‐Ray peaks at 17.5 and 19.6 keV

•  Rhodium (Rh) produces characteris8c X-‐Ray peaks at 20.2 and 22.7 keV

Filters installaAon

•  depend on the anode and are used to achieve op8mal energy spectra

•  a k-‐edge filter is used •  a filter is made in layers •  a filter uses two or more materials which match each other in their absorbing abili8es

•  filters increase the penetra8on of beam •  reduce the exposure 8me •  reduce the dose

CombinaAons of Targets and Filters

•  a Mo target with Mo filter is used for thick breast

•  a Mo target with Rh filter is used for imaging thicker and denser breasts

•  a Rh target with Rh filter gives the highest effec8ve energy beam (thickest and densest breasts)

X-‐Ray Spectrum

1. X-‐ray Spectrum from Mo/Mo 2. X-‐ray Spectrum from Rh/Rh

Half Value Layer (HVL)

•  is the thickness of any given material where the 50% of the incident energy has been a:enuated

•  depends on kVp, compression thickness, tube filtra8on, target material and tube’s age

•  increases with higher kVp and higher atomic number of targets and filters

•  is approximately 1 – 2 cm in the breast 8ssues

Collimator

•  is a device that narrows the X-‐Rays •  it cause the direc8on of mo8on to become more aligned in a specific direc8on

•  determines the X-‐Ray beam area •  increases the resolu8on but reduces intensity

Photon InteracAons with MaDer

•  in this energy range we have the photoelectric effect and sca:ering processes

•  elas8c sca:ering leaves no energy and produces no signal

•  inelas8c (Compton) sca:ering reduces the available image contrast and resul8ng in the loss of spa8al resolu8on

•  the photoelectric effect is the dominant interac8on (below 22keV)

Image Quality

•  mono-‐energe8c X-‐rays are op8mal choice to achieve high subject contrast at a low radia8on dose

•  polychroma8c beam gives high energy X-‐rays in the bremsstrahlung spectrum and reduce the subject contrast

•  or low energy X-‐rays in the bremsstrahlung spectrum which have small penetra8on and require more dose without providing a be:er image

Image quality

nA=n0 e-‐μz

nB=n0 e-‐μ(z-‐a)-‐μ’a

Image Quality

•  for mono-‐energe8c X-‐ray beam:

nA=n0 e-‐μz (Path A)

where: §  n0 is the average number of X-‐rays incident on the breast,

§  z is its thickness §  μ is the X-‐ray a:enua8on coefficient of the 8ssue

Image Quality

nB=n0 e-‐μ(z-‐a)-‐μ’a (Path B) where: §  n0 is the average number of X-‐rays incident on the breast,

§  z is its thickness §  μ is the X-‐ray a:enua8on coefficient of the 8ssue §  where a is the thickness of the structure in the direc8on of travel of the X-‐rays.

Image Quality

•  the signal difference produced by the presence of the structure is:

SD= nA – nB

•  the resultant radia8on contrast is: Crad = (nA – nB)/(nA + nB)

Image Quality

•  low energy photons deliver the maximum possible subject contrast

•  the presence of sca:er reduces the available image contrast

•  it is possible to minimize the sca:er by: §  Compression

§  An8-‐Sca:er Grids § Magnifica8on

Image Quality

•  the maximum contrast with sca:er is:

•  where the S: amount of sca:er •  where the P: amount of primary radia8on •  where the S/P: is the sca:er to primary ra8o

Cprimary+scatter =Cprimary

1+ ScatterdPrimary

=Cprimary

1+ SP

Compression Plates

•  there is a large varia8on in the size and composi8on of the female breast

•  compression is necessary for the examina8on •  it tries to press the breast in order to reduce the volume

•  be:er image quality •  lower exposure dose

Image Detectors

•  image receptors must provide enough spa8al resolu8on, radiographic speed and image contrast

•  there are two types of mammography units •  analog detectors (film / screen) – older technology

•  digital detectors (CCD – flat panels)

Film / Screen Detectors

•  in this method the the image is captured, displayed and archived with film

•  mammography film / screens are more advanced than the conven8onal detectors

•  it has a high spa8al resolu8on (20 line pairs per millimeter)

•  it can demonstrate micro-‐calcifica8ons very well •  it has high contrast (very crucial for mammography)

Film / Screens

•  mammography casse:es are made of low-‐a:enua8on carbon fiber

•  have a single hd phosphor screen in contact with, but behind the emulsion film

•  gadolinium oxysulphide (Gd2O2S) is mostly used for the fluorescent screen – rare earth phosphor

•  this screen gives a green light emission and it is needed green sensi8ve film emulsions


•  in this case the film is situated near to the X ray tube

•  most interac8ons of X -‐ photons occur at the top of the screen

•  most of the interac8ons are near the ac8ve film emulsion

•  the generated visible light photons have less travel path to the film


•  this configura8on brings the point of produc8on the light photons emi:ed by the phosphor as close as possible to the emulsion

•  a smaller path to travel reduces the lateral spread and increases the spa8al resolu8on

* the design of the screen takes into account the

resolu8on, the dose and the noise

Film / Screen Detectors Problems

•  if the phosphor thickness increases, it absorbs more X-‐Ray photons

•  the un-‐sharpness become worse because of the larger lateral spread of the light photons

•  lateral spread can be reduced by the addi8on of dyes to the phosphor – but it is increasing the dose


•  the performance of screen / film detectors is limited because of the film emulsion

•  it has a non-‐liner response to the light photons

•  it shows a flat varia8on with exposure at high and low points

•  they increase the noise •  there is a limited dynamic range (40:1)


•  there is no op8on for post-‐processing and op8miza8on

•  imbalance between dynamic range and contrast resolu8on

•  Loss and damage of films


Digital Mammography: an overview Mahadevappa Mahesh, MS, PHD

Digital Mammography

•  uses full field – of view – digital receptors •  post-‐processing capability of the images •  permit computer-‐aided detec8on (CAD) •  it has linear response over a wide range of X-‐Ray intensi8es

•  low system noise •  wide dynamic range (1000:1) •  dynamic image manipula8on

Digital Mammography

•  there are two types of design of flat panels detectors

•  Indirect Capture (IC) •  Direct Capture (DC)

Indirect Capture (IC)

•  it is the earliest design •  it is a two-‐step process •  step 1: X -‐ photons absorbed within fluorescent material such as cesium iodide (CsI) and visible light photons generated

•  step 2: the light photons interact with photo-‐diodes

•  there are two layers of arrays: the photo-‐diode array and the Thin Film Transistor (TFT) array

Indirect Capture (IC)

•  IC designs has problems like film/screens systems

•  there is light spread problem •  thicker fluorescent screens reduce spa8al resolu8on

•  the photo-‐diode and transistor arrays are not transparent to X photons, unlike film

Direct Capture (DC)

•  has a single-‐step process in produc8on of the image signals

•  uses a photo-‐conduc8ve layer •  the X-‐Ray photons are directly captured by the photoconductor

•  the photoconductor converts absorbed X-‐Rays directly to a digital signal

Different approaches of Digital Mammography

•  1. Slot scanning with scin8llators and CCD arrays

•  2. a single flat-‐ panel scin8llator and an amorphous silicon diode array

•  3. a flat panel a-‐Se array •  4. 8led scin8llators with fiber-‐op8c tapers and mosaic CCD arrays

•  5. photos8mulable phosphor plates (computed radiography)

Analog and digital mammograms of a dense breast

a) Analog image demonstrates poor penetra8on in the dense region.

(b) Digital image has improved contrast, shows a suspicious mass more clearly, and allows be:er visualiza8on of peripheral 8ssue and the skin line.

References •  The Essen8al Physics of Medical Imaging, 2nd edi8on,

Bushberg – Seibert – Leidholdt – Boone •  Biomedical Imaging Processing, Deserno Th. •  Digital Mammography, Bick U. – Diekmann F. •  The physics of Radiology, 4th edi8on, Johns – Cunningham •  Digital Mammography: an overview, Mahadevappa Mahesh •  Physical principles of mammography, D. R. Dance •  Physics of Mammography: Image Recording Process, Mar8nJ.

Yaffe

physics behind mammography

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