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 or softcopy workstations.

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

dose and exposure time.

increases patient



So breast imaging needs a special designed x ray 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 Conventional x ray tube

Tungsten (Z=74) E K =70Kev E L =12Kev Ch. Rad.= 58kev

Photoelectric Mammography

Molybdenum (Z=42) E K =20Kev E L =2.5Kev

Ch. Rad.= 17.5kev

Focal spot

 Small focal spot is used for best contrast (0.1 0.3mm)  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 For large dense breast, implant Focal spot, Film λ Beryllium window, Not glass Air gap Grids, GR SID Tube voltage Screen Main source of contrast Effective dose

Molybdenum 17.5-19.5

tungsten target , Rhodium filter 0.1-0.3mm, tube cooling prob, 3 min. filtration (z=4) due to low U used Moving 65cm 25-35kv single screen 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 Single screen and single emulsion film • Film processing

Questions

1.

Tabulate the differences between conventional and mammographic x ray machine?

2.

Mention the conventional, radiography?

main difference computerized and between digital 3.

Define heel effect & mention its importance?