Presentation on theme: "Advanced Biomedical Imaging Dr. Azza Helal A. Prof. of Medical Physics Faculty of Medicine Alexandria University Lecture 5 Advanced X ray machines &Mammogram."— Presentation transcript:
Advanced Biomedical Imaging Dr. Azza Helal A. Prof. of Medical Physics Faculty of Medicine Alexandria University Lecture 5 Advanced X ray machines &Mammogram
is revolutionizing medicine in the same way that digital cameras changed photography. Computed Radiography (CR) Advanced X ray machines
CR uses instead of ordinary cassette, an imaging plate coated with storage phosphor to capture x-rays as they pass through the patient. CR uses instead of ordinary cassette, an imaging plate coated with storage phosphor to capture x-rays as they pass through the patient. imaging plate imaging plate When irradiated, the enhanced phosphor absorb and store x-ray energy. When irradiated, the enhanced phosphor absorb and store x-ray energy. This trapped energy comprises a latent image 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. CR cassette is then placed in a digitizer where the phosphor plate is scanned causing release of trapped energy into visible light.digitizer This light is captured and converted into an electrical signal, which is converted into digital image 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. Digital image can be displayed on laser-printed films or softcopy workstations.
Lower radiation dose Higher contrast. Better image quality. Rapid result No need to repeat examination. Post-processing capabilities… Enables workstation display & archiving. Advantages of computed Radiography (CR)
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 Radiography (DR)
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. Mammogram imaging modalities
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. Mammography
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 x- ray tube. 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: Photo timer: Radiation sensor used to control the exposure. It sets the optimal kV from a short test exposure.
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 Mammography Molybdenum (Z=42) E K =20Kev E L =2.5Kev Ch. Rad.= 17.5kev Photoelectric Target
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 of characteristic x-ray energies. (Mo/Mo).
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
Molybdenum (Mo)Target MolybdenumFilter 17.5-19.5Mo ch, rad tungsten target, Rhodium filterFor large dense breast, implant 0.1-0.3mm, tube cooling prob, 3Focal spot, Film λ min. filtration (z=4) due to low UBeryllium window, Not glass usedAir gap MovingGrids, GR 65cmSID 25-35kvTube voltage single screenScreen photoelectric effectMain source of contrast 0.5-1msvEffective dose
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 main difference between conventional, computerized and digital radiography? 3. Define heel effect & mention its importance?