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Advanced Biomedical Imaging Lecture 3

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Presentation on theme: "Advanced Biomedical Imaging Lecture 3"— Presentation transcript:

1 Advanced Biomedical Imaging Lecture 3
Dr. Azza Helal A. Prof. of Medical Physics Faculty of Medicine Alexandria University

2 X ray Machine Slide 13 Slide 5 X ray tube

3 1. Focal spot Actual focal spot Effective focal spot Effective focal spot: Face of target is slanted from vertical (6-150). As angle increased effective focal spot increased but (actual focal spot) remains the same.

4 A small focal spot produces:
Sharper images, but unable to dissipate heat rapidly, can not sustain high current required for short exposure required to decrease patient motion, used in mammography. A large focal spot: can tolerate more heat, but un-sharpness increases, used in fluoroscopy. 4 4 DR Azza Helal

5 This can be done through photoelectric interaction process.
2. Filters Slide 2 A quality filter: thickness of material placed in radiation beam to attenuate lower energies photons (responsible for patient dose) more than higher energies (responsible for image). This can be done through photoelectric interaction process.

6 Uα1/E3 As u (attenuation coefficient) is α to 1/E3, it attenuates low energy photons & does not affect high energy photons So filtration:- ↑photon energy , ↑ exit dose / entry dose ratio so ↑ film dose / skin dose ratio, but it decreases the contrast.

7 3. Collimators & Grids I=I0e-ux
Slide 10 I=I0e-ux

8 The problem is scattered radiation?
Primary radiation carries information to be imaged & but Scattered radiation obscures it. S/P depend on thickness of part, it is 4:1 in PA chest & 9:1 in lateral pelvis. Large patient creates more scatter. It ↓ contrast which produced by primary beam. 8 8 DR Azza Helal

9 To reduce amount of scatter produced by patient
Field size cones & diaphragm → ↓ volume of scattering tissues → ↓ scatter & ↑ contrast. Compression of patient: moving overlying tissues laterally reduces volume of scatter & reduce patient dose. ↓ scatter & ↑ contrast Kv : ↑Kv → ↓ μ, ↓ scatter in pt and ↑ forward scatter reach film ↓ contrast 9 9 DR Azza Helal

10 To reduce amount of scatter after it left pt:
Grid between patient & film, air gap & flat metal filter. 1. Grid: anti-scatter grid: thin strips of lead sandwiched between plastic, carbon fiber & AL. Lead absorb 90% of scattered rays while allow 70% of 1ry beam to pass through gap to reach film. ↑ contrast The beam becomes more penetrating as low energy beam can not reach the film so the mean of the energy increase. Slide 7 10 10 DR Azza Helal

11 Contrast improvement factor
Grid ratio (GR)= Depth / Width of interspaced channel GR, efficiency to absorb scatter radiation & scatter / primary rays, contrast. , & dose Contrast improvement factor = Contrast with grid / contrast without grid=3-5. Bucky factor or grid factor =Exp necessary with grid/Exp necessary without grid=2-3 Frequency is the number of strips per inch. High ratio & frequency more dose required. No grid is used with thin part of body (extremities) & child 11 11 DR Azza Helal

12 2. Air gap Film moved 30 cm a way from patient so much oblique scatter will miss the film & so contrast improves Need ↑Kv or mAS as image magnified (mammography). Patient dose ↑ by grids, air gap, grids require higher dose than air gap so they are not used in pediatrics. 3. Flat metal filter: placed on cassette, absorb softer & obliquely traveling scatter > harder direct rays. 12 12 DR Azza Helal

13 4. Screen film cassette A) x ray cassette is a flat light box with film between a pair of screen. Front of cassette (AL low Z=13) to minimize attenuation of beam so decrease required pt dose. Back of cassette is thin lead sheet to absorb remnant radiation so no back scatter irradiated pt nor screen. Slide 2 13 13 DR Azza Helal

14 B) Intensifying screen: phosphor crystals (↑z)
Crystals absorb x-ray and emit light of intensity α intensity of x rays. It converts 1 photon to 1500 light photons. So allows Low pt dose & High contrast But with Noise & low resolution 14 14 DR Azza Helal

15 Silver halide crystals
C) Films Silver halide crystals exposed to light → electrons attach to silver ions →silver metal. The more silver metal the more film blackening. Screen is more sensitive than film exposed alone: why? The screen phosphor layer is more effective than film emulsion at absorbing x-ray. Use of screen & film reduces pt dose by a factor of 50. Film is used alone to obtain sharp image in thin part where fine details is required (dental) 15 15 DR Azza Helal

16 Factors affect x ray emission: (quantity & quality)
It is α Kv2 mAs & z & exp time. Quality: (penetrating power) It is affected by KV & tube filtration Tube filtration: Affects both quantity & quality, it affect quality by removal of low energy photons. 16 16 DR Azza Helal

17 Limitation of x ray tube
Heat with x ray Patient dose 1. Heat with x ray: 99.5% of energy converted to heat It is avoid by using: Rotating anode Copper anode & oil circulating through anode.

18 2. Patient dose: (limiting factor) X ray is absorbed by patient.
Emerged x-ray beam carries pattern of intensity dependent on thickness & composition of organs Acceptable patient dose of radiation is a limiting factor; it should be as low as possible Minimum exit dose emerges from pt is required to produce satisfactory image. 18 18 DR Azza Helal

19 Factors affect Patient dose Effect of tube Kv:
↑ Kv → ↓u → → ↓ skin dose, ↑ penetration & ↑ proportion of high energy photon reach film screen. So entrance dose decreases to acquire same exit dose. Effect of tube mA mA increases no of photons, and so signals but also amount of exposure (dose). ↑ patient dose Effect of Focus – Film distance on patient dose: ↑FFD, beam diverge and no of photons distributes over larger area →↓ patient dose ↑ noise in image, to compensate more x rays is needed to maintain (MAS ↑ but it ↑ skin dose). U α Z3/E3 19 19 DR Azza Helal

20 So to reduce scatter radiation reaching the film: Use low KV,
Moving grid, Air gap. Conning, placing thin sheet of zinc on film cassette, To reduce patient exposure: adding 2mm Al filter, increase KV ( increase penetration, if obese) decrease mA ↑x-ray target –object distance, small object film distance, compress patient. 20 20 DR Azza Helal

21 Questions Enumerate the factors that affect patient dose in x ray image? Enumerate the factors that affect x ray quantity & quality? Enumerate the factors that used to reduce the effect of scatter radiation on the film? What are the advantages of using the grids? What is the GR (grid ratio)? Mention one case in which grid is not recommended?

22 Thank you

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