3 1. Focal spotActual focal spotEffective focal spotEffective focal spot:Face of target is slanted from vertical(6-150).As angle increased effectivefocal 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.44DR Azza Helal
5 This can be done through photoelectric interaction process. 2. FiltersSlide 2A 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/E3As u (attenuation coefficient) is α to 1/E3, it attenuates low energy photons & does not affect high energy photonsSo filtration:-↑photon energy , ↑ exit dose / entry dose ratio so ↑ film dose / skin dose ratio, but it decreases the contrast.
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.88DR 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 & ↑ contrastKv : ↑Kv → ↓ μ, ↓ scatter in pt and ↑ forward scatter reach film ↓ contrast99DR 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 leadsandwiched between plastic,carbon fiber & AL.Lead absorb 90% of scattered rays while allow 70% of 1ry beam to pass through gap to reach film. ↑ contrastThe beam becomes more penetrating as low energy beam can not reach the film so the mean of the energy increase.Slide 71010DR Azza Helal
11 Contrast improvement factor Grid ratio (GR)= Depth / Width of interspaced channelGR, efficiency to absorb scatter radiation & scatter / primary rays, contrast. , & doseContrast improvement factor= Contrast with grid / contrast without grid=3-5.Bucky factor or grid factor=Exp necessary with grid/Exp necessary without grid=2-3Frequency is the number of strips per inch.High ratio & frequency more dose required.No grid is used with thin part of body (extremities) & child1111DR Azza Helal
12 2. Air gapFilm moved 30 cm a way from patient so much oblique scatter will miss the film & so contrast improvesNeed ↑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.1212DR Azza Helal
13 4. Screen film cassetteA) 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 21313DR 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 allowsLow pt dose & High contrastBut with Noise & low resolution1414DR Azza Helal
15 Silver halide crystals C) FilmsSilver halide crystalsexposed 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)1515DR 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 filtrationTube filtration:Affects both quantity & quality, it affect quality by removal of low energy photons.1616DR Azza Helal
17 Limitation of x ray tube Heat with x ray Patient dose1. Heat with x ray: 99.5% of energy converted to heatIt is avoid by using:Rotating anodeCopper 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 organsAcceptable patient dose of radiation is a limiting factor;it should be as low as possibleMinimum exit dose emerges from pt is required to produce satisfactory image.1818DR 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 mAmA increases no of photons, and so signals but also amount of exposure (dose). ↑ patient doseEffect 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/E31919DR 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.2020DR Azza Helal
21 QuestionsEnumerate 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?
Your consent to our cookies if you continue to use this website.