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Projection radiography. Projection imaging refers to the acquisition of a two-dimensional image of the patient's three-dimensional anatomyRadiography is a transmission imaging procedureIn screen-film radiography, the optical density (OD) at a specific location on the film is (ideally) determined b
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1. Screen-Film Radiography I Geometric principles
Film cassettes
Screen characteristics
2. Projection radiography Projection imaging refers to the acquisition of a two-dimensional image of the patient’s three-dimensional anatomy
Radiography is a transmission imaging procedure
In screen-film radiography, the optical density (OD) at a specific location on the film is (ideally) determined by the x-ray attenuation characteristics of the patient’s anatomy along a straight line through the patient between the x-ray source and the corresponding location on the detector
4. Basic geometric principles Two triangles that have the same shape (the three angles of one are equal to the three angles of the other) but have different sizes are said to be similar triangles
If two triangles are similar, the ratio of the corresponding sides and heights are equal
Similar triangles are encountered when determining image magnification and when evaluating image unsharpness
6. Magnification Magnification occurs because the beam diverges from the focal spot to the image plane
Magnification given by:
Largest when object close to focal spot
8. Extended source With magnification, geometric blurring of the object occurs in the image
Similar triangles allow calculation of the edge gradient blurring, f, in terms of magnification, M, and focal spot size, F:
9. Extended source (cont.) Blur increases with the size of the focal spot and with the amount of magnification
Focal spot blur can be minimized by keeping the object close to the image plane
11. Screens Film by itself can be used to detect x-rays
Relatively insensitive
A lot of x-ray energy is required to produce a properly exposed x-ray film
X-ray screens are used to reduce the x-ray dose to the patient
Screens are made of a scintillating material, called a phosphor
X-rays interact in the phosphor; visible or ultraviolet light is emitted
12. Materials For much of the 20th century, calcium tungstate (CaWO4) most commonly used
In early 70s, rare earth phosphors introduced
Most common used today is gadolinium oxysulfide (Gd2O2S)
Others are lanthanum oxybromide (LaOBr) and yttrium tantalate (YTaO4)
Cesium iodide (CsI) is used in fluroscopy and digital radiography; too moisture sensitive and fragile for use in screen-film radiography
14. Phosphor thickness Usually expressed as the mass thickness (product of thickness and density) of the phosphor, excluding the binder
For general radiography, two screens are used, with each screen having a thickness of about 60 mg/cm2, for a total thickness of 120 mg/cm2
For high-resolution requirements and low energies used in mammography, a single screen of approximately 35 mg/cm2 is used
15. Screen function Screen function is two-fold:
Absorbing (detecting) incident x-rays
Emitting visible (or UV) light, which exposes the film
Conversion efficiency defined as the fraction of the absorbed energy that is emitted as light
CaWO4 about 5%
Gd2O2S:Tb about 15%
Not all emitted photons reach the film emulsion after diffusing through the phosphor and being reflected at interface layers
16. Quantum detection efficiency QDE of a screen is defined as the fraction of incident x-ray photons that interact with it
Easiest way to increase QDE is to make the screen thicker
18. Thickness effects After an x-ray absorption event, visible light given off in the depth of a screen propagates towards the screen’s surface to reach the adjacent film emulsion layer
Spreads out in all directions with equal probability (isotropic diffusion)
For thicker screens, light photons propagate greater lateral distances
20. Resolution Increasing the screen’s thickness increases detection efficiency and thereby improves the screen’s sensitivity
Increased thickness causes undesirable loss of spatial resolution
Classic compromise between sensitivity and resolution seen in many imaging systems
Modulation transfer function (MTF) is the technical description of spatial resolution for most imaging systems
22. Conversion efficiency Total conversion efficiency of a screen-film combination refers to the ability of the screen or screens to convert the energy deposited by absorbed x-rays into film darkening
Depends on intrinsic conversion efficiency of the phosphor, efficiency of light propagation through screen to emulsion layer, and efficiency of film emulsion in absorbing the emitted light
Light-absorbing dye may be used to reduce light propagation distance and preserve spatial resolution
24. Conversion efficiency (cont.) For faster (i.e., higher conversion efficiency), lower-resolution screen-film system, a reflective layer can be placed between the screen and its support
Intensifying screen by itself is a linear device at a given x-ray energy
If number of x-ray photons is doubled, light intensity produced by screen also doubles
27. Overall efficiency Total efficiency of a screen-film system is the product of the conversion efficiency and the absorption efficiency
Screen-film system provides vast increase in x-ray detection efficiency over film by itself
At 80 kVp, a 120 mg/cm2 thickness of Gd2O2S detects 29.5% of the incident x-ray energy
Film detects only 0.65% of the incident x-ray energy
29. Other benefits Output requirements of the x-ray system are reduced, substantially reducing the requirement for powerful generators and high heat capacity tubes and reducing costs
Exposure times are shorter and motion artifacts are reduced
Exposure to personnel from scattered x-rays is reduced