Geometry of Projection Radiography PDF

GEOMETRY OF PROJECTION RADIOGRAPHY ❑ The Primary Effect of projection radiography is to record an image of a 3D object (the patient) in 2D, resulting in superposition of the anatomy along each ray ❑ This leads to a number of effects that need to be considered in: the Design of equipment the Pro...

GEOMETRY OF PROJECTION RADIOGRAPHY ❑ The Primary Effect of projection radiography is to record an image of a 3D object (the patient) in 2D, resulting in superposition of the anatomy along each ray ❑ This leads to a number of effects that need to be considered in: the Design of equipment the Production of the images and their Interpretation ❑ In particular, for each projection there will be a region of clinical interest, Somewhere between the entrance and exit IAEA of the region to be imaged surface EFFECTS OF PROJECTION GEOMETRY Geometrical Distortion - Position ❑ All objects are magnified by an amount related to the OID ❑ The further away from the OID the larger the object appears ❑ In diagram all objects A, B and C are the Same size, but they appear progressively larger due to differences in position Effect of depth of objects on their projected size EFFECTS OF PROJECTION GEOMETRY Geometrical Distortion - Shape Tilted object is shown projected at a range of angles, illustrating the increasing degree of foreshortening as the angle increases Effect of angulation on projected length of an angled object EFFECTS OF PROJECTION GEOMETRY Geometrical Unsharpness # Ideal image Sharpness would be produced by a Point Source # The spatial resolution in such a case being limited by the image receptor factors such as ▪ Phosphor layer Thickness, ▪ Lateral Spread of light in scintillators, and the ▪ Image Matrix # ❑ The Spatial Resolution depends on the focal spot size. ❑ Typically the fine focal spots are 0.3-1.0 mm, but must use lower mAs to protect the tube from heating effects. EFFECTS OF PROJECTION GEOMETRY Geometrical Unsharpness (Ug) ❑ the magnification (m): 𝑋𝐹 𝑆𝐼𝐷 𝑚= 𝑆𝑂𝐷 where SID is the Source-Image Distance SOD is the Source-Object Distance OID is the Object-Image Distance Ug 𝑂𝐼𝐷 𝑆𝐼𝐷 − 𝑆𝑂𝐷 𝑈𝑔 = 𝑋𝐹. = 𝑋𝐹. = 𝑋𝐹. (𝑚 − 1) 𝑆𝑂𝐷 𝑆𝑂𝐷 EFFECTS OF PROJECTION GEOMETRY Geometrical Unsharpness ❑ Optimization of projection radiographs involves choosing an appropriate focal spot size ❑ This requires a Compromise between the exposure time and the resolution ❑ For Example, a very small focal spot will provide good spatial resolution, but only permit a low tube current, therefore requiring a long exposure time, leading to increased risk of motion blur