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 Producti...

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 MAGNIFICATION IMAGING ❑ Magnification is achieved by increasing the OID which generally requires an increase in the FID as well. ❑ The actual magnification achieved varies with depth in the patient. Example: Patient thickness is 20 cm, th FID 140 cm and the FSD 80 cm the magnification varies between 1.4 at the Exit side of the patient to 1.75 at the Entrance side ~ MAGNIFICATION IMAGING Magnification requires employment of a Larger image receptor For large body regions this may Not be possible The use of magnification has consequences for: ▪ Dose ▪ Spatial Resolution and ▪ SNR ~ MAGNIFICATION IMAGING - Dose A number of Effects occur when increasing the OID There is a substantial reduction in the Scatter Fraction at the image receptor, because the scattered rays are generally directed away from the receptor To maintain the dose to the image receptor, an increase in the mAs and hence the patient dose would be required MAGNIFICATION IMAGING - Unsharpness ❑ An increase in the OID leads to a reduction in image sharpness due to the geometric blur of the focal spot ❑ Use of magnification techniques requires a Significant Reduction in focal spot size compared to contact methods MAGNIFICATION IMAGING - Unsharpness Improvement in the overall sharpness of the complete system is generally because of the increase in size of the image compared to the Unsharpness of the image receptor - From effects such as: ▪ Light Spread for screen-film systems and the ▪ Pixel Size for digital systems Magnification can therefore Improve Spatial Resolution, compared to the result of a simple zoom of a digital image which enlarges the pixels as well as the image TECHNIQUE SELECTION - ❑ With Screen-Film systems, technique selection is relatively straightforward: ▪ The choice of kV setting is based largely on the required contrast ▪ the mAs is chosen to produce a suitable optical density for the region of clinical interest. ❑ With Digital systems, the direct link between technique setting and image appearance has been lost, making correct technique selection much more difficult TECHNIQUE SELECTION - Effect of Tube Voltage on Contrast, Noise & Dose To determine whether a detail will be detectable in the image, Noise must be considered The primary Source of noise is generally the random arrival of photons at the image receptor, a Poisson process From Rose’s expression, the number of detected photons required per unit area, to image a detail of size d and contrast C with a signal to noise ratio of k, is: N = k2/C2d2 The value of k is often taken to be 5 TECHNIQUE SELECTION - Effect of Tube Voltage on Contrast, Noise & Dose As C is increased, the number of photons required at the image receptor is reduced so that a Reduction in kV will Produce an Image of Satisfactory Quality at a Lower Image Receptor Dose. IAEA - TECHNIQUE SELECTION Technique Selection & 15% Rule However, This Reduction in kV will Require an Increase in the mAs, Leading to an Increase in Patient Dose ❑ The patient dose (Ki), is proportional to mAs and approx to kV2. ❑ The penetration through the patient is proportional to kV3. ❑ Total dose to the image receptor (detector Signal) depends approximately on kV5, and is linear with mAs. ❑ So If the kVp reduce by 15%, the new mAs will be: 𝑚𝐴𝑠𝑛𝑒𝑤 = 𝑚𝐴𝑠 𝑘𝑉𝑝 5 = 𝑘𝑉𝑝𝑁𝑒𝑤 𝑚𝐴𝑠 1.15 5 = 𝑚𝐴𝑠 × 2 TECHNIQUE SELECTION ~ For Example A Lateral Cervical spine radiograph was produced using 32 mAs and 80 kVp at 180cm. The C7-T1 area is not penetrated well and the image needs to be repeated. The kVp is being increased to 92 kVp, what new mAs should be used to maintain the original exposure? 1. The mAs will be increase or decrease? 2. What will be the new value of mAs? 3. Explain the your result?