Magnification Imaging PDF

MAGNIFICATION IMAGING A M ❑ Magnification is achieved by increasing the OID which generally requires an increase in the FID as well. 2 ❑ The actual magnification achieved varies with depth in the patient. 1 Example: Patient thickness isO 20 cm, th FID 140 cm and the FSD 80 cm the magnification varie...

MAGNIFICATION IMAGING A M ❑ Magnification is achieved by increasing the OID which generally requires an increase in the FID as well. 2 ❑ The actual magnification achieved varies with depth in the patient. 1 Example: Patient thickness isO 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 z 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 ⑭ There is a substantial reduction in the Scatter Fraction at the image receptor, because the scattered rays are generally directed away from the receptor nAS To maintain the dose to the image receptor, an increase in the / mAs and hence the patient dose would be required * - - m s e i e n m e - MAGNIFICATION IMAGING X Unsharpness wil - & ❑ 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