Chapter 14: Geometrical Qualities of the Image

10. An image measures 16 cm across. A 60 cm SID was used with an SOD of 40 cm. What is the size of the original object?

10.67

9. With an SID of 60 and an OID of 40, by what factor will the image be magnified?

13.33 (3 times)

11. What is the percentage magnification for an image with a magnification factor of 1.44?

44%

Summary 2

Every projected image consists of a central, clear umbra component and a blurry penumbra component at the edges. Penumbra can be causes by the projection geometry or by motion. As it grows outward, it also spreads inward, invading and shrinking the umbra.

Summary 7

Radiographic magnification is proportional to the SID/SOD ratio. For magnification to be present, the umbra portion of the image must expand (not just the penumbra).

Summary 4

Radiographic penumbra or unsharpness can be predicted and measured by penumbra diagrams or by the formula.

Summary 5

Relative sharpness in a radiographic image is proportional to the ration SOD/OID.

Summary 9

Resolution indicates the overall quality of an image, and is specifically defined as the ability to distinguish adjacent details as being separate and distinct from each other. Although all image characteristics bear upon resolution, its two most important components are contrast and sharpness of detail.

Summary 8

Shape distortion is present only when the shape ration changes because one dimension of the image (length or width) is changed by a different amount than the other.

Summary 3

Sharpness is defined as the abruptness with which the edges of a particular detail "stop" as on scans across an image

Summary 1

The geometrical components of an image that affect its recognizability are sharpness of recorded detail, magnification, and shape distortion. Sharpness should be maximized and shape distortion minimized. Magnification is generally undesirable, except when a detail is too small to be recognized.

Summary 6

To the human eye, high contrast can mimic improved sharpness, and poor contrast can be mistaken as blur. They are unrelated image characteristics and must not be confused.

2. Why is penumbra present to some degree in all practical images?

because it is not a point source; its an area source

13. The resolution of line pairs from a test template can be reduced by either of which two processes?

blurring and loss of contrast

3. The umbra of very small details in an image can completely disappear by what geometrical process?

extreme distance from the screen

1. The recognizability factors in an image depend upon the __ integrity of the projection.

geometrical

14. The ability to distinguish any two adjacent details as separate and distinct details is the definition for what aspect of an image?

resolution

4. Misalignment of the xray beam, part, and image receptor results in what undesirable image quality?

shape distortion