RAD 1226

RADIOGRAPHIC DETAIL AND FACTORS AFFECTING IT

A. Voluntary and involuntary motion

1. Definitions

2. Methods of control

B. Geometric unsharpness

1. Definitions

2. Umbra, Penumbra

3. Three controlling factors

C. Object‑film distance

1. Definition

2. Importance

3. Influence on sharpness

D. Focal‑film distance

1. Definition

2. Importance

E. Effective focal‑spot size

1. Definition

2. Line focus principle

3. Actual focal spot

4. Target angle

F. Calculation of geometric unsharpness

1. Formula

2. Applications

G. Screen influences

1. Contact

2. Phosphor thickness

3. Crystal size

4. Reflectance of backing

5. Presence of optical attenuator in backing

6. Screen mottle

H. Film influences

1. Grain

2. Duplitized vs. single‑coated

I. Quantum mottle

1. Definition

2. Influence of system speed

J. Visibility of detail

1. Influence of density

2. Influence of contrast

K. Resolution

1. Definition

2. Methods of measuring

Voluntary motion

Patient motion is either voluntary or involuntary. When patients move their bodies, arms, or legs, or breathe during the exam, that's considered voluntary movement.

Involuntary motion

It's more difficult for you to control involuntary motion because it's not something the patient can control either. It can happen in the gastrointestinal tract or the cardiovascular system. Or you might have an unconscious patient who can't hold his breath

Controlling Motion

Patient motion can usually be adequately controlled by observing these three principles:

use a short exposure time when necessary

explain the examination to the patient and communicate throughout the exam

immobilize when necessary

Umbra - Penumbra

Geometric unsharpness or penumbra can be measured if these three factors are known: SOD, OID, and FS size

Geometric unsharpness

Geometric unsharpness or blur is affected by three factors:

focal spot (FS) size -- using the small focal spot reduces unsharpness

object-image receptor distance (OID) -- using a small OID reduces unsharpness

source-image receptor distance (SID) -- using a large SID reduces unsharpness

Of the three factors, the focal spot size is probably the least important; it is often difficult to differentiate between images produced using different focal spots

Geometric unsharpness

More specifically, geometric sharpness is affected by the size of the focal spot, the distance between the x-ray source and the image receptor (SID), and the distance between the part of the body to be examined and the image receptor (OID).

Focal Spot

With the larger focal spot, x-rays strike the end of the object (or the edge of any structure inside the object) at more widely diverging angles because they begin farther apart at the anode. The sharp edge of the object blurs into a broader, fuzzier edge as the x-rays continue to spread out between the object and the image receptor

Object‑film distance

object-image receptor distance (OID) -- using a small OID reduces unsharpness

Focal‑film distance (SID)

source-image receptor distance (SID) -- using a large SID reduces unsharpness

Effective focal‑spot size

Modern x-ray tube anodes are designed according to the line-focus principle to achieve the best recorded detail. The actual focal spot is the area of the anode bombarded by electrons, and the effective focal spot is the area from which the beam projects onto the patient or image receptor.

Effective focal‑spot size

The disadvantage of a smaller angle anode, in addition to the increased anode heel effect, is that the primary beam covers a smaller area and therefore limits your choices of film size

calculation of geometric unsharpness

Geometric unsharpness or penumbra can be measured if these three factors are known: SOD, OID, and FS size

Penumbra = focal spot size X OID

SOD What is the Penumbra for an image taken with a 1.0 mm focal spot, at a 72 inch distance and an OID of 3 inches 0.04

One More

If a structure of interest is 8" from the film, the SID is 40", and the FS size is 0.6 mm, then the SOD is _____inches

32 FORMULA ANSWER

image receptor (material) unsharpness

Image receptor unsharpness refers to the image unsharpness introduced by (1) intensifying screens and (2) film .

With computed radiography - the influences include (1) the phosphor plate, (2) the reader/digitizer, and (3) the pixel size of the display monitor. The monitor is usually the limiting factor

Intensifying screen factors

The unsharpness introduced by intensifying screens is much more important that any influence of the film itself. As you learned last semester, the two important influences are (1) film-screen contact and (2) the thickness of the phosphor layer . Closer contact between the screens and film results in less unsharpness, but a thicker phosphor layer (which increases screen speed) usually causes more unsharpness.

Intensifying screen factors

(3) screen crystal size (increase causes more unsharpness), (4) presence of a reflective backing (increases unsharpness), and (5) presence of yellow dye in "detail" screens (decreases unsharpness).

Slower speed screens provide better resolution but also a higher patient radiation dose. That is a classic dilemma in radiography. As a radiologic technologist this decision often is yours to make, so it is important to be familiar with the pros and cons in every case.

Film Factors

The film is an unimportant influence on the unsharpness of the image; all types and brands of x-ray film, by themselves, provide excellent detail. Two factors which are sometimes mentioned are (1) film grain and (2) parallax unsharpness . Film grain is clumping or uneven distribution of the silver bromide crystals in the emulsion; it results in a small-scale variation in density.

Parallax unsharpness refers to misalignment (misregistration) of the images on each side of a double-coated film. It occurs when the photons striking the film are not perpendicular to it. The remedy for parallax unsharpness is to use single-coated film, which unfortunately increases patient dose. The only modern exam which uses single-coated film is mammography.

Noise

Noise is interference with the transfer of information . It occurs to some degree in any information transfer; examples include "static" in radio reception and "snow" in television reception. Noise in film-screen image production may be due to film grain or may be seen as mottle (variation in density).

Screen Mottle

Screen mottle is density variation caused by uneven distribution of screen phosphor . This may be due to wear in old screens, or to substandard manufacture. Screen mottle is not much of a problem with modern screens.

Quantum mottle

Is density variation caused by random fluctuation in photon distribution. It can be seen in both film-screen images and CR images. At any moment in time, the number of photons in a particular area of the beam may be higher or lower than in an adjacent area. If enough exposure (i.e. enough photons) are used, these small differences in exposure will be equalized, and the image will exhibit little or no mottle. However, when very low exposures are used, there are not enough photons to demonstrate all structures adequately.

Not Enough Photons

Assessing Recorded Detail