Chapter 19

Chapter 19 – Image Quality Radiographic Quality Film Factors Characterist ic curve • Densi ty •

Contr ast

•

Spee d

•

Latitu de

Geometric Factors • Distor tion • Magni ficatio n • Blur

Subject Factors Contrast •

Thic kne ss • D ensi ty • A tomi c num ber Motion

Processing • Time • Temp eratur e

Optical Density – the degree of blackness of a radiograph. Optical density is the numerical value calculated between the level of light incident on a processed step wedge image. mAs – Exposure Factors mAs is directly proportional to x-ray quality. 2x mAs = 2x x-ray exposure ½ mAs = ½ x-ray exposure

Visibility of detail – the ability to see the detail or the sharpness of the radiographic image sharpness of image detail refers to the structural lines or borders of tissues in the radiographic image.

Influencing Factors Regarding Density 1. kVp – primary controlling factor for contrast 2. Distance 3. Heel effect 4. Intensifying screens 5. Beam limiting device 6. Grids 7. Filtration 8. Patient Consideration 9. Processing 10.Contrast Media ↑kVp 15% = 2x density ↓kVp 15% = ½ density I.e. Hand X-Ray Technique: 50 mA, 1/30 sec, 60 kVp In order to reduce motion, what new technique can be employ using the same density? Direct Square Law New mAs Old mAs

= New Distance2 Old Distance2

If 25 mAs at 40 inches produces an optical density of 1.0, what mAs is needed to produce the same density at 32 inches? X X

new

= =

25 mAs X 1024 1600 16 mAs

Density – Intensifying screens Screens act as an amplifier of the remnant radiation reaching the film. Density – optical density is the numerical value calculated between the levels of light incident on a processed step wedge image. (lo) and the level of light transmitted through the film (lt) Optical density is defined as follows: OD = log10 X lo/ lt An optical density of 1 = 1/10 or 10% of light transmitted through the film (refer to table 19-1). Characteristic curve, sensitometric curve, H&D curve & Dloge curve. H&D Curve

Shoulder

Straight-line Portion

Toe

Every 0.3 on the densitometer (x-axis) →doubling the mAs - x-ray output. Useful range of OD is approximately 0.25 to 2.5 = straight line portion of H&D curve. Film speed 1. The ability of an image receptor to respond to a low x-ray exposure is a measure of its sensitivity or its speed. 2. Film speed and screen speed are considered the same; therefore the film and screen combination speeds are referred to as the image-receptor speed. 3. When changing from one IR speed to another, a change in mAs may be necessary to maintain the same optical density. • • • • •

Rare earth (200-1200 speed) = ¼ X mAs Fast (CaWO4, 200 speed) = ½ X mAs Par (CaWO4, 100 speed) = 1 X mAs Detail (CaWO4, 50 speed) = 2 X mAs Ultra Detail (CaWO4, 100 speed) = 4 X mAs

Increase the screen film, it will increase density. To maintain the same density, use relative speed factor. The steeper the slope, there is less latitude. Speed and latitude are inherent. 1. Film latitude – the emulsion must be able to record a relatively long range of densities, from the blackest black (gas or air) to the whitest white (dense osseous structures or barium filled organs), with all shades in between. 2. Exposure latitude – the capability of a film to record differences in density. Therefore, density changes in mAs are less critical with higher kVp values. There is more margin of error. Optical density/ radiographic density OD is controlled by two major factors:

1. mAs – milliamperes X seconds 2. Distance – source-to-image distance (SID) mAs is the primary controlling factor manipulation of OD. mAs – Exposure Factors mAs is directly proportional to x-ray quantity 2x mAs = 2x x-ray exposure ½ mAs = ½ x-ray exposure mAs/ mA = seconds mAs/seconds = mA ±30 – 50% of the original mAs will make a change in radiographic density Screen speed Rare earth phosphors (200-1200) ¼ X mAs High speed (CaWO4 - 200) ½ X mAs Par/medium speed (CaWO4 - 100) mAs mAs

=

new old

Screen speed Screen speed

old new

200 mA, 1/40 sec, 70 kVp, using 200 speed screen/film Grid – 1. 2. 3. 4.

effects on density Absorption of scattered radiation Decrease fog Decreases radiographic density mAs must increase to compensate for the increase in absorption of the x-ray energy 5. Grid conversion factor: (see lab manual p.87)

mAs mAs

new old

=

Grid conversion new Grid conversion

old

A technique of 200 mA, 1/3 sec, 80 kVp with 8:1 grid is used to reach an optimum density, what change in technique should be made using a 16:1 grid? X

=

X

=

X

=

66.6 mAs 4 399.6 4 99.9

X

6