Chapter 27 - Contrast PowerPoint

When assessing contrast, two photographic factors allow detail to be seen

• IR exposure

• Contrast

What affects IR exposure?

mAs

What affects Contrast?

kVp + LUT (Look Up Table)

When assessing contrast, what affects the dynamic range?

Window width (digital factor)

A low contrast image would have

lots of gray in image

A high contrast image would have

very distinct difference between the black and light parts of image

contrast

difference in brightness levels between adjacent areas (on a digital monitor)

Digital Image Contrast primarily controlled by

look-up table (LUT)

mAs is primary controller of

Receptor Exposure

AART definition of Dynamic Range

the range of exposures that may be captured by a detector

Dynamic Range is a digital factor that is not tech controlled

true, considered a pre-processing computer function

dynamic range describes

the number of gray shades that an imaging system can reproduce

Digital imaging has a _______ dynamic range

wide (tied to low contrast)

Look-up Table (LUT)

a series of mathematical equations that are used for image processing in radiography

LUT allows the gray-scale values in a recorded image to be remapped, improving the final image with by adjusting the

appropriate brightness and contrast

LUTs are specific to

anatomic areas

LUT is the controller of

digital contrast (main controller of final contrast)

Not referring to a specific region, LUT values are determined by

manufacturer

Windowing is not tech controlled

false, it is tech-controlled manipulation and considered post-processing

Windowing definition

ability to alter the brightness and contrast of a digital image following processing

Window level

adjusts brightness and the type of tissue to be imaged

Increasing window level

will decrease image brightness

Decreasing window level

will increase image brightness

Window Width

adjusts image contrast and the gray-scale representation of the tissue

Increasing window width

increases contrast scale, decreases image contrast

Decreasing window width

decreases contrast scale, increases image contrast

Short scale

• few shades of gray

• short/narrow dynamic range

High contrast

Long scale

• More shades of gray

• Large/wide dynamic range

Low contrast

Main controller of subject contrast (patient) (Pre-Processing)

kVp

What type of relationship does kVp have with contrast?

Inverse

When is it recommended to use Short Scale Contrast?

Bone Work (fractures)

• low kVp (high contrast)

• longer wavelengths (low energy)

• reduced scatter

• demonstrates fine bone markings

When is it recommended to use Long Scale Contrast?

Chest/Abdominal Images

• High kVp (low contrast)

• increased scatter

• shorter wavelengths (high energy)

• photons able to pass through thicker anatomy

Bit Depth

number of shades of gray that can be used to define each pixel

Greater Bit Depth

greater the number of tones (gray-scale) that can be represented

Increasing bit depth increases

image quality

How to find how many shades of gray housed in a pixel?

2^{bit depth}

low kVp

more attenuation and absorption

• high contrast

• black and white

high kVp

less attenuation and absorption

• more gray

• less difference in brightness levels

• low contrast

kVp and scatter have what type of relationship?

direct

High scatter = low contrast

Scatter will fog an image resulting in a

gray cloud

Causes of Scatter Radiation?

• High kVps

  • increased compton interactions

• Patient Thickness

• Increased field sizes

mAs is not related to what type of interactions?

scatter

Large Hypersthenic

patients have a higher fat content which creates a longer scale of gray values

• (long scale contrast)

• (more matter more scatter)

Contrast agents

can be used to increase contrast of areas such as the GI and urinary tracts

Positive Contrast Agent

Barium or Water Soluble (Iodine)

• higher atomic number

• appears white

• higher rate of attenuation/absorption

Negative Contrast Agent

Air or CO2 crystals (fizzies)

• lower atomic number

• appears black

• less attenuation/absorption

Unlike ________, there is no numerical indicator to quantify image contrast

exposure intensity

Exposures use exposure indices

• EI#

• S# (CR)

(quantity to plate, not related to contrast)

Air gap technique must be at least

6” OID to be effective

(Object to image receptor distance)

To overcome the magnification created by the air gap technique

SID must be increased

(Source to Image distance)

When using the Air Gap technique, a 10 inch air-gap =

15:1 grid

Disadvantage of using air gap technique?

loss of detail (magnifies anatomy)

• image magnification with associated focal-spot blur

Air gap technique primarily used in

magnification radiography and chest radiography

• lateral C-Spines

• chests

  • dissipates scatter before reaching Image Receptor

Factors affecting Subject Contrast (patient)

• kVp

• Pathology

• patient size

• contrast media

• OID

• Filtration

• Collimation