X-Ray Physuics Module 7

What primarily causes Scatter radiation?

Compton effect

Somewhat coherent scatter

What is contrast?

degree of difference in OD between areas of an image

What would only be in an ideal image?

Transmittance and absorption

What does scatter do?

Reduce image contrast by exposing parts of film

What are the main factors that contribute to scatter radiation?

kVp (energy)

Patient thickness

X-ray field size

What increases both Compton and photoelectric effect?

Energy (kVp)

T/F: Photoelectric decreases much faster than compton?

True

Is there more compton or photoelectric at higher energies?

Compton

Why would we ever raise the kVp?

Patient dose

Transmittance

What occurs to patient dose is kVp is to low?

Increases

What occurs to transmittance if kVp is to low?

Not enough x-rays will go through to create image

What happen to scatter radiation as the field size increases?

Increases

What is the most common beam-restricting device?

Light-localizing variable aperture collimator

What is the first part that a collimator controls?

off-focus radiation

What is the benefit of a adjustable collimator?

Variable x-ray beam size

How thick are the lead leaves in a collimator?

3mm

What are the devices used to reduce scatter?

Beam restricting devices
Radiographic grids

What are beam-restricting devices?

Effect what reaches the patient

What do radiographic grids effect?

Remnant beam (after patient)

How much scatter can a high-quality grid reduce?

80-90%

What is the interspace material in a grid made of?

Aluminum or plastic

What are the important dimensions of a grid?

Height of grid

Thickness of strips

Width of interspace material

What determines the grid ratio?

Height of grid by the interspace width

Which are the more effective grids?

High ratio

What is the downside to a higher ratio grid?

More radiation exposure necessary

What ratio do most systems use?

8:1 to 12:1

Which grid will clean up 85% of scatter?

5:1

Which grid will clean up near 97%?

16:1

What is the range of grid rations?

5:1 to 16:1

What is grid frequency?

Grid strips per centimeter

What range of frequencies do most grids have?

25-45 lines per centimeter

What are the results of using high-frequency grids?

Less distinct grid lines on radiograph

Requires high exposure, higher patient dose

What is the principal function of a grid?

Improve image contrast by absorbing scatter

What is the contrast improvement factor? (k)

image contrast with grid / image contrast without grid

What value in contrast improvement factor indicates no improvement?

1

How does a higher grid ratio and grid frequency affect contrast improvement factor?

Increase

What must be increased to maintain OD when using a grid?

mAs

What is the Bucky factor? (B)

The amount of increase is expressed

Higher the grid ratio or grid frequency, higher the B

What is another name for the Bucky factor?

Grid factor

What is the equation for the Bucky factor?

Patient dose grid / patient dose without grid

What is the simplest type of grid?

Parallel

What is a result of decreased SID?

More potential for grid cutoff

What can also influence grid cutoff?

X-ray field size

What occurs if there is short SID?

OD decreases towards the edge of image receptor

What is the goal of focused grids?

Minimize grid cutoff

How are strips aligned in a focused grid?

With divergence of x-ray beam

What is the purpose of moving grids?

Eliminate grid lines

What kinds of grids will have less noticeable lines?

Higher frequency

What is the name for the moving grid?

Potter-Bucky Diaphragm

What is the largest problem with grids?

Misalignment

What can cause misalignment of a grid?

Off center

Off level

Off focus

Upside down

What is the air-gap technique?

Increase OID (object to image receptor distance)

What grid is the air gap technique equivalent to?

8:1

What is the main disadvantage to air-gap technique?

Image magnification

What still needs to be increased with air-gap technique?

mAs

At what rate does mAs need to be increased with air-gap technique?

10% for every cm

When was computed radiography introduced?

1980s by Fujifilm Medical

What other names did the computed radiograph have?

“Photostimulable Phosphor Plate Radiography”

PSP plate radiography

What is photostimulable luminescence?

Receiving x-rays and storing the energy as trapped electrons, then releasing them as light when exposed to a different light source

How is Photostimulable luminescence accomplished?

Photostimulable phosphors (PSPs) in the plate

What are storage phosphor screens?

Latent image is stored for a period

What needs to occur once an image plate is exposed and the latent image is formed?

Stimulated to obtain the manifest image

What causes the PSP to emit light?

Finely focused laser beam

In what light region does the PSP emit light?

Blue

What is used to clear the image plate to prepare for a new image?

Intense white light

What contains the laser photomultiplier tubes and photodiodes?

Computed radiography reader

What does the Computed radiography reader do?

Removes PSP plate, scan it, erase it, and replace it

When was digital radiography introduced?

1980s by Fujifilm Medical after CR

What does digital radiography not have compared to computed radiography?

No film

No PSP

What does digital radiography use?

Flat panel detectors (FPDs)

What is direct digital radiography?

Converts x-ray photons into electrical signal

What is indirect digital radiography?

Converts x-ray photons into visible light, then into an electrical charge

What are charge-coupled device detectors? (CCDs)

Use indirect conversion of x-ray photons into visible light and then into electrical charge

What do direct flat panel detectors use?

Amorphous selenium photoconductor to convert x-rays into electrical signals

What processes the electrical signals in direct flat panel detectors?

Thin film transistor (TFT)

What kind of flat panel detectors are most commonly used in mammography where high resolution is required?

Direct

What does indirect flat panel detectors use?

Phosphor layer for producing visible light, which is read by a photodiode

What is a downside to CCD detectors?

Potential to produce high amount of image noise due to demagnification process

What is bit depth?

System is the number of discrete sampling levels used to interpret and convert analog signal into a digital one

What does a high bit depth allow for?

More accurate representation of the incoming signal

Requires more memory

What represents the number of bit depth levels?

2^n

n = image bit size

2^16 (bit) = 65, 536 levels

Why is a small bit ranges beneficial?

Accuracy

What is another way to make bits smaller?

Shrink total range of energy values

Reduces dynamic range

What is the dynamic range of detector?

Range of exposure values over which the signal will be properly digitized

What will result from a low dynamic range?

Limit range of exposure values able to be represented

What will result from a high dynamic range?

Each bit covering a greater exposure value range

What is a DICOM image file

Standardized image file type for medical imaging

Digital Imaging and Communications in Medicine

What is the filename extension for a DICOM file?

.dcm