Scatter Control Fauber Pages 151-174

What is scatter radiation?

Scatter radiation is primarily the result of Compton interactions, where an incoming x-ray photon loses energy and changes direction.

How does scatter radiation affect radiographic images?

Scatter radiation adds unwanted exposure (fog) to the image without providing anatomical information, negatively impacting image quality.

What factors affect the amount and energy of scatter radiation exiting the patient?

The kilovoltage peak (kVp) and the volume of irradiated tissue.

What is the purpose of beam-restricting devices?

To decrease the x-ray-beam field size and the amount of tissue irradiated, thereby reducing scatter radiation production.

What is collimation?

Collimation refers to the decrease in the size of the projected radiation field, often accomplished with beam-restricting devices.

What is the relationship between kVp and scatter radiation?

Higher kVp examinations produce a greater proportion of higher-energy scattered x-rays compared to lower kVp examinations.

What is the air gap technique?

A method used to reduce scatter radiation reaching the image receptor by increasing the distance between the patient and the receptor.

What is a radiographic grid?

A device used to improve radiographic image quality by absorbing scatter radiation exiting the patient.

What does grid cutoff refer to?

Grid cutoff occurs when the grid is improperly aligned, resulting in a loss of image density.

What is the moiré effect in radiography?

An artifact that occurs when the grid lines interfere with the digital image, causing a wavy pattern.

What is the Bucky factor?

A measure that indicates how much the exposure must be increased when using a grid compared to using no grid.

What is the grid conversion factor (GCF)?

A factor used to calculate the necessary increase in exposure when switching from a non-grid to a grid technique.

What is the function of a focused grid?

To allow for better alignment of the x-ray beam with the grid lines, reducing grid cutoff.

What are the types of beam-restricting devices?

Aperture diaphragms, cones, cylinders, and collimators.

What is the relationship between beam restriction and patient dose?

As beam restriction increases, the field size and patient dose decrease; as it decreases, the field size and patient dose increase.

What is the purpose of automatic collimators?

To automatically adjust the x-ray beam size to match the dimensions of the image receptor.

What is a crossed grid?

A type of grid that has two sets of grid lines oriented at right angles to each other, improving scatter absorption.

What is the significance of grid frequency?

Grid frequency refers to the number of grid lines per unit length, affecting the grid's ability to absorb scatter.

What is the difference between focused and nonfocused grids?

Focused grids are designed to match the divergence of the x-ray beam, while nonfocused grids do not.

How does the thickness of the part being imaged affect scatter radiation?

Thicker parts increase the volume of irradiated tissue, leading to greater scatter radiation production.

What is the role of interspace material in grids?

Interspace material separates the lead strips in a grid, allowing for better absorption of scatter radiation.

What is the long dimension in grid design?

The orientation of the grid lines that is parallel to the long axis of the x-ray beam.

What is the short dimension in grid design?

The orientation of the grid lines that is perpendicular to the long axis of the x-ray beam.

What is a variable aperture?

A beam-restricting device that allows for adjustment of the x-ray field size.

What is the purpose of a grid cap?

To cover the image receptor and hold the grid in place during radiographic procedures.

What is a wafer grid?

A thin, flat grid that can be placed directly on the image receptor to absorb scatter radiation.

What is the purpose of beam restriction in radiography?

To limit the x-ray field size to just beyond the area of interest, protecting patients from unnecessary exposure.

How does collimation affect scatter radiation?

Increasing collimation decreases the x-ray field size and the quantity of scatter radiation produced.

What is the relationship between collimation and radiographic contrast?

As collimation increases, the quantity of scatter radiation decreases, resulting in increased radiographic contrast.

What happens to exposure to the image receptor (IR) when collimation is increased?

Exposure to the IR decreases as collimation increases.

What is the recommended increase in mAs when significant collimation is applied?

It is recommended to increase mAs by 30% to 50% to compensate for decreased IR exposure.

What is an aperture diaphragm?

The simplest type of beam-restricting device, made of a flat piece of lead with an opening to limit the x-ray field.

What are cones and cylinders in radiography?

Beam-restricting devices that extend the aperture diaphragm to limit unsharpness surrounding radiographic images.

What is the most sophisticated type of beam-restricting device used in radiography today?

The collimator, also known as a variable aperture.

What is the effect of postexposure electronic masking on patient exposure?

It does not reduce scatter production, improve image contrast, or reduce patient radiation exposure.

How does increasing collimation affect the volume of tissue irradiated?

Increasing collimation decreases the volume of tissue irradiated.

What is the significance of the relationship between collimation and scatter radiation?

As collimation increases, scatter radiation production decreases, enhancing image quality.

What is the primary function of beam-restricting devices?

To absorb x-rays and limit the exposure area, thereby reducing patient dose.

What is the disadvantage of using an aperture diaphragm?

It cannot be adjusted from the designed size, leading to a large area of unsharpness around the image.

What is the role of entrance shutters in a collimator?

They limit the x-ray beam similar to how an aperture diaphragm would.

What happens to patient dose when the x-ray field size is increased?

Patient dose increases as the x-ray field size increases.

Why should preexposure collimation not be replaced by postexposure electronic masking?

Because it does not effectively reduce scatter production or improve image quality.

What is the effect of decreased collimation on radiographic contrast?

Decreased collimation increases scatter radiation and decreases radiographic contrast.

What materials are beam-restricting devices typically made of?

Most are made of metal or a combination of metals that readily absorb x-rays.

How do adjustable cylinders benefit sinus imaging?

They allow collimation closer to the patient's skin surface, reducing exposure and improving image quality.

What is the impact of collimation on the number of x-ray photons reaching the IR?

Increasing collimation decreases the number of x-ray photons that reach the IR.

What is the relationship between field size and scatter radiation intensity?

As field size increases, the relative quantity of scattered radiation increases.

What is the primary benefit of using collimation in radiographic procedures?

It enhances image quality by reducing scatter radiation and improving contrast.

What should be included in the interpretation by the radiologist?

All anatomy that has been irradiated.

What is the effect of collimation on the number of photons striking the patient?

Increasing collimation decreases the number of photons that strike the patient.

What is the primary purpose of beam restriction?

To minimize patient radiation exposure and improve image quality.

What is the function of entrance shutters in an x-ray collimator?

They limit the x-ray beam similar to an aperture diaphragm.

Where are the adjustable lead shutters located in relation to the x-ray tube?

8 to 18 cm (3-7 inches) below the tube.

What shape is the x-ray field produced by a collimator?

Rectangular or square, unless modified by an aperture diaphragm, cone, or cylinder.

What is the purpose of the white light source in a collimator?

To project a light field onto the patient, indicating where the primary x-ray beam will be projected.

What should be done if the light field in a collimator fails?

Refer to the x-ray field measurement guide present on the front of the collimator.

What is the maximum acceptable misalignment for collimator settings?

Less than ±2% of the Source-to-image receptor distance (SID).

What is an automatic collimator?

A device that automatically limits the size and shape of the primary beam to match the size and shape of the image receptor (IR).

What is the purpose of positive beam-limiting devices?

To protect patients from overexposure to radiation by limiting the x-ray field size.

What is the significance of the radiographic grid?

It limits scatter radiation that reaches the image receptor, improving radiographic contrast.

Who invented the radiographic grid and when?

Gustave Bucky in 1913.

What is the typical thickness of a radiographic grid?

Approximately 0.625 cm (0.25 inch).

What are the drawbacks of using a radiographic grid?

They require additional mAs, resulting in a higher patient dose.

When should grids be used in radiography?

When the anatomic part is 10 cm (4 inches) or greater in thickness and more than 60 kVp is needed.

How do radiographic grids work to improve image quality?

They absorb scatter radiation while allowing transmitted photons to reach the image receptor.

What materials are typically used for the interspace in grids?

Aluminum or carbon fiber composite.

What is grid frequency?

The number of lead lines per unit length, typically expressed in lines/cm or lines/inch.

What is grid ratio?

The ratio of the height of the lead strips to the distance between them.

What is the relationship between scatter radiation and image quality?

Scatter radiation adds unwanted exposure (fog) to the image receptor and decreases image quality.

What is the purpose of the plastic template with crosshairs in collimation?

To indicate where the center of the primary beam (central ray) will be directed.

What happens if the x-ray central ray is not perpendicular to the table?

Radiographic quality may be compromised.

What is the role of the mirror in a collimator?

To reflect the light down toward the patient, indicating the projected light field.

What is the potential issue with the light field accuracy in collimators?

The mirror or light bulb may be slightly out of position, leading to inaccurate projections.

What is the function of the collimator and beam alignment test tool?

To evaluate proper alignment of the collimator and beam.

What is a common feature of modern x-ray tube heads?

Displays of SID, x-ray field dimension, and video image of the patient anatomy.

What does SID stand for?

Source-to-image receptor distance.

What is the purpose of the override mechanism in automatic collimators?

To allow the radiographer to disengage the automatic collimation feature.

Why is it important to limit the x-ray field size to the anatomic area of interest?

To prevent unnecessary radiation exposure to the patient.

How is grid ratio mathematically expressed?

Grid ratio = h/D

What is the grid ratio when the lead strips are 2.4 mm high and separated by 0.2 mm?

12 or 12:1

What is the typical range of grid ratios?

From 5:1 to 16:1.

How does a high-ratio grid affect scatter radiation?

It removes more scatter radiation than lower-ratio grids, increasing radiographic contrast.

What is the relationship between grid ratio and radiographic contrast?

As grid ratio increases, scatter cleanup improves and radiographic contrast increases.

What information is typically found on a grid's label?

Type of interspace material, grid frequency, grid ratio, grid size, and SID range.

What are the two types of grid patterns?

Linear and crossed (crosshatched).

What is a linear grid?

A grid with lead lines that run in only one direction.

Why are crossed grids limited in application?

Because the x-ray tube cannot be angled without producing grid cutoff.

What is grid cutoff?

Absorption of transmitted x-rays due to misalignment with the grid.

What is the difference between parallel and focused grids?

Focused grids have lead lines angled to match the divergence of the primary beam, allowing more transmitted photons to reach the IR.

What is the focal distance of a focused grid?

The distance between the grid and the convergent line or point.

What is the focal range of a focused grid?

The recommended range of SIDs that can be used with a focused grid.

What are stationary grids?

Grids that do not move during the radiographic procedure, such as wafer grids and grid cassettes.

What is a moving or reciprocating grid?

A grid that moves during the x-ray exposure to blur grid lines and improve image quality.

What is the difference between long-dimension and short-dimension grids?

Long-dimension grids have lead strips running parallel to their long axis, while short-dimension grids have strips running perpendicular.

Why might a short-dimension grid be used?

When it is difficult to correctly center or angle the central ray for a long-dimension grid.

What is the purpose of using grids in radiography?

To reduce scatter radiation reaching the image receptor (IR) and increase image contrast.

What happens to the total amount of x-rays reaching the IR when using a grid?

It reduces the total amount of x-rays reaching the IR.

How is the GCF mathematically expressed?

GCF = mAs with the grid / mAs without the grid.

What is the relationship between grid ratio and exposure to the IR?

As the grid ratio increases, exposure to the IR decreases; as the grid ratio decreases, exposure to the IR increases.

What is the GCF for a 5:1 grid ratio?

2