Chapter 11

Off-center grid

- A grid that is perpendicular to the CR, but shifted laterally can produce this type of grid cutoff - Mainly a problem with focused grids... Any lateral shift results in grid cutoff across the entire radiograph... Known as lateral decentering - A result of poor positioning of the x-ray tube rather than the grid. - Grid cutoff across image; underexposed, light image

Off-focus grid

- Arises when radiographs are taken at SIDs unspecified for that grid - Grid cutoff is more severe toward the edges of the image

Focused grid

- Designed to minimize grid cutoff - Lead strips coincide with the divergence of the x-ray beam - Exhibit no grid cutoff - Usually are moving grids

Off-level grid

- Grids should be perpendicular to the CR - Produced with an improperly positioned x-ray tube... if the CR is not perpendicular to the plane of the grid, grid cutoff will occur - Grid cutoff across image; underexposed, light image

Parallel grids

- all lead strips run parallel - attenuation of primary x-rays becomes greater as the x-rays approach the edge of the IR. - Can result in grid cutoff - Grid cutoff is most pronounced when the grid is used at a short SID, or with a larger-area IR

Crossed Grid

- have lead grid strips that run parallel to the long and short axes of the grid. - More efficient in cleaning up scatter than parallel grids - Positioning the grid is critical (CR must coincide with the center of the grid) - Tilt-table techniques are possible only if the x-ray tube and the table are properly aligned. - The exposure technique required is substantial and results in a higher patient dose.

Upside-down grid

- shows severe grid cutoff on either side of the CR - Resultant radiograph appears dark on one side and light on the other.

Grid Selection Factors

1.) Pt dose increases with increasing grid ratio 2.) High-ratio grids are used for high-kVp examinations 3.) The Pt radiation dose at high kVp is less than that at low kVp

Which kVp selection would result in the most scattered x-rays in the image-forming beam? A) 90 kVp B) 75 kVp C) 60 kVp D) 50 kVp

A) 90 kVp pg. 188: when kVp is increased, the level of scatter radiation also increases, leading to reduced image contrast.

Beam restriction with a(n)_____ is only accurate at a fixed SID. A) aperture diaphragm B) cone C) collimator D) PBL device

A) aperture diaphragm pg. 192

The use of _____ improves contrast and reduces patient dose. A) collimation B) high kVp C) low kVp D) less filtration

A) collimation pg. 187: collimation reduces patient dose and improves contrast resolution.

Decreasing the kVp will increase _____. A) patient dose B) image noise C) Compton scatter D) optical density

A) patient dose pg. 187: radiographs taken at the lowest, reasonable kVp would result in minimum scatter and thus higher image contrast. However, the resulting increase in photoelectric absorption results in a considerable increase in patient dose... fewer x-rays reach the IR at low kVp, which is compensated for by increasing the mAs = higher patient radiation dose.

The x-rays that are transmitted through the patient without interaction contribute to _____. A) useful information B) film fog C) image noise D) all of the above

A) useful information

Federal regulations passed in _____ required that positive beam limiting devices be on all manufactured x-ray collimators. A) 1954 B) 1974 C) 1994 D) 2000

B) 1974 pg. 194: PBL was mandated by the U.S. Food and Drug Administration in 1974. That regulation was removed in 1994.

How can you improve image contrast with a heavy patient without increasing patient dose? A) Lower kVp and raise mAs. B) Use tight collimation. C) Raise kVp and lower mAs. D) Do both A and B.

B) Use tight collimation. pg. 193: collimation reduces patient dose and improves contrast resolution.

Scatter radiation increases as _____ increases. A) photoelectric absorption B) field size C) filtration D) contrast

B) field size pg. 189: as field size is increased, scatter radiation also increases.

Compton scatter contributes to _____. A) useful information B) image noise C) image contrast D) all of the above

B) image noise pg. 187: Compton scatter x-radiation produces noise, reducing image contrast and contrast resolution. It makes the image less visible. pg. 153: scattered x-rays result in image noise, a generalized dulling of the image by x-rays not representing anatomy.

The most commonly used beam restricting device is the _____. A) extension cone B) variable collimator C) aperture diaphragm D) compression device

B) variable collimator pg. 193: the light-localizing variable-aperture collimator is the most commonly used beam-restricting device in radiography.

A fixed aperture diaphragm should leave a _____ cm unexposed border on the film. A) 0.1 B) 0.5 C) 1 D) 2

C) 1 pg. 192: fixed aperture diaphragms - are designed to expose all of the IR except for a 1 cm border.

The use of a compression device will increase _____. A) film fog B) patient dose C) contrast D) scatter

C) contrast pg. 190: compression of anatomy improves spatial resolution and contrast resolution and lowers the patient radiation dose.

The three primary factors influencing the intensity of scatter in the image-forming beam are _____. A) mAs, kVp, and collimation B) mAs, filtration, and grids C) kVp, field size, and patient thickness D) filtration, patient thickness, and mAs

C) kVp, field size, and patient thickness pg. 187: three primary factors influence the relative intensity of scatter radiation that reaches the IR: kVp, field size, and patient thickness.

The x-ray interaction that contributes to the clear parts of the image is _____. A) coherent scatter B) Compton interaction C) photoelectric interaction D) pair production

C) photoelectric interaction pg. 187: photoelectric interaction = x-rays that interact with the patient and are totally absorbed.

The positive beam limiting device (PBL) assures that the x-ray beam is collimated to _____. A) the exact part size B) 1 cm inside image receptor size C) the image receptor size D) 1 cm outside image receptor size

C) the image receptor size pg. 194: APBL - a signal transmitted to the collimator housing actuates the synchronous motors that drive the collimator leaves to a pre-calibrated position, so the x-ray beam is restricted to the IR in use.

Approximately _____% of the x-ray beam incident on the patient is transmitted through the patient without interaction to become part of the image-forming beam. A) 90-99 B) 80-89 C) 20-29 D) 0-9

D) 0-9 pg. 188: Approximately 1% of the x-ray beam incident on the patient reaches the IR. See also table 11-1 "percent transmission"

Contrast resolution is improved by _____. A) tight collimation B) lowering kVp C) patient compression D) all of the above

D) all of the above pg. 187: collimation reduces patient dose and improves contrast resolution. pg. 190: reduced image contrast results from scattered x-rays. pg. 190: compression of anatomy improves spatial resolution and contrast resolution and lowers the patient radiation dose.

Image-forming x-rays include those which have been _____. A) transmitted without interaction B) scattered through Compton interaction C) absorbed through photoelectric interaction D) both A and B

D) both A and B pg. 187: Two types of x-rays are responsible for the optical density (OD) and contrast on a radiographic image: those that pass through the patient without interaction and those that are Compton scattered within the patient. pg. 191: in a normal situation, however, image-forming xrays consist of both transmitted and scattered x-rays.

Lowering kVp _____ patient dose and _____ image contrast. A) lowers, decreases B) increases, decreases C) lowers, increases D) increases, increases

D) increases, increases pg. 187: radiographs taken at the lowest, reasonable kVp would result in minimum scatter and thus higher image contrast. However, the resulting increase in photoelectric absorption results in a considerable increase in patient dose... fewer x-rays reach the IR at low kVp, which is compensated for by increasing the mAs = higher patient radiation dose.

Photoelectric interactions increase when _____ is decreased. A) mAs B) patient thickness C) filtration D) kVp

D) kVp pg. 187: as x-ray energy increases, the absolute number of Compton interactions decreases, but the number of photoelectric interactions decreases much more rapidly. Therefore, the relative number of x-rays that undergo Compton scattering increases.

T/F. Patient radiation dose decreases with increasing grid ratio

False pg. 195: high-ratio grids increase the patient radiation dose. pg. 196: the use of high-frequency grids requires a high radiographic technique and results in a higher patient radiation dose.