Chapter 7: Quality Assurance

Question 1

List the three basic tenets of a quality assurance program.

Answer 1

Quality assurance programs should be designed around three basic concepts:
1. The tests that make up the program must be performed on a regular basis
2. The results from all tests must be recorded using a consistent format
3 . Documentation should indicate whether the tested parameter is within specified guidelines

Question 2

TRUE or FALSE

Most phantoms are designed with many components so that a single phantom can be used to examine a broad range of scanner parameters.

Answer 2

TRUE

Question 3

the American College of Radiology (ACR) CT accreditation phantom is a solid phantom that contains four modules, and is constructed primarily from a water-equivalent material. Each module is 4 cm deep and 20 cm in diameter, with external alignment markings to allow centering of the phantom in the x, y, and z axes. What are the four modules?

Answer 3

Module 1 is used to assess positioning and alignment, CT accuracy and slice thickness.
Module 2 is used to assess low-contrast resolution. Module 3 is used to assess CT number uniformity and the accuracy of in-plane distance measurements. Module 4 is used to assess spatial resolution

Question 4

Why is low-contrast resolution evaluated at different mAs settings?

Answer 4

The phantoms are scanned at different mAs settings. The most common method requires an observer to detect objects as distinct. At the minimum, contrast resolution should be such that with a density difference of 0.5% a 5-mm object can be displayed. This test is performed monthly in most quality assurance programs.

Question 5

Explain how noise is measured using a water phantom.

Answer 5

Noise is measured by obtaining the standard deviation (SD) of the CT numbers within a region of interest (ROI).
For noise measurements, the standard deviation of an ROI in a water phantom should not exceed 10. For uniformity measurements, there should be no more than a ±2 HU variation from an ROI placed at the center of the water phantom to those placed at the periphery. These tests should be performed on a weekly basis.

Question 6

Refers to the ability of the scanner to yield the same CT number regardless of the location of an ROI within a homogeneous object. Like noise, it is most commonly measured using a water phantom

Answer 6

Uniformity

Question 7

This refers to the relationship between CT numbers and the linear attenuation values of the scanned object at a designated kVp value.

Answer 7

Linearity

Question 8

What tools do medical physicists use to measure radiation dose?

Answer 8

Measurements are made using standard head and body CT dose index (CTDI) phantoms and a pencil ionization chamber. Using the CTDI measurement, the physicist must calculate several descriptors of dose for an adult head, pediatric abdomen, and adult abdomen examination. These measurements are compared with reference dose values.

Question 9

Any object seen on the image that is not present in the object scanned is considered an _________.

Answer 9

Artifact

Question 10

The best strategy available to the operator to avoid beam hardening is to

Answer 10

select the appropriate SFOV to ensure the correct filtration, calibration, and beam-hardening correction software is used.

Question 11

What factors affect beam hardening artifact?

Answer 11

☑️Composition of part examined

☑️extent the beam must travel through various tissues

Question 12

The best strategy available to the operator to avoid beam hardening is to

Answer 12

☑️select the appropriate SFOV to ensure correct filtration, calibration and beam hardening correction software is used.

Question 13

List strategies to reduce or eliminate motion artifacts?

Answer 13

Voluntary motion
☑️reduce or eliminated by adequately preparing the px for examination
☑️explaining procedure - px understands
☑️ positioning aids/immobilization devices

Involuntary
☑️shortest scan time possible

Question 14

What steps can be taken to avoid out of field artifacts? When are these artifacts unavoidable?

Answer 14

☑️can be avoided when an SFOV can be selected that is larger than the px
☑️encouraging px to raise their arms out of the way of the SFOV will also avoid artifact
☑️unavoidable for exceptionally large px’s who exceed even at largest SFOV

Question 15

Occurs when more than one tyoe of tissue is contained within a voxel

Answer 15

Partial volume effect

Question 16

Occurs when a dense object lies to the edge of FOV.

Answer 16

Partial volume artifact

Question 17

Best method of reducing partial volume artifacts is to

Answer 17

Used thinner slices

Question 18

Aliasing artifacts can be combated by

Answer 18

☑️Slowing gantry speed rotation (increasing scan time)

☑️By reducing the helical pitch