CT Ch. 1 Quizzes

The main advantages of CT over conventional radiography are the:

- Elimination of superimposed structures - The ability to differentiate small differences in density of anatomic structures and abnormalities - Superior quality of the images

An object is slightly less dense than water. What is the expected Hounsfield measurement?

-10

How many pixels are contained in a 1024 matrix?

1,048,576

What is the thickness of a cross-sectional CT slice called?

A CT slice represents a specific plane in the patient's body. The thickness of the plane is referred to as the Z axis.

___________ refers to movement towards the sides of the body.

All directional terms are based on the body being viewed in the anatomic position. The term lateral refers to movement toward the sides of the body.

What are image artifacts?

Artifacts are objects seen on the image but not present in the object scanned. They always degrade the quality of the image.

__________ artifacts appear as dark streaks or vague areas of decreased density on a CT image.

Artifacts that result from preferential absorption of the low-energy photons, which leaves higher-intensity photons to strike the detector array, are called beam-hardening artifacts.

What is another name for step-and-shoot scan mode?

Axial Conventional Serial

Which of the following is a logical consideration when determining an appropriate slice thickness for studies of the internal auditory canal? - The thickest slice available should be used to reduce the radiation dose to the radiosensitive tissues of the eye. - Because the auditory ossicles are quite small, a thin slice will be necessary to reduce the chance that volume averaging will obscure their appearance on the image. - The appropriate slice thickness will vary considerably from patient to patient. Therefore, each technologist should be free to adjust the slice thickness as he or she deems necessary for the particular patient. - A slice thickness of 5 to7 mm is adequate because the structures of interest are not particularly small and the examination is most often ordered as a screening study for asymptomatic patients.

Because the auditory ossicles are quite small, a thin slice will be necessary to reduce the chance that volume averaging will obscure their appearance on the image.

What can be used to create a temporary artificial density difference between two objects on a CT image?

Contrast agents fill a structure with a material that has a different density the structure itself.

What is density?

Density is the mass of a substance per unit volume. Put more simply, it is the degree to which matter is concentrated.

(T/F) The larger the object being scanned, the thinner the CT slice required.

False

(T/F) To differentiate adjacent objects on a CT image, there cannot be a density difference between the two objects.

False

Which is an advantage of filtering the x-ray beam?

Filtering reduces the radiation dose to the patient.

Which of the following is not a scan mode? First level controlled operating mode. Spiral (Helical) Multidetector Row Step and Shoot (Axial)

First level controlled operating mode. This is an MRI operating mode.

What is the name for the area of the anode where the electrons strike and the x-ray beam is produced?

High voltage propels the electrons from the x-ray tube filament to the anode. The area of the anode the electrons hit and where the x-ray beam is produced is called the focal spot.

What determines the shades of gray on a CT image?

In CT, the x-ray beam passes through the patient's body and is recorded by detectors. Then the computer creates the CT image. The quantity of x-ray photons that pass through the body determine the shades of gray on the image.

Which is a drawback of using thicker CT slices when imaging? - They increase the likelihood of missing very small objects. - They increase the number of slices that are produced. - They increase radiation dosage. - They decrease the pixel size.

In general, the smaller the object being scanned, the thinner the CT slice required. Thicker slices increase the likelihood of missing very small objects on the scan.

In which part of the CT process are the data converted into shades of gray for viewing?

In the creation of the CT image, the processed data are converted into shades of gray for viewing during the image display phase.

Which of the following is a low attenuation structure?

Lungs

What are used to quantify the degree that a structure attenuates an x-ray beam?

Measurements of the beam attenuation capability of an object are expressed in Hounsfield units (HU), named after Godfrey Hounsfield, one of the pioneers in the development of CT. Hounsfield units are also referred to as CT numbers, or density values.

How are metals represented on a CT image?

Metals are dense and have the greatest capacity for beam attenuation. Consequently metals are represented as white on a CT image.

Which is an alternate name for raw data?

Scan data

__________ resolution describes the ability of a system to define small objects distinctly.

Spatial resolution is one of the criteria for evaluating CT image quality.

Which is NOT a synonym for the preliminary (or localizer) image taken at the start of the CT examination?

Spiral

Why does using a small pixel size reduce the likelihood of volume averaging?

The X and Y dimensions of the pixel affect the likelihood of volume averaging. The larger the X and Y dimensions the larger the pixel and therefore the more likely that the pixel will contain tissues of different densities causing volume averaging.

What is the term for the ability of an x-ray tube to withstand resultant heat?

The ability of the x-ray tube to withstand the resultant heat is called its heat capacity.

The terms ___________ and/or ___________ are used interchangeably to refer to the computer data waiting to be processed to create an image.

The bits of data acquired by the system with each scan are called scan data and/or raw data.

What is the most common matrix size in CT?

The most common matrix size is 512. This means there are 512 rows of pixels down and 512 columns of pixels across.

What is the primary reason for changing the image plane from axial to coronal? What is the secondary reason?

The primary reason is when the anatomy of interest lies vertically rather than horizontally. The secondary reason is to reduce artifacts created by surrounding structures.

What is the name of the process in CT by which different attenuation values are averaged to produce one less accurate pixel reading?

The process in CT by which different tissue attenuation values are averaged to produce one less accurate pixel reading is called volume averaging. The slice thickness has a significant effect on the degree of volume averaging that is present on an image. The X and Y dimensions of the pixel also affect the likelihood of volume averaging.

Which directional term refers to movement towards the feet?

The term inferior refers to movement toward the feet (down) and is synonymous with the term caudal.

Which directional term refers to movement towards the head?

The term superior defines movement toward the head (up) and is used interchangeably with the term cranial or cephalic.

What is the name for each two-dimensional square in a CT slice?

The word pixel is used for the term picture element. A composite of thou-sands of pixels creates the CT image that displays on the CT monitor.

(T/F) The degree to which an x-ray beam is reduced by an object is known as attenuation.

True

(T/F) The sagittal plane divides the body into right and left sections.

True

An object that appears on the image but is not present in the object scanned is called a (an)

artifact

When in anatomic position, the arms are

down by the sides, palms facing forward

The ability of a system to differentiate, on the image, objects with similar densities is known as:

low contrast resolution

How many CT numbers are assigned to each pixel in the image matrix?

one

An x-ray beam consists of bundles of energy known as ________.

photons

Each two-dimensional square of data that make up the CT image is called a:

pixel

The x-ray beam sources for CT produce x-ray energy that is polychromatic. This means that:

the beam is comprised of photons of varying energies

Beam attenuation can be defined as

the phenomenon by which an x-ray beam passing through a structure is decreased in intensity or amount because of absorption and interaction with matter