COMPUTED TOMOGRAPHY

Question 1

CT scan parameters can be altered or optimized in order to

Answer 1

– reduce patient radiation dose
– improve image quality

Question 2

This is the size of the FIELD in the gantry aperture.

Answer 2

SFOV Scan Field of View

Question 3

■ Tip: The field size is NOT the entire visible area you see within the aperture.
■ The size of the scan parameter tells the computer which data to use and which data to ignore.
■ The__ determines the number of detectors required to collect data for a particular procedure.
■ The __ should always be larger than the area of interest.

Answer 3

SFOV Scan Field of View

Question 4

When the anatomical area of the patient is OUTSIDE the SFOV, out-of field artifacts will occur. These artifacts can present as:

Answer 4

■ Streaking
■ Shading
■ Miss-assignment of CT numbers

Question 5

Too Large a SFOV may also produce artifacts such as:

Answer 5

■ Shading
■ Streaking at the skin surface

Question 6

Also known as the Reconstructed field of view (RFOV)
■ __ plus matrix size determine the limitations of perceived detail
■ __ should be equal to or smaller than the SFOV NEVER LARGER
■ The ___ also impacts image noise and resolution.
■ Wider __ increase the quantity of the photons from which data is retrieved.
■ Image noise is reduced, however, at the expense
of resolution

Answer 6

DFOV Display Field of View

Question 7

Determines the size of the image viewed on the monitor

Answer 7

DFOV Display Field of View

Question 8

is normally set to include other structures or pathology that may also be located in the scan plane.
■ ___ - best for imaging tissue types that carry greatly
■ ___ - tissues with similar densities

Answer 8

Window width; Wide window width (500- 2000 HU); Narrow window width (50- 500 HU)

Question 9

determines the center of the gray scale and is generally set at the average tissue density of the structures within the anatomy being scanned.

Answer 9

Window level

Question 10

Basically, the __ shows the range of densities you will be seeing, and the __ depicts the average density you will be seeing on the final reconstructed image.

Answer 10

width; level

Question 11

The manipulation of window width and window level to optimize image contrast is referred to as

Answer 11

windowing

Question 12

Width and Level: Head examinations

Answer 12

Posterior Fossa: 150-40
Brain: 100-30
Temporal bone: 2,800-600

Question 13

Width and Level: Neck examinations

Answer 13

Neck: 250-30

Question 14

Width and Level: Chest examinations

Answer 14

Mediastinum: 350-50
Lungs: 1,500- -600

Question 15

Width and Level: Abdominal examinations

Answer 15

Soft tissues: 350-50
Liver (high contrast): 150-30

Question 16

Width and Level: Pelvis examinations

Answer 16

Soft tissues: 400-50
Bones: 1,800 - 400

Question 17

Width and Level: Spine examinations

Answer 17

Soft tissues: 250-50
Bones: 1,800-400

Question 18

■ Primarily responsible for ensuring that the scanned anatomy is accurately represented
■ Secondarily responsible for enhancing the spatial or contrast resolution of the final image

Answer 18

Reconstruction Filters

Question 19

provide definitive borders and edges, used for high contrast areas, musculoskeletal (increases image noise)

Answer 19

High-pass (sharp) filters

Question 20

do not define borders and edges to the same extent as High-pass filters. Used for low contrast areas such as brain, abd. etc.

Answer 20

Low-pass (soft) filters

Question 21

■Primarily responsible for accurate anatomy representation on the final image
■ Secondary responsibility for enhancing the spatial resolution
■ High pass filters
■ Low pass filters

Answer 21

Reconstruction Filters

Question 22

■ Represent segment of the anatomy scanned
■ Contain numerical information based on tissue density
■ Two-dimensional representation
■ Slice thickness turns pixels into voxels (volume elements)
■ Expressed in __

Answer 22

Pixels; Hounsfield units

Question 23

Hounsfield units are __ to the attenuation coefficient of the tissue

Answer 23

directly proportional

Question 24

512 x 512 matrix has __ than a 1024 x 1024 matrix

Answer 24

fewer pixels

Question 25

= faster reconstruction = decreased information to be archived

Answer 25

Fewer pixel

Question 26

the __ the size of the pixel the less detail is presented on the image. __ the pixel the greater the detail.

Answer 26

larger; Smaller

Question 27

In general, __ slices produce sharper images because to create an image the system must flatten the scan thickness (a volume) into two dimensions (a flat image). The __ the slice, the more flattening is necessary

Answer 27

thinner; thicker

Question 28

To reduce image noise more __ is required.

Answer 28

mAs

Question 29

The slice thickness is determined by the:

Answer 29

■ Beam-width (BW)
■ Pitch
■ Shape and width of the reconstruction filter in the Z-axis

Question 30

Defined as the distance of couch top travel, per one revolution of the x-ray tube, divided by the slice thickness or the beam collimation.

Answer 30

Pitch

Question 31

A __ pitch, with increased overlap of anatomy and increased sampling at each location, results in an __

Answer 31

smaller ; increased radiation dose.

Question 32

__ suggests overlap between adjacent acquisitions, __ implies gaps between adjacent acquisitions, and __ suggests that acquisitions are contiguous, with neither overlap nor gaps

Answer 32

Pitch <1; pitch >1; pitch of 1

Question 33

Radiation dose is affected by the pitch of the __

Answer 33

helical scan.

Question 34

___ is associated with less image noise, fewer artifacts, and improved signal-to-noise and contrast-to noise ratios

Answer 34

Low-pitch technique

Question 35

■ 1 : 1 changed to 1.5 : 1 results in a __ in patient exposure.
■ May be significant when scanning pediatric patients.

Answer 35

33% reduction

Question 36

■ Newer scanners have __
■ The scanner detects the remnant radiation and will
reduce the mA accordingly.

Answer 36

“smart scan” or “smart mA”

Question 37

__ result in improved image quality, decreased image noise, and increased patient dose

Answer 37

Increase in tube current or the product of tube current and scan time (mAs)

Question 38

Is directly proportional to the mA (tube current).

Answer 38

Patient Dose

Question 39

have the ability to produce more than one set of reconstructed images using the same Raw Data

Answer 39

Scanners

Question 40

Remember not enough sampling causes artifact called __

Answer 40

aliasing

Question 41

■ The rate at which a CT machine samples data is selectable at purchase.
■ This rate does affect the scan time select ability.

Answer 41

Sampling and scan time

Question 42

Only half of the otherwise available data are available to reconstruct the image with partial scans; therefore, they are inferior to standard 360° scans.

Answer 42

180° (Partial scan)

Question 43

The most common selection in rotation arc

Answer 43

360°

Question 44

Allows some overlap of data from the first and last tube positions and reduces motion artifacts.

Answer 44

400° (Overscan)

Question 45

the __ cause more geometric unsharpness in the image and reduce spatial resolution.

Answer 45

larger focal spots