Computed Tomography: Scan Modes, Image Quality

Question 1

What is Topograms?

Answer 1

Done prior to CT scanning to select the position of scan regions

X-ray tube is kept in a fixed angular position

Question 2

What is the result of Topogram?

Answer 2

Digital survey radiograph with low spatial resolution

Question 3

What happens in a sequential CT?

Answer 3

after scanning a single slice over 360

the patient is transported for a defined distance or scan increment

Question 4

What continues to require the capability for single slice sequential scanning?

Answer 4

Dynamic CT and interventional CT

Question 5

What are the scan parameters generally used?

Answer 5

Kilovoltage: from 80 kVp to 140 kVp
Tube current: from 10 mA to 800 mA
Slice thickness S: from 0.5 mm to 10 mm
Scan time per 360o rotation: 0.3 s - 2 s
Number of simultaneous slices M: 1 to 128
Scan increment: arbitrary, mostly equal to total slice collimation M x S
Scan range: 1 to 100 cm

Question 6

What is dynamic CT used to record?

Answer 6

Temporal changes in the attenuation characteristics of the object examined

Question 7

What is dynamic CT used for in the clinical applications?

Answer 7

Assessment of contrast medium dynamics in various body regions

Question 8

What is the process of dynamic CT typically?

Answer 8

1. A representative slice, or M slices for multi-slice scanners are selected
2. Contrast medium is adminstered
3. The selected slices are scanned repeatedly

Question 9

What does the time interval between scans depend on?

Answer 9

physiological process to be examined and the type of contrast medium

Question 10

How are monitoring interventions by CT done?

Answer 10

in real time with continuous scanning of a slice or a number of slices (for multi-slice CT) with the fastest possible reconstruction and presentation of images

Question 11

What is performed under CT guidance?

Answer 11

1. Biopsies
2. Drainage’s
3. Numerous other therapeutic interventions

Question 12

What allows for data acquisition for entire head or chest?

Answer 12

Transition from fan beam geometry to cone beam geometry

Question 13

What is the principal problem inherent in cone beam geometry?

Answer 13

details in the object are projected onto different detector rows for different projection angles, depending on their distance from the central plane

Question 14

What allows the assessment of vascular anatomy and pathology, but not of soft tissues, which exhibit low contrast?

Answer 14

3D cone beam CT angiography

Question 15

What does spiral scans encompass?

Answer 15

many rotations of the tube-detector system, while the patient is transported continuously through the gantry.

Question 16

What is pitch of great importance for?

Answer 16

1. Image quality

2. Dose considerations

Question 17

What does the choice of pitch also depend on?

Answer 17

the available algorithms and parameters for image reconstruction.

Question 18

What does the spatial resolution describe?

Answer 18

the capability of an imaging system to display fine details separately

Question 19

What is spatial resolution generally determined for?

Answer 19

High-contrast structures in order to eliminate the influence of noise in measurement

Question 20

What is necessary to distinguish between?

Answer 20

The resolution in the x/y-scan plane and the resolution in the z direction

Question 21

What are the possibilities to increase signal resolution?

Answer 21

1. Choosing a small focus

2. Choosing a high-resolution image reconstruction kernel

Question 22

What can be used to visually assess high contrast resolution in the scan plane?

Answer 22

A bar pattern test object (shown below) containing lines of high contrast

Question 23

The resolution along the Z-axis

Answer 23

mostly limited to specifying the slice thickness

Question 24

What can be used to visually assess low contrast resolution?

Answer 24

Low contrast detail detect ability test objecy

Question 25

How cam you increase contrast resolution?

Answer 25

Decrease image noise and use contrast agents