CT Principles part 1

Question 1

Name the steps that the xrays go through from generation to what is seen at the console

Answer 1

Xrays are produced by the tube, emitted, filtered, and collimated. Then xray penetration occurs (through the patient), passes through post patient collimators, striking detectors. The computer system then converts that to a digital signal that the computer processes.

Question 2

What is the relationship between the number of bits of the ADC and it’s accuracy?

Answer 2

Direct - the more bits the higher the accuracy

Question 3

What is the term that describes the reduction in intensity of the x-ray beam as it passes through the patient by either absorption or scatter?

Answer 3

Attenuation

Question 4

When discussing Beam Quality - how do homogeneous and heterogeneous differ?

Answer 4

A homogeneous beam is composed of photons with the same amount of energy while a heterogeneous beam possesses varying energies

Question 5

Name a term that describes a quantitative measurement of attenuation per cm of an absorber (ie. bone or soft tissue)

Answer 5

Linear Attenuation Coefficient

Question 6

What two main factors affect the Linear Attenuation Coefficient?

Answer 6

The energy of the beam and the atomic number of the absorber

Question 7

What is another name for ‘CT numbers’?

Answer 7

Hounsfield Units

Question 8

What is the HU of water?

Answer 8

zero

Question 9

What is the HU of average bone?

Answer 9

1000

Question 10

What is the HU of fresh blood?

Answer 10

20-50

Question 11

What is the HU of Air?

Answer 11

-1000

Question 12

What is the HU of fat?

Answer 12

-100

Question 13

What is meant by “beam hardening”

Answer 13

This is when low energy, long wavelength photos are absorbed as the xray beam passes through an object (such as a filter or the patient) leaving the remaining beam with a higher average energy or penetrating power.

Question 14

A single measurement of the x-ray attenuation along a path between the source and a detector is know as:

Answer 14

a ray or single ray

Question 15

What is a view?

Answer 15

A collection of rays from one viewing angle

Question 16

What is the equation for determining the total number of data samples?

Answer 16

Data samples = # of detectors x # of samples at each detector

Question 17

What is the formula to calculate volume coverage aka Z axis coverage?

Answer 17

pitch x collimation x scan time / gantry rotation time.

Question 18

The reduction in intensity of the x-ray beam as it passes through the patient by either absorption or scatter of photons from the beam is termed:

Answer 18

Attenuation

Question 19

A quantitative measurement of attenuation per cm of an absorber for example bone or soft tissue is called:

Answer 19

Linear attenuation coefficient

Question 20

The absorption of low energy, long wavelength x-ray photons within the anatomical structure with higher energy photons continuing through the body with an increasing mean beam energy or penetrating power is referred to as:

Answer 20

Beam Hardening

Question 21

The movement of the tube & detectors with x-ray transmission is termed:

Answer 21

scanning

Question 22

As a result of the scanning process, a single measurement of the x-ray attenuation along a path between the source and detector is termed:

Answer 22

ray

Question 23

A view generates a ______ which is an electric signal relative to the amount of attenuation that has occurred as the x-ray is transmitted through the patient.

Answer 23

Profile

Question 24

The number of detectors multiplied by the number of samples at each detector result in the:

Answer 24

Data Sample Total

Question 25

What year was helical (spiral) scanning introduced? [think Taylor Swift]

Answer 25

1989

Question 26

Gaps between slices, inter scan delay, variability of contrast media enhancement, inconsistent respiration or motion, and inaccurate 3D or reformations are all limitations of what kind of scanning?

Answer 26

Axial or step-and-shoot

Question 27

What is the term for the geometric corrections that take place to convert the fan beam information to parallel beam information?

Answer 27

interpolation

Question 28

What are three types of interpolation that are used in CT?

Answer 28

1. Fan beam to parallel beam interpolation 2. helical to single slice interpolation 3. helical to multislice interpolation

Question 29

Increasing pitch ______ coverage

Answer 29

increases

Question 30

Increasing pitch ______ overall scan time

Answer 30

reduces

Question 31

Increasing pitch ______ patient dose

Answer 31

reduces

Question 32

Increasing pitch ______ resolution in the slice (Z) direction.

Answer 32

reduces

Question 33

What is the definition of pitch in a single slice helical scan

Answer 33

PITCH = TABLE TRAVEL PER GANTRY ROTATION DIVIDED BY BEAM COLLIMATION

Question 34

What is the definition of pitch in a multislice helical scan?

Answer 34

Pitch - table travel per gantry rotation divided by the number of channels times the detector size

Question 35

How is pitch calculated differently in single slice CT vs MDCT?

Answer 35

Single slice is divided by the beam collimation where in MDCT you must divide by the (number of channels x detector size)

Question 36

How do you calculate Z axis coverage?

Answer 36

pitch x section thickness x total scan time

Question 37

What could be one negative consequence of reducing you scan time by increasing pitch?

Answer 37

You might miss small lesions or fractures.

Question 38

Gapless examination of Anatomy, rapid volume scanning, reduced exam time, reduced motion artifacts, reconstruction of arbitrary slices, no mis-registration due to respiration level changes, less contrast required, uniform contrast enhancement, better multiplanar and 3D images, allows CT fluoro, CT angio, CT endoscopy, 3DCT ARE ALL ADVANTAGES OF WHAT KIND OF SCANNING (helical or axial)?

Answer 38

Helical

Question 39

After x-rays are emitted from the CT x-ray tube, they pass through a ______ to shape the energy of the beam.

Answer 39

Beam filter

Question 40

Slice thickness is impaced by (a. collimator system b. beam filter system c. DAC d. ADC)

Answer 40

a. collimator system

Question 41

After the x-ray beam passes through the patient it is directed through the ______.

Answer 41

detector collimator

Question 42

The x-ray beam is picked up by the _____ device that measures the amount of attenuation that occurs.

Answer 42

detector