Chapter 4 ( Physics)

Question 1

describe the general process of CT imaging

Answer 1

Data acquisition
Data Reconstruction
Multidimensional Image display
image archival and communication

Question 2

azimuth

Answer 2

the angle of the tube and detectors in relationship to the patient position during scout acquisition

Question 3

What is housed in the CT gantry

Answer 3

the majority of the mechanical components including the generator, x-ray tube, assorted collimators, the data acquisition system (DAS), slip-rings, and the detectors

Question 4

focal spot size range

Answer 4

0.5 to 1.2mm

Question 5

flying focal spot technology

Answer 5

involves the electromagnetic steering of the electron beam emitted from the cathode. The beam is directed towards two seperate locations on the anode resulting in two sources of x-radiation

Question 6

Photon fluence

Answer 6

quantity of x-radiation

Question 7

photon flux

Answer 7

the rate at which the photon fluence passes through a unit area over unit time.

Question 8

effective mAs

Answer 8

calculated mAs value per acquired slice. effective mAs= mAs/Pitch

Question 9

streak artifact

Answer 9

occurs when x-rays pass through very dense parts. example:posterior fossa during head imaging

Question 10

kVp range of CT tube

Answer 10

80-140

Question 11

dual energy CT systems

Answer 11

as the single x-ray tube rotates around the patient, the kVp is varied at a very high rate.

Question 12

dual source CT systems

Answer 12

systems consisting of two completely seperate x-ray tubes and detector arrays positioned 90 Degrees from each other.

Question 13

beam hardening

Answer 13

as the beam traverses the patient, low energy photons are absorbed first increasing the average intensity of the beam as it travels along its path. This change in beam quality can have an artifactual result on the image.

Question 14

What type of generators are used?

Answer 14

high frequency, power output = 60-100 kW

Question 15

two distinct components of Multi Slice C T (MSCT)

Answer 15

beam collimation

detector collimation

Question 16

beam collimation

Answer 16

restricts the beam to the are of interest. reduces dose, improves quality.

Question 17

section width

Answer 17

also called the slice, directly controlled by beam collimation

Question 18

section interval

Answer 18

describes the spacing between two adjacent CT images. Measure by the distance from the center of one section to the center of the next

Question 19

contiguous images

Answer 19

acquired with equal section thickness and interval.. example: 5X5

Question 20

overlapping images

Answer 20

section interval is less than section width. 1mm X 0.5mm =50% overlap

Question 21

detector pitch formula

Answer 21

pitch= table feed per rotation/section width

Question 22

beam pitch formula

Answer 22

beam pitch=table feed per rotation/total collimation

Question 23

total collimation formula

Answer 23

total collimation= No. of sections X section width

Question 24

what type of ionization chambers were primarily used for third-generation scanners?

Answer 24

gas

Question 25

gas ionization chambers

Answer 25

detectors contain small high-pressure chambers of xenon gas. X-rays cause ionize the xenon gas and cause the chambers to discharge small electrical signals.

Question 26

modern MSCT system detectors

Answer 26

solid state detectors consisting primarily of a scintillating crystal material. Crystal emits a quantity of light energy proportional to incident x-rays. light is measured by a photodiode device and converted to an electronic signal.

Question 27

Crystal materials

Answer 27

cesium iodide, cadmium tungstate, bismuth germanate, rare earth materials…etc

Question 28

First generation CT scanner

Answer 28

• prototypical head only CT scanner
• developed by Dir Godfrey Hounsefield
• put to clinical use in 1972
• two detectors positioned adjacently along z-axis
• pencil beam of x-rays
• translate-rotate

Question 29

Second Generation CT scanner

Answer 29

• translate-rotate
• increase in number of detectors
• first use of fan beam to expose a wider detector array
• expanded clinical capabilities beyond head imaging

Question 30

Third Generation CT scanner

Answer 30

• expanded curvilinear array of detectors rotates with the x-ray tube around the patient
• rotate-rotate geometry eliminating the need for translation
• larger fan beam used to expose the expanded detector array

Question 31

Fourth Generation CT scanner

Answer 31

• co-evolved with third generation systems and based upon a rotate-stationary geometry.
• the gantry houses a stationary, circular ring of detectors and the x-ray tube rotates around the patient.

Question 32

Uniform Matrix Array detector

Answer 32

utilizes multiple detectors in the longitudinal direction, each the same length.

Question 33

adaptive-array detector

Answer 33

detectors configured with the thinnest widths at the center, surrounded by detectors of incrementally increasing widths along the z-axis

Question 34

hybrid array detector

Answer 34

consists of two detector size. The narrower detectors are positioned midline, flanked by the wider detectors.

Question 35

Cine CT acquisition

Answer 35

involves multiple axial scans obtained at a single anatomic level over a predetermined period. Example, perfusions and also bolus tracking.

Question 36

most widely accepted standard for detector configuration ?

Answer 36

64-slice MSCT systems

Question 37

Data Acquisition System (DAS)

Answer 37

responsible for measuring the transmitted x-radiation absorbed by the detectors.

Question 38

parallel processing

Answer 38

the ability of the computer system to perform multiple tasks simultaneously.