Computed Tomography: Main principles, CT Scanner Design

Question 1

How is the human body viewed in CT?

Answer 1

Built up of a finite number of discrete slices and volume elements (voxels)

Question 2

In CT, what is the x/y plane?

Answer 2

Transverse plane

Question 3

In CT, what is the z axis?

Answer 3

Orientated perpendicular to the scan and image plane

Approximately parallel to the body’s longitudinal axis

Question 4

What is S?

Answer 4

Slice thickness

Question 5

What has to be measured to calculate the attenuation value P along each ray from the X-ray source to detector?

Answer 5

1. Intensity I of x-rays attenuated by object

2. Primary intensity Io of X-rays

Question 6

What is the simplest case given in CT?

Answer 6

Measurement of a homogenous object with monochromatic radiation

Question 7

What does the intensity of X-rays fall off exponentially with?

Answer 7

Absorber thickness d

Question 8

What is the attenuation P defined as?

Answer 8

Natural logarithm of the radio of primary intensity Io to attenuated intensity I

Question 9

For Inhomogenous object, what does the total attenuation resulting from each ray path interval depend on?

Answer 9

Local value of the attenuation coefficient μi

Question 10

What does CT consist of?

Answer 10

Measuring many such line integrals exactly

Question 11

How can a two-dimensional distribution of an object characteristic be determined?

Answer 11

Exactly if an infinite number of line integrals are given

Question 12

What is sufficient to compute an image to a good approximation?

Answer 12

A finite number of measurements of the distribution of attenuation coefficient

Question 13

What is used in real CT scanners?

Answer 13

Polychromatic X-ray radiation

Question 14

What does Linear attenuation coeffucuent depend strongly on?

Answer 14

X-ray energy E

Question 15

What are examples of dependence on energy may yield?

Answer 15

1. Beam hardening effects that lead to image artefacts

Question 16

How is an object measured in CT?

Answer 16

A sufficiently high number of attenuation integrals or projection values have to be recorded

Necessary to carry out measurements in all directions

Question 17

What is the simplest measurement set up?

Answer 17

1. A radiation source with adequate collimation emits a pencil beam
2. The intensity, attenuated by the object, is registered by the detector placed outside
3. For a given angular position, this set up of radiation source and detector is moved linearly (transition) and the intensity is measured

Question 18

How is an attenuation profiled created which is termed as a projection?

Answer 18

Determining the ratios of primary intensity recorded in periphery and the attenuated intensities recorded behind the object and taking their logarihms

Question 19

How are projections measured?

Answer 19

Successively for successive angular positions

Question 20

Where is the complete set of projections transferred to?

Answer 20

Data processing unit

Question 21

How is a CT image computed?

Answer 21

Back-projection is usually utilized

Question 22

How does back-projection work?

Answer 22

Each projection value is added to all the picture elements in the computer memory along the direction in which it was measured

Question 23

What does each detail in the object and that is represented in the attenuation profile contribute to?

Answer 23

Pixel value at the desired image point

Entire imge as well

Question 24

When will an unsharp image result from?

Answer 24

Considering only 2 or 3 projections

Question 25

How do you avoid the unsharpening of each projection?

Answer 25

Each projection has to be convoluted before back projection with a mathematical function, the convolution kernel

Question 26

What does the convolution kernel constitute?

Answer 26

A pointwise multiplication of the convolutional kernel and attenuation profile and addition of resulting values

Question 27

What does convolution represent?

Answer 27

A high-pass filtering procedure which generates overshoots and undershoots at object boundaries

Question 28

What does the convolution additionally offer the possibility to do?

Answer 28

Influence image characteristics by the choice and design of the convolution kernel - from soft or smoothing to sharp or edge enhancing

Question 29

What does CT measure and compute?

Answer 29

spatial distribution of linear attenuation coefficient

Question 30

How is the computed attenuation coeficient displayed as?

Answer 30

CT number relative to the attenution of water

Question 31

What are CT numbers measured in?

Answer 31

Hounsfield units (HU)

Question 32

What is windowing?

Answer 32

Values above the chosen window will be displayed as white Values below the window as black

Question 33

When is a narrow window chosen?

Answer 33

Display of very small attenuation differences as given in the brain

Question 34

What is an increase in CT number assigned to?

Answer 34

Increased density and/or an increase in effective atomic number

Question 35

What does Dual-energy CT make use of?

Answer 35

the energy dependence in μ due to the materials’ atomic numbers two scans with different spectra are carried out and the attenuation values and the differences in attenuation for the two spectra have to be evaluated

Question 36

What is the aim of Dual energy CT?

Answer 36

Generate material-selective images and to determine material density as accurately as possible

Question 37

What do modern CT scanners mainly use?

Answer 37

Fan-shaped X-ray beam

Question 38

What are X-ray tubes capable of producing?

Answer 38

Tube currents up to 800mA and with significantly improved cooling systems

Question 39

Why is having an improved cooling system important?

Answer 39

Long continuous exposures can be made on a regular basis during the day

Question 40

Gantry tilt

Answer 40

The gantry has a capability to tilt, most CT scanners allow a tilt of the scan plane up to ±30% with respect to the axis of rotation. This allows the selection of scan planes directly through the structures of interest which is important for critical regions, such as the base of the skull or the lumbar spine.

Question 41

What do real X-ray tubes have?

Answer 41

Extended focus sizes | for example 0.8mm x 1.2mm

Question 42

What does Collimators in front of the detector serve?

Answer 42

to minimize signal contributions from scattered radiation

Question 43

What does the bow-tie filters reduce?

Answer 43

the demands on the dynamic range of the detector system and scattered radiation intensities arising from peripheral object zones

Question 44

What does the detector transform?

Answer 44

Incident X-ray intensity into a corresponding electrical sinal

Question 45

What can double the number of simultaneously measurable slices?

Answer 45

Flying focal spot