CT

Question 1

Tomographic acquisition

Answer 1

Ray: single transmission measurement through patient made by single detector at a given moment

Projection: series of rays passing through patient at same orientation

2 projection geometry in CT: parallel beam and fam beam

Question 2

Current CT geometry

Answer 2

3rd generation. Wide fan beam, rotate/rotate

Question 3

Detector systems

Answer 3

1. Multiple scintillation detectors with photomultiplier tubes (PMTs)
2. Multiple scintillation detectors with photodiodes
3. Single multi-chamber inert has detector (Xe)

Question 4

Scintillators

Answer 4

Type of chemical that produce visible light when interacting with high-energy photons

Question 5

Gas detectors

Answer 5

Made of inert gas chambers sensitive to high energy photons

Question 6

Projection data

Answer 6

Summ attenuation values along measured line

Question 7

Slice sensitive profile

Answer 7

Perfect SSP: rectangular

Real SSP: due to divergence, cause steepness

Question 8

Scanned projection radiograph

CT radiograph

Answer 8

First X-ray image taken before actual CT: sets boundaries

Question 9

Basic sequential acquisition

Answer 9

Step and shoot

Slice dose profile (SDP) overlap= increase dose to patient at skin surface

Question 10

Spiral (helical) acquisition

Answer 10

Patient moving throughout gantry while fan beam continuously rotating

Question 11

Slip rings

Answer 11

Replace cables and transfer the necessary power and signal data

Question 12

Pitch

Answer 12

Describes coarseness of spiral shaped slices (higher pitch is more spacing- less dense)

Question 13

Cone beam acquisition

Answer 13

Multi row detectors used for multi slice Ct

Question 14

Image data

Answer 14

Ray sum: sum of individual voxel data

Pixel: picture element

Voxel: same 2D dimensions of pixel and extra dimension of slice thickness

Question 15

Image reconstruction techniques

Answer 15

1. Interative
   - finds exact mathematical solution ; slow rounding errors
2. Back projection
   - ray sum put into matrix for whole ray, creates star burst effect
3. Filtered back projections
   - removes star burst effect

Question 16

Mask signal

Answer 16

Removed back projected high frequency (too much detail) interference pattern

Question 17

Factors determining image quality

Answer 17

1. Uniformity
   - measure of homogeneity of entire cross section
2. Resolution
   - ability to resolve small details
3. Contrast
   - ability to discriminate object from background
4. Noise
   - standard deviation of CT numbers in a uniform image
5. Image artifacts
   - sampling accuracy, partial volume, beam gardening

Question 18

Radiation dose

Answer 18

Public limit of 1 mSv/year

Question 19

Factors influencing dose

Answer 19

Beam energy
Filtration
Collimation
Slice number and spacing
Image quality
Nose
Scan pitch factor