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FRCR physics > CT extra flashcards > Flashcards

CT extra flashcards

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1
Q

Describe 1st generation CT scanners

A

Translate rotate system
1. The x-ray beam is picked-up by a single detector.
2. The x-ray source and detector then move together (translate)
3. The two then rotate together to image a different angle
4. This is repeated until a single slice is scanned
5. The two then move down the patient to start imaging a different slice

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2
Q

Describe 2nd generation CT scanners

A

translate rotate
1. The x-ray beam is picked-up by a row of up to 30 detectors.
2. The x-ray source and detector then move together (translate)
3. The two then rotate together to image a different angle
4. This is repeated until a single slice is scanned
5. The two then move down the patient to start imaging a different slice in the patient

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3
Q

Describe 3rd generation CT scanners

A

Rotate rotate
1. The x-ray beam hits a row of detectors wide enough to image the whole slice
2. The two then rotate together to image a different angle
3. This is repeated until a single slice is scanned then the array is moved to a different slice (axial scanning). Alternatively, the detector array is continually moved down the patient as it rotates (spiral scanning), This is the most commonly used method today and takes about 0.3 seconds to image a single slice
*High spec scanners use solid state scintillation
**Can be helical

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4
Q

Describe 4th generation CT scanners - what type of detector is used in these?

A

Rotate fixed
1. There is a fixed complete ring of detectors
2. The x-ray source rotates around to capture a slice
3. Both then move down the patient to begin imaging a different slice
*Uses solid state scintillants - eg cadmium tungstate

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5
Q

How many revolutions per second can be achieved with a modern CT scanner?

A

2-4 revolutions per second

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6
Q

How much sampling occurs in CT per rotation?

A

-Continuous beam is sampled approx. 1000 times per rotation

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7
Q

Slice thickness: what determines thickness in single slice and multi-slice CT?

A

-Single slice CT = determined by collimation. Limited by detector row width.
-Multisclice CT = determined by width of detector rows

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8
Q

What is the equation for calculation of HU? What is a HU?

A

-CT image: made of pixels along greyscale – level of grey is determined by density of material (also expressed as LAC), which is represented numerically by Hounsfield unit.
-HU: set so that water measures 0 and everything else is relative to this.

HU = 1000 x (μt – μw) / μw

where:
μt = attenuation coefficient of tissue
μw = attenuation coefficient of water

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9
Q

How does iterative reconstruction compare with FPB re streak artefact reduction, noise sensitivity

A

Interactive reconstruction in CT
-Adjusts image activity content in steps until calculated activity is as close as possible to measured activity
-Reduces streak artefact in comparison with filtered back projection
-Is less sensitive to noise compared to filtered back projection
-Corrects accurately for attenuation-Enables reconstruction of an image, even if projections at some angles are missing.

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10
Q

T/F An edge enhancing convolution kernel will reduce spatial reduction. What is the effect on noise?

A

-False: edge enhancement increases the differences between neighbouring voxels thereby increasing spatial resolution, this
also increases noise.

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11
Q

True or false: FBP is used to remove the blurring inherent in tomographically acquired data

A

True

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12
Q

What factors can affect trans-axial resolution in CT?

A

-Focal spot (smaller the better)
-Detector size: smaller detectors give higher spatial resolution but more detectors mean more partitions (dead space) and reduced overal detection efficiency
-Detector design properties

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13
Q

What factors can affect Z plane resolution in CT?

A

-Detector slice thickness: wider (in z axis) the detector row, the lower the resolution
-Pitch: data with overlapping slices improves z sensitivity
-Small focal spot is better

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14
Q

What is the HU of air?

A

(-1000)

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15
Q

What is the HU of lung?

A

(-300)

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16
Q

What is the HU of fat?

A

(-90)

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17
Q

What is the HU of water?

A

0

18
Q

What is the HU of CSF?

A

15

19
Q

What is the HU of white matter and grey matter?

A

-White: 30
-Grey: 40

20
Q

What is the HU of muscle?

A

50

21
Q

What is the HU of cortical bone?

A

+1000

22
Q

What is an example of a soft tissue window setting?

A

-window width 400
-Window centre: 30 HU

23
Q

What is an example of a lung window setting?

A

-Width: 1500
-Centre: -600

24
Q

What is cone beam artefact?

A

-Occurs in multislice scanners
-As the section scanned increases per rotation, a wider collimation is used. Because of this the x-ray beam becomes cone-shaped instead of fan-shaped and the area imaged by each detector as it rotates around the patient is a volume instead of a flat plane. The resulting artefact is similar to the partial volume artefact for off-centre objects. This is particularly pronounced at the edges of the image. With modern scanners cone beam reconstruction algorithms correct this artefact.

25
Q

-T/F: cone beam artefact does not occur for objects situated close to the centre of rotation of a multi-slice scanner

A

-False: objects at the centre of rotation are always represented by the same central detectors but as the scan progresses the
same objects will move out of the central plane and then be represented by the outer detectors, thus contributing to the potential cone beam artefact. The cone beam artefact will be less at the rotation axis compared to objects at scan periphery.

26
Q

What is the photoelectric effect?

A

Ejection by a photon of an electron from the innermost K-shell of an atom. An electron from the next shell fills the empty space. The energy from this is released in the form of a photoelectron. The photoelectric event can only occur if the incident photon has enough energy to overcome the binding energy of the electrons within the K-shell.

27
Q

Hydrogen: atomic number and k-edge

A

-1
-0.01 keV

28
Q

Iodine: atomic number and k-edge

A

-53
-33 keV

29
Q

Barium: atomic number and k-edge

A

-56
-37 keV

30
Q

Gadolinium: atomic number and k-edge

A

-64
-50 keV

31
Q

What is the underlying principle of dual energy CT?

A

Dual-energy CT (DECT) utilises the photoelectric effect to separate out different materials within the voxel based upon their different attenuations at different beam energies.

32
Q

Dual source DECT

A

-Uses the beam energy spectrum to separate out materials based on different attenuation at a high (140 kVp) and low (80 kVp) potentials
-Two x-ray tubes of different voltages with two sets of detectors paired to the x-ray tubes. The x-ray tubes are at 90° to each other.

33
Q

Pros of dual source DECT

A

-Simultaneous acquisition and processing at the two energies leads to quicker acquisition, good overlap of images acquired at the two voltages.
-Can independently optimise the signal-to-noise ratio for each x-ray tube-detector pair

34
Q

Cons of Dual source DECT

A

-Increased dosage (two scans acquired)
-Scattered radiation from one tube may be detected by the detector for the other tube

35
Q

What is single source DECT?

A

-Single x-ray tube and two detectors
-Can be consecutive, sequential or fast kVp switch

36
Q

What detector based spectral ct?

A

-Single x-ray tube with a single high tube potential beam
-Single detector but made of two layers (sandwich detector) that simultaneously detects two energies.
-Dual-energy analysis can be performed on every data set acquired

37
Q

What temporal resolution and spatial resolution allow cardiac gated CT imaging?

A

-Temporal resolution 75ms and spatial resolution <0.5mm

38
Q

What is retrospective cardiac CT?

A

-Retrospective: pt irradiated throughout the cardiac cycle (very high dose)

39
Q

What is restrospective with dose modulation cardiac CT?

A

-Retrospective with dose modulation: variant of retrospective, tube current is reduced (usually during systole) when the ECG cycle is not within the acquisition window – less dose than retrospective

40
Q

What is prospective cardiac CT?

A

-Prospective: ECG monitors timing of cycle – makes a prediction and the scanner is triggered at a certain point of the partial cycle. Better with steady HRs and can be prone to errors with ectopics. Acquisition scan can be padded out. Use step and shoot and 3cm blocks combined to form final image.

41
Q

What is ultrahigh pitch cardiac CT?

A

Uses dual source scanners – rapid table movements cover entire heart in 1 heartbeat. Each detector array covers 90 degrees – need steady HR <60 BPM.

FRCR physics (13 decks)

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