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CT Principles > Basic Principles > Flashcards

Basic Principles Flashcards

1
Q

What is the Mylar window?

A

Narrow transparent window though which X-ray beam passes

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2
Q

Ct gantry contains…

A

Detectors, collimators, data acquisition system ( DAs )

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3
Q

System emergency off (seo)

A

Located on wall

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4
Q

What are slip rings attached to gantry of helical ct?

A

Electro - mechanical devices made of parallel electrically conductive rings and brushes

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5
Q

What is helical/spiral scanning?

A

Continuous rotation of tube and detector elements around the pt. And continuous movement of table. Rotate- rotate scanning

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6
Q

Axial scanning?

A

’ Tube moves 360°, scanning halts, tube moves 360° opposite, scans again
Known as rotate/ translate scanning

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7
Q

Axial acquisition produces…

A

Discrete packages of image data, with locked thickness, position and orientation - manipulation is Ltd

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8
Q

Helical advantages?

A

Speed
Data acquired as volume- not broken into slices
Increased temporal resolution
MPR - multiplanar reformation + 3D possible

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9
Q

What is a data set?

A

A. Volume of data

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10
Q

X-ray generation in helical scanners?

A

High frequency solid state generators - stable high power output to tube

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11
Q

Describe Ct tube

A

Stating anode tube mounted on gantry
Subjected to great thermal loads
Design helps heat, storage and dissipation

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12
Q

Why do Ct s have metal envelope?

A

no risk of electrical arcing caused by tungsten deposits vaporising

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13
Q

Ct rotating anode are…

A

’ Thick ,increase heat storage, dissipation = higher tube currents possible
Brazed graphite anode with tungsten - rhenium focal track
Graphite = high heat capacity

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14
Q

Contrast resolution?

A

Ability to differentiate between small differences in tissue density

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15
Q

Beam hardening…

A

Artefact reduced by copper flat filters

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16
Q

Bow tie filters…

A

Teflon , Shaped to reduce beam intensity at periphery _ correspond with thinner parts of pt. Anatomy

17
Q

Ct detectors located on…

A

Rotating gantry

18
Q

Ct attenuated beam enters the…

A

Detector chamber and energy is amplified, converted to electrical signal

19
Q

Optimal Ct detector is…

A

High efficiency, scatter suppression ‘ stability
Low after glow, crosstalk between elements cheap & serviceable

20
Q

2 types of Ct detector …

A

Ionisation chamber
scintillation detector / solid state detector

21
Q

Ionisation detectors are…

A

Thin tungsten plates, - Many detectors (chambers) used, pressurised xenon ‘ more gas molecules/ unit path length ,
Ions collected by electric field between plates - signal produced and amplified - processed by comp as raw data

22
Q

Ct Ionisation chamber advantages…

A

Cheaper than ssd
Easier to calibrate
Stable

23
Q

. Ct Ionisation chamber disadvantages..

A

Kept under pressure in Al casing
This reduces system efficiency by filtering beam
Bulky

24
Q

Ct scintillation detector…

A

X-ray photons absorbed by caesium iodide or ceramic Crystal
Beam energy converted into light pulse
Detected by photo diode & converted into signal
Low afterglow reduces contrast resolution!

25
Q

What is. Crosstalk?

A

’ Photon partially absorbed by detector , then enters adjacent detector + detected again = 2 weak signals and reduced image resolution

26
Q

How to minimise crosstalk?

A

Using a highly efficient photon absorptive Crystal

27
Q

Scintillation / solid state detectors absorb….. % of photons

A

100 %

28
Q

Scintillation / ssd disadvantage?

A

Sensitive to temp fluctuations and moisture

29
Q

Multi slice Ct…

A

Most widely used clinically ‘
Multiple parallel rows of detectors = detector array
16, 32,64,320 rows!

30
Q

Multiple detector rows mean…

A

Rapid scan speed
Better coverage is single breath hold
Reduced pt. movement artefact
Scanner can keep up with contrast through vessels - peak contrast enhancement
Less req, to sedate
Good temporal resolution - imp. For Cardiac Ct

31
Q

Where is das?

A

Data acquisition system close to detectors
Samples detector elements, amplifies signal, converts analogue to digital = raw = image data

CT Principles (1 decks)

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