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CT Flashcards

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1
Q

What are the approximate fan and cone angles in modern CT units?

A
  • Fan angle = 60 degrees
  • Cone angle = 2.4 degrees
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2
Q

List 2 advantages of arc shaped CT detector arrays

A
  • Minimal variation in fluence in detectors due to inverse square law
  • Struck in nearly normal manner -> no lateral positioning errors (parallax)
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3
Q

How does cone beam geometry differ from fan-beam / narrow cone beam? What challenges does this present?

A
  • Use of flat panel detector
  • Cone angle near to fan angle
  • Differences in distance of rays striking detectors, requires additional calculation steps to negate below effects:

Inverse square law

Heel effect

Parallax (positioning errors)

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4
Q

What defines the max FOV?

A

FAN ANGLE / curved extent of detectors

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5
Q

Briefly detail different scanner generations

A
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6
Q

What type of detectors are used in CT? What are they made from?

A
  • Indirect scinitllators (solid state) -> used to use ionisation chambers
  • Use crystal e.g. caesium iodide + ceramic material e.g. Gadolinium oxysulfide Gd2O2S (and other materials) which have undgone ‘sintering’ to make more sensitve to xray detection
  • FORMED INTO CERAMIC
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7
Q

What dictates slice thickness in single array and MDCT scanners?

A
  • Single array - > XRAY BEAM COLLIMATION
  • MDCT -> DETECTOR CONFIGURATION / WIDTH

Faster scanning and thinner slices available with MDCT

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8
Q

A 64 slice MDCT scanner (n) with a detector width (T) of 0.625mm is used to scan 20cm of abdomen, pitch = 1, scan time = 1sec. How many scans are required and how long does it take?

A

64 x 0.625 = 40mm

20cm/4cm = 5 scans

5 x 1 sec -> 5 sec

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9
Q

What downside is there to having multiple thin slices in z direction?

A
  • Decreased photon interaction results in INCREASED NOISE
  • > Need to increase dose to negate this.

TRADE OFF between dose, noise and z axis resolution

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10
Q

How do CT and XR tubes compare?

A

CT:

More powerful - 5-7 Megajoules (XR = 0.3-0.5 Megajoules)

Continuous source

Typically run at 120kV for generic scanning (can alter)

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11
Q

The heel effect is present along which axis in CT scanners?

A
  • z axis
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12
Q

Which radiological interaction predominantes in CT?

A

COMPTON SCATTER

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13
Q

Describe the HU scale: air, fluid, water, fat, ST, contrast, bone

A
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14
Q

HU equation

A

Reflects LINEAR ATTENUATION COEFFICIENTS

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15
Q

What does the bow tie filter do?

A
  • Reduced patient dose at periphery (if applied correctly)

IF TOO SMALL FILTER USED, will concentrate / increase dose centrally

  • Reduced image noise due to decreased statistical variation of beams with markedly different attenuation
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16
Q

What are the pitch equations x 3?

A

Single slice collimator pitch:

= Ftable / collimation width at isocentre

Multislice collimator pitch

= detector pitch / number of selected detectors or binned combinations

Multislicer detector pitch =

Pitch = Ftable / detector width

Gantry rotation time important too for calculation!

E.g. 0.5s rotation time, 40mm detector width -> what table feed required to achieve pitch of 1? = 80mm/s

17
Q

How does pitch relate to dose?

A

INVERSELY PROPORTIONAL TO PITCH

=> higher pitch, lower dose

Pitch = 1, equiavelent to contiguous

Pitch < 1, OVERSCANNING

  • Increased patient dose
  • Cardiac imaging, very large patients

Pitch > 1, UNDERSCANNING

  • Decreased patient dose
  • Paediatric imaging (speed important)
18
Q

Define PITCH. What is normal pitch range?

A

Relative advancement of table per tube rotation

THINK SPEED -> HIGHER PITCH, FASTER TABLE MOVEMENT

0.75-1.25 typically

19
Q

List approximate WW/WL for ST, bone, lung, brain

A
20
Q

ROUGHLY, what do Ij and Ir refer to?

A

Ij = Detector value following projection through subject.

Ir = detector value outside of subject

=> IN BROAD TERMS compared to each other during calibration to adjust for e.g. variation secondary to bow tie filtration, distance from source etc

21
Q

Define forward projection. Describe simple back projection vs filtered back projection

A

Forward projection = Result of linear attenuation data recieved at detectors. RAW DATA as yet not used to produce CT image

Simple back projection = Smearing of forward projected data across matrix -> results in 1/r blurring due to inherent geometric factors

Filtered back projection = DECONVOLUTION KERNEL applied -> Shapens edges to control for 1/r blurring.

22
Q

Describe relationship of convolutional methods with fourier transform in CT recon. What is a ramp filter and why is it used?

A

Convolution = mathematical method to accurately describe physical act of blurring in SPATIAL DOMAIN. 1/r blurring in this method controlled with deconvolution kernel

Fourier transform = Method of transformation of convolutional data into FREQUENCY DOMAIN -> allows for faster data handling.

Ramp filter = same as deconvolution kernel, but as plotted in FREQ DOMAIN (see shape in pic). Requires roll off, because inherently amplified high freq signal -> small objects with predominance of quantum noise! Thus roll-off controls noise.

23
Q

Briefly describe differences in iterative and analytical reconstruction techniques

A

Analytical = e.g. simple / filtered backprojection, fourier transform based models. Use of convolutional methods and filtration to control for noise

Iterative = uses base image (may be FBP image) and through numerous iterations improves image accuracy -> Utilised error matrix comparing forward projected data and acquisition data, serially minimising over time

=> BETTER, but numerically intensive. Allows for control of real world factors e.g. focal spot blurring, xray spectrum. In principal -> Better SNR, or equivalent SNR with reduced dose.

24
Q

Explain the role of sharp / smooth filters in reconstruction -> THINK RAMP

A

Sharp edge filters:

  • Used to increase signal from small (high frequency) objects -> however, inherent increase in NOISE
  • Used in regions of HIGH CONTRAST, as demonstrated by lung and bone window characterisitics -> big differences in attenuation e.g. lung vs air, bone vs ST
  • LOOKING AT SMALL THINGS

=> Lung, bone

  • Smooth edge filters: Used to decrease NOISE, with predominant focus on larger objects features
  • Used in regions of LOW CONTRAST, e.g. ST windows, need to reduce noise
  • LOOKING AT BIG THINGS

=> e.g. mild differences in attenuation in liver lesion

25
Q

What is the difference between SFOV and DFOV

A

SFOV - Max volume scanned within bore

DFOV - Volume reconstructed. Can be smaller than SFOV -> Smaller DFOVwill result in higher spatial resolution.

26
Q

Explain the effects of matrix size, image depth (pixel bit depth), FOV, slice thickness, and mA/kVp on image quality

A
27
Q

List 5 factors that limit spatial resolution in CT. Which factor is most important?

A
  • Size of detector measurements / spacing of detector measurements
  • Focal spot size
  • Motion
  • Matrix size
  • Reconstruction filter
28
Q

What is contrast resolution? What are 4 factors that directly affect it?

A

= NOISE!!!!

  • Technical factors (e.g kV, mA, time and pitch -> Determine dose and thus noise)
  • Slice thickness -> thinner slices more noise
  • Reconstruction filter (e.g. sharp / bone / lung -> more noise)
  • Reconstruction method (e.g. iterative less noise)

Physics (10 decks)

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