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VetRad Boards prep > CT physics > Flashcards

CT physics Flashcards

1
Q

What is filtered back projection?

A

most commonly used algorithm - reverses the attenuation process and builds up the matrix where areas of high and low attenuation tend to reinforce eachother

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2
Q

What are the disadvantages of 1st generation scanners?

A
  • poor spatial resolution
  • after glow (NaI detector) occured secondary to slow decay
  • large change in x-ray flux between the patient’s head and exterior to the head
  • slow spead
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3
Q

What are advantages and disadvantages of the third generation scanners?

A

Advantages

  • scan time is significantly reduced
  • there is better scatter rejection than 4th generation scanners

Disadvantages

  • suffers from ring artifacts and aliasing
  • need perfect alignment of the tube and detectors
  • scanning velocity limited by mechanical factors
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4
Q

What are two major ways the 4th generation scanner differs from the 3rd generation?

A
  • x-ray tube rotates around stationary detectors
  • detector fan instead of a source fan
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5
Q

What determines slice thickness in single detector arrays?

A

Collimation

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6
Q

In single slice CT, what is the trade off between SNR and spatial resolution when increasing slice thickness?

A
  • Number of photons detected increases linearly with slice thickness, thus higher SNR
  • Thinner slices increase spatial resolution and reduce partial volume averaging
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7
Q

For single slice detector arrays, the slice sensitivity profile is a consequence of what?

A
  • detector width
  • focal spot size and distribution
  • collimator prenumbra
  • combined influence of all the projection angles encircling the patient
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8
Q

How do you calculate collimator pitch (single detector helical scanners)?

A

Collimator pitch = table movement (mm) per 360 gantry rotation ÷ collimator width (mm)

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9
Q

WHat does a pitch of 1 imply?

A

the table movement per gantry rotation is equal to the collimator width

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10
Q

What does a pitch < 1 imply

A

over scanning- higher image quality at higher dose to patient

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11
Q

What does a pitch >1 imply?

A

partial scanning - reduces scan time, patient motion, patient exposure, but increases volume averaging

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12
Q

What is binning?

A

electronic signals generated by adjacent detector elements are electronically summed

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13
Q

WHat determines slice thickness in multiple detector array scanners?

A

determined by the width of the detectors in the slice tickness dimension (z-axis) and not by collimation

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14
Q

How do you calculate detector pitch (multidetector scanners)?

A

detector pitch = table movement (mm) per 360 gantry rotation ÷ detector width (mm)

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15
Q

How do you calculate collimator pitch of MDA scanners?

A

Collimator pitch = detector pitch ÷ N (number of detector arrays)

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16
Q

What are some major advantages of MDA scanners?

A
  • True isotropic spatial resolution
    • cubic voxels can be realized - image equally sharp in any plane traversing the scanned volume
  • Improved temporal resolution - less motion artifact
  • Increased SNR (decreased noise)
    • higher tube current used because more patient length is scanned
  • Increased speed - 1 rotation/0.5 seconds
  • CTA advantage - higher contrast rate for more separation of phases
  • less unusable penumbra radiation
    • higher umbra:penumbra ratio
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17
Q

What are disadvantages of MDA?

A
  • larger the number of detectors, the more cone beam artifacts
  • more scatter radiation between adjacent slices
    • even with thin septa
  • tube cooling capabilities
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18
Q

What are advantages of Helical CT scanning?

A
  • faster
  • sampling along the z-axis is uniform
  • images can be reconstructed at arbitrarily detremined positions and intervals along the z-axis
  • optimizes enhancement effect of contrast
  • image quality is equivalent to that of conventional CT
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19
Q

What are disadvantages of helical scanning?

A
  • inherent inconsitency in the acquired projections because of the constant parient translation leading to artifacts
  • Increased heat loading
  • increased processing time due to z-interpolation
  • High pitch - decreased z-axis resolution
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20
Q

In axial aquisition - what is the Multiple Scan Average Dose?

A

The does to tissue that includes the dose attibutable to scattered radiation eminating from all adjacent slices

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21
Q

How can you calculate/estimate the multiple scan average dose (MSAD)?

A

can be estimated with the CT dose Index (CTDI)

CTDI(mGy) = [reading (mGy) x length of chamber (mm)] ÷ beam width

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22
Q

Compare CTDIFDA to CTDI100

A
  • CTDIFDA - the radiation dose to any point in the patient including the scattered radiation from 7 CT slices in both directions
    • significantly underestimates the MSAD for small slice thickness
  • CTDI100 - provides a better estimation of MSAD for thin slices
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23
Q

How do you calculatre radiation dose with helical acquisition?

A

Helical dose = CTDI dose ÷ collimator pitch

  • increasung the collimator pitch will decrease the patient dose
  • Helical scanning usually uses less mAs
24
Q

What two variable affect image resolution in CT?

A
  • Circumferential resolution
  • Radial resolution
25
Q

How do circumferential and radial resolution differ?

A
  • Circumferential resolution is affected by number of views
    • reducing the number of views creates aliasing
  • Radial resolution is affected by the number of rays
    • reducing ray sampling results in low resolution, blurred images
26
Q

In CT interpolation algorithsm, how many full gantry rotations are required to reconstruct a slice?

A

2 full gantry rotations are required to reconstruct a slice

27
Q

What is the major difference between simple backprojection reconstruction and filtered back projection reconstruction?

A

In filtered, the raw data are mathmatically filtered before being back projected onto the image matrix, reversing blurring.

This is not done in simple back projection so there is blurring, 1/r

28
Q

What is the difference between bone kernal and soft tissue kernals?

A
  • bone kernals - less high frequency roll-off
    • higher spatial resolution, decreased SNR
  • Soft tissue kernals - more high frequncy roll off
    • use when high contrast more importal than spatial resolution (e.g. looking for mets)
    • increased SNR, lower spatial resolution
29
Q

What is the CT level?

A

CT number at the center of the window - all numbers above will be white, all below will be black

30
Q

What is Compton scatter dependent on?

A

Physical density (g/cm3) and electron density in tissue (pe)

31
Q

How do you calculate CT number (HU)?

A

CT (x,y) = 1000 x [mu(x,y) - muwater)] ÷ muwater

32
Q

How does CT compare to radiography as far as spatial resolution and contrast resolution?

A
  • CT has significantly worse spatial resolution
  • Comparting MTF for radiography and CT
    • Screen-film: 7 lp/mm
    • Digital radiography: 5 lp/mm
    • CT: 1 lp/mm
33
Q

What size of an object can a scanner resolve?

A

absolute size = reciprocal of spatial frequency (1/s)

34
Q

What is the mean equine pituitary height?

A

1.0 cm

35
Q

Which cheek tooth is most commonly involved in equine alveolitis/sinusitis and how does it appear on CT?

A

The first molar -

  • hypoattenuation of the cementum, destruction of the enamel and filling of the infundibular cavity with gas
  • gas bubble in the root area or fragmentation of the root
  • see page 41 of CT notes for more changes
36
Q

How do 4th generation scanners eliminate rign artifact?

A

the reference beam is measured by the same detector used for transmission measurement

37
Q

What are disadvantages of helical scanners/acquisition?

A
  • motion artifacts
  • increased chance of partial volume averaging
  • reduced resolution in the z-direction when using large pitch factors
  • increase in processing time due to z-interpolation required
38
Q

What causes ray aliasing and what dies it looks like?

A
  • objects in the image with sharp, well-defined edges that contain strong, high-frequency components exceeding the Nyquist limit
  • looks like pattern of radial dak and bright streaks emanating directly from a high frequency object
39
Q

How do you correct ray aliasing?

A
  • quarter detector off-set
  • dynamic focal spot
40
Q

When does view aliasing occur and how does it appear?

A
  • cause by insufficient number of views per beam rotation
  • fine radial streaks in the image periphary
41
Q

How do you correct view aliasing?

A
  • increase the tube rotation time (e.g. from 1s to 2s)
  • avoidance of high density objects (not really practical)
42
Q

What are examples of artifacts that occur due to movement?

A

ghosting, streaks, bluring and slice mismatch

43
Q

What causes windmill artifacts? How is it minimized?

A
  • Due to the complex interpolation of image data from helical multislice CT
  • minimized by applying different types of image interpolation (z-filter interpolation) or using non-integer pitches in relation to detector width
44
Q

What is cone beam artifact? What does it look like?

A
  • Due to the relative small focal spot and relatively larger detector width, x-ray beam is more cone/wedge shaped
    • small objects in periphery may be detected in one scan field but not the opposite, leading to conflicting density information
  • leads to streak formation
45
Q

What does zebra artifact look like?

A

faint parallel stripes in the x-y dimension on multiplanar and 3D reformatted images (caused by helical image interpolation)

46
Q

How can partial volume averaging atrifact appear?

A
  • blurring of object margins, erroneous attenuation measurements, pseudolesions, and streaks
47
Q

How does truncated view/out-of-field artifact appear? What causes it?

A

Characterized by a bright rim along the periphery and/or streaks originating from the area of structures outside the scan field of view

48
Q

What causes partial volume averaging?

A

occurs if and object is only partially intruding, traversing obliquely or is smaller than the scanning plane

49
Q

What causes rebound artifact?

A

edge enhancement

50
Q

Under what circumstances will point spread effect cause a object to appear thicker? Thinner?

A
  • Thicker:
    • widest with a low freq algorithm protocol and low and narrow settings (e.g. brain)
    • when inherent object contrast is reduced (e.g. fluid filled bulla or sinus)
  • Thinner:
    • high frequency algorithm, viewed with high wide setting
51
Q

What is photon starvation? How do you correct it?

A
  • Streaking artifacts that occur in highly attenuating body parts such as the shoulders or thighs (insufficient photons reach the detector)
  • increase current or increase slice thickness
52
Q

What causes beam hardening?

A

selective resorption of low-energy photons from the polychromatic x-ray beam by a highly attenuating structure

53
Q

How can you reduce beam hardening?

A
  • pre-patient filtering
    • bow-tie filter - minimize cupping
54
Q

How can you correct non-linear partial volume effect?

A

thick section reformating of thinly collimated slices

55
Q
A

VetRad Boards prep (11 decks)

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