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ABR Nuc Med > Gamma Cameras and SPECT > Flashcards

Gamma Cameras and SPECT Flashcards

1
Q

The resolution of a collimator is expressed as the

A

FWHM of a line source

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2
Q

Collimators generally involve a tradeoff between

A

spatial resolution

sensitivity

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3
Q

parallel-hole collimators may be classified as

A

high sensitivity (low resolution)

general-purpose

high resolution (low-sensitivity)

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4
Q

Typical collimator for Tc-99m and Tl-201

A

low energy

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5
Q

Typical collimator for Ga-67, In-111

A

Medium energy

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6
Q

Typical collimator for I-131 and F-18 (non-PET)

A

High-energy

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7
Q

Typical count range for planar image

A

500,000 to 1 M

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8
Q

SPECT images are typically reconstructed using this matrix

A

128 x 128

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9
Q

For brain SPECT, typical collimators may include

A

hybrid convergin and parallel hole

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10
Q

Typical bit depth in terms of gray levels

A

8 bit, 256 gray levels

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11
Q

Gamma cameras typically have this number of PMTs

A

37, 61, 91 PMTs

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12
Q

A typical window for Tc-99m imaging

A

20%

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13
Q

Efficiency of the detector crystal increases with

A

thickness

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14
Q

Inclusion of scattered photons in a gamma camera acquisition causes

A

decrease in spatial resolution

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15
Q

Primary cause of limited intrinsic resolution in a gamma camera

A

statistical fluctuation in the distribution of light photons among photmultiplier tubes from one scintillation to the next

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16
Q

As gamma or x-ray energy decreases, intrinsic resolution

A

decreases do to larger relative statistical fluctuations in the light photons per scintillation event.

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17
Q

As a rule of thumb, intrinsic resolution in a gamma camera and energy are related as

A

1 / √E

(similar to relative uncertainty in count statistics)

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18
Q
A
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19
Q

Thicker detectors result in

A

increased sensitivity

decreased intrinsic resolution

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20
Q

The system resolution is largely determined by the

A

collimator resolution

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21
Q

Efficiency for 140 keV with NaI(Tl)

A

70% to 90% for 6.4mm and 12.7 mm thick crystals

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22
Q

Efficiency of NaI(Tl) for 500 keV photons

A

less than 20%

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23
Q

The energy resolution improves according to photon energy as

A

1 / √E

due to relatively decreased statistical fluctuations at higher energy

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24
Q

For Tc-99m, a lower energy threshold of 130 keV should remove photons that have been scattered through angles greater than

A

45 degrees, but only at about 50% due to the blurring of the spectrum (for scattered and unscattered photons)

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25
Q

As count rates increase, the accuracy of dead time corrections

A

decreases

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26
Q

True or False

In NM imaging studies,most are not impaced by dead time losses

A

True, with the exception of high-counting rate applications such as first-pass cardiac studies

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27
Q

In a NM image with high count rates from two sources, the resulting image may look like

A
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28
Q

An undesirable effect of pulse pile-up in terms of the image

A

image distortion

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29
Q

Modern gamma cameras employ this approach for pile-up correction

A

pulse-tail extrapolation

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30
Q

Which happens between the PMTs and over the PMTs?

A

Pincushion occurs over the PMT due to nonlinearities in sensitivity as you approach the center. Barrel distortion occurs between PMTs

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31
Q

Cause of artifacts

A

nonlinearities: pincushion and barrel effects seen with line pattern and homogeneity test

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32
Q

Two causes of gamma camera nonuniformities

A
  1. Differences in pulse-height spectrum for different PM tubes (tube response can be adjusted with careful tuning)
  2. position-dependent collection efficiency of scintillator light
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33
Q

Edge packing, due to reflection of scintillator light from the sides of the detector crystal back towards the PM tubes is mainly masked by

A

Useful Field of View

UFOV

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34
Q

For higher energy photons, interactions occur closer to the PMTs, resulting in a more narrow light spread and a __________ of detector nonuniformity

A

worsening

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35
Q

An intrinsic flood image is used to apply

A

spatially varying energy corrections

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36
Q

spatially varying energy corrections include

A
  1. spatially varying PHA settings
  2. normalized intensity correction (count skimming) : certain percentage of counts in hot spots are thrown out {older approach}
  3. use of lead sheet with uniformly spaced array of holes (~1mm diam, ~4 mm separation) at known locations, placed without collimator - lookup table corrects for offsets in x and y {mfg}
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37
Q

In newer cameras, tuning is performed this way

A

LED attached to each PMT is used to produce a light signal, PMTs are then automatically adjusted.

38
Q

A reasonable amount of septal penetration is

A

about 5%

39
Q

Upper limit for low energy collimators

A

about 150 keV

40
Q

upper limit for medium energy collimators

A

about 400 keV

41
Q

System resolution combines resolution for each part as

A
42
Q

Intrinsic resolution may be determined by

A

linearity test pattern with point source at 5 x UFOV

acquire with matrix so that min pixel size is less thatn 1/10 expected resolution (usually < .35 mm)

43
Q

Typical intrinsic spatial resolution of a gamma camera

A

2.5 to 3.5 mm

44
Q

System resolution with a line source

A

2, 1 mm line sources spaced about 5 cm apart, 10 cm from collimator surface, several million counts

45
Q

Typical system (extrinsic) spatial resolution for Tc-99m

A

8 to 14 mm

46
Q

Integral uniformity

A

100 % * (max - min)(max + min)

pixel counts

47
Q

Differential uniformity

A

Over 5 consecutive pixels

100% x (high - low)/(high + low)

48
Q

Typical integral uniformity in the UFOV and CFOV

A

2% to 4%

49
Q

Typical energy resolution for Tc-99m

A

8% to 11%

50
Q

Typical sensitivity for gamma camer

A

2 to 3 x 10-4 cps/Bq

or .02% to .03%

51
Q

System sensitivity can be measured using

A

flask with small amount of Tc-99 (10 ml or so) to minimize selb-absorption

52
Q

NaI(Tl) has a thin aluminum casing to

A

protect it from moisture

53
Q

Most general purpose cameras have a crystal thickness of

A

about 9.5 mm

54
Q

This image is

A

gamma camera head with the cover removed showing PMTs above sinctillator

55
Q

To form an image, gamma cameras use

A

absorptive collimation rather than a lens like a traditional camera uses

56
Q

For a parallel hole camera, the image and the object size as a function of distance are

A

the same

57
Q

For a pinhole collimator, the image and object size are related by

A

I/O = f/b

f = height of collimator

b = distance from collimator to object

58
Q

In a pinhole collimator, the lead cone is about this high

A

20 to 25 cm

59
Q

For a diverging collimator, image and object size are related in this way

A

I/O = (f-t)/(f+b)

f = focal length (collimator to point)

b = distance collimator to object

t = thickness of collimator

60
Q

Approaches to spatial resolution testing may include

A

bar phantoms

line sources (line spread function)

61
Q

The FWHM of a line spread function used in system resolution is

A

approximately 1.4-2 x width of the smallest resolvable bar pattern

62
Q

the most detailed specification of spatial resolution is provided by the

A

modulation transfer function

63
Q

The equation for the modulation transfer function

A

MTF = Min / Mout

Min =(Imax - Imin)/(Imax + Imin)

Mout =(Omax - Omin)/(Omax + Omin)

I and O are intensities, you would plot as a function of varying spatial frequencies

64
Q

Contrast of a lesion is defined as

A

Cl = (Rl - Ro)/Ro

Ro is the counting rate outside the lesion

65
Q

the percent contrast is

showing that a high background count rate relative to count rate outside the lesion will decrease percent contrast

A
66
Q

Background counts can results from

A

septal penetration

scattered radiation

inadequately shielded sources

67
Q

As the energy window is increased, spatial resolution

A

decreases

68
Q

statistical noise or quantum mottle can be reduced by

A

summing measurements, unlike structured noise

69
Q

The Rose criterion

A

An object’s CNR must exceed 3 to 5 to be detectible (shown in multiple research studies based on observers viewing lesions or objects in noise generated contrast patterns)

70
Q

Lesion contrast to noise ratio is defined as

A
71
Q

In a sinogram, each row represents

A

a projection at a specific angle

72
Q

A break in the sinogram may represent

A

patient movement

73
Q

The effect of the ramp filter is to

A

enhance high spatial frequencies and suppress low spatial frequencies to eliminate 1/r blurring

A rounded shape ramp filter will help supress unwanted high spatial frequency amplification (i.e., Hann Filter)

74
Q

According to sampling theory, in order for the highest spatial frequency to be recovered, the data must be sampled at

A

twice this frequency, otherwise aliasing will occur.

75
Q

As a rule of thumb, the sampling requirement for an imaging detector is

A

sampling distance <= FWHM / 3

76
Q

Spoke-like artifacts occur around small high entensity objects when

A

the number of angles is insufficient

77
Q

In a SPECT system, the minimum number of views required for a linear sampling distance, Δr is

A
78
Q

The minimum number of angular views should be acquired over a __________ degree arc

A

180

79
Q

This is an example of

A

image distortion due to insufficient sampling

80
Q
A

Sinogram showing parts of object are not included in all projections

81
Q
A

missing / damaged projection element

82
Q

In SPECT, to keep the same SNR while increasing the resolution by a factor of 2…

A

the total number of counts must increase by 8 (inverse cube of pixel size)

83
Q

Partial volume effect

A

The sum of all pixels containing the object reflect the radioactivity, but the intensity for a given pixel may be reduced due to averaging

84
Q

spillover in SPECT occurs when

A

an area of low activity is surrounded by higher activity regions - reducing contrast and apparent size

85
Q

The recovery coefficient in SPECT is

A

ratio of apparent concentration to true concentration, usually plotted as a function of cylinder diameter/FWHM (x) and recovery coefficient (y). It goes to 1 as the cylinder diameter / FWHM increases to > 2

86
Q

In SPECT, transaxial resolution refers to

A

in-plane resolution

87
Q

In SPECT, axial resolution refers to

A

component that is perpendicular to the slice along the axis of rotation (z direction) - sometimes referred to as slice thickness

88
Q

For SPECT, resolution degrades as you move

A

away from the collimator

89
Q

A typical resolution for SPECT is

A

12 mm FWHM

90
Q

A major benefit of SPECT

A

ability to resolve underlying structures

91
Q

The most frequent use of SPECT is for

A

myocardial perfusion for assessing coronary artery disease and heart muscle damage following infarction

92
Q
A

ABR Nuc Med (18 decks)

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