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CT - Chapter 1 Flashcards

1
Q

Based on dose minimization, which patients would CCTA be LEAST appropriate?

A

68 year-old man with high risk for CAD + acute chest pain

  • Appropriate patient selection is an important first step in dose reduction
  • Due to High sensitivty and negative predictive value
  • Good at ruling out CAD in low-intermediate risk individuals
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2
Q

What patient population is CCTA best ruling out CAD?

A

Low-Intermediate risk individuals

  • High sensitivity
  • High negative predictive value
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3
Q

What does effective radiation dose compare?

A

cancer risk from:

a non-uniform exposure of ionizing radiation

to

a uniform exposure of the whole body

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4
Q

What is the equivalent radiation dose for 10 mGy?

  • mSV
  • Sv
A
  • 10 mGy = 10 mSV
  • Sv = 0.01 Sv​
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5
Q

Why are radiation doses between gray and sievert interchangeable for radiation from X-rays?

A

Because XR’s have a weighting factor of 1

  • not the case for some other radiation sources such as neutrons or alpha particles
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6
Q

What radiation dose reduction can be expected with CT angiogram acquisition?

  • Propsective ECG-triggering
  • Retrospective ECG-triggering
A

80-90% reduction

  • depends on phase window chosen
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7
Q

Describe Computed Tomography Dose Index (CTDI)

A
  • provides an estimate for the radiation dose to the patient based on:
    • scan parameters entered and
    • geometric assumptions derived from a phantom
  • typically does not provide information about actual received doses since those are only available when placing a dosimetry vest on the patient
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8
Q

Describe ALARA and its importance

A

As Low As Reasonably Achievable

  • alludes to the paramount objective for any X-ray procedure in humans
  • to apply only the minimum radiation necessary to adequately address the examination question posed in view of the potential risk from any radiation dose
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9
Q

What is the estimated average annual radiation exposure from natural sources to individuals living in the USA?

A

3 mSv in USA (2.4 mSv worldwide)

  • estimated average annual radiation exposure from natural sources
  • somewhat higher radiation exposure in the USA derives from higher radon concentrations
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10
Q

What is the predominant mechanism for X-ray generation for diagnostic imaging?

A

Bremsstrahlung

  • German for “braking radiation”
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11
Q

How is tube current time product commonly derived in the axial scan mode (prospectively ECG-triggered CT)?

What results in oversimplification of this calculation?

A

tube current time product (mAs) = tube current (milliamperes) x gantry rotation time (seconds)

  • scan angle is not accounted for in this equation and may lead to over- or underestimation of actual radiation exposure
  • to factor in scan angle
    • TCT (mAs) = [tube current (milliamperes) x gantry rotation time (seconds)] x [scan angle / 360]
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12
Q

What is the legal limit for radiation doses in pregnant women working with radioactive material for the entire duration of pregnancy?

A

5 mSv

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13
Q

What is the legal limit for radiation doses in an adult non-pregnant radiation worker over one year?

A

50 mSv

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14
Q

What is the average annual radiation dose contribution among individuals living in the USA from these sources?

  • Medical X-rays
  • Nuclear Medicine
  • Consumer Products
  • Occupational Hazards
A
  • Medical X-rays –> 70%
  • Nuclear Medicine –> 15%
  • Consumer Products –> 5%
  • Occupational Hazards –> 2%
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15
Q

What is the radiation dose reduction between:

  • Prospective triggering
  • Retrospective gating
A

71-83% reduction

  • PROTECTION I study
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16
Q

What is the effect of the following on radiation dose?

  • Decreasing kVp from 120 to 100
A

40% radiation dose reduction

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17
Q

What is the effect of the following on radiation dose?

  • EKG dose-modulation
A

40% radiation dose reduction

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18
Q

What is the effect of the following on radiation dose?

  • Decreasing pitch
A

Increased radiation exposure

  • higher radiation dose due to greater overlap
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19
Q

What adjustment is the best way to minimize radiation dose in this patient?

  • 43 year old male with atypical chest pain
  • BMI 21
  • HR 64 bpm
A

Decrease peak tube voltage from 120 to 100 kVp

  • reduction will likely not negatively affect image quality while having the greatest effect (up to 50% reduction) in reducing radiation exposure
  • patients with low BMI, generally require less X-ray beam energy to achieve adequate tissue penetration
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20
Q

What are the estimated radiation doses for the given procedures?

  • Coronary calcium scanning using retrospectively ECG-gated MDCT
  • Propsectively ECG-triggered CCTA
  • Myocardial stress testing using dual isotope protocol (thallium and sestamibi)
  • Diagnostic cardiac catheterization and coronary angiography including ventriculogram
A
  • ​24-29 mSv - Myocardial stress testing using dual isotope protocol (thallium and sestamibi)
  • 6-8 mSv - Diagnostic cardiac catheterization and coronary angiography including ventriculogram
  • 1-7 mSv - Propsectively ECG-triggered CCTA
  • 2 mSv - Coronary calcium scanning using retrospectively ECG-gated MDCT
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21
Q

What is the most commonly used method of estimating radiation dose from CT imaging?

A

Dose-length-product (DLP) x conversion factor of 0.014

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22
Q

What is the major limitation when estimating radiation dose from CT imaging utilizing DLP x conversion factor?

A

Substantially underestimate the dose

  • these guidelines apply for chest imaging in general
  • do not consider many of the specific aspects of cardiac CT –>
  • leads to underestimation of such derived doses compared to measurements utilizing dosimetry
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23
Q

What organ typically receives the greatest radiation dose during cardiac CT?

A

Breast and Skin

  • followed by:
    • lungs
    • heart
    • esophagus
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24
Q

What CT scan aquisition adjustment generally leads to the greatest reduction of radiation dose to the patient?

A

Prospective ECG-triggering with a 70-75% phase exposure window

  • leads to greatest dose reduction
  • particularly if a short exposure window is chosen
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25
Q

Describe how effective radiation dose is derived?

A

taking the sum of radiation doses to individual organs adjusted by the tissue weighting factors

  • using effective dose allows comparison of radiation exposures in various parts of the body among individuals for risk assessment
  • most of the data for risk assessment were derived from whole body exposure (atomic bomb survivors)
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26
Q

What is the predominant mechanism of harm from ionizing radiation administered with diagnostic X-ray imaging?

A

DNA damage

  • predominant risk –> increased risk of cancer
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27
Q

Three CT examinations of the head in childhood was associated with an increase of the probability of brain cancer observed later in life by a factor of:

A

3

  • three CT scans of the head –> associated with triplingthe risk of brain tumors in a large retrospective cohort of more than 176,000 young adults in the UK
  • causality cannot be proven from the data –> theory is that X-ray radiation increases the risk of malignancy, particularly, when applied early in life
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28
Q

How does equivalent radiation dose account for the carcinogenic potency of X-ray radiation applied to a given individual?

A

The energy absorbed per unit mass x adjustments for the type of radiation

  • Equivalent dose = (energy per unit mass) x (adjustments for the type of radiation)
  • different types of radiation have different biologic effects –> different carcinogenic potency
  • Equivalent dose accounts for these differences by adjusting the energy absorbed for its biologic effects in tissue
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29
Q

How are X-rays generated in an X-ray tube?

A

Electrons are emitted from the cathode and hit the anode where X-rays (photons) are created

  • electrons are emitted from a cathode (heated coil) –>
  • accelerated by a voltage gradient –>
  • directed towards the anode –>
  • electrons entering the anode create X-rays
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30
Q

When generating X-rays, the amount of current in the cathode coil determines this?

A

the amount of electrons (mA)

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31
Q

When generating X-rays, the voltage gradient between the cathode and anode determines this?

A

peak tube voltage

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32
Q

What is one major beneficial area of shielding in Cardiac CT?

A

Breast shielding

  • does reduce organ dose at the expense of image quality
  • increases image noise (similar to lowering tube current) but in a more unpredictable fashion
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33
Q

What is one method of reducing radiation that is more reliable than breast shielding?

A

reduce overall tube current

  • more effective at maintaining image quality
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34
Q

What materials can be utilized to shiled high energy electromagetic radiation (gamma radiation or XR’s)?

A

Lead

or

Concrete

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35
Q

What type of radiation are clothes sufficient at blocking?

A

alpha radiation

36
Q

What type of radiation is glass sufficient at blocking?

A

beta radiation

  • cannot block XR’s
37
Q

What information is typically derived form the dose length product (DLP)?

A

Integrated radiation dose for an entire CT examination

  • defined as: CTDLvol x scan length
  • does not represent the effective radiation dose, which requires consideration of a conversion factor for the body region imaged
38
Q

What is the difference between:

  • DLP
  • Effective radiation dose
A

Conversion factor (for body region imaged)

  • DLP does not represent the effective radiation dose,
  • requires consideration of a conversion factor for the body region imaged
39
Q

What patient group is believed to be at highest risk of developing cancer from Cardiac CT?

A

Women

  • breast receives the highest radiation doses
40
Q

What are advantages of iterative image reconstruction over filtered back projection?

A
  • Reduces image noise
    • improved SNR
  • Reduced radiation doses
  • Requires longer processing (reconstruction) times than traditional FBP
  • Improves image quality
  • Can potentially be used with any CT system
41
Q

What is the weighting factor for XR’s to express their biologic effect in tissue?

A

1

  • XR’s and gamma radiation have a weighting factor of 1
  • net dose in gray and sievert are the same for these types of radiation
42
Q

What factor in patient preparation is most critical for allowing the lowest radiation dose scan acquisition for CT angiography?

A

HR control

  • critical factor for allowing CT acquisition techniques that are associated with lower radiation dose
    • prospective scan triggering
    • single heartbeat acquisition
43
Q

Describe the appropriateness of CCTA:

  • 67 year old female for pre-operative evaluation of non-cardiac surgery
  • low-to-intermediate risk, non-cardiac surgery evaluation
  • no active cardiac conditions
A

Inappropriate because radiation risk outweights benefits

44
Q

Define Automatically Adjusted Tube Current Modulation (AATCM)

A
  • method which automatically lowers the tube current during imaing of tissue with less density resulting in lower radiation exposure
  • Modest effect for Cardiac CT
45
Q

Describe Automatically Adjusted Tube Current Modulation (AATCM) effect on Cardiac CT?

A

Modest effect for Cardiac CT

  • because of the rather homogenous scan conditions for cardiac imaging (surrounded by lung tissue and ribs)
46
Q

What is the expected radiation dose for CT abdomen?

A

15-25 mSv​

47
Q

What are the main advantages of increased z-axis coverage with 128-, 256- and 320- slice scanners?

A

Large detectors enable z-axis coverage with lesser number of heartbeats

  • decreases misregistration artifacts
  • decreases banding artifacts
  • decreased radiation
    • restoration of reduced CNR related to scatter is postulated to require 5-25% more radiation
48
Q

What is one disadvantage of increased z-axis coverage with large detectors (128, 256 and 320 slice)?

A

Increased streak artifacts

  • related to radiograph scatter
  • scattered radiation may also result in decreased contrast-to-noise ratio (CNR) by 5-10% for a single-source system or more for a dual-source system.
49
Q

What is the average radiation dose that is MOST likely to be realistic for coronary CT angiography among US centers?

A

12 mSv

  • progressive laboratories –> 2-3 mSv
    • high utilization of prospective scan triggering is the strongest predictor of lower doses
  • PROTECTION I and PROMISE studies
50
Q

How is an estimate for effective radiation dose derived from the dose length product (DLP)?

A

DLP x conversion factor for body region imaged

  • weight is not directly considered in the equation
  • ​cardiac CT conversion factor = 0.017 (newer literature = 0.014)
51
Q

How is dose reduction with high-pitch dual-source scanners achieved?

A

Decreased X-ray exposure time

  • performed with 2 radiograph tubes and 2 broad 128 x 0.6 mm detectors
  • Using a pitch of 3.4 on these scanners results in the ability to acquire data covering the entire heart in a single heartbeat ( < 260 ms)
  • shortened X-ray time + minimized overlap (with increased pitch) –> significant radiation reduction
52
Q

What unit is used to express the effective radiation dose resulting from exposure of parts of the body with different sensitivities to radiation to an equivalent whole body radiation exposure?

A

Millisievert (mSv)

53
Q

Dual-source scanners have demonstrated the feasibility of acquiring coronary angiography with doses less than 1 mSv without compromise of image quality. The image acquisition for such scanners uses approximately what pitch values?

A

3.4

  • high-pitch scans performed with 2 radiograph tubes and 2 broader (128 x 0.6 mm) deterctors + pitch of 3.4 –>
    • acquire data covering the heart in a single heartbeat
    • data acquisition time of < 260 ms
54
Q

What adjustment to radiation dose does not result in a proportional change?

A

Peak tube voltage

  • changes with the square of the peak tube voltage alteration
  • Proportional change with:
    • tube current
    • exposure time
    • pitch (inversely proportional)
55
Q

What did the Biologic Effects of Ionizing Radiation Committee VII conclude in regards to the relationship between radiation exposure and risk in humans?

A

Current evidence supports a linear, no-threshold dose - response relationship between exposure to ionizing radiation and the development of cancer

  • suggests that any exposure to ionizing radiation poses risk
56
Q

Describe the benefits of ECG-triggered tube current modulation

A
  • Reduces tube current during pre-specified parts of the cardiac cycle –> reduced radiation dose
  • More effective at lower HR’s
    • time is required for the tube current to ramp up and down
  • Should be considered in every patient to allow dose savings
57
Q

What is one major disadvantage of dose ECG-based tube current modulation?

A

reduced image quality during the phase with lowered tube current (mostly systole)

58
Q

What is the average dose of sestamibi SPECT MPI?

Thallium SPECT MPI?

A

11 mSv

24 mSv

59
Q

Compare the differences in radiation dose between:

  • iterative image reconstruction
  • filtered back projection
A

Does not reduce radiation dose but allows to lower X-ray tube settings during image acquisition due to lower image noise than with traditional FBP

  • IIR does not directly influence radiation dose
    • leads to lower levels of image noise compared with FBP
    • X-ray tube settings can be lowered –> lower tube current and/or tube voltage –> would ordinarily lead to increased background noise with FBP
    • noise levels significantly reduced (with 50% lower radiation dose) –> similar image quailty
60
Q

What are the major sources of natural radiation average annual exposure among individuals living in the US?

A

Radon gas (70%)

  • Cosmic radiation (11%)
  • Ingestion (10%)
  • Terrestrial radiation (6%)
61
Q

What are the average radiation doses:

  • MDCT (64-row CT with retrospective ECG-gating, without tube current modulation)
  • Cath (diagnostic study)
  • SPECT (sestamibi)
  • CAC
  • CXR
A
  • MDCT - 15-18 mSv
  • SPECT (sestamibi) - 10-12 mSv
  • Cath - 5-7 mSv
  • CAC - 1-2 mSv
  • CXR - 0.01 mSv
62
Q

Define absorbed radiation dose

A

amount of radiation energy absorbed into a given mass of tissue

  • does not account for the destructive potency of the type of radiation applied
63
Q

X-rays are what type of radiation?

A

electromagnetic radiation

  • similar to gamma radiation but of somewhat lesser energy
64
Q

What is the associated risk with regards to radiation doses to a human fetus from cardiac CT?

A

Low dose and low risk

  • substantial distance between the scan range for cardiac CT and the position of the fetus in the abdomen
  • estimated doses exposure (from chest CT, which is likely higher than cardiac CT) to the embryo are 0.14 mSv –> not more than exposure from three weeks from natural sources
65
Q

Define tube current time product (helical scan)

A

tube current (milliamperes) x gantry rotation (seconds) /

pitch

  • accounts for repeatedly irradiated tissue which occurs with a pitch < 1 (overlap)
  • used interchangeably with “effective tube current time product”
66
Q

Depending on the scanner type used, what is the maximum dose savings that can be expected when using ECG-based tube current modulation?

A

30-50%

  • depending on phase selection, HR, scanner type, extent of tube current reduction
67
Q

What is the expected dose increase for a bypass protocol (including internal mammary arteries) compared to standard cardiac CT?

A

40%

  • due to increase scan length, effective radiation dose on coronary CTA scans to evaluate bypass grafts
68
Q

Increased conspicuity of vessels seen with decreased tube voltage is the result of this?

A

Increase photoelectric effect and Decreased Compton scattering

  • increases opacification of vessels
  • attenuation of coronary arteries is higher at 100 kVp compared to 120 kVp
69
Q

What is the difference between computed tomography dose index (CTDI) and dose length product (DLP)?

A

CTDI does not account for scan length, DLP does

  • DLP provides an estimate for radiation dose to the patient based on a CT examination
    • CTDIvol x scan length = DLP
70
Q

What are the different types of gating/protocol’s that are performed for CT scanning?

A
  • Retrospective helical
  • Dose-modulated helical
  • Prospective
    • Helical
    • Step & shoot (axial)
    • Volume
    • Hi-pitch helical
71
Q

What are Pros/Cons of Retrospective helical scanning?

A
  • Cons:
    • Highest radiation dose (18-20 mSv)
    • Scan is continuous throughout multiple cardiac cycles
  • Pros:
    • Functional assessment
    • Redundant data (averaging, PVC’s)
      • reduce noise
      • throw out PVC’s
    • Select alernative locations in the cardiac cycle
      • Diastasis
      • Isovolumic relaxation
72
Q

Define Pitch

A
  • unitless ratio of:

Table Movement / Rotation

Beam Coverage / Rotation

  • Pitch < 1
  • Overlapping coverage in Retrospective helical scanning
  • Example:
    • [1.0 cm/rotation] / [3.2 cm/rotation] = 0.32
73
Q

Describe the image

A

Helical Retorspective - Dose modulation

74
Q

Describe Retrospective helical - dose modulated scanning

A
  • Reduction in radiation by 30%
    • ​reduction if HR are low
  • Pros:
    • Functional assessment
    • Redundant data (averaging, PVC’s)
  • Cons:
    • Radiation
79
Q

What are the protocols for Prospective gating in CT?

A
  • Helical
  • Step & Shoot
  • Volume
  • Hi-pitch helical
80
Q

As pitch decreases (e.g. from 0.3 to 0.2), if all other variables are constant, what will be the effect on:

  • table speed
  • radiation dose
A
  • table speed –> decrease
  • radiation dose –> increase
81
Q

What are the advantages of sequential (prospectively triggered) scan mode compared to conventional retrospective gating?

A
  • Reduced radiation
  • Reduced scan time
  • Accommodating functional reconstructions

***Temporal resolution is decreased

82
Q

Explain the basic principle of CT technology

A
  • use of ionizing radiation within a gantry rotating around the patient
  • in which X-rays are detected on a detector array
  • and converted through reconstruction algorightms to images
83
Q

What are the techincal advances that have led to more accurate, motion-free imaging of the heart in Cardiac CT?

A
  • Increase in the number of detector rows (or “slices”)
  • Image data (voxels) of equal sizes on all sides, or “isotropism”
    • leads to distortion-free, multiplanar imaging
    • presently, the narrowest commercial detector width is 0.32mm, or “high-definition” CT
  • Higher temporal resolution
    • obtaining motion-free cardiac images requires fast gantry rotation (presently max rotation times are 270-330 msec) and performing image acquisition or reconstruction during periods of limited cardiac motion (end-systole to mid-late diastole)
84
Q

What are the two modes of CT scanning?

What is the trial that demonstrated similar diagnostic image quality between the two?

A
  • Helical scanning
  • Axial Scanning
  • PROSPECT I trial
    • Prospective Multicenter Study on Radiation Dose Estimates of Cardiac CT Angiography
85
Q

Describe the finding

A

PFO

  • small PFO which is demonstrated by a small puff of contrast from the “brighter” left atrium to the right atrium
86
Q

Describe the findings

A

Lipomatous hypertrophy

  • classic appearance of lipomatous hypertrophy of interatrial septum
  • Septum is hypertrophied and the denisty is similar to fat
  • Care should be taken not to misdiagnose as a lipoma
87
Q

Describe Helical CT scanning?

A
  • continuous radiation exposure and table movement
    • patient is moved through the rotating XR beam
  • Relies on collection of a redundant or overlapping data set
  • so that complete image data can be reconstructed using the timing of the ECG after CT data acquisition (“retrospective” reconstruction)
88
Q

What is high-pitch helical CT?

A
  • provides a rapidly acquired CT data set with no overlap
  • This scan mode requires very low heart rates
  • (when utilized) leads to very low radiation exposure ( < 1-2 mSv) with retained image quality and diagnostic accuracy
89
Q

Describe Axial CT scanning

A
  • involves sequential scanner “snapshots”
  • in between which the XR tube is turned off
  • and the table is moved to a different position for the next image to be acquired
  • CT data are then reconstructed in a series of slices akin to slices of a loaf of bread
90
Q

Describe the findings and view:

A

Short Axis

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