CT - Chapter 1 Flashcards
Based on dose minimization, which patients would CCTA be LEAST appropriate?
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
What patient population is CCTA best ruling out CAD?
Low-Intermediate risk individuals
- High sensitivity
- High negative predictive value
What does effective radiation dose compare?
cancer risk from:
a non-uniform exposure of ionizing radiation
to
a uniform exposure of the whole body
What is the equivalent radiation dose for 10 mGy?
- mSV
- Sv
- 10 mGy = 10 mSV
- Sv = 0.01 Sv
Why are radiation doses between gray and sievert interchangeable for radiation from X-rays?
Because XR’s have a weighting factor of 1
- not the case for some other radiation sources such as neutrons or alpha particles
What radiation dose reduction can be expected with CT angiogram acquisition?
- Propsective ECG-triggering
- Retrospective ECG-triggering
80-90% reduction
- depends on phase window chosen
Describe Computed Tomography Dose Index (CTDI)
- 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
Describe ALARA and its importance
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
What is the estimated average annual radiation exposure from natural sources to individuals living in the USA?
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
What is the predominant mechanism for X-ray generation for diagnostic imaging?
Bremsstrahlung
- German for “braking radiation”
How is tube current time product commonly derived in the axial scan mode (prospectively ECG-triggered CT)?
What results in oversimplification of this calculation?
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]
What is the legal limit for radiation doses in pregnant women working with radioactive material for the entire duration of pregnancy?
5 mSv
What is the legal limit for radiation doses in an adult non-pregnant radiation worker over one year?
50 mSv
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
- Medical X-rays –> 70%
- Nuclear Medicine –> 15%
- Consumer Products –> 5%
- Occupational Hazards –> 2%
What is the radiation dose reduction between:
- Prospective triggering
- Retrospective gating
71-83% reduction
- PROTECTION I study
What is the effect of the following on radiation dose?
- Decreasing kVp from 120 to 100
40% radiation dose reduction
What is the effect of the following on radiation dose?
- EKG dose-modulation
40% radiation dose reduction
What is the effect of the following on radiation dose?
- Decreasing pitch
Increased radiation exposure
- higher radiation dose due to greater overlap
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
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
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
- 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
What is the most commonly used method of estimating radiation dose from CT imaging?
Dose-length-product (DLP) x conversion factor of 0.014
What is the major limitation when estimating radiation dose from CT imaging utilizing DLP x conversion factor?
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
What organ typically receives the greatest radiation dose during cardiac CT?
Breast and Skin
- followed by:
- lungs
- heart
- esophagus
What CT scan aquisition adjustment generally leads to the greatest reduction of radiation dose to the patient?
Prospective ECG-triggering with a 70-75% phase exposure window
- leads to greatest dose reduction
- particularly if a short exposure window is chosen










