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
Describe how effective radiation dose is derived?
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)
What is the predominant mechanism of harm from ionizing radiation administered with diagnostic X-ray imaging?
DNA damage
- predominant risk –> increased risk of cancer
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:
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
How does equivalent radiation dose account for the carcinogenic potency of X-ray radiation applied to a given individual?
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
How are X-rays generated in an X-ray tube?
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
When generating X-rays, the amount of current in the cathode coil determines this?
the amount of electrons (mA)
When generating X-rays, the voltage gradient between the cathode and anode determines this?
peak tube voltage
What is one major beneficial area of shielding in Cardiac CT?
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
What is one method of reducing radiation that is more reliable than breast shielding?
reduce overall tube current
- more effective at maintaining image quality
What materials can be utilized to shiled high energy electromagetic radiation (gamma radiation or XR’s)?
Lead
or
Concrete
What type of radiation are clothes sufficient at blocking?
alpha radiation
What type of radiation is glass sufficient at blocking?
beta radiation
- cannot block XR’s
What information is typically derived form the dose length product (DLP)?
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
What is the difference between:
- DLP
- Effective radiation dose
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
What patient group is believed to be at highest risk of developing cancer from Cardiac CT?
Women
- breast receives the highest radiation doses
What are advantages of iterative image reconstruction over filtered back projection?
- 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
What is the weighting factor for XR’s to express their biologic effect in tissue?
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
What factor in patient preparation is most critical for allowing the lowest radiation dose scan acquisition for CT angiography?
HR control
- critical factor for allowing CT acquisition techniques that are associated with lower radiation dose
- prospective scan triggering
- single heartbeat acquisition
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
Inappropriate because radiation risk outweights benefits
Define Automatically Adjusted Tube Current Modulation (AATCM)
- method which automatically lowers the tube current during imaing of tissue with less density resulting in lower radiation exposure
- Modest effect for Cardiac CT
Describe Automatically Adjusted Tube Current Modulation (AATCM) effect on Cardiac CT?
Modest effect for Cardiac CT
- because of the rather homogenous scan conditions for cardiac imaging (surrounded by lung tissue and ribs)
What is the expected radiation dose for CT abdomen?
15-25 mSv
What are the main advantages of increased z-axis coverage with 128-, 256- and 320- slice scanners?
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
What is one disadvantage of increased z-axis coverage with large detectors (128, 256 and 320 slice)?
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.
What is the average radiation dose that is MOST likely to be realistic for coronary CT angiography among US centers?
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
How is an estimate for effective radiation dose derived from the dose length product (DLP)?
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)
How is dose reduction with high-pitch dual-source scanners achieved?
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
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?
Millisievert (mSv)
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?
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
What adjustment to radiation dose does not result in a proportional change?
Peak tube voltage
- changes with the square of the peak tube voltage alteration
- Proportional change with:
- tube current
- exposure time
- pitch (inversely proportional)
What did the Biologic Effects of Ionizing Radiation Committee VII conclude in regards to the relationship between radiation exposure and risk in humans?
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
Describe the benefits of ECG-triggered tube current modulation
- 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
What is one major disadvantage of dose ECG-based tube current modulation?
reduced image quality during the phase with lowered tube current (mostly systole)
What is the average dose of sestamibi SPECT MPI?
Thallium SPECT MPI?
11 mSv
24 mSv
Compare the differences in radiation dose between:
- iterative image reconstruction
- filtered back projection
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
What are the major sources of natural radiation average annual exposure among individuals living in the US?
Radon gas (70%)
- Cosmic radiation (11%)
- Ingestion (10%)
- Terrestrial radiation (6%)
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
- MDCT - 15-18 mSv
- SPECT (sestamibi) - 10-12 mSv
- Cath - 5-7 mSv
- CAC - 1-2 mSv
- CXR - 0.01 mSv
Define absorbed radiation dose
amount of radiation energy absorbed into a given mass of tissue
- does not account for the destructive potency of the type of radiation applied
X-rays are what type of radiation?
electromagnetic radiation
- similar to gamma radiation but of somewhat lesser energy
What is the associated risk with regards to radiation doses to a human fetus from cardiac CT?
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
Define tube current time product (helical scan)
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”
Depending on the scanner type used, what is the maximum dose savings that can be expected when using ECG-based tube current modulation?
30-50%
- depending on phase selection, HR, scanner type, extent of tube current reduction
What is the expected dose increase for a bypass protocol (including internal mammary arteries) compared to standard cardiac CT?
40%
- due to increase scan length, effective radiation dose on coronary CTA scans to evaluate bypass grafts
Increased conspicuity of vessels seen with decreased tube voltage is the result of this?
Increase photoelectric effect and Decreased Compton scattering
- increases opacification of vessels
- attenuation of coronary arteries is higher at 100 kVp compared to 120 kVp
What is the difference between computed tomography dose index (CTDI) and dose length product (DLP)?
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
What are the different types of gating/protocol’s that are performed for CT scanning?
- Retrospective helical
- Dose-modulated helical
- Prospective
- Helical
- Step & shoot (axial)
- Volume
- Hi-pitch helical
What are Pros/Cons of Retrospective helical scanning?
- 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
Define Pitch
- 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
Describe the image
Helical Retorspective - Dose modulation
Describe Retrospective helical - dose modulated scanning
- Reduction in radiation by 30%
- reduction if HR are low
- Pros:
- Functional assessment
- Redundant data (averaging, PVC’s)
- Cons:
- Radiation
What are the protocols for Prospective gating in CT?
- Helical
- Step & Shoot
- Volume
- Hi-pitch helical
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
- table speed –> decrease
- radiation dose –> increase
What are the advantages of sequential (prospectively triggered) scan mode compared to conventional retrospective gating?
- Reduced radiation
- Reduced scan time
- Accommodating functional reconstructions
***Temporal resolution is decreased
Explain the basic principle of CT technology
- 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
What are the techincal advances that have led to more accurate, motion-free imaging of the heart in Cardiac CT?
- 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)
What are the two modes of CT scanning?
What is the trial that demonstrated similar diagnostic image quality between the two?
- Helical scanning
- Axial Scanning
- PROSPECT I trial
- Prospective Multicenter Study on Radiation Dose Estimates of Cardiac CT Angiography
Describe the finding
PFO
- small PFO which is demonstrated by a small puff of contrast from the “brighter” left atrium to the right atrium
Describe the findings
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
Describe Helical CT scanning?
- 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)
What is high-pitch helical CT?
- 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
Describe Axial CT scanning
- 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
Describe the findings and view:
Short Axis
