Cardiac Ct - Physisists 5.3.24 Flashcards
What are the 3 main methods of cardiac scanning?
• Sequential scanning
•Cone beam CT (single rotation capture)
• 2 tube system and rapid table feed
Why do we need Heart rate control?
Slower the heart rate, the longer the period of heart stasis at the end diastole
This maximises the chance of success under prospective gating utilising a single cardiac phase without phase segmentations and summation.
Consider breath hold and/or beta blockers to slow the heart rate
Which phase of the cardiac cycle do we scan during?
Need to image during phase of least cardiac motion (diastole or end systole)
How is the R-R interval used in cardiac scanning?
R- R internal defined as 100%
Reconstructed image position
- usually given as % of R - R interval (eg:70%) from first R peak
- or sometimes as Ms before next R wave
Extent of data acquired given as % of R-R
For data acquisition how much data is needed?
Opposing projections provide the same information so to reconstruct only 180° of scan data is required
Image time = rotation time /2
: 150ms = 300ms rotation / 2
What is the optimal phase for reconstruction for CTA?
Diastole @~ 70% ( position of least motion)
What is the benefit of Axial scanning with padding?
Why is padding axial scanning used?
Allows more flexibility with reconstructed phase position
Over scanning to have more chance of getting the heart in the correct phase
What does Helical cardiac CT need?
Requires an overlapping pitch ~ 0.2 - 0.3 to eliminate gaps in coverage
Eg: 0.3 sec ( 300Ms) Scan ~ 3 rotations per heartbeat
Which gating is axial scanning?
Prospective gating
What gating triggers helical scanning?
Retrospective gating
What is the Description of axial cardiac CT?
Snap shot
What is a Description of helical cardiac CT ?
Movie - select image afterwards
What are the BSCI standards?
Standard of practice of computed tomography coronary angiography (CTCA) in adult patients
What units does absorbed dose have?
Gray
What is absorbed dose?
Energy imparted per unit mass of the medium
What are the Conversions of grays?
1 gray =1 joule /kg
1 centigram - cGy
1 milligray -mGy
1 microgray -1 uGy
What is the Equivalent dose calculation?
Equivalent dose (sv) = absorbed dose (gy) x radiation weighting factor(Wr)
Wr =1 for diagnostic X-rays, electrons and B particles
We = 20 for alpha particles
Therefore one grey = one sivert for X-rays
But 1 gray = 20 Sv for alpha radiation
What is Effective dose?
What is the calculation?
Effective dose (sv) = equivalent dose (sv) x tissue weighting factor (w T)
” that uniform whole body exposure that would give the same risk of harm as a non-uniform partial body exposure.
what are Tissue weighting factors?
Defined for different organs based on their individual radio sensitivity
Based on epidemiological studies on cancer
Why are there Errors in use of generic conversion factors?
• Varies with body habitus
• dependent on kVp (little difference above 100 kv)
• K dependent on scan volume ie 100/140/160mm (higher for smaller volumes)
What is Prospective gating ?
~ lower radiation dose ( provided limited padding used)
~ requires low, stable and predictable heart rate
~ better Z-axis resolution
~use of padding can return information at other cardiac phases
What is Retrospective gating ?
Higher radiation dose, but not substantial way careful choice of MA modulation
Fast, irregular heart rates can still yield diagnostic images
What to do for The difficult patient ?
> calcium protocol
- best spatial resolution needed to reduce bloodying
- Used HD more
- edge enhancing post processing
- Lower KVp not a major issue
HD struggles with BMI over 33
What ate Limitations of cardiac CT?
Temporal resolution
Movement
Resolution
Radiation dose
Erratic heart rates - Af or ectopic beats
Fast HRs
Large patients
High calcium yield
