Cardiac Ct - Physisists 5.3.24

Question 1

What are the 3 main methods of cardiac scanning?

Answer 1

• Sequential scanning
•Cone beam CT (single rotation capture)
• 2 tube system and rapid table feed

Question 2

Why do we need Heart rate control?

Answer 2

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

Question 3

Which phase of the cardiac cycle do we scan during?

Answer 3

Need to image during phase of least cardiac motion (diastole or end systole)

Question 4

How is the R-R interval used in cardiac scanning?

Answer 4

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

Question 5

For data acquisition how much data is needed?

Answer 5

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

Question 6

What is the optimal phase for reconstruction for CTA?

Answer 6

Diastole @~ 70% ( position of least motion)

Question 7

What is the benefit of Axial scanning with padding?
Why is padding axial scanning used?

Answer 7

Allows more flexibility with reconstructed phase position

Over scanning to have more chance of getting the heart in the correct phase

Question 8

What does Helical cardiac CT need?

Answer 8

Requires an overlapping pitch ~ 0.2 - 0.3 to eliminate gaps in coverage

Eg: 0.3 sec ( 300Ms) Scan ~ 3 rotations per heartbeat

Question 9

Which gating is axial scanning?

Answer 9

Prospective gating

Question 10

What gating triggers helical scanning?

Answer 10

Retrospective gating

Question 11

What is the Description of axial cardiac CT?

Answer 11

Snap shot

Question 12

What is a Description of helical cardiac CT ?

Answer 12

Movie - select image afterwards

Question 13

What are the BSCI standards?

Answer 13

Standard of practice of computed tomography coronary angiography (CTCA) in adult patients

Question 14

What units does absorbed dose have?

Answer 14

Gray

Question 15

What is absorbed dose?

Answer 15

Energy imparted per unit mass of the medium

Question 16

What are the Conversions of grays?

Answer 16

1 gray =1 joule /kg

1 centigram - cGy
1 milligray -mGy
1 microgray -1 uGy

Question 17

What is the Equivalent dose calculation?

Answer 17

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

Question 18

What is Effective dose?
What is the calculation?

Answer 18

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.

Question 19

what are Tissue weighting factors?

Answer 19

Defined for different organs based on their individual radio sensitivity

Based on epidemiological studies on cancer

Question 20

Why are there Errors in use of generic conversion factors?

Answer 20

• 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)

Question 21

What is Prospective gating ?

Answer 21

~ 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

Question 22

What is Retrospective gating ?

Answer 22

Higher radiation dose, but not substantial way careful choice of MA modulation

Fast, irregular heart rates can still yield diagnostic images

Question 23

What to do for The difficult patient ?

Answer 23

> 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

Question 24

What ate Limitations of cardiac CT?

Answer 24

Temporal resolution
Movement
Resolution
Radiation dose
Erratic heart rates - Af or ectopic beats
Fast HRs
Large patients
High calcium yield

Question 25

What do we want?

Answer 25

Visualisation of the coronary vessels to the smallest structure at the lowest radiation dose for the fast erratic HR in a patient with high coronary artery calcium

This would permit CTCA as a first line test in the rule out and assess CAD