CTCA Dose Reduction &
Image Quality Improvement
Strategy in NDH
Speaker: Au Chun Yu Edmund
Chong Siu King Windy
HKRA AGM 2011
North district Hospital
 CT machine: GE Lightspeed VCT, 64MSCT
 Over 500 cases done (since 2008):
 Cardiac CT booked daily
 several sub – stages:
Protocol selection
Scanning
parameters
Scanning range
Breath-hold
preparation
premedication
Contrast volume
NDH vs other standards:
GE suggestion:
International:
NDH:
kVp:
120
100
80-100
Tube current
modulation:
~650mA;
30%-80%
~600mA;
30%-80%
<500mA;
40%-80%
Scan coverage:
Superior: 2cm
above carina
Inferior: base of
heart
Superior: sufficiently include LAD
Inferior: sufficiently include PLB &
PDA
Contrast volume: 80ml; 5ml/s
80ml; 5ml/s
60-65ml;
5-5.5ml/s
Protocol
selection:
Pulse
Pulse/Segment
Segment
Breathing
technique:
inspiration
Not applicable
Inspiration/
suspension
Beta-blocker:
HR:>70bpm
HR>70bpm
HR:>65bpm
mSv
11
7-12
7.88
 Analyzed statistically
 Maintain diagnostic quality
 Radiation protection
Effective Radiation Dose (mSv)
Effective Radiation Dose
of CTCA in 2010 in NDH
14
12
10
8
6
4
2
0
1
2
3
4
5
6
7
8
Month
9
10 11 12
Limited conditions:
 Limited pre-medication:
 Beta-blocker prescribed by Cardiac department
 CT machine: GE Lightspeed VCT, 64MSCT
 Maintain high image quality for reporting
Outcome:
 International standard dose for CTCA in 64MSCT :
 7-12mSv
 Average effective dose in NDH (2010):
 7.88mSv
 ~10% Dose reduction throughout 2010
 Organized, structured & optimized protocol agreed
with radiologists
 Successful training program for junior radiographers
Image Quality:
Subjective: (analyzed by the radiologists)
Image noise
Overall image quality with diagnostic confidence level
Objective:
Image noise

standard deviation of the density (in HU) within a large region of interest.
Contrast-to-noise ratio (CNR):


Signal-to-noise ratio
Noise:


Contrast –to-noise ratio
CNR = (HU LV Chamber – HU LV wall)/noise
Signal-to-noise ratio (SNR):

SNR = HU coronary artery lumen/noise
Before & After… …
>80kg
<80kg
Stage 1:
Stage 2:
Stage 3:
120 kVp
80 kVp
100kVp
80kVp
 kVp adjustment according to patient’s body weight
 Radiation dose is proportional to the square of kVp
Stage 1:
mA:
Mean Dose:
(DLP/mGY-cm)
Stage 2:
Manual mA:
>500
Tube current
modulation:
<500; 40%-80%
635.76
507.90
Mean dose reduction: 20%
Stage 1:
Stage 2:
Superior aspect:
2 cm superior to
carina of trachea
Sufficiently include
LAD
Inferior aspect:
Base of heart
Sufficiently include
PLB & PDA
Mean Dose:
(DLP/mGY-cm)
637.5
512.05
 Reduction of 1cm: dose savings of 1 mSv
Radiation dose reduction: 20%
Contrast volume:
GE suggestion:
International:
NDH:
80ml; 5ml/s
80ml; 5ml/s
60-65ml;
5-5.5ml/s
 Reduction of contrast :
 Decrease probability of allergic reaction
 Faster contrast rate:
 Better contrast resolution
Snapshot pulse
(HR 30-65BPM)
Prospective ECG gating
Snapshot segment
(HR 30-74BPM)
Snapshot burst
(HR 75-113)
Retrospective ECG gating
Case #
Mean dose
(DLP/mGy-cm)
Lowest
Highest
Average
DLP/slice
Burst
21
723.94
548.67
899.75
3.73
pulse
4
197.46
105.10
315.56
0.99
segment
136
548.54
349.54
879.08
2.78
 The most dose-efficient method of ECG-synchronized:
 Snapshot pulse
 Dose reduction by 64% (compared with segment)
 X-ray on/off is triggered by ECG R-peak with user
selectable time off
Radiation exposure is about 4 times less
 HR <60bpm
 Stable rhythms dependence
 Allow limited phase reconstruction:
 only 3-4 % phase
 Insufficient for functional analysis & Electrophysiology (EP)
 Pros:
 Helical continuous data acquisition
Favor retro-reconstruction
Option for different cardiac pattern;
Enable cardiac function analysis
 Larger volume coverage
i.e. bypass graft
Patient preparation:
Breathing technique:
Breathing technique:
Stage 1:
Stage 2:
Inspiration only
Inspiration/suspension
 Options for different types of patient:
Important in evaluation of time for stable
HR after breath-hold
Beta-blocker:
Heart rate:
Stage 1:
Stage 2:
>70 bpm
>65 bpm
 Lower heart beat and stabilize rhythm :
 Improve temporal resolution
 Options for scanning protocol selection
 Flowchart of beta-blocker standardized
Flow chart of beta blocker:
<65 bpm
>65 bpm
(1)
Stable
Pulse
1st β medication
irregular
Segment
30 mins
<65 bpm
>65 bpm
(1)
2nd β medication
30 mins
<65 bpm
>65 bpm
(1)
Calcium score + consult radiologist
 No caffeine & smoking 12 hrs before exam
 Prepare for contrast CT scan
 i.e. fasting, Metformin, LMP
 Steroid cover
 Measure resting heart rate (HR):
 Below 70 bpm: preferable
 >70 bpm: consult radiologist for medication
 Breathing instruction rehearsal:
 Evaluate the time of stable HR after breath-hold
 IV access: 18 gauge(5ml/sec), right-sided preferable
1. Scout view:
2. Calcium score:
If score >400  consult radiologist
3. Test dose:
20ml IOP370 at 5ml/s + saline at 5ml/s
 Test dose:
 Calculation of delay time
 Contrast volume depends on delay time
4. Contrast scan protocol selection:
Burst/Segment/Pulse mode
Grafting:
 Bypass grafting implant of left internal mammary artery (LIMA) to LAD
 Right IMA or inferior epigastric artery grafting to RCA
 Increase scan coverage superiorly
 Only segment protocol applicable
Future directions in NDH:
Future directions in NDH:
 Cross-departmental communication:
 Improve pre-medication prescription
 Pulse scanning protocol trial
 Further radiation dose reduction
 BMI (body mass index) dependent:
 Develop all-rounded & more precise kVp modification
Conclusion:
 Radiation dose reduction with satisfactory image
quality
 Structured ,organized & optimized protocol
 Ease the workflow of CT cardiac exam
 Improve efficiency and effectiveness for both radiologists
and radiographers
 Junior radiographers gain confidence in Cardiac CT
training program
Acknowledgements:
 Mr Ho (DM), Mr Wong (SR) & Mr Leung (SR) of NDH
 Ms Tracy Chan, Mr Eddy Chan & Mr Wayne Li
 Staff of NDH Radiology department
 Cardiac team of NDH
 HKRA
 Patients involved…
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