ctca dose reduction & image quality improvement strategy in ndh

Speaker: Au Chun Yu EdmundChong Siu King WindyNorth district Hospital

CTCA Dose Reduction & Image Quality Improvement Strategy in NDH

HKRA AGM 2011

In NDH: 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 80-100Tube current modulation:

~650mA; 30%-80%

~600mA; 30%-80%

<500mA; 40%-80%

Scan coverage: Superior: 2cm above carinaInferior: 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

Results:

Analyzed statistically Maintain diagnostic quality Radiation protection

1 2 3 4 5 6 7 8 9 10 11 120

2

4

6

8

10

12

14

Effective Radiation Dose of CTCA in 2010 in NDH

MonthEffec

tive

Rad

iati

on D

ose

(mSv

)

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:

Analysis of Image quality:

Noise:

standard deviation of the density (in HU) within a large region of interest.

Contrast-to-noise ratio (CNR):

CNR = (HU LV Chamber – HU LV wall)/noise

Signal-to-noise ratio (SNR):

SNR = HU coronary artery lumen/noise

Subjective: (analyzed by the radiologists)Image noise Overall image quality with diagnostic confidence level

Objective:Image noise Contrast –to-noise ratio Signal-to-noise ratio

Before & After… …

kVp modification:

kVp adjustment according to patient’s body weight

Radiation dose is proportional to the square of kVp

Stage 1: Stage 2: Stage 3:>80kg 120 kVp 80 kVp 100kVp<80kg 80kVp

mA modification:

Mean dose reduction: 20%

Stage 1: Stage 2:mA: Manual mA:

>500

Tube current modulation:

<500; 40%-80%

Mean Dose:(DLP/mGY-cm)

635.76 507.90

Scan length modification:

Reduction of 1cm: dose savings of 1 mSv

Radiation dose reduction: 20%

Stage 1: Stage 2:Superior aspect: 2 cm superior to

carina of tracheaSufficiently include

LADInferior aspect: Base of heart Sufficiently include

PLB & PDAMean Dose:

(DLP/mGY-cm)637.5 512.05

Contrast volume:

Reduction of contrast : Decrease probability of allergic reaction

Faster contrast rate: Better contrast resolution

GE suggestion: International: NDH:80ml; 5ml/s 80ml; 5ml/s 60-65ml;

5-5.5ml/s

Protocol selection:

Snapshot pulse (HR 30-65BPM)

Prospective ECG gating

Snapshot segment(HR 30-74BPM)

Snapshot burst(HR 75-113)

Retrospective ECG gating

Protocol selection:

The most dose-efficient method of ECG-synchronized: Snapshot pulse Dose reduction by 64% (compared with segment)

Case # Mean dose (DLP/mGy-

cm)

Lowest Highest Average DLP/slice

Burst 21 723.94 548.67 899.75 3.73pulse 4 197.46 105.10 315.56 0.99

segment

136 548.54 349.54 879.08 2.78

Snapshot pulse: X-ray on/off is triggered by ECG R-peak with user

selectable time off

Radiation exposure is about 4 times less

Limitation: HR <60bpm Stable rhythms dependence Allow limited phase reconstruction:

only 3-4 % phase Insufficient for functional analysis & Electrophysiology (EP)

NDH DECISION:SEGMENT

Snapshot segment: Pros:

Helical continuous data acquisitionFavor retro-reconstruction

Option for different cardiac pattern;Enable cardiac function analysis

Larger volume coverage i.e. bypass graft

Patient preparation:

Breathing technique:

Options for different types of patient:Important in evaluation of time for

stable HR after breath-hold

Stage 1: Stage 2:Breathing technique: Inspiration only Inspiration/suspension

Beta-blocker:

Lower heart beat and stabilize rhythm : Improve temporal resolution Options for scanning protocol selection

Flowchart of beta-blocker standardized

Stage 1: Stage 2:Heart rate: >70 bpm >65 bpm

Flow chart of beta blocker:<65 bpm

(1)>65 bpm

Stable irregular

Pulse

Segment

1st β medication

30 mins

<65 bpm(1)

>65 bpm

2nd β medication30 mins

<65 bpm(1)

>65 bpm

Calcium score + consult radiologist

Patient Preparation checklist: 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

Scanning Protocol checklist:

Test dose: Calculation of delay time Contrast volume depends on delay time

1. Scout view:

2. Calcium score:If score >400 consult radiologist

3. Test dose:20ml IOP370 at 5ml/s + saline at 5ml/s

4. Contrast scan protocol selection:

Burst/Segment/Pulse mode

Technique Modification:

(pathology-related)

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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