Software and Hardware in CCTA

Elly Castellano PhD

2IMIC 2016

Outline

• technical requirements for coronary CTA• the modern cardiac CT scanner• ECG-gating technology• image reconstruction algorithms

3IMIC 2016

Technical requirements for coronary CTA

4IMIC 2016

Technical requirements for coronary CTA

• ECG-gating capability– freezes cardiac motion

• high temporal resolution – reduces blurring due to cardiac motion

• high contrast and spatial resolution– enhances conspicuity of narrow vessels

• large detector coverage per rotation– facilitates short scan times

without ECG gating

with ECG gatingimages courtesy of Willi Kalender

5IMIC 2016

ECG gating

• HR < 65

– end diastole

• HR > 65

– end systole

V

t

V

t

R-R interval 100%

~ 65-75% ~ 25-35%

• two physiological windows of opportunity

• choice in timing and width of acquisition window• to capture one / several / all phases of cardiac cycle

6IMIC 2016

Temporal resolution• determined by time taken to acquire

projection data– rotation time– 180º+ or 360º scan– conventional CT or DSCT

• at 60 bpm require < 200 ms– possible with 0.4 s rotation time and

180º+ scan

180º+ scan

360º scan

7IMIC 2016

Spatial resolution• determined by

– in-plane: reconstruction kernel– out-of-plane: slice thickness

• require – sharp reconstruction kernels

• increase spatial resolution, but increase image noise

– < 1 mm slices• increase spatial resolution, but increase

image noise

Corresponding CT and conventional angiogram images of RCA

8IMIC 2016

Contrast resolution• determined by

– difference in HU between arteries and background• kV setting• partial volume effects

– image noise• require

– contrast enhancement – low kV

• to increase sensitivity to iodine, but image noise and likelihood of photon starvation increase

– high maximum tube mA• for use with lower kV settings

– < 1 mm slices• to reduce partial volume effects, but image noise increases

9IMIC 2016

Coverage per rotation• determined by

– width of detector array with thin detector elements• require

– > 30 mm of sub-mm detector rows• 4 - 5 rotations to cover heart• all data acquisition during optimal contrast enhancement• image-stitching artefacts reduced

10IMIC 2016

The modern cardiac CT scanner

11IMIC 2016

The basic model: cardiac-enabled 64-DCT scanners*

supplier example scanner model

detector arrayrows x element mm

z-coverage mm

slices per rotation x thinnest slice mm

minimum rot s

GE Optima CT660 64 x 0.625 40 128 x 0.625 0.35

Siemens Definition Edge 64 x 0.6 38.4 128 x 0.6 0.285

Philips Brilliance 64 64 x 0.625 40 64 x 0.625 0.4

Toshiba Aquilion CXL 64 x 0.5 32 64 x 0.5 0.35

NB manufacturers with z-FFS or detector resampling double number of slices per rotation

* NB scanner specifications are continually changing

12IMIC 2016

Top MDCT scanners

13IMIC 2016

Revolution series: Rev CT• 512-slice scanner

– 256 detector rows with resampling• improved anatomical coverage

– 160 mm detector covers heart• improved spatial resolution

– new x-ray tube– HD mode using xy-FFS

• improved contrast resolution– “Gemstone Clarity” detector in 3 sections

• increased detector efficiency• lower detector afterglow image courtesy of GE

14IMIC 2016

Brilliance iCT / iCT Elite• 256-slice scanner

– 128 detector rows with z-FFS• improved anatomical

coverage– 80 mm detector

• faster rotation time– 0.27 s with AirGlide

technology and iMRC tube

images courtesy of Philips

15IMIC 2016

Definition Force• 2 imaging chains mounted

at ~94º– 50 and 35 cm FOV

• improved temporal resolution– faster acquisition of

projection data set• improved anatomical

coverage– 96 detector rows, 57.6 mm– “Turbo Flash” mode

• high pitch helical mode

image courtesy of Siemens

16IMIC 2016

Aquilion One ViSION• 640-slice scanner

– 320 detector rows with double sampling

• improved anatomical coverage– 160 mm detector

covers heart– volume mode

• no table feed image courtesy of Toshiba

17IMIC 2016

Top MDCT scanners: summary*

supplier scanner model

detector designrows x element mm

z-coverage mm

slices per rotation x thinnest slice mm

minimum rot s

GE Revolution CT 256 x 0.625 160 512 x 0.625 0.28

Siemens Definition Force 96 x 0.6 x2 57.6 192 x 0.6 0.25

Philips iCT / Elite 128 x 0.625 80 256 x 0.625 0.27

Toshiba Aquilion One Vision 320 x 0.5 160 640 x 0.5 0.275

NB manufacturers with z-FFS or detector resampling double number of slices per rotation

*from manufacturers information. NB scanner specifications are continually changing

18IMIC 2016

ECG gating technology

19IMIC 2016

ECG gating of acquisition• essentially two types:

– prospective– retrospective

• different image sets generated• different radiation burden to patient

20IMIC 2016

Prospective gating• acquisition triggered by ECG

– set time after R wave• acquisition window matched to

minimum acquisition time– “no padding” acquisition– image sets in selected phase

• acquisition window extended beyond minimum acquisition time– “with padding” acquisition– at full or reduced mA– image sets in 1+ selected phases– can adjust for heart rate changes

mA

t

prescribed mAs

padding mAs

mA

t

prescribed mAs padding mAs

21IMIC 2016

Prospective gating• choice of table motions

– step-and-shoot– slow helical– high-pitch helical (Flash)– none

• axial images reconstructed from partial projection data sets• MPRs, VR image sets• patient dose 1 – 5 mSv

– 50 – 250 CXRs

partial projection data set

22IMIC 2016

Prospective gating

LAD with stent

Image courtesy of Universität Ulm, Germany, and Philips

images courtesy of Institute of Medical Physics / Erlangen, Germany, and Siemens

Flash helical mode

23IMIC 2016

Retrospective gating• long acquisition

– fast rotation time

• choice of table motion– slow helical

• pitch typically 0.2 – 0.3– none

• projections tagged with ECG signal

• ECG-gated mA pulsing– selectable temporal window

position and width– restricts reconstruction to

selected cardiac phase(s)

24IMIC 2016

Retrospective gating• images reconstructed from

partial projection data set– to improve temporal resolution

• single or multi-segment recon– determines temporal resolution

• reconstruction window can be adjusted– essential for arrhythmic patients

• typical patient dose 10 – 30 mSv– 500 – 1500 CXRs

multi-segment recon

single-segment recon

25IMIC 2016

Retrospective gating• coronary CTA from

images reconstructed at one cardiac phase

• cardiac function from images reconstructed at several cardiac phases

Functional Analysis, EF 80%

images courtesy of Dr Rybicki, BWH, and Toshiba

26IMIC 2016

Dealing with irregular heart rates

• arrhythmia detection algorithms– acquisition delayed until rhythm re-established

• compensation for heart rate changes– acquisition window automatically widened

• to capture required phase(s)

– may impact on radiation dose to patient

27IMIC 2016

Image reconstruction algorithms

28IMIC 2016

Filtered back-projection

• choice of convolution kernels

images courtesy of Willi Kalender

29IMIC 2016

Iterative reconstruction

from Beister et al 2011

30IMIC 2016

IR: vendor solutions

supplier IR algorithms

GE ASiR Veo ASiR-V

Siemens SAFIRE ADMIRE

Philips iDose4 IMRToshiba AIDR 3D

proprietary black boxes!

31IMIC 2016

IR solutions in raw data and image domains

• used by Philips (iDose), Siemens and Toshiba• based on statistical modelling of x-ray detection

from Willemink et al 2013

32IMIC 2016

IR solutions in raw data domain• used by GE, Philips (IMR)• based on

– statistical modelling of x-ray detection– geometric modelling of imaging chain (Veo)– physics modelling (ASiR-V)

from Willemink et al 2013

33IMIC 2016

Iterative reconstruction• images presented

– different IR “strengths”– mixed with FBP

• lower image noise than FBP– opportunity for dose reduction

images courtesy of Siemens

FBP

SAFIRE

34IMIC 2016

Iterative reconstruction• slower recon than FBP• IR-specific artefacts may be introduced

from Beister et al 2012

disappearance of hypodense seam

35IMIC 2016

Conclusions• coronary CT angiography requires cardiac-

enabled scanners– 64+ DCT– choice of ECG gating techniques

• iterative reconstruction algorithms should be carefully validated

36IMIC 2016

Acknowledgements

• Dee Mears, RMH• Nina Arcuri, RBH• Willi Kalender• ImPACT group• Philips• Siemens• Toshiba• GE