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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
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