Download How to optimize your CT scan and image reading.

How to optimize your CT scan and image reading
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How to Perform and Interpret
Computed Tomography Coronary
Angiography Cardiac CT
L.Davin
CHU Liege Belgium
Cardiology Department
Euroecho 2012
How to optimize your CT
 Since the late 1990, research has shown that CT
imaging allows for non invasive imaging of the heart
and great vessels, especially the coronary arteries
 Imaging the coronary arteries is challenging because
of their small dimension and motion.
 Cardiovascular CT requires high temporal resolution
to « freeze » cardiac motion
 Data acquisition and image reconstruction fully
synchronized with the ECG signal are also necessary
to obtain image datasets from the desired cardiac
phase.
 Furthermore, imaging of the coronary arteries requires
high spatial resolution and the reconstruction of thin
slices.
Data Acquisition
Patient selection
 Patient should be screened for any
contraindications to contrast-enhanced CT
 Fully briefed about the CT scan to reduce any
apprehension
 Able to perform a breath hold maneuver at least
as long as the scan time.
 Intravenous cannula of 18-20 gauge in the
anticubital fossa
Engelken FJ. Acad Radiol 2009
Data acquisition
Patient preparation
 Exclude patients with « significant » arrhythmia, compared to patients
with normal sinus rhythm, patients with AF had the highest radiation
exposure
Techasith T. J Cardiovasc Comput Tomogr 2011
Yang L. AJR 2009
 Another crucial point is optimization of patient‟s rate if the rate is
considered to high (the level of this heart rate for using BB depends
on the protocol used for the acquisition but also on the technical
characteristics of the scanner ( if ≥65-90 beats/min)
 Metoprolol oral (25-100mg) or IV (5-75mg)
Johnson PT. AJR 2008
 Sublingual Nitroglycerin can be used: 2 spray=0,8 mg , it widens the
coronary arteries and may also help in differentiating true lesion from
temporary spasms
Hamon M. N Engl J Med 2006
Data Acquisition
Contrast administration protocols
 Two methods are used for timing scan acquisition
 Bolus tracking technique: a region of interest (ROI) is positioned in
the ascending aorta. At a predefined threshold within the ROI is
reached, the patient is automatically instructed to perform a
breath hold maneuver after which the scan will start
 The test bolus method to determine individual circulation time can
be used
 The contrast agents used include different commercially
available preparations with iodine content of 300 to 400 mg/ml
Cademartiri F. Invest Radiol 2006
 A flow volume of 5 ml per second (4-6ml/s), can be adjusted to
the patient‟s body weight or body index
A saline chaser after contrast administration
100 ml CP – 40 ml saline
triphasic protocol:30%:70% contrast
media-saline mixture in 2d phase
A saline chaser following intravenous contrast injection is usually used.
The main effect is the reduction of artifact that might otherwise result from high
amounts of contrast medium in the right atrium and ventricle
Zhu X. Int J Cardiovasc Imaging 2012
Kim DJ Radiology 2008
Non-enhanced scan
Imaging of coronary calcification by non-enhanced computed tomography.
Coronary calcium is clearly depicted because of its high CT attenuation.
Not only atherosclerotic coronary plaque is calcified, but within a
coronary artery,the amount of coronary calcium roughly correlates to
the extent of atherosclerotic plaque burden.
Agatston score
Based on the Agatston score, more pronounced calcium is associated with
higher risk: in the same patient population, a calcium score between 1 and 100
was associated with a hazard ratio for major coronary events of 3.89 whereas
the risk is considerably much higher with a score of more than 300
„Agatston score‟
0
1–100
101–300
≥301
Hazard ratio (major
coronary events)
Number of individuals followed 3.8years
With events
Total
1
8
3409
3.89
25
1728
7.08
24
752
6.84
32
833
Similar to several previous trials, the results of this study confirmed that coronary
calcium provides incremental prognostic information beyond traditional risk factors
Detrano R. N Engl J Med 2008
Single source CT
Temporal resolution
Temp. Resolution =
Rotation Time
2
= 135 ms
The time required to collect all data needed for reconstruction of cardiac images
is approximately one-half the gantry rotation time
Multicycle reconstruction
92,5ms
92,5ms
Temporal Resolution= ¼ rotation time
To overcome insufficient temporal resolution at high heart rates, single source CT
scanners use a software solution called multisegment reconstruction. Two or more
subsequent heartbeats are used to collect 180 degrees of cardiac data.
Scan modes
 Retrospective
 ECG-Gated Spiral (helical)CT
 Continuous x-rays with
simultaneous bed translation
 8 – 16 mSv
 Tube current modulation
With low-pitch helical scanning, data are retrospectively gated to the patient‟s
recorded ECG signal. X-ray data are acquired throughout the cardiac cycle
with continuous rotation of the gantry and movement of the table until the entire
scan length is covered. This mode retrospective may be preferred for patients
with high and/or irregular heart rates. A significant decrease in radiation dose
can be achieved with tube current modulation.
Desai MY. Heart 2011
Halliburton SS. J Cardiovasc Comuted Tomogr 2011
Scan modes
 Prospective ECG-triggered
Axial scan
 “Step-and-shout” acquisition
 low dose, diagnostic scan (4-6
mSv)
 equal image quality as helical
scanning.
 Regular , low heart rates
For the prospective ECG triggered axial mode, scanning is initiated at a
predefined time after the detection of an R peak while the patient table is
stationary. Multislice CT coronary angiography with prospective ECG-gating
leads to a significant reduction of radiation dose when compared to that of
retrospective ECG-gating, while offering comparable image quality and
diagnostic value. In prospective mode, heart rate and rhythm can provoke
different types of scanner performance, which can significantly alter radiation
exposure and scan time.
Desai MY. Heart 2011
Halliburton SS. J Cardiovasc Comuted Tomogr 2011
“ECG-Pulsing” with dose reduction
Used to limit radiation exposure: limits high
dose to diastolic phase
Disadvantage: late systolic images can
not be adequately reconstructed.
Because CT data are typically needed only from the cardiac phase with the
least motion (the mid-diastolic or end-systolic phase) for image reconstruction,
a significant decrease in radiation dose can be achieved by modulating the
tube current according to the patient‟s ECG signal to a maximum value during
the desired reconstruction phase of the cardiac cycle and a minimum value
during the ramaining phases
Leschka S. Invest Radiol 2007
Reconstruction
 A slice thickness of 0.625 mm ( 0.5-2mm)
 Increment of 0.5 mm (0.2-4mm)
 Small reconstruction field-of-view: 18 cm (12-50 cm)
 Reconstruction using a single interval at a specific phase:
for heart rates under of 60 beats/min, optimal image
quality is usually found during late diastole, during a time
window between 65 and 75% of the cardiac cycle
 Reconstruction with the phases from 0% to 90% in 10%
intervals from the RR interval: select best series
Reading
 Interpret with axial source images or coronal and sagittal reconstructed
images
 A workstation is capable of automatically generating multiplanar
reconstructions (MPRs) for interpretation: make Curved MPR along each
vessel
 Interpret with help of cross-sectional images perpendicular to centerline
 Thin(thick)-slice in maximum intensity projection (MIPs)
 3D volume-rendered reconstruction
 Angiographic emulations
 Cath views
 Evaluation of pulmonary and other extracardiac structures
 Total time required for interpretation and reporting is about 15-30 min
Curved Multiplanar
reconstruction
Hardware : Wide-Detector CT
 64-row is regarded as the minimum required technical standard for
robust clinical applications of coronary CT. One factor is the limited TR
as compared with the rapid motion of the heart. Another potential
problem is caused by the fact that image acquisition requires several
heart beats with some vulnerability to breathing artifacts and to the
occurrence of arrhythmias.
 128/256-row: Most manufacturers increased the number of
simultaneously acquired slices which will shorten the number of
heartbeats that are required to complete the acquisition of a cardiac
CT data set
 320-detector row CT, with improved longitudinal coverage of
detector, permits prospectively triggered axial acquisition with a
stationary table and ideally covers the volume of the heart within a
single cardiac cycle. It resolves step artefact and high patient dose
caused by irregular heart rate.
Weigold W. Int J Cardiovasc Imaging 2009
Abbara S. J Cardiovasc Comptud Tomogr 2009
Li Y. Eur Radiol 2012; Sun G.Br J Radiol 2012;
Lee AB. AJR 20112; de Graaf FR. EHJ 2010
Hardware: Dual Source CT
The gantry contains 2 x-ray tubes and 2 detectors. The tubes are arranged in ±90°
angle. It permits the collection of x-ray data in 180° of projections during only a
quarter rotation of the gantry, 2 times faster than for single-source CT.
100 bpm single source CT
Slow Acquisition Speed
100 bpm Dual Source CT
Fast Acquisition Speed
The higher TR as compared with single-source CT makes this technology less vulnerable
to high heart rates which suggest that heart-rate control is no longer required
 X-ray focal spot:
to achieve improved through-plane spatial
resolution, some systems use an x-ray focal spot
that alternates between 2 Z-positions to acquire
2 overlapping slices for each detector row
 High pitch mode:
Marwan M. EHJ 2010
Achenbach S. J cardiovasc Comput Tomogr
2012
Recently, a new image-acquisition protocol has
been described, with a rapid movement of the
table with a prospectively ECG triggered highpitch spiral acquisition
Achenbach S. JACC Cardiovasc Imaging 2011
Stolzmann P. Invest Radiol 2010
Flohr TG. J Cardiovasc Computed Tomogr 2009
 Dual Energy
 2 x-ray source/detector systems, each operated at different peak
tube potentials, at different energy levels with potentiel advantages:
The improved delineation of vascular calcium (and its separation
from the contrast-enhanced lumen) and an improved visualisation of
differencies in myocardial contrast enhancement when myocardial
perfusion is studied.
 A single x-ray source/detector with novel detector material that
permits rapid tube potential switching
 Single x-ray source and dual layers of energy-sensitive detectors to
acquire both low-energy and high-energy x-ray photons
simultaneously
Achenbach S. JACC Cardiovasc Imaging 2011
Stolzmann P. Invest Radiol 2010
Flohr TG. J Cardiovasc Computed Tomogr 2009
New detectors
 improve the ratio signal/noise of images and the
spatial resolution without impact on the
radiation dose
 Stellar Siemens (0.3 mm)
 Gemstone GE Healthcare (0.22mm) with possibility
to get spectral imaging by the technique of
Energy
Koo BK. JACC 2011
Software: Iterative reconstruction
 CT imaging is a 2-step process:
 first, x-ray data are acquired,
 a second step, images are reconstructed from the collected x-ray
information
 Both steps critically influence image quality
 Image reconstruction from the numerical x-ray data has
conventionnally been performed by a process called filtered
back projection, which does not make full use of the
information contained in the x-ray data that were acquired.
Improvements in computer processing power now permit the
use of such algorithms for this kind of image reconstruction.
ASIRE/ VEO (GE Healthcare: adaptive statistical iterative
reconstruction) SAFIRE/IRIS (Siemens) AIDR (Toshiba) iDose
(Philips)
Nuyts J. Phys Med Biol 1998
Bittencourt MS. Int J Cardiovasc Imaging 2011
Leipsic J. AJR 2010; LaBounty TM. Am J Cardiol 2010
Conclusion
 Selection of the patient
 Preparation of the patient
 Several different scan modes
 Number of x-ray sources, detector geometry, and
gantry rotation time
 Advances in CT sofware including various image
data-, projection data-, and model basediterative image reconstruction technique
 Dual energy
 Delay time is influenced by HR, gender and
transversal cardiac diameter ; coronary arterial
density changes with HR and weight
Tang L. Acta Radiol 2011
 The 100-kV setting allows significant reductions in
contrast material volume and effective radiation
dose while maintaining adequate
Zhang C.diagnostic
Int J Cardiovasc Imaging
image quality
 Contrast material dose may need to be tailored
individually by body weight and BMI
Zhu X. Acta Radiol 2012
Select best series