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151
Reconstruction of CT Data in 64-Detector CT
Systems
With 64-detector CT systems, each revolution of the gantry
enables imaging of 3.2–4.0 cm of the heart (64 detectors
times the width of each detector). To cover the entire heart,
the required length of most cardiac scans is 9–10 cm; this
necessitates combining data from multiple table positions to
yield a fi nal volume of data. The greatest challenge in the
reconstruction of this volumetric cardiac CT data is the
maintenance of temporal uniformity: that is, one should be
able to provide cardiac images at each table position from the
same part of each cardiac cycle. A breakdown of temporal
uniformity can lead to characteristic “stairstep” artifacts.
The most commonly used reconstruction algorithm in
spiral cardiac CT is the half-scan reconstruction method ,
which involves the use of scan data from a single gantry
rotation to generate an axial CT image. Half-scan
reconstruction excludes the fan beam width (approximately
30°), so that approximately 210° of rotation are necessary to
generate a single axial image. Using the half-scan method,
ab
cd
Fig. 8.7 Sharp and smooth convolution kernels are employed to
improve visualization of stents and calcifi ed vessels while reducing
blooming artifacts. ( a and c ) Depict long axis and short axis oblique
thin-slice projections of an LAD stent using a standard smooth (B26f)
kernel. ( b and d ) are similar projections using a sharp (B46f) convolu-
tion kernel. Blooming artifacts are reduced, and the edge of the stent is
more clearly delineated at the expense of increased noise in the remainder of the image
Pitch
Pitch 1
Pitch 2
itch 0.5
Pitch:
(in Multislice CT)
Table feed per rotation
Total width of collimated beam
(n detectors ¥ collimation)
Fig. 8.8 Pitch refers to the table feed per gantry rotation during a spiral
acquisition divided by the width of the collimated beam. A pitch of
exactly 1 ( central diagram ) implies that there are no data gaps and there
is no overlap of data. When pitch is greater than 1 ( top diagram ), there
are gaps in data acquisition, whereas when pitch is less than 1 ( bottom
diagram ), there is data overlap. Retrospectively gated cardiac CT examinations are performed with a pitch of approximately 0.2
8 Post-processing and Reconstruction Techniques for the Coronary Arteries
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the temporal resolution is approximately 60 % of the
rotational speed of the scanner (due to the fan beam width
exclusion). For modern single source 64-slice CT scanners
which have gantry rotation times ranging from 330 to
375 ms, the temporal resolution using the half-scan reconstruction method is approximately 200–225 ms. In patients
with heart rates of 60 beats per minute or less, the mid-diastolic
diastasis period of minimal coronary motion is long enough
to allow effective reconstructions in mid diastole in the
majority of patients.
In patients with heart rates faster than 80 beats per minute, the duration of the diastasis period decreases considerably and may be only 100–200 ms. This makes it nearly
impossible to reconstruct motion-free images using the halfscan reconstruction method. In these cases, multi-segment
reconstruction may be utilized to improve the effective
temporal resolution of the CT scanner. Multisegment reconstruction relies on additional data overlap with slower table
movement and decreased pitch during CT acquisition
[ 11 , 12 ]. This overlap results in the same table position being
available for imaging at multiple heart beats from multiple
detectors (Fig. 8.9 ). By combining views at a single table
position from different subsequent gantry rotations, one
simulated half-scan rotation is generated. This results in
improved image quality with fewer motion artifacts. By
combining images from 3 cardiac cycles, the effective
temporal resolution can be improved to as much as 65 ms.
Multi-segment reconstruction relies heavily on a consistent
R-R interval on the consecutive beats used to generate the
fi nal axial image. Irregularities from atrial fi brillation or
sinus arrhythmia during breath holding may cause misregistration artifacts.
Reconstruction of CT Data in 320-Detector CT
Systems
With 320-detector CT systems, one axial rotation can cover
up to 16 cm of tissue, although typically all 320 detectors
cannot be employed simultaneously to image the heart. To
accomplish this, the use of a wide X-ray beam and a wide
cone angle is required. Figures 8.10 and 8.11 demonstrate
the concept of the fan angle and the cone angle. The cone
angle determines the coverage of the X-ray beam in the longitudinal, or z-axis while the fan angle determines the coverage in the x/y plane. The use of a wide cone angle gives the
320-detector scanner the ability to cover up to 16 cm of tissue
60°
60°
60°
67 ms 67 ms
67 ms
67 ms
180°
Continuous
spiral scan & feed
Image data
Delay
Z – Position
slower
pitch
Volume
gaps
Recon
Recon
Recon
Fig. 8.9 ( a and b ) Multisegment reconstruction of a CT acquisition
involves the acquisition of data at a single table position over several
cardiac cycles. The volumetric data are combined to yield a fi nal
summed volume. The major requirement for multisegment reconstruction is data overlap, which results in a slower pitch and higher radiation
dose during the CT acquisition. Using this technique, effective temporal
resolution can be improved to 67 ms for a scanner with a half-scan
acquisition time of 200 ms
Fig. 8.10 The fan angle represents the spread of the X-ray beam in the
x/y plane and is a factor in determining the diameter of the fi eld of view
(Reproduced with permission from Toshiba America Medical Systems)
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153
in one axial rotation [ 13 ]. As a result, it is not necessary to
move the patient through a spiral scan line. Instead, the
patient can remain stationary and the entire heart to be
imaged in a single gantry revolution. This has the potential to
substantially improve image clarity by eliminating stairstep
and misregistration artifacts that are sometimes seen with
64-detector systems, but will have no impact on temporal
resolution, based upon the gantry rotation speed (as described
above).
While use of a wide cone angle offers the capability of
whole organ coverage, it also presents a challenge to image
reconstruction algorithms employed in 64-detector systems.
Traditional reconstruction algorithms cannot successfully
incorporate the highly tilted X-ray planes created by the cone
angle trajectory. As a result, one can get cone-beam artifacts
including shading, ghosting, and diminished resolution as
shown in Fig. 8.12a . With more advanced reconstruction
algorithms specifi cally designed for 320-detector systems
that account for the cone angle, these artifacts can be eliminated, shown in Fig. 8.12b [ 14 ].
Post Scan Related Post-processing
Parameters
After the scan has been completed and datasets have been
generated, additional post-processing techniques on a cardiac CT workstation are essential to accurately interpreting
cardiac morphology as well as coronary artery anatomy and
disease burden. The presence of an isotropic data set in
which the spatial resolution is identical across all planes of
examination facilitates manipulation of the data on a
workstation.
Fig. 8.11 The cone angle quantifi es the spread of the X-ray beam in
the z-direction, along the length of the patient (Reproduced with permission from Toshiba America Medical Systems)
ab
Fig. 8.12 Panel A ( top ) illustrates cone beam artifact. The image
demonstrates shading, ghosting, and diminished resolution. This is a
result of a reconstruction algorithm that does not properly take into
account the wide cone angle. Panel B ( bottom ) shows an identical
image reconstructed with a novel reconstruction algorithm that takes
into account the wide cone angle, thereby eliminating the artifact
(Reproduced with permission from Toshiba America Medical
Systems)
8 Post-processing and Reconstruction Techniques for the Coronary Arteries
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The raw axial images are the most reliable for diagnosis,
as they refl ect the source data in the order the images were
acquired. To assist the reader in processing large volumes of
data and illustrating key fi ndings, additional rendering
techniques have been developed [ 15 , 16 ]. These include the
multiplanar reformatting, maximum intensity projection,
the volume averaging and volume rendering techniques, and
the curved multiplanar projection. All involve rendering data
contained within a 3 dimensional slab (the thickness of
which the user can change) of data as a single 2-dimensional
projection.
Multiplanar Reformatting (MPR)
Multiplanar reformatting involves the selection of an
arbitrary image plane in a cardiac CT volume. This technique requires an isotropic volumetric data set with equal
spatial resolution in the X, Y, and Z axes. The plane may not
conform to the conventional axial, coronal, and sagittal
imaging planes and can be modifi ed by the user, as shown in
Fig. 8.13 . MPR is the mainstay for analysis of cardiac CT
data sets and is the most reliable method for the reader to
arrive at the correct diagnosis. It can be performed using a
single slice or with differing numbers of stacked slices.
When more than one slice is selected, a rendering option
must be selected to display a composite image of the multiple slices.
Maximum Intensity Projection
The maximum intensity projection (MIP) involves the projection of data in a 3-dimensional slab so that only the voxels
of highest HU are displayed on a 2-dimensional image
(Fig. 8.14 ). Initially developed by Rubin and colleagues
[ 17 , 18 ]. for use in peripheral CT angiography, the MIP is now
used for nearly all CT angiography applications and is the
mainstay of coronary artery interpretation. The MIP is ideal
for the display of coronary artery images from a contrast CT
examination as the maximum intensity in the coronary arteries is usually the intraluminal contrast. The coronary arteries
are surrounded by low-attenuation epicardial fat, resulting in
an angiogram- like image with excellent edge defi nition
(Fig. 8.14 ). Due the selection of the highest intensity voxels
within a slab, the MIP tends to overestimate stenosis severity
in calcifi ed vessels and stented segments (bright objects like
calcium and metal get further enhanced). Overreliance on
MIPs can also lead the reader to overlook subtle fi ndings in
the coronary arteries, including motion and misalignment
artifacts. In general, readers should always reconfi rm fi ndings on MIP with the source axial data.
Volume Averaging and Volume Rendering
Volume averaging (VA) involves the projection of data in a
3-dimensional slab so that the intensity of all the voxels in
the slab is averaged on a fi nal 2-dimensional image
(Fig. 8.15 ). While edge defi nition is poorer than with MIP,
VA enables the reader to “see through” a dense object in a
slab and can be useful in interpreting stenoses in calcifi ed
vessels. When specifi c colors are assigned to specifi c ranges
of HU in a 3-dimensional volumetric slab, this is termed volume rendering (VR); this is available on virtually every cardiac workstation. The relative position and 3-dimensional
relationship of the coronary arteries, cardiac veins, and cardiac chambers is possible using 3-dimensional VR
(Fig. 8.16 ). While especially helpful in evaluating coronary
Objects in volume Image plane
Selectsd plane
Fig. 8.13 Multiplanar Reformat Projection (MPR)
Objects in volume
Image plane
Selected “Slab” of images
Fig. 8.14 Maximum Intensity Projection (MIP)
Objects in volume Image plane
Selected “Slab” of images
Fig. 8.15 Volume averaging
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anomalies and bypass grafts, the reader should never attempt
to interpret stenoses purely on the basis of a 3D VR images:
as calcium and intraluminal contrast have attenuation ranges
that are near one another, signifi cant coronary artery stenoses
in calcifi ed vessels can be misinterpreted.
Curved Multiplanar Projection
The curved multiplanar projection (CMP) is a centerline
method for analysis of vessels whereby a virtual plane is created using the center of the column of contrast visualized on
a series of consecutive axial slices (Fig. 8.17 ). A virtual
plane in which the vessel is “stretched out” can then be displayed. CMP is useful for confi rming and illustrating the
appearance of stenoses identifi ed using standard axial multiplanar projections. It is particularly useful for tortuous vessels and those vessels that cannot be easily tracked in a single
plane or thin slab using MIP. The CMP is only as reliable as
the accuracy of the centerline, and should not be used as a
fi rst-line assessment of coronary stenosis severity. Artifacts
from inaccurate centerlines can lead to incorrect assessment
of stenosis severity.
References
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P. Noninvasive angiographic evaluation of coronary stents with
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heart. Med Phys. 2000;27(8):1881–902.
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of electrocardiographically gated multisection spiral CT: initial
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Fig. 8.16 3D volume rendering
Objects in volume Image plane
Fig. 8.17 Curved Planar Reformat (CPR)
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16. Pavone P, Luccichenti G, Cademartiri F. From maximum intensity
projection to volume rendering. Semin Ultrasound CT MR.
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© Springer International Publishing 2016
M.J. Budoff, J.S. Shinbane (eds.), Cardiac CT Imaging: Diagnosis of Cardiovascular Disease,
DOI 10.1007/978-3-319-28219-0_9
Coronary CT Angiography:
Native Vessels
Stephan Achenbach
Abstract
Coronary CT Angiography has become increasingly stable and robust and accuracy to identify and rule out high-grade stenosis of the coronary arteries is high. The main clinical
application is to rule out signifi cant coronary artery disease in patients with a relatively low
pre-test likelihood of disease. For this application, several guidelines in the US and Europe
endorse the use of cardiac CT. Other applications of coronary CTA include the support of
coronary interventions, especially in the context of chronic total coronary occlusion, the
identifi cation of coronary anomalies, and, to some extent, the assessment of non-obstructive
coronary atherosclerosis. However, the use of coronary CTA for screening purposes is currently not supported by offi cial recommendations.
Keywords
Coronary CT Angiography • Coronary CTA • Coronary Artery Disease • Computed
Tomography • Stenosis • Atherosclerosis • Plaque
Introduction
Visualization of the coronary arteries has been the major
focus of cardiac CT in the past years. Non-invasive “coronary
CT angiography” has tremendous clinical potential for detecting or ruling out coronary artery stenoses in selected patients
(see Figs. 9.1 and 9.2 ). As a consequence of continuous and
substantial progress regarding image quality and robustness
of the investigation, it is incorporated in several recent offi cial
guidelines and recommendations. In addition, imaging of
coronary atherosclerotic plaque may play a potential role in
risk stratifi cation. However, spatial resolution and temporal
resolution of CT imaging, even with the latest scanner generations, are not equal to those of invasive coronary angiography. Interpreters of coronary CT angiography data sets must
therefore be aware that artefacts can occur and may lead to
false-positive and, less frequently, to false- negative results.
Diagnostic accuracy is impaired when image quality is
reduced and image quality, in turn, is infl uenced by many factors such as the patient’s heart rate, body weight, ability to
cooperate, and extent of coronary calcifi cation. Therefore, the
clinical utility of coronary CT angiography signifi cantly
depends on the specifi c clinical situation and patient under
investigation. The specifi c advantages and disadvantages of
coronary CT angiography must be carefully considered
before using this method in the workup of a patient with
known or suspected coronary artery disease.
Imaging Protocol
Since the small dimensions and the rapid motion of the coronary vessels pose tremendous challenges for non-invasive
imaging, high-end CT equipment and adequate imaging protocols must be used. Currently, 64-slice CT is considered the
minimum requirement for coronary artery imaging, and newer
S. Achenbach , MD
Department of Cardiology , University of Erlangen ,
Ulmenweg 18 , Erlangen 91054 , Germany
e-mail: Stephan.Achenbach@uk-erlangen.de
9
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ab
cd
e
Fig. 9.1 Normal anatomy of the coronary arteries in transaxial images. ( a )
Level of the left main origin from the aortic root. The bifurcation of the left
main into the left anterior descending ( large arrow ) and left circumfl ex
coronary artery ( small arrow ) can be seen. The arrowheads point at a coro-
nary vein. ( b ) A few mm further distal, the left anterior descending coro-
nary artery ( large arrow ) has given rise to a diagonal branch ( arrowhead ).
( c ) Level of the right coronary ostium. A short section of the right coronary
artery is visible ( double arrows ). Large arrow : Mid left anterior descending
coronary artery, small arrow : left circumfl ex coronary artery. ( d ) Mid-
ventricular level. The left anterior descending coronary artery ( large
arrow ), left circumfl ex coronary artery ( small arrow ), and right coronary
artery can be seen ( double arrows ). ( e ) Distal segment of the right coronary
artery ( double arrow ), which ends in the posterior descending artery ( small
arrow ). The arrowhead points at a right ventricular branch
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technology [ 1 ], such as Dual Source CT and scanners that
allow simultaneous acquisition of 256 or 320 cross- sections,
provide for more robust image quality and further improved
image quality.
A basic prerequisite for CT imaging of the coronary
arteries is the patient’s ability to understand and follow
breathhold commands. Even slight respiratory motion
during data acquisition will cause substantial artefact and
may render the coronary arteries (or parts of them) unevaluable. Therefore, patients should be able to reliably hold their
breath for approximately 10 s. Otherwise, coronary CT
angiography should not be performed. Heart rate should be
regular and preferably low (optimally below 60/min, even
though this is not as strictly required for Dual Source CT)
ab
c
d
Fig. 9.2 Same patient as in Fig. 9.1 . Various forms of post-processing
have been used to visualize longer segments of the coronary arteries. ( a )
Curved multiplanar reconstruction (curved MPR) of the right coronary
artery. ( b ) Maximum Intensity projection (MIP) of the right coronary
artery ( arrows ) in a double-oblique plane. ( c , d ) 3-dimensional, surface-
weighted Volume Rendering Technique (VRT) reconstructions in two
different angulations
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[ 1 , 2 ]. It is usually recommended that patients receive pre-
medication with short acting beta blockers to lower the heart
rate. Beta blockers can be administered orally approximately 1 hr prior to scanning, or intravenously immediately
before the scan. Sometimes, a combination of both is necessary. Nitrates should be given to all patients who have no
contraindications in order to achieve coronary dilatation,
which substantially improves image quality [ 1 ].
Intravenous contrast enhancement is necessary for coronary CT angiography and typically, 50–100 ml of iodinebased, high concentration contrast agent are injected. High
fl ow rates are recommended, injection should be between 4
and 7 ml/s. Synchronization of contrast injection and data
acquisition can be achieved either through a “bolus tracking”
method or by using a separate “test bolus” acquisition to
measure the contrast transit time.
Subsequent data acquisition can follow various principles. The obtained data need to be synchronized with the
heart beat, and this can either be achieved through retrospective ECG gating or prospective ECG triggering [ 1 ].
Retrospectively gated scans are acquired in spiral mode
and usually provide for robust and high image quality, fl exibility to choose the cardiac phase during which images are
reconstructed, as well as the ability to reconstruct “functional” data sets throughout the cardiac cycle in order to
analyse left ventricular function and regional wall motion. In
order to limit radiation exposure, the output of the x-ray tube
can be “modulated” during the acquisition, with lower output in systole, and higher output in diastole, when the most
relevant image reconstructions are usually performed.
Prospectively triggered scans are associated with substan-
tially lower radiation exposure. Images are acquired in axial
mode without table movement and the patient table is
advanced by one detector width following the acquisition,
with subsequent images acquired in the next or second to
next cardiac cycle. Less fl exibility to reconstruct data at different time instants in the cardiac cycle as well as greater
susceptibility to artefacts caused by arrhythmia are trade-offs
for the advantage of lower dose. Especially in young
patients – in whom radiation dose may be of major concern –
prospectively triggered scans should be strongly considered
[ 2 – 5 ]. Heart rate must be low so that artefact-free images can
be guaranteed at the time instant of radiation exposure. The
lowest radiation dose is achieved when the x-ray exposure
window in each cardiac cycle is only as long as one-half
rotation of the gantry requires (just long enough to reconstruct one set of transaxial slices), but “padding” (x-ray
exposure over a longer time period in each cardiac cycle)
may be used to provide some fl exibility regarding the time
instant of image reconstruction [ 6 , 7 ].
A combination of spiral acquisition and prospective triggering is the so-called “Flash Mode” ( prospectively ECG
triggered high pitch spiral acquisition ). It is only available
with a limited number of scanners that have either two detectors or a very wide detector. Images are acquired during continuous, very fast motion of the table, and the volume of the
heart is typically covered within 150–250 ms. In the craniocaudal direction, the data for each subsequent image are
acquired with a very slight temporal offset as compared to
the previous image (approximately 0.5 ms), so that the consecutive images represent ever so slightly different time
instants within the cardiac cycle. Since the transition is
smooth and image acquisition is typically performed in diastole with very little cardiac motion, this offset is not noticeable in the data set and in reconstructed images. This mode
of data acquisition provides high image quality at very low
doses, but requires stable heart rates below 60 beats/min to
avoid artefact [ 8 – 11 ].
Radiation Exposure
Unless specifi c measures are taken to limit radiation dose,
the exposure during coronary CTA can be high. A landmark
study performed several years ago demonstrated that in
individual centers, the average estimated effective radiation
exposure was as high as 30 mSv (while in the same study,
sites at the lower end of the spectrum performed coronary
CTA with an average exposure of only 4–5 mSv) [ 12 ]. Since
then, substantial progress has been achieved regarding radiation exposure (see Table 9.1 ). ECG-based tube current
modulation in spiral acquisition and prospectively ECGtriggered image acquisition avoid x-ray exposure during the
entire cardiac cycle and limit x-ray tube output to those
phases of the heart beat which are likely to be used for
image reconstruction. This limits the fl exibility of reconstructing images during different parts of the R-R interval,
which would be desirable to assess ventricular function (a
question, however, that is rarely relevant in coronary CTA),
and which is also advantageous when motion artefacts are
present, to identify a phase with no or little artefact. Hence,
low heart rates, which make it extremely likely that artefactfree images are obtained in diastole, facilitate the use of
these techniques and in this way, reducing the heart rate by
premedication contributes to lower radiation exposure [ 2 ].
While the tube voltage for coronary CTA used to uniformly
be 120 kV, it has been observed that depending on patient
size, it is possible to reduce tube voltage to 100 or 80 kV, or
in very selected cases even 70 kV [ 13 – 17 ]. The increase in
noise is tolerable depending on patient size, and to some
extent is offset by higher iodine contrast. A “rule of thumb”
is that tube voltage can be reduced to 100 kV for all patients
with a body weight below 100 kg. Finally, there is a linear
relationship between tube current and image noise, so that
again, especially in patients with low body weight there is
potential to reduce exposure.
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