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Cardiovascular Computed Tomography: Current and Future
Scanning System Design
Wm. Guy Weigold
Introduction
The heart can be visualized in gross form on any standard
chest computed tomography (CT), but for detailed evaluation of cardiac anatomy exacting specifications of scanner
hardware and software must be in place because: the heart
is continually in motion, the structures of interest (i.e., coronary arteries) are small, and their curvilinear course
requires multiplanar reformatting for analysis.
For these specifications to be met, the scanner hardware
must be robust, the scanner must be fitted with ECGmonitoring equipment, and cardiac-specific software and
image reconstruction servers must be installed.
Elements of Scanner Design
The general design of a CT scanner equipped for cardiac
imaging is the same as that of any modern scanner: an
X-ray tube mounted on a rotating gantry opposite a detector array generates X-rays which are emitted across the
bore, are attenuated as they pass through the patient’s chest,
and are scintillated by the detectors as the gantry rotates
around the patient obtaining attenuation data from multiple view angles. These data are transmitted to an image
reconstruction server which then computes the axial
images. To perform cardiac CT, however, a number of additional capabilities are required, [1, 2] namely, high spatial
resolution, high temporal resolution, and fast scan speed,
while delivering an acceptable radiation exposure.
Special Design Considerations
for Cardiac CT
Spatial Resolution
Spatial resolution is the product of multiple variables including detector characteristics and collimation, sampling rate,
and reconstruction methods. State of the art scanners use
thin 0.5–0.625 mm collimation. This reduces volume averaging thus increasing image detail of small objects, and allows
reconstruction of isotropic datasets, thereby preserving
z-axis resolution and permitting multiplanar reformatting
necessary for analysis of cardiac anatomy.
GE Healthcare has focused their system development on
spatial resolution, developing a new detector scintillation
material with a fast decay time (30 ns), minimal afterglow
(one-quarter that of conventional Gd2O2S crystals), and
increased efficiency [3]. This new detector is one element of a
multipronged approach to increasing image resolution, which
also includes new electronics that permit increased sampling,[4] and a new iterative reconstruction algorithm [5, 6]
(Figure 2.1). These innovations increase spatial resolution by
3–4.5 line pairs per centimeter (lp/cm), which is substantial
given that current resolution of systems is approximately
12–15 lp/cm. The enhanced resolution is particularly useful
for imaging stented and/or calcified coronary arteries.
Temporal Resolution
In order to reconstruct an axial image, attenuation data
from multiple view angles that encompass half a rotation
around the patient are required. The amount of time
needed to obtain these data is the nominal, heart-rate
independent temporal resolution of the scanner. Hence, a
gantry that requires 330 ms to make one complete rotation has a baseline temporal resolution of 165 ms. This is
analogous to shutter speed: just as shutter speed must be
sufficiently fast to capture motion-free images of a moving subject, temporal resolution must be sufficiently fast
to capture the data needed for image reconstruction
within the relatively brief period of cardiac diastasis. If
temporal resolution is insufficient, motion artifact occurs
(Figure 2.2). A temporal resolution of 165 ms is relatively
slow for cardiac imaging, so systems overcome this by
various means: faster gantry rotation, multicycle reconstruction, and a second X-ray tube.
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Cardiac CT Imaging: Diagnosis of Cardiovascular Disease
Figure 2.1. Increased sampling improves image quality. Axial images of line-pair phantom demonstrating improved spatial resolution using increased sampling density (on the right) compared to
standard sampling frequency (on the left). Note the improved edge detail and resolution of the line
pairs of the image derived from the high density sampling dataset. (Reproduced with permission
from Chandra [4]).
Figure 2.2. Better temporal resolution improves image quality of moving structures. 3D volume
reconstructions of the right coronary artery in three CT scans representing three generations of CT
scanner (4-, 16-, and 64-slice, from left to right, respectively), possessing temporal resolution of 400,
250, and 180 ms from left to right, respectively. Note that the vessel appears distorted and dis-
jointed, due to motion artifact, when temporal resolution is insufficient (far left), while motion
artifact is minimally present, and small anatomic details are clearly imaged when temporal resolution is improved (far right). (Reproduced with the kind permission from Springer Science + Business
Media from Hurlock et al [2]).
Currently, the fastest gantry on the market resides in the
Philips iCT, which has a rotation time of 270 ms using an
innovative frictionless design in which the gantry is actually suspended in air [7]. As this design has only recently
been released, it is likely that future iterations will allow
even faster rotation.
Multicycle reconstruction combines data from multiple
contiguous heart beats to complete the half-scan of views
(Figure 2.3). This technique is heart-rate dependent: it can
only be used within certain ranges of heart rates, since the
cardiac cycle and gantry rotation must be asynchronous.
An approach adopted by Siemens in 2006 uses two X-ray
tubes mounted on the gantry at 90° angles; therefore, only a
quarter turn of the gantry is required to collect the 180° of
attenuation data (Figure 2.4). Hence, a dual-source system
with a gantry rotation time of 330 ms has a baseline central
temporal resolution of approximately 85 ms. This obviates
the need for multicycle reconstruction, making the system
less susceptible to variation in heart rate, and providing
high heart rate independent temporal resolution. Initial
reports demonstrated very good image quality, [8] and, in a
comparison of 64 MDCT and DSCT scans, the dual-source
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Cardiovascular Computed Tomography: Current and Future Scanning System Design
z
t
180°
z
45°
45°
45°
t
45°
Figure 2.3. Multicycle reconstruction. Single cycle recon (a) the duration of the acquisition window (gray bar) is approximately equivalent to one-half the gantry rotation time, since this is the
time required to obtain 180° of attenuation data. Multicycle recon (b) When multiple detector rows
are present, the axial slice position in the z-position can be imaged at multiple different times, using
multiple, shorter, acquisition windows distributed across multiple contiguous beats. These data can
then be combined to provide 180° of attenuation data and used to reconstruct the axial image.
Temporal resolution of the image is improved because the duration of the each acquisition window
is shorter. (Reproduced with permission from Wolters Kluwer from Vembar et al [1]).
system produced better image quality at higher heart
rates [9]. Image quality is still improved, however, by reducing heart rate before scanning. Siemens recently announced
a new scanner with an even faster (280 ms) gantry which is
capable of a central temporal resolution of 75 ms [10].
Detector Coverage and Scan Speed
Since the advent of multislice CT, manufacturers have
increased z-axis coverage by the addition of more rows of
detectors. By increasing coverage, scan times have been
reduced to just a few seconds, and even to single-beat scanning. This has been achieved by Toshiba’s Aquilion One scanner, which currently has the largest z-axis coverage [11]. This
scanner has 320 rows of detectors, with 0.5 mm collimation,
16 cm of coverage, and can perform an axial 1-beat acquisition of the entire heart when the rate is well controlled.
The Philips iCT is a 128-row scanner with 8 cm of coverage, which allows axial acquisition of the entire heart
in 2 beats [7]. GE offers a 64-row scanner with 4 cm of
Figure 2.4. Dual source CT. Schematic diagram of dual-source CT scanner: two X-ray source and
corresponding detector arrays are offset by approximately 90°. In this way, the half-scan of data
required for axial image reconstruction can be acquired over a quarter-rotation of the gantry,
thereby halving the nominal heart rate independent temporal resolution of the scan. (Reproduced
with permission from the RSNA from McCollough et al [22]).
coverage. The current Siemens system is a 32-row scanner,
with approximately 2 cm of coverage, but Siemens has
recently announced a new 64-row scanner offering approximately 3.8 cm of coverage.
Radiation Exposure
Traditional cardiac CT uses a helical scan mode and very
low pitch to perform retrospective ECG-gated reconstructions. This delivers an undesirably high radiation exposure,
so various methods have been designed to reduce radiation
exposure.
One of the first was ECG-based tube current modulation,
which fluctuates tube current in sync with the heart rate,
maintaining high tube current during diastasis and providing best image quality in that phase, then lowering tube
current during other phases when high-resolution detail is
not required (Figure 2.5). This results in a dose savings of
approximately 40%.
A great advance in dose reduction was made with the
advent of prospective, ECG-triggered, axial cardiac CT
(Figure 2.6). By keeping the X-ray tube off during most of the
scan, and only turning it on during diastasis, as triggered by the
ECG, radiation exposure is dramatically reduced (Figure 2.7).
GE first published results using this method [12] with good
clinical results and radiation doses of 1–2 mSv, [13] and this
approach is now offered by all manufacturers. Using a 64-slice
system, the entire heart can be covered in 3–4 acquisitions;
however this leaves the system vulnerable to fluctuations in
heart rate and rhythm during the scan. Larger coverage mitigates this vulnerability by reducing the number of beats
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Cardiac CT Imaging: Diagnosis of Cardiovascular Disease
Tube output
Figure 2.5. ECG-based tube current modulation
reduces radiation exposure. Once the desired
acquisition phase is determined, based on heart
rate, in this case 75% phase, scanning proceeds
with maintenance of 100% tube output during
that phase, while tube current is reduced outside of
that phase.This can reduce effective radiation dose
by 40% or more; however, note that images reconstructed from the low tube output phases (left
image) will be excessively noisy, and are usually
considered unusable for coronary interpretation.
(Reproduced with permission from Wolters Klower
from Vembar et al [1]).
100%
20%
Time
40
PGA
RGH
Number of Patients
35
30
25
20
15
10
5
20
18
16
14
12
10
8
6
4
Effective Dose (mSv)
Table move
75%
2
0
0
75%
Figure 2.6. Prospective ECG-triggered acquisition method. Cardiac rhythm is monitored while
table remains stationary. When cardiac cycle reaches predetermined acquisition phase (in this case
75%, diastolic, phase), X-ray source is briefly turned on (<1 s, blue bars) and acquires attenuation
data of a length equivalent to the craniocaudal coverage of the detector array, and is then turned off.
The table is advanced almost the length of craniocaudal coverage (minus a small amount of overlap), and the process repeats again. Depending on the craniocaudal coverage of the scanner, the
entire heart can be scanned in 1–4 acquisitions. (Reproduced with the kind permission from
Springer Science + Business Media from Weigold et al [7]).
required for data acquisition. However, larger coverage lends
to higher radiation doses, as the detectors are less efficient
when having to expose a wider detector array. Hence, Philips’
128-row iCT system can cover the entire heart in 2 beats, with
Figure 2.7. Prospective cardiovascular computer tomographic angiographty (CCTA) reduces
radiation dose. Histogram of effective radiation dose, grouped by acquisition method. Using prospectively gated axial scanning, average effective dose is dramatically reduced to 2–3 mSv. Even the
highest dose PGA scan still delivers a lower effective dose than the lowest-dose RGH scan. PGA
prospectively gated axial; RGH retrospectively gated helical. (Reproduced with permission from
RSNA from Earls et al [13]).
a reported radiation exposure of 3–5 mSv [7]. Toshiba’s
­320-row Aquilion One scanner can cover the entire heart in
one beat, with a reported radiation exposure of approximately
6 mSv. However, lower radiation doses and dependable coronary scanning using 1-beat acquisition requires heart rate
reduction. At higher heart rates (>65 bpm), optimal image
quality requires a 2- or 3-beat acquisition (and use of multicycle recon) which increases radiation exposure to approximately 13 (2-beat) to 19 (3-beat) mSv [14, 15]. Acquiring with
a wide exposure window (“padding”) can ensure the capture
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Cardiovascular Computed Tomography: Current and Future Scanning System Design
a
Low heart rate
Table move
Figure 2.8. Prospective CCTA phase selection
and padding. Acquisition phase and padding
depend on heart rate: For low heart rates (a),
target late diastole (75% phase) for prospective
acquisition, but if heart rate is high (b)
(>75 bpm), target end-systole (40% phase)
which is more likely to produce motion-free
images. By restricting acquisition to the minimum exposure window (dark gray), radiation
dose is minimized, but ability to reconstruct
adjacent cardiac phases is obliterated. Widening
the acquisition window (“padding,” light gray
shading) allows reconstruction of a small number of additional phases, which can help interpretation of motion artifact, but increases the
radiation dose. (Reproduced with the kind permission from Springer Science + Business Media
from Weigold et al [7]).
75%
b
75%
High heart rate
Table move
40%
40%
83ms
Figure 2.9. Method of high-pitch coronary CCTA. Single source helical CT requires a pitch £1.5; a
faster pitch results in gaps in the data (left panel). A dual-source system can scan at a higher pitch
(up to 3.2), using the second X-ray source-detector system to fill in what would otherwise be gaps
in the data. Since the table speed is so high, the entire heart can be imaged in a fraction of a second,
from data derived from a single heartbeat (right panel). If the image acquisition is triggered appropriately to synchronize acquisition with diastasis, and the heart rate is sufficiently low, motion-free
images can be obtained with a very low radiation dose. (Reproduced with permission from Elsevier
from Achenbach et al [18]).
of motion-free data, but increases radiation exposure
(Figure 2.8). A narrower exposure window can be used with
preservation of image quality if heart rate is ­controlled [16]
and reduces radiation exposure.
Siemens has recently introduced a new innovative scanning method using a high pitch helical mode which takes
advantage of the dual-source design [17, 18] (Figure 2.9).
The high pitch (3.2–3.4) would normally produce gaps in
the attenuation data using a single-source system, but, in a
dual-source system, these are compensated for by data
gathered from the second detector. Scan time is less than
1 s, with a radiation exposure of approximately 2 mSv.
Initial studies performed on the first-generation dualsource system proved the feasibility of the method, though
the authors noted that these first-generation systems are
not suitable for this high pitch technique. High pitch scanning with their new scanner, with faster gantry rotation
(280 ms) and larger coverage (64 rows), has been reported
to produce very good coronary image quality with radiation exposure of approximately 1 mSv [10]. Of note, in this
initial investigational phase, low heart rates (<60 bpm) are
absolutely required, as the entire data collection takes place
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Cardiac CT Imaging: Diagnosis of Cardiovascular Disease
within a 250–270 ms acquisition window of one heart beat;
hence diastasis must be at least this long to provide motion
free data acquisition, and therefore the requirement for a
very low heart rate.
The previously described new detector and reconstruction system offered by GE aims to lower radiation exposure
by preserving image quality while scanning with less radiation (<1 mSv), because images are of higher resolution and
lower noise than would otherwise be achievable with a lowexposure scan.
energy CT could take the field to a new level of diagnostic
power if it can be used for refined tissue characterization,
such as differentiating plaque characteristics. This is difficult to do in the coronary arteries, and initial studies have
not yielded any breakthroughs, but at the least it has the
potential to enhance coronary lumen visualization in heavily calcified vessels by differentiating calcium and iodine
[20] (Figure 2.11). Dual-energy CT may also yield new
applications for noncoronary imaging, such as imaging of
myocardial perfusion [21].
Future Directions
Conclusions
Future scanner designs are closely guarded industry secrets,
but some concepts have been openly discussed. Flat-panel
volume CT systems replace detector rows with a large area
detector, and provide coverage of the entire heart in one
axial acquisition and extremely high spatial resolution of
up to 26 lp/cm, comparable to invasive angiography [19]
(Figure 2.10). However, the contrast resolution is inferior to
that of multidetector CT, a high radiation exposure is needed
to achieve a sufficient contrast-to-noise ratio, and scintillation times are slow and temporal resolution insufficient for
cardiac imaging.
Dual energy scanning is being developed by all vendors
by various means, including rapid fluctuation of tube
energy, stacked detectors, or the dual-tube design in which
each tube emits photons of different energy levels. Dual
Since the turn of the twenty-first century, there has been an
explosion in technological development of cardiac CT systems, with concomitant gain in reliability and accuracy,
especially of coronary imaging. The theme has been one of
progressively improved spatial and temporal resolution,
reduced scan time, and, most recently, a focus on driving
down radiation exposure. In appropriately selected patients,
using careful technique, it is easily achievable to perform a
cardiac CT using less radiation exposure than that of a
standard chest CT or an invasive coronary angiogram. The
goal of future systems will be to make this more widely
achievable in a larger group of patients without requiring
patient selection or stringent patient prep. Given the history of rapid technological advancement, we can expect to
see this goal achieved in the near future.
X-Ray
Tube
57 cm
Source
Isocenter
93 cm
Detector
Panel
Detector
Figure 2.10. Flat panel CT. Schematic of flat
panel scanner: In a flat panel system, the traditional multirow detector array is replaced by an
area detector consisting of millions of detector
elements, each with dimensions <0.2 × 0.2 mm.
Large coverage allows whole-organ imaging
with tremendous spatial resolution (up to
150 mm). Limiting factors of these prototypes
for cardiac imaging have been insufficient temporal resolution, and requirement of high radiation dose. (Reproduced with permission from the
RSNA from Gupta et al [19]).
Cardiovascular Computed Tomography: Current and Future Scanning System Design
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Figure 2.11. Dual energy CT. Dual energy CT
can be used to characterize tissue: vessel crosssectional images derived from high- (140 keV)
and low-energy (90 keV) attenuation profiles (a,
b) demonstrate the influence of photon energy
on photoattenuation. The contrast-filled lumen
(arrow) exerts greater photoattenuation on lowenergy photons, and hence appears brighter in
(b), while calcification (asterisk) appears highdensity in both images. In the subtracted image
(c), dense calcification has been “removed.” By
adding in the low-energy (90 keV) attenuation
data to this subtracted image, depiction of the
lumen edge is enhanced (because of reduced
contrast blooming), and visualization of a small
side branch (arrowhead) is improved (d).
(Reproduced with permission from Wolters
Kluwer from Boll et al [23]).
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