CT Coronary Angiography: 256-Slice and 320-Detector Row Scanners

Abstract

Multidetector computed tomography (MDCT) has rapidly evolved from 4-detector row systems in 1998 to 256-slice and 320-detector row CT systems. With smaller detector element size and faster gantry rotation speed, spatial and temporal resolution of the 64-detector MDCT scanners have made coronary artery imaging a reliable clinical test. Wide-area coverage MDCT, such as the 256-slice and 320-detector row MDCT scanners, has enabled volumetric imaging of the entire heart free of stair-step artifacts at a single time point within one cardiac cycle. It is hoped that these improvements will be realized with greater diagnostic accuracy of CT coronary angiography. Such scanners hold promise in performing a rapid high quality "triple rule-out" test without high contrast load, improved myocardial perfusion imaging, and even four-dimensional CT subtraction angiography. These emerging technical advances and novel applications will continue to change the way we study coronary artery disease beyond detecting luminal stenosis.

Full text

CT Coronary Angiography: 256-Slice and 320-Detector Row
Scanners
Edward M. Hsiao, Frank J. Rybicki, and Michael Steigner
Applied Imaging Science Laboratory, Brigham and Women’s Hospital Radiology & Harvard Medical
School, 75 Francis Street, Boston, MA 02115, USA
Frank J. Rybicki: frybicki@partners.org
Abstract
Multidetector computed tomography (MDCT) has rapidly evolved from 4-detector row systems in
1998 to 256-slice and 320-detector row CT systems. With smaller detector element size and faster
gantry rotation speed, spatial and temporal resolution of the 64-detector MDCT scanners have made
coronary artery imaging a reliable clinical test. Wide-area coverage MDCT, such as the 256-slice
and 320-detector row MDCT scanners, has enabled volumetric imaging of the entire heart free of
stair-step artifacts at a single time point within one cardiac cycle. It is hoped that these improvements
will be realized with greater diagnostic accuracy of CT coronary angiography. Such scanners hold
promise in performing a rapid high quality “triple rule-out” test without high contrast load, improved
myocardial perfusion imaging, and even four-dimensional CT subtraction angiography. These
emerging technical advances and novel applications will continue to change the way we study
coronary artery disease beyond detecting luminal stenosis.
Keywords
Computed tomography; Coronary artery disease; Wide area detector; Imaging; Technology
Introduction
This article highlights the role of wide-area detector CT for cardiac imaging. Technical
specifications of the hardware are described, as are novel potential clinical applications.
Nomenclature of MDCT
Single-source multidetector computed tomography (MDCT) uses a single x-ray source
mounted opposite to a detector array. The x-ray tube and detector array system rotates around
the patient to generate tomographic images. To reconstruct a transverse CT image, the gantry
requires a rotation of approximately 180°. Dual-source CT uses two x-ray tubes with opposing
detector arrays mounted 90° from each other. The main advantage of this system is that the
temporal resolution is effectively halved because each x-ray tube/detector array system only
needs to rotate half of the angle that would otherwise be required by a single-source system.
The number of detector rows in the longitudinal axis (z-axis) and the number of slices of an
MDCT system are not interchangeable terms because multiple systems with an alternating
focal spot allow the same z-axis coverage to be sampled twice, and thus the number of image
Disclosure Dr. Frank Rybicki has received research grants from Toshiba Medical Systems and Bracco Diagnostics. No other potential
conflicts of interest relevant to this article were reported.
NIH Public Access
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Published in final edited form as:
Curr Cardiol Rep. 2010 January ; 12(1): 68–75. doi:10.1007/s11886-009-0075-z.
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slices generated is double the number of detector rows. However, the volume coverage (ie, z-
axis coverage) remains the same; for example, a 128-detector row scanner with two alternating
z-focal spot positions can be referred to as 256-slice CT. It is important to specify the number
of detector rows in z-axis, with or without alternating focal spot positions, and single versus
dual source.
Development in CT Coronary Angiography
CT coronary angiography became clinically practical with retrospective electrocardiogram
(ECG) gating to freeze cardiac motion plus the z-axis coverage from 16-detector row scanners
[1]. Following 16-detector row technology, the clinical accuracy of MDCT in coronary artery
disease (CAD) detection is now recognized by two multicenter trials based on 64-detector row,
single-source, single focal spot MDCT. The ACCURACY (Assessment by Coronary
Computed Tomographic Angiography of Individuals Undergoing Invasive Coronary
Angiography) trial based in the United States demonstrated the sensitivity, specificity, positive
predictive values (PPVs), and negative predictive values (NPVs) for greater than 50% stenosis
to be 0.95, 0.83, 0.64, and 0.99, respectively, on a per-patient basis [2••]. The Core-64
multicenter trial subsequently showed the sensitivity, specificity, PPV, and NPV of 085, 0.90,
0.91, and 0.83, respectively [3••]. The slightly lower than expected NPV for Core-64 can be
at least partly attributed to the high prevalence of obstructive CAD (56%) in the study
population. The 2006 American College of Cardiology appropriateness guidelines considered
the use of MDCT as an appropriate indication to exclude CAD in low- and intermediate-risk
individuals [4].
Using those systems tested in the multicenter trials, cardiac motion artifacts, stair-step artifacts,
and small vessel diameter less than 1.5 mm render around 20% of the coronary segments
uninterpretable [3••,5]. Heavily calcified vessels and coronary stents also impose diagnostic
challenges. In the “post-64” era, the MDCT technology started branching out in various
directions to overcome these limitations. The first direction is to increase the number of detector
elements and, therefore, the volume coverage along the z-axis of detector block. The second
is to increase sensitivity of detector material. Next is the use of iterative image reconstruction
algorithms [6,7]. The last are dual-source CT [8,9] scanners that use a high-pitch acquisition
strategy [10] to capture the entire heart within one heartbeat. To date, most new technology
developments are available only on different systems, although they may be combined in future
CT releases.
State-of-the-Art Wide-Coverage MDCT Scanners
320-Detector Row, Single-Source, Single Focal Spot
This hardware (Aquilion One Dynamic Volume CT; Toshiba Medical System, Tochigi-ken,
Japan) currently has the largest z-axis detector coverage. It was released shortly after
experiments with a 256-detector row MDCT prototype [11–14]. Each detector element is 0.5
mm wide, yielding a maximum of 16-cm z-axis coverage. This configuration allows three-
dimensional volumetric wholeheart imaging during the diastole of one R-R interval. In 320-
detector row CT, the entire heart is imaged with temporal uniformity (ie, at the same time point
without temporal delay from the base to apex). Furthermore, if the x-ray beam is turned on for
a longer period, the scanner can capture the heart over one or more cardiac cycles. This has
been described as four-dimensional CT or volumetric cine imaging [14].
The temporal resolution of an MDCT scanner reflects the ability to freeze cardiac motion, thus
producing motion-free images. The 320-detector scanner has a standard temporal resolution
of approximately 175 ms, one half the gantry rotation time. This remains significantly longer
than the 33 ms of catheter coronary angiography operating at 30 frames per second. Therefore,
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to achieve excellent image quality, meticulous heart rate control is mandatory. Coronary
opacification gradients are linked to blood flow and fractional flow reserve, and may have
implications particularly for patients with indeterminate lesions, or for patients with diffuse
CAD without focal stenosis [15]. In the latter case, a graded, continuous pressure drop along
the arterial length results in flow resistance. In diffuse CAD, the lumen area is diffusely smaller
than normal, and thus there is no reference segment for qualitative measurements for visual
assessment.
For patients with higher heart rate (>65 bpm) and contraindications to β blockers, multisegment
reconstruction can be used at the expense of higher radiation dose. For example, in two-segment
reconstruction, data required for image reconstruction are acquired over two cardiac cycles.
Therefore, only data from 90° rotation during each of the two cardiac cycles are used, improving
the effective temporal resolution by a factor of 2.
128-Detector Row, Single-Source, Dual Focal Spot
At the November 2007 annual meeting of the Radiological Society of North America, Philips
introduced the 256-slice MDCT (Brilliance iCT; Philips Healthcare, Cleveland, OH), a
128×0.625-mm detector row system with dual focal spot positions to double the number of
slices within the 8-cm (width) z-axis gantry coverage. The iCT has 270-ms gantry rotation
time, which translates to an approximate temporal resolution of 135 ms. Prospectively ECG-
gated cardiac CT typically covers the entire heart in two axial acquisitions over three heartbeats.
During the diastole of the first heartbeat, the upper half of the heart is imaged. During the
second heartbeat, the patient table translates 62.4 mm. Subsequently, the lower half of the heart
is acquired during the diastole of the third heartbeat. The scanner is equipped with several
radiation reduction capabilities, including a dynamic helical collimator and an adaptive axial
collimator to reduce z-overscanning [16,17].
64-Detector Row, Dual-Source, Dual Focal Spot
The second-generation dual-source MDCT (Somatom Definition FLASH; Siemens Medical
Solution, Forchheim, Germany) introduced at the end of 2008 is equipped with two 64-detector
row units, each with an alternating focal spot. The 360° gantry rotation time is 280 ms,
translating to a temporal resolution of approximately 75 ms when the scanner operates with
both x-ray tubes collecting data at the same energy. The vendor has proposed a high-pitch
prospectively gated scanning acquisition. In single-source MDCT, the maximum pitch is
roughly 1.5 for gapless image reconstruction. The pitch can be increased up to 3.2 in dual-
source systems. For coronary CT angiography, the typical phase window required for a
diagnostic quality examination regarding motion artifact is 10% of the R-R interval [18]. The
pitch required for multiphase acquisition ranges from 0.2 to 0.5 (depending on the heart rate).
With the high-pitch acquisition mode, only one “phase” is acquired, which gradually increases
with the z-axis table translation (ie, the phase at the top of the scan range is different and earlier
than the phase acquired at the bottom of the scan range). The influence on image quality for
different clinical scenarios and heart rates will be evaluated with second-generation dual-source
MDCT. Achenbach et al. [10] recently demonstrated the feasibility of this new scanning
method using first-generation dual-source CT. Slow and regular heart rates are the prerequisites
for this acquisition that is prospectively triggered by ECG and is anticipated to scan the entire
heart (12 cm) in 270 ms, with a pitch of 3.2 [10].
Another potential advantage of dual-source CT is tissue characterization with both detector
systems operating at different kilovolts, so-called “dual-energy CT” [19]. Although this has
not been clinically realized to date, two x-ray beams of different energy spectra in theory could
better demonstrate varying attenuation characteristics of different tissues [20,21••]. In this
approach the temporal resolution is sacrificed, and scanning requires a larger number of
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subvolumes, or slabs. In the case of differentiating myocardial perfusion defects from normal
myocardium [19,20,22], this will likely be problematic because the scan time can extend to
over 5 s.
Advantages
Elimination of Stair-Step and Misalignment Artifacts
Larger detector row widths limit the number of subvolumes, or slabs, needed for cardiac CT,
with the limit of z-axis coverage as 16 cm. For this technology, axial CT cardiac acquisitions
eliminate artifacts inherent in subvolume imaging over several cardiac cycles. An initial report
of 40 patients using this method showed that 89% and 99% of all coronary segments were of
excellent and diagnostic quality, respectively. The effective dose ranged from 4.9 to 9.4 mSv
with prospective ECG gating, and 60% to 100% phase window in one cardiac cycle acquisition
[21••].
Lowering Radiation Dose
Increasing the number of detector rows in an MDCT scanner generally increases radiation dose
to patients. However, shorter scanning time, elimination of redundant radiation from helical
oversampling, or overlapping of sequential axial acquisitions translate to lower than expected
radiation dose despite the higher number of detector rows in these wide-coverage MDCT
scanners. Early 4-detector coronary CT angiograms reported patient doses of 3.9 to 5.8 mSv
[23]. Using retrospective ECG gating on a 64-detector single-source CT had doses near 18.4
mSv, with a range from 15 to 21 mSv [3••,24–26•]. With ECG-controlled tube current
modulation, the average dose can be reduced to 9.4 mSv for 64-detector MDCT [27,28•].
Prospective ECG gating allows the x-ray beams to be turned on during preselected phases in
the cardiac cycles, and has been shown by various studies to further reduce radiation dose by
52% to 85% [17,21••,29–31•,32–36•] while maintaining equivalent diagnostic accuracy
compared with retrospective scanning [31•,35,37••,38].
The initial experience with 128×2 detector row CT showed a mean effective dose of 4 mSv,
ranging from 2.1 to 7.0 mSv [39]. The initial experience with 320-detector row CT had a mean
effective dose of 7.2 mSv, ranging from 4.9 to 16.5 mSv [21••], with reductions based on
careful selection of phase window and modification of x-ray output based on the patient’s body
habitus.
Reduced Intravenous Contrast Requirement
With rapid volumetric coverage, the duration of vascular contrast opacification required for
image acquisition is significantly reduced. In patients with normal cardiac output, some have
reported the contrast volume for CT coronary angiogram can be as low as 45 mL at 5 mL/sec
[39]. This could benefit patients with renal impairment and decrease the risk of contrast-
induced nephropathy.
Applications Beyond Coronary CT Angiography
Volumetric Myocardial Imaging for Function, Perfusion, and Viability
Volumetric imaging using wide-area coverage scanners can acquire data from the cardiac apex
to the base with minimal, or in the limit of 16-cm z-axis coverage, no time delay. For functional
assessment, the left ventricle ejection fraction (LVEF) is calculated with the x-ray beam
evaluating a single cardiac cycle from end systole to end diastole. The radiation dose for such
a protocol using a wide-area detector scanner is estimated to be 4 to 12 mSv with 60 to 80 mL
of contrast [5]. This method estimates the end-systolic and end-diastolic volumes and thus the
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LVEF. However, the effects of shunting, aortic or mitral valve regurgitation cannot be assessed
as is available using echocardiography and cardiac MRI.
Because iodinated contrast has similar extracellular kinetics to gadolinium contrast used in
MRI [40], several early studies [40,41••,42–46] have performed first-pass myocardial
perfusion imaging and/or delayed contrast enhancement using 16-, single-source 64-detector
row, or 32×2 dual-source CT to detect myocardial infarct or scar. Myocardial scar is
characterized by hyperenhancement on delayed imaging (>10 min after contrast
administration) and does not require rapid data acquisition. For perfusion imaging, fast imaging
techniques and temporal resolution are important to capture the first pass of contrast in the
myocardium. Although further investigation is required, rapid volumetric imaging with
temporal uniformity will likely help delineate small or subtle perfusion defects by allowing
comparison of the abnormally perfused myocardium to normal myocardium imaged at the
same time point.
Early experiments reported that coronary imaging, functional assessment, and rest and stress
myocardial perfusion can be achieved over three to five heartbeats with 14 to 16 mSv as an
effective dose and 120 to 140 mL of contrast [5]. The stress perfusion is a pharmacologic test,
and high temporal resolution is important because the heart rate will be increased from the
resting state of 60 to 70 bpm. Selection of an agent with less chronotropic effect is desirable
and the use of a multisegmented approach can improve the temporal resolution and the signal-
to-noise ratio at the expense of a higher radiation dose.
Evaluation for Acute Chest Pain and “Triple Rule-Out” Test
In the 2006 National Health Statistics Report, acute chest pain accounted for over 6 million
emergency department visits and close to 2 million hospital admissions in the United States
[47]. Patients with suspected acute coronary syndrome routinely undergo serial ECG, serial
troponin tests, and stress tests with prolonged emergency department stays or admissions. The
health care cost is estimated to be more than $8 billion per year, of which $6 billion is spent
on negative cardiac evaluation [48•].
Studies have shown that coronary CT using 64-detector row single-source, single focal spot
CT is a safe and efficient method to triage patients with acute chest pain who have a low to
intermediate likelihood of CAD due to its high NPV, diagnostic efficacy, time efficiency, and
cost-effectiveness [49,50•,51•]. A negative study or result with clinically insignificant stenosis
(≤ 50%) in these patients can lead to prompt exclusion of acute coronary syndrome. Complex
plaques with high-risk features or severe coronary artery stenosis on CT can expedite patients
to invasive coronary angiography. Patients with an intermediate (50% to 70%) degree of
stenosis by should undergo further testing, such as a conventional nuclear stress. In addition,
approximately 10% of the CT evaluations are inconclusive or nondiagnostic (retrospective
ECG gating 64-detector row single-source, single focal spot) [50•,51•]. Wider detector
coverage can potentially decrease stair-step and misalignment artifacts leading to a lower
number of inconclusive or nondiagnostic studies.
Several “triple rule-out” protocols have been proposed to provide high-quality images covering
the thoracic aorta, coronary, and pulmonary arterial trees. The examination must be able to
exclude life-threatening conditions, including pulmonary embolism, aortic dissection, and
acute coronary syndrome, among patients with acute chest pain and low to intermediate risk
of acute coronary syndrome. These protocols remain challenging because of 1) limited ability
of subvolume MDCT scanners to rapidly cover a large volume (at least from aortic arch to
cardiac inferior wall); 2) the requirement of large volume iodine contrast; and 3) high radiation
dose.
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Hein et al. [52] exploited the wide-area coverage of a 320-detector row scanner and performed
a triple rule-out protocol in 30 patients. The protocol included three axial nongated volume
acquisitions to cover the chest with triggering at the pulmonary arteries followed by a
prospectively ECG-gated cardiac scan for the coronary arteries. Both phases used 45 mL of
iodinated contrast respectively. The average effective radiation dose was 7 to 9 mSv for heart
rates less than 65 bpm (ie, single-segment image reconstruction for the coronary scan).
Endothelial Shear Stress and Coronary Vascular Profiling
Endothelial shear stress (ESS) is the frictional force exerted on the endothelial surface by the
blood flowing through the artery. ESS is determined by geometric variations of the coronary
anatomy as it courses around the heart. Low ESS (<0.5 Pa) is known to predispose the
development and progression of atherosclerotic plaque. Such change culminates in high-risk
vulnerable plaque, which is likely to rupture and cause acute coronary events [53••]. Currently,
“vascular profiling” of the entire length of a coronary artery is performed invasively using
intravascular ultrasound and catheter angiography to recreate the individual coronary lumen
[54]. This is followed by simulations of the blood flow in the coronary artery using
computational fluid dynamics. The methodology can provide accurate assessment of local
hemodynamic forces, such as ESS, and the local plaque size, morphology, and tissue
characterization. However, the invasive nature of the test limits its widespread use.
Early noninvasive identification of low ESS coronary artery segments at risk would be
invaluable for risk stratification and coronary event prevention. It has been proposed that highly
selective interventions to segments at risk could prevent future cardiac events. Early attempts
at performing CT-derived shear stress maps from fluid dynamic simulations have used
subvolume CT scanners that are prone to various artifacts leading to limited precision in
determining coronary anatomy and distorted coronary hemodynamics [55]. High-quality
single-heartbeat volumetric CT coronary angiography presents significant potential in the
noninvasive assessment of ESS (Fig. 1) [56].
Cine Volumetric Imaging and Four-Dimensional Subtraction Angiography
With volumetric cine scanning mode, it is feasible to perform CT subtraction angiography.
Having the x-ray beam being turned on throughout numerous gantry rotations without ECG
gating, dynamic CT angiographic images can be acquired by reconstructing transverse source
images and subsequently three-dimensional reformats at very short time intervals (eg, 0.1 s).
Continuous imaging over a large volume without patient table motion allows more accurate
subtraction of background from angiographic phase images. However, the first challenge for
this application is the huge dataset (320 slices×10 frames/sec) that can cause significant delay
in the calculation time (up to 76 min in an animal model with 256-detector MDCT prototype)
[57•]. A raw data-based subtraction method has been developed; therefore, image subtraction
can be performed with manipulation of raw data before completing image reconstruction with
filtered back projection [14,57•]. The second challenge is cardiac motion that makes this
technique more difficult than in the central nervous system vasculature [58].
Coronary Artery Opacification Gradients
As opposed to invasive catheterization, CT does not have a direct means to measure coronary
pressure changes. A novel application of wide-area coverage cardiac CT may extend beyond
anatomic plaque assessment by capitalizing on the temporal uniformity of the acquired images
by measuring contrast opacification (density) gradient across atherosclerotic lesions.
Initial experiences have shown that Hounsfield unit (HU) measurements near coronary ostia
were greater than those measured distally [21••]. A follow-up study demonstrated statistically
larger gradient change of contrast density across stenotic lesions [15]. For example, the
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difference in HU between the ostium and where an angiographically normal artery tapers to
2.5 mm was approximately 39 HU (n=84 arteries). This difference increased to 55 HU higher
in arteries with greater than 20% stenosis.
Furthermore, the contrast gradient within a coronary artery has been shown to be gradual along
the length of the artery. The contrast concentration at any point of an end artery is intrinsically
related to the cumulative resistance to the flow of contrast-opacified blood up to that location.
Thus, contrast opacification gradient between two points in the artery may carry information
related to flow characteristics. It may be that coronary opacification gradients are linked to
blood flow and fractional flow reserve, and may have implications particularly for patients
with indeterminate lesions, or for patients with diffuse CAD without focal stenosis. In the latter
case, a graded, continuous pressure drop along the arterial length results in flow resistance. In
diffuse CAD the lumen area is diffusely smaller than normal, and thus there is no reference
segment for qualitative measurements for visual assessment.
Conclusions
In the “post-64 era,” MDCT technology has branched into wide-area detector coverage and
dual-source acquisition strategies. Both directions of evolution have different potentials and
clinical applications. Beyond the elimination of helical and stair-step artifacts, new
technologies are one step closer to a more comprehensive cardiac evaluation. It is likely that
wide-area coverage and dual-source technology will not remain mutually exclusive with further
technology improvements.
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Fig. 1.
Whole intracoronary endothelial shear stress (ESS) mapping from single-heartbeat 320-
detector row CT (Toshiba Aquilion One Dynamic Volume CT; Tochigi, Japan) after injection
of 80 mL of iopamidol, 370 mg I/mL (Isovue-370; Bracco Diagnostics, Princeton, NJ),
followed by 40 mL of normal saline injected with a dual injector (EZEM Empower CTA DUAL
Injector; EZEM Inc., Lake Success, NY). Three-dimensional volume-rendered images
generated from Vitrea 4.1 software (Vital Images, Minnetonka, MN) (left side). Corresponding
ESS maps (right side). Simulated blood flow uses a computational fluid dynamic technique.
(Adapted from Ramkumar et al. [59]; with permission)
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