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ORIGINAL ARTICLE
Iodine Concentration and Optimization in Computed
Tomography Angiography
Current Issues
Lorenzo Faggioni, MD, PhD and Michela Gabelloni, MD
Abstract: Computed tomography (CT) technology has seen a dramatic evolution
in the recent past that has deeply changed the face of this diagnostic modality.
Since the early days of helical single-slice and then multislice CT, CT angiography (CTA) has been one of the most technically demanding applications, both
in terms of scanning technique and contrast medium (CM) injection protocol,
due to the need to acquire a large amount of high-resolution data over a limited
period corresponding to the peak contrast enhancement of the arterial system. Iodine concentration is one of the main determinants of arterial enhancement in
CTA, and current low-osmolar and iso-osmolar nonionic CM for intravascular
administration still come in a handful of molecules, but a relatively wide range
of different iodine concentrations. This gives the opportunity to optimize CTA
protocols as a function of several factors such as patient characteristics, CT technology, and CM features in an attempt to maximize the diagnostic yield of CTA
examinations while considering patient safety and avoiding unnecessary extra
costs. Our aim is to provide an up-to-date overview of the existing evidence on
how changing iodine concentration can have an impact on CTA performance, especially with the use of state-of-the-art CT and power injector technology, in the
perspective of improving patient care while minimizing overall exposure to iodinated CM and ionizing radiation.
Key Words: iodinated contrast medium, iodine concentration, CT angiography,
iodine delivery rate, viscosity, signal-to-noise ratio, contrast-to-noise ratio,
protocol optimization
(Invest Radiol 2016;00: 00–00)
O
ver the last decade, computed tomography (CT) has been revolutionized by a number of huge technological advancements, leading to the current possibility to acquire thousands of thin-slice images
with voxel isotropy in a few seconds.1,2 Among modern CT applications, CT angiography (CTA) is one that has gained the most benefit
from such evolution in terms of improved diagnostic performance and
broadened clinical indications. In fact, while in nonvascular CT, intravenously administered iodinated contrast medium (CM) tends to accumulate into the microcirculation and then in the interstitial space relatively
slowly after the beginning of intravenous injection, the following conditions must be fulfilled for optimal depiction of the arterial system:
• narrow slice width (usually ≤1 mm for peripheral CTA, or even
narrower down to approximately 0.5 mm for coronary CTA) for
proper evaluation of the smallest arterial vessels and high quality of
2-dimensional and 3-dimensional postprocessing;
• fast imaging time (usually in the order of seconds), so as to acquire
CTA data at the peak of arterial contrast enhancement before venous
enhancement occurs;
Received for publication January 31, 2016; and accepted for publication, after revision, March 21, 2016.
From the Department of Diagnostic and Interventional Radiology, University of Pisa,
Pisa, Italy.
Conflicts of interest and sources of funding: none declared.
Correspondence to: Lorenzo Faggioni, MD, PhD, Department of Diagnostic and Interventional Radiology, University of Pisa, Via Paradisa 2, 56100 Pisa, Italy.
E-mail [email protected].
Copyright © 2016 Wolters Kluwer Health, Inc. All rights reserved.
ISSN: 0020-9996/16/0000–0000
DOI: 10.1097/RLI.0000000000000283
Investigative Radiology • Volume 00, Number 00, Month 2016
• high selective contrast enhancement of the arteries throughout the
whole acquisition volume.3–7
In order for those requirements to be met, accurate coupling between CM injection and CTA image acquisition is of paramount importance to fully exploit the contrast bolus over the entire scan duration. In
other terms, both the bolus geometry and the scan parameters should be
tailored to achieve optimal, homogeneous enhancement of the arterial
lumen as well as an accurate assessment of vessel walls, taking into account several factors related to patient (eg, size, cardiac output [CO],
circulating blood volume [BV]), CT scanner (eg, scan speed, tube voltage, radiation dose), CM properties (eg, iodine concentration, viscosity,
safety issues), and CM injection protocol (eg, flow rate and volume, administration of a saline flush or multiple CM/saline boluses).3–7 Among
them, iodine concentration (defined as the mass of iodine per unit volume of CM in terms of grams per milliliter) is a key element because it
is directly related to contrast enhancement in CTA, as outlined in the
following sections.
While CT technology has been evolving quickly and offers several ways to optimize scan acquisition for CTA examinations, iodinated
CM technology has remained substantially unchanged since the introduction of low-osmolar and iso-osmolar nonionic CM for intravascular
administration.8 Therefore, on the CM side, optimization of CTA protocols can currently rely on the most appropriate choice of a limited number of iodinated CM, each of which is commercially available at
different iodine concentrations (Table 1).
This short review will focus on how iodine concentration can
have an impact on the optimization of CTA studies depending on the
available CT scanner technology and diagnostic scenario, based on evidence from the existing literature.
Iodine Concentration and Iodine Delivery Rate
As pointed out previously, CTA is technically more demanding
than CT venography or CT imaging of parenchymal tissues (especially
those with predominantly venous perfusion, where contrast enhancement depends on the gradual accumulation of iodine over time into a
capacitive system), as image acquisition occurs during the arterial
first-pass of CM for maximum arterial enhancement with no or minimized venous overlap. As blood opacified by iodinated CM is continuously flushed away from the vascular segment under examination as
an effect of systolic pressure and tends to be quickly replaced by unenhanced blood coming from the heart, CM with an adequate amount
of iodine per injected unit of volume (hence, iodine concentration) must
be supplied at high enough a rate to keep luminal enhancement above
a given target level. Therefore, for arterial contrast enhancement to be
increased for CTA in a given patient, one can increase iodine concentration, the flow rate at which CM is injected, or both.
The aforementioned considerations can be synthesized by the
concept of iodine delivery rate (IDR), which is defined as:
IDR ¼ ½I f
ðEquation1Þ
where [I] is the iodine concentration of CM and f is the flow rate at
which it is injected into the vessel (eg, via intravenous injection, as in
the case of CTA). In other terms, IDR represents the rate at which iodine
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1
Investigative Radiology • Volume 00, Number 00, Month 2016
Faggioni and Gabelloni
TABLE 1. Nonionic Iodinated CM Currently Available for
Intravascular Administration
Chemical Chemical
Class
Name
Trade
Name
Monomer, Iobitridol Xenetix (Guerbet)
LOCM
Monomer,
LOCM
Iohexol
Omnipaque
(GE Healthcare)
Monomer, Iomeprol Iomeron (Bracco)
LOCM
Monomer, Iopamidol Iopamiro (Bracco)
LOCM
Monomer, Iopromide Ultravist
LOCM
(Bayer Schering)
Monomer,
LOCM
Dimer,
IOCM
Ioversol
Optiray (Covidien)
Iodixanol Visipaque
(GE Healthcare)
Iodine
Concentration* Viscosity†
250
300
350
240
300
350
300
350
400
300
370
240
300
370
240
300
350
270
320
4.0
6.0
10.0
3.4
6.3
10.4
4.5
7.5
12.6
4.7
9.4
2.8
4.7
10.0
3.0
5.5
9.0
6.3
11.8
*Iodine concentration is expressed in milligram iodine per milliliter.
†Viscosity is expressed in millipascal seconds and is referred to a temperature
of 37°C.
CM indicates contrast medium; LOCM, low osmolar CM; IOCM, iso-osmolar CM.
is delivered into the arterial system and is the main determinant of arterial enhancement in CTA.3–7 As a matter of principle, for a fixed IDR
value, a given CM at a higher iodine concentration may thus be administered at a slower flow rate or lower concentration CM can be used with
a higher injection rate. A target IDR of approximately 1.2 to 1.6 gI/s and
up to 2.0 gI/s is usually recommended for noncoronary and coronary
CTA applications, respectively.5–7 On this basis, a wide range of combinations of iodine concentration and flow rate for a given CM can be selected to achieve the desired IDR value (Fig. 1).
Of course, different combinations of iodine concentration and
flow rate affect the magnitude of peak enhancement and contrast bolus
geometry, as well as the injection pressure due to CM viscosity and
flow rate, and the timing and duration of the CTA acquisition, with
a substantial impact on the CTA protocol.3–7 Ideally, contrast bolus
should have a rectangular shape with instantaneous rise and fall after
intravenous administration for infinite bolus compactness and full exploitation of iodine-related enhancement strictly over the CTA scan
time. Yet, the time-to-peak enhancement varies as a function of injection rate, with higher injection rates leading to a steeper contrast bolus
geometry and faster arrival of the enhancement peak, as well as shorter
duration of the arterial enhancement plateau (ie, the range of intravascular attenuation values deemed useful for a diagnostic CTA study).3,4,9
Thus, administering CM at a higher flow rate with other injection parameters kept constant results in a shorter bolus duration, allowing to
achieve good bolus compaction (which is essential to optimize contrast usage and mitigate beam hardening artifacts related to pooling of
hyperconcentrated CM into the venous system, especially for chest
and cardiac CT applications) and avoid waste of CM with the fast scan
times afforded by modern CT equipment (Fig. 2). Putting things into
context, with fast CTA scan protocols (acquisition time <10 seconds,
accounting for the majority of CTA examinations in current practice),
2
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the bolus shape can be approximated to a pulse function and injection
duration does not significantly affect arterial enhancement due to absence of bolus recirculation. Therefore, with a fast protocol, CTA scanning should start approximately 6 to 8 seconds after the CM transit
time (as determined, eg, by bolus tracking with a threshold of approximately 100 HU) to reach the peak enhancement and allow for the
scan delay of newer-generation multidetector CT scanners.5
Conversely, increasing iodine concentration while keeping other
parameters the same does not alter the time-to-peak enhancement, but
broadens the duration of the enhancement plateau, which may provide
some extra enhancement as a potential safety margin in case of suboptimal bolus timing and can be helpful with slower scan protocols (acquisition time >10 seconds), such as those for CTA of the lower limb
vessels.3–5 With slow CTA protocols, it is essential that the duration
of CM injection covers the relatively long time needed to scan the entire
anatomy up to the most distal arteries to avoid outrunning the CM bolus. In such cases, an increase in injection duration with other parameters kept constant leads to increased and more homogeneous arterial
enhancement over the entire acquisition volume due to the cumulative
effects of bolus recirculation (contributing to a 10% to 20% increase
in peak arterial enhancement), and modern dual-head power injectors
allow the usage of a biphasic CM bolus technique that may be beneficial to further prolong and linearize the enhancement plateau.3–7,10,11
However, whenever a fast CTA protocol is feasible, the combination
of a shorter injection time at constant IDR and a narrower scan window results in higher arterial enhancement due to greater bolus compactness and thus is better than a slower CTA protocol over the same
acquisition volume.12
On the other hand, for a given CM volume, a larger iodine dose
(expressed as [I] V, where V is CM volume) would be delivered using
higher concentration CM. Therefore, a lower volume of high concentration CM should be used to keep the same iodine dose and improve
bolus compaction while potentially improving patient safety and reducing overall costs. To this latter purpose, evidence exists that volumes
as low as 25 to 40 mL of high concentration CM are sufficient to provide diagnostic quality CTA examinations, for example, of the intracranial arteries (25 mL of iomeprol 400 mg I/mL at 5 mL/s flow rate13),
coronary arteries (30 mL of iopromide 400 mg I/mL at 5 mL/s flow
rate14), and for the diagnostic workup of noncardiac chest pain
(40 mL of iomeprol 400 mg I/mL at 3 mL/s flow rate15). In parallel,
low-volume, high-flow rate protocols have been devised with low concentration CM that yield diagnostic CTA image quality, for example,
for TAVI (trancatheter aortic valve implantation) planning (down to
FIGURE 1. How it is possible to select different combinations of flow rate
and iodine concentration values to get the desired IDR (eg, 1.6 g I/s).
IDR values in the matrix are expressed in terms of gI/s.
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Investigative Radiology • Volume 00, Number 00, Month 2016
Iodine Concentration in CT Angiography
FIGURE 2. A, How variations in CM flow rate, volume, and concentration lead to a variation in both magnitude of peak arterial enhancement (expressed
in Hounsfield units) and bolus duration. B, How variations in IDR and injection time affect peak arterial enhancement and bolus duration.
40 mL of iodixanol 320 mg I/mL with a multiphasic injection up to
5 mL/s flow rate),16 CTA of the thoracoabdominal aorta (50 mL
of iopromide 300 mg I/mL at 6 mL/s flow rate17), and CT pulmonary
angiography (CTPA) (30 mL of iodixanol 320 mg I/mL at 5 mL/s flow
rate18). Of note, in this latter work CTPA examinations performed
with 30 mL and 100 mL of the same CM injected at the same flow rate
were compared, and no significant differences were found between
the 2 groups in terms of visual image quality and attenuation of the
pulmonary arteries down to subsegmental level, showing that CM
volume does not affect contrast enhancement in CTA provided that
scan parameters are properly set. Interestingly, while in the 100 mL
group, bolus tracking was performed conventionally in the main pulmonary trunk with a 120-HU threshold; in the 30 mL group, the superior vena cava was chosen for bolus tracking with a 75-HU threshold
to account for the shorter bolus duration and a fixed scan delay as
long as 4 seconds, leading to a more efficient usage of the shorter bolus that may contribute to explain the study findings. In this setting
and as a general rule, a strict optimization of the CTA scan protocol
including accurate timing between scan and CM injection is always
mandatory to attain diagnostic image quality with the least amount
of iodine.
The choice of the most appropriate iodine concentration and
flow rate to reach the desired IDR is also influenced by several
patient- and CM-related factors, including the following:
• circulating BV (which is proportional to patient's lean body weight),
leading to lower arterial enhancement in patients with higher BV due
to greater CM dilution in the intravascular compartment3,4;
• CO, with slower time-to-peak arterial enhancement, greater enhancement magnitude, and longer enhancement plateau in patients with reduced CO due to slower CM circulation and clearance3,4;
• CM viscosity and the maximum pressure that can be generated to
deliver contrast at a given flow rate. On one hand, injecting CM at
too low an iodine concentration may be impractical due to the extremely high flow rate required to reach the target IDR value, so
that using high concentration CM with an iodine concentration of at
least 350 mg I/mL seems attractive to keep flow rate lower. On the
other hand, high concentration CM have greater viscosity than the
same CM at lower iodine concentration, which can actually limit
the usable flow rate substantially, and hence IDR (eg, due to high resistance of the venous line or poor injector performance).3,4
In practice, the actual relationship between CM injection parameters and degree of arterial enhancement that can be obtained is
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more complex than predicted by IDR alone, and the level of vascular attenuation needed for a CTA examination to be of diagnostic quality can
vary depending on the available CT technology and acceptable contrastto-noise ratio (CNR), as discussed later.
CTA and Iodine Concentration: Higher or Lower?
Which Concentration for a Given IDR?
As mentioned previously, one advantage of high concentration
over low iodine concentration CM is the possibility to achieve the same
degree of arterial enhancement with a lower flow rate or, equivalently,
greater arterial enhancement with the same flow rate in CTA. This is
a logical consequence of the IDR definition, and there is wide agreement on this point in the literature regarding CTA of several arterial territories, ranging from CTPA19,20 to chest CT including CTA of the
thoracic arteries,21 CTA of the infrarenal aorta and runoff vessels,22
CT coronary angiography,23 and CTA of the abdominal vasculature
for planning of pancreatic surgery24 and evaluation of renal transplant
donors.25 In all of the aforementioned listed papers, high and low iodine
concentration CM were compared using the same flow rate and CTA
scanning technique, expectedly resulting in greater arterial enhancement with higher concentration CM, as well as greater subjective image
quality and preserved diagnostic accuracy where assessed. Of interest, a
partial exception was reported in the work by Cademartiri et al,23 where
iodixanol 320 mg I/mL (a dimeric, iso-osmolar CM) was found to yield
vascular attenuation comparable to that of monomeric, low-osmolar
350 mg I/mL CM injected at the same flow rate of 4 mL/s, likely due
to its different physicochemical properties. In general, the use of high
concentration CM allowed for a reduction of injected volumes compared with lower concentration CM administered with the same flow
rate and iodine load,21,22 along with up to 5.5% cost savings21 and increased bolus compaction due to a shorter bolus duration. Furthermore,
CM administration at a lower flow rate allows the usage of smaller-bore
needles for intravenous injection, which may be beneficial in patients
with a poor venous access (eg, after chemotherapy), provided that
CM viscosity be kept reasonably low by preheating contrast before
introduction.3,4
Conversely (and in agreement with theory), there is extensive
evidence that when CM at different iodine concentration and matched
IDR are compared, injection of lower concentration CM leads to comparable26,27 or even greater arterial enhancement than higher concentration CM28,29 without an increase in iodine load. Mühlenbruch et al
compared intravascular attenuation of the aorta and pulmonary vessels
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Faggioni and Gabelloni
Investigative Radiology • Volume 00, Number 00, Month 2016
in 300 chest CTA examinations obtained by administering 3 different
CM (iopromide 300 mg I/mL, iopromide 370 mg I/mL, and iomeprol
400 mg I/mL) at matched IDR of 1.3 g I/s and iodine load of 33 g I
via a 18-gauge needle placed in a cubital vein. In this setting, no statistically significant differences were found in terms of arterial enhancement, image quality, and discomfort during CM injection.27 Yet,
Behrendt et al28 compared equi-IDR, equi-iodine dose injections of
the same CM for CTA of the pulmonary arteries and thoracic aorta
and showed that significantly greater vascular enhancement could be
achieved with the lowest iodine concentration. Similarly, Behrendt
et al29 showed as well that higher vascular enhancement could be obtained for abdominal and chest CTA with iopromide 300 mg I/mL
than with iopromide 370 mg I/mL using an equi-IDR, equi-iodine
dose protocol. These findings can be explained by the lower viscosity of lower concentration CM, allowing for reduced flow resistance
and injection pressure. Therefore, CM with lower viscosity would distribute more easily and more evenly in the vessels, potentially resulting in higher vascular attenuation during the arterial first-pass of
CM.28,30,31
As a matter of fact, a greater bolus compactness is relevant for
CTA regardless of iodine concentration because it allows the contrast
bolus to be fully exploited. To this latter respect, it may be interesting
to mention the finding by Matoba et al32 that, whereas higher arterial
enhancement can be obtained in liver CT for depiction of hypervascular hepatocellular carcinoma using 300 mg I/mL rather than
370 mg I/mL CM administered with the same iodine load and injection
duration without saline flush, the addition of 40 mL of saline solution
after the CM bolus negates the enhancement gain with the 300 mg I/mL
CM. This was presumably due to the increased bolus compactness
and clearance of the “dead space” between the brachial vein and superior vena cava provided by the saline flush, which should regularly
be administered after CM injection using modern dual-head power
injectors.
iodine concentrations between 240 and 300 mg I/mL. In the authors'
opinion, this finding may be explained by the fact that too low an iodine
concentration results in poor compactness and excessively low concentrative flow pattern of the contrast bolus, whereas a moderate concentration CM would distribute faster and more homogeneously in
the vessels than high concentration CM. Moreover, administration of
iodinated CM with lower viscosity (and hence, iodine concentration
for a given molecule) may potentially limit the risk of acute kidney injury compared with higher viscosity CM by preventing renal hypoperfusion and hypoxia induced by excessive tubular fluid viscosity.36
As to the risk of contrast extravasation related to high speed injection, there is no evidence in the literature of a correlation between
injection rate and extravasation events, especially if intravenous
needles with a diameter larger than 22 gauge are used.31,37 The availability of modern power injector technology is certainly beneficial, as
state-of-the-art systems allow to accurately calibrate flow rate within
a wide range of injection speeds, monitor CM injection pressure, and
keep track of injection parameters and potential adverse events (such
as extravasation or flow limitation issues).33
However, pushing things to the extreme, it is unlikely that an injection rate higher than 10 mL/s would further increase the magnitude
of arterial enhancement because of CM dispersion and reflux from
the right atrium, the pressure performance envelope of power injectors,
and the risk of stressing the venous injection site at such high flow
rates.4 Thus, whenever high flow injection is impossible for any reason
(eg, poor venous access), increasing CM iodine concentration remains
an option to keep IDR up.
Iodine Concentration and CM Viscosity: A Closer Look
With the evolution of CT and power injector technology, as well
as the introduction of PACS-connected hardware and software platforms for automated monitoring of CM administration protocols,33 interest has arisen on the development of CTA protocols based on the
injection of ultralow (ie, less than 300 mg I/mL) iodine concentration
CM at very fast flow rates, provided that a valid venous access is available.30,31,34 This is possible owing to the decreased viscosity of lower
concentration CM compared with a higher concentration CM passing
through a tube (eg, a venous access or a vessel) at a given flow rate
according to Poiseuille's law:
Q ¼ πΔpr4=8ηl
ðEquation2Þ
where Q is the flow rate, Δp is the pressure gradient, r is the radius of
the tube, η is the viscosity, and l is the tube length.35 In particular, Mihl
et al showed that CT coronary angiography is feasible using iopromide
with an iodine concentration as low as 240 mg I/mL injected at 9 mL/s
via a 18-gauge needle (ie, a commonly used needle size for CTA). In
their experience, subjective and objective image quality as well as peak
pressures as measured by an injector-connected monitoring system
were comparable to those achieved by administering an equi-iodine
dose of iopromide 300 mg I/mL at the same IDR of 2.16 g I/s (resulting
in a flow rate of 7.2 mL/s), opening a doorway toward applicability of
a broad variety in flow rates and IDRs and subsequently more individually tailored injection protocols34 (Fig. 3). Interestingly, in a previous
study, Behrendt et al31 compared the arterial enhancement obtained in
pigs by injecting iopromide over a wide range of iodine concentrations
(ranging from 150 to 370 mg I/mL) at the same iodine dose and IDR,
and found that contrast enhancement was significantly improved using
4
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Does IDR Always Need to Be “So” High for CTA?
A high IDR for CTA allows to generate a vascular attenuation
(ie, signal) high enough to obtain a sufficient signal-to-noise ratio and
CNR for optimal depiction of the arterial system and postprocessing
of CTA data sets. More generally speaking, maximizing image contrast
plays a pivotal role in technical optimization of CTA examinations
(where high-contrast objects such as iodine-enhanced blood into the
vessel lumen versus the vessel wall are the diagnostic focus), with the
target being to produce CTA images of diagnostic quality with the least
amount of ionizing radiation (especially in younger patients) and/or
iodinated CM. This latter issue is especially important in patients at
higher risk for contrast-induced nephropathy (making up a substantial fraction of those referred for CTA), and despite recent reports that
such risk for all patients receiving intravenous iodinated CM may be
overstated,38 up-to-date guidelines recommend usage of the lowest
dose of CM consistent with a diagnostic result.39
To this purpose, image acquisition at a tube voltage as low as
80 to 100 kV or even less on newer-generation CT scanners (low kV
scanning) is a common strategy to improve vascular attenuation and
CNR while substantially lowering radiation exposure in CTA examinations of nonobese patients, with other scanning parameters kept
unchanged.40–43 In fact, x-rays generated by a lower tube voltage have
a lower average energy, closer to the k-edge of iodine (33.2 keV),
resulting in greater photoelectric effect as well as greater noise and
susceptibility to photon starvation artifacts,40,44 which, however,
can usually be tolerated unless unacceptably severe (such as in obese
patients or patients with metal hardware) owing to the net increase in
CNR.45 In this setting, high concentration CM can be advantageous
over low concentration CM injected at the same flow rate to improve
vessel contrast and actually reduce radiation dose in low kV CTA
studies. In particular, Iezzi et al46 showed that compared with a
300 mg I/mL CM, 120 kV protocol for CTA assessment of abdominal aortic aneurysms after endovascular repair, the combination of
low kV scanning and the same iodine dose (36 g I) of high concentration CM (400 mg I/mL) yielded comparable subjective and
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Investigative Radiology • Volume 00, Number 00, Month 2016
Iodine Concentration in CT Angiography
FIGURE 3. Image quality, iodine load, and radiation exposure in 2 patients undergoing CTA of the thoracic aorta. A, Good attenuation (455 HU)
of the aortic root obtained by injecting 90 mL of iobitridol 350 mg I/mL at 5.5 mL/s flow rate, resulting in an IDR of 1.92 g I/s and an iodine load
of 31.5 g. Tube voltage was set at 120 kV, and radiation dose in terms of dose-length product was 626 mGy · cm. B, Comparable (if slightly greater)
attenuation of the aortic root (500 HU) at the same level in another patient who underwent CTA of the thoracic aorta with 100 kV tube voltage and
80 mL of iobitridol 250 mg I/mL injected at 7 mL/s flow rate, resulting in an IDR of 1.75 g I/s and an iodine load of 20 g. Radiation dose was almost
halved (dose-length product 323 mGy · cm). C, Despite the higher flow rate, injection pressure for the lower concentration CM was lower (152 psi)
than for the higher concentration CM (172 psi) owing to its reduced viscosity. Figure 3 can be viewed online in color at www.investigativeradiology.com.
objective image quality along with substantially reduced radiation
exposure.
Yet, when comparing high (370 mg I/mL) and low (300 mg
I/mL) concentration CM injected with the same volume (110 mL) and
flow rate (5 mL/s) for CTA of the renal arteries performed at 120 kV
and 80 kV, respectively, the combination of low concentration CM
and 80 kV was associated with significantly higher arterial enhancement and superior image quality along with a smaller amount of iodine and a lower radiation dose,47 underscoring the prevailing impact
of kV reduction on boosting iodine signal. Of interest, some modern
power injectors can actually reduce both IDR and iodine load without changing CTA timing strategy by simply mixing saline to high
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concentration CM, potentially improving individualization of CTA
protocols and workflow with a single iodine concentration.
How Conservative CM Technology Can Keep Up With
Fast-Changing CTA Technology
More recently, the development and increased availability of
novel CT technology including iterative reconstruction (IR) algorithms,48,49 dual-energy imaging,50,51 and ultrafast CT scanners52 have
opened new perspectives toward a more and more aggressive reduction
of iodine and radiation exposure, progressively pushing the limit at
which diagnostic CTA images can be acquired with low kV, low iodine
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5
Investigative Radiology • Volume 00, Number 00, Month 2016
Faggioni and Gabelloni
protocols, even in larger patients.43,52 In fact, the extremely short acquisition time of newer-generation CT scanners (in the order of a couple
of seconds for prospectively electrocardiographically [ECG]-triggered
CTA of the entire aorta),53 combined with the availability of tube settings as low as 70 kV and state-of-the-art IR algorithms enabling maximization of signal-to-noise ratio and CNR via a strong reduction of
image noise, can naturally be matched and even strengthen all of the
previously discussed concepts of maximum bolus compaction, as low
as possible iodine dose, and as low as possible radiation exposure for
CTA. As a potential additional tool, dual-energy scanning coupled with
IR techniques allows to even halve iodine exposure by reconstructing lowenergy monochromatic images (hence with dramatically improved photoelectric effect), allowing flexibility in optimizing image contrast.50
In this context, several papers have recently been published
showing the feasibility of submillisievert CTA examinations with modest amounts of iodine, both from high-14,43 and low-concentration
CM.53,54 For instance, Zhang et al14 obtained diagnostic quality highpitch ECG-triggered CTA examinations of the aorta on a dual-source
CT scanner with only 30 mL of 370 mg I/mL at 70 kV tube voltage.
On the other hand, Zheng et al54 compared a high iodine concentration (370 mg I/mL), 100 to 120 kV protocol without IR and a low
concentration (270 mg I/mL), 80 to 100 kV IR protocol for prospectively ECG-triggered dual-source CT coronary angiography, showing
that this latter protocol yielded comparable arterial enhancement and
image quality along with reduced radiation dose. As for dual-energy
scanning, it may be worthwhile reporting the finding by Xin et al51
of better image quality for upper abdominal CTA examinations performed with 28% less iodine load and reduced radiation burden
using 270 mg I/mL CM and monochromatic dual-energy scanning,
compared with 350 mg I/mL and conventional 120 kV scanning.
Of course, the advantages seen with the lower concentration protocols in the 2 papers mentioned previously51,54 are not due to the
lower iodine concentration itself but rather to the higher inherent image contrast of the improved CT scanning techniques, leading to a
more efficient use of iodine.
CONCLUSIONS
Image quality and overall diagnostic performance of CTA examinations in the various areas of the arterial tree are a function of
multiple variables related to patient characteristics, available CT technology, and CM properties. Out of these latter, iodine concentration is
one of the main determinants of arterial enhancement and plays a major
role in the optimization of CTA protocols in terms of diagnostic quality,
iodine exposure, and radiation dose. To the best of our knowledge, there
is currently no evidence that any given iodine concentration is in itself
superior to the others as to the overall optimization of CTA protocols.
Rather, it is the operators' duty to make any effort to choose the most
appropriate CTA protocol (ie, scanning and CM injection parameters,
including iodine concentration), tailored to any single patient for any
single diagnostic query by leveraging the wide variety of available iodine concentrations and scanning techniques and the flexibility of modern power injectors, in line with the ALARA (as low as reasonably
achievable) principle.
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