Introduction
Cardiovascular disease (CVD) is the leading cause of mortality worldwide. According
to the European Cardiovascular Disease Statistics 2012 report, CVD and coronary heart
disease (CHD) account for over 4 million deaths (42 % of all deaths) in Europe each
year, and the impact of mortality and morbidity on European society and healthcare
systems remains at a challenging level [1].
Not surprisingly, coronary imaging has been the main focus of cardiac computed tomography
(CT) with its main indication still being “known or suspected coronary artery disease”
– according to the European Society of Cardiac Radiology Cardiac MR/CT registry 2018
[2]. Rapid developments in scanner technology and acquisition protocols have made cardiac
CT a safe, reliable, and widely applicable tool for coronary imaging [3]
[4]. Innovations and advanced post-processing tools have generated increasing potential
for applications beyond the anatomical imaging of coronary arteries including functional
and structural assessment [5]. Furthermore, there has been an increasing demand to use cardiac CT for pre-interventional
planning in minimally invasive procedures such as transcatheter valve implantation,
mitral valve repair, and pulmonary vein ablation [6]
[7]
[8]. As all of these applications are expected to gain increasing importance in the
clinical routine, this review intends to give an overview of the current use of cardiac
CT as well as emerging techniques with a specific focus on applications beyond coronary
arteries and their potential for future clinical application.
Current Guidelines for the Appropriate Use of Cardiac CT
As with all techniques using ionizing radiation, patient safety and risk/benefit assessments
are crucial elements which need to be considered when applying CT in clinical settings.
Therefore, over the past years, several guidelines for the appropriate use of cardiac
CT have been issued by different societies to guide decision making [9]
[10]
[11]
[12]
[13].
Dependent on the patient’s risk profile and previous test results, ruling out/detecting
coronary artery disease (CAD) in symptomatic or asymptomatic patients without previously
diagnosed heart disease or in patients prior to “noncoronary cardiac surgery” [9] is one of the main indications of cardiac CT [9]. Non-contrast cardiac CT allows the quantification of coronary calcification (to determine
a calcium score/Agatston score [14]) at very low-radiation doses (< 1 mSv). This score serves for further risk stratification
as patients with a calcium score of zero present a very low risk for adverse cardiac
events, whereas the presence of calcifications indicates an elevated risk of events
in the future [15]
[16].
While the absence of coronary calcification does not entirely rule out the presence
of CAD, coronary computed tomography angiography (CTA) can reliably detect atherosclerotic
plaque as well as subsequent luminal narrowing [3]
[4].
Functional assessment, specifically of the left ventricle, is recommended in patients
with heart failure, post-myocardial infarction, or inconclusive images from prior
noninvasive testing, while right ventricular function plays an important role in arrhythmogenic
right ventricular dysplasia [9]. Functional and structural assessment is intended for “adult congenital heart disease”
[9], native/prosthetic valves suspicious for dysfunction, the anatomy of pulmonary or
coronary veins before intervention, or the visualization of bypass grafts [9]
[11].
As outlined above, coronary artery imaging and specific functional analyses are well
implemented in clinical guidelines, whereas several secondary analyses of the primary
CT-data – especially with regard to function and structure – are currently predominant
in research rather than the daily routine. However, with the continuous evolution
of CT imaging towards physiological assessment raising the demand for these techniques,
a broader spectrum of clinical indications of cardiac CT can be expected within the
next years.
Cardiac CTA – Image Acquisition
Apart from non-contrast cardiac CT for the detection of coronary artery calcifications,
coronary CTA is the workhorse of cardiac imaging and prerequisite for most post-processing
analyses.
The acquisition of a coronary CTA ([Fig. 1]) is performed ECG-gated either in a retrospective or prospective way. The retrospectively
ECG-gated approach captures the heart throughout the whole cardiac cycle, acquiring
images at multiple cardiac phases and, thus, delivering robust, high-quality images
even at high heart rates, yet at increased radiation exposure compared to prospective
modes. For prospectively ECG-gated scans, sequential and high-pitch helical techniques
are available, which are chosen depending on heart rate and rhythm, as well as patient
habitus (with maximum established thresholds of 65 heartbeats/minute, sinus rhythm
and a body-mass index < 30 kg/m2). The sequential mode acquires images of different anatomic regions during preselected
cardiac phases, the so-called “step-and-shoot” method, whereas high-pitch helical
scans cover the entire heart with a single gantry rotation, so that the image is obtained
in a single cardiac cycle [17]
[18]. The latter is associated with a drastic dose reduction to an average effective
dose of 1.2–3.2 mSV [19] – in comparison, modern retrospective spiral acquisitions comprise 5.7–10.7 mSV
[20], and diagnostic invasive coronary angiography typically uses 2–7 mSV [21] – yet requires optimal timing during acquisition to ensure a motion-free image.
Another limitation of this approach is the inability to assess cardiac function, which
is done in retrospectively gated scans [22] and will be further addressed in the corresponding section.
Fig. 1 Coronary CTA of a 62-year-old patient with a non-obstructive calcified plaque in
the proximal LAD. CX and RCA were patent on images. CTA: computed tomography angiography,
CX: circumflex artery, LAD: left anterior descending Artery, RCA: right coronary artery.
Abb. 1 Koronare CTA eines 62-jährigen Patienten mit nicht obstruktivem kalzifiziertem Plaque
des proximalen Ramus interventricularis anterior. Ramus circumflexus und Arteria coronaria
dextra waren durchgängig. CTA: Computertomografie-Angiografie.
Cardiac CT in Functional Imaging
Fractional Flow Reserve (FFR)
Several studies in the past years have demonstrated the high diagnostic value of coronary
CTA in patients with acute and stable anginal symptoms [3]
[4]
[23]
[24]
[25]
[26]
[27].
In this context, coronary CTA has proven to have a high negative predictive value
for the occurrence of acute coronary events in patients with acute chest pain [24] and has been evaluated as safe for ruling out CAD in low/intermediate risk patients
with stable angina and suspected coronary syndrome [23]. Also, most recent long-term results in patients with stable chest pain confirmed
that coronary CTA in addition to standard care versus standard care alone leads to
a significant decrease in death from coronary heart disease and nonfatal myocardial
infarction without resulting in an increase in invasive catheterization and revascularization
[25]. However, the extent of coronary plaque on CTA may not always correlate well with
the functional significance of a lesion as measured by invasive FFR [28], which is one of the gold standard methods for identifying such lesions and is considered
a class IIa recommendation for admitting a patient to coronary revascularization [29]. In fact, the positive predictive value of coronary CTA in the evaluation of functionally
significant lesions (defined by > 50 % luminal narrowing) compared to FFR (defined
by a ratio ≤ 0.75) demonstrated a sensitivity of 79 % and a specificity of 86 % [30], making management of patients with stenoses on CTA difficult [31].
In this respect, novel computational fluid dynamic (CFD) modeling techniques now allow
the noninvasive calculation of a CT-derived FFR by using coronary CTA-images to evaluate
the functional significance of a given lesion without additional application of contrast
or vasodilator agents. For example, HeartFlow Inc. (Redwood City, California, USA)
is a company approved by the US Food and Drug Administration (FDA) to provide the
CT-FFR value in an online service by calculating the values three-dimensionally similar
to what is described in [Fig. 2]. Alternatively, Siemens Healthcare (Forchheim, Germany) recently developed a technique
for on-site workstations to calculate CT-FFR values using a 1D approach, which, however,
is currently only available for research purposes [32].
Fig. 2 64-year-old male patient with a two-day history of episodes of unstable angina. He
underwent coronary CTA which demonstrated moderate luminal narrowing of the LAD with
a calculated CT-FFR of 0.98 (orange arrow) and subtotal occlusion of RCA with a CT-FFR
of 0.49 (blue arrow). Representative case performed by Dr. Eslami: CFD patient-specific
models were created to simulate hemodynamics in coronary arteries. CT-FFR was calculated
in post-processing before and after the lesion using a spherical probe with a radius
of 0.005 cm to measure the pressure. CFD: computational fluid dynamics, CTA: computed
tomography angiography, CT-FFR: CT-derived fractional flow reserve, LAD: left anterior
descending artery, RCA: right coronary artery.
Abb. 2 64-jähriger Patient mit seit 2 Tagen bestehenden Episonden einer instabilen Angina
pectoris. Die koronare CTA zeigte eine moderate luminale Einengung des Ramus interventricularis
mit einer berechneten CT-FFR von 0,98 (orangener Pfeil) und einem subtotalen Verschluss
der Arteria coronaria dextra mit einer CT-FFR von 0,49 (blauer Pfeil). Repräsentativer
Fall berechnet von Dr. Eslami: Patientenspezifische CFD-Modelle zur Simulation der
hämodynamischen Verhältnisse der Koronararterien wurden erstellt, die CT-FFR wurde
mittels post-processing proximal und distal der Läsion durch eine sphärische Messsonde
mit einem Radius von 0,005 cm zur Druckmessung berechnet. CFD: Computational Fluid
Dynamics, CTA: Computertomografie-Angiografie, CT-FFR: CT-basierte fraktionelle Flussreserve.
Thus far, several studies have described good correlations of both computational methods
to invasive FFR when compared to CTA alone [33]
[34]
[35]. For instance, a study by Norgaard et al. demonstrated a strong correlation with
coronary CTA (r = 0.82) with a sensitivity and specificity in detecting a functionally
significant lesion of 86 % and 79 %, respectively [34]. Moreover, Ko et al. reported that CT-FFR compared to invasive measurements had
a higher specificity (87 % vs. 74 %) with similar sensitivity (78 % vs. 79 %) [35].
While comparative studies demonstrate encouraging results, the long-term outcomes
of FFR-guided interventions, in general, appear to be unclear. Recently presented
data from the FUTURE trial comparing FFR-guided intervention to traditional angioplasty
in patients with acute and stable chest pain and multivessel disease (> 50 % stenosis)
suggests a higher mortality rate in the FFR group (interim analysis at 12 months:
4 % all-cause death in the FFR group versus 2 % in angioplasty group, p = 0.02), which
led the investigators to end enrollment prematurely [36]. This is a very critical aspect warranting additional investigation. With regard
to these results and the currently limited availability of CT-FFR calculations, further
research on clinical performance and cost-effectiveness is needed.
Myocardial Perfusion Imaging
The detection of myocardial ischemia is of utmost importance for the diagnosis, treatment,
and prognostic outcome of patients and has been the method of choice for viability
assessment in obstructive atherosclerosis [37]. Currently, stress-induced electrocardiogram (ECG), echocardiography, cardiac magnetic
resonance imaging (MRI), as well as single photon emission computed tomography (SPECT)
and positron emission tomography (PET) are the gold standard methods for the assessment
of LV myocardial viability [38]. Yet, none of these techniques can assess myocardial perfusion while at the same
time delivering anatomical information on coronary arteries, which is a hallmark of
CT. The combination of functional and anatomical information can be especially important
as a high number of events have been shown to occur in patients with non-obstructive
CAD (1–69 % stenosis), which is not captured by perfusion imaging alone [39].
The theory behind CT perfusion imaging is the distribution of iodinated contrast media
through the myocardium by the coronary arteries. Thus, first-pass perfusion defects
– as present in high-grade stenosis or occlusions – are visualized as areas of hypoattenuation
in the myocardial muscle ([Fig. 3]) [40].
Fig. 3 54-year-old male patient with history of diabetes mellitus type 2, hypertension,
hyperlipidemia, and a family history of CAD as well as know CAD in the LAD. The patient
presented with stable anginal symptoms. Coronary CTA and CT perfusion imaging were
performed. CTA demonstrated 70 % stenosis in the medial LAD, perfusion imaging showed
a subendocardial perfusion defect in the LAD territory (white arrow). a temporal average, b myocardial blood flow, c myocardial blood volume, d perfused capillary blood volume, CAD: coronary artery disease, CTA: computed tomography
angiography, LAD: left anterior descending artery.
Abb. 3 54-jähriger Patient mit Diabetes mellitus Typ 2, Hypertension, Hyperlipidämie, positiver
Familienanamnese einer KHK und einer bekannter KHK des Ramus interventriucularis.
Der Patient zeigte Symptome einer stabilen Angina pectoris, worauf eine koronare CTA
sowie eine CT-Perfusion durchgeführt wurden. In der CTA zeigte sich eine 70 %ige Stenose
im mittleren Abschnitt des Ramus interventricularis mit Perfusionsdefekt im entsprechenden
Versorgungsgebiet (weißer Pfeil) in der Perfusionsmessung. a Zeitmittelwert, b myokardialer Blutfluss, c myokardiales Blutvolumen, d perfundiertes kapillares Blutvolumen. CTA: Computertomografie-Angiografie, KHK: koronare
Herzerkrankung.
In general, CT myocardial perfusion imaging can be performed at rest or under pharmacologically
induced stress in two different ways: static versus dynamic. Static image acquisition
takes place at the time of maximum contrast concentration in the myocardium of the
left ventricle allowing a “visual qualitative assessment of a single snapshot of myocardial
iodine contrast attenuation” [40]. In dynamic perfusion imaging, the scan is repeated sequentially during contrast
passage through the myocardium, thus, allowing direct measurement of myocardial perfusion.
While the correct timing of this technique is less crucial, its limitations are the
risk of motion artifacts (caused by both motion of the patient and the heart) as well
as increased radiation exposure compared to static image acquisition [40]
[41].
The introduction of dual-source scanners permits a third approach to perform CT perfusion
imaging (either static or dynamic) by combining two different tube voltages, and thus,
two different energy spectra of CT photons – typically 100 kV and 140 kV [42]
[43]. As CT attenuation values for different tissues are specific depending on the energy
spectrum used, dual-energy CT improves tissue characterization (especially of iodine)
[43], rendering the differentiation of iodine attenuation and cardiac tissue possible.
Iodine distribution in the myocardium can be mapped (usually in a color-coded manner)
and superimposed on the naïve image to aid identification of perfusion deficits [42]. Another feature of dual-energy CT is the simultaneous acquisition of two data sets
(high- and low-kV). While the low-kV images present better tissue contrast (as they
are closer to the k-edge of iodine), the consecutive high noise limits its routine
acquisition in single-source scanning. Yet, the improved differentiation of iodine
uptake in these images is a potential advantage, which yielded a higher sensitivity
(80 % vs. 77 %) in a study investigating 100 kV images compared to a virtual 120 kV
series in the detection of chronic myocardial infarction [43].
In terms of radiation exposure, recent literature reports very low radiation doses
of just 2.5 mSv in stress/rest perfusion using a 128-slice dual-source scanner [44].
Image analysis in CT perfusion imaging is performed on multiplanar reconstructed images
in short axis stacks of the left ventricle as well as in the orthogonal axis. The
location of mal-perfused areas is described using the standard 17-segment model [41]. In static imaging, evaluation is mostly performed visually. While an option to
evaluate the images semi-quantitatively (by calculating the ratio of subendocardial
and subepicardial enhancement) exists, the visual approach has been described as more
accurate [41]. In dynamic imaging, the semiquantitative evaluation can be performed by calculating
a time-attenuation curve [40].
Several studies have addressed the additional value and diagnostic accuracy of CT
perfusion versus CT angiography alone. In a meta-analysis, investigating 12 studies
with a total of 920 patients, CT perfusion showed a “favorable diagnostic performance”
[45] when compared to invasive coronary catheterization with a small increase in specificity
(without altering the sensitivity or overall performance) [45]. A multicenter study including 381 patients found that coronary CTA in combination
with static CT perfusion imaging was able to correctly identify patients with known
CAD ≥ 50 % (results from invasive coronary angiography) and perfusion defects as detected
by stress single photon emission computed tomography [46]. Osawa and colleagues further described a significant added value of CT perfusion
(under resting conditions) to coronary CTA in the diagnosis of CAD with an increase
of the area under the receiver operating characteristic curve from 0.84 to 0.89 (p = 0.02)
[47]. Also in single-energy studies, several investigations report an incremental diagnostic
value when combining dual-energy perfusion imaging with coronary CTA using SPECT or
invasive coronary angiography as a reference [48]
[49]
[50]. Additionally, there is evidence that the dual-energy approach may be favorable
in tissue characterization when compared to single-energy CT (especially when using
reconstructed monochromatic images at 70 kV to eliminate beam-hardening artifacts)
[51].
Viability and Fibrosis
Secondly to first-pass perfusion imaging, delayed enhancement imaging (performed 5–10 min
after contrast injection) has been introduced. With this, cardiac CT has proven feasible
in viability assessment (i. e., the detection of necrosis, fibrosis, and microvascular
obstruction) in a selected patient population, which has thus far been a domain of
cardiac MRI. An infarcted territory can be characterized based on hyper- and hypoenhancement
on delayed enhancement images signaling an infarcted territory or microvascular obstruction.
In the case of hyperenhancement in acute infarction, membrane dysfunction lets iodine
molecules pass into the intracellular space where contrast accumulates. Hyperenhancement
in scar tissue, however, is believed to be caused by an increase of the intercellular
space due to cell necrosis. Microvascular obstruction, on the other hand, appears
as hypoattenuation due to blockage of capillaries caused by cell debris despite restored
flow [52].
While focal myocardial scar tissue can be reliably detected on CT images, diffuse
myocardial fibrosis has mainly been quantified using MRI (specifically T1-mapping)
[53]. However, with the increasing use of CT, several groups have developed methods to
quantify diffuse tissue scarring using CT images [54]
[55]
[56]. The idea behind both techniques is the calculation of the extracellular volume
(ECV) fraction of the myocardium (representing equal distribution of contrast material
between muscle and blood on delayed enhancement images), which is increased in myocardial
fibrosis and associated with various cardiomyopathies and heart failure [54]
[57]
[58].
Nacif et al. have published a method to identify myocardial fibrosis on cardiac CT
using unenhanced and contrast-enhanced images. For the calculation of ECV, HU attenuation
values in the myocardium and blood pool were measured in pre- and postcontrast images
and the ratio of these changes (change in myocardial attenuation/change in blood pool
attenuation) was set in relation with the patient´s hematocrit level. CT-obtained
ECV values demonstrated good correlation with MRI measures (r = 0.82) and were elevated
in patients with heart failure [54].
In a different approach, Lee et al. evaluated the feasibility of contrast-enhanced
dual-energy CT for the quantification of myocardial fibrosis by measuring overlay
attenuation values of the myocardium and blood pool on iodine attenuation maps. Again,
the results were comparable with MRI, which served as a reference standard, and an
increase in ECV was associated with cardiomyopathy (hypertrophic and dilated), amyloidosis,
and sarcoidosis [55].
Overall, these results encourage the use of CT-based tissue characterization in the
future.
Cardiac Functional Imaging
Especially in patients with chest pain but an uncertain diagnosis of an acute coronary
syndrome, the detection of a dysfunctional myocardium is of high prognostic value
and could guide further patient management [5]. Functional assessment is readily available for every retrospectively acquired ECG-gated
cardiac CT examination. However, dedicated post-processing tools are needed for image
analysis [17]. Previous studies have shown a close correlation between end-diastolic and end-systolic
LV volume and ejection fraction and regional wall motion abnormalities [5] obtained by multislice CT compared to two-dimensional echocardiography, and acceptable
correlation for the computed LV stroke volume [59]. While the temporal resolution of CT is still inferior to that of transthoracic
echocardiography (TTE) (CT as low as 66 ms [60] vs. TTE < 5 ms [61]), limited echocardiographic windows are not an issue. Cury and colleagues described
a comparable accuracy of CT (of 96 %) and TTE in the diagnosis of an acute myocardial
infarction combined with higher interobserver reliability for the quantification of
the ejection fraction in CT (interobserver reliability CT r = 0.83, TTE r = 0.68)
[5]. In patients with acute chest pain, the CT LV function demonstrated an incremental
value in the detection of an acute coronary syndrome with an 89 % sensitivity and
86 % specificity for significant stenosis (> 50 %) and a 60 % sensitivity and an 86 %
specificity in patients with inconclusive coronary CTA [62]. This is especially crucial in patients post-myocardial infarction as LV function
is an important marker for prognosis and treatment [63].
For the assessment of right ventricular (RV) function, good opacification of the right
ventricular lumen is required, which can be achieved by alternating the standard injection
protocol (i. e., by extending the standard duration of contrast application or using
multiphase protocols with a combination of contrast and saline flush) [64]. Results of RV lumen measurements and ejection fraction have demonstrated similar
results in comparison to cardiac MRI [65].
While impairment of ventricular function often resembles global or later-stage disease
(i. e., systolic heart failure), CAD, but also other cardiac diseases such as myocarditis
can be limited to certain territories, leading to regional abnormalities. These regional
functional impairments can be assessed using advanced secondary analyses including
strain measurements (longitudinal, circumferential, and radial as well as sheer strain).
Strain measurements are typically assessed with TTE or cardiac MRI (currently considered
the gold standard), but practical and technical limitations have been hindering its
implementation in clinical practice. Functional assessment on cardiac CT is rapidly
evolving with comparable results to MRI measures in initial studies [66]
[67]. With the broad availability of CT scanners, fast acquisition times (compared to
MRI studies) and a wider window (compared to TTE), CT strain measurements resemble
a promising but still developing alternative given that dedicated software packages
for evaluation are available [66]
[67].
Cardiac CT for Pre-Interventional Planning
Transcatheter Valve Implantation
With the introduction of catheter-based minimally invasive methods to treat valvular
disorders, this procedure has gained increasing interest throughout Europe in the
past years. Transcatheter aortic valve implantation (TAVI), as well as transcatheter
mitral valve implantation (TMVI), have become established alternatives to open heart
surgery specifically for high-risk patients with symptomatic valve disease [6]
[7]. For instance, in Germany the number of TAVI procedures has increased 20-fold from
2008 (with 637 procedures) to 2013 (with 13 264 procedures), thus outnumbering surgical
aortic valve replacements and becoming the most commonly performed procedure in the
treatment of aortic valve stenosis in patients of age [68].
To be able to perform this intervention successfully, CT is essential for selecting
suitable candidates, including assessment of the valvular anatomy and the peripheral
vessels (as access routes) for the lack of intraprocedural visualization. An expert
consensus issued in collaboration with the American Heart Association (AHA) in 2012
recommends a multidetector system with 64 or more slices, high spatial resolution
(0.5–0.6 mm) and a scan ranging from the ascending aorta to the iliofemoral branches.
While image acquisition protocols may vary depending on site, vendor, and scanner,
it is essential to capture the aortic root without motion artifacts using an ECG-synchronized
mode. Assessment of the following vascular segments can be performed in a non-gated
fashion to reduce radiation dose and the amount of iodinated contrast material needed
[13].
In aortic valve replacement, it is crucial to evaluate the aortic valve, aortic annulus,
aortic root, ascending aorta, and the aortic run-offs to ensure appropriate prosthesis
selection and the availability of sufficient access routes ([Fig. 4]). Especially measuring the effective diameter of the aortic annulus (formed by the
lowest points of each of the three aortic cusps and their connection to the wall of
the left ventricular outflow tract) in an appropriate cardiac phase (late systole)
is of importance, as the prosthesis is fitted to this ring-like structure [6]
[69]. One advantage over the traditional measurements performed with 3D echocardiography
(which only delivers a single diameter) is the possibility to assess the minimal as
well as the maximal diameter and the area of the aortic annulus, which oftentimes
resembles an oval shape. As this shape is likelier understood with multiple measurements,
CT assessment may be beneficial in prosthesis sizing [13]. Furthermore, the distance from the aortic annulus to the ostia of the coronary
arteries as well as the diameter of the ascending aorta are required for planning
the intervention to avoid injury or occlusion [6]. Additionally, pre-procedural CT has been proven beneficial in determining optimal
fluoroscopic angulations for an orthogonal view of the aortic valve leading to a significant
reduction of contrast use during intervention [70].
Fig. 4 Images of an 87-year-old female patient scheduled for transcatheter aortic valve
implantation. Measurements of aortic root and area of aortic annulus (with distance
of annulus to ostium of coronary arteries) to ensure appropriate device selection.
Abb. 4 Bilder einer 87-jährige Patientin vor geplantem Katheter-gestützten Aortenklappenersatz.
Messungen der Aortenwurzel und der Fläche des Anulus (mit Abstandsmessung vom Anulus
zum Ostium der Koronararterien) zur Interventionsplanung.
In mitral valve replacement, it is especially important to pay close attention to
the mitral annulus and mitral valve leaflets, the morphology of the papillary muscles
and the anatomic relation to the left circumflex coronary artery and coronary sinus
[7].
Mitral valve annuloplasty is another approach for transcatheter valve repair in patients
with secondary mitral regurgitation, in which the septolateral diameter of the mitral
annulus is reduced to improve leaflet coaptation [71]. Currently, different catheter-based devices for direct annuloplasty using a transvenous-transseptal
or transarterial route are available. Again, pre-procedural CT imaging to screen for
suitable anatomy and appropriate device selection is critical [72].
The overall strength of pre-procedural measurement with CT is the high spatial and
temporal resolution and the unlimited availability of the data set once the CT images
are acquired. Dedicated post-processing software allows the reconstruction of the
images along predefined planes including left anterior oblique (LAO) and right anterior
oblique (RAO) resembling angiographic planes [7]. To enable the surgeon to select the most suitable and safe pathway to access (i. e.,
transfemoral, transapical, subclavian or transaortic), the entire vascular system
can be mapped three-dimensionally to visualize the course and tortuosity of vessels
(from the aortic arch to the femoral arteries) and measure minimal and maximal vessel
diameter to perform the intervention ([Fig. 5]). Furthermore, the aortic valve and vessel wall calcifications can be quantified,
which may present contraindications for the implementation of certain devices [6]
[7]
[73]
[74].
Fig. 5 Images of the same 87-year-old female patient scheduled for transcatheter aortic
valve implantation. 3D volume rendering technique of the vascular anatomy as well
as 2D images of the iliac arteries to select the most suitable pathway to access.
Measurements include the minimal and maximal vessel diameter.
Abb. 5 Bilder der gleichen 87-jährigen Patientin vor Katheter-gestütztem Aortenklappenersatz.
3D-Volume-Rendering-Technik der Gefäßanatomie sowie 2D-Bilder der Iliakalarterien
zur Interventionsplanung. Messungen beinhalten minimalen und maximalen Gefäßdurchmesser.
Pulmonary Vein Isolation
Pulmonary vein (PV) isolation is an established catheter ablation procedure in atrial
fibrillation (AF) for patients with recurrent and drug-refractory symptoms [8]. AF is the most common cardiac arrhythmia affecting 2–3 % of the general population
in Europe and North America with a prevalence of 10–17 % in patients over the age
of 80 and it significantly increases the risk of cardiac and non-cardiac deaths [75]. Common causes for AF include ectopic electrical foci in the atria or the muscular
sleeves of the distal PV, of which 50 % are located in the left superior PV. To isolate
these foci by interrupting the conduction pathways, PV isolation has become an established
non-surgical treatment option. Although pre-procedural planning can be performed with
electrocardiography, pulmonary venography, or MRI, cardiac CT features numerous benefits
compared to other imaging techniques. With volume rendering, a 3D-model of the left
atrium and the PVs can be calculated to provide accurate 3D information. Furthermore,
ostium size of the PV and its distance from the first side branch as well as the location
of the esophagus, and vagal nerve structures can be determined, which is crucial to
eliminate complications associated with this procedure. Lastly, CT images can be imported
directly into the ablation workstations and fused with electrophysiology maps to guide
the procedure and assure maximum success [76].
Conclusion
With CVD being the leading cause of mortality worldwide, coronary artery imaging has
been the main focus of cardiac CT. However, with recent technical developments, cardiac
CT is emerging beyond coronary imaging for functional and pre-interventional assessment.
Innovations, rendering CT-derived FFR calculations and CT perfusion imaging possible,
enable the noninvasive assessment of the functional significance of coronary lesions
and help tackle significantly diseased coronary artery segments. Dual-energy, CT perfusion,
and delayed-enhancement imaging allow CT-based tissue characterization and diagnosis
of ischemia. Ventricular volume and function assessment as well as the emerging possibility
to measure myocardial strain with CT are of significance in regional versus global
disease. Ventricular volume is also an important marker for prognosis and treatment.
Furthermore, cardiac CT has become a highly valuable tool for planning complex interventions.
CT images provide accurate 3D anatomic models for pre-interventional planning of catheter-guided
interventions such as TAVI, interventional mitral valve therapies, and PV isolation.
While all of these techniques are of growing importance, on-site expertise as well
as appropriate hardware and software for the acquisition and analysis of each CT data
set is required.
It is expected that these techniques will be increasingly implemented in the clinical
routine and that some indications such as the diagnosis of myocardial ischemia and
viability might be redirected from MRI to cardiac CT within the next years.
