In the last decade, there has been increasing evidence that comprehensive evaluation of the left atrium is of utmost importance. Numerous studies have clearly demonstrated the prognostic value of left atrial volume for long-term outcome. Furthermore, advances in catheter ablation procedures used for the treatment of drug-refractory atrial fibrillation require the need for detailed knowledge of left atrial and pulmonary venous morphology as well of atrial wall characteristics. This review article discusses the role of cardiac magnetic resonance and computed tomography in assessment of left atrial size, its normal and abnormal morphology, and function. Special interest is paid to the utility of these rapidly involving noninvasive imaging methods before and after atrial fibrillation ablation.
In recent years there has been increased interest in the comprehensive evaluation of the left atrium (LA). The main role of the LA is to modulate left ventricular (LV) filling through several mechanical functions: reservoir function, conduit function, and active contraction [
The aim of this paper is to give a detailed overview on the utility of cardiac magnetic resonance (CMR) and cardiac computed tomography (CT) in the assessment of LA anatomy, size, and function.
Cardiac magnetic resonance has been considered the gold standard for assessment of the heart chambers [
Representative slice demonstrating the tracing of the left atrial boundary at ventricular end-systole in short-axis view on steady-state free precession magnetic resonance image. The left atrial volume is then calculated using the disk area summation method (Simpson’s method) from contiguous slices covering the whole left atrial cavity.
Measurement of the left atrial area at ventricular-end systole in four-chamber view on steady-state free precession magnetic resonance image.
Normal anatomy of four pulmonary veins (red stars) on three-dimensional contrast-enhanced magnetic resonance angiography.
The main limitations of CMR examination are the length of the complete heart imaging procedure (about 45 minutes), the financial burden, possible evocation of claustrophobia, limited availability, and the incompatibility of certain prosthetic materials [
Accurate visualization of LA morphology is essential in many clinical situations, such as assessment of the feasibility of radiofrequency ablation (RFA) in patients with atrial fibrillation (AF), planning cardiovascular surgery, evaluation of left atrial appendage (LAA) thrombosis, and evaluation of various other pathological masses like tumours and infiltration of the cardiac wall. Conventionally, transthoracic echocardiography (TTE) has been the most useful and practical method for atrial investigation in daily clinical practice due to its wide availability and cost-benefit ratio. Cardiac CT currently represents a gold standard for assessment of the anatomy of the LA complex, LAA shapes and lobes, LA wall thickness, atrial septal anomalies such as septal defects, patent foramen ovale, and diverticula. Cardiac CT is generally performed rapidly, with acceptable temporal resolution and high spatial resolution. Nevertheless, standard CMR techniques also allow assessment of some LA morphological and functional parameters with reasonable accuracy which may be of particular importance in patients undergoing RFA of atrial fibrillation as described in more detail below.
Cardiac magnetic resonance allows excellent characterization of various intra-atrial masses. The technique enables superior tissue characterisation and thus differentiation between cardiac tumours and thrombi. Cardiac magnetic resonance can also be helpful in differentiating between benign and malignant tumours. In a study comprising 116 patients with various cardiac masses, Pazos-López et al. used CMR to show that tumours tend to be larger, heterogeneous, and more mobile compared to thrombi [
Increased LA size has been shown to be a marker of LV diastolic dysfunction as well as a predictor of conditions like myocardial infarction, heart failure, stroke, and atrial fibrillation [
The standard evaluation of LA size in daily clinical practice is performed by transthoracic echocardiography. However, echocardiographic assessment is often inaccurate due to geometric assumptions and foreshortening of the LA cavity [
The current European heart failure guidelines recommend assessment of LA volume in all patients with heart failure regardless of the aetiology [
Established CMR reference values of LA parameters are based on data derived from several studies [
The MESA (Multi-Ethnic Study of Atherosclerosis) demonstrated that LA enlargement and deterioration of LA function precede the development of heart failure [
However, all these data raise an intriguing question regarding the need for highly accurate measurement of LA size [
Cardiac magnetic resonance assessment of LA function is currently not used in routine practice, but it has been attracting more research attention due to the technical development of imaging methods. The technique is highly suitable for studying parameters like LA phasic volumes and emptying fractions (total, passive, and active). Although not as high as in 2D echocardiography, the temporal resolution of steady-state free precession sequences of 25 to 50 ms is sufficient for the evaluation of LA volumes during cardiac cycle. It was demonstrated that LA passive volumetric contribution to LV filling decreases with age, while its active component increases with age. These age-related differences are augmented by dobutamine challenge, as shown by Ahtarovski et al. [
In a study of 210 subjects with a history of long-lasting arterial hypertension, Kaminski et al. proved a strong association between decreased LA contractile function and patient mortality, nonfatal events, and all major adverse cardiovascular events (MACE) [
Quite recently, Kowallick et al. reported results from a small study assessing LA deformation by CMR myocardial feature tracking. In this study 10 patients with hypertrophic cardiomyopathy, 10 individuals with heart failure with preserved LV ejection fraction, and 10 healthy volunteers were studied using CMR assessment of LA longitudinal strain and strain rate parameters derived from steady-state free precession cine images. The results showed reliable quantification of LA conduit, reservoir, and contractile function both in disease states and in healthy individuals. Moreover, excellent intra- and interobserver agreement was noted [
Detailed assessment of the LA by CMR is increasingly used in the field of atrial arrhythmias, particularly AF. It is well known that episodes of paroxysmal AF are triggered by an ectopic beat in the muscular layer of the pulmonary veins in more than 90% of cases [
Radiofrequency ablation of both paroxysmal and persistent AF has been performed with increasing frequency since 2003 [
Preprocedural CMR images of the LA help to identify the anatomical relationship of the LA to other structures in thorax, thus enabling easier orientation within the LA for RFA operators. The location of the oesophagus is highly variable due to its peristaltic activity, but most often it lies directly behind the LA, very close to the left pulmonary veins, posing the risk of creating an atrial-esophageal fistula with possibly catastrophic consequences. Meng et al. showed that one-third of patients treated with RFA had oesophageal LGE on CMR performed 2 months following the ablation procedure [
The presence of thrombus in the LA appendage represents an absolute contraindication for RFA. Transoesophageal echo (TOE) is the standard method for excluding the presence of thrombus in the LA appendage with high negative predictive value (96–100%) [
Preprocedural identification of pulmonary vein anomalies—their number, location, or branching–could potentially decrease the recurrence rate of AF and the risk of pulmonary vein stenosis. Pulmonary vein stenosis occurs in about 2–5% of patients undergoing RFA [
Variation in pulmonary vein anatomy occurs in approximately 40% of patients indicated to the ablation procedure [
Surprisingly, several studies have failed to support the hypothesis that assessment of pulmonary vein number, location, and branching prior to RFA affects long-term efficacy of RFA [
Extensive structural remodelling of the LA seems to play an important role in the prediction of the phenotype of AF, recurrence of arrhythmia, thromboembolic events, and mortality [
The examples of inversion recovery gradient echo sequencies in four-chamber view demonstrating the absence (left) and the presence of late gadolinium enhancement (right; arrows) in atrial walls.
A unique scoring system called The Utah Staging System has been developed to quantify the degree of LA structural remodelling based on the extent of LGE: Utah stage I is defined by less than 5% LGE, Utah stage II as 5–20% LGE, Utah stage III as 21–35% LGE, and Utah stage IV as more than 35% LGE [
Scarring of the LA wall visualized by LGE also seems to be associated with increased risk of thromboembolism. According to Wylie et al., this risk is related to the decreased contractile function of the LA, even in the absence of AF [
Based on these findings, CMR could aid in the stratification of AF patients, allowing for optimal choice of treatment and individualized care. Patients with low grade LGE are likely to benefit from the standard ablation protocol, while patients with extensive LA fibrosis should probably be treated conservatively with adequate rate-control management with long-term anticoagulation or with an extensive ablation strategy (i.e., ablation of the posterior wall and septal LA debulking) [
A further challenge is the use of real-time CMR for online navigation of the ablation catheter. The use of CMR could give the operator immediate feedback of lesion formation during ablation, thus distinguishing between true scar formation and tissue oedema that leads to a temporary electrical gap, with delayed pulmonary vein reconnection [
Postablation CMR enables visualisation of LGE as a correlate of postablation scar formation, which is likely to be predictive of RFA success [
The integration of data from electroanatomic mapping with CMR or cardiac CT images into so-called hybrid maps represents an intriguing application of CMR. However, the results of existing studies seem to be quite controversial. Bertaglia et al. described significantly improved outcomes of patients after RFA using merged CMR hybrid maps [
Computed tomography imaging of the heart requires minimization of cardiac motion artefacts. For this reason, cardiac CT scanning is almost exclusively performed with simultaneous ECG registration. Two basic methods are recognized: prospective triggering and retrospective gating. Prospective ECG triggering is a method in which the data are acquired at a prespecified phase of the cardiac cycle (usually during the phase with minimal heart motion and therefore minimal coronary artery motion—thus in mid-diastole or in end-systole in patients with accelerated heart rate). In retrospective ECG gating, data are acquired throughout the entire cardiac cycle, and the only data obtained during the cardiac phase with the least motion artefacts are used for image reconstruction [
In comparison with CMR, cardiac CT has worse temporal resolution (75–250 ms) but better spatial resolution (0.5–1 mm) [
Cardiac CT is always associated with a certain radiation dose that usually varies from 1 to 15 mSv, depending on the scanning protocol. Iodinated contrast agents are used in most cardiac CT examinations.
Beta blockers and other drugs with negative chronotropic properties, such as ivabradine and verapamil, are administered in order to achieve a target heart rate, usually less than 65 beats per minute, minimizing motion artefacts.
The LA represents a complex structure that consists of three compartments of different embryonic origin: the anterior LA (ALA), the venous LA (VLA), and the LA appendage (LAA).
The ALA forms the anterior and central parts of the LA. The LAA is a tubular structure that bulges out superiorly and in a leftward direction. The VLA receives blood from the pulmonary veins and represents the remaining part of the LA [
The average wall thickness of the LA varies between subjects in sinus rhythm and those with various types of atrial fibrillation (AF). In patients with paroxysmal AF, the LA wall measured by CT is thicker compared to patients with long-standing, persistent AF as well as individuals in sinus rhythm. This is reflective of the LA wall structural remodelling that occurs in paroxysmal AF prior to LA dilatation, which occurs as the AF becomes persistent [
The LAA has variable shapes and number of lobes. Four different shapes have been described: cactus, chicken wing (Figure
Contrast-enhanced CT image showing the most common subtype of left atrial appendage called chicken wing.
The risk of stroke or transient ischemic attack (TIA) due to paradoxical embolism is associated not only with atrial septal defect but also with the presence of a patent foramen ovale (PFO) which is found in approximately 25% of the general population mainly. Higher risk of stroke or TIA is also connected with interatrial septal aneurysm and with the relatively recently described atrial septal pouch. An atrial septal pouch is a lesion characterized by incomplete fusion of the septal components and thus might serve as a site for thrombus formation and subsequent embolization [
Apart from abnormalities of the interatrial septum, the most common morphological variations of the LA are diverticula (Figure
Contrast-enhanced CT image demonstrating a diverticulum (arrow) at the lateral part of the left atrium, close to the mitral valve.
Accessory LA appendages represent less common abnormalities with prevalence between 3 and 10% of subjects undergoing cardiac CT. LA accessory appendages are defined as sac-like structures with irregular contours that resemble the pectinate muscle and have narrower ostia than diverticula [
Left atrial size is associated with cardiovascular events and mortality [
In patients in sinus rhythm, there is clear evidence regarding the excellent correlation between LA size and function measured by CMR and CT. It is also well known that transthoracic echocardiography (TTE) tends to underestimate LA size and to overestimate LA systolic function in comparison with both CT and CMR [
Data regarding CT and CMR correlation in patients with ongoing AF have been sparse. Very recently Agner et al. conducted a study that included 34 patients with persistent AF, comparing TTE, CMR, and CT in the assessment of LA size expressed as LA maximal and minimal volumes (LAVmax and LAVmin, resp.) and LA systolic function represented by fractional area change (FC) [
Recently, the advantage of unenhanced CT evaluation of the LA size was shown in a large cohort study. Mahabadi et al. reported the results of The Heinz Nixdorf Recall Study, which included 3958 subjects aged 45–75 years without coronary artery disease, stroke, atrial fibrillation, or flutter and without pacemaker or implanted defibrillator [
The LA represents a complex structure, with three compartments that are subject to differing degrees of involvement in AF patients. Park et al. conducted a study comprising 53 patients with paroxysmal AF, 43 individuals with persistent AF, and 48 control subjects in sinus rhythm who had no history of arterial hypertension, diabetes mellitus, ischemic heart disease, cardiomyopathy, or moderate to severe valvular disease. The authors assessed LA volumes indexed to body surface area (BSA) and ejection fraction of the entire LA as well as ejection fraction of each of its three components [
According to this study and a report by Christiaens et al. [
Thrombi are by far the most common form of LA mass and are found predominantly in the LAA. Well-known factors precipitating thrombus formation include structural and functional cardiac abnormalities, artificial cardiac implants, and hypercoagulable states [
Currently, TOE is considered to be the gold standard for detection of intra-atrial thrombi. The main advantages of TOE are related to its high temporal as well as spatial resolution, as well as the lack of radiation and nephrotoxic contrast agent exposure. However, TOE is a semi-invasive method, which needs to be performed with sedation in most patients and, even more importantly, is relatively highly operator-dependent.
Contrast-enhanced cardiac CT allows visualisation of the entire LA, including the LAA, and thus represents a diagnostic alternative to TOE for detection of intra-atrial thrombosis.
On CT, a thrombus appears as a homogeneous, low-attenuation lesion that classically does not enhance (Figure
ECG-gated contrast-enhanced CT image depicting a thrombus in the left atrial appendage (arrow).
In various studies, different results in terms of sensitivity and specificity of intracardiac thrombus detection have been reported. Wu et al. performed a meta-analysis to explore the potential diagnostic value of CT in detecting LA or LAA thrombosis [
The main issue in CT is to differentiate between true thrombi and a filling defect corresponding to circulatory stasis. Feuchtner et al. suggested four characteristic imaging features on cardiac CT that could be helpful in characterizing an incomplete filling defect of the LAA corresponding to stasis. The first of these features is the flow phenomenon, defined as the inhomogeneous appearance of mixed blood and contrast agent; the second feature is hypostatic layering, which is a sharp horizontal borderline between the contrast agent and nonmixed blood; the third feature is Hounsfield unit (HU) runoff, with decreasing density dorsally to ventrally; and the last feature is higher intralesional density in comparison with the density of thrombi (
In order to improve the diagnostic accuracy of cardiac CT, Hur et al. suggested performing two-phase cardiac CT angiography [
Furthermore, quantitative analysis of the LAA/ascending aorta (AA) HU ratios was performed. The mean HU ratios significantly differed between thrombus and circulatory stasis (0.15 versus 0.27;
With its ability to acquire a three-dimensional dataset accurately depicting LA and pulmonary venous anatomy, cardiac CT is currently the most commonly used imaging modality for pulmonary vein assessment in many centres, especially if interventional AF treatment is planned.
Approximately 70% of the general population has the classical configuration of four pulmonary veins: left superior and inferior pulmonary veins and right superior and inferior pulmonary veins, with four independent ostia (Figure
Contrast-enhanced CT scan showing two left-sided and two right-sided pulmonary veins.
The clinical utility of CT imaging prior to AF ablation procedures includes preablation imaging of the LA and pulmonary veins to exclude intra-atrial thrombosis and to detect anatomical variants; depiction of the anatomical relationship between the LA, oesophagus, and adjacent vascular structures; creation of a 3D dataset of the LA and pulmonary veins for fusion with electroanatomical mapping data; assessment of morphological remodelling of the LA and pulmonary veins; and, finally, acquisition of a dataset useful for the detection of postprocedure complications (e.g., pulmonary vein stenosis) [
The presentation and clinical course of pulmonary vein stenosis vary widely from asymptomatic to highly symptomatic cases. Symptoms usually correlate with the number of the stenotic veins, the severity of the stenosis, the time course of the stenosis, compensatory mechanisms, and development of collaterals. The frequency of right superior, left superior, and left inferior pulmonary veins involvement is similar. The right inferior and right middle pulmonary veins are infrequently involved [
Cardiac CT represents an alternative or complementary technique to echocardiography and CMR for the characterization of masses in the LA.
Myxomas are the most common primary cardiac tumours, predominantly localized in the LA. Typically, myxomas are solitary, pedunculated lesions attached to the interatrial septum (Figure
Contrast-enhanced CT image depicting left atrial myxoma attached to the interatrial septum.
Secondary cardiac tumours are at least thirty times more common than primary tumours. Metastases to the heart are mostly associated with lung and breast tumours. Heart involvement is also frequently associated with the progression of various types of lymphomas. Features suggestive of a malignant mass in the LA are a filling defect with poorly defined margins, a lesion crossing tissue planes, an associated pericardial effusion, or the presence of an extracardiac tumour. Metastases are usually multiple and in most cases they involve pericardium and, less frequently, the myocardium or endocardium. Isolated LA involvement is uncommon in secondary tumours.
Both CMR and cardiac CT currently represent very important imaging modalities used for the comprehensive evaluation of the LA in various clinical settings. Cardiac magnetic resonance is considered to be the gold standard for volumetric assessment of left atrial size. Late gadolinium enhancement CMR offers unique information about the presence of LA scaring that may be used for optimizing the treatment of patients suffering from atrial arrhythmias. Cardiac CT is regarded as the method of choice for evaluation of pulmonary vein and LA anatomy before catheter ablation procedures for AF and also the most useful imaging modality for detection of pulmonary vein stenosis after RFA. Cardiac CT represents an alternative to TOE in exclusion of LA thrombosis. Cardiac magnetic resonance as well as CT also plays very important role in characterizing LA morphological variants and pathological intra-atrial masses.
The authors declare that they do not have any conflict of interests.
Petr Kuchynka and Jana Podzimkova contributed to this review equally.
The study was supported by PRVOUK-P35/LF1/5, PRVOUK-P27/LF1/5, and Project Registration no. CZ.2.16/3.1.00/24012 from OP Prague Competitiveness and European Regional Development Fund—Project FNUSA-ICRC (no. CZ.1.05/1.1.00/02.0123).