Author + information
- Received March 12, 2014
- Revision received May 2, 2014
- Accepted June 4, 2014
- Published online September 1, 2014.
- Pascal Thériault-Lauzier, PhD∗,
- Ali Andalib, MD∗,
- Giuseppe Martucci, MD∗,
- Darren Mylotte, MD∗,
- Renzo Cecere, MD†,
- Rüediger Lange, MD, PhD‡,
- Didier Tchétché, MD§,
- Thomas Modine, MD, PhD‖,
- Nicolas van Mieghem, MD¶,
- Stephan Windecker, MD#,
- Jean Buithieu, MD∗ and
- Nicolo Piazza, MD, PhD∗,‡∗ ()
- ∗Department of Medicine, Division of Cardiology, McGill University Health Centre, Montreal, Quebec, Canada
- †Department of Surgery, Division of Cardiovascular Surgery, McGill University Health Centre, Montreal, Quebec, Canada
- ‡Department of Cardiovascular Surgery, German Heart Centre Munich, Munich, Germany
- §Department of Interventional Cardiology, Clinique Pasteur, Toulouse, France
- ‖Department of Cardiovascular Surgery, University Hospital of Lille, Lille, France
- ¶Department of Cardiology, Thoraxcentre, Erasmus Medical Centre, Rotterdam, the Netherlands
- #Department of Interventional Cardiology, University Hospital, Bern, Switzerland
- ↵∗Reprint requests and correspondence:
Dr. Nicolo Piazza, Division of Cardiology, Department of Medicine, McGill University Health Centre, The Royal Victoria Hospital, 687 Pine Avenue West, Montreal, Quebec H3A 1A1, Canada.
With the introduction of transcatheter structural heart therapies, cardiologists are increasingly aware of the importance of understanding anatomical details of left-sided heart structures. Understanding fluoroscopic cardiac anatomy can facilitate optimal positioning and deployment of prostheses during transcatheter valve repair/replacement, left atrial appendage occlusion, septal defect closure, and paravalvular leak closure. It is possible to use multislice computed tomography to determine optimal fluoroscopic viewing angles for such transcatheter therapies. The purpose of this paper is to describe how optimal fluoroscopic viewing angles of left-sided heart structures can be obtained using computed tomography. Two- and 3-chamber views are described and may become standard in the context of transcatheter structural heart interventions.
- cardiac imaging
- fluoroscopic anatomy
- interventional imaging
- multislice computed tomography
- transcatheter cardiac intervention
Since the introduction of transcatheter structural heart therapies, cardiologists have become increasingly aware of the importance of understanding anatomical details of left-sided cardiac structures (1). Many critical anatomical structures comprise several components and are arranged in a complex tridimensional geometry. Even though these anatomical and functional components have been the topic of numerous publications (1,2), little consideration has been given to understanding their configuration as appreciated under fluoroscopy (3). Understanding fluoroscopic cardiac anatomy can facilitate optimal positioning and deployment of prostheses during transcatheter valve repair/replacement, left atrial appendage occlusion, septal defect closure, and paravalvular leak closure. Commonly, these therapies are conducted using standard fluoroscopic angulations irrespective of variations in anatomy. It is possible that patient-specific fluoroscopic viewing angles can improve procedural safety and efficacy.
Multislice computed tomography (MSCT) multiplanar reconstruction of the aortic valvular complex has enhanced patient selection and procedural planning for transcatheter aortic valve replacement (4). For example, MSCT affords physicians the opportunity to pre-select optimal x-ray fluoroscopic viewing angles for deployment of the valve prosthesis (5–15). Such angle optimization may decrease procedure time, radiation exposure, and injected contrast agent volume (15). It may reduce the risk of acute kidney injury (15) and paravalvular regurgitation (14). MSCT can also be used to determine optimal fluoroscopic viewing angles for transcatheter therapies targeting the mitral valvular complex, the left atrial appendage, the pulmonary veins, and the atrial septum.
Therefore, the purpose of this paper is to describe how optimal fluoroscopic viewing angles of left-sided heart structures can be obtained using computed tomography (CT). We describe 2- and 3-chamber views that may become standard in the context of left-sided structural heart interventions.
Attitudinal Description of Anatomy
In our discussion, structures will be termed according to their attitudinally correct anatomical position (16). This implies that the subject is facing the observer and standing upright. Thus, structures closer to the observer are described as being anterior and those relatively farther away within the body are posterior. Components lying closer to the head are superior (i.e., cranial [CRA]) and those toward the feet are said to be inferior (i.e., caudal [CAU]). Structures to the left-hand side of the observer are right-sided and those to the observer's right are left-sided. Discussing heart structures in their attitudinal position is in perfect agreement with nomenclatures used for CT and x-ray fluoroscopic imaging; this is not necessarily true with echocardiography. The fluoroscopic screen portrays the thorax in an upright orientation despite the patient being in a supine position. Superior and inferior structures are appreciated in the upper and lower halves of the screen. The direction of fluoroscopic projections is described based on 2 conventional angles, CRA/CAU and left anterior oblique (LAO)/right anterior oblique (RAO) (Figures 1A and 1B). In the anteroposterior viewing angle (CRA/CAU 0°, LAO/RAO 0°), right- and left-sided structures are found on the left and right sides of the screen, respectively.
Fluoroscopic Anatomy Using MSCT
Fluoroscopy is used to guide the vast majority of transcatheter cardiac procedures. Inherently, it is a 2-dimensional imaging modality that requires the user to select a viewing angle that provides accurate information on device positioning. For a particular structure, an optimal viewing angle should minimize positioning errors due to parallax. The goal of many transcatheter procedures—such as valve implantations, left-atrial appendage occlusions, septal defect closures, and paravalvular leak occlusions—is to implant a quasicylindrical device inside a highly variable anatomical structure. The fluoroscopic projection that minimizes parallax during deployment is such that the source-to-detector direction is orthogonal to the axis of symmetry of the anatomical feature of interest (Figure 1C). Based on this criterion, it is possible to determine an optimal CRA/CAU angle for any given LAO/RAO angle. The plot of the optimal combinations is called the optimal projection curve (7,8,10–12,15). MSCT is a 3-dimensional imaging modality that is not affected by parallax. MSCT images can be used to define the direction of structures and produce an optimal projection curve (Figure 1D).
In addition to parallax errors, optimal view angles should also minimize the overlap of anatomic structures, a problem that also stems from the 2-dimensional nature of fluoroscopy. In the context of transcatheter interventions, some cardiac structures must be distinguished in order to accurately implant devices. The overlap of a highly attenuating anatomical structure with the region of implantation may reduce the contrast-to-noise ratio and compromise visualization. Therefore, it is crucial to understand which fluoroscopic angulations provide maximal separation between structures of interest. This understanding can be obtained from MSCT angiography, which depicts cardiac structures with relatively high soft-tissue contrast, as well as high temporal and spatial resolution.
Fluoroscopy and MSCT share a common image contrast mechanism: x-ray attenuation. Consequently, it is possible to use MSCT volumetric data to simulate fluoroscopic images. A ray-casting method based on this principle was used to generate the fluoroscopic images presented in this paper. To demonstrate how MSCT may provide optimal fluoroscopic viewing angles for left-sided heart structures, we used an MSCT scan from a 66-year-old male patient with moderate functional mitral regurgitation. It is important to note that fluoroscopic angulations are dependent on the specific orientation of the heart within the thorax, a parameter that may vary considerably between patients (7,8,10–12,15). Although we describe general angulations, exact angulations may be determined for a specific patient prior to an intervention. Measurement of fluoroscopic angulation from MSCT data can be done with visualization software packages that offer double-oblique multiplanar reconstruction; the exact CRA/CAU and RAO/LAO angulations for a particular structure can be obtained by analyzing the oblique sagittal and oblique transverse views, respectively.
Aortic Root, Valve, and Left Ventricular Outflow Tract
The aortic root extends from the basal attachment points of the aortic valve leaflets to their superior attachment at the level of the sinotubular junction (17). The 3 leaflets form the limits of the right, left, and noncoronary aortic sinuses. Optimal fluoroscopic views of the aortic root can be selected such that the view is perpendicular to the plane of the aortic annulus (Figures 2A to 2C). During transcatheter aortic valve replacements, physicians often select a view that shows the right coronary sinus between the noncoronary and left coronary sinuses (Figures 2D and 2F). In the current patient, this view is obtained using a mild LAO and mild CAU angulation. Alternatively, a view with the noncoronary or left coronary sinus in the central position can also be obtained (Figures 2E and 2G). In this case, an LAO and CRA angulation is selected.
The aortic valve annulus is defined as the planar ring of tissue that unites the most proximal point of each leaflet attachment line. The annulus is rarely circular (18). Rather, it has a long and a short axis that can be appreciated using MSCT (19). Under fluoroscopy, the aortic annulus’s apparent diameter varies depending on the view angle (Figure 3A). In the current patient, a mild LAO projection with minimal CRA/CAU angulation provides a view of the maximum diameter (Figures 3B and 3D), whereas an extreme RAO and CAU view shows the minimum diameter (Figures 3C and 3E).
The mitral valve has 2 major leaflets commonly described as being anterior and posterior. Relative to the anatomical axes of the body, however, the leaflets are in an anterosuperior and posteroinferior orientation. For simplicity, we describe these leaflets as aortic and mural, respectively. The Carpentier classification recognizes 3 scallops in the mural leaflet (P1 to P3) and 3 corresponding segments in the aortic leaflet (A1 to A3) (20). A1P1 and A3P3 segments are located superoposterior and inferoanterior, respectively, with respect to A2P2 (Figure 4A). Emerging transcatheter interventions require the positioning of the delivery catheter in a precise position relative to each leaflet and each segment. Understanding the leaflet configuration under fluoroscopy may ease this process. In the patient studied, an extreme RAO and CAU view can be used to maximally separate the mural and aortic leaflets of the mitral valve (Figures 4B and 4C). In this view, however, the aortic segments A1,2,3 overlap one another; such an overlap also exists for mural segments P1,2,3. To separate the leaflet segment (1,2,3), we use an RAO and CRA projection. In this view, mural and aortic leaflets overlap such that A1 overlaps P1, A2 overlaps P2, and A3 overlaps P3 (Figures 4D to 4F).
The fibrous ring of the mitral valve annulus extends from the right and left trigones and encircles the mural leaflet of the mitral valve. The area of fibrous continuity between trigones is commonly referred to as the aortic-mitral curtain (Figure 5A). Some designs of transcatheter mitral valve replacement devices are not axially symmetric and must be positioned at a particular angle relative to the mitral commissure. In the patient studied, the intercommissural diameter of the mitral annulus, which is often the maximum diameter of the annulus, can be appreciated using a mild RAO and CRA angulation (Figure 5B). This angulation is analogous to an echocardiographic 2-chamber view. Also in the patient studied, an extreme RAO and CAU view shows the aortomural diameter of the mitral annulus—often considered to be the minimum diameter of the annulus (Figure 5C). This view is analogous to a 3-chamber echocardiographic view. The aortic-mitral curtain is observed perpendicularly and clearly separated from the left atrium and the aortic root. This view may be of particular interest to those implanting transcatheter mitral valve bioprostheses. Indeed, it can be used to assess the interaction of the prosthesis with the left ventricular outflow tract and could potentially reveal an obstruction or a deformation of the aortic-mitral curtain.
Two papillary muscle divisions arise adjacent to each other within the left ventricular cavity. Using attitudinal description, it is illogical to describe these divisions as being either posteromedial or anterolateral (16); in the vast majority of cases, the papillary muscles lie in the posterior one-half of the left ventricular cavity and are relatively inferoanterior and posterosuperior to each other (Figure 6A).
The exact number, positioning, morphology, and thickness of papillary muscles can vary considerably (21). They are considered to be potential obstacles to the deployment of transcatheter devices that protrude into the left ventricle such as mitral valve prostheses or ventricular septal defect closure devices. Therefore, it is important to understand their configuration as observed under fluoroscopy. In the subject studied, the papillary muscle divisions can be maximally separated in a mild CRA projection with or without mild left or right angulation (Figures 6B and 6C) (2-chamber view). An extreme RAO and CAU projection creates an overlap between the papillary muscles (Figures 6D to 6F) (3-chamber view). The mural attachment of the papillary muscles is diametrically opposed to the anteriorly located left ventricular outflow tract.
Left Atrial Appendage
The left atrial appendage is a contractile extension of the left atrium that lies in its anterior, superior, and left aspect. It varies considerably in shape and size, as well as in the number of lobes formed within its lumen (22). The orifice of the left atrial appendage is oval (Figure 7A) and opens in the left atrium adjacent to and at an oblique angle with the mitral valve annulus (23). Transcatheter left atrial appendage closure devices are deployed in the orifice of the appendage to prevent thrombus formation in patients with atrial fibrillation. Some physicians size these devices based on fluoroscopic images acquired in standard angulations such as CRA 25° and RAO 25°. In the subject studied, the orifice of the appendage is appreciated en face in a highly LAO and CRA fluoroscopic projection (Figures 7B and 7C). The minimum diameter of the appendage orifice is appreciated in an RAO and mild cranial fluoroscopic view (Figures 7D to 7F). This angulation can also be used to visualize the ridge of tissue that separates the left atrial appendage orifice from the left superior pulmonary vein ostium. The long diameter of the appendage orifice can be appreciated in an LAO and CAU projection (Figures 7G to 7I). This angulation results in an overlap of the proximal segment of the left superior pulmonary vein with the left atrial appendage.
The atrial septum is a tissue partition between the right and left atria. In a majority of patients, the periphery of the septum is constantly thick and becomes thin toward its center at the fossa ovalis, the embryologic remnant of the foramen ovale (24). In normal individuals, the septum is oriented obliquely with the right atrium lying anterior, right, and inferior to the left atrium (25) (Figure 8A). The aortic root lies left and anterior to the atrial septum. In many procedures, a transseptal puncture is used to gain access to the left atrium from the right atrium. An RAO view shows the septum with minimal overlap with the aortic root. Having said this, a standard CRA/CAU 0° RAO 30° view is often used to confirm that the puncture needle is positioned over the septum and not the aortic root. In the subject studied, CRA/CAU 0° RAO 35° achieves minimal overlap (Figures 8G to 8I). An RAO and CAU angulation is selected to view the septum en face (Figures 8B and 8C). A puncture needle would point toward the image in this angulation. In an LAO view (Figures 8D to 8F), the septum is seen perpendicularly but overlaps the aortic root. In this view angle, the curvature of a puncture needle would appear maximal because it would point directly to the right of the image.
The spatial configuration of cardiac anatomy as depicted using fluoroscopy is difficult to understand. In effect, physicians often rely on pattern recognition rather than tridimensional mental visualization when performing image-guided interventions. Fluoroscopic cardiac anatomy has been previously described in the context of electrophysiological interventions (26–28). This paper also describes fluoroscopic cardiac anatomy, but it focuses on anatomy relevant for structural heart interventions. This topic has not previously been covered in a systematic fashion.
As noted throughout the paper, several fluoroscopic views may be particularly relevant for transcatheter therapies. Among the different projections described, 2 angulations are particularly noteworthy: 1) the extreme RAO and CAU view, which we call 3-chamber view; and 2) the mild RAO and CRA view, which we refer to as 2-chamber view. Figure 9 summarizes the anatomical structures that can be visualized in these 2 fluoroscopic views. We believe that these 2 angulations may become a standard in the context of left-sided structural heart interventions, and in particular, transcatheter mitral valve therapies.
In particular, a single MSCT scan in diastole was used, which explains why only approximate angulations were described. Normal anatomical variations as well as pathological states can have a strong influence on optimal viewing angles. In particular, ventricular hypertrophy, ventricular or atrial dilation, pulmonary hyperinflation, pulmonary obstructive disease, obesity, and changes associated with aging are among potential factors influencing fluoroscopic angulations. Furthermore, the use of a pre-operative MSCT scan may not accurately reflect the spatial position of anatomical structures once in the catheterization laboratory. Respiratory motion may affect the orientation of the heart. The patient’s arm position also differs between the 2 procedures; the arms are generally placed overhead during MSCT scanning, whereas they are placed along the body during the intervention. New technology such as intraoperative C-arm cone-beam CT, also known as rotational angiography, might alleviate these limitations. Furthermore, tridimensional renderings may be overlaid over planar fluoroscopic images to visualize the anatomy of structures targeted by transcatheter therapies. In the future, the concept of CT-based fluoroscopic viewing angles can be applied to images obtained using these new technologies.
Herein we present a review of the fluoroscopic anatomy of left-sided heart structures relevant for transcatheter therapies. Optimal fluoroscopic angulations, which minimize parallax or overlap with other clinically relevant structures, were described for the aortic valve, the mitral valve, papillary muscles, and the atrial septum.
Dr. Martucci is a proctor for Medtronic. Dr. Lange receives consulting and lecture fees from Medtronic. Dr. Modine receives consulting fees from Medtronic. Dr. van Mieghem receives consulting fees from Medtronic and Boston Scientific. Dr. Windecker has received institutional research grants from Biotronik and St. Jude Medicalhttp://dx.doi.org/10.13039/100006279; and serves on the scientific advisory board of Cardialysis BV. Dr. Piazza is a proctor for and receives consulting fees from Medtronic. All other authors have reported that they have no relationships relevant to the contents of this paper to disclose.
- Abbreviations and Acronyms
- left-anterior oblique
- multislice computed tomography
- right-anterior oblique
- Received March 12, 2014.
- Revision received May 2, 2014.
- Accepted June 4, 2014.
- American College of Cardiology Foundation
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