Pulmonary Vein and Left Atrial Imaging

Imaging Methods: Pulmonary Veins

The pulmonary veins can be identified by using standard anatomic and functional CMR imaging sequences. Although these methods are sufficient for identifying the anatomic relationship of the pulmonary veins to the heart and the other major vascular structures, CE-MRA is usually used for a volumetric three-dimensional (3D) understanding of the intricate pulmonary venous anatomy. A 3D spoiled gradient echo sequence is acquired during the first pass of gadolinium (Gd) contrast. Clinical protocols vary but have mainly common elements. The technique uses short repetition times (TR; 2–5 ms), a high flip angle (25–60 degrees), and fractional echoes, all of which provide T1 weighting and minimal flow artifacts. The spatial resolution varies from 1–2 × 1–2 mm in-plane with 2 to 4 mm slices, before interpolation. A single 3D volume requires a 10 to 20 second breath-hold to suppress ventilatory motion, but scan time can be shortened using smaller fields of view, shorter TRs, partial Fourier, lower spatial resolution, parallel imaging, or compressed sensing. Electrocardiogram (ECG) triggering is not employed, although it is recognized that the position and shape of the pulmonary veins changes throughout the cardiac cycle. Images obtained with this method reflect the pulmonary veins at their maximal size. Axial or coronal slabs are usually acquired, using either sequential or centric k -space filling. For the pulmonary vasculature, the arterial-venous transit time is very short (4–7 seconds) and therefore artery-vein separation is highly challenging and generally not targeted. Contrast is injected with a gadolinium dose of 0.1 to 0.2 mmol/kg at a rate of 1 to 2 mL/s, followed by a saline flush. A precontrast mask can be acquired, although mask subtraction is not essential for pulmonary venography because the background lung signal is very low. Often a second time frame is acquired immediately after the first-pass image to ensure acquisition during peak contrast. Timing of the acquisition to the first pass of contrast through the pulmonary veins is critical, and is achieved using either a bolus timing scan or with fluoroscopic triggering. For either method, imaging is timed to begin with the appearance of contrast in the left atrium (LA). Time-resolved imaging using view-sharing methods is also valuable, providing multiple 3D volumes during the passage of contrast.

More intravascular contrast agents (gadobenate dimeglumine, gadofosveset trisodium, or ferumoxytol) are available to extend the duration of shortened blood T1, and improve quality, but they may reduce the contrast between blood and scar/fibrosis, if LGE CMR is acquired.

Image Display

Once the 3D MRA dataset is obtained, the images can be transferred to a workstation for further manipulation and analysis ( Fig. 42.1 ). The simplest and often most informative is to dynamically view two-dimensional (2D) slices within the 3D dataset in the axial, coronal, and sagittal planes. The axial images usually provide a good overview of the pulmonary veins and their relationship to the LA, but the coronal and sagittal images are frequently required to determine specific anatomic findings, such as a left common or anomalous pulmonary vein.

Although 2D slices are very useful for viewing the individual pulmonary veins, it is difficult to produce a single summary image of the anatomy. Maximal intensity projection (MIP) and 3D reconstructions displayed as shaded surface or volume-rendered images take full advantage of the 3D dataset and provide very good summary images. By convention, the LA and pulmonary veins are viewed in the posterior-anterior orientation. These 3D views are most useful when the displayed volume is limited to the LA and pulmonary veins. Because the aorta is directly posterior to the left-sided pulmonary vein, it frequently obscures them from view in the MIP images. Three-dimensional reconstructed images are frequently preferred because the aorta can be excluded from the displayed volume. Three-dimensional rendered images can also be rotated, to better appreciate the anatomy. Software is also available for generating an endovascular reconstruction, simulating the view of the pulmonary vein ostia from “inside” the LA. Direct anatomic measurements should not be obtained from these postprocessed images but from the 2D slices instead.

Pulmonary Vein Embryology

A clear understanding of pulmonary vein embryology is important for understanding both normal pulmonary vein anatomy, nonpathologic variations from the normal anatomy, and congenital anomalies. The pulmonary veins and associated apical LA are derived from the primitive common pulmonary vein. The primitive pulmonary venous system initially has no connection with the heart and drains into the cardinal veins and the umbilico vitelline system. At approximately the fourth week of gestation, the pulmonary venous drainage coalesces into a single vessel. At the same time, an outgrowth of the primitive LA extends toward the pulmonary venous system to meet this vessel to form the primitive common pulmonary vein and the venous connections to the cardinal veins and the umbilic vitelline system degenerate. The common pulmonary vein then expands to form the smooth-walled body of the LA, whereas the primitive LA forms the trabeculated left atrial appendage. The branches of the primitive common pulmonary vein form the adult pulmonary veins. The development of the LA and pulmonary veins is asymmetrical, with the two right-sided pulmonary veins developing first whereas the left-sided pulmonary venous drainage enters the LA through a single trunk that eventually bifurcates to form two veins.

Normal and Variant Pulmonary Venous Anatomy

Most commonly, there are four pulmonary veins that enter the LA: right superior, right inferior, left superior, and left inferior (see Fig. 42.1 ). Each of the veins is directed laterally, with the inferior veins directed posteriorly and the superior veins directed anteriorly. The left superior pulmonary vein frequently has a cranial angulation and may appear to arise from the superior portion of the LA.

Variant, nonpathologic pulmonary vein anatomy is very common, present in approximately 40% of patients. Although numerous variations have been described, the most common variations in the usual anatomy are a single left common pulmonary vein or an additional right middle pulmonary vein ( Fig. 42.2 ). These variations occur because of more or less incorporation of the primitive common pulmonary vein into the LA. Less incorporation leads to apparent fusion of pulmonary veins before entering the LA, whereas more incorporation results in additional pulmonary veins ( Fig. 42.3 ). Because the right-sided pulmonary veins form first and have more developmental time to be incorporated into the LA, it is more common to have additional veins on the right. Conversely, the left-sided pulmonary veins form later and are more likely to have a common trunk. These variations in pulmonary venous anatomy have not yet been identified as a cause of pathology.