Chest: Thoracic Venous Structures
Leo P. Lawler, MD, FRCR1, Elliot K. Fishman MD, FACR2
1 Assistant Professor, Diagnostic Imaging and Body CT 2 Professor of Radiology and Oncology, Director Diagnostic Imaging and Body CT
Both authors.
The Russell H. Morgan Department of Radiology and Radiological Science
601 North Caroline Street, Rm. 3254,
Baltimore, MD 21287-0801.
Introduction
CTA has been shown in a number of series to be at least comparable to digital subtraction angiography for arterial imaging and in many cases it is superior. Due to its slow flow and large circulating volume venous imaging is poor with conventional angiography. The contrast and spatial resolution of CTA have much to offer for venous angiography with the added benefit of extraluminal information. The particular attributes of multidetector row (multislice) CT (MDCT) provide data sets, which enhance the established role of spiral CT for venous imaging and provide a vehicle for the latest post-processing techniques.
Multidetector Row CTA and Thoracic Veins
Multidetector row CT is more than just a scanner with increased numbers of detectors. Its superior speed is provided through numerous detectors, faster gantry rotation and table translation, faster data processing and greater tube heat loading capacity. This increased speed may be harnessed and translated into faster z-axis coverage, increased spatial resolution or increased temporal resolution. Compared with single detector spiral CTA, scanning and contrast efficiency are increased, scan slices are thinner and scan duration is reduced. For thoracic venous imaging breath hold imaging from the jugular veins to the intra-hepatic IVC is easily obtained. Current scanner types may be categorized as either fixed or adaptive array design. The former array consists of same sized detectors and slight loss of dose efficiency due to inter-detector dividers. Adaptive array design permits greater choice of detector combination, better dose efficiency and larger range of pitch choices. For the majority of thoracic venous conditions either scanner type will suffice though this paper will allude mainly to adaptive array parameters. Most current scanners use four channel data acquisition systems with four 1mm or four 2.5mm detectors. The former we employ for high resolution imaging whereas the latter we use for greater speed in more unstable patients such as those with superior vena cava obstruction who cannot lie supine for long periods. With table translation speeds of up to 12.5 cm per half-second gantry rotation pitches of 5 or 6 may be used without compromise of image quality as slice sensitivity profile is independent of pitch due to the simultaneous contiguous acquisition and weighted interpolation (pitch defined as table increment per gantry rotation divided by single slice collimation). Unlike single detector scanning slice thicknesses that are larger than the original detector collimation may be obtained. For venous imaging we routinely obtain either 1mm or 3mm slice widths for image reconstruction from a beam width collimated to four 1mm detectors. Wide slice thickness artifact is reduced due to the fusion of smaller slices. MDCT radiation dose is modulated based on patient geometry and absorption. MDCT has given us the possibility of improved z-axis resolution so that near isotropic and isotropic (equal in all dimensions) are now possible. This means image display and interpretation is becoming independent of the acquisition plane or the plane of anatomy. Volume acquisition and interpretation has significant benefit for thoracic venous imaging where naturally tortuous and variable vessels rarely conform to a single predictable plane.
We rarely find non-contrast images necessary. 120cc of non-ionic contrast with 350mg/ml of iodine is administered with power injection at 2-4cc/second through 18-20G antecubital access. This is similar to the protocol for pulmonary artery and thoracic aortic imaging and seems to work well for both systemic vein and pulmonary vein opacification. With a 20-30s delay immediate rapid marked enhancement of the ipsilateral innominate vein and superior vena cava (SVC) is obtained. Subsequently less marked enhancement of the remaining veins due to continued administration and recirculation is observed. The goal is to match the short MDCT scan duration to the relative plateau of venous opacification. In our experience empiric timing gives consistent venous opacification. Though bolus timing and bolus tracking techniques may produce more accurate and consistent numerical density measurements this may not significantly affect interpretation of venous abnormalities where the question usually does not related to subtle caliber change and measurement. Similarly though administration of a saline bolus to circulate pooled venous contrast does increase the density plateau duration it must be hand injected and is less important with the large circulating volume of venous imaging. For patients with presumed decreased cardiac output or more peripheral access (e.g. foot) 5-10seconds is added to imaging time and we decrease injection rate for power injections into the hand. Central line injection is dictated by catheter type and institutional policy and we routinely hand inject with imaging after 80cc.
Volume Rendering Techniques and Thoracic Veins
Multiplanar reconstruction (MPR) and curved MPR are the simplest form of multi-dimensional imaging. Though not truly a volume post-processing this technique is reordering of voxel values with elimination of non-planar data to provide a single alternate plane perspective. The tortuous venous systems of the thorax however do not conform well to any single two-dimensional plane which limits both axial and MPR reconstructions. Maximum intensity projection displays Houndsfield (HU) unit values above an assigned threshold and reduces them to a single imaging plane. No depth cues are provided and this technique is perhaps conceptually closest to DSA. The thoracic cage bony detail will be super-imposed on the opacified veins of interest unless laborious editing is performed. Any post-processing that requires labor intensive editing is not viable in current clinical practice. Shaded surface display (SSD) creates a binary classification with polygon reconstruction of interfaces highlighted by lighting models. Critics have questioned the possibility for error in stenosis measurement due to inaccurate choice of threshold. Both SSD and MIP suffer in thoracic vein imaging as both these techniques depend on high venous contrast values, which are often not obtained.
Volume rendering is the latest form of three-dimensional image reconstruction, which has become intimately linked to MDCT acquisition. The latest in processing hardware and software allows this computer intensive-technique to be performed in real-time with easy manipulation of large data sets involving 3-400 slices of 512 matrix. Volume rendering casts a ray through the data and assigns a value to the voxels, which is weighted by percentage classification to faithfully represent the tissue components within the voxel. Thus 100% of the data and volume-averaged voxels are ultimately displayed in the final image. Trapezoid histograms are used to select window width, level, opacity and brightness and a depth is conferred on the images. Clip plane slab editing expeditiously removes overlying thoracic structures and limitless clip planes and projections permit veins of interest to be shown to best effect. We have pre-set algorithms for optimal display of mediastinal vascular structures and with experience one can rapidly select the appropriate planes and perspectives for the vein of interest. Alternate trapezoids can be utilized to demonstrate the relationship of thoracic veins to the airway, lung and other thoracic tissues. The imaging concept of pathophysiology affecting the thoracic veins can often be reduced to a few well-chosen volumes rendered reconstruction, which greatly aids communication with clinical services.
3D volume rendered MDCT of the thoracic venous vasculature is a symbiotic process depending on optimal execution of the contrast enhanced CT and subsequent appropriate selection of reconstruction and post-processing parameters.
The Thoracic Veins
The thoracic veins are highly variable and tend to be quite tortuous with complex branching patterns especially with especially in the setting of collateral development. MDCT is able to resolve even very small thoracic veins and volume rendered CT is able to provide comprehensive maps of the drainage pattern. It is difficult to design a protocol or single imaging plane that will demonstrate all venous anatomy in different patients so it is preferable to have a display and post-processing that may be tailored to the individual patient. The thoracic veins can be divided into those that return oxygenated blood to the left atrium (i.e. pulmonary veins) and those that return de-oxygenated blood to the right heart(i.e. systemic veins). The systemic veins can be divided into those that drain the chest wall and paraspinal region (superficial) and those that drain everything else including the head and neck, extremities, heart, pericardium and the body below the diaphragm (deep). These latter categories are somewhat arbitrary and here is indeed much overlap in their drainage pattern however it serves as a model for orderly discussion of each component of the thoracic venous drainage.
Systemic veins
Superficial
Azygos, hemiazygos and Batson’s plexus
Internal mammary and lateral thoracic
Chest wall and intercostals
Deep
Superior and inferior vena cava
Innominate, jugular and subclavian
Thymic, inferior thyroid and inter-brachiocephalic
Pericardiophrenic, cardiac, coronary sinus, saphenous vein grafts
Pulmonary veins
Deep Systemic Veins
Superior and inferior vena cava. The superior vena cava (SVC) is one of the more common veins within the thorax that is referred for CT imaging. It is formed at the confluence of the brachiocephalic veins and travels posterior to the right lateral margin of the sternum. The azygos vein arches across the right mainstem bronchus to enter the SVC posteriorly. The most common reason for CT imaging is to address a question of superior vena cava obstruction. This condition presents with head and neck swelling and symptoms of light-headedness during changes in posture. Most commonly the etiology is internal lumen blockage from thrombus or extrinsic compression from primary or secondary tumor encroachment. Thrombus internally is usually due to paraneoplastic hypercoagulable states (Trousseaus syndrome) or direct extension of tumor thrombus. Tumor thrombus may be associated with enhancing vascularity within the thrombus. Clot in the SVC can also occur due to long standing central line placement or secondary to line infection. Localized discrete small clot is frequently seen at the end of indwelling catheters but rarely is of clinical consequence. Clot manifests on CT as a low HU filling defect within a pool of contrast and a gradient is suggested by vein enlargement and collateral development. Post-therapy radiation fibrosis can cause significant lumen compromise through vessel distortion. Abnormal enhancement of the quadrate lobe (segment 4B) is seen and is thought to be due to collateral formation altering flow patterns through the liver parenchyma. Systemic to pulmonary venous communication has also been documented. On occasion false positive diagnoses of tumor thrombosis can arise due to poorly opacified veins or inflow of non-opacified blood. 3D reconstructions may demonstrate that this low density has a character more in keeping with flowing unopacified blood. Repeat immediate second phase imaging or a second later scan with injection of the contralateral vessel can also help discriminate clot and flow phenomenon.
Once SVC compromise has been detected and diagnosed full evaluation requires 3D renderings to map the full extent of vessel involvement and the feasibility of stenting or surgical thrombectomy or decompression. SVC thrombus often propagates superiorly into the brachiocephalic veins, which can adversely affect interventional approaches. Venous mapping is critical to avoid catastrophic hemorrhage from tissue biopsy or therapeutic intervention.
A persistent left SVC is a normal variation due to incomplete resorbtion embryonic vasculature, which occurs, in 3% of normal patients and 4.3% of those with congenital heart disease. It usually occurs in the presence of a normal right-sided SVC. It drains the from the left subclavian vein and travels inferiorly to enter the left atrium posteriorly through the oblique vein of Marshall. It is rarely of clinical consequence but can give rise for concern to clinicians due to the unexpected course of a central line.
The inferior vena cava (IVC) is short and begins at the confluence of the iliac veins. After receiving the hepatic veins it traverses the diaphragm through a foramen in the central tendon at T8 and is separated from the phrenic nerve by fibrous pericardium. In the setting of raised right heart pressures the IVC enlarges and backflow of contrast into it may be seen with tricuspid regurgitation. However in isolation this finding is not reliable and can be seen in normal patients when power injection is used or the tip of a central line is placed inferiorly in the right atrium. Coronal 3D CT reconstruction of renal cell carcinoma IVC extension at this level is critical for management. If these images reveal IVC extension above the diaphragm a thoracotomy is required whereas tumor thrombus limited to the subdiaphragmatic IVC may be approached by abdominal incision alone. Interrupted IVCs still receive hepatic veins and drain into the right atrium though the remaining venous return occurs through an enlarged azygos vein.
Subclavian, jugular and brachiocephalic veins
The subclavian and jugular veins serve the venous drainage of the head and neck and upper extremities. The subclavian veins travel medially from the axilla and receives the jugular and lateral thoracic veins. The subclavian veins are commonly used for access to the central veins and thus can be obstructed through line complications. Neo-intimal hyperplasia can cause stenosis in dialysis patients and thoracic inlet symptoms can be ascribed to cervical ribs or muscular body habitus causing lumen compromise. The latter condition may require imaging with arms raised and by the side for full evaluation of dynamic change and coronal 3D reconstructions of this axial in-plane vessel are superior for assessing caliber change. After receiving the jugular drainage they continue to the superior vena cava as the brachiocephalic (innominate veins). The brachiocephalic vein tributaries include the internal mammary, thymic, inferior thyroid, intercostal and pericardiophrenic veins as well as unnamed anomalous branches. Dedicated subclavian and brachiocephalic questions are better served with ipsilateral venous injection and dilute contrast may decrease beam hardening. Only small portions of the internal jugular veins are routinely imaged. They are usually asymmetric in size and when obstructed collaterals develop through the chest wall and external jugular veins. The internal mammary veins course medial to the arteries and lateral to the sternal border. Both internal mammary and lateral thoracic veins receive drainage from the intercostal and chest wall superficial veins and can be enlarged in the setting of venous obstruction. If there is a known or suspected venous thrombosis or narrowing in a brachiocephalic vein, ipsilateral injection or delayed imaging during recirculation can be beneficial to opacify the region of interest.
Pericardiophrenic, cardiac, coronary sinus, saphenous vein grafts
With a MDCT temporal resolution of around 250ms the motion artifact of cardiac imaging is reduced. The cardiac and pericardiac vessels run in multiple oblique planes and are better seen with 3D rendering. It can be further improved with prospective or retrospective gating as needed to image the relatively motionless diastolic phase of the heart cycle. The pericardium is drained by right and left pericardiophrenic veins. Though usually one on each side these veins may be duplicated. They can be identified in normal individuals coursing with the phrenic nerve on the lateral aspects of the pericardium. They usually drain to both the brachiocephalic veins and the phrenic veins and are markedly enlarged in the setting of SVC obstruction with brachiocephalic extension. Venous drainage to superior intercostal, internal mammary, thymic and jugular veins has also been identified.
Cardiac vein imaging is not usually requested. Their importance lies in differentiation from coronary arteries as MDCT increasingly moves to provide coronary artery imaging for calcium scoring and angiography. The great and small cardiac veins run in the anterior interventricular groove and the middle cardiac vein runs in the posterior interventricular groove. All venous drainage ultimately drains into the coronary sinus at the right atrium. Saphenous vein grafts can be imaged with CT and 3D imaging which has potential with further refinements for establishing patency. To date most work in this area has been done with ultrafast electron beam CT but the gap is narrowing with MDCT.
Superficial Systemic Veins
Azygos , hemiazygos and accessory hemiazygos systems The azygos, hemiazygos and accessory hemiazygos systems together provide a large caliber pathway for venous drainage to the right heart. These vessels provide drainage for both the thoracic and abdomino-pelvic regions as well as communication between the superficial and deep veins. They receive intercostal, esophageal, mediastinal, bronchial and lumbar venous blood. They have a highly variable branching pattern and individuals differ in vessel dominance. The azygos forms after the right subcostal vein joins the ascending lumbar vein and then it enters the thoracic cavity posterior to the crus of diaphragm and through the aortic hiatus. It ascends lateral to thoracic duct and to the right of descending thoracic aorta and finally ends at the superior vena cava after crossing the right mainstem bronchus. These vessels will be opacified in routine imaging. The extent of opacification varies between normal patients and this is due to the variable branching patterns and communications as well as injection techniques. These vessels usually become conspicuous in the setting of high grade superior vena cava obstruction but the azygos vein may also enlarged with congenital vena cava interruption as previously mentioned. They are best appreciated with coronal and oblique sagittal reconstruction perspectives with clip plane editing of the thoracic cage. They tend to be obscured against the bony spine with MIP and SSD techniques but volume rendering can assign different levels of gray to the bone and the vessels to separate them.
Intercostal chest wall and paraspinal veins
Each intercostal space is served by a two anterior and a single posterior vein. The anterior veins communicate with internal mammary and lateral thoracic veins. The lower eight posterior veins drain to the azygos system on the right and the accessory hemiazygos and hemiazygos veins on the left. The first intercostal space drains to the brachiocephalic and vertebral veins via the supreme intercostal vein. Second, third and possibly fourth spaces drain to the superior intercostal vein which is a tributary of the brachiocephalic vein. The left superior intercostal vein may produce a left sided contour bulge at the aortic arch, sometimes termed the ‘aortic nipple’. Most of the intercostal veins are not appreciated on axial 2D imaging due to their small size and volume averaging. They can be consistently depicted with oblique coronal 3D reconstructions of data containing 1 or 2mm reconstructions. A radiating network of vessels including the superficial and thoraco-epigastric veins drain from the umbilicus communicating with the lateral thoracic and axillary veins. The systemic veins along the spinal column are made up of a complex system of veins both internal and external to the spinal canal. This plexus, termed Batson’s plexus communicates freely with the vessels of the cord, vertebral, intercostal, lumbar and lateral sacral veins. Metastatic spread to the vertebral bodies has been attributed to hematogenous spread through these veins. Though they can be visualized this plexus is not routinely imaged.
Pulmonary veins
Usually there are two pulmonary veins returning oxygenated blood to the left atrium from above and below the oblique fissure on each side. Their orifices define the anterior aspect of the oblique sinus of the heart. They lie anterior to the pulmonary arteries and antero-inferior to the bronchi at the hilum. These vessels arise in embryologic development from gradual assimilation of primitive pulmonary veins into the developing left auricle. The distinctively smooth left atrium is due to absorption of these veins. Depending on the absorptive process there can be a wide variation in the number and branching pattern of the pulmonary veins.
The pulmonary veins are best seen in health and disease through MDCT and volume rendering. Their oblique course and variable branching and size cannot be assessed by 2D axial imaging alone and are poorly seen with conventional angiography. Sagittal reconstructions will clearly depict their bronchovascular relationships at the hilum and posterior coronal views of the left atrium will demonstrate their anatomy around the left atrium.
Pulmonary veins can be affected by congenital anomalies that give rise to partial anomalous venous return (e.g. Scimitar syndrome) or sequestration. Developmental anomalies can lead to absent pulmonary veins or pulmonary venous vein varix. Pulmonary veins may provide the venous drainage for isolated or multiple (e.g. Hereditary Hemorrhagic Telangiectasia) arteriovenous malformations. We rarely perform multiphase imaging but with slow flow through a venous anomaly it may be indicated under radiologist direction. All of these conditions are better appreciated with reformatted 3D imaging that is customized to demonstrate the vessel course to best effect. There is increasing use of radiofrequency ablation of potential arrhythmogenic foci in the pulmonary veins. Pre-procedure planning is aided by mapping of the venous anatomy and measurement of vessel caliber for post-procedure follow-up. One of the complications of this therapy is vessel stenosis and lung infarct, which can be easily assessed with reproducible 3D CTA studies of the pulmonary veins. Hilar mass resectability depends on the extent of medistinal invasion and involvement of vital structures such as vena cava and pulmonary veins. The extent of medial invasion and vessel compromise can be demonstrated with 3D imaging using perspectives similar to the operative approach, which better assess the potential for sleeve-resection.
Conclusion
Two dimensional single detector imaging and interpretation of the complex and variable branching pattern of the thoracic systemic and pulmonary venous systems has limitations. The combination of MDCT and VRT provides higher quality data sets and a method to fully harness their potential for image display and assessment. Though this may not radically alter sensitivity for detection of systemic or pulmonary venous pathology it does provide a more comprehensive and sophisticated evaluation through volume acquisition and interpretation.