Chest: Multidetector Computed Tomography of Thoracic Pathology with an Emphasis on 3D Volume Rendering and CT Angiography

Leo P. Lawler, M.D.1
Elliot K. Fishman, M.D., FACR1 1From the Russell H. Morgan Department of Radiology and Radiological Science

Abstract

The advent of multidetector computed tomography (MDCT) and the latest three-dimensional volume rendering computer platforms represent a powerful combination providing a unique perspective of thoracic pathology. MDCT not only permits shorter acquisition times, greater coverage and image resolution but also provides a substrate to better exploit the potential of the most recent volume rendering techniques. These techniques now permit real-time, interactive modification of relative pixel attenuation in an infinite number of planes and projections. We have employed these tools to address diverse thoracic clinical questions and this paper will illustrate their application to the normal and diseased states of thoracic vasculature, lung and chest wall.

One sentence synopsis

Multidetector computed tomography and volume rendering of thorax.

Key Words

Multidetector CT, Volume rendering, Thorax

Introduction

Among the various thoracic anatomical structures and the pathologies that affect them, there now remain few entities that can elude the combined potential of multidetector computed tomography (MDCT) and three-dimensional volume rendering (3DVR). The result of progress in this area is the capability of the fast acquisition of unprecedented high quality data sets and the potential to view them in three dimensions in what is a practical, real time and powerful way [1,2]. We shall show how these advances may be applied to enhance the conventional roles of thoracic CT, to challenge other imaging modalities and to change the approach to disease processes themselves. In many cases, although axial images may be adequate for diagnosis, more information about the nature of the disease and better communication with clinicians are obtained with volume rendered 3D images. For our purpose we shall arbitrarily divide the thorax into the neurovascular structures, the lung and the chest wall.

Technique

The protocols herein reflect our personal experience which is largely with a Siemens Plus 4 Volume Zoom (Siemens Medical Systems, Iselin, NJ) with an adaptive detector array design and highly flexible pitch. Our images are volume rendered on a prototype Siemens 3D Virtuoso (Siemens Medical Systems, Iselin, NJ) workstation. This system is commercially available. When used, the contrast was non-ionic Omnipaque 350 (Nycomed Amersham, Princeton,NJ).

The adaptive array design peculiar to Siemens permits multiple choices of detector sizes and variable pitch from 1 to 8. The choice of collimators will determine the range of slice widths that are available after the scan is performed. Its main advantage rests in the ability to get thinner slices without Z-axis coverage compromise. In many conditions MDCT and 3DVR represents an advance in the classic two-dimensional study and in all cases the near-isotropic matching of in-plane resolution and slice thickness means that alternatives to trans-axial interpretation are not only possible but in many conditions are more logical.

In pulmonary imaging we routinely use 1 or 2.5-mm thick detectors depending on the clinical indication. With 1mm detectors we shall acquire 1.25mm slice thickness and with the 2.5mm collimators we get 3mm slice thickness. With our 0.5-second rotation times we commonly use a pitch of 6 (pitch=travel per gantry rotation/slice collimation). In the thorax, breath-hold imaging coverage and temporal resolution has been enhanced by the peculiar ability of MDCT to get high pitch without widening the slice width profile. With the ability to cover a 24cm thorax in 8 seconds the need for hyperventilation techniques is eliminated and the volumes of intravenous contrast and the need for pediatric sedation have lessened. [3].

We use various reconstruction or volume rendering algorithms to highlight the structure or pathologies of interest. The 3-D images are interactively displayed and edited in real time. Volume rendering applies shades of gray to the pixels of varying attenuation that are found in a data set and trapezoidal transfer functions allow user modification of the relative contribution of various pixel values for the area of interest. Thus, all the information initially acquired is utilized for the final reconstruction. Unlike a threshold technique such as maximum intensity projection (MIP) or shaded surface display (SSD), volume rendered angiography will take account of pixels that are only partially filled with contrast [4,5]. Though there is no data directly comparing volume rendering to other 3D techniques in thoracic application it has been shown advantageous in non-thoracic applications [6,7]. Maximum intensity images will lack depth perception and inter-structure relationships unless the movable images are used (Fig 1). Likewise all tissues will be represented based on their houndsfield units so that, unlike a threshold technique, simultaneous depiction of vessel and airway on individual images is possible. Stereo imaging display and endo-luminal perspectives on occasion may further enhance the appreciation of three-dimensional relationships.

On a practical note the images are immediately sent to the workstation when the patient is scanned. Depending on the institution the images may be produced by the radiologist or a trained technologist. There is a steep learning curve and most training courses complete orientation in three days. Time-consuming regions of interest are not required with the use of clip editing planes and there is true, real-time change of perspective with use of the mouse allowing the three-dimensional images to be produced in a timely and efficacious manner. For example we will routinely produce four or five perspectives of a thoracic aorta in under 10 minutes along with the conventional axial study so that 3D information is available the same day if required. We have reached a point now with MDCT and volume rendering flexibility that there are many more tailored, high quality techniques for the chest than are necessary in an individual patient, so protocols have been designed to rationalize what we do for each case (examples tables 1 and 2).

Clinical Applications

Thoracic vasculature It is probably in vascular imaging that MDCT and 3DVR has found widest clinical acceptance [8]. Whether it be normal anatomical structures and their variations (Figs.2-5) [9] or the pathologies that affect them, the images now possible are equal to or surpass the quality of conventional angiography which had been held as the gold standard. As the volume rendered technique faithfully incorporates all the acquired data one may see vessels that would otherwise be hidden in threshold based shaded surface display or maximum intensity projection which sacrifice some data. The maximum intensity projection studies also lack three-dimensional information unless motion display is used.

Aorta

Soon after its introduction the role for spiral CT to non-invasively image the aorta became clear [10]. Subsequent studies and reviews confirmed this role and suggested the likely enhancements with development of multislice imaging [11,12,13]. 3D-volume rendering has expanded the use to fulfill the needs of diagnosis and operative planning [14]. In many cases this negates the need for conventional angiography and cost savings have been demonstrated [15].

For aortic aneurysm or dissection we routinely get 1.25-mm slice thickness using four 1mm collimators and reconstruct at 1mm intervals. When required, imaging from the arch to the bifurcation in a single bolus of contrast is possible. The temporal resolution now eliminates artifact from aortic pulsation and breathing and cardiac pulsation misregistration is minimized. The caliber and course of normal and diseased aorta and branch vessels can be well depicted as well as their internal architecture (Fig. 6-8). The infinite internal and external perspectives from a single contrast bolus confer great advantage over conventional angiography [16]. By modifying opacity one can choose to optimize visualization of either the vasculature or the viscera they supply.

In the setting of thoracic aneurysms the rendered images can enhance the understanding of the vessel dilatation, mural thrombus and branch vessel patency over axial imaging alone. It has become almost imperative for endovascular stenting of abdominal aneurysms and this will likely be true for interventional management of thoracic aneurysms. The postoperative aorta may also benefit from these techniques with the added benefit of clear depiction of stent deployment [17].

The definition of the extent of an intimal flap and its relationship to arch vessels as well as the establishment of patency of both the true and false lumens are well defined with imaging planes customized to the aortic course and angioscopic views (Fig. 6) [16, 17-20]. CT has been found more sensitive and specific than transesophageal echocardiogram or MRI [21] and though 3D image quality may be improved with MDCT there is no data as yet to show it confers any greater sensitivity in detecting the intimal tears.

In the patient with suspected thoracic aorta trauma the ability to also produce high quality lung parenchyma and chest wall images from the same data set is invaluable in the emergency department (Fig. 7). Three-dimensional imaging has also been suggested as more sensitive for pseudoaneurysm detection than multiplanar reconstruction (Fig. 8) [18] though axial will often suffice.

Coronary arteries

Quantification of coronary calcium was introduced by Agatston [22] and was shown to be related to the presence of coronary artery disease. The finding that lipid-lowering drugs could alter the calcified plaque burden [23] and possibly change the natural history of atherosclerosis intensified efforts to detect and quantify coronary calcification. High correlation between conventional and electron beam CT was noted [24] leading the way for the use of spiral imaging. Cardiac gated MDCT is proving a worthy competitor of electron-beam CT in this area (Fig 9).

The high temporal resolution of 250ms with 500ms rotation times, multislice acquisition and EKG gating decrease the effects of cardiac and respiratory motion and possibly inter-study variability is reduced. There is still a need for large prospective cohort studies to evaluate the whether this technique should be implemented as a cost-effective screening tool in addition to conventional risk factor assessment [25]. MDCT and 3DVR will likely play a leading role research into coronary CT angiography and into imaging of non-calcified lipid laden plaques [26].

Pulmonary embolism

Pulmonary embolism accounts for 10% of all hospital deaths and is a contributing factor in another 10% [27] and yet 50% of pulmonary emboli are not diagnosed ante-mortem [28]. Part of the problem is our diagnostic tests are simply not good enough. 70-75% of V/Q scans are read as indeterminate and the often recommended ‘problem solving’ pulmonary angiogram is not performed [29]. Increasingly too the problem solving conventional angiogram is being questioned as a true gold standard [30].

Electron beam CT and subsequently spiral CT have been found reliable in depicting clot [31,32] and the vasculature [33] even in patients not clinically suspected of having thromboembolism [34,35]. Mayo [36] in 1997 showed CT rivaled scintigraphy and Remy-Jardin showed CT rivaled conventional angiography [37]. Though there is continued debate on the methods employed in such studies CT pulmonary angiography has established itself as a quick, reliable, safe and cost effective technique [38] and incorportated in the latest PIOPED study. In cases where one rules out pulmonary thromb-oembolism the CT technique is advantageous in finding alternative conditions that may mimic such a presentation.

There is better visualization of smaller pulmonary artery branches with smaller collimation of 2mm [39]. MDCT with 1mm collimation and a pitch of 6 can cover from the aortic arch to the diaphragm in 10 seconds. Single detector spiral will take 20 seconds for such z-axis coverage with 2mm collimation. Thus the single detector limitations of not seeing clot in the smaller segmental and subsegmental branches should be improved. When a very quick study is required, such as in a poor breath-holder, to rule out clot in larger vessels we employ 2.5mm collimation. If the clinical question relates to segmental or sub-segmental clot 1mm collimation is chosen. MDCT temporal resolution limits artifacts especially in lower lobe pulmonary artery branches near the heart. The breath-hold of single detector spiral CT (SDCT) is reduced by almost half for MDCT and the high pitch means CT venography of the lower extremities at the same time is feasible [40]. Although good axial imaging will suffice in the majority of cases the ability to render images in three-dimensions helps in viewing those vessels that course obliquely through the imaging plane and in following vessels to their origin to differentiate arteries and veins (Fig. 10) [36]. Also we find it useful to define the extent of clot in a vessel to understand the clot burden and to plan surgery in chronic thrombo-embolic pulmonary hypertension (Fig. 11).

Venous anatomy

MDCT/3DVR has been utilized for venous angiography imaging of the liver and kidney especially for tumor resection or transplant surgery planning. Similarly the venous structures of the thorax may be visualized and many of the advantages discussed in arterial imaging apply. The inherent tortuous, variable caliber nature of veins is well suited to volume imaging

Superior vena cava obstruction, often related to tumor, is a frequent presentation. MDCT and 3DVR may establish the extent of tumor involvement and document the extent of collateral formation (Figs. 12,13). Similarly other causes of venous obstruction such as thoracic outlet syndrome, subclavian thrombosis can be evaluated [41]. This technique is an aid to studies of venous anomalies such as duplications and pulmonary arteriovenous malformations and in planning therapy (Fig.14). CT with shaded surface display was shown to be accurate to define the angioarchitecture of pulmonary arteriovenous malformations and volume rendering has improved on this with better depiction of the small caliber structures [42, 43]. The interactive change in projection to separate vessels enables categorization into simple or complex malformations, a fact that is important in management decisions [43].

Airway and lung parenchyma

The ability to get multi-planar imaging of the lung with nearly identical image quality independent of the plane chosen is useful in lung airway and parenchyma imaging. Likewise the ability to select either less noisey, thicker slice widths for survey studies or thinner high resolution images for diffuse disease from the same scan is of particular interest. Chest coverage of 24cm in 8seconds at a pitch of 6 is of obvious benefit in poor breath-holders.

Airway

Since the demise of bronchography whole lung airway imaging has been limited largely to axial slice imaging at selected levels of interest. Attempts at three-dimensional imaging have been hampered by poor quality of the data acquired and sub-optimal techniques to render such data. Stair step and other artifacts understandably led to some skepticism for its role in thoracic imaging. However now high quality, high fidelity images are produced in an efficient manner and has been used for problem solving in airway disease [44,45,46].

Unlike its single detector predecessor, the narrow collimation and overlapping reconstructions permit routine images beyond central airways to the sub-segmental bronchi of the peripheral airways (Fig. 15). Airway disease often affects multiple disparate branches of the tracheobronchial tree. It is important to be able to perform whole lung volume rendered images with ease, as opposed to being confined to a pre-elected small area of interest as in the past. Unlike the all-or-none thresholding binary classification, volume rendering allows the percentage of different tissue types to be reflected in the final image while maintaining three-dimensional spatial relationships. The trapezoidal transfer function creates a histogram of displayed houndsfield units permitting control over window width and level, opacity and brightness so that particular combinations of tissues may be displayed (Fig. 15-19). It is far less susceptible to volume averaging artifacts, which are a particular problem with airway imaging due to the air-soft tissue interface.

The ability to visualize beyond severe narrowing and the ease with which small pediatric airways can be imaged compliments bronchoscopy (Figs. 16,17). A pre-procedure bronchoscopic map has clear potential in defining the number, location and extent within an individual bronchus as well as the overall lobar and segmental distribution of both areas of stenoses and bronchiectasis. Such lumen abnormalities of obliquely coursing airways are better seen with volume rendered views. Functional imaging is possible so that the airway caliber at various stages in the respiratory cycle may be interpreted. As with vascular structures endoluminal views may also be generated. Anomalous branching of the airway is better appreciated in volume rendered views and simultaneous CT angiography may clearly define the interrelationship of vascular structures in cases of rings and slings (Fig. 16,17). We have found airway imaging to be of particular use in the pre-procedure planning and post-procedure follow-up of stent placement. (Fig. 18)[47,48].

Diffuse lung disease

For diffuse lung diseases we have found that whole lung imaging and interpretation facilitate understanding and communication of the disease extent (Fig.19). Minimum intensity projection has shown some merit is the evaluation of the patient with emphysema [49,50] both in disease detection and describing its extent. The near isotropic data sets provide near identical imaging quality in any chosen plane which when describing distribution of diffuse lung diseases is of value where many of the axial landmarks of airway and fissures are distorted. One can use the choice of larger 2.5-mm detectors to do a rapid evaluation of lung at the cost of some spatial resolution or incorporate the 1mm detectors for high-resolution images such as in interstitial lung disease.

This article would not be complete without a comment on the current interest in lung cancer screening [51,52]. It is not yet clear if the ability to screen for cancer will change clinical outcomes or if current CT technology is overly sensitive and lacking in specificity for many nodules. However if a practical, high quality test is considered the MDCT modality is a consideration and low dose population studies may be feasible. Three-dimensional imaging has been shown to increase nodule detection and can help separate small nodules and vessels [53,54].

Chest wall

MDCT and 3DVR has proven worthwhile in musculoskeletal imaging and an extension of this is its application to the thoracic cage (Fig. 20)[55,56]. It has been shown to have value in congenital and post surgical anomalies and similar principles may be applied to other chest wall conditions [57,58]. Whether it be the bony thorax or the soft tissues it supports we have applied these techniques in imaging congenital changes (Figs. 21,22), tumor (Fig.23), trauma (Figs. 24,25) or infectious diseases (Figs. 26,27) with variable trapezoids to highlight structures of interest. Unlike MIP imaging, one preserves the 3D interrelationship of data. The complex curving, overlapping anatomy of the shoulder girdle and thoracic cage are well suited to three-dimensional imaging. In the trauma setting complicated chest wall fracture evaluation and a CT aortogram can be performed in a timely manner by a single study using these techniques. CT is well established as a modality for lung cancer staging and the question of chest wall involvement can be aided by three-dimensional imaging [58].



Conclusions

The latest advances in MDCT and 3DVR have much to offer the thoracic imager. Volume rendering is suitable to exploit the advantages of the high quality MDCT data and is a necessity due to the large data sets involved. Not only are these techniques adding to the established conventional roles of thoracic CT imaging but also we see their potential in a host of new areas.



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TABLES

Table 1. Multidetector airway stenosis protocol.

Table 2. Multidetector aortic dissection protocol.