As mentioned in the preceding paragraph, volumetric rendering techniques of computed tomographic angiography (CTA) and magnetic resonance angiography (MRA) data can display anatomic structures in a manner similar to that experienced on direct visualization. Such techniques emphasize surface representations but have substantial limitations. Many vascular abnormalities are best assessed by using cross-sectional representations through the structures of interest. However, the routine primarily reconstructed cross-sectional images are usually arbitrarily oriented relative to these structures. For example, although the abdominal aorta courses in a cranial to caudal direction nearly parallel to the CT or MR scanner table, it is not truly parallel. Therefore a “transverse,” or “axial,” primary reconstruction would not be quite perpendicular to the longitudinal axis of the aorta, resulting in oblique cross sections of this structure. Moreover, because even the normal aorta subtly curves with the kyphosis of the thoracic spine and the lordosis of the lumbar spine, a planar reformation perpendicular to the axis of the aorta at one point when translated cranially and caudally may no longer be perpendicular to the curved centerline axis of the aorta (Fig. 6-2). Because the vascular system is a network of tubes oriented with complex and at times highly variable spatial relationships, and because diagnosis and characterization of vascular diseases necessitate an assessment of a vessel’s lumen, wall, and surrounding tissue, a diverse menu of visualization options have been developed to enable physicians to maximize their understanding of the imaged vasculature. The flexibility afforded by volumetric visualization techniques facilitates comparison of changes over time despite subtle shifts in a patient’s position between serial examinations. These techniques allow assessment of anatomy by using landmarks and axes that are intrinsically relevant to structures of interest rather than to the orientation of the imaging table (2,3,4,5,6,7,8) (Fig. 6-3).

A single volumetric CT angiogram of the aorta through the lower extremity arterial system can consist of over 2,000 primary reconstructions (Fig. 6-4). Assuming that only 3 seconds are spent examining each image in a CT scan of 2,400 primary transverse reconstructions, an impractical 2 hours would be required for a physician to examine each volumetric data set (3,6,9,10).

A preferable paradigm to consider for the interpretation of such volumetric data arises from ultrasonography. A single sonographic image is a cross-sectional reconstruction similar to a primary planar reconstruction from CTA or MRA; however, the sonographer does not typically save


entire stacks of cross sections for interpretation while sweeping the transducer from top to bottom of the imaging volume. Rather, the sonographer watches the image display changing in real time as the transducer is swept over the surface of the patient. Based on knowledge of normal and abnormal anatomy, only relevant and representative images in standard planes are captured for further review and archive. Primary diagnosis is made based on direct real-time observation by the sonographer; the saved static images serve primarily to document clinically relevant information in familiar organ-specific planes.

CTA and MRA data can be explored in an analogous fashion. If one considers the volumetric data as a virtual patient lying on a gurney in the ultrasound suite and conceptualizes the workstation as a virtual sonographic transducer, the interpreter of such examinations can rapidly scan through and interactively explore this volumetric data set both to perform primary interpretation and to record views that are clinically relevant. Using a spectrum of visualization tools in this fashion, the volumetric data can be examined more efficiently than a simple review of individual cross sections.

It should be noted, however, that primary interpretation using interactive volumetric visualization techniques cannot completely replace the routine examination of the primary reconstructions. The pitfalls of each visualization technique utilized should be considered when scanning through the primary reconstructions to assure that all abnormalities have been detected and that volumetric representations are truly representative of the anatomy. Moreover, although the primary goal of vascular imaging is to investigate vascular structures and the organs perfused or drained by those structures, it is incumbent on any interpreter of CTA or MRA data to examine the entirety of the volumetric data set, including structures that are not primarily relevant to the purpose of the examination so that important incidental nonvascular findings may be identified and reported (2,3,4,5,7,9,10,11).

Individual “screen shots” saved during real-time exploration of volumetric data provide a basis for communication to referring physicians of anatomic findings with complex spatial relationships. The adage that a picture is worth a thousand words is certainly applicable in the case of communicating imaging results (Fig. 6-4). Although a CT or MR angiogram may be composed of thousands of primary reconstructions, key findings can typically be represented in a handful of dedicated views (Fig. 6-5). The advantage of providing this focused output is that it allows the referring clinician to be most efficient in understanding the primary interpretation while reducing the need to wade through piles of film or virtual stacks of cross-sectional images on a computer monitor (3,12). In certain circumstances, the ability for the referring physician to then use those representative images to illustrate the severity and nature of disease to a patient has been shown to alter a patient’s treatment choices (e.g., to accept a therapeutic option or undergo lifestyle modifications) (13).

The final major rationale for volumetric analysis is the ability to quantify the complex geometric relationships and features of the vasculature. Length, area, and volume measurements of vascular disease serve as objective metrics for evaluating disease severity as well as change over time. In an era where percutaneous delivery of stents, stent grafts, and
other therapeutic devices has significantly reduced the direct visualization and measurement afforded by an open procedure, vascular quantitation based on imaging has become a critical process both for triaging patients to appropriate therapeutic options as well as for determining the appropriate size and type of medical devices to be used when intervention is planned (14).

While many sophisticated visualization techniques critical to interpretation of cardiovascular imaging are the focus of this chapter, the importance of review of primary planar reconstructions both with CTA and MRA cannot be understated.
The “source data” provide the highest spatial resolution and the lowest likelihood of motion-related misregistration and off-axis artifacts.

Artifacts related to beam hardening in CT and magnetic susceptibility in MR are easiest to recognize on the primary image reconstructions. These effects frequently result from metallic clips and prostheses and can cause vascular pseudo-lesions due to localized loss of detail in affected regions. A similar effect can occur where the high concentration of intravenous contrast medium along the course of venous access results in x-ray beam hardening or magnetic susceptibility. While 3D volume-rendered images may deceivingly suggest stenosis or occlusion due to susceptibility, review of the primary reconstructions show affected regions as signal voids that cross normal anatomic boundaries (Fig. 6-6).

A critical consideration in the visualization process lies in the setting of the grayscale window and level for image review. When performing cardiovascular imaging, the vessel lumen should never be rendered with the highest gray values (white). If the lumen is rendered at these highest gray values, then the grayscale is truncated, which can misrepresent vessel dimensions. This consideration is particularly critical for visualization of arterial wall calcifications or metallic stents and stent grafts in CTAs—truncation may render mural calcifications and metal indistinguishable from the lumen (Fig. 6-7). In fact, in the presence of mural calcification in an image, the window and level should be adjusted so that the calcification is not truncated. If the calcium is allowed to be represented as white, its dimensions may be overrepresented and arterial lumina


adjacent to calcifications underrepresented, suggesting higher grades of stenosis than are actually present (Fig. 6-8). Similarly, when the lumina of metallic stents or stent-grafts need to be assessed, a window and level should be selected to avoid rendering the metallic components of the stents or stent grafts as bright white (Fig. 6-9). These principles for selecting grayscale window and level are equally applicable and clinically important when assessing multiplanar reformations and maximum intensity projections (5,7,10).

Multiplanar reformations (MPR) refer to planar cross sections that are oriented through planes other than those of the primary reconstructions. If the primary reconstruction is transverse, then simple sagittal or coronal MPRs orthogonal to these transverse images may be created; however, just as primary transverse reconstructions that are perpendicular to the MR or CT table are generally oriented arbitrarily to human anatomic structures, coronal and sagittal MPRs suffer from the same limitation. MPRs become most valuable when obliqued
parallel or perpendicular to the primary axes of a structure to be examined (Fig. 6-10), again in a manner analogous to the cross-sectional images or drawings typically shown in anatomic diagrams. Because blood vessels have a curved course, MPRs are rarely the best means of summarizing vascular anatomy. Nonetheless, MPRs can be useful for examining the cross-sectional characteristics of short segments of targeted vasculature, particularly in the vessel wall (5,7,10) (Fig. 6-11).

An important limitation of evaluating planar CT or MR images (whether primary reconstructions or MPRs)
occurs because most structures are only partially represented within a single image. When examining blood vessels, planar images must be evaluated in a stacked cine mode rather than in a tiled presentation as would traditionally be represented on photographic film. In this manner, cine paging is performed on a workstation to progressively scroll through a stack of planar images. By dynamically paging through the stack, the interpreting clinician can develop a much better understanding of the spatial relationships of imaged structures than when the stack of images are spatially dissociated by the tiled format (5,7,10).

Unlike MPRs, where planes are created via the propagation of a straight line through the imaging volume, curved planar reformations (CPR) are created via the propagation of a curved line through the imaging volume. The curved line typically represents a trace through the centerline of a
blood vessel or other curvilinear structures such as the pancreatic duct or biliary tree, which is then extruded through the volume. Intuitive representations of curved planar reformations, illustrated in Figure 6-12, have been euphemistically referred to as “magic-carpet views,” but are not standard in clinical practice. Instead, CPRs are represented on a flat plane as if the folds of the magic carpet have been laid out on a flat surface. This powerful form of display provides the best overview for evaluating wall irregularities along the longitudinal course of blood vessels (Fig. 6-13). It can also be very useful for evaluating the relationship of filling defects such as thrombi or intimal flaps to the vessel wall and the origins of vessel branches (Fig. 6-14). CPRs in CTA are particularly useful for assessing the luminae of arteries with high attenuation material in or along the arterial wall, such as calcifications or metal stents (Fig. 6-15). One might wonder why CPRs are not more widely used in the assessment of vascular structures, given these capabilities (5,7,10,15).

While CPRs are routinely utilized at the Stanford University 3D Laboratory, many practices eschew this technique for several reasons. Until recently, curved planar

reformations could only be achieved through manual tracing of a vessel centerline (Fig. 6-16). This labor-intensive process is both time consuming and operator dependent, with potential for substantial variability in the quality of the CPR. Over the past several years, software algorithms to automatically extract the median centerline of blood vessels have resulted in the introduction of automated techniques for creating CPRs (Fig. 6-17). Such automation eliminates both the time-consuming aspects of tracing the curve as well as the operator dependence associated with creating the curve. The interpreter must be cautioned, however, that different implementations of centerline algorithms have varying degrees of effectiveness in finding the true median path, and thus, the interpretation of a CPR should always be made in association with the review of the centerline trace. Another limitation of CPRs is that to the unfamiliar eye, they can present bizarre spatial representations of structures outside the vessels of interest. Particularly when blood vessels course in one direction and then double back on themselves, unusual artifacts called “loop artifacts” can be created (Fig. 6-18). As long as the observer focuses on the primary structure through which the curve is created (while ignoring unusual appearances of other structures), mistakes in interpretation can be avoided. Just as we typically think of MPRs as a collection of parallel planes stacked through the imaging volume, CPRs are more effectively created as a series of radially oriented planes through the curved centerline. By rotating through a series of CPRs created at regular intervals (e.g., 5 degrees) around the centerline, eccentric manifestations of mural thickening or disease deposition are readily demonstrated (Fig. 6-19) and provide greater analytical efficiency when compared with planar reformation translated through a cartesian axis (15).

An important adjunct to CPRs is stacks of cross sections oriented orthogonal or perpendicular to the median centerline of a vessel. The creation of a stack of cross sections in this fashion results in planes that are not parallel to one another but that maintain a perpendicular relationship to the median axis of the vessel as these images propagate along the length of the vessel (Fig. 6-20). This allows for a true assessment of variations in cross-sectional area (CSA) or cross-sectional wall thickening and forms the basis for many quantification techniques described later in this chapter. The stacked review of these orthogonal cross sections has been referred to as a “slice-through” display (15).

Although the current connotation of “volume rendering” (VR) in medical imaging implies a specific 3D visualization technique that uses sophisticated algorithms to represent MR signal or CT attenuation characteristics, volume
rendering also more broadly refers to a family of visualization techniques generally linked by “ray casting,” a process which creates 2D images derived from the information in 3D volumetric data sets. Ray casting is a process where the computer projects rays along a specified viewing direction through a volume or subvolume of the CT or MR data and uses characteristics of voxels encountered along each of these rays to compute pixel characteristics that compose an output image. By doing so, ray casting produces 2D images that represent projections from 3D volumes. Commonly seen ray casting techniques include maximum- and minimum-intensity projection (MIP and MinIP, respectively), ray sum, shaded-surface display (SSD), and VR (2,3,4,5,6,7,8,10,12,15,16,17).