Advanced MRA Rendering Techniques: A Pictorial Review

Lawrence N. Tanenbaum, MD

Disclosures

Appl Radiol. 2002;31(5) 

In This Article

Three-dimensional Rendering

Two-dimensional MR (and CT) source data can also be rendered with 3D techniques. Three-dimensional rendering requires operator-specified determination of the range of intensities (or on CT, densities) that will contribute to or be eliminated from the model. The thresholding procedure removes undesirable information from the data set such as noise (and bone on CT) by virtue of intensity (and density on CT) and allows a greater range of rendering possibilities. Traditional 3D techniques require "hard-thresholding" in which ranges are excluded "permanently" from the data set at the onset of model manipulation. Rendering options include shaded-surface display (SSD), ray sum, and MIP.[2,3]

Shaded-surface display offers an extremely dynamic rendering, corresponding closely to superficial features of the anatomy under study (figure 9). This has the greatest application in the depiction of vascular disease, such as aneurysms of the intracranial vasculature and thoraco-abdominal aorta. While appropriate for the depiction of vascular stenosis with MR data, the process of thresholding can affect accuracy and the information provided should be interpreted with caution. Because calcified plaque and contrast opacified blood fall into a similar density range on CT, SSD is an inappropriate technique to depict stenosis with CTA.

Shaded-surface display (SSD) time-of-flight magnetic resonance angiogram (MRA) of a left middle cerebral artery aneurysm. Thresholded maximum intensity projection (upper left) and SSD of MRA data set. Note the superior SSD depiction of surface features of the left middle cerebral artery aneurysm (arrows), corresponding closely to that seen with direct inspection at surgery.

Thresholding-based removal of unwanted intensities before application of MIP algorithms has the benefit of eliminating information such as noise, hemorrhage, and fat that ordinarily may maintain sufficient intensity and manifest on projected images. The resultant MIP renderings are typically more dramatic and appealing to referring physicians (figure 10). CTA data is improved by the removal of bone that can obscure pathology in the brain, body, and extremities.

Thresholded maximum intensity projection (MIP) of aortic dissection. Removing the signal from artifact and "shine-through" of background tissues improves the MIP display. Note the involvement of the arch and descend-ing aorta with sparing of the arch and aortic root. Blood supply to the superior mesenteric artery and celiac axis is via the true lumen.

Thresholded data sets can be rendered with a technique that is a variation on MIP called ray sum. Similar to MIP, this 2D ray tracing also requires viewing of multiple projections to obtain a complete depiction of anatomy. Ray-sum techniques offer a summation of maximum intensities along the traced ray. As a result, when structures overlap there is an increase in depicted opacity (figures 11 and 12). This type of rendition is very similar to that provided by traditional radiographic angiography studies and is superior to MIP in depicting complex vascular anatomy and thrombus.

Time-of-flight magnetic resonance angiogram of a middle cerebral artery aneurysm: ray sum (upper) and shaded-surface display (SSD). Note the changing intensity of the ray-sum display with variations in vascular structure, orientation, and overlap. Ray-sum rendering improves the depiction of complex vascular anatomy and thrombus. Note the dramatic depiction of surface anatomy and variation of display at different view perspectives with SSD rendering.

Ray-sum rendering of contrast-enhanced magnetic resonance angiography of the neurovasculature. Study was performed after administration of 0.05 mmol/kg of gadoteridol and was acquired with a fast three-dimensional gradient recalled echo sequence and elliptical centric k-space ordering. Ray-sum renderings have an appearance similar to that of conventional X-ray angiography, which improves clinical acceptance.

Thresholding a data set before rendering introduces the very significant factor of operator dependency into the imaging process, however. Constant vigilance to avoid overzealous processing in the effort to reduce background information (or remove bone on CT) is essential to maintain model accuracy (figure 13). Because of the potential for operator error, thresholded renderings are best utilized as a supplement to, rather than a replacement for, traditional, non-thresholded MIP techniques.

Computed tomography angiogram of an internal carotid bifurcation aneurysm: Pitfalls of thresholding. CTA studies of the brain are performed typically with 50 mL of a 370 mg/mL concentration of iodinated contrast agent injected at 2.5 to 5 mL/sec. (A) Note the eroded, "apple-core" appearance of the right-sided aneurysm with shaded-surface display (arrow). The hard thresholding process, which assists in segmentation of the bones of the skull base, can also adversely affect anatomic accuracy of the rendered model. (B) Note the true, more "pear-shaped" morphology (arrow) depicted with non-thresholded overlapping, limited-volume maximum intensity projection techniques. Retention of bone information is useful in operative planning.

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