Advanced MRA Rendering Techniques: A Pictorial Review

Lawrence N. Tanenbaum, MD

Disclosures

Appl Radiol. 2002;31(5) 

In This Article

Volume Rendering

More recently, volume-rendering techniques have become available that allow integration of the full range of image data and depiction with variable opacity. Traditional, non-thresholded, whole-volume MIP is a form of volume rendering in which only the brightest pixels have been rendered opaque. Unlike traditional "hard" thresholded 3D techniques, which require removal of unwanted ranges, volume-rendering techniques allow all ranges to remain within the model under study.[4,5]

The operator is presented with a menu of preset rendering options under which intensity (and density) ranges have been designated a variable opacity (figures 14 through 19).

Volume renderings of contrast-enhanced magnetic resonance angiography of the hand. Note the varied and dramatic three-dimensional (3D) appearance of the vasculature in this patient with hypothenar-hammer syndrome. The study was performed after determination of contrast transit time via a test injection timing run. After acquisition of a subtraction mask, a contrast-enhanced 3D gradient recalled echo acquisition with elliptical centric k-space ordering was performed.

Contrast-enhanced magnetic resonance angiogram of an arteriovenous malformation. Study is fast three-dimensional gradient recalled echo image acquired with 0.05 mmol/kg of gadoteridol. Note the more dramatic depiction of anatomy with volume rendering when compared with the two-dimensional maximum intensity projection image (upper image). Note how the vol-ume renderings, in this instance, highlight surface anatomy allowing only minimum transparency.

Comparison of (A) volume-rendered contrast-enhanced magnetic resonance angiography and (B) three-dimensional conventional X-ray angiography studies of an arteriovenous malformation (AVM). All components of the AVM are well depicted on the "steady-state" magnetic resonance angiography study, albeit with significant overlap of arterial and venous information.

(A) Ray-sum-like volume rendering of a contrast-enhanced magnetic resonance angiogram in a patient with diminished pulses in the right upper extremity reveals a right subclavian artery stenosis (arrow). Note the partial transparency of the model with clear delineation of summation information similar to that of conventional X-ray techniques. (B and C) Shaded-surface-display (SSD)-like volume renderings assist in delineating the right subclavian artery origin stenosis (arrows). Some transparency is retained, a feature not available with SSD.

Magnetic resonance angiography (MRA) of thoracic outlet syndrome. (A and B) Study obtained with arms down (volume renderings) is unrevealing. (C) Volume rendering of contrast-enhanced MRA study acquired with arms elevated over the head (Adson's maneuver) demonstrates right subclavian artery impingement. One of the strengths of MRA over computed tomography angiography and conventional angiography is the ability to repeat exams in multiple phases or conditions due to the absence of ionizing radiation and an extremely well tolerated contrast agent.

Volume-rendered computed tomography angiogram of an abdominal aortic aneurysm. Note the excellent depiction of surface features with retained partial transparency as well as differentiation of calcified wall, thrombus, and patent opacified lumen combining features of shaded-surface display, ray-sum, and maximum intensity projection.

Since thresholding is "soft," creative model manipulation is rapid and interactive, providing the best possible dynamic display of pathology and anatomy.

Initially advocated for CTA data, these techniques offer significant benefits when rendering MRA. Renderings with features of hard-thresholded SSD, MIP, and ray sum can be created separately or in combination on a free-standing workstation (figures 19 and 20). Hard- and soft-thresholding techniques can also be used in combination to facilitate display.

Three-dimensional time-of-flight magnetic resonance angiography of a bilobed aneurysm at the origin of a fetal posterior cerebral artery. Hard thresholding removes signal from background tissues, allowing more flexibility in application of volume-rendering algorithms. Here the rendering has many of the features of shaded-surface display.

Recently, automated bone segmentation algorithms have become available that facilitate bone removal. Alternatively, bone information can be superimposed on the completed vascular model to provide useful localization information (figure 21).

Maximum intensity projection (left), partially transparent volume rendering (right), and combination of bone and vascular volume rendering (center) of computed tomography angiography-depicted left renal artery stenosis. Automated bone segmentation algorithms augment bone removal and replacement enhancing display. A combination of rendering techniques is often employed to best demonstrate the features of a lesion.

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