Advanced MRA Rendering Techniques: A Pictorial Review

Lawrence N. Tanenbaum, MD

Disclosures

Appl Radiol. 2002;31(5) 

In This Article

Two-dimensional Rendering

Two-dimensional (2D) projection and 3D rendering of MR (and computed tomography [CT]) source images are accomplished with a variety of techniques. The most commonly used is maximum intensity projection. With MIP techniques, a ray tracing is created in which the highest intensity pixels along the ray are depicted in a 2D fashion and intensities below an arbitrary threshold are eliminated. MIP algorithms determine the threshold for inclusion by considering the full range of intensities in the imaging volume, including quality signal and (interfering) noise (figure 1). All information included in the model is rendered at the same opacity--residual noise is as conspicuous as anatomy. Benefits of MIP rendering include rapid essentially automatic reconstruction with minimal operator input and dependence.

Calf station on a contrast-enhanced magnetic resonance angiography runoff study rendered with maximum intensity projection. Partially saturated background tissues are eliminated by the algorithm, which renders only the maximum intensity along the ray tracing. Note that MIP is a two-dimensional projection, so review of a full range of projections is critical to thorough interrogation.

While dynamic range is limited on MR MIP images, rendering of CT data does produce projections that reflect the inherent relative gray scale (Hounsfield units) of the source images. This allows calcified plaques to appear more intense than the contrast-opacified vessel lumen. Since MIP creates 2D depictions, projections must be created over 180° to integrate 3D information fully (figure 2).

CT angiography maximum inten-sity projection (MIP) of partially thrombosed abdominal aortic aneurysm. Note that CT MIP display easily differentiates calcified plaque from the contrast opacified lumen (see figure 19). Parameters critical to stent graft placement such as distance between the proximal aspect of the aneurysm and the lowest renal artery, the presence or absence of accessory renal arteries and the distance between the distal aspect of the aneurysm and the aortic bifurcation are easily gleaned. Particularly in the absence of mural calcification, review of source images remains essential for confirmation of aneurysm diameter.

MIP rendering is limited in areas of vascular overlap since there is no summation information. This potential for partial obscuration can limit evaluation of complex and tortuous anatomy, such as the circle of Willis and aortic branch vessels.

Limiting the volume under consideration can improve pixel selection and enhance the accuracy of maximum intensity pixel projection. The ability to create a "scalpel" or freehand trace circumscribing regions of anatomy for inclusion or exclusion in the model is widely available and commonly employed. Isolating individual structures under evaluation, eg, a single carotid artery on an MRA of the neck both improves the accuracy of rendering and reduces overlap with adjacent structures (figure 3). In some circumstances with complex, overlapping, tortuous anatomy, or combined venous and arterial enhancement (as well as close proximity to bone on CT), isolation of the structures of interest in a volume may be extremely time consuming and problematic.

Free-hand trace limited volume maximum intensity projection (MIP). Removing background signal and noise enhances the integrity of MIP. Elimination of the posterior circulation vascular structures permits visualization of the posterior communicating artery aneurysm in multiple projections. Note that the aneurysm is obscured on inferior projection or submental-vertex view (see figure 8).

Overlapping, limited volume (OLIVE) MIP rendering can overcome many of the limitations of full-volume and regionally circumscribed MIP. These studies, also known as "sliding thin-slab MIPs" or "multiplanar volume reformations" (MPVR) are essentially a hybrid between multiplanar reformation and MIP. Limiting the volume improves the integrity of MIP and limits overlap from adjacent vascular (and on CT, bony) structures[1,2] (figures 4 and 5).

Right renal artery stenosis due to an anteroposteriorly oriented eccentric plaque (arrow). Overlapping, limited volume maximum intensity projection rendering in at least two orthogonal planes are critical to the high-resolution interrogation of small structures such as the renal arteries, avoiding overlap and the overestimation of stenosis.

Contrast-enhanced magnetic resonance angiography of a cerebral arteriovenous malformation (AVM). Study is performed with a fast three-dimensional gradient recalled echo sequence during administration of 0.05 mmol/kg of gadolinium contrast. Routine multiplanar overlapping, limited volume maximum intensity projection display is critical for delineation of the critical components of an AVM (arterial feeders, presence of aneurysms and varices, and venous drainage), as well for investigation of possible associated aneurysms..

Narrow interval, overlapping sub-volume MIP slabs offer a valuable tomographic assessment augmenting critical evaluation of intracranial pathology such as aneurysm, and vascular malformation as well aortic pathology such as dissection and aneurysm using both MR and CT data sets (figures 6 and 7).

Patient with subarachnoid hemorrhage and no detected aneurysm with conventional angiography. Full-volume maximum intensity projection (MIP) image demonstrates "shine-through" from hemorrhage methemoglobin. Anything with a short T1 (eg, hemorrhage or fat) will be evident on the MIP of a magnetic resonance angiography study. Due to the proximity of blood there was concern about a possible abnormality in the region of the anterior communicating artery (A Com) (arrow).

Anterior communicating artery aneurysm. Overlapping, limited volume maximum intensity projection images in the oblique sagittal plane (left) indicate the presence of a blind sac (arrow) extending antero-inferiorly from the region of the anterior communicating artery. Note the large amount of methemoglobin surrounding the aneurysm. Intraoperative photograph (right) confirms the presence of an aneurysm (arrows).

While many advocate interrogation of source images for similar reasons, employing thicker slabs with MIP processing provides useful information about vascular structures in the proximity and a more realistic depiction of anatomy. As opposed to single-pixel thick source and thin section unrendered reformat images, OLIVE MIPs typically have greater appeal to referring clinicians (figure 8).

Overlapping, limited volume maximum intensity projection (MIP) assessment of an aneurysm (arrow) at the origin of a fetal posterior communicating artery. Tomographic overlapping thin-slab MIPs depict the aneurysm as well as critical proximity structures (see figure 3) without interference from overlapping structures. In-plane source images may not provide the ideal obliquity for analysis, may suffer from low signal-to-noise ratio, and may yield limited information about structures surrounding the vascular lesion..

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