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Volume Intensity Projection - a new post-processing technique for evaluating CTA

White Paper
Dr. Neuman, Joel, M.D. Zoom Imaging

Joel Neuman, M.D.
Zoom Imaging, Bethlehem, Pennsylvania, USA

Mariana Meyers, M.D.

University Hospitals of Cleveland, Cleveland, Ohio, USA 

Introduction

The advent of multislice CT (MSCT) has facilitated a number of post-processing techniques that have enhanced imaging in a range of areas - CT angiography (CTA) in particular. Typical axial slices provide only a limited view of a vessel so that it cannot be viewed in its entirety, and little information can be obtained regarding its volume, diameter or degree of calcification or stenosis. In contrast, the large number of slices generated with MSCT allows for 3-D reconstruction of a vessel, as well as other structures.

Maximum Intensity Projection

One technique that takes advantage of volumetric imaging is Maximum Intensity Projection (MIP). MIP displays the highest intensity (maximum Hounsfield unit) voxel along a particular projection through the volume dataset, creating a 2-D image of the brightest voxels. This process enhances the viewing of vascular structures and allows for greater accuracy in evaluating vessel diameter. The resulting images can be viewed at any arbitrary angle, and can be windowed, zoomed, and paned. However, MIP displays the maximum sample values as the ray is projected through a volume of data. This is then displayed as a flat, 2-D image without depth cues and the ability to differentiate spatial detail of the surrounding structures. In addition, the highest intensity voxels may be from very dense structures - such as underlying bone - and these superimposed structures can obscure the vessel, limiting the utility of MIP.
Figure 1 Voxel processing by VIP. Voxels closest to eyepoint are assigned highest brightness values, while those
further away become faded.Figure 2 Voxels in front and behind target slab thickness are faded, whereas those within the slab are gradually darkened
based on their relative distances from the eye-point.
Figure 1
Figure 2
Voxel processing by VIP. Voxels closest to eyepoint are assigned highest brightness values, while those further away become faded.
Voxels in front and behind target slab thickness are faded, whereas those within the slab are gradually darkened based on their relative distances from the eye-point.

Volume Rendering

Volume rendering is a technique that provides simultaneous viewing of vasculature, soft tissue, and bone. It offers realtime, interactive control of the image with regard to opacity and transparency values. Volume rendering thus allows visualization through and beyond contiguous structures like stents, arterial calcifications, and bone. In this manner, the complex relationship of overlapping, penetrating, and connected tissues can be discerned. Because selected structures can be rendered opaque, translucent, or invisible to obtain a desired view, this technique reduces large data sets to concise images.
Figure 3 A VIP vs. MIP image of the aorta. Conventional MIP displays the brightest voxel along a ray on the screen. In the VIP image of the same anatomy, background bony structures are faded, allowing a high-density view of the aorta and its main branches.Figure 4 MIP and VIP images of the thoracic aorta. The VIP image shows the branches of the thoracic aorta that cross the descending aorta, whereas the MIP image does not.Figures 5 Renal arteries, abdominal aneurysm, and related venous structures viewed with (a) MIP and (b) VIP imaging techniques.
Figure 3
Figure 4
Figures 5
A VIP vs. MIP image of the aorta. Conventional MIP displays the brightest voxel along a ray on the screen. In the VIP image of the same anatomy, background bony structures are faded, allowing a high-density view of the aorta and its main branches.
MIP and VIP images of the thoracic aorta. The VIP image shows the branches of the thoracic aorta that cross the descending aorta, whereas the MIP image does not.
Renal arteries, abdominal aneurysm, and related venous structures viewed with (a) MIP and (b) VIP imaging techniques.

Volume Intensity Projection

Philips Healthcare's exclusive Volume Intensity Projection (VIP) takes the best of both traditional MIP (ease of use) and volume rendering (three dimensional display and spatial relationships) and incorporates them into a single rendering. Similar to MIP, it displays the highest intensity voxel along a projection through a volume dataset. In contrast to MIP, however, VIP assigns the highest intensity, or greatest brightness value, to voxels closest to the eye-point, while those further away will be faded according to their distance from the eye-point (Figures 1 & 2).

 

Therefore, structures of moderate density, such as contrastfilled vessels, will preferentially be depicted even when higher density structures such as bone or metal are superimposed (Figure 3). Images can be rotated so that targeted structures are closest to the eye-point and thus appear brighter.

 

The VIP technique has considerably less difficulty than MIP does with the super-imposition of comparable or higher density structures along specific projections, and it retains excellent low-contrast resolution. Moreover, it saves time by allowing rapid processing of large MSCT databases, because removal of bone images is often unnecessary. VIP essentially combines the basic principals of MIP, the spatial details of multiplanar reconstruction (MPR), and the techniques of volume rendering. Therefore, in addition to providing exceptional visualization of vascular anatomy, regardless of overlapping bone or other high contrast structures, VIP facilitates the visualization and differentiation of spatial relationships in complex vascular anatomy (Figure 4).

 

VIP helps reduce data overload, because it displays a level of detail in a single image that would require many images to be visualized with MIP and MPR. Figures 5a and b demonstrate this difference. Conventional MIP (Figure 5a) requires many more thinner slices to view the full extent of renal arteries, an abdominal aneurysm, and related venous structures. By comparison, the VIP image (Figure 5b) displays the pertinent data in a thicker slab that contains all critical information in the proper spatial relationship. VIP saves time by allowing the physician to view fewer images. It also facilitates 3-D viewing for referring physicians who cannot integrate planar images into 3-dimensions as readily as radiologists can. In addition, data handling becomes more efficient with VIP. For example, a technician preparing reformations to be sent to a Picture Archiving and Communications System (PACS) station, can send a few, thicker slab VIPs compared to the many thin slabs that would have to be sent with MIP or MPR.

 

Compared with MIP, VIP imaging retains excellent lowcontrast detail. A VIP cardiac image, for example, demonstrates a soft plaque lesion in a patient's right coronary artery (Figure 6a), whereas the 35 mm wide MIP image of the same view (Figure 6b) failed to identify this lesion. To see the lesion with MIP it would be necessary to decrease slab thickness to 2 mm, in which case only a small portion of the vessel and very little surrounding anatomy would be seen (Figure 7a). In fact, the pulmonary embolus (PE) seen on the large MSCT dataset with fewer 35 mm VIP slabs (Figure 7b) was not detected with a dataset of many 2 mm MIP images.

 

Similarly, the ability of VIP to preserve low-contrast visibility can be seen in Figure 8a. This VIP image clearly displays lymph nodes, a small dissection of the left iliac artery, and provides excellent visualization of the perfused bowel while maintaining 3-D spatial detail. In comparison, the MIP image (Figure 8b) fails to show these structures in detail, nor does it differentiate their spatial relationships.

 

These comparative MSCT images demonstrate the ability of VIP to surpase traditional MIP in its ability to:

  • Give greater emphasis to structures closer to the eye-point
  • Visualize critical anatomy regardless of superimposed structures
  • Maintain low-contrast visibility
  • Differentiate and define spatial relationships between structures.
Figure 6 Comparison of (a) 35 mm slab MIP and (b) 35 mm slab VIP images of right coronary vessel.Figure 7 (a) A 35 mm slab VIP image of a right coronary vessel compared with (b) a 2 mm-slab MIP image of the same view.Figure 8 Abdominal CTA images produced by (a) VIP and (b) MIP.
Figure 6
Figure 7
Figure 8
Comparison of (a) 35 mm slab MIP and (b) 35 mm slab VIP images of right coronary vessel.
(a) A 35 mm slab VIP image of a right coronary vessel compared with (b) a 2 mm-slab MIP image of the same view.
Abdominal CTA images produced by (a) VIP and (b) MIP.


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White Paper
abdomen CTA, aneurysm, aorta CTA, chest CTA, coronary arteries, Extended Brilliance Workspace v2.1, Extended Brilliance Workspace Version 1.0, Extended Brilliance Workspace Version 2.0, head CTA, kidney/renal CTA, MIP, neck CTA, pediatric CTA, peripheral runoff CTA, Vascular, VIP
 

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