Web Log

Bolus tracking is a technique used in computed tomography imaging, to visualise vessels more clearly. A bolus of radio-opaque contrast media is injected into a patient via a peripheral intravenous cannula. Depending on the vessel being imaged, the volume of contrast is tracked using a region of interest at a certain level and then followed by the CT scanner once it reaches this level. Images are acquired at a rate as fast as the contrast moving through the blood vessels. This method of imaging is used primarily to produce images of arteries, such as the aorta, pulmonary artery, cerebral and carotid arteries. The image shown illustrates this technique on a sagittal MPR (multi planar reformat). The image is demonstrating the blood flow through an abdominal aortic aneurysm or AAA. The bright white on the image is the contrast. You can see the lumen of the aorta in which the contrast is contained, surrounded by a grey 'sack', which is the aneurysm. Images acquired from a bolus track, can be manipulated into a MIP (maximum intensity projection) or a volume rendered image.

B. What is Maximum Intensity Projection? Explain how it works, and provide images as examples of the technique.
A maximum intensity projection (MIP) is a computer visualization method for 3D data that projects in the visualization plane the voxels with maximum intensity that fall in the way of parallel rays traced from the viewpoint to the plane of projection. This implies that two MIP renderings from opposite viewpoints are symmetrical images. This technique is computationally fast, but the 2D results do not provide a good sense of depth of the original data. To improve the sense of 3D, animations are usually rendered of several MIP frames in which the viewpoint is slightly changed from one to the other, thus creating the illusion of rotation. This helps the viewer's perception to find the relative 3D positions of the object components. However, since the projection is orthographic the viewer cannot distinguish between left or right, front or back and even if the object is rotating clockwise or anti-clockwise.

C. What is Segmentation? Explain how it works, and provide images as examples of the technique.
Segmentation algorithms are often based on the principle of region growing. Placing one or more seed points initiates the segmentation of the target structure. From these seed points, more and more neighboring voxels that fulfill predefined criteria are included in the segmentation. The technique can be defined in two ways: segmentation of the desired tissue or segmentation of the undesired tissue with subsequent removal from the data. The latter method removes only interfering tissue from the CT angiography data and retains soft tissue. The process of segmentation is somewhat automatic, an operator is needed to set additional seeding points.
The Web log is due on April 23rd. Once it is complete, send me an e-mail with the exact web address in the body of the e-mail. The subject of the e-mail should be "Web Log" (exactly that, and nothing else).