CT ANGIOGRAPHY PRINCIPLES AYMAN OSAMA

CT angiography (CTA) was described as a new minimally invasive method for volumetric display of contrast filled vessels. It is a software package that reconstructs angiogram like images from the images of spiral CT examination.

CT angiography has rapidly become the imaging modality of choice in many clinical situations, including the evaluation of aortic aneurysm, dissection, trauma, and penetrating atherosclerotic ulcer. CT angiography is clearly well-suited for the pre- and post-procedural evaluation of stent grafts. Other aortic abnormalities, including congenital anomalies, arteritis (such as Takayasu's), and intramural hematoma, are evaluated well by CT angiography. It also shows great promise in the imaging of renal and visceral arteries and the peripheral arterial system.

Additionally, as with other cross-sectional techniques and in contrast to conventional angiography, CT angiography can depict extravascular structures, soft-tissue inflammatory changes and anatomic relationships with adjacent structures displaying a strength of CT that cannot be matched by conventional angiography.

Technique of CT Angiography : 1- Data Acquisition. 2- Image Reconstruction and Post-processing of CT Data. 3- Image interpretation.

Image Reconstruction and Post-processing of CT Data: This is the second step in CT angiography. Image reconstruction, is nearly as critical as the technique by which data is acquired in determining the diagnostic quality of the final product.

Image Reconstruction and Post-processing of CT Data: A) Section thickness. B) Reconstruction increment. C) Interpretation of axial images. D) Image transfer. E) 3D reformatting techniques.

A) Section thickness:- Image reconstruction begins with the choice of a desired section thickness, which is generally the thinnest possible for a given acquisition in order to optimize 3D reconstruction and provide the best chance at visualizing small structures or abnormalities.

The section thickness is determined by the width of a single active detector channel for multidetector-row scanners and by the slice collimation for single-detector–row scanners.

B) Reconstruction increment:- The influence of the reconstruction increment (RI), i.e. the distance between the positions of neighbouring imaging planes, is of great importance

The reconstruction of overlapping images brings fundamental advantages with regards 3D spatial resolution by reducing stair-step artifact.

A 50% overlap, that is a reconstruction increment of half a slice thickness, is a good value for orientation in spiral CT.

C) Interpretation of axial images:- An important consideration when interpreting CT angiography studies is that all the diagnostic information contained within the study is present on the axial source images, whereas the same cannot be said for the various 3D reconstructions that may be generated.

For this reason, it is imperative that the axial source images be carefully reviewed in all cases in order to detect vascular pathology and incidental, nonvascular abnormalities. To improve the efficiency of CT angiography interpretation, thicker axial slices (eg, 5- to 7-mm thick) can be reconstructed from the CT data for the purpose of evaluating the nonvascular structures.

D) Image transfer:- Once the scan is complete and the data are reconstructed, the information is typically transferred over the network to an imaging workstation. The time for data transfer will obviously be dependent on the type of network used and the volume of information transferred over the network. Once the information reaches the workstation, the post-processing or image analysis step truly begins.

E) 3D reformatting techniques:- a) Multi Planer Reformation (MPR). b) Curved planer reformation (CPR) . c) Shaded-surface display (SSD). d) Maximum Intensity Projection (MIP). e) Volume Rendering (VR).

a) Multi Planar Reformation (MPR) : Multiplanar reformation in which from a given angle of view a plane is reconstructed in a defined depth of the volume

Multi-planar reformations are sagittal, coronal and oblique single voxel thick images created from stacked transverse sections. Multi planar reformations preserve all of the relative attenuation information and don’t suffer from the limitations that overlapping vessels have on 3D renderings.

They are valuable in visualizing the true walls of blood vessels, their lumen and the interior of metallic stents. This technique is operator dependent, so inclusion or exclusion of voxels due to inaccurate plane selection can result in misdiagnosis.

b) Curved planer reformation (CPR) : Curved planar reformations display the entire course of a vessel through a 3D volume in a collapsed 2D image. Points are designated in the center of the structure of interest on each of a stack of transverse images. This collection of points defines a curved line, containing the vessel of interest, from which various curved planes can be defined. This curved plane is then projected onto a flat 2D image.

CPR images are useful for the evaluation of vessel lumen, vessel walls, and immediately adjacent structures, thus making them useful for evaluating vessels for stenoses and for quick communication of information regarding the extent of other processes such as aneurysms, intramural hematoma, vasculitis, and dissections.

CPR images are highly operator dependent meaning that inaccurate in the selection of points that define a vessel may result in artifactual pseudo-stenosis. Also, CPR images are only one voxel thick; thus, small or thin structures such as a dissection flap may be missed. To overcome this at least two CPR images of a given structure in orthogonal planes should be acquired.

c) Shaded-surface display (SSD) Surface rendering was one of the earliest methods of 3D display. In this method, each voxel within the data set is determined to be a part of or not a part of the object of interest, usually by comparing the voxel intensity to some user-chosen threshold value.

This defines the “surface” of the object This defines the “surface” of the object. With the surface determined, the rest of the data are discarded. Surface contours are typically modeled as a collection of polygons and displayed with surface shading. By converting the data from a volume to a surface, a large portion of the data available is forfeited in exchange for faster, easier computation.

The thresholding assignment of the voxels that will be visible is both critical and sometimes difficult to reproducibly define. If the thresholding process is too aggressive, actual protruding structures can be lost from view because of partial volume effects for example “clipping” of the edges of vessels resulting in pseudostenoses. If the thresholding process is too lax, nontissue materials (eg, calcified plaques) can be rendered as if they were tissue, causing masking of stenoses.

d) Maximum Intensity Projection (MIP): maximum intensity projection images which means that from a given angle of view only the brightest voxels of a volume are collected and used to create an image

In the maximum-intensity projection method, viewing rays are traced from the expected position of the operator through the object to the display screen, and only the relative maximum value detected along each ray path is retained by the computer and displayed on the resultant 2D image . This method tends to display bone and contrast material–filled structures preferentially, and other lower-attenuation structures are not well visualized.

The presence of such structures especially bones often requires extensive manual editing of the dataset prior to MIP rendering, a process that can turn a theoretically simple process into a tedious one, particularly in regions of complex vascular and bony anatomy such as the extremities. This attribute of MIP images can be used in preoperative evaluation of patients with atherosclerotic occlusive disease by mapping the global distribution of arterial calcification.

One of the drawbacks of MIP images as compared to conventional angiography is its lack of depth information resulting in ambiguity in interpreting overlapping vessels. The resulting images are typically not displayed with surface shading or other devices to help the user appreciate the "depth" of the rendering, making three-dimensional relationships difficult to assess.

e) Volume Rendering (VR) : Volume rendering is to select several groups of voxels according to their attenuation in the Hounsfield units and to assign them a color and a so –called opacity.

Volume rendering has replaced most earlier applications of surface rendering with a few notable exceptions (ie, virtual colonoscopy and bronchoscopy and interior vessel analysis). In volume rendering, the CT numbers that make up the image are assigned to be either visible or invisible, to be displayed with varying colors, and often to be displayed with varying opacity levels (transparency).