Aus: MR Angiography with Vasovist. Mathias Goyen (Ed.) ABW Wissenschaftsverlag GmbH, Kurfürstendamm 57, Berlin. Image post-processing

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1 Image post-processing

2 2 Image post-processing Joachim Graessner 2.1 Introduction Image post-processing for the evaluation and diagnosis of MR angiography (MRA) using intravascular contrast agents with a large number of acquired images (approx ) has the task of sorting and structuring for diagnostic purposes, compressing for documentation, preparing for quantitative evaluation. Depending on the capability of the software, certain working steps can be integrated at the measurement protocol stage to relieve the user of time-consuming work and to provide MIP images for immediate quality control of the examination: online subtraction of post contrast and unenhanced series of the first pass bolus, online maximum intensity projection (MIP) of the subtracted series in the main orientations (coronal, sagittal, transversal), online standard deviation (STD) calculation of dynamic or triggered series to accentuate the vascular system (largest change in signal within the image series leads to the highest intensity in the resulting image), composing of morphological series and MIP images at different stages. Figure 2.1 shows a typical whole-body MRA procedure including the optional postprocessing steps. The measurement process can, of course, vary depending on the investigation and may be supplemented with special measurements following administration of contrast agent. The start of the post-contrast measurement can also take place as a direct observation of the entire bolus (CareBolus, SmartPrep, BolusTrak). The blood pool measurements are of high resolution with isotropic or almost isotropic voxel size and measuring times of several minutes. Non-enhanced measurements for these series for background subtraction are usually not performed owing to the time required and the risk of subtraction errors that exist due to movement of restless patients. 2.2 Image post-processing techniques Introduction Since the introduction of tomographic techniques in medical imaging, the tools available for post-processing and data presentation have expanded continuously. MR and CT have mutually enhanced each other in the application and utilization of the various techniques. As a result of their variable data quality, CT images with their normalized Hounsfield units for 22

3 Preparation Measurement Post-processing Register Patient Localisation and slice positioning Opt.: manual subtraction 4 5 stages w/wo 3D elastic Corr. Position patient and colls Determination of transit time 3D MIP/MPR/VRT Setup online post-processing Viewing first pass and blood-pool phase Pre-CM measurement 4 5 stages CM administration with timing Vesselview and arteryvein separation Composing Post-CM first pass 4 5 stages (Fly through) blood-pool phase measurement 4 5 stages Figure 2.1 Workflow of whole-body MR angiography. Documentation more complex SSD programs such as VRT (volume rendering technique), are easier to segment; MR images, as simple intensity images, do not offer such sharp segmentation boundaries, however, this does not exclude the application of this technique. In the special case of MR angiography, the lumen of the vascular system is accentuated with a contrast agent thereby simplifying post-processing. The greatest challenges in post-processing MRA images include the correct depiction of the vascular lumen, optimal suppression of the tissue background signal to achieve high contrast, and good separation of arteries and veins. Maximum intensity projection (MIP) and multiplanar reconstruction (MPR) with their sub-variants represent the most important programs in 3D post-processing of MRA data. The volume rendering technique (VRT) and surface-shaded display (SSD) supplement imaging with a three-dimensional aspect, which is very advantageous for assessing the positional relationship of vessels. Table 2.1 provides an overview of the techniques explained later on, their resulting output and a subjective assessment of the importance of each technique MIP and Thin MIP The maximum intensity projection (MIP) [1, 2] is the most important tool used for postprocessing MR angiograms. The MIP image delivers a similar impression as a conventional X-ray angiogram. The MIP data can be presented in positive contrast (bright lumen, dark background) or inverted (dark lumen, bright background). 23

4 Table 2.1 Overview of the techniques and results Technique Image Result Importance 1 Subtraction Vessel data set with signal reduction of the background + 2 MIP Vessel image + 3 Thin MIP Vessel image of a thin slice of the 3D volume + 4 MPR Multiplanar reconstruction + 24

5 Table 2.1 Continuation Technique Image Result Importance 5 Thick MPR Vessel image of a thick slice of the 3D volume ~ 6 VRT 3D visualization of the vessel tree 7 Thin slice VRT VRT of a partial volume 8 Vessel analysis/ Vesselview Mapping of vessel sections to determine degree of stenosis 25