Magnetic resonance angiography (MRA) is a medical imaging modality used to reveal the shape of vessels for diagnosis and therapeutic purposes. This technique receives much attention because it is non-invasive and provides three-dimensional (3D) data sets as opposed to the planar or two-dimensional (2D) projections of conventional x-ray digital subtraction angiography (DSA) [1-7]. Like DSA, contrast-enhanced MRA (CE MRA) uses contrast agents to enhance the vascular lumen.

The term post-processing refers to a vast number of image manipulation techniques that facilitate the assessment of arterial and venous structures at an independent console. It refers to all operations from data transfer and image visualization to automatic quantification of vessel lesions. For accurate image interpretation, knowledge of the available image processing tools is mandatory. A varietyof reformatting techniques are now available, and it is advantageous to be well versed in as many of these as possible. Each technique has its own strengths and weaknesses, which can lead to pitfalls and artifacts in inexperienced hands. The main challenges for MRA image processing include proper visualization of vessel lumen, opti mized thresholding of vessel-to-background image contrast, and arterial-venous separation. The most widely available methods for post-processing MRA data sets are multiplanar reformatting (MPR), maximum-intensity projection (MIP), subvolume MIP, surface-rendering (SR), volume-rendering (VR) and virtual intraluminal endoscopy (VIE).

This article will focus on the three main areas of the MRA post-processing systems: data handling, image visualization and vascular analysis (Fig. 1).

Data Handling

Any post-processing system for medical images should be compatible with the DICOM (Digital Imaging and Communications in Medicine) standard [8-12]. DICOM defines a complex communication protocol and requires that two connected workstations agree on the exchanged services. The unit requesting a service is called a "Service Class User" (SCU) and the unit supplying the requested service is called the "Service Class Provider" (SCP). Typically, a SCU (a post-processing workstation) performing a "Query/Retrieve" on a SCP (an imag

Data handling Image Vascular analysis * \ \

C A Dicom viewer

Multiplanar reformatting Maximum intensity projection Surface rendering Volume rendering Virtual intraluminal endoscopy

Vessel tracking Automatic quantification V )



Fig. 1. Tools for the MRA post-processing

Fig. 1. Tools for the MRA post-processing ing unit) first asks for a list of patients; then, once the patient is locally selected, the SCU asks for a set of images (series). Thereafter, the SCP initiates transfer through the network and the SCU locally stores the received files on a local hard disk. DI-COM also defines the file format in which the images are encoded, allowing local visualization and post-processing of the data with no loss of quality. A DICOM file consists of two parts: 1) a header containing a set of Data Elements (Patient Name, Acquisition Protocol, Image Matrix, Pixel Size, etc.), and 2) the original pixel/voxel data.

Initially, DICOM functions were limited to the exchange of data between proprietary workstations. Recently, however, some DICOM-compliant applications have become available on personal computers and their easier diffusion allows medical institutions or laboratories to perform specific post-processing tasks (measurements, MPR, MIP, archiving) with lower investment. Some of these applications can provide SCP/SCU functions [13].

Many general purpose post-processing systems are commercially available. There is a great variety of programs called "Dicom Viewers" which include the data handling and the image visualization functions (e.g. eFilm Workstation [13], DicomEye [36], Osiris [37], MediMatic Dicom Viewer [38], DicomWorks etc.). More sophisticated post-processing algorithms are available on independent workstations (e.g. Vitrea [21],Advantage Windows [39], Leonardo Workstation [40], EasyVision [41], etc.). However, none of those was designed specifically for use on standard office personal computers. Few of the available software packages, moreover, include dedicated quantitative vascular analysis tools.

In response to the specific medical need for MRA-targeted image analysis software, we have developed our own post-processing program called MARACAS (MAgnetic Resonance Angiog-raphy Computer ASsisted analysis). It is an interactive software package for visualization and analysis of blood vessels using 3D MRA data which provides automated quantification of the arterial stenosis.

Image Visualization

Although assessment of source images acquired by the scanner gives access to a precise local information on vessel diameter, area, etc., mental reconstruction of the global morphology and appreciation of spatial relationships is often difficult. Moreover, this local information may not be available when the slices are not perpendicular to the vessel of interest [14]. Several post-processing techniques are available to improve rendering of the overall morphology and/or the spatial relationships of the anatomical structures. Their purpose is to allow the user to simultaneously view the MRA data sets in different display formats such as maximum intensity projections, multiplanar reformats, surface and volume rendered images and virtual intraluminal endoscopic views. This section briefly explains the principles of these techniques and discusses their principal advantages and disadvantages.

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  • Tanja Vogel
    How to postprocess MRA image?
    8 years ago

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