ct血管造影对动脉瘤的诊断

EDUCATION EXHIBIT 637 CT Angiography of Intracranial Aneu- rysms: A Focus on Postprocessing1 LEARNING OBJECTIVES FOR TEST 1 After reading this article and taking the test, the reader will be able to: � Describe the tech- nique of CT angiog- raphy performed for detection of intracra- nial aneurysms. � List the technical considerations in and limitations of the most common meth- ods of image postpro- cessing. � Discuss when to use CT angiography as the sole diagnostic test in a patient with subarachnoid hemor- rhage before therapy. Bernd F. Tomandl, MD ● Niels C. Ko¨stner ● Miriam Schempershofe Walter J. Huk, MD ● Christian Strauss, MD ● Lars Anker, MD ● Peter Hastreiter, PhD Computed tomographic (CT) angiography is a well-known tool for detection of intracranial aneurysms and the planning of therapeutic intervention. Despite a wealth of existing studies and an increase in image quality due to use of multisection CT and increasingly sophisti- cated postprocessing tools such as direct volume rendering, CT an- giography has still not replaced digital subtraction angiography as the standard of reference for detection of intracranial aneurysms. One rea- son may be that CT angiography is still not a uniformly standardized method, particularly with regard to image postprocessing. Several methods for two- and three-dimensional visualization can be used: multiplanar reformation, maximum intensity projection, shaded sur- face display, and direct volume rendering. Pitfalls of CT angiography include lack of visibility of small arteries, difficulty differentiating the infundibular dilatation at the origin of an artery from an aneurysm, the kissing vessel artifact, demonstration of venous structures that can simulate aneurysms, inability to identify thrombosis and calcification on three-dimensional images, and beam hardening artifacts produced by aneurysm clips. Finally, an algorithm for the safe and useful applica- tion of CT angiography in patients with subarachnoid hemorrhage has been developed, which takes into account the varying quality of equip- ment and software at different imaging centers. ©RSNA, 2004 Abbreviations: DSA� digital subtraction angiography, dVR� direct volume rendering, FOV� field of view, ICA� internal carotid artery, MCA�middle cerebral artery, MIP�maximum intensity projection, MPR�multiplanar reformation, PICA� posterior inferior cerebellar artery, SAH� subarachnoid hemorrhage, SSD� shaded surface display, 3D� three-dimensional, 2D� two-dimensional Index terms: Aneurysm, CT, 17.12116, 17.73 ● Aneurysm, intracranial, 17.73 ● Computed tomography (CT), angiography, 17.12116 ● Computed tomography (CT), image processing, 17.12117 RadioGraphics 2004; 24:637–655 ● Published online 10.1148/rg.243035126 1From the Department of Neurosurgery, University of Erlangen-Nuremberg, Schwabachanlage 6, D-91054 Erlangen, Germany. Presented as an edu- cation exhibit at the 2002 RSNA scientific assembly. Received May 5, 2003; revision requested June 13 and received July 30; accepted July 31. All au- thors have no financial relationships to disclose. Address correspondence to B.F.T. (e-mail: tomandl@neuroradiologie-erlangen.de). ©RSNA, 2004 R a d io G ra p h ic s CME FEATURE See accompanying test at http:// www.rsna.org /education /rg_cme.html Introduction Sudden onset of vigorous headache typically is the leading symptom in patients with subarach- noid hemorrhage (SAH) caused by the rupture of an intracranial aneurysm. Computed tomography (CT) is the first step in the examination of these patients. Once SAH is confirmed, it is paramount to detect the source of bleeding in order to initiate therapy. Digital subtraction angiography (DSA) is still the most sensitive tool for the detection of intracranial aneurysms. The selective intraarterial injection of contrast medium ensures optimal en- hancement of the intracranial arteries with supe- rior resolution compared with that of CT or mag- netic resonance (MR) angiography. However, DSA has the disadvantage of being an invasive study. The risk of acquiring a permanent neuro- logic deficit with cerebral angiography in patients with SAH is below 0.1% (1). Despite this rela- tively low risk, a noninvasive method yielding three-dimensional (3D) information for the plan- ning of therapeutic intervention is desirable. The reported sensitivity of CT angiography lies in the range of 80%–97% (2–8) depending on the size and location of an aneurysm (3). In all of these studies, some kind of 3D visualization was used to analyze the CT angiography data. Little is known about the influence of postprocessing methods like maximum intensity projection (MIP), shaded surface display (SSD), and direct volume rendering (dVR) on the detection rate of intracranial aneurysms. However, it can be as- sumed that the same CT angiography data may lead to varying detection rates when different vi- sualization strategies, computer platforms, and graphics hardware are used (9–12). In the first section of this article, technical as- pects of CT angiography with a focus on data ac- quisition are discussed. In the second section, different methods for the postprocessing of CT data are presented, including the analysis of source images and the methods currently avail- able for two-dimensional (2D) and 3D postpro- cessing, such as high-resolution dVR. Then, typi- cal pitfalls encountered while working with CT angiography data are demonstrated. Finally, we propose a reasonable paradigm for the use of CT angiography in patients with SAH, taking into account that this method is still not a standard- ized procedure. Technique of Intra- cranial CT Angiography CT angiography can be defined as a fast thin-sec- tion volumetric spiral (helical) CT examination performed with a time-optimized bolus of con- trast medium in order to enhance the cerebral arteries (13). In order to visualize the intracranial arteries, the examination includes the region from the first vertebral body up to the vertex. It is im- portant to include the atlas in the study to ensure incorporation of the posterior inferior cerebellar artery (PICA), which has an extracranial origin from the vertebral arteries in about 18% of cases (14). On our four-row multisection scanner (Soma- tom 4 Volume Zoom; Siemens Medical Solu- tions, Erlangen, Germany), we used the following parameters: 120 kVp, 200 mAs, collimation of 4� 1 mm, table feed of 2.7 mm per rotation, and rotation time of 0.5 seconds. Image reconstruc- tion parameters were as follows: section thickness of 1.25 mm, overlapping steps of 0.5 mm, and field of view (FOV) of 120 mm2. The applied narrow FOV of 120 mm2 leads to an excellent in-plane resolution (0.23� 0.23 mm2) and repro- duces all relevant information (Fig 1). In addi- tion, lateral parts of the skull are already elimi- nated, which simplifies the postprocessing of source data. It is possible to perform reconstruc- tions in steps of 0.23 mm to produce isotropic data (15), thus yielding voxels of equal extent in all three dimensions. In our experience, this does not noticeably increase image quality while dou- bling the number of source images, thus leading to an extension of time spent on postprocessing source data. For enhancement of intracranial arteries, 100 mL of contrast medium (Ultravist 300; Schering, Berlin, Germany) was injected intravenously at a flow rate of 4 mL/sec by using a power injector (EnVision CT injector; Medrad, Indianola, Pa). A bolus tracking method (16) was used routinely to achieve optimal synchronization of contrast medium flow and scanning. Once the injection is started, the bolus tracking software measures at- tenuation values within one internal carotid artery (ICA), and the spiral scan is automatically started as soon as a threshold of 100 HU is exceeded. If bolus tracking is not available, the test bolus method should be applied to calibrate timing of the data acquisition (17): Ten seconds after bolus injection of 20 mL of contrast medium, a dy- namic single-axial-section study (one scan every 638 May-June 2004 RG f Volume 24 ● Number 3 R a d io G ra p h ic s 2 seconds) at the level of the first cervical verte- bral body is started until the contrast material appears as hyperattenuating spots in the ICAs. By using this technique, the time interval between bolus administration and the beginning of data acquisition can also be determined individually. Analysis of CT Angiograms The examples shown in this article were created with the regular software of the workstation sup- plied with a Somatom Volume Zoom CT scanner (Syngo Wizard version VA 40C; Siemens Medical Solutions). The dVR images were created on a separate workstation (Syngo Leonardo 2002B; Siemens Medical Solutions). Prior to any kind of postprocessing, such as 3D visualization, a detailed review of the source im- ages that are the basis of CT angiography is man- datory (18). These source images contain the en- tire information that is available from the data. Even the most sophisticated methods for 3D im- aging will lead to a distinct loss of data and, thus, potentially important information. Partial throm- bosis or calcification of an aneurysm will be missed if the source images are not reviewed in a meticulous way. The interactive analysis of the source images should be done on a workstation rather than by looking at hard copies in order to develop a better perception of the course and the relationships of the intracranial arteries of inter- est. A wide window setting is necessary to enable differentiation between arteries filled with con- trast medium, bone, and calcifications (Fig 2). Figure 1. Effect of FOV on image quality of 3D imaging with SSD in a patient with two aneurysms (arrows in b–d) at the bifurcation of the left middle cerebral artery (MCA). (a) CT image shows the areas covered by three different val- ues for FOV: 200, 120, and 60 mm2. (b) SSD image obtained by reconstructing the data with an FOV of 200 mm2. Arteries appear blurred. (c) SSD image obtained by reconstructing the data with an FOV of 120 mm2. Vascular anatomy is shown more clearly than in b. This FOV contains all relevant intra- cranial arteries from which an- eurysms usually originate while providing good in-plane resolu- tion. Thus, we always use this FOV for detection of intracra- nial aneurysms. (d) SSD image obtained by reconstructing the data with an FOV of 60 mm2. There is even better demonstra- tion of vascular anatomy than in c. It is sometimes useful to perform a second reconstruc- tion with a narrow FOV such as this when CT angiography is used for therapy planning and very detailed information is re- quired. RG f Volume 24 ● Number 3 Tomandl et al 639 R a d io G ra p h ic s Figure 2. Importance of an adequate win- dow setting to demonstrate the intracranial arteries within the skull base. CT image ob- tained with a window width of 500 HU and a center of 150 HU. Both ICAs can be clearly differentiated within the carotid canals (ar- rows). Figure 3. Preparation of the volume for analysis. (a) Posterosuperior image shows large veins (arrowheads), which are typically also visible in CT angiography of intracranial vessels and preclude an unobstructed view of the circle of Willis and the basilar artery (ar- row). (b) Left lateroposterior image shows easy elimination of the most obscuring venous structures by using a clip plane (dotted white line) parallel to the clivus. (c) Posterosuperior image obtained after application of the clip plane (dotted white line) shows that the basilar artery is demonstrated completely (arrowheads). 640 May-June 2004 RG f Volume 24 ● Number 3 R a d io G ra p h ic s Many aneurysms can already be detected by analyzing the source images. Smaller aneurysms below 5 mm in diameter are often difficult to de- tect on the basis of source images alone. There- fore, several methods for 2D and 3D postprocess- ing have been developed that allow more detailed analysis and in addition an “angiographic” repre- sentation of CT angiography data. Postprocessing of CT Angiography Data The basic principle of 2D and 3D postprocessing is to input cross-sectional images into a computer and thereby to create a so-called volume. One can imagine that this procedure is like putting back together the pieces of a sliced potato. Once a vol- ume is created, several methods for 2D and 3D visualization exist (19). The easiest way to ana- lyze a volumetric data set is multiplanar reforma- tion (MPR), in which from a given angle of view a plane is reconstructed in a defined depth of the volume. This way it is possible to create coronal, axial, sagittal, as well as any kind of oblique sec- tions. The quality of the reconstructions depends on the voxel size. With the use of isometric data (ie, voxels have the same depth, length, and height), all images are of the same quality as the basic source images (20). In contrast to MIP and the 3D methods discussed later, the recon- structed planes contain all information that is contained in the source images. Therefore, MPR should always be the method of first choice for the further examination of CT angiography data (21). To create useful “angiographic” representa- tions from CT angiography data, it is always nec- essary to eliminate disturbing structures from the volume in order to ensure an unobstructed view of the circle of Willis and its related arteries. For this purpose, several graphical tools exist that vary depending on the software of the workstation used. To eliminate the straight sinus and other veins that always prohibit an unobstructed view of the basilar artery, a so-called clip plane can be applied parallel to the clivus (Fig 3). This kind of data manipulation is always necessary when MIP or 3D visualization of CT angiography data is performed by using one of the following methods. Maximum Intensity Projection The term maximum intensity projection (MIP) means that from any given angle of view only the brightest voxels of a volume are collected and used to create an image (22). Therefore, MIP is not a 3D method, as it creates 2D images in which voxels from different locations within the volume are collapsed into one plane. Thus, depth information is lost and it is not possible to tell whether a structure is located in the front or back on the basis of a single MIP image. Because calci- fications and bone are brighter than contrast ma- terial–filled arteries, it is possible to differentiate levels of attenuation (eg, to recognize a calcified artery). The use of MIP as a method to create CT an- giograms is limited due to the fact that the skull base has a much higher attenuation than the in- tracranial arteries and therefore has to be elimi- nated when MIP is used for image reconstruction (23). When dealing with intracranial aneurysms, it is often not possible to clearly depict the rela- tions of the aneurysm to its adjacent arteries. Fur- thermore, with the use of MIP, small aneurysms will often be missed as they are eclipsed by the signal of their parent vessels averaged into the same 2D plane (Fig 4). Figure 4. Influence of 3D visualization techniques on the detection of intracranial aneurysms. (a) MIP image (su- perior view) shows the bifurcation of the left MCA (arrow). Owing to the lack of depth information, the image does not allow visualization of two aneurysms at this site. (b, c) SSD (b) and dVR (c) images (superior views) of the bifur- cation of the left MCA show the two aneurysms (arrows). RG f Volume 24 ● Number 3 Tomandl et al 641 R a d io G ra p h ic s Figure 5. Analysis of CT angiography data with MPR and thin-section MIP. (a–c) Sagittal (a), coro- nal (b), and axial (c) MPR images show a small aneurysm at the bifurcation of the right MCA (arrow). Note the large intracerebral hematoma (arrowheads in a), which is usually not demonstrated on thresh- old-based 3D images. (d–f) Sagittal (d), coronal (e), and axial (f) MIP images obtained with thin sec- tions of 20 mm show the aneurysm more clearly (arrow) and show the intracerebral hematoma as well (arrowheads in d). 642 May-June 2004 RG f Volume 24 ● Number 3 R a d io G ra p h ic s In contrast to the two methods for 3D visual- ization described later, MIP is not threshold de- pendent and therefore is relatively easy to use. It has to be kept in mind that MIP uses only about 10% of the information contained in a given vol- ume. In our experience, MIP is of minor use for the creation of CT angiograms in order to search for and analyze aneurysms but is often very help- ful when used interactively on the workstation in thin sections of about 10–20 mm in addition to MPR (24) (Fig 5). In contrast to threshold-de- pendent methods, smaller arteries are displayed without user interaction (22). Threshold-dependent Methods for 3D Visualization Shaded surface display (SSD) and direct volume rendering (dVR) are more difficult to use than MIP. They require the user to define thresholds for the selection of voxels on the basis of their at- tenuation (measured in Hounsfield units). For SSD, typically upper and lower thresholds are defined and from a chosen angle of view the first layer of voxels with an attenuation within the de- fined parameters is displayed. Therefore, the im- ages show the surface of these structures and pro- vide valuable information about the 3D shape of an object (25). On the other hand, all structures are shown in the same color and information about the attenuation of a structure is lost com- pletely. For example, it is not possible to see calci- fications within an artery on SSD images. Since MIP retains information about the attenuation of objects yet does not allow the depth perception provided by SSD, both methods may be used to complement one another. The definition of the thresholds is performed interactively by the user and significantly influ- ences the appearance of the vascular structures (7) (Fig 6). Setting the lower threshold to a low value (eg, 100 HU) will result in an image show- ing many vascular structures, including the veins and small arteries. When the lower threshold is increased (eg, to 200 HU), structures of low at- tenuation such as intracranial veins and small ar- teries will disappear completely and the major arteries will appear smaller. The “ideal” threshold to depict intracranial arteries has to be found in- teractively and depends on several parameters, including the injection rate of the contrast me- dium and cardiac output, both of which influence the attenuation of the contrast material–filled vas- culature (26). Figure 6. Threshold-dependent 3D visualization with SSD. (a) Superoposterior view obtained with a lower threshold of 100 HU shows smaller arteries like the left PICA (arrow) and venous structures (arrowheads). (b) Superoposterior view obtained by increasing the lower threshold to 200 HU shows arteries that appear thinner compared with those in a and even demonstrate dis- continuities (arrow). The venous structures are nearly eliminated (arrowheads), resulting in a less complex image. RG f Volume 24 ● Number 3 Tomandl et al 643 R a d io G ra p h ic s From the point of view of a computer scientist, MIP and SSD are types of “volume rendering” in which only one layer of voxels is used for visual- ization. In the medical literature, the term volume rendering is reserved for the technique described next, in which all voxels of a volume are consid- ered for visualization (27). Direct volume rendering (dVR) is the most sophisticated method for 3D visualization. The basic principle is to select several groups of voxels according to their attenuation in Hounsfield units and to assign them a color and a so-called opacity (28,29) (Fig 7). When dVR is used to create CT angiograms, the voxels of high attenuation con- taining information about bony structures are selected separately from those voxels with an at- tenuation bet