Copyright #ERS Journals Ltd 2001 Eur Respir J 2001; 18: 381–392 European Respiratory Journal Printed in UK – all rights reserved ISSN 0903-1936 SERIES 0THORACIC IMAGING 0 . Edited by P.A. Gevenois, A. Bankier and Y. Sibille Number 6 in this Series Virtual tools for imaging of the thorax G.R. Ferretti, I. Bricault, M. Coulomb Virtual tools for imaging of the thorax. G.R. Ferretti, I. Bricault, M. Coulomb. #ERS Dept of Radiology, Hoˆpital Michallon, Journals Ltd 2001. Centre Hospitalier Universitaire, Gre- ABSTRACT: Helical computed tomography (HCT) allows for volume acquisition of noble Cedex 09, France. the entire thorax during a single apnoea. Combination of HCT acquisition with Correspondence: G.R. Ferretti, Service synchronous vascular enhancement gives rise to HCT angiography (HCTA). Central de Radiologie et Imagerie Me´d- In the last decade, HCT and HCTA have revolutionized the diagnosis of thoracic icale, Centre Hospitalier Universitaire, diseases, modifying many diagnostic algorithms. Because HCT provides for a true Grenoble BP 217, F-38043 Grenoble volume acquisition free of respiratory misregistration, three-dimensional (3D) rendering cedex 09, France. techniques can be applied to HCT acquisitions. Fax: 33 476765901 As these 3D rendering techniques present the HCT information in a different format to the conventional transaxial CT slices, they can be summarized as virtual tools. Keywords: Computed tomography The purpose of this review is to give the readers the most important technical aspects helical computed tomography maximum intensity projection of virtual tools, to report their application to the thorax, to answer clinical and scientific minimum intensity projection questions, and to stress their importance for patient management, clinical decision virtual rendering making, and research. volume rendering Eur Respir J 2001; 18: 381–392. Received: October 10 2000 Accepted after revision February 21 2001 Volume acquisition of the thorax during one perception of the anatomy. These reconstructions breath-hold is possible using helical computed tomo- include multiplanar reformation (MPR), maximum graphy (HCT) [1]. HCT of the thorax has become a intensity projection (MIP) and minimum intensity major noninvasive technique for the evaluation of the projection (IPmin), three-dimensional surface shaded airways, the mediastinum, the major thoracic vessels, display (3D SSD), and volume rendering (VR). the chest wall, and, in some instances, the pulmonary The purpose of this review is to give the readers the parenchyma [2–4]. It has replaced conventional or most important technical aspects of 3D virtual slice-by-slice computed tomography (CT) acquisition, images, their strengths and weaknesses, to report except for high resolution computed tomography their application to clinical and scientific questions, (HRCT) of the pulmonary parenchyma. Because and to stress their importance for patient manage- HCT provides a thorough volumetric acquisition of ment, clinical decision making, and research. the thorax, HCT acquisitions can be reconstructed as contiguous or overlapped transaxial CT images (the conventional way to display CT images) or as three- Technical aspects dimensional (3D) images by using various 3D soft- ware [5–7]. 3D reconstructions are now widely HCT acquisition is a major technical evolution of available and have gained popularity among radio- CT that was introduced in the early 1990s [1]. It logists who use them to answer specific clinical combines the continuous rotation of the X-ray tube problems [8]. Although 3D images contain no more with the advancement at a constant speed of the information than the axial CT images, they provide patient through the gantry during the emission of information in other formats that can enhance the X-rays. Therefore, the X-ray beam describes a helical Previous articles in this series: No. 1: Ghaye B, Dondelinger RF. Imaging guided thoracic interventions. Eur Respir J 2001; 17: 507–528. No. 2: Vansteenkiste JF, Stroobants SG. The role of positron emission tomography with 18F-fluoro-2-deoxy-D-glucose in respiratory oncology. Eur Respir J 2001; 17: 802–820. No. 3: Kauczor HU, Chen XJ, van Beek EJR, Schreiber WG. Pulmonary ventilation imaged by magnetic resonance: at the doorstep of clinical application. Eur Respir J 2001; 17: 1008–1023. No. 4: Hansell DM. Small airways diseases: detection and insights with computed tomography. Eur Respir J 2001; 17: 1294–1313. No. 5: Franquet T. Imaging of pneumonia: trends and algorithms. Eur Respir J 2001; 18: 196–208. 382 G.R. FERRETTI ET AL. course around the patient and gives rise to a volume Image processing of data. Virtual tools can be divided into techniques that create planar images such MPR and techniques that produce 3D images. Helical computed tomography protocols Multiplanar reformation. MPR is the most popular Whatever software is used, generating 3D images tool in clinical practice [7]. It consists of creating requires three main steps: data acquisition, data images of 1 voxel thick in any arbitrary plane of processing, and data display. the volume of acquisition. MPRs eliminate super- imposition of voxels lying outside the selected plane. Planar or curvilinear MPR can be interactively and Data acquisition instantaneously created along a structure of interest. Images are displayed in the conventional Hounsfield9s The choice of HCT parameters is of paramount scale, which precludes the loss of information due to importance in generating accurate visualization of the segmentation. thoracic anatomy. Using different scanner models, many reports [9, 10] have emphasized the importance Three-dimensional reconstructions. 3D reconstruction of selecting the thinnest collimation, pitch value (ratio software uses three main algorithms: maximum and of the table increment per rotation and slice collima- minimum intensity projection, surface rendering, and tion) around 1, and to reconstruct axial slices with an VR. overlap of ¢30%. In clinical practice, the radiologist has to find a compromise between the slice thickness Maximum intensity projection and minimum intensity and the pitch value compatible with the acquisition of projection. MIP and IPmin are based on the same the thoracic volume of interest during a single breath- principal: imaginary rays are projected through a hold. Of these parameters, slice thickness has the subvolume of original data set [12]. The rays are greatest impact on the quality of 3D-reconstructions parallel (orthogonal projection) and select some [10]. Therefore, using state of the art single detector specific voxels along their route inside the volume. HCT scanners with subsecond rotation time (0.75 or MIP algorithm selects the voxel with the highest 0.8 s), an acceptable compromise is reached using attenuation along each ray, while IPmin algorithm 2-mm or 3-mm collimation, a pitch value ranging from selects the voxel with the lowest attenuation along 1.5–2.30% to 50% overlap index, and a low spatial the rays. This process is instantaneous. The final resolution algorithm. In some cases, a HCT scanner image is, therefore, a projection of the highest or using thinner collimation (1 mm) is required to lowest attenuation voxels of the initial volume with analyse the lung parenchyma or the subsegmental no depth information. MIP has been the most popu- bronchi. Introduction of the multiple detector-row lar method of obtaining CT angiographic images technology [11] permits the performance of faster [13]. acquisitions for a given volume while using thinner MIP and IPmin algorithms have been applied to collimation, thereby increasing the overall quality of high-resolution (1 mm) volume acquisition of a 3D simulations. Using this new technology, it is now selected volume of lung parenchyma without contrast possible to acquire the entire thorax during one media injection [14–16]. This technique is called apnoea with 1-mm slice sections. sliding thin slab (STS), or multiplanar volume recon- Contrast media administration is usually needed to struction (MPVR), MIP or IPmin. The main value of analyse the vessels of the thorax and gives rise to HCT both STS MIP and STS IPmin is that they extract angiography. A review of the protocols of contrast specific information based on density value within media administration is beyond the scope of this the volume. The main drawbacks of MIP, STS, MIP, paper and can be found in a recent state of the art and STS IPmin are the absence of depth information, study [4]. Analysis of the airways does not require the superimposition of complex anatomic structures, intravascular contrast injection, because of the great and the loss of density information [14]. natural contrast between the airways and their environment. Three-dimensional surface shaded display. 3D SSD Radiation dose to the patients is a great concern is the most frequently used algorithm to create 3D and should be minimized at the lowest level possible views of the central airways, the pulmonary vessels, without making compromises in image quality. and the thoracic wall. The goal of the 3D SSD Because of the great natural contrast (air-tissue inter- algorithms is to isolate the surface of an anatomical faces) of the thorax, low kilovoltage (100–120 kV) structure of interest from the initial volume of data. and milliamperage (100–150 mA) can be used. HCT The voxels constituting the structure to display are offers the opportunity to reduce the radiation dose isolated from the surrounding voxels based on their as compared to slice-by-slice contiguous