2009, Vol.47, Issues 2, Imaging of Airway Diseases.pdf Imaging of Airway DiseasesPrefacePhilippe A. Grenier, MD Guest EditorIn the past 20 years, remarkable technologic advances in CT imaging have revolutionized noninvasive imaging of all thoracic structures, including the airways. CT has assumed a central position in the modern management of both focal and diffuse airway diseases. The combination of thin collimation and helical acquisition during a single breath-hold at full inspiration multidetector CT (MDCT) provides high-resolution volumetric data sets that allow the generation of high-quality multiplanar and three-dimensional images of the airways. Accurate assessment and anatomical display of proximal and distal airways are routinely obtained. In addition, dynamic acquisition during a forced expiratory maneuver is highly appreciated to detect and assess obstruction on small airways and abnormal collapse of large airways. Nowadays, MDCT is used not only to detect neoplastic and non-neoplastic endotracheal and endobronchial lesions and to assess the extent of tracheobronchial stenosis for planning treat- ment and follow-up, but also to diagnose and assess the extent of bronchiectasis and small- airway disease, and, in addition, to detect bron- chial fistula, cysts, or dehiscences. In parallel, there have also been important advances in diagnostic and interventional bronchos- copy and surgery. In this respect, the information provided by CT has become increasingly essential for establishing accurate diagnoses, for guiding and planning procedures, and for assessingRadiol Clin N Am 47 (2009) xi doi:10.1016/j.rcl.2009.01.007 0033-8389/09/$ – see front matter ª 2009 Elsevier Inc. Allresponse to therapy. Recent improvement in image analysis techniques has made possible accurate and reproducible quantitative analysis of airway wall and lumen areas, as well as lung volume and attenuation, leading to better insights in physiopa- thology of obstructive lung disease, particularly chronic obstructive pulmonary disease and asthma. The authors of this issue of Radiologic Clinics of North America were chosen for their focused expertise in airway imaging. I thank those chest radiologists for sharing their experience and insights to provide a comprehensive update on practical imaging for airway disease. While high- resolution CT and MDCT have tremendously improved our ability to assess large- and small- airway diseases, I anticipate even more develop- ments in the future. Rapid-volume scanning and new postprocessing techniques may promote sophisticated functional imaging and advanced interventions. MR imaging in this respect may become an additional modality to MDCT. Philippe A. Grenier, MD Service de Radiologie Polyvalente Diagnostique et Interventionnelle Hopital Pitie-Salpetriere 47-83, boulevard de I’Hopital 75651 Paris cedex 13, France E-mail address: philippe.grenier@psl.aphp.fr (P.A. Grenier)rights reserved. ra di ol og ic .th ec li ni cs .c om MDCT of the Airways: Technique and Normal Results Catherine Beigelman-Aubry, MDa, Pierre-Yves Brillet, MD, PhDb, PhilippeA. Grenier, MDa,*KEYWORDS  Trachea anatomy  Bronchi anatomy  Airway dimensions  Airway MDCT techniquePreviously, the trachea and main bronchi were as- sessed with a variable slice thickness up to 5 mm with sequential or volumetric CT, and small airways diseases were explored with high-resolu- tion CT (HRCT), based on a 1.5-mm slice at 10-mm intervals. Currently, the new generation of multidetector CT (MDCT) by combining volumetric CT acquisition and thin collimation during a single breathhold provides an accurate continuous assessment from the trachea to the most distal airway visible. Isotropic voxels allow image recon- structions in which the z dimension is equivalent to the x and y (in plane) resolution.1 This approach creates multiplanar reformations of high quality along the long axis of the airways2 and three- dimensional volume rendering, including extrac- tion of the airway and virtual endoscopy without any distortion in any orientation. Whatever their nature and severity, excellent assessments of stenoses may be obtained by a combination of various reconstructions, especially the determina- tion of the morphology, including the identification of horizontal webs and the length and exact loca- tion from the vocal cords and carina.3,4 Airway stents and extrinsic airway compression are also assessed perfectly. Preprocedural planning before stent placement or surgery3 and posttherapeutic aspects also benefit from the same techniques. Despite images usually being obtained during suspended inspiration for analysis of airways, a complementary acquisition during forceda Service de Radiologie, Hôpital Pitié-Salpêtrière, Assist Pierre et Marie Curie, 47/83 boulevard de l’Hôpital, 7565 b Service de Radiologie, Hôpital Avicenne, Assistance Pu UPRES EA 2363, Hôpital Avicenne, 125, route de Stalingr * Corresponding author. E-mail address: philippe.grenier@psl.aphp.fr (P.A. Grenie Radiol Clin N Am 47 (2009) 185–201 doi:10.1016/j.rcl.2009.01.001 0033-8389/09/$ – see front matter ª 2009 Elsevier Inc. Allexpiratory maneuver may be requested to assess the degree of tracheobronchomalacia and the extent of air trapping.IMAGE ACQUISITION AND RECONSTRUCTION Because the lung parenchyma offers a unique natural contrast, low radiation dose may be used without significant loss of information (100–120 kV, 60–160 mAs). Using a detector size of 0.625 mm with MDCT, images are reconstructed with a slice thickness of approximately 1 mm and over- lapped with a reconstruction interval of approxi- mately one-half slice thickness. This produces a resolution voxel of almost cubic dimensions of approximately 0.4 mm in each direction by using a spatial resolution algorithm. Experts recommend using a 512 or even a 768 matrix, which permits fields of view of 265 mm and 400 mm, respec- tively. The pixel size at the workstation, which is defined as the ratio between the field of view and the matrix, has to be lower than the intrinsic reso- lution in the plane of image to benefit from the intrinsic resolution capabilities of the equipment.5 A rotation time of approximately 500 msec allows an important decrease in cardiac pulsation artifacts and allows a good analysis of all bronchi, including the paracardiac areas. Breathholding for acquisition of the entire chest lasts approximately 6 to 8 seconds using a 40 or 64 detector row CT scanner, which avoids respiratory motion artifactsance Publique-Hôpitaux de Paris (APHP), Université 1 Paris cedex 13, France blique-Hôpitaux de Paris (APHP), Université Paris XIII, ad, 93009 Bobigny, France r). rights reserved. r ad io lo gi c. th ec li ni cs .c om Beigelman-Aubry et al186in most cases. The use of cardiac gating is not rec- ommended because of the higher radiation dose delivered and short rotation time available with the last generations of MDCT.READING AND POSTPROCESSING TOOLS Cine Viewing Visualization of overlapped thin axial images sequentially in a cine mode allows analysis of bronchial divisions from the segmental origin down to the smallest bronchi that can be identified on thin section images. Particular attention must be paid to the analysis of the lumen of theFig. 1. (A) Down and backward 1.41-mm oblique reform bronchi, and some segmental and subsegmental bronchi 54 mm (C)—allows reproduction of previous tomographic pathology, especially for the airways. (D) Slab average of of the frontal chest radiograph. The right tracheal stripetracheobronchial tree, the airway walls, and the spurs at the same time. Moving up and down through the volume at the monitor has become a useful alternative to film-based review. This viewing technique helps indicate the exact loca- tion of any airway lesion and may serve as a road- map for the endoscopist. Reading of chest MDCT goes actually far beyond the standard assessment of axial slices, because multiplanar reformats are easily performed in real time in all directions6 and slabs with various rendering modes. Once any abnormality has been detected, an oblique refor- mat plane may be chosen with the swivel mode by focusing a rotation center on the abnormalityat allows visualization of the trachea, carina, main . Progressive thickening of the slabs—17 mm (B) and aspects with better understanding of the underlying 180 mm thickness allows reproduction of the aspect is clearly explained by the correlation on (A). MDCT of the Airways: Technique and Normal Results 187found. A combination of slabs of various thick- nesses with minimum intensity projection (mIP) or maximum intensity projection (MIP) or both usually is obtained. Two-Dimensional Reformats and Multiplanar Volume Rendering Slabs Reformations and reconstructions are easy to generate and may be interactively performed in real-time at the console or workstation. Multipla- nar reformation images are single-voxel sections with a 0.6- to 0.8-mm displayed image. They are the easiest reconstructions to generate and permit creation of images oriented in any plane, especially along the long axis of any airway (eg, in a coronal oblique orientation for the trachea and the carina). On the other hand, multiplanar volume reformation consists of a slab of adjacent thin slices of various thicknesses that may be combined with the use of intensity projection techniques. The reformation plane may be selected by focusing a rotation center on the abnormality and using the swivel mode or using a three-dimensional reconstructed image of the airways.7 A significant decrease in the number of slices to be analyzed is achieved by analysis of longitudinal reformats compared with the axial images with a complementary role of both viewing techniques. Analysis of various large and small airway diseases may be enhanced with this technique. In fact, multiplanar volume reformation images combine the excellent spatial resolution of multi- planar reformats images with the anatomic display of thick slices8 and the possibility of using various rendering tools:Fig. 2. Coronal mIP 60-mm slab allows display of the normal bronchial tree to the subsegmental level. Average: the mean attenuation value of the voxels in every view throughout the volume explored is projected on a two-dimensional image. A less noisy image may be obtained de facto. Tomographic equivalent images may be obtained by thickening the slabs with equivalent of plain films in the coronal and lateral views with the thickest slabs (Fig. 1).  mIP imaging is a simple form of volume rendering (sliding thin slab or multiplanar volume reformation mIP technique) that is able to project the tracheobronchial air column onto a viewing plane by projecting the pixels with the lowest attenuation value. This technique enhances the visibility of the airways within lung parenchyma below the subsubsegmental level because of lower attenuation of air contained within the tracheobronchial tree compared with thesurrounding pulmonary parenchyma (Fig. 2), with a difference of density between 50 and 150 HU.9 The overall morphology of the tracheobronchial tree is particularly well displayed on longitudinal views combined with a multiplanar volume reformation mIP technique. Three- to 7-mm slabs are partic- ularly adapted for the assessment of central airways stenosis, but the slab thickness may be chosen according to the complexity and morphology of the abnormality and may be increased up to several centime- ters. Abnormal lucencies, including bron- chial wall diverticula observed in patients who have chronic obstructive pulmonary disease and bronchial anastomosis, dehis- cence, or fistula during or after lung trans- plantation, may be assessed using the same technique. Multiplanar volume refor- mation mIP is also used for a systematic analysis of the parietal wall and lumen of the bronchi. This analysis also includes the assessment of peribronchial thickening encountered in case of lung diseases with a perilymphatic distribution. In chronic bronchial disease, bronchial wall thickening is often irregular and associated with thick- ening of the spurs and irregularities in the morphology and caliber of the bronchi. This technique may help plan the correct bronchoscopic pathway toward a distal lesion for biopsy. Postexpiratory mIP ...