15 MDCT in Mediastinal Imaging
A. R. Hunsaker
A. R. Hunsaker, MD
Department of Radiology, Brigham and Women’s Hospital, Boston, MA 02115, USA
CONTENTS
15.1 Introduction 215 15.2 Imaging Techniques 215 15.3 Clinical Applications 217 15.3.1 Invasion 217
15.3.2 Assessment of Lymph Nodes 217 15.3.3 Extent and Origin of a Lesion 218 15.3.4 Tissue Characteristics 221 15.3.5 Airway Masses 221 15.4 Conclusion 221
References 224
15.1 Introduction
The usefulness of MDCT in imaging vascular, tra- cheobronchial, lung parenchymal and chest wall lesions has been described in the recent literature by several investigators (Lawler and Fishman 2001;
Ravenel et al. 2001; Remy-Jardin et al. 1998a, 1998b;
Bhalla et al. 1996; Johnson et al. 1996, 1998; Cal- houn et al. 1999; Kauczor et al. 1996). Very little has been written about the usefulness of MDCT in other non-vascular and non-airway mediastinal lesions as we have traditionally studied the mediastinum. In this chapter we show relevant clinical applications of MDCT in mediastinal lesions.
The mediastinum is composed of the heart, vessels, airways, esophagus, fat, lymph nodes and lymphatics, thymic tissue, and nerves. These normal structures, in isolation or in consort, can be involved in a variety of disease processes which originate directly from them, metastasize to them, or invade them from adja- cent structures. These disease processes can be fl uid, soft tissue, fatty material, or a combination of these.
Imaging of the mediastinum, therefore, is aimed
at providing relevant information regarding tissue characteristics, enhancement patterns, location of a lesion, and invasion of structures.
Computed tomography (CT) has been deemed the primary modality for imaging the mediastinum (Tecce et al. 1994), particularly the anterior medi- astinum; however, magnetic resonance imaging (MRI) has been used as a modality which clarifi es ambiguous fi ndings on CT (Bittner and Felix 1998;
Webb and Sostman 1992) because of its multiplanar capabilities its ability to image vessels, and its ability to distinguish between different tissues. Prior to the advent of spiral CT imaging in the late 1980s, MR imaging was very comparable to CT for evaluating the mediastinum (Bittner and Felix 1998; Webb and Sostman 1992) but lost equal ground in its use once spiral CT was introduced. Multidetector CT (MDCT) has the potential to put even further distance between it and MR imaging because of its ability to perform isotropic scanning and thin-section imaging through the entire thorax in a very short space of time and the capabilities of applying postprocessing multiplanar reconstructions and three-dimensional (3D) tech- niques. For this reason, all of the apparent advantages of MRI have been supplanted by MDCT.
15.2 Imaging Techniques
In general, MDCT offers several advantages over single-detector CT. One advantage is that the exami- nation can be performed with thinner sections, lead- ing to higher spatial resolution along the patient z- axis. Additionally, scanning can be performed much faster, resulting in improved temporal resolution and reduced motion artifacts (Rydberg et al. 2000;
Berland and Smith 1998). Intravenous contrast material is delivered at a higher fl ow rate, increasing the vascular enhancement of detected abnormalities.
Rydberg et al. (2000) found that these factors com-
bined to improve the spatial, temporal, and contrast
resolution of the images, signifi cantly increasing the diagnostic accuracy of an examination (Rydberg et al. 2000). These advantages are particularly useful for demonstration of subtle areas of tumor enhancement in demonstrating vascular supply of lesions preop- eratively (Fig. 15.1). Postprocessing techniques offer a great advantage in that postprocessing techniques can be adapted to the underlying disorder (Schoepf et al. 2001); thus, the focus in planning a CT study is directed away from the data acquisition and towards postprocessing techniques. From a single thin slice MDCT study, 2D multiplanar reformatted images, maximum intensity projection and minimum intensity projection images, and 3D imaging can be attained at a workstation. Interactive displays allow for demonstration of relationship between mass and normal mediastinal structures.
Although many postprocessing techniques are available, only those with potential usefulness in the mediastinum are described in order of their practical- ity from the least to the greatest. The fi rst techniques to be described are 3D maximum intensity projec- tion imaging (3D MIP) and 3D minimum intensity projection imaging (3D min-IP). The MIP images are created by a series of parallel rays projected through a volume of data. Only the highest attenuation voxels are made part of the image. In the process of creat- ing these images, over 95% of the original data is lost (Ravenel et al. 2001), because only the brightest
voxels or voxels with the highest attenuation values along a projection ray are used; thus, there is a ten- dency to misrepresent spatial relationships. For this reason, it is of limited usefulness in the mediastinum, particularly in imaging mediastinal vessels. The min-IP images are acquired in the same way as MIP images with the exception that the lowest attenuation voxels are projected to create an image.
A second technique is that of 2D multiplanar refor- matted images (2D MPR). With this technique MPR images can be created in any plane with the same spatial resolution as the original sections because of the isotropic acquisition of data. Reformatted images in the coronal, sagittal, and axial planes can be created from one spiral acquisition and have the same isotropic spatial resolution as sections from the original acquisition (Rydberg et al. 2000). This type of reconstruction is the simplest and most frequently performed reconstruction and is achieved by stack- ing the axial images and cutting them in different planes (Ravenel et al. 2001). One simply places a line in either the sagittal or coronal planes and then obtains reformations from these images. Oblique or curved reformatted images can also be applied along the axis of a lumen, thus decreasing interpretive errors (Ravenel et al. 2001). Thin-collimation and overlapping sections also decrease artifacts along the z-axis (Fleischmann et al. 2002). The MPR images are very useful in the mediastinum, as we show later,
Fig. 15.1a, b. Invasive thymoma. a Contrast-enhanced image shows an intensely enhancing nodular mediastinal mass with adja- cent atelectasis. b Color-enhanced 3D volume-rendered image effectively shows enlarged internal mammary vessels and other thoracic collaterals in this very vascular tumor. This preoperative information was invaluable to surgeons in surgical planning.
The extent of vessels were not demonstrated on the axial images
a b
in demonstrating craniocaudal extent of disease and possibly origin of disease. Most importantly, MPR images eliminate the need for direct imaging in the sagittal or coronal planes, thus eliminating increased radiation dose to the patient. In a study performed by Honda et al. (2002), the image quality of coronal multiplanar reconstructions from isotropic voxel data obtained using 0.5-mm collimation, with or without overlapping reconstruction, was found to be similar to that of direct coronal thin-section CT scans. This was found to be the case in a previous study by Caldemeyer et al. (1999).
The last technique of value to be discussed in the mediastinum is 3D imaging, which may be either surface-shaded display (3D SSD) or volume rendered. Three-dimensional surface-shaded tech- niques display a subset of the volumetric data by including voxels in a range of attenuation values to determine surface (Ravenel et al. 2001). These images are acquired by two methods. An attenua- tion range for reconstruction can be provided and the computer will generate an image, or images can be obtained by manually drawing around an object’s boundaries by selecting points in a data set from which the computer will automatically connect these areas (Ravenel et al. 2001). Since this is a shaded surface, one cannot „see through“ the images, and like the MIP displays, more than 90% of the data is lost making this technique not as useful as volume- rendering techniques. The 3D volume rendering, in contrast to 3D SSD, uses all the data in creating a fi nal image. Data from all voxels are summed and displayed as a composite image on the monitor. By changing parameters, the data can be segmented by attenuation values to display the area of interest such as vessels, airways, or chest wall (Ravenel et al. 2001). Unlike SSD, one can „see through“ with volume rendering.
In imaging a patient for suspected mediastinal dis- ease at our institution, patients are scanned on either a 4- or 16-channel multidetector scanner (Siemens, Malvern, Pa.). Images are acquired with thin sec- tions (1 mm on the 4-channel and 0.75 mm on the 16-channel scanners) through the entire thorax and then reconstructed at 5-mm slice thickness using a mediastinal algorithm. Most of our mediastinal pro- tocols require the intravenous administration of con- trast material. We use 100 cc of Ultravist 300 (Wayne, N.J.) injected at a rate of 3 cc per second following a 30-s scan delay. The data are then transferred to a workstation (Voxar, Edinburgh, Scotland, UK) where a trained technologist performs 2D MPR or 3D imag- ing as requested.
15.3 Clinical Applications
15.3.1 Invasion
Bronchogenic carcinoma is one of the leading causes of cancer death in the U.S. and imaging plays an important role in assessing prognosis and resectabil- ity based on metastasis to nodes and local invasion of the mediastinum or chest wall. In a state-of-the-art paper by Webb and Sostman (1992), MR imaging was considered to provide information which was superior to that of CT in certain situations. They suggested that MR should be used as the primary imaging modality in these situations. Lung cancer was one of those situations. Specifi cally, it was felt that MR may be superior in the diagnosis of chest wall or mediastinal invasion by the primary lung tumor because of better contrast on T1- and T2-weighted images between tumor and fat and vessels in the mediastinum. With the ability to obtain 2D MPR images with very thin sections and with the increase in concentration of intravascular contrast material, MDCT is certainly now at least equal to MR imaging in showing invasion (Figs. 15.2, 15.3) or lack thereof (Fig. 15.4), and likely will be shown to be superior.
Because CT is less expensive, more readily available, takes less time to perform, and is now much easier for sick patients to tolerate, it is the imaging method of choice for invasion into the mediastinum. This is likely to be true in the evaluation of not just bron- chogenic carcinoma but of malignant mesothelioma.
Volume rendering can also aid in determining tumor resectability based on invasion of adjacent structures and pre-operative planning for tumor resection (Calhoun et al. 1999).
15.3.2 Assessment of Lymph Nodes
Lymph nodes are frequently seen in the mediastinum,
and when larger than 1 cm in short axis or when many
in number, they are a cause for concern as evaluation
of mediastinal nodes is important in the work-up of
non-small cell lung carcinoma. Routine CT studies
of the thorax on single-detector CT (SDCT) scanners
usually do not routinely include thin slices through
the mediastinum. The MDCT offers an advantage in
that by using the smallest collimation (1 mm on the
4-channel and 0.75 mm on the 16-channel scanners)
one is able to retrospectively examine a worrisome
Fig. 15.2. Bronchogenic carcinoma. Con- trast-enhanced 2D coronal multiplanar reformatted images (MPR) image dem- onstrates a large aggressive mass which involves the mediastinum, particularly the left superior pulmonary vein. It also clearly shows complete obstruction of the left upper lobe (LUL) bronchus resulting in partial LUL atelectasis
node to evaluate its tissue content (Fig. 15.5). The ability to obtain thin-section images retrospectively is limited by the collimation; hence, routinely using the smallest collimator can prevent additional unnec- essary imaging or invasive procedures. In a study by Boiselle et al. (1998), CT, MRI, and 2-[fl uorine-18]
fl uoro-2-deoxy-D-glucose (FDG) positron emission tomography (PET) were compared for their ability to identify tumor-positive nodes. They concluded that FDG-PET showed improved diagnostic accuracy in nodal status when compared with anatomic imaging with CT and MR. As demonstrated in Fig. 15.5, thin- section MDCT imaging through nodes followed by postprocessing MPR techniques will at least prevent further work-up of nodes which are clearly benign by CT, i.e., nodes which clearly show low-attenuation centers.
15.3.3 Extent and Origin of a Lesion
At times the origin of a lesion within the mediasti- num can be confusing if only one plane is provided.
By using MPR reconstructions in the sagittal and coronal planes in conjunction with the axial images, a more accurate assessment of a lesion can be made with regard to its origin and possibly its diagnosis (Fig. 15.6).
Fig. 15.3a, b. Bronchogenic carcinoma. Contrast enhanced 2D coronal images show extensive tumor mass encircling the a right lower lobe bronchus and b bronchus intermedius. Inva- sion of the diaphragm is also demonstrated
a
b
Fig. 15.5a–c. Benign node. Axial images at the level of the carina show a 1-cm 4R (lower right paratracheal node) which on the a 5-mm thick slice is indeterminate and can possibly be interpreted as malignant. b The 2.5-mm image shows a low-attenuation center which is confi rmed on the c 1.25-mm thin-section image. This confi rms the benignity of this node a
c
b Fig. 15.4a, b. Non-Hodgkin’s lymphoma. Contrast-enhanced a coronal and b sagittal 2D MPR images show a large heterogeneous mass in the anterior mediastinum which is shown to abut rather than invade the vessels. Note the smooth contour of the vessels as opposed to the superior pulmonary invasion shown in Fig. 15.2
a b
Fig. 15.6a–e. Bronchogenic cyst. a–c Three contrast-enhanced axial images show a cystic lesion which appears inseparable from the thyroid gland and, in fact, based on these images was thought to be a thyroid cyst; however, d coronal and e sagittal 2D MRR images strongly suggested otherwise. These images led to a confi dent preoperative diagnosis that this was not of thyroid origin but rather a mediastinal cyst and prob- ably a bronchogenic cyst. This was confi rmed surgically and pathologically
a
c
e
b
d
15.3.4 Tissue Characteristics
Many lesions within the mediastinum are of mixed attenuation including teratoma, lymphoma, broncho- genic carcinoma, thymolipomas and other thymic tumors, thyroid lesions, and others. Previous reports have indicated that internal characteristics of a mass as well as invasion into the mediastinum or its rela- tionship to vessels are best shown with MR imaging (Webb and Sostman 1992). Magnetic resonance is felt to be superior for tissue characterization when compared with CT. Because of the improvement in spatial temporal and contrast resolution of the MDCT images, the diagnostic accuracy of CT has been improved when compared with MR imaging (Fig. 15.7). Coronal and sagittal MPR images can be more useful for diagnostic purposes when compared with axial sections. (Rydberg et al. 2000). The 3D techniques may be useful in showing the relationship of lesions to vasculature and the airways (Fig. 15.7).
15.3.5 Airway Masses
A brief description of airway lesions and imaging is presented as the airways are included in mediastinal differential diagnoses. Both 2D and 3D reconstruc- tion techniques provide useful information in evalu- ation of the central airways (Fig. 15.8), particularly when used with axial imaging (Ravenel et al. 2001;
Boiselle et al. 2002). Sagittal and coronal 2D MPR images aid in assessing degree of stenosis as well as craniocaudal extent. Modest improvement in airway analysis is achieved with 3D SSD and volume-ren- dered images (Ravenel et al. 2001; Boiselle et al.
2002; Remy-Jardin et al. 1998). Boiselle et al. (2002) and Remy-Jardin et al. (1998) concluded that 2D MPR and 3D volume rendering does not create new information but rather presents complementary ways
of viewing information already present on the origi- nal axial images. Although it has been shown that in the majority of uncomplicated cases 3D images are of minimal usefulness, 3D images are moderately or highly valuable in assessing complex airway lesions (Kauczor et al. 1996; Remy-Jardin et al. 1998). It has been suggested that 2.5- to 3.0-mm-collimation images be used for better defi nition of multiplanar reformatted and 3D images. This should be done with overlapping reconstruction intervals (Boiselle et al.
2002). The 3D volume-rendered images are useful for extrinsic airway compression as well as a wide vari- ety of complex tracheobronchial anomalies. The 3D images also have the advantage of assessing airways distal to an impassable structure or mass (Ravenel et al. 2001) when compared with bronchoscopy. The 2D MPR images can be displayed in a curved fashion along the axis of an airway, thus better displaying the length of an abnormality.
15.4 Conclusion
Multidetector CT imaging shows great potential in
mediastinal imaging, not merely in the vascular,
cardiac, and airways lesions, but also in evaluation
of more typical mediastinal lesions. Specifi cally, due
to improvement in temporal, spatial, and contrast
resolution, and the ability to obtain 2D multiplanar
reformations and 3D reconstructions, the ability of
CT to characterize tissues and assess for invasion into
mediastinal structures may potentially be equal to that
of MR imaging. Both 2D and 3D imaging in medi-
astinal imaging also contribute to surgical planning
and to assessing resectability of lesions. The need for
2D multiplanar reformations and 3D reconstructions
should be considered at the time of planning the CT
scan so that adequate imaging is obtained which will
optimize exquisite reconstructions.
Fig. 15.7a–d. Non-Hodgkin’s lymphoma. a Sagittal contrast- enhanced and c coronal enhanced 2D MPR images are com- pared with b contrast-enhanced sagittal and d coronal gradi- ent-echo images. Both images show well the complex solid and cystic components of this large mass, but the CT reconstruc- tions show to much better advantage the vessels as they relate to the tumor and show the relationship of the mass to the pericardium much better. e The 3D color-enhanced volume- rendered image shows the solid and cystic components of this mass as well as its relationship to the vessels
a
c
e
b
d
Fig. 15.8a–e. Squamous cell carcinoma of trachea. a Axial, b coronal 2D MPR, and c sagittal 2D MPR images interpreted in a complementary fashion show the circumferential and cephalocaudad extent of this tumor. They all show marked narrowing of the trachea; however, the d coronal and e sagittal 3D volume-rendered sculpted images show more dramatically the extent of airway narrowing relative to the more normal luminal size of the trachea above and below the lesion a
c
e
b
d
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