Thoracic Imaging 2003 - Society of Thoracic Radiology

Virtual Endoscopy in the Thorax: Present and Future
Applications
R.C. Gilkeson, M.D.
University Hospitals of Cleveland
CWRU Medical School
Learning Objectives:
1. To understand the imaging appearances of large and small
airway disease
2. To gain a basic understanding of virtual bronchoscopy and
its clinical utility.
3. To understand the virtual bronchoscopic appearance of a
variety of airway diseases.
In the clinical evaluation of pulmonary disease, fiberoptic
bronchosopy is a crucial tool in the diagnosis of a variety of chest
diseases. Though often instrumental in the diagnosis of a variety
of neoplastic, inflammatory and infectious diseases, fiberoptic
bronchscopy (FOB) has important limitations: It is invasive, time
consuming, and requires sedation. It is often not well tolerated in
the young, the critically ill, or in patients with coagulopathies. In
patients with significant airway disease/stenoses, the bronchoscopic
evaluation of the airway distal to areas of stenoses/narrowing
is technically difficult and can often signicantly compromise
patient oxygenation. Equally important, the evaluation of
extraluminal pathology is significantly limited in fiberoptic bronchoscopy.
Moreover, the diagnostic capabilities of FOB in the
evaluation of extraluminal pathology is limited. The last decade
has seen incredible advances in thoracic imaging. The advent of
spiral CT, and the acquisition of volumetric data sets, has allowed
anatomic depiction of axially acquired data.. MPR, MIP, and volume
rendering techniques are now standard in chest imaging.
With increasingly sophisticated software, axial images can be
reconfigured to display this data from an endoscopic perspective.
This has clearly been important in the evaluation of bowel pathology,
and virtual colonoscopy offers promising results in the noninvasive
screening evaluation of colon cancer. Similar advances
have been made in other gastrointestinal organs, the paranasal
sinuses and vascular structures Virtual bronchosopy has similarly
become an important tool in the evaluation of chest diseases.
While original virtual bronchoscopy programs were often time
consuming and impractical to the practicing radiologist, increasingly
sophisticated post processing techniques have improved the
speed and accessibility of the user interface, enabling rapid virtual
endoscopic depiction of the airways. This presentation will
outline these advances, and present the use of virtual bronchoscopy
in the evaluation of a variety of neoplastic and non
neoplastic processes. Other endoscopic applications in the thorax
will be presented, along with a discussion of their possible future
in chest imaging.
Several principles in image acquisition are crucial in the successful
performance of virtual bronchosopy. A number of studies
have demonstrated the importance of narrow collimation and
reconstruction overlap of at least 50% to optimize any type of
three dimensional imaging; and these parameters are similarly
important to virtual bronchoscopic evaluation. For single slice
CT scanners a slice thickness of 3-5mm is preferred. While pitch
values up to 2 are usually acceptable for the volumetric depiction
of pathology, most published protocols prefer a pitch of 1 for virtual
bronchscopic rendering using a single slice scanner. With the
advent of the dual slice CT scanner, slice collimation of 2.5mm
with 1mm slice reconstruction has been published. With the
advent of multislice scanning slice acquisitions of 4-32 slices and
gantry rotation times of 500msec, virtual bronchoscopic imaging
can now be routinely performed with 1mm slice collimation.
While there is not a significant amount of data on the role of multislice
CT in virtual bronchosocpy, examination of these
improved imaging parameters would strongly suggest their superiority
in VB imaging. While the use of IV contrast is not necessary
in most virtual bronchscopy, it is often used for the depiction
of extraluminal pathology. To optimize contrast opacification of
extraluminal vascular structures, 100-140cc of IV contrast is used
at injection rates that range from 2-5cc sec. As will be discussed
later, accurate integration of extraluminal pathology with the virtual
bronchscopic data helps optimze virtual bronchosopy diagnostic
capabilities. Accurate segmentation of extraluminal structures
is optimized with IV contrast, and is therefore important to
advanced VB protocols. At our institution, cases in which virtual
bronchscopy is considered is now performed entirely on our multislice
scanners.. Decisions on 1 or 2.5mm scanning is depends on
the clinical question and anatomic coverage required. The speed
of the multislice scanner allows dynamic inspratory/expiratory
endoscopic imaging, which has proven important to our referring
pulmonologists in the evaluation of tracheomalacia and suspected
upper airway collapse Future research in image optimization
in VB includes the role of cardiac gating in the accurate depiction
of airway stenoses, and will be presented here .
Following the acquisiton of the spiral CT data, transfer of the
DICOM data to advanced imaging workstations can be performed.
There are a variety of advanced imaging products capable
of virtual endoscopic imaging. These products include
General Electric Navigator (General Electric Medical Systems,
Milwaukee, Wis), Vital Images Voxel View (Vital Images,
Fairfield Connec,ticut), Iris Explorer (Silicon Graphics,
Mountain View, USA). A number of advanced, hybrid imaging
techniques have been developed at individual institutions to further
optimize the VB data. At our instituition we have used
Voyager Software (Phillips Medical Systems) for virtual
bronchscopy. The recent transition to a Windows NT workstation
has significantly improved the speed for image reconstruction of
this data and made virtual bronchoscopic imaging increasingly
accessible to the less experienced operator. The effective use of
virtual bronchoscopy necessitates an understanding of the anatomy
seen during fiberoptic bronchoscopy. . In particular, the perspective
of the bronchoscopist, with the bronchosocope displaying
the anatomy in a cranial-caudad direction with the patient in
a supine position, needs to be easily understood by the thoracic
radiologist. For those radiologists involved with virtual bronchoscopy,
a solid understanding of the anatomy seen at fiberoptic
bronchoscopy is important, and participating/correlation with
fiberoptic bronchoscopy is important in understanding the capabilities
and limitations of virtual bronchoscopy. In most virtual
bronchoscopy software packages, both surface and volume rendering
can be performed. Surface rendering is most often used
because with traditional software, reconstruction times for virtual
bronchoscopy is faster than volume rendering. Standard
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THURSDAY