Thoracic Imaging 2003 - Society of Thoracic Radiology

curves (TDCs) are generated by manually tracing regions of
interest (ROIs) over lung segments showing flow-deficits. A
ROI over the main pulmonary artery or the right ventricle can
be used as input function. Regional pulmonary blood-flow in
each segment can then be assessed according to the indicator
dilution theory 73 74 using a basic flow equation 64, 69 where
PBF / V = DPul / ÚC RC dt (PBF/V = pulmonary blood flow per
unit volume of lung tissue; DPul = peak HU change during contrast
injection; ÚC RC dt = area under a g variate-fit of a TDC of
the right side of the circulation).
Advantages and Disadvantages of Functional EBCT Perfusion
Imaging of the Lung:
Once a suspicion of pulmonary embolism arises, crucial
questions need to be answered: Is it indeed PE that causes the
patient’s symptoms, or are there other reasons for the patient’s
discomfort? If it is PE, where are the emboli located and how
extensive is the disease? Does anticoagulation suffice, or are
thrombolysis or invasive measures warranted? And are there
conditions that prohibit thrombolysis? Where does the disease
originate? Contrast enhanced CT provides high spatial resolution
and allows the objective, non-invasive visualization of thoracic
anatomy. Sources of chest pain other than PE can be identified.
The location of pulmonary emboli and the extent of the
disease can be assessed to determine the need for and feasibility
of anticoagulation, thrombolysis or more invasive measures.
However, to date, computed tomography has not permitted the
functional evaluation of pulmonary microcirculation during pulmonary
embolism. CT perfusion measurements as described in
this chapter may represent a valuable adjunct to CT pulmonary
angiography by providing both structural and functional information
using the same modality. The well-established accuracy
of CT for the depiction of emboli and thoracic anatomy is thus
supplemented by an effective means to quantitatively assess the
functional effect of thromboemboli on lung perfusion. This way,
a comprehensive diagnosis is feasible within few minutes, without
having to subject a patient to multiple expensive and timeconsuming
tests requiring transportation and advanced logistics.
In a recent study 32 we were able to show that EBCT can successfully
be used for the functional analysis of pulmonary blood
flow and therefore allows to differentiate between segments
with normal and reduced capillary perfusion. This allows to
estimate the percentage of lung parenchyma with impaired
microcirculation. Thus, a decision whether anticoagulation or
thrombolysis is warranted is facilitated. During the course of
treatment, the effect of therapy may be monitored by comparing
pre- and post-therapy blood-flow values by repetitive scanning.
Up to two thirds of patients with initially suspected PE receive
other diagnoses including unknown malignancies or life-threatening
conditions such as aortic rupture or dissection. By initially
performing a contrast enhanced thin-slice volume study, the
presence of pulmonary embolism can be verified and other or
additional underlying diseases are readily recognized. The
patient with previously unknown small-cell lung cancer in our
patient group illustrates the importance of a thorough analysis
of thoracic structure. A major limitation is that the 7.6 cm scan
volume of the dynamic study does not cover the entire chest.
The additional radiation dose for the dynamic study amounts to
an effective dose equivalent of 7.2 mSv, which about equals the
radiation exposure usually applied during nuclear scanning 75 .
While there is good correlation between volume and functional
scans for the detection of segmental emboli, a potential
limitation of this technique arises from partial occlusion of vessels
with maintained blood flow on a capillary level despite of
the presence of thrombi. Such findings may be regarded as
“false negative” results. However, valuable information can be
gained by assessing the actual effect of small emboli on lung
microcirculation. Completely or partially maintained perfusion
despite of the presence of emboli revealed by functional scanning
may influence the decision whether or not to start thrombolytic
therapy in a patient, the latter carrying a small but definite
risk 59 60 . Thus the quantitative assessment of the effect of
PE on tissue perfusion may bear more important information for
patient management than the direct visualization of emboli by
CT angiography alone. Non-occluding emboli are also a wellknown
problem in lung scintigraphy, where even extensive central
emboli frequently go undiagnosed if they are only partially
occluding and do not cause localized perfusion defects. Similar
pitfalls can be avoided by the combined use of both CT angiography
and CT perfusion measurement for a comprehensive diagnosis.
Our model for the assessment of pulmonary blood flow in
patients with suspected PE has been previously described 64 69
73 . However, numerous different models have been developed
in the past for the derivation of different blood flow parameters
from non-invasive imaging modalities.
Functional Multidetector-Row CT Imaging of Lung Perfusion:
Dedicated image processing tools are currently being developed
for use with high-resolution Multidetector-row CT. Based
on EBCT experiences we currently pursue several approaches to
non-invasively visualize blood-flow parameters in the lung with
a dedicated software platform (LungLab, Siemens).
LungLab is an image processing tool for diagnosis of pulmonary
embolism using high-resolution Multidetector-row CT
protocols. For visualization of lung attenuation “Perfusion
weighted display” allows color encoded display of lung
parenchyma enhancement. This kind of display is generated by
acquiring a high resolution Multidetector-row CT data set of the
entire thorax with 500 msec gantry rotation, pitch of 6 at 120
kV and 120 mAs. Contrast enhancement is achieved with a
sharp (5 cc/s) bolus of 120 cc low-viscosity, non-ionic contrast
material with 16 s delay via a cubital vein. Images are reconstructed
with 1-mm sections with a soft body kernel. The lung
parenchyma is segmented from the enhanced thin slice reconstructions
using threshold based contour finding to extract the
lung volume and HU range selection to suppress major vascular
structures. The segmented images are low pass filtered with an
adaptive 3D filter. The filter can be selected out of a range of
dedicated image filters. The resulting images are interactively
mapped on a spectral color scale in order to optimally display
differences in enhancement. Additional overlay of the parenchymal
color map onto the original CT-image is used to enhance
visualization of spatial relationships. Due to the isotropic nature
of high resolution Multidetector-row CT data set, color-coded
images can be viewed in arbitrary imaging planes. This allows
for semi-quantitative evaluation of blood flow in the lung
parenchyma. We anticipate these methods to evolve in a valuable
adjunct to CT pulmonary angiography by providing both
structural and functional information using the same modality.
The well-established accuracy of CT for the depiction of emboli
and thoracic anatomy is thus supplemented by an effective
means to quantitatively assess the functional effect of thromboemboli
on lung perfusion. This way, a comprehensive diagnosis
is feasible within few minutes, without having to subject a
patient to multiple expensive and time-consuming tests requiring
transportation and advanced logistics.
Problems and Limitations of Computed Tomography for PE
Diagnosis
Contrast Media Injection and Artifacts:
Despite advances in CT technology, there are still several
factors that can render CT pulmonary angiography inconclusive.
The most common reasons for non-diagnostic CT studies are
poor contrast opacification of pulmonary vessels, patient motion
and increased image noise due to excessive patient obesity.
The advent of multidetector-row CT necessitates an exten-
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