Thoracic Imaging 2003 - Society of Thoracic Radiology

MONDAY
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Imaging the Subsegmental Embolus
While traditional technical limitations of CT in the diagnosis
of pulmonary emboli appear successfully overcome by multidetector-row
CT, we are now facing new challenges that are a
direct result of our high-resolution imaging capabilities. Small
peripheral clots that might have gone unnoticed in the past are
now frequently detected, often in patients with minor symptoms.
While, based on a good quality multidetector-row CT scan,
there may be no doubt in the mind of the interpreting radiologists
as to the presence of a small isolated clot, such findings
will be increasingly difficult to prove in a correlative manner.
Animal experiments that use artificial emboli as an independent
gold standard indicate that high-resolution 4-slice multidetectorrow
CT is at least as accurate as invasive pulmonary angiography
for the detection of small peripheral emboli 47 . However, it
appears highly unlikely that pulmonary angiography will be performed
on a patient merely to prove the presence of a small (2-3
mm) isolated embolus. Additionally, given the limited interobserver
correlation of pulmonary angiography discussed earlier
11, 12 it appears doubtful that this test, even if performed,
would provide as useful and conclusive proof as high-resolution
multidetector-row CT. Broad based studies such as PIOPED II,
which set out to establish the efficacy of multidetector-row CT
in suspected PE, account for this latter fact by using a composite
reference test based on ventilation/perfusion scanning, ultrasound
of the lower extremities, pulmonary angiography, and
contrast venography to establish the PE status of the patient 48 .
Perhaps more importantly there is a growing sense of insecurity
within the clinical community how to manage patients in
whom a diagnosis of isolated peripheral embolism has been
established. It has been shown that 6% 15 to 30% 49 of patients
with documented PE present with clots only in subsegmental
and smaller arteries, but the clinical significance of small
peripheral emboli in subsegmental pulmonary arteries in the
absence of central emboli is uncertain. It is assumed that one
important function of the lung is to prevent small emboli from
entering the arterial circulation 25 . Such emboli are thought to
form even in healthy individuals although this notion has never
been substantiated 50 . Controversy also exists, whether the
treatment of small emboli, once detected, may result in a better
clinical outcome for patients 37, 51, 52 . There is little disagreement
though, that the presence of peripheral emboli may be an
indicator for current deep vein thrombosis thus potentially
heralding more severe embolic events 27,49,53 . A burden of
small peripheral emboli may also have prognostic relevance in
individuals with cardio-pulmonary restrictions 25,49,52 and for
the development of chronic pulmonary hypertension in patients
with thromboembolic disease 49 .
Perhaps the most practical and realistic scenario for studying
the efficacy of computed tomography for the evaluation of
patients with suspected PE is to assess patient outcome. There is
a growing body of experience concerning the negative predictive
value of a normal CT study and patient outcome if anticoagulation
is subsequently withheld 16, 43, 52, 54-58 . According to
these retrospective studies the negative predictive value of a normal
CT study is high, approaching 98%, regardless whether
multidetector-row technology is used 43 or whether underlying
lung disease is present 57 . The frequency of a subsequent clinical
diagnoses of PE or DVT after a negative CT pulmonary
angiogram is low and lower than that after a negative or lowprobability
V-Q scan 52 . Thus even single-slice CT appears to
be a reliable imaging tool for excluding clinically relevant PE so
that it appears that anticoagulation can be safely withheld when
the CT scan is normal and of good diagnostic quality 52, 58 .
CT Functional Imaging of PE
To date, CT has not permitted the functional evaluation of
pulmonary microcirculation during pulmonary embolism. Yet,
the choice of the adequate therapeutic regimen critically hinges
on an accurate evaluation of the functional effect of the embolic
event on lung perfusion. If large percentages of the lung
parenchyma are affected by embolic occlusion, imminent right
heart failure warrants a more aggressive regimen, such as
thrombolysis that carry 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.
It has been shown that with the advent of fast CT scanning
techniques functional parameters of lung perfusion can be noninvasively
assessed by means of CT imaging 31, 32, 61, 62 . In
the following we would like to discuss different experimental
approaches for visualization and quantification of pulmonary
perfusion, based on various CT techniques. 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 the embolic event 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.
Electron-Beam CT:
Functional EBCT Scan Protocol:
A unique feature of Electron Beam CT (EBCT) is that it can
be used both for volume scanning for the depiction of structure
63 and for functional analyses by acquiring high temporal resolution
data sets simultaneously on multiple sections of an organ.
EBCT has successfully been used for perfusion measurements
in the heart 64 65 66 , the brain 67 and the kidneys 68 . The feasibility
of pulmonary blood flow measurements with EBCT has
been validated in a number of controlled animal studies 69 70 .
However the value of this method in the diagnostic work-up of
patients with suspected PE has never been assessed. In a
prospective study we were able to demonstrate the usefulness of
EBCT as a single modality to image both thoracic structure and
function in patients with suspected acute PE.
The technical design of the electron beam scanner is
described in detail elsewhere 71 72 . In the multidetector-row
mode of the scanner eight slices in a 7.6-cm volume at 20 consecutive
time-points can be acquired without patient table movement
to monitor the passage of a contrast material bolus through
the lung parenchyma. To improve the quality of the data, scans
can be ECG triggered to the quiet diastolic phase of the heart
cycle. For measuring pulmonary perfusion, contrast material is
intravenously injected with a flow rate of 10 cc/s for 4 s.
Functional EBCT analysis:
For dynamic blood flow evaluation we use an approach that
comprises a qualitative analysis by selectively coding lung pixel
attenuation in a color-coded cold-to-hot spectrum. This way
maps can be generated for visualization of parameters such as
peak Hounsfield Unit (HU) change, time to peak or mean transit
time of contrast material. In our experience peak HU change is
most suitable for identification of flow deficits. Using this
parameter a qualitative analysis of lung perfusion can be performed
by generating a color-coded map for the eight scan levels
that are simultaneously acquired by EBCT. On color-coded
maps flow deficits are defined by predominance of cold-spectrum
colors with segmental distribution. Guided by color-coded
maps, a quantitative analysis for the assessment of regional pulmonary
blood flow can be performed. To this end, time-density