Thoracic Imaging 2003 - Society of Thoracic Radiology

terminate scintigrams, CT correctly identified the final diagnosis
in 29 (13 PE, 11 alternative, 5 normal). These authors suggested
that CTA replace the VQ scan.
MULTI-CHANNEL CT: THE DEFINITIVE TEST
The advantage of multi-channel CT is speed. Thus, much
larger volumes can be covered with the same or even thinner
collimation than possible with a single slice scanner. In practice,
we use 1.25 mm collimation, single breath hold, HS mode
(pitch=6), and cover the entire chest in one breath hold. Images
are reconstructed at 1 mm intervals. Therefore, we get both
thinner sections (better resolution) and a larger coverage area
than with single channel scanning. This facilitaties depiction of
smaller peripheral vessels, increases the resolution and quality
of multiplanar reformations, and leads to potential applications
of dynamic repeated acquisitions through questionable areas if
necessary. We assess the pelvic and upper leg veins routinely
with 5 or 10 mm collimation 2-3 minutes after the start of the
contrast injection. Tube cooling, often problematic in single
channel scanning, does not limit multi-channel acquisitions.
Image processing is an area of active research. Cardiac gating
software is now available for multi-channel scanners that
facilitates data acquistion only during quiescent phases of the
cardiac cycle as well. Huge data sets, gated, with 1.25 mm
overlapping reconstructions can be sent to powerful computers;
software engineers in concert with radiologists are only beginning
to develop new means of processing this information to
extract pertinent diagnostic value with the click of a mouse.
Axial images will probably be only a small or nonexistent part
of CT display in the not-so-distant future. Examples of colorbased
threshold techniques and pulmonary perfusion maps will
be shown.
More “conventional” image processing tools, including
maximum intensity projection (MIP) angiography, can also be
used as needed. These images project the brightest pixel at a
given location in a preselected slab of images onto a final summation
image- we often perform 3.5-5 mm MIP angiography
using our CTA data set- this reduces the number of images for
review and increases the degree of apparent vascular enhancement.
Examples and pitfall;ls will be illustrated.
In sum, multi-channel CT represents a major advance in our
ability to scan large volumes rapidly with collimation sufficiently
small to resolve small vessels. Processing volumetric data in
novel ways is the key to use of the full potential of this exciting
technological advance.
CLINICAL LITERATURE: CONTROVERSIES
Despite the above, much clinical literature continues to question
the use of CTA in comparison to old (old, not gold) standards
(scintigraphy, conventional angiography, and even best
clinical guess). In part, this is due to an incomprehensible simple
“meta-analysis” of the CT “accuracy” literature despite its
clear limitations. Other problematic factors relate to the specialty-of-origin
of the authors of recent major publications: in many
cases, judgments on CT angiography are rendered without any
input from radiologists. Editoral boards in many cases appear to
be impressed by patient numbers and have little expertise in
radiologic techniques. We must be aware of these studies and
must be very clear and pointed in our criticism as needed.
It is vital for radiologists to peruse the major clinical journals
for such “reviews” or “panels” and especially for the trendy
“meta-analysis”- such are often the basis for physician testordering
patterns. It is our job to maintain diligence in the
appropriate education of nonradiolgist physicians and physician
extenders- technology changes are quite rapid and will continue
to be so.
ACCP Consensus Committee on PE, 1998
Based on pooled “CT accuracy” data, the panel concluded
that CTA “still under investigation” and that a normal CTA
“does not exclude PE.” (In fairness, these recommendations
preceded recent CTA outcomes studies.) The expert panel concluded
that “further studies are necessary to delineate the diagnostic
role” of CTA. There were no CT/CTA specialists on the
panel, but the recommendations are now familiar to most practicing
pulmonary physicians and have not yet been updated
[14]. Still, simple combination of reported accuracy values
without a consideration of the variables that explain these quoted
ranges seems superficial for such an expert group.
Annals of Internal Medicine CTA Accuracy Meta-analysis,
2000
Rathbun and colleagues concluded that no sound methodologic
studies prove that CTA should be used clinically, and cite
the “wide range of accuracy values” as well as an “interobserver
variability problem” (as if this is not problematic with scintigraphy
or best clinical guess or even conventional angiography).
Stunningly, this Oklahoma group of nonradiologists concludes
that “scintigraphy combined with best clinical guess” is still the
best way to diagnose or exclude pulmonary embolism. Maybe
in Oklahoma [15].
Annals of Internal Medicine Repeats Controversial CTA
Manuscript, 2001
Perrier and colleagues published a disastrous review of 299
ER patients who had abnormal d-dimer levels and CTA [16].
Gold standards fo PE included a positive conventional
angiogram, DVT on ultrasound with clinical symptoms of PE,
or a high probability VQ scan. The gold standard for “no PE”
was a negative angiogram (in few patients), a normal or low
probability VQ scan, or a low clinical suspicion of PE with negative
leg ultrasound and an indeterminate VQ scan. Any event
(DVT or PE) in a 3-month follow-up period (diagnosed in the
same ways as above) indicated that the initial CTA was a false
negative.
In other words, clot noted on CTA was wrong if the VQ was
low probability. A negative CTA was wrong if the patient had a
DVT within 3 months of follow-up or had a DVT and symptoms
of possible PE at initial diagnosis.
The authors’ unavoidable conclusion: the sensitivity of CTA
(70%) is too low to exclude PE. Worse, “false positives” occur
“even in the lobar (15%) and segmental (38%) levels.” Againin
other words- 15% of the lobar clots and 38% of all the segmental
clots identified at CTA were reported as false positives
of CTA rather than false negatives of the other testing combinations.
Alternative explanation: the VQ scan and best clinical
guess can miss clots, even big ones.
Archives of Surgery CTA Condemnation, 2001
Velmahos and colleagues (no radiologists) assessed CTA
results in 22 SICU patients who also underwent pulmonary
angiography. CTA techniques were limited, with a single slice
machine, 3 mm collimation, only 100cc of intravenous contrast-
worse, there was no induced apnea in these intubated
patients [17].
CT sensitivity values in comparison to the angiography gold
standard were low, but angiography showed isolated segmental
or subsegmental in clots in 6 of the 11 patients with “proven”
PE. The authors conclude that “CTA has not stood up to the
challenge” and cannot be relied upon in ICU patients.
What can we as radiologists say? It is not surprising that
technically bad CTA exams performed in moving patients miss
tiny clots diagnosed with great certainty by angiography readers
in tiny vessels where the interobserver variability is similar that
of predicting the results of a coin flip. This cannot, however,
rationally lead to the conclusion that CTA is not accurate.
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