Helical CT pulmonary angiography (HCTPA) is increasingly used for the evaluation of patients suspected of having acute pulmonary embolism (PE). This technique offers many advantages over other modalities traditionally used to examine such patients.
is Assistant Professor-In-Residence and Director of Thoracic
Imaging, San Francisco General Hospital, and Director, Radiology
Residency Training Program, University of California, San
is Associate Professor-In-Residence, University of California,
San Francisco, and Chief of Abdominal Imaging, Veterans Affairs
Medical Center, San Francisco, CA.
Helical CT pulmonary angiography (HCTPA) is increasingly used
for the evaluation of patients suspected of having acute pulmonary
embolism (PE). This technique offers many advantages over other
modalities traditionally used to examine such patients. Helical CT
pulmonary angiography is widely available and is performed rapidly.
In cases interpreted as negative, HCTPA frequently provides
alternative diagnoses for symptoms suggestive of acute PE.
To use this modality effectively, radiologists must be familiar
with proper HCTPA protocols for imaging suspected pulmonary
embolism, HCTPA findings of acute PE, and commonly encountered
diagnostic pitfalls. In addition, it is becoming increasingly
important to have an understanding of the controversies and
limitations of HCTPA.
HCTPA technique for acute PE
Helical CT pulmonary angiography protocols require attention to
several technical parameters to optimize study quality. These
parameters include: beam collimation, pitch, volume of coverage,
scanning direction, field of view, tube amperage (mA), and contrast
injection timing and rate.
Single-detector helical CT systems
Thin collimation, typically 2 or 3 mm, should be used for PE
imaging on single-detector helical CT (SDCT) systems.
Narrow collimation provides improved spatial resolution, and allows
the necessary volume of tissue to be covered in a period of 15 to
20 seconds, and provides improved visualization of smaller
pulmonary arteries compared with thicker collimation studies.
The pitch should be increased as needed to scan the required volume
of tissue in an expedient manner. A pitch of 2 on SDCT is usually
adequate, and the pitch may be increased further to provide more
coverage. Increasing the pitch from 1 to 2 increases effective
slice thickness by about 30%. In general, it is preferable to
increase pitch, rather than collimation, to provide more volume of
Volume of coverage--
The volume of coverage should range from the superior aspect of the
aortic arch to approximately 3 to 4 cm below the level of the
inferior pulmonary veins. To minimize scan degradation due to
respiratory motion (which will be maximal in the lung bases),
scanning should proceed in the caudal to cranial direction.
Field of view--
The field of view (FOV) should encompass the width of the chest at
the widest part of the thorax. The FOV should not be collimated
tightly, because this will reduce photon flux and contribute to
increased image noise. Additionally, excessively tight collimation
may obscure potential alternative diagnoses in the peripheral lung,
pleural space, or chest wall.
Tube amperage (mA)--
While every effort should be made to reduce tube current, and
thereby limit the radiation dose to the patient, PE studies require
high-contrast resolution, and dose reduction should be thus be
Contrast injection rate and timing--
HCTPA technique requires injection rates of 3 mL/sec or more. While
a standard contrast injection delay of 20 seconds may suffice in
most patients, such a standard delay may not be appropriate for
patients with congestive heart failure or venous stenoses. When
possible, the use of bolus timing software will provide properly
timed, high-quality studies routinely.
Automated bolus timing may be performed with a cursor placed on the
main pulmonary artery.
Multidetector row helical CT systems
Multidetector row helical CT (MDCT) systems have additional
detectors that allow for rapid scanning of the pulmonary vascular
bed using very narrow collimation during a reasonable scan time.
This technology allows for narrow collimation (1.25 mm collimation
on GE LightSpeed scanners, GE Medical Systems, Milwaukee, WI) to be
utilized, providing increased spatial resolution. The improvement
in spatial resolution may translate to improved visualization of
small (subsegmental) pulmonary arteries.
A reconstruction increment may be utilized (0.6 to 0.7 mm), which
probably adds little to the scan quality but may improve the
quality of reformations and renderings, although at the cost of
increased data storage requirements.
A high pitch (table transport speed) should be used for MDCT
pulmonary angiography (a pitch of 6, or "high speed" mode on GE
Because MDCT scanners (4-ring systems) acquire data at least 3
times faster than SDCT systems, the entire chest may be scanned,
rather than limiting the volume of coverage as is required with
SDCT scanners. Similar to SDCT protocols, scanning should proceed
from caudal to cranial, and high injection rates and bolus timing
software should be used.
HCTPA findings of acute PE
The single most important finding of acute PE on HCTPA studies
is an intraluminal filling defect surrounded by contrast (figures 1
Ancillary findings suggestive of acute PE include an expanded,
unopacified vessel (figure 3); eccentric filling defects (figure
4); peripheral, wedge-shaped consolidations (figure 5); oligemia
(figure 6); and pleural effusion.
However, ancillary findings are not as specific for acute PE as are
intraluminal filling defects surrounded by contrast because some
ancillary findings may also be seen with chronic PE (eccentric
filling defects; peripheral, wedge-shaped consolidations; and
oligemia), or are not specific for PE (peripheral, wedge-shaped
consolidation; oligemia; and pleural effusion).
Pitfalls in the HCTPA diagnosis of acute PE
There are several anatomic findings or technical difficulties
that may either simulate or obscure the diagnosis of acute PE on
Anatomic pitfalls in the HCTPA diagnosis of acute PE include lymph
nodes, impacted bronchi, pulmonary veins, pulmonary arterial
catheters, and pulmonary artery sarcomas. Technical causes of
pitfalls in the diagnosis of acute PE are usually related to
respiratory or cardiac motion, poor bolus timing, quantum mottle,
and edge-enhancing reconstruction algorithms.
Anatomic pitfalls in the HCTPA diagnosis of acute PE
--Lymph nodes are among the most commonly encountered findings on
HCTPA studies that may be mistaken for acute PE. Lymph nodes are
extraluminal and tend to form obtuse angles with the adjacent
opacified pulmonary arteries. True intraluminal filling defects,
such as acute PE, will form acute angles with the opacified
arterial blood pool
(figure 7). Knowledge of the normal locations of intrathoracic
lymph nodes is essential for avoiding this pitfall. Lymph nodes
typically reside lateral to the adjacent pulmonary arteries in the
upper lobes, whereas they are usually medial to the adjacent
pulmonary arteries in the lower lobes.
Mucous impacted within bronchi may create the appearance of
intraluminal filling defects, particularly when the bronchial walls
are calcified. The absence of air-filled bronchi when the images
are reviewed in lung windows assists in demonstrating the true
nature of the abnormality (figure 8).
Apparent filling defects within pulmonary veins can simulate the
presence of acute PE. One may simply follow the vessel in question
into the left atrium to identify the true nature of the
abnormality. For more peripherally located apparent filling
defects, it is helpful to remember that pulmonary arteries and
bronchi course together, whereas veins run independently. If the
vessel in question is unaccompanied by a bronchus when the images
are reviewed in lung windows, it is a pulmonary vein rather than an
artery (figure 9).
Pulmonary artery catheters--
Pulmonary arterial catheter tips may create small filling defects
within the arteries simulating PE. Usually the relationship of the
catheter to the filling defect is obvious, although the catheter
itself may not be seen if the contrast bolus is bright. Examination
of the patient, consultation with the referring physician, or a
review of the scout image should clarify the situation.
Pulmonary artery sarcoma --
Pulmonary artery sarcomas are rare primary neoplasms that may
closely simulate PE.
Subtle enhancement of the filling defect itself and ipsilateral
lung nodules may be a clue to the correct diagnosis, although often
the true nature of the abnormality is not discovered until surgery
Technical pitfalls in the HCTPA diagnosis of acute PE
Respiratory motion is the most problematic technical difficulty
encountered with HCTPA imaging. Respiratory motion may obscure an
embolism (figure 11), and may also cause an apparent decrease in
arterial opacification, simulating the presence of acute PE.
Although shallow breathing during the scan is not usually
problematic, every effort should be made to suspend respiration
completely to optimize study quality.
Cardiac motion is usually less of a problem than respiratory
motion. Cardiac motion may result in a ghosting artifact, which is
usually readily recognizable.
Improper bolus timing--
The use of bolus-timing software usually results in a properly
timed study. If the contrast bolus arrives too early or too late,
an embolus may not be seen easily. If the bolus timing is improper,
a diligent search for the cause of the suboptimal timing should be
undertaken and the study may be repeated after the difficulties are
corrected (figure 12).
Quantum mottle, or image noise, may result in unsatisfactory study
quality. Mottle is more likely to be encountered if the FOV is
small and if the collimation is very thin (as with MDCT). To reduce
mottle, the FOV should be set properly, and the mA must be
increased appropriately, although these maneuvers obviously come at
the expense of increased radiation dose (figure 13).
Edge-enhancing reconstruction algorithms--
Edge-enhancing reconstruction algorithms can create the
false-positive appearance of acute PE. While edge-enhancing
algorithms are appropriate for review of images in lung windows,
smoothing algorithms are more appropriate for review of the images
in soft-tissue windows.
Accuracy of HCTPA for diagnosing acute PE
Adequately performed HCTPA studies are essentially 100%
sensitive and specific for large central emboli.
Numerous studies evaluating the accuracy of HCTPA have been
each with somewhat different techniques and study design. The
results of these studies suggest an overall sensitivity of HCTPA
for acute PE of approximately 90% when HCTPA is compared with
pulmonary angiography directly.
The major controversy surrounding the use of HCTPA revolves around
its accuracy for smaller clots, particularly subsegmental emboli.
In general, studies showing a lower sensitivity for HCTPA have
involved the use of HCTPA after nondiagnostic scintigraphy, a
situation that may tend to pre-select patients who are more likely
to have smaller emboli.
When evaluating the use of HCTPA for the diagnosis of small
pulmonary emboli, four issues must be addressed: 1) What is the
frequency of isolated subsegmental emboli? 2) How accurate is
pulmonary angiography for the diagnosis of small clots? 3) Are
small clots clinically significant? and 4) Can patients be managed
safely on the basis of negative HCTPA results alone?
What is the frequency of isolated subsegmental emboli?
The reported frequency of isolated subsegmental emboli is 6% to
In the Prospective Investigation of Pulmonary Embolism Diagnosis
approximately 6% of all patients were diagnosed with isolated
subsegmental clot only.
However, in other studies in which angiography was performed after
nondiagnostic scintigraphy, the frequency of isolated subsegmental
emboli ranges as high as 30%.
The latter finding tends to support the idea that strategies
directing patients to HCTPA after nondiagnostic scintigraphy tend
to select patients that are more likely to have smaller clots. In
unselected patients, the frequency of isolated small clots is
How accurate is pulmonary angiography for the diagnosis of
Pulmonary angiography has played a major role in evaluating the
accuracy of HCTPA in several studies.
However, the accuracy of pulmonary angiography for the
identification of small clots is questionable. In one study
addressing this issue, consensus readings of pulmonary angiograms
changed initial interpretations in 30% on a per-patient basis and
37% on a per-embolus basis.
Recently, pulmonary angiography was compared to 1-mm and 3-mm HCTPA
studies in pigs, after the pigs were injected with
methylmethacrylate beads ranging in size from 3.8 to 4.4 mm to
simulate the presence of small clots.
The investigators found no statistically significant difference in
sensitivity and positive predictive value between pulmonary
angiography and 1-mm and 3-mm HCTPA. Such data should be kept in
mind when reviewing studies that used pulmonary angiography as the
reference standard for evaluating HCTPA.
Are small clots clinically significant?
The clinical significance of small emboli is unclear. In one
large study of 627 patients suspected of acute PE who had
nondiagnostic scintigraphy and negative lower extremity venous
examinations (a protocol that tends to select patients with small
clots), and who were not treated, only 1.9% had evidence of acute
PE on long-term follow-up.
Such data implies that small, untreated clots may not be associated
with poor outcome. In the PIOPED series, 20 patients with acute PE
were not treated; 84% had segmental or smaller clots. Among these
patients, 1 died and 1 suffered nonfatal PE. However, there was no
difference in patient outcome when patients were grouped according
to the size of the vessel involved.
Although small clots may be reasonably well tolerated, patients
with impaired cardiopulmonary reserve may not fare so well. Such
patients have been noted to have a mortality approaching 7.8% when
small clots are untreated.
Can patients be managed safely on the basis of negative
HCTPA results alone?
Because of the controversies outlined above, the ultimate answer
to the question of the utility of HCTPA for the diagnosis of acute
PE is whether or not patients can be managed safely on the basis of
negative HCTPA results. Larger scale studies are currently under
way. The results of several smaller investigations have found that
the negative predictive value of HCTPA is >= 97%.
A major advantage of HCTPA over other studies traditionally used
to investigate patients suspected of acute PE is the ability of
HCTPA to make alternative diagnoses in cases interpreted as
negative. Alternative findings for symptoms simulating acute PE
were visible on HCTPA studies in at least 66% of cases interpreted
as negative for acute PE.
Helical CT venography
CT venography (CTV) offers the benefit of evaluating patients
for suspected venous thromboembolism, as opposed to limiting the
evaluation to only thoracic manifestations of this disease. Because
at least 90% of clinically significant acute PEs originate from the
a technique that has the ability to evaluate patients for PE and
deep venous thrombosis (DVT) simultaneously, using a
single-contrast bolus, offers obvious advantages.
CT venography technique
CT venography protocols differ among institutions. Some
investigators use 5-mm collimation helical imaging with a high
beginning the scan from either the level of the diaphragm or the
iliac crest and extending to the level of the tibial plateaus.
Other investigators have used 5- to 10-mm axial imaging every 2 cm
instead of helical imaging.
A scan delay of 2.5 to 3 minutes is adequate.
Findings of DVT on CT venography
As with acute PE, the single most reliable finding of acute DVT
on CTV studies is an intraluminal filling defect surrounded by
contrast (figures 14 and 15). Ancillary findings include
non-opacified venous segments, acute venous distension, enhancement
of the venous walls, prolonged arterial enhancement, and perivenous
Pitfalls in the CT diagnosis of acute DVT
Findings that may simulate the appearance of acute DVT on CTV
studies include venous valves, volume averaging of obliquely
oriented pelvic veins, and arterial thrombi mistaken for DVT.
Situations that may obscure the diagnosis of acute DVT include
arterial insufficiency resulting in poor venous enhancement, streak
artifacts from orthopedic hardware obscuring the venous system,
bilateral extensive DVT, and the presence of indwelling venous
Accuracy of CT venography
Several studies evaluating the performance of CTV techniques
have found that the sensitivity of CTV for the detection of acute
DVT ranges from 93% to 100%.
These studies have also found that >= 95% of CTV studies are
Clinical utility of CTV techniques
A good test of the utility of CTV is how often the addition of
CTV results to HCTPA studies changes clinical management. In one
study addressing this issue, the addition of CTV to HCTPA protocols
increased the yield for the diagnosis of venous thromboembolism
from 69% to 90%.
Another investigation found that the addition of CTV to HCTPA
examinations increased the yield for the diagnosis of venous
thromboembolism by 18%.
Recently, Loud and colleagues
reported their experience in 650 patients. These investigators
found that CTV techniques increased the yield over HCTPA techniques
alone by 36%.
Among the cases of DVT, these investigators reported 17% of cases
were limited to the abdominal or pelvic veins (figure 15), and thus
would likely not have been detected by ultrasound.
Such data highlights the fact that CTV techniques have the added
benefit of evaluating patients for nonlower-extremity sources of
DVT (pelvic and renal veins and inferior vena cava).
HCTPA, particularly with the addition of CTV, is a powerful tool
in the investigation of suspected venous thromboembolism. In
unselected patients, the sensitivity of HCTPA is approximately 90%,
and this figure will likely improve with the addition of CTV
protocols and the widespread introduction of MDCT. The negative
predictive value of HCTPA is high, and HCTPA studies often provide
alternative diagnoses accounting for clinical presentations
simulating acute PE for examinations that are interpreted as
negative. Attention to numerous technical parameters as well as
familiarity with a host of pitfalls will ensure high-quality
examinations and accurate diagnosis.