Introduction
Pulmonary embolism (PE) is widely acknowledged as an
emergency vascular disease (1) that
is frequently neglected and misdiagnosed, and can result in
mortality. Early symptoms are atypical, while 30–65% of patients
may be asymptomatic. Dyspnea, pectoralgia and hemoptysis are the
triad of symptoms of the disease, although they only occur in 3–20%
of patients (2). Acute PE can occur
in patients with acute myocardial infarction and aortic dissection
(3). Although therapy, including
anticoagulation, thrombolysis and thrombectomy, is available, its
efficacy are not satisfactory (4).
Lack of awareness in the early diagnosis is an
important risk factor (5). The
development of diagnostic technologies such as multislice computed
tomographic pulmonary angiography (msCTPA) and MRPA have led to
improvement in the diagnosis of PE, reaching up to 80–95% (6). 3D dynamic contrast-enhanced magnetic
resonance pulmonary angiography (3D-DCE-MRPA) surpasses msCTPA in
screening the pulmonary artery for emboli, reduces X-ray exposure
and the negative effects of contrast agent (7).
The aim of the present study was to examine the
characteristics of imaging in the diagnosis of PE.
Patients and methods
Patients
Sixty patients suspected highly of having acute PE
were selected between January, 2013 and January, 2016 in the
People's Liberation Army 107th Hospital. The exclusion criteria for
the study were: Severe cases; <72 h of expected survival;
complications including acute myocardial infarction, aortic
dissection or sudden death; chronic PE; primary lung diseases such
as chronic obstructive pulmonary disease, lung cancer or
bronchiectasia; failure to end emergent computed tomography (CT) or
magnetic resonance imaging (MRI); unqualified image and undiagnosed
PE. Thirty cases, including 18 men and 12 women, aged 37–76 years
with an average age of 56.4±13.8 years, were analyzed using
3D-DCE-MRPA. Twenty-two patients had increasing D-dimer levels
while 14 had deep venous thrombosis (DVT). Thirty cases, including
17 men and 13 women, aged 39–79 years, with an average age of
57.2±15.6 years, were analyzed using msCTPA. Four had increasing
D-dimer levels while 12 had DVT.
The study was approved by the Ethics Committee of
the the People's Liberation Army 107th Hospital. Informed consent
was obtained from the patients and/or their families.
3D-DCE-MRPA
The 3.0T MRI system (MAGNETOM Avanto, Siemens,
Erlangen, Germany) and its software INDY R500 Silicon Graphic
station were used. The contrast agent used was 469 mg Gd-DTPA
(Guangzhou Consun Pharmaceutical Co., Ltd., Guangzhou, China),
which was administered intravenously (22GA1.001N, 0.9×25 mm)
(Becton-Dickinson, Suzhou, China). Cardiac gating was applied, and
standard transverse, coronal and sagittal scans were taken
including T1WI, fast spin echo (FSE) and 3D fast field echo (FFE)
under the coronal section (Table I).
The region of interest (ROI) was found, and a scan slice was set
from the pulmonary trunk to the posterior thoracic vertebra. Breath
holding was key for the scanning and patients were instructed to
hold their breaths for 20 sec at 10-sec intervals. Four groups of
images, including arterial-phase and delayed-phase, were obtained.
Maximum intensity projection (MIP) was used for high resolution
dimensional reconstructed radiography. Scans were rotated 15° along
their axes, resulting in 13 multi-planar rotated images for
observation.
 | Table I.MRI sequence scanning parameter
settings. |
Table I.
MRI sequence scanning parameter
settings.
| Sequence | TR (msec) | TE (msec) | TI (msec) | FA (degree) | FOV (cm) | Matrix | Collection time |
|---|
| T1WI | 583 | 15 |
| 90/180 | 42×42 | 224×256 | 2 |
| FSE, T2WI | 3,000 | 100 |
| 90/160 | 18×18 | 192×256 | 2 |
| 3D FFE | 9 | 3 | 100 | 20 | 40×40 | 128×256 | 1 |
MSCTPA
A Somatom Definition CT scanner was used for
examinations from the thoracic inlet to the level of the
costophrenic angle. Iopromide (370 mg) was used as the contrast
agent, and the scans were taken automatically with the pulmonary
trunk as the ROI (threshold, 90–100 HU). The scanning parameters
were as follows: Collimation, 0.6 mm; changing pitch, 1.35–1.55
with heart rate; slice thickness, 0.75 mm; reconstructive interval,
0.5 mm; voltage, 120 kV; tube current, 180–250 mAs, rotation time,
0.33 sec; and scanning time, 4–6 sec. Original data were used for
multi-planar reconstruction (MPR), MIP and volume rendering
(VR).
Observation indexes
Direct signs including the location, number, and
formation of emboli, as well as indirect signs including pulmonary
infarction, pneumonia, and pleural effusion, were analyzed. Image
quality was contrasted, the pulmonary artery branch was revealed,
and differences of sensitivity, specificity and signal-to-noise
ratio (SNR) were assessed.
Following embolism, the pulmonary trunk, left and
right pulmonary arteries, transverse diameter of the phrenic
artery, signal strength of corresponding surface and cervical
muscles, and the standard deviation of background noise, were
analyzed in the original and MIP reconstructed images. The SNR of
corresponding vessels and carrier-to-noise ratio (CNR) between
vessels and cervical muscles were calculated as follows: SNR =
average signal strength of ROI/noise standard deviation, and CNR =
SNRvessel - SNRcervical muscle.
The quality of artery images were divided into four grades. Grade I
was poor quality, with blurring of the pulmonary artery segment and
part of the subsegment in addition to considerable vein overlap
which affected the diagnosis. Grade II appeared normal, with rough
segment of the pulmonary artery and part of the subsegment in
addition to a moderate amount of pulmonary vein overlap which
affected the diagnosis slightly. Grade III images were of good
quality, with clearly visible segment of the pulmonary artery part
of the subsegment as well as minor vein overlap which conformed to
the diagnosis. Grade IV images were optimal, with clear pulmonary
artery segment and part of subsegment with little vein overlap
which made for accurate diagnosis. The levels of the pulmonary
branch were classified as: Pulmonary trunk was level 1, left and
right pulmonary arteries were level 2, interlobar artery was level
3, segmental artery was level 4, subsegmental artery (grade I) was
level 5, and grade II subsegmental artery was level 6. The
pulmonary artery tapered and any interruption or filling defects
that contrasted with its normal formation were considered
diagnostic criteria of PE.
Statistical analysis
Data were analyzed using SPSS. Quantitative data are
presented as mean ± standard deviation. Differences among the
groups were assessed by t-test and χ2 test. Countable
data were expressed as case or percentage (%). Ranked data were
evaluated by Kruskal-Wallis. P<0.05 was considered to indicate a
statistically significant difference.
Results
Comparison of direct signs
The average number and different formations of
emboli in the two groups were compared, and there were no
significant differences (P>0.05). There were more emboli
appearing in segmental or subsegmental arteries in the MRI group,
and the difference was significant (P<0.05) (Table II).
| Table II.Comparison of direct and indirect
signs [cases (%)]. |
Table II.
Comparison of direct and indirect
signs [cases (%)].
|
|
| Location of
embolus |
| Formation |
|---|
|
|
|
|
|
|
|---|
| Groups | Cases | Subsegment
artery | Segmental artery |
| Interlobar
artery | Left, right and trunk
of artery | Numbers | Central | Intraluminal | Totally
obstructive |
|---|
| CT | 30 | 5 (16.7) | 8 (26.7) |
| 16 (53.3) | 1 (3.3) | 1.5±0.2 | 8 (26.7) | 15 (50.0) | 7 (23.3) |
| MRI | 30 | 10 (33.3) | 13 (43.3) |
| 6 (20.0) | 1 (3.3) | 1.6±0.3 | 8 (26.7) | 13 (43.3) | 9 (30.0) |
| t
(χ2)-test |
|
|
| 7.024a |
|
| 0.627 |
| 0.393 |
|
| P-value |
|
|
| 0.030 |
|
| 0.849 |
| 0.822 |
|
Comparison of indirect signs
We compared the indirect signs between the two
groups, but there were no significant differences (P>0.05)
(Table III).
| Table III.Comparison of indirect signs [cases
(%)]. |
Table III.
Comparison of indirect signs [cases
(%)].
| Groups | Cases | Infarction | Pneumonia | Hydrothorax | Enlargement of
pulmonary artery |
|---|
| CT | 30 | 4 (13.3) | 16 (53.3) | 3 (10.0) | 7
(23.3) |
| MRI | 30 | 3 (10.0) | 15 (50.0) | 2 (6.7) | 10 (33.3) |
| χ2 |
|
|
|
0.909 |
|
| P-value |
|
|
|
0.823 |
|
Comparison of image quality, pulmonary
artery branches and SNR
Image quality in the two groups was not
statistically different (P>0.05). There were more level 5 and 6
pulmonary arteries in the MRI group than in the CT group, which
also had higher SNR and CNR. The differences were significant
(P<0.05) (Table IV).
| Table IV.Comparison of image quality, pulmonary
artery branches and SNR [cases (%)]. |
Table IV.
Comparison of image quality, pulmonary
artery branches and SNR [cases (%)].
|
| Image quality | Branch of pulmonary
artery | SNR |
|---|
|
|
|
|
|
|---|
| Groups | Cases | I | II | III | IV | Level 1–3 | Level 4 | Level 5 and 6 | SNR | CNR |
|---|
| CT | 30 | 3 (10.0) | 8 (26.7) |
13
(43.3) | 6 (20.0) | 3 (10.0) | 20 (50.0) | 7 (23.3) | 41.3±7.6 | 32.5±5.9 |
| MRI | 30 | 2 (6.7) | 6 (20.0) |
14
(46.7) | 8 (26.7) | 4 (13.3) | 10 (33.3) | 16 (53.3) | 45.6±8.2 | 36.7±6.0 |
| t
(χ2)-test |
|
|
| 0.812 |
|
| 7.159 |
| 5.632 | 5.487 |
| P-value |
|
|
| 0.847 |
|
| 0.028 |
| 0.030 | 0.033 |
Comparison of sensitivity and
specificity in diagnosis
Twenty-six cases of PE were confirmed in the CT
group, in which sensitivity was 90.5% and specificity was 86.7%.
Twenty-five cases were confirmed in the MRI group, in which
sensitivity was 92.3% and specificity was 84.2%.
Discussion
The process of imaging by 3D-DCE-MRPA was performed
as follows: Paramagnetic contrast agent was injected which
shortened the T1 time to adjacent tissues, including adipose
tissue. FFE was selected for scanning of the targeted vessels while
patients held their breath. After reconstruction of MIP,
well-defined and high-resolution vessel images were obtained. The
images were observed at every angle and there was no need to apply
spatial pre-saturation, which eliminated disadvantages such as
unenhanced MRA, making it suitable for large vessels such as the
pulmonary artery. Pulmonary emboli were revealed with filling
defects in the pulmonary trunk or branch or with disconnection,
blocking, narrowing or deficiency of pulmonary arterial branches
(8). It has been shown that if
emboli are located in the center of the lumen, a ‘track sign’
appears (9). Compared to msCTPA,
3D-DCE-MRPA allowed for the acquisition of multidimensional images,
and the pulmonary vessels in the macropinacoid, middle lobular and
lingular artery were seen with precision (10). Therefore, 3D-DCE-MRPA is superior to
msCTPA in revealing smaller pulmonary arterial branches. Similarly,
examination of the deep vein in the pelvis and lower limbs is
convenient and precise (11). PE and
deep vein thrombosis are defined as different stages of venous
thromboembolism (VTE), advancing the screening of potential PE.
Sensitivity in diagnosing PE approached 100%, specificity was 95%,
the positive predictive value was 86% and the negative predictive
value was 100% (12).
Use of msCTPA in diagnosing emboli in the pulmonary
trunk, interlobar and segmental artery is highly accurate, with a
sensitivity of 94–98%, and specificity of 96–99%. However,
diagnosis in the subsegmental and peripheral vessels proved more
difficult (13). In addition, more
contrast agent was needed for enhanced scanning, which is toxic and
can have adverse effects on patients with liver or kidney
dysfunction (14). High resolution
time leads to monocyclic reconstruction time being shortened to 83
msec. This avoids artifacts caused by the heartbeat and breathing
and enhances image quality and efficiency of diagnosis. In
addition, peripheral vessels, such as the pulmonary artery and
aorta were clearly presented (15).
Original images, with high spatial resolution provided a high
guarantee for post-processing techniques. The value of isotropic
projection reconstruction was obtained, without the misdiagnosis of
vertical and subsegmental arteries (16). The requirements for diagnosis and
treatment of acute chest pain were met. Standard diagnostic
requirements, such as examining for the triad of symptoms of PE,
obtaining images of the coronary artery, pulmonary artery and other
large vessels in the thorax were unnecessary and were accomplished
by a single, rapid scan in order for the diagnoses to be made
rapidly and with precision (17).
Various post-processing techniques of images guaranteed definite
identification of emboli within the pulmonary artery.
Intra-arterial filling-defects were shown in MPR. The subtypes were
identified and pulmonary artery filling-defects were revealed in
the multi-directional and multi-angle phase. Emboli in the
horizontal segmental and subsegmental pulmonary arteries were
precisely diagnosed (18). The
pulmonary artery has been directly visualized with the assistance
of MIP and VR technology. In particular, the location of the
pulmonary segmental artery has been directly visualized (19), providing exact objective evidence for
physicians.
In conclusion, the average number and formations of
emboli in the two groups were compared, and no significant
differences were observed. There were more emboli appearing in the
segmental or subsegmental arteries in the MRI group, and the
difference was statistically significant. A comparison of indirect
signs between the two groups revealed no significant differences.
The image quality in the two groups was not statistically
different. Level 5 and 6 pulmonary arteries were observed more in
the MRI group than in the CT group, with higher SNR and CNR. The
differences were significant. Twenty-six cases of PE were confirmed
in the CT group, with sensitivity of 90.5% and specificity of
86.7%. Twenty-five cases were confirmed in the MRI group, with a
sensitivity of 92.3% and specificity of 84.2%. Compared with
msCTPA, more segmental or subsegmental pulmonary arteries were
observed using 3D-DCE-MRPA, with enhanced SNR.
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