Pulmonary embolism (PE) occurs when a blood clot lodges in the pulmonary arteries. It can be difficult to diagnose due to non-specific symptoms. Imaging plays a key role in the evaluation of suspected PE. Computed tomography pulmonary angiography (CTPA) has become the initial imaging test of choice since it can directly visualize thrombi and rule out other potential causes. It has high sensitivity and specificity. Ventilation-perfusion scintigraphy and pulmonary angiography are older modalities that remain useful in certain settings. New techniques like dual-energy CT may improve detection of subsegmental emboli. Right heart strain on CT suggests worse prognosis.

1. Pulmonary Embolism
Imaging
Dr. Soe Moe Htoo

2. Introduction
• Pulmonary thromboembolism (PTE) is a common clinical entity that
results in morbidity and mortality in a large number of patients.
• Because of same signs and symptoms with a large number of
diseases & nonspecific risk factors, the diagnosis continues to be a
great clinical challenge.

3. Pulmonary Thromboembolic Disease
• Pulmonary embolus lodges within branch pulmonary arteries
• usually arise from pelvic or lower limb vein
• Saddle’ embolus
• a thrombus lodged at the main pulmonary arterial bifurcation
• Pulmonary infarction
• relatively rare as there is a second ‘systemic’ arterial supply to the
lungs from the bronchial arteries
• Infarction therefore requires compromise of both arterial supplies

4. • Diagnosis of PTE is based on the following:
1) Clinical pre-test probability
2) D-dimer level
3) Imaging findings

5. EVALUATION OF PATIENTS WITH PTE
• Analysis of the pre-test probability
• D-dimer level

6. Risk factors for PTE
• Advance ages
• Hypercoagulable state
• Orthopaedic surgery
• Pregnancy
• Malignant disease
• Prolong immobilization
• High oestrogen level

7. D-dimers
• breakdown product of crosslinked fibrin
• a measure of fibrinolytic activity
• ng/ml (<500)
• highly sensitive but non-specific test with a high false-positive rate
but a very high negative predictive value
• false-negative tests can occur (particularly with subsegmental emboli)

8. CLINICAL PRESENTATION
• Dyspnoea
• Chest pain (which may be pleuritic)
• Cough
• Haemoptysis
• Hypotension
• Tachycardia
• Pulmonary oedema (due to left ventricular failure precipitated by a
large PE)
• ECG changes are non-specific and only seen in patients with a severe
embolism: right bundle branch block right axis deviation and
ventricular hypertrophy

9. Two criteria
• Wells score
• Geneva score

12. IMAGING EVALUATION OF PATIENTS
WITH
SUSPECTED PTE
1) Conventional chest X-ray
2) Pulmonary angiography
3) Ventilation-perfusion lung scintigraphy
4) Computed tomography pulmonary angiography (CTPA)
5) Dual-energy CT (DECT)
6) Magnetic resonance imaging (MRI) of the chest
7) Ultrasonography

13. • Usually not useful in diagnosis of PE
• Useful in excluding other cause of acute chest pain (pneumonia, pul.
oedema or pneumothorax)
Conventional chest X-ray

14. Most common signs (without infarction):
• Regional oligemia Westermark sign (hypovolemia in the region of the lung
irrigated by the occluded vessel)
(Localized reduction in the peripheral blood flow, with or without
distension of the proximal vessels)
• Peripheral airspace opacification: this represents pulmonary haemorrhage
• Linear atelectasis: ischaemic injury to type II pneumocytes leads to
surfactant deficiency
• Pleural effusions: often small
• Central pulmonary arterial enlargement (Fleischner sign) : this is secondary
to chronic repeated embolic disease

15. Signs associated with infarction:
• Hampton’s hump:
• a pleural based, wedge-shaped opacity usually seen within the lateral or posterior
costophrenic sulcus
• the apex of the triangle points toward the occluded feeding vessel with its base
against the pleural surface, rarely contains air bronchograms
• Consolidation
• may be multifocal, predominantly lower lobes
• can be seen from 12 h to several days post embolism
• Cavitation: secondary infection at the infarction site or following a septic
embolus
• Haemorrhagic pleural effusions: this is seen in 50% of patients
• Serial CXRs:
• rapid resolution of any parenchymal changes is associated with a non-infarcting
• PE – infarction normally heals with scarring and localized pleural thickening

16. Frontal chest radiograph in a 55-year-old male
shows a wedge-shaped opacity in the periphery of
the right lateral lung (red arrows) concerning for
infarction, dubbed a “Hampton hump”
CTPA shows a filling defect
within an enlarged right
lower lobe lateral segmental
pulmonary artery consistent
with occlusive thrombus

17. Pulmonary thromboembolism.
(A) CXR demonstrating a moderate right pleural effusion on the day of symptom onset.
(B) CT taken 2 days after symptom onset reveals a filling defect (representing thrombus) in the right
lower lobe pulmonary artery.
(C) Further CXR taken 10 days after (A) with the patient on anticoagulant therapy reveals resolution of
the previous effusion. As a sequelae, a thin band of atelectasis is seen in the right lower zone.*

18. Causes of Pulmonary arterial hypertension
• Chronic lung disease
• Pulmonary embolic disease
• Pulmonary venous hypertension
• Cardiac shunts (left to right and bidirectional)
• Pulmonary arteritides

19. CXR
• Cardiac enlargement (right atrial and ventricular enlargement)
• central pulmonary arterial enlargement
• ‘Peripheral pruning’: tapering of the peripheral arterial vessels beyond the
segmental level
CT
• A smoothly marginated hilar arterial outline (cf. a lobulated border with
lymphadenopathy)
• Pulmonary arterial calcification (due to atheroma) can be seen with chronic
disease
• A transverse diameter of the mid-right descending pulmonary artery >
17mm
• The diameter of the main pulmonary artery is greater than the adjacent
ascending aorta

20. Pulmonary Angiography
• Invasive diagnostic method
• Intravenous catheter is introduced into the proximal pulmonary artery and
the contrast medium is injected rapidly.
• High spatial resolution, direct evaluation of the pulmonary arterial tree
• Although pulmonary angiography is considered the gold standard method,
it can lead to complications, mainly anaphylaxis, contrast-induced
nephrotoxicity, cardiac events, and pulmonary complications
• Now used only when a concomitant endovascular treatment is planned
• Findings:
• occlusion with abrupt cut-off
• filling defects, slow flow, and
• regional hypo-perfusion

22. Large right upper lobe filling defect consistent with acute PE
(arrow).
Also noted are areas of decreased perfusion within the
peripheral upper and lower lobes consistent with sub segmental
emboli.

23. Ventilation-perfusion lung
scintigraphy
• Although CTPA is the current gold standard, VQ scan is preferred,
particularly renal failure, contrast material allergies, young females, and
patients who cannot fit into the CT scanner.
• VQ scan has 50-fold lower radiation dose to the breast (0.28–0.9 vs. 50–80
mSv in 64 slice CT) , which makes it useful in young females, including
those who are pregnant.
• Ventilation agents
• aerosolized technetium-99m (Tc-99m) labeled agents [diethylene-
triamine-penta-acetic acid (DTPA), sulfur colloid, and ultrafine carbon
suspensions] and
• radioactive noble gases [Krypton-81m and Xenon-133
• Perfusion portion is performed following the intravenous injection of
200,000–700,000 particles of Tc-99m labeled macro-aggregated albumin
(MAA).

24. 81mKr
• optimal imaging agent
• emit high energy photons (190keV) & allows the ventilation images to
be obtained after the perfusion images  allowing matching of both
image sets without moving the patient
• short half-life (13 s)  it can be continuously administered (including
during perfusion imaging) although it means that no washout images
are possible
• Disadvantages:
• decreased resolution due to collimator penetration by the high-
energy photons
• it is expensive

25. 133Xe
• cheaper than 81mKr
• it is a less optimal imaging agent owing to its longer half-life (5.3 days)
and low photon energies (80keV)
• ventilation studies need to be performed prior to any perfusion
studies (thus preventing Compton scatter from the 99mTc into the
lower 133Xe photopeak)
• Single breath inhalation image: a cold spot is abnormal
• Equilibirum phase: tracer activity corresponds to aerated lung
• Washout phase: tracer retention corresponds to areas of air trapping
(e.g. COPD)

26. • Multiple planar images are obtained in upright position.
• Ventilation scans can be performed before or after the perfusion
scan.
• If perfusion scan is performed first and it is normal, then the
ventilation scan can be avoided, particularly in pregnant patients
• A peripheral wedge-shaped perfusion defect in a lobar, segmental, or
sub-segmental distribution without a corresponding ventilation
defect (i.e., a mismatched defect) raises the concern for the presence
of PE.

27. • Identifying ventilation in regions without perfusion at a location distal
to obstructing emboli is suggestive of PTE.
• Probability of embolism is classified as follows:
• High
• Intermediate
• Low
• Very low or
• Non-existent
• High probability confirm the diagnosis of PTE
• very low or non-existent probability allow the diagnosis to be
excluded.

28. • Anatomical data obtained with single-photon emission CT (SPECT)/CT
can be associated with the functional data obtained with scintigraphy.
• SPECT/CT has a high (99%) accuracy for the diagnosis, with a
sensitivity of 97–100% and a specificity of 83–100%

30. Modified Prospective Investigation of Pulmonary Embolism Diagnosis Criteria

31. Normal V/Q study
Ventilation [v] images on top row and perfusion [p] images beneath.
No defects are seen in either series.

32. Matched defects – multiple foci of nontracer uptake seen in ventilation and
perfusion series. Images reveals that the defects are well matched for position on
both series.

34. Perfusion defect (wedge-shaped peripherally) seen on perfusion imaging
which is not replicated on ventilation imaging.
This suggests a high probability for the presence of pulmonary embolus.*

36. Case 1

37. Case 2

38. Case 3 : middle aged female was referred with shortness of breath

39. CT Pulmonary Angiography (CTPA)
• CTPA is the imaging modality of choice for the workup of patients
with suspected acute PE
• high sensitivity and specificity, readily available, minimally invasive,
and fast with scan duration, cost-effective
• sensitive method of detecting main, lobar and segmental pulmonary
arterial emboli
• reliably detect emboli in up to 4th-order vessels (which are 7mm in
diameter)

40. CT windows for PE
• Lung window - width/level of 1500/600 HU
• Mediastinal window - width/level of 400/40 HU
• Pulmonary embolism window - width/level of 700/100 HU

41. Advantages
• Direct visualization of thrombus
• Can reveal other etiologies of chest pain and shortness of breath
(musculoskeletal injuries, pericardial abnormalities, pneumonia,
vascular pathologies, and even coronary artery disease)
Limitation
• not be suitable for patients with a low glomerular filtration rate (GFR)

42. Diagnostic criteria for acute PTE
• Arterial occlusion with filling defects throughout the lumen
• Diameter of occluded artery - increased in comparison with the
adjacent vessels
• Partial contrast filling defect:
• “polo mint” sign in images perpendicular to the long axis of the vessel
• “tram-track” sign in images acquired along that axis
• Pulmonary infarct
• wedge-shaped, peripheral opacity commonly with a “reverse-halo”
appearance consisting of central ground glass and a rim of consolidation
• Pleural effusions can also be seen with acute PE

43. Diagnostic criteria for chronic PTE
• intraluminal webs, calcification, thrombus recanalization, and filling defects
adherent to the wall that form obtuse angles and concave surfaces.
• vessels are typically smaller than normal, exhibit abnormal tapering, and
may show complete cut-off of the segmental vessel
• Parenchymal changes (mosaic perfusion, band-like opacities, and bronchial
dilation in abnormal areas)
• arterial recanalization; and the presence of a “web” within a contrast-filled
artery.
• Infarction: a peripheral wedge-shaped region of consolidation (analogous
to a Hampton’s hump on CXR) this is only a specific sign if the vessel can be
traced to the apex of the wedge
• Parameters for estimating the severity of PE and risk-stratification (right
heart strain, clot burden and lung perfusion)

44. • Features of right heart strain
• increased right ventricle (RV)/left ventricular (LV) ratio (>1 in axial
plane, >0.9 in 4-chamber reconstruction),
• flattening of interventricular septum and
• reflux of contrast material into the IVC and hepatic veins.
• RV/LV ratio >1.1 has been associated with increased risk of death
within 30 days. Four-chamber RV/LV ratio >0.9

45. Acute PTE in a 62-year-old female patient. CT slices, in axial and coronal
views (A and B, respectively), showing an extensive irregular filling defect in
the right and left pulmonary arteries, extending to its segmental branches.

46. Chronic PTE in an 86-year-old female patient with a history of breast
cancer. Axial CT scan showing a filling defect with obtuse margins in
the right pulmonary artery, with patent flow in the distal bed.

47. Correlation between a coronal CT slice (A) and a T2-weighted fast-spin-echo
coronal MRI sequence (B) showing a filling defect at the pulmonary artery
bifurcation.

48. CTPA demonstrating multiple bilateral
pulmonary emboli.
CTPA demonstrating a large filling
defect (thrombus) within the right main
pulmonary artery

49. CTPA identifying additional features that can be detected
supporting a diagnosis of pulmonary embolism. There is a
right-sided pleural effusion and dilatation of the right
ventricle (arrow)

50. Chronic pulmonary embolus.
CTPA showing mural thrombus (arrows) lining
the left and right pulmonary arteries

51. Dual Energy CT (DECT)
• based on the premise that materials behave differently when exposed
to X-ray photons of different energies.
• Higher molecular weight materials show a greater difference in X-ray
attenuation when exposed to low and high energy levels as compared
to lower molecular weight materials, due to the higher probability of
the photoelectric effect in high molecular weight materials when
interacting with lower energy X-rays
• Current two DECT techniques:
1) Dual-source CT scanners
2) Single-source CT scanners with rapid voltage switching

52. Iodine Mapping
• Data sets derived from DECT can be used to generate iodine maps
• allow visualization of the distribution and amount of iodine within the
lung after intravenous contrast administration (pulmonary perfusion)
• The use of iodine mapping in DECT is
• improve the accuracy of the diagnosis of PTE, especially for
segmental and subsegmental PTE which may not be detected by
CT
• Wedge-shaped perfusion defects are seen in acute PE, which has
been shown to correlate well with pulmonary perfusion on
scintigraphy

53. CT slices, in coronal and sagittal
views (A and B, respectively),
showing extensive filling defects
affecting the pulmonary artery
branches, mainly in the left lower
lobe.
Dual-energy CT, in axial and sagittal
views (C and D, respectively),
demonstrating an extensive
perfusion defect in the left lower
lobe due to acute PTE.

54. Axial CT slice (A) and iodine map created by the subtraction technique (B)
showing bilateral filling defects in the subsegmental branches in
correlation with the associated perfusion defects.

55. Limitations of DECT
• Scanner-related: high cost; relatively long image processing time; and
smaller field of view of the B tube, which may not include the
peripheral portion of the thorax
• Patient-related: obesity can which can increase image noise,
compromising the structural and functional analysis, and, in some
cases, patient weight exceeds the allowable limit of the DECT scanner.
• Interpretation: there are a limited number of radiologists who are
familiar with the technique; and the terminology has yet to be
standardized.

56. CTV
• CTV can also be used in the detection of deep vein thrombosis in the
extremities.
• It can be obtained at the same time of CTPA
• CTV can be performed adequately in patients with overlying casts, surgical
material, or wounds.
• Acute DVT on CTV appears as a complete or partial filling intraluminal
hypodense filling defect in a deep vein.
• The vein is typically expanded, and a rim of enhancing venous wall may be
seen. Secondary signs include edema in the adjacent tissues
• Chronic DVT manifests on CTV with small vessels, recanalization of
thrombus, calcifications, and thickened venous veins
• The mean CT attenuation of DVT is between 31 and 65 HU

57. MR angiography (MRA)
• provide both morphological and functional information.
• MRI protocols:
• free-breathing steady-state free precession
• post-contrast T1-weighted 3D-contrast enhanced MRA for pulmonary
angiogram
• T1-weighted 4D-contrast enhanced first pass perfusion study for lung
perfusion
• T1-weighted volumetric interpolated 3D gradient-echo for mediastinal and
pleural disease
• MRPA findings of PE include filling defects, complete absence of
vessel enhancement, dilatation of the main pulmonary artery, and
caliber change with post-stenotic dilatation

58. • In comparison with CTPA, MRPA had lower sensitivities for detecting
PE, especially in peripheral pulmonary arteries
• MRPA should be considered only in well-experienced facilities and
only for patients with contraindications to standard tests
• MRPA might be appropriate instead of CTPA and VQ scintigraphy only
in patients with intermediate pretest probability with a positive D-
dimer or high pretest probability

59. Axial MR image of the heart showing multiple filling defects
in the main pulmonary arteries, suggestive of pulmonary
embolism (arrows).

60. Lower extremity ultrasound
• Important step in the diagnostic algorithm for suspected VTE
• Acute deep vein thrombus manifests on US as hypoechoic intravascular
material that expands the venous lumen.
• DVT may be fully occlusive or non-occlusive, and the affected vessel lumen
will not collapse under compression
• Color Doppler may be useful to demonstrate a lack of flow, but caution
with this approach is recommended as blooming of color maps may
obscure small thrombus.
• Spectral Doppler waveforms can be observed for appropriate response to
calf compression; a lack of response being indicative of intervening
thrombus between the point of observation and the calf.
• Lack of normal respiratory phasicity of the more proximal veins may
indicate central venous thrombus.

61. Echocardiography
• Transthoracic echocardiography (TTE) has limited sensitivity and
specificity for diagnosis of acute PE.
• A negative echocardiogram cannot exclude a diagnosis of PE, and
similarly positive findings can be secondary to cardiorespiratory
disease in the absence of PE.
• Presence of right sided cardiac thrombus may be seen
RV overload criteria (seen in 30–40% of patients with PE)
• dilated diastolic RV diameter >30 mm or RV/LV >1
• systolic flattening of interventricular septum or
• acceleration time <90 ms

62. CONCLUSION
• CTPA is the current gold standard in the diagnosis of acute PE with high
accuracy, wide availability, and rapid turnaround time.
• Combining CTPA with clinical pre-test probability of PE yields superior
sensitivity and specificity than imagining alone.
• VQ scanning is indicated in patients who are young, pregnant or cannot get
contrast.
• Chest radiographs are useful in excluding other causes of chest pain.
• MRPA can provide good accuracy in centers with adequate expertise.
• In pregnant patients, lower extremity ultrasound is recommended as the
initial imaging modality.
• Echocardiography is useful in triaging high-risk PE patients.
• Invasive pulmonary angiography is reserved for those patients needing
endovascular intervention.

63. THANKS