Imaging of Pulmonary Embolism-Traditional and Contemporary

Anjali Agrawal , MD Consultant ,Teleradiology Solutions

SER 2016, Bangalore

Objectives To underscore the importance of imaging

in diagnosis of PE To familiarize with the various imaging

modalities for PE To discuss the basic principles of each,

sensitivity,  specificity, drawbacks and current utility

Illustrate the key imaging findings with case examples

Imaging approach to PE in special clinical situations

 

Disease burden: PE

Third most common acute cardiovascular disease after MI and stroke

Incidence 1:1000 10% of symptomatic PE fatal in the first

hour Timely diagnosis and treatment can

decrease hospital mortality from 30% to 8%

PE at a younger age in the Indian population1.Wells PS, Rodger M. Diagnosis of pulmonary embolism:when is imaging needed? Clin Chest Med

2003;24:13-282.Kearon C. Natural history of venous thromboembolism. Circulation 2003;107:I-22-30.3.Giuntini C, et al. Pulmonary embolism: epidemiology. Chest 1995; 107:3S-9S4. Pulmonary and Pathology Departements, PGIMER , Chandigarh

Diagnosis of PE

Clinical history and exam D-dimer assay Imaging

Chest radiography Pulmonary angiography V/Q scintigraphy Venous doppler Echocardiography including TEE MDCT pulmonary angiography MRI and MR Angiography Dual energy CT

Clinical findings

Neither sensitive nor specific Overlap with many entities Combination of physical exam,

laboratory and imaging tests Imaging-mainstay

Arch Intern Med 1991; 151:933

D-dimer assay

High sensitivity but poor specificity

If positive, further evaluation is needed. Often positive in ICU and pregnant patients

Negative ELISA has >95% negative predictive value and can be used to r/o PE in low risk patients (less than 2 points Wells’ score)

Chest radiography

Findings nonspecific and not sensitive

Most common- atelectasis or linear opacities in lower lungs

Atelectasis

Chest radiography

Hampton’s humpWestermark’s and Palla’s Signs

Focal oligemia and a prominent right descending pulmonary artery

Specific, not sensitive

Chest radiography-Current Role

To exclude other entities

To help decide the choice of modality in specific situations

Ventilation-perfusion scintigraphy Introduced in the 60’sMain imaging modality for PE prior to

CTPADiagnosis of PE = Wedge-shaped perfusion defects with the

base toward the pleura Preserved ventilation in these areas

produces V/Q mismatches

Modified PIOPED criteria

High probability (high specificity of 97%)- usually indicates a PE, but only 40% of patients with PE have a high probability scan

Intermediate probabilityLow probability- 15% patients might

still have PENormal- excludes clinically

significant PEThe PIOPED Investigators. Value of the ventilation/perfusion scan in acute pulmonary embolism: results of the prospective investigation of pulmonary embolism diagnosis. JAMA 1990

Normal scan (NPV 96-100%)

Fleischner’s sign

High probability scan (PPV 83-92%)

Large matched ventilation perfusion defect in the right lower lobe with corresponding radiographic opacity.

By the modified PIOPED criteria, intermediate probability for pulmonary embolism

Significant number in the indeterminate category

Interobserver variabilityLong examination times and

radiotracer availabilityAlternate diagnoses?

Ventilation-perfusion scintigraphyVQ scintigraphy- Drawbacks

Renal insufficiencyAllergy to IV iodinated contrastYoung female patientsFollow-up Pregnancy-if chest radiograph

normal

Ventilation-perfusion scintigraphyVQ scintigraphy- Current role

Venous sonography

Mainstay for evaluation of venous thrombosis. First-line test in suspected PE in certain clinical situations

Combination of Grey scale compression sonography, color flow and spectral Doppler evaluation: high negative predictive value

Venous sonography-Limitations

Technical expertise Isolated calf vein

thrombus Compression

challenging Anatomic variants Rationale for venous

doppler in PE

CT diagnosis of pulmonary embolism

MDCT with fast caudocranial acquisition of volumetric data. Reference standard since 2007 (ACR and Fleischner Society)*.

PE = intraluminal filling defects showing sharp interface with the intravascular contrast

Normal study excludes PE (NPV 99%) Study may be indeterminate due to

ambiguous findings or suboptimal quality

*Remy-Jardin M, et al. Management of suspected acute PE in the era of CT angiography: a statement from the Fleischner Society. Radiology 2007

Acute PE

Arterial occlusion: failure to enhance the

entire lumen due to a large filling defectOccluded artery may be enlarged

Acute PEA partial filling defect surrounded by contrast material, producing the "polo mint" sign

or

the "railway track" sign

Saddle PEIntraluminal filling defect forms acute angles with the arterial wall

Acute PE

Segmental PE with infarct

Pulmonary infarcts

Distribution-segmental, peripheral wedge-shaped

Lower lobe involvement, pleural fluid

Subsegmental PE –MIP reformations

Hyperdense material in the pulmonary artery on unenhanced CT

30 F, S/P hysterectomy, SOB

Chronic PE

Complete occlusion of a vessel that is smaller than the adjacent arteries

Peripheral intraluminal filling defect forms obtuse angles with the vessel wall

Thickened small caliber arteries with calcification

Pulmonary artery flap or web Bronchial or systemic collaterals PAH

Dx

Heterogeneous lungs, with larger geographic peripheral hyperlucent areas Peripheral pruning of the vessels and variable vessel caliber indicate a vascular etiology

Dilated main pulmonary arteries with wall calcifications

Chronic pulmonary thromboembolism

Peripheral intraluminal filling defect forms obtuse angles with the vessel wall.

Thickened small caliber arteries with calcification.

Bronchial or systemic collaterals.

Courtesy: Sridhar P.G., MD

Pulmonary artery flap or band

PAH

Occlusion of a vessel that is smaller than the adjacent arteries

CTA chest and Indirect CT venogram

CTV is useful as a one-stop-shop in sick /ICU patients or those with a high risk for DVT

Pitfalls

Beam-hardening streak artifact from dense contrast in the SVC

Partial volume averaging artifact

Lung algorithm Standard algorithm

Flow artifact in the right lower lobe pulmonary artery due to right lower lobe consolidation

Respiratory motion artifact

Respiratory motion artifact creating the “seagull” sign, inconclusive study

Prognostic CT findings-varied results Ventricular septal bowing predictive of death

due to PE but low sensitivity and high interobserver variability.

Ratio between the diameter of the RV and LV may be predictive

Embolic burden independently not associated with short term death due to PE.

Pulmonary perfusion defect score, CTA clot load score and cardiovascular manifestations helpful in assessing the severity of PE

Araoz et al. Pulmonary Embolism: Prognostic CT Findings. Radiology 2007

Atasoy MM, et al. Nonsevere Acute PE:Prognostic CTPA Findings. JCAT 2015

Zhou Y, et al. Assessment of correlation between CTA clot load score, pulmonary perfusion defect score and global right ventricular function with dual source CT for acute PE. BJR 2012

Right ventricular strain

Right ventricular dilatation

Deviation of the interventricular septum toward the left ventricle

Contrast reflux into the hepatic veins

Electron Beam CTUseful for volume scanning to depict

structure and functional analyses of pulmonary blood flow by acquiring high temporal resolution data sets

Drawback: increased radiation exposure, a limited portion of the lung evaluated for blood flow

Dual energy CT: Physics

Exploit differences in the mass attenuation coefficients of different materials as a function of energy:

Energy subtraction-a weighted subtraction of images taken at two different energies

Basis material decomposition-decomposition of the measured data into contributions due to the “basis materials”.

Can offer information on tissue composition

Dual energy CT in PEData post processed to determine

the amount of iodinated contrast in a voxel. Iodine and soft tissue attenuate X-ray differently at 80 and 140kVp settings.

Hybrid image used for diagnosis of morphology and anatomy

Iodine map reflects microvascular circulation in the lung and provides morphological and functional informationLu G-M, et. Dual energy CT in PE. Br J Radiol.2010

Basis of utility of DECT3-5 mm subsegmental emboli can

create 3-5 cm diameter contrast enhancement defects in the distal parenchyma

Compromised physiology -anatomical obstruction, release of chemical from platelets cause V/Q mismatch.

BFI and CTPA: complimentary roles in diagnosis of PE

Pulmonary embolism DECT with infarction

Why only infarction? Why not pneumonia or aspiration?

Size of PBV defect with infarct bigger than size of abnormality on lung windows!

Courtesy: Alexi Otrakji, MDMGH

60 kev60 kev PBV

Pulmonary embolism with lobar defect on PBV image (no infarction)

60 kev

Generally lung windows are normal

or subtly darker with PBV defects

without infarction.

Rule out: Air-trapping!

60 kev

PBV

Segmental Pulmonary Emboli with NO infarction

60 kev

PBV

PBV

Pulmonary emboli with segmental defects on PBV (no infarction)

MRI/MRAAlternative technique in PE diagnosisAllergy to iodinated contrast. Lack of

ionizing radiationProblems-subsegmental PE.

Increased time of imaging.Combination of MRI with Gadolinium

enhanced MRA and MR perfusion comparable to MDCT

Alexander Kluge et al.Acute Pulmonary Embolism to the Subsegmental Level: Diagnostic Accuracy of Three MRI Techniques Compared with 16-MDCT. AJR 2006

Alexander Kluge et al. Acute Pulmonary Embolism to the Subsegmental Level: Diagnostic Accuracy of Three MRI Techniques Compared with 16-MDCT. AJR 2006

MR perfusion

MR angiogram MRI (TrueFISP)

Conventional angiography-RoleCatheter directed intervention

Special situations ICU or high risk patients in shock or hypotension-> MDCT

D-dimer often positive due to other reasons

Iodinated contrast allergy - mild cases-> premedicate -> MDCT -severe allergy-> V/Q scan or MRA

Renal impairment- V/Q scan and venous dopplerGadolinium can lead to nephrogenic systemic fibrosis

Pregnancy-Venous doppler as the first-line test. If normal CxR, can consider a perfusion scan with reduced dose or CTPA.

Conclusion

Imaging cornerstone for diagnosis and risk stratification

CTA gold standardVenous doppler and VQ scans

important roles in specific situationsDECT-comprehensive anatomic and

functional informationMRI/MRA problem solving in certain

clinical settings

Anjali Agrawal , MD anjali.agrawal@telradsol.com

Imaging of Pulmonary Embolism-traditional and contemporary