5. Modified Wells Criteria
Clinical symptoms of DVT: 3
Other diagnosis less likely than PE: 3
Heart rate >100 beats/min: 1.5
Immobilization or surgery <4 weeks: 1.5
Previous DVT or PE: 1.5
Hemoptysis: 1
Malignancy: 1
CLINICAL PROBABILITY ASSESSMENT
High: >6 (41%)
Moderate: 2-6 (16%)
Low: <2 (0.5-2.7%)
6.
7. Pulmonary arteriography
• Gold standard
• Sensitivity: 98%; Specificity: 97%
• Invasive, 0.5% risk for death
and 2% risk for other major
complication
8. Chest radiographs
• Can reveal other causes
• Normal radiographs are
common
• Pleural-based Hampton
hump
hemorrhage filling the
alveolar spaces
9. Nuclear medicine ventilation (V) /
perfusion (Q) scan
• The PIOPED trial found that the V/Q to be accurate
• But the interpretation criteria were complex
• Effective dose about 2 mSv or less
• Many sites utilize very low-dose techniques
involving perfusion only techniques
• Since then, a number of modifications to the
criteria have been suggested
PIOPED, the Prospective Investigation of Pulmonary Embolism Diagnosis
10. Computed tomography
arteriography, CTA
• Widely used as the primary method for PE
• In PIOPED II with MDCT:
Sensitivity
83%, 78% (if indeterminate scan included)
Specificity
96%
Positive predictive value
86%
Negative predictive value
95%
• Nondiagnostic CT scans average 6% but may be
higher in pregnancy, where it might reach as high
as 25%
11. Computed tomography
arteriography, CTA
• Patients with renal failure or allergy to iodinated
contrast – CTA (X)
• Effective dose of CT: 3 to 5 mSv
is associated with increased risk of cancer
12. Magnetic resonance angiography,
MRA
• Attempts to replace CTA with MRA have not yet
been successful.
• The PIOPED III trial reported a sensitivity of only
63% for pulmonary emboli
13. It should be noted that
• In PIOPED II trial, failed to prove the superiority of
CTA over V/Q
• Both CTA and V/Q lose value if the objective clinical
evaluation is discordant or does not match the
imaging results.
14. It should be noted that
• In PIOPED I, V/Q were performed on old equipment
• New, simplified interpretation criteria are being
considered in the United States and are already
recommended in Europe.
20. Many Diseases Affect Ventilation
• Asthma
• Bronchitis
• Pneumonia
• Emphysema
The normal regional
response to acute hypoxia
is vasoconstriction, which
shunts blood flow away to
other aerated areas of
lung. This results in regions
of reduced perfusion and
reduced ventilation that are
“matched.”
21. In a pulmonary embolus
• Pulmonary arterial perfusion
is reduced
• The ventilation is unaffected
in such cases, so the alveolar
spaces remain aerated.
Ventilation and perfusion
are uncoupled or
“mismatched” in territories
affected by the pulmonary
embolus.
24. Perfusion Radiopharmaceuticals
• Iodine-131 macroaggregated albumin
• Tc-99m labeled human albumin microspheres
• Tc-99m macroaggregated albumin – U.S.A
• Only Tc-99m MAA is available for clinical
use in the United States
25. Tc-99m MAA
• Size ranges from 5 to 100 µm
• Generally 60% to 80% between 10 and 30 µm
• Biological half-life in the lung: 4 to 6 hours
• Number of particles is important
Particles
Adult
200,000 to 500,000
Neonates
10,000
<5 y/o
50,000
Pulmonary hypertension
100,000 to 250,000
Right to left shunt
100,000 to 150,000
Pregnancy
Minimum 100,000
26. Tc-99m MAA
• Many sites advocate
performing a reduced-dose
perfusion-only scan with 1
to 2 mCi (37-74 MBq) of
Tc-9m MAA in pregnant
patients to reduce fetal
exposure
27. Tc-99m MAA
• Because there are an estimated 300 million
precapillary arterioles and more than 280 billion
pulmonary capillaries, this should result in no ill
effects with obstruction of only 0.1% to 0.3% of
vessels
28. Ventilation Radiopharmaceuticals
An ideal ventilation radiopharmaceutical
• Could be used after the perfusion examination
• Optimal radiolabel for gamma camera imaging
• Closely model respiration
• For SPECT, it should not clear too rapidly from the
lungs
31. Xenon-133
Several disadvantages
• Rapid washout limits the number of views and
projections obtainable
• Photopeak of 81-keV is not optimal for gamma
camera imaging
• Ventilation scan must be performed first
• Need special equipment
32. Tc-99m DTPA
• Tc-99m DTPA aerosol
particles are cleared by
crossing the airway
membrane and entering
the circulation, where
they are cleared by the
kidney. The biological
half-life is
approximately 80 ± 20
minutes in healthy
people and 24 ± 9
minutes in healthy
smokers.
33. Tc-99m DTPA
• The ideal particle size is
in the range of 0.1 to 0.5
μm. Particles larger than
1 to 2 μm tend to
settle out in the large
airways - possibly
“shining through” onto
the subsequent
perfusion examination
34. Tc-99m DTPA
• Particle clumping
may still occur in
patients unable to
cooperate with deep
breathing or who
have asthma or
COPD.
35. Tc-99m Technegas
• The aerosol Technegas consists of Tc-99m–
labeled solid graphite particles with a submicron
diameter of approximately 100 nM (0.005-0.2 μm)
in argon carrier gas
36. Tc-99m Technegas
• Like a gas, the particles penetrate far into the lung
periphery without the central clumping often
seen in COPD with Tc-99m DTPA.
• In addition to having the advantages of a gas,
multiple views or SPECT imaging is possible, adding
appeal
37. Tc-99m Technegas
• Produced onsite by a generator. This generator
requires regular maintenance
• Tc-99m Technegas is widely used outside of the
United States, and the images appear superior to
those obtained with Tc-99m DTPA and Xe-133
38. Dosimetry
• Reports describe the effective whole-body dose
for a V/Q scan to be 1.4 to 2 mSv compared to
2.2 to 6.0 mSv from CTA
• In pregnant patients, fetal doses from V/Q are low,
reportedly 350 to 570 μGy using a reduced 1 to 2
mCi (37-74 MBq)-MBq Tc-99m MAA dose.
However, this is generally higher than with CT
