Radionuclide Evaluation for Interventional Radiologists
Don C. Yoo
Sabah S. Tumeh
Ventilation-Perfusion Pulmonary Scintigraphy
Precautions
1. Severe pulmonary hypertension (the only reported instances of death following infusion of 99mTc-labeled macroaggregated albumin [MAA] have occurred in such cases). The dose (number of particles) should be appropriately reduced.
2. Pregnant or pediatric patients: The radiation dose should be reduced to minimize exposure to the fetus and to children.
3. Patients with known right to left shunts: The dose should be appropriately reduced.
4. Typically for dose reduction, the standard dose is decreased by a half.
Preprocedure Preparation
1. Chest radiographs (CXRs) taken within 24 hours of performing the lung scan should be available. The CXR should show clear lungs for an optimal ventilation/perfusion ([V with dot above]/[Q with dot above]) scan. Significant airspace disease or consolidation can make it difficult to interpret [V with dot above]/[Q with dot above] scans and can often result in intermediate probability studies. Therefore, if possible, checking the CXR prior to performing a [V with dot above]/[Q with dot above] scan would be ideal.
2. If the patient has had prior [V with dot above]/[Q with dot above] scans, it is also important to review them when interpreting the new study because chronic PE from uncanalized clot can have the same appearance as acute clots (3).
Procedure
1. Labeling of MAA with 99mTc
2. Intravenous (IV) injection of 3 to 5 mCi (200,000 to 1,000,000 particles) 30 to 100 µm diameter with patient supine and taking deep breaths
3. Images performed in the anterior, posterior, right lateral, left lateral, left posterior oblique, right posterior oblique, right anterior oblique, and left anterior oblique projections (minimum counts/image 750,000)
4. If the perfusion images are normal or heterogeneous but show no large wedgeshaped defect, the examination can be terminated because the probability of PE is very low.
5. If the perfusion scan is abnormal and xenon-133 is going to be used for ventilation, the projection that depicts the defects to the best advantage should be used to ventilate the patient. Radioxenon is introduced while the patient takes a deep breath for a single breath image. This is followed by rebreathing for about 2 to 4 minutes into a closed system to achieve equilibrium of distribution of radioxenon in the lungs. The patient then breaths into a tubing system connected to a radioxenon trapping container for washout images. The length of the ventilation scan should be no less than 6 minutes.
6. The initial image is acquired for 100,000 counts while the equilibrium and washout images are acquired for 45 to 60 seconds each.
7. If 99mTc-labeled diethylenetriamine pentaacetic acid (DTPA) aerosol is used as the ventilation agent, this scan should precede the perfusion scan. About 30 to 40 mCi of 99mTc are used to label the radioaerosol, whereas about only 800 to 1,000 µCi are actually inhaled by the patient. The advantage of this technique is the ability to acquire images in multiple projections and directly compare to the perfusion images. However, in many patients who cannot take a deep initial breath, there is a high incidence of deposition of the radioaerosol in the tracheobronchial tree, resulting in significant artifacts.
8. Lung scintigraphy for PE can also be performed using single-photon emission computed tomography (SPECT)/computed tomography (CT), which can improve sensitivity. In one study, the sensitivity of SPECT in four or five investigations was higher than that of planar [V with dot above]/[Q with dot above] imaging. When planar [V with dot above]/[Q with dot above] scans are “intermediate” (20% to 80% chance of PE), then SPECT/CT may increase the certainty of the presence or absence of PE. SPECT may further improve the performance of [V with dot above]/[Q with dot above] imaging, but a larger prospective evaluation is necessary (4).
Postprocedure Management
Usually none is needed.
Results
1. Revised Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) [V with dot above]/[Q with dot above] scan interpretation criteria (1)
a. High probability: At least two segmental perfusion defects without ventilatory or CXR abnormality. These could be divided into
(1) Two or more large (>75% of a segment) segmental perfusion defects without corresponding ventilation or chest radiographic images abnormality
(2) One large segmental perfusion defect and two or more moderate-size (25% to 75% of a segment) segmental perfusion defects without corresponding ventilation or CXR abnormality
(3) Four or more moderate-size segmental perfusion defects without corresponding ventilation or CXR abnormality
b. Intermediate probability
(1) One moderate to less than two large segmental perfusion defects without corresponding ventilation or CXR abnormality
(2) Corresponding [V with dot above]/[Q with dot above] defects and CXR parenchymal opacity in lower lung zone
(4) Difficult to categorize as normal, low, or high probability
c. Low probability
(1) Multiple matched [V with dot above]/[Q with dot above] defects, regardless of size, with normal CXR
(2) Corresponding [V with dot above]/[Q with dot above] defects and CXR parenchymal opacity in upper or middle lung zone
(3) Corresponding [V with dot above]/[Q with dot above] defects and large pleural effusion
(4) Any perfusion defects with substantially larger CXR abnormality
(5) Defects surrounded by normal-perfusion lung (stripe or rim sign)
(6) Single or multiple small (<25% of a segment) segmental perfusion defects with normal CXR
(7) Nonsegmental perfusion defects (round or non-wedge-shaped defects)
d. Normal: no perfusion defects; homogeneous distribution of radioactivity
2. Correlation of [V with dot above]/[Q with dot above] scan category (PIOPED) with the clinical likelihood of PE (1)
3. The Biello categorization of [V with dot above]/[Q with dot above] scans for the probability of PE (5)
a. High probability
(1) Single large (>90% of a segment) [V with dot above]/[Q with dot above] mismatch
(2) Perfusion defect substantially larger than density on CXR
(3) Multiple medium (25% to 90% of a segment) or large [V with dot above]/[Q with dot above] mismatches without matched density on CXR
b. Intermediate probability
(1) Severe diffuse obstructive pulmonary disease with perfusion defects
(2) Perfusion defect same size as CXR abnormality
