Egyptian J. Nucl. Med., Vol. 1, No. 1, 3-15 (2009)
LUNG REVIEW
DIAGNOSIS OF PULMONARY EMBOLISM: A CONTINUING DILEMMA.
ELGAZZAR, A.H. MD, FCAP and AL ENIZI, E. MD, KBNM
Department of Nuclear medicine, Kuwait University
Basic considerations: 1 completely. This can happen even within The vast majority of pulmonary a few minutes. Second, partial venous emboli are thromboemboli originating obstruction may occur. Blood flow therefore continues over the thrombus from deep veins. Fat, air, or tumor emboli are rare [1]. Fat emboli are surface. Under this circumstance, reported with long bone fractures and thrombus growth tends to occur by the liposuction while air emboli occur with progressive layering of platelets and cardiac and neurosurgeries. Renal cell fibrin on the clot surface, pathologically carcinoma with invasion reaching seen as the lines of Zahn. Third, probably inferior vena cava is a clinical setting that the most common scenario, a small may lead to tumor emboli. Data indicate thrombus is swept away before it reaches that 90% of pulmonary thromboemboli an appreciable size. It lodges in the originate from the lower extremities and pulmonary vasculature without pelvis. The remainder come from symptoms. thrombi that occur in the right side of the Unless fibrinolytic resolution is heart or in bronchial or cervical veins. prompt, organization of the thrombus Embolization and symptomatology are begins within hours of formation. What proportional to how proximal is the vein was a thrombus is slowly replaced by that contains the thrombus. The vast granulation tissue. This process anchors majority of pulmonary thromboemboli the thrombus to the venous wall. The originating from thrombi of the lower dynamic battle between fibrinolysis and extremities come more frequently from thrombus formation is fought out over a the thigh and pelvis (75%) than from period of 7–10 days, at the end of which smaller veins of the calf and feet [2,3]. time either complete resolution has The risk of pulmonary embolus is also occurred or an endothelialized residual is directly related to the presence of a present. At any time during this period, a residual clot at the site of a venous portion or all of the thrombus can detach thrombus [4]. as an embolus. This risk is highest early, One of three events can happen before significant dissolution or during the natural history of venous organization occurs [3]. thrombi. First, the red thrombus grows Pulmonary embolism is potentially explosively and obstructs the vein fatal and the most common pathological condition involving the lungs of hospitalized patients. The majority of Correspondence and reprints: Adelhamid fatal emboli are not recognized or H Elgazzar, Professor and chairman, suspected prior to death. The best Department of Nuclear medicine, Kuwait solution to the problem of embolism is to University. P.O.Box 24923, safat, Kuwait prevent it. However, prevention requires 13110, Email:[email protected], identification of those at risk (table 1). Fax: 965 5338936 4 Egyptian J. Nucl. Med., Vol. 1, No. 1, 3-15 (2009)
Pulmonary thromboemboli occur embolism. If the embolus completely more commonly in the lower lobes occludes an artery or an arteriole and the because of the preferential blood flow to collateral bronchial circulation is these regions. This also applies to the insufficient to sustain tissue viability, right lung because of the straighter infarction occurs over 24–48 h. course of the pulmonary artery. Pulmonary infarction with coagulative necrosis results in an area of radiographic Table (1): Risk factors for pulmonary opacity that requires an average of 20 thromboembolism. days to resolve but occurs in less than 1. Recent surgery specially operations 10%–15% of patients with pulmonary on the abdomen and pelvis embolism. There is significant 2. Trauma inflammatory component in pulmonary 3. Neoplasms infarcts which is the basis behind the 4. Prior history of thromboembolic significant FDG uptake in recent lung disease infarcts as reported recently and can 5. Venous stasis cause false positive interpretation for 5. Hypercoagulability states lung malignancy [5]. More frequently, 7. Immobilization incomplete infarction with hemorrhage 8. Infection adjacent to veins but without necrosis occurs. This type of 9. Heart disease particularly injury resolves quickly and produces only myocardial infarction and transient radiographic opacities. congestive heart failure Infarction always involves the pleural 10. Following cerebrovascular surface of the lung (peripheral) and more accidents frequently involves the lower lobes than 11. Pregnancy other sites. 12. Certain drugs such as oral The regional decrease in ventilation contraceptives, estrogens is due to local bronchoconstriction with a 13. Polycythemia tendency for redistribution of ventilation 14. Bleeding away from the hypoperfused segment. 15. Vascular spasm This probably occurs due to decreased 16. Intimal injury regional alveolar and airway carbon 17. Obesity dioxide tension, which is the usual 18. Old age stimulus for bronchodilation. The 19. Varicose veins ventilation of the hypoperfused areas Immediately after acute embolism, returns to normal within several hours there is a decrease of perfusion distal to after acute embolism [6, 7]. This concept the occluded vessel along with a transient is the pathophysiological basis for the decrease of ventilation to the affected scintigraphic interpretation of ventilation segment. The blood flow is diverted to and perfusion scans, which show the other portions of the lung and segmental perfusion defects with pulmonary artery pressure may increase, preserved ventilation as a typical although cardiac output usually remains scintigraphic pattern for pulmonary stable. The resultant tissue ischemia embolism. Those showing only regions disturbs certain metabolic functions of of matched perfusion and ventilation the lung such as the production of defects carry a low probability of surfactant. Reduction of the surfactant pulmonary embolism if no chest X-ray concentration reduces the alveolar abnormalities are noted at the same sites, surface tension and may cause the since this pattern is more likely atelectasis that often accompanies associated with nonembolic conditions and is more typical of parenchymal lung
Egyptian J. Nucl. Med., Vol. 1, No. 1, 3-15 (2009) 5 disease. Because patients with pulmonary or a pulmonary artery mean pressure emboli usually arrive at the hospital after higher than 20 mm Hg. normalization of the ventilation at the site Pulmonary hypertension may have of pulmonary emboli, the mismatching no cause (primary) which is rare or may pattern is typical of pulmonary emboli. follow cardiac or pulmonary disorders However in patients may have their V/Q (secondary). Pathphysiologically, three scans within a short time after predominant mechanisms may be presentation and matching abnormalities involved in the pathogenesis of may be associated with pulmonary secondary pulmonary hypertension, (1) emboli. This has to be borne in mind, and hypoxic vasoconstriction, (2) decreased the duration of symptoms should be a area of the pulmonary vascular bed, and factor in decision-making regarding the (3) volume/pressure overload. management of pulmonary embolism. Normally, the lower zones of the lungs When pulmonary hypertension are better perfused and ventilated because occurs it indicates that at least 25% of the effect of gravity. This gradient is obstruction of pulmonary vascular tree as more pronounced in perfusion than in assessed by angiography [8]. The higher ventilation. This physiological fact will the degree of obstruction the more severe usually cause the perfusion to appear less the abnormalities of the cardiopulmonary than the ventilation in the lung apices on hemodynamics become. When over 50% scintigraphy. This should not be confused of the pulmonary vasculature is included with a mismatching pattern. 99mTc Macro (massive pulmonary embolism), acute aggregated albumin (MAA) is injected pulmonary hypertension and /or right for perfusion imaging while the patient is ventricular failure (cor pulmonale) occurs in the supine position to minimize the [8]. gradient. Injection while the patient is Pulmonary emboli may, taking a deep breath also helps. spontaneously or with treatment, An increase in the resistance of the fragment into smaller portions that travel pulmonary arterial circulation, due distally and block smaller arterioles. This primarily to mechanical blockage by may create new, smaller perfusion numerous small emboli in the pulmonary defects that are more peripherally located vasculature and also to humorally in comparison to the original defect mediated vasoconstriction, may follow caused by the original embolus. This pulmonary emboli. These hemodynamic pattern should not be mistaken for consequences may include increased recurrent pulmonary emboli on a follow- pulmonary arterial resistance with up scan. If this pattern is the only interval elevated pulmonary arterial and right change with no other defects seen in ventricular systolic pressures and areas other than those in the vicinity of hypoxemia. Normal pulmonary artery the distribution of the original embolus, it systolic pressure at rest is 18 to 25 mm does not suggest recurrent emboli [7]. Hg, with a mean pulmonary pressure Resolution of pulmonary ranging from 12 to 16 mm Hg. This low thromboembolus may start within hours. pressure is due to the large cross- It can be seen on perfusion scans as early sectional area of the pulmonary as 24 h and is progressively noted up to circulation, which results in low 3 months, with insignificant change after resistance. An increase in pulmonary 6 months. This is the basis of the vascular resistance or pulmonary blood recommendation that follow up flow results in pulmonary hypertension. ventilation and perfusion scans is It is defined as a pulmonary artery performed three months after the initial systolic pressure higher than 30 mm Hg incident for evaluation of resolution and
6 Egyptian J. Nucl. Med., Vol. 1, No. 1, 3-15 (2009) function as a baseline for future incidents commonly more difficult and atypical in to differentiate between acute and older age group compared to younger unresolved old emboli. This resolution is patients [18, 19]. Accordingly only 24% dependent on the age of the patient, with of fatal emboli were diagnosed complete resolution in young age-groups antemortem [20]. Data indicate that the and less complete and less significant mortality of pulmonary embolism is more resolution in older age-groups [9]. Other than 30% if untreated. Promptly factors include age of the diagnosed and treated, emboli have a thromboembolus or length of time mortality of 2.5%–8% [9, 12, 21]. The between formation of the embolus and mortality of PE was found to vary among the institution of proper anticoagulation. patients with or without cardiac disease. This is the basis behind the relatively Paraskos et al [22] reported survival rates recent trend of starting anticoagulant at a mean follow up period of 29 months therapy in most patients with pulmonary of 19% among patients with prior emboli who have no contraindication for congestive heart failure and 86% for anticoagulation immediately when a those with no prior congestive heart pulmonary thromboembolus is suspected failure. Pulmonary angiography is the before finishing the workup for the most accurate modality for the diagnosis condition. Anticoagulant therapy may of pulmonary emboli with an accuracy of then be stopped if the condition is 96% [23]. However, angiography is excluded. invasive and is not suitable as a screening Pulmonary thromboemboli recur in imaging modality for the disease. up to 50% of patients [10], although the Scintigraphic diagnosis: incidence in treated PIOPED patients was Scintigraphy remains the most cost- only 8.3% [11]. The vast majority of effective noninvasive screening modality. deaths among pulmonary embolism The major advantages include its ability patients are due to recurrent emboli. In to provide regional and quantitative the PIOPED study population, it was information useful for the diagnosis, as found that nine of ten people who died well as for mapping to guide selective had a recurrent pulmonary embolus [12]. angiography if needed for the diagnosis, Recurrence has been reported to occur at after noninvasive methods including the same site as the original Doppler and Duplex studies for deep thromboembolus [13]. venous thrombosis have been exhausted. Diagnosis of pulmonary emboli: Additionally it determines the disease The clinical diagnosis of severity and monitors its progress [24]. pulmonary thromboembolism is difficult Several agents have been used for and unreliable, due to the nonspecificity ventilation (Table 2). Every agent has of its symptoms and signs as well as the certain advantages and limitations. laboratory and chest X-ray findings Xenon 133 is useful in evaluating [14,15]. Chest x-ray however must be obstructive airway disease. Crypton 81, obtained since it may show many Tc99m DTPA and Technegas provides parynchymal diseases [16] and must be ability to perform ventilation studies after available for lung scan interpretation. the perfusion particularly Crypton-81. Pulmonary embolism may also be Tc99m Macroaggregated albumin is used asymptomatic. In a study using spiral CT for perfusion. For proper interpretation of 24% of pulmonary emboli were lung perfusion/ventilation study chest X asymptomatic among patients with ray must be available and should be moderately to severely injured patients obtained within 12 hours of the time of [17]. Furthermore the presentation is the scans. Technegas is ventilation
Egyptian J. Nucl. Med., Vol. 1, No. 1, 3-15 (2009) 7 aerosol agent that gained popularity recently. It is ultrafine labeled carbon particles produced by heating 99mTc- pertechnetate to very high temperatures of approximately 2500o C in the presence of 100% argon gas. An ash material is produced that acts like a gas with good peripheral deposition because the particles are so small, with a median size of 0.05 to 0.15 um. Technegas has a half- clearance time of 4-6 hours. Another agent, pertechnegas, which is a vapor of pertechnetate, is prepared similarly but in the presence of 2 to 5% oxygen, has a shorter clearance time, and shows excellent deposition in the lungs.
Table (2): Patterns of perfusion defects associated with higher probability of pulmonary emboli
Size Moderate and large Larger than that of corresponding chest X-ray densities Location Pleural based defects Lower lobes Shape Wedge-shaped
Type Segmental Relation to ventilation pattern Mismatching
Number Multiple Figure (1): Tc99m MAA perfusion Normal perfusion study (Figure 1) study showing no segmental or rules out any clinically significant subsegmental perfusion defects pulmonary emboli. Since the ventilation indicating normal perfusion ruling and perfusion lung scans lack specificity out the possibility of any clinically (Table 4), probabilities are used for the significant pulmonary emboli interpretation of abnormal studies. Based on the pathophysiological changes and moderate defect between 25% and 75%, scintigraphic observations, several while a large defect takes up 75% or scintigraphic features of perfusion more. Using these features, several abnormalities are known to affect the retrospective and prospective studies were probability of a scan for pulmonary conducted to refine the interpretation of emboli (Table 5). One of the important ventilation and perfusion scans and assess features is the size of segmental their value in managing patients suspected perfusion defects. A small defect of having embolic disease [25-26]. occupies up to 25% of the segment, a
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Table (3): Summary of major ventilation agents
Agent Advantages and limitations Aerosols
Tc99m-DTPA aerosol Lung half clearance time = 58 minutes Pre or post perfusion
Multiple projections
Technegas Multiple projections Good peripheral deposition
Gases:
Xenon-133 Ability to obtain single breath, equilibrium and washout images Very sensitive for obstructive airway disease
Only posterior view is possible in most patients
Low energy of 81 keV Pre-perfusion acquisition
Krypton-81m Expensive - available only in some areas
Energy: 190 keV Half life: 13 seconds Multiple views
Pre or post perfusion
Table (4): Characteristics of Tc-99m- Macro-aggregated albumin (Tc99m-MAA)
Size 10-90 Microns (mostly 20 to 50)
Minimum number of particles to be used in adults 100,000 unless pulmonary hypertension or right to left shunt is present
Ideal number of particles 200,000 - 500,000 Biologic half life 4 - 8 hours
Injection Care should be taken not to cause particle aggregates that can produce hot spots
Safety Particles block <1/1000 of the capillaries and pre-capillary arterioles
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Table (5): Causes of abnormal perfusion lung scan
Emphysema Pneumonia
Abscess Granulomatous disease (Sarcoidosis, Tuberculosis)
Pulmonary fibrosis
Bronchial obstruction
Acute and chronic Asthma Mucus plug
Foreign body
Rib fractures (Reduced lung excursion)
Congenital hypoplasia or absence of the pulmonary arteries
Peripheral pulmonary artery stenosis Thromboembolic disease
Extrinsic vessel compression (Tumor, inflammation)
Left ventricular failure Mitral valve disease Venoocclusive disease
Prior lung resection
Radiation
The most popular retrospective study is analysis utilizing the clinical pre-scan that by Biello et al. [25], which compared and scan probabilities to figure the post- the ventilation and perfusion scans and scan or diagnostic probability. The study chest X-rays with angiography to showed clearly that when the clinical produce a set of criteria for interpreting odds agree with the scan probability in ventilation and perfusion studies. The the low and high probability categories, most recent and largest prospective study pulmonary embolism can be ruled out or is the PIOPED [11]. Despite its confirmed with a high degree of shortcomings, which originated mainly certainty. from the use of a pre-established nonstandard set of criteria for Based on the modifications of interpretation of ventilation and perfusion PIOPED criteria and the other validated scans, the PIOPED provided a wealth of criteria, a simplified set is shown in table information. The study established the 6. Small perfusion defects indicate low value of normal and high probability probability of pulmonary emboli as well scans in excluding and diagnosing as matching perfusion and ventilation pulmonary embolism. defects regardless of size with no matching x ray abnormalities. Non It validated the segment equivalent segemental defects also indicate low concept and clarified the use of Bayesian probability. When perfusion defects
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Table (6): Criteria for interpretation of ventilation/Perfusion lung scans Category Pattern on V/Q images Normal No perfusion defects. Allow for impressions explained by
enlarged heart or other hilar structures as seen on chest x-ray.
Near normal heterogenous uptake with no definite segmental or subsegmental perfusion defects
Low - Non segmental perfusion defects other than those explained by cardiomegaly or other prominent hilar structures
- Matching V/Q defects with no corresponding CXR abnormalities - Any number of only small defects regardless of ventilation and CXR patterns - Perfusion defect substantially smaller than CXR abnormality - Stripe sign Intermediate -Perfusion defect matching chest x-ray abnormality and of the same approximate size.
-Single moderate up to less than 2 segmental mismatching defects with no corresponding chest x-ray abnormalities.
-Difficult to categorize as low or high. High - Two or more large mismatching defects or their equivalent (4 moderate or 1 large plus 2 moderate defects) with no corresponding CXR abnormalities. - Perfusion defect substantially larger than CXR abnormality * 1.5 in patients with no prior cardiopulmonary disease can be considered high probability.
match the X ray abnormalities it may The minimum number of indicate low, intermediate or high mismatching perfusion defects is two probability based on the relative size of segment equivalent defects to make high perfusion compared to the x ray densities. probability interpretation (Figure 3). When the perfusion defect is of the same However a study analyzing PIOPED data approximate size of the matching x ray indicated that defects equivalent of 1.5 density (Figure 2) it indicates segments are indicating high probability intermediate probability (approximately among patients with no prior 25%). However, Worley et al suggested cardiopulmonary diseases [31]. that when the perfusion defect matches Till further development, proper the chest X-ray density in the upper or utilization of V/Q scans along with the intermediate lung zones it indicates low DVT tests and Spiral CT solves most probability while if it is in the lower diagnostic problems and decreases the zones it indicates intermediate need for angiograms probability [27]. Other studies however did not support this categorization and it Other diagnostic modalities: is debatable [28]. Size of pleural effusion Noninvasive DVT studies are also was interpreted as low probability if useful in decision making in equivocal large and intermediate if small by cases. Positive DVT studies increases Worsley, et al [29] but this was not probability to up to 93%. proved by Goldberg et al [30].
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Figure (2): Representative images of a tc99m MAA perfusion scan (a) and Tc99m DTPA arerosol ventilation scan (b) showing perfusion defect matching the ventilation abnormality and is od the same approximate
size as the chest X ray density. This pattern indicates intermediate probability of pulmonary emboli.
Figure (3): Ventilation and perfusion lung scans showing mulitiple large and medium sized defects equivalent to more than two segments and are mismatching with the ventilation pattern. The chest X ray (not shown) is normal. The pattern indicates high probability of pulmoonary emboli.
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