Interpretation of V/Q Scan for Pulmonary Embolus

Defect Descriptors, as Used in PIOPED:

V/Q ratio: Perfusion defect that has any ventilatory abnormality of comparable size is called a V/Q Match. If the perfusion defect has no corresponding ventilatory abnormality, or if it is either much more sever or larger than ventilatory abnormality, it is considered a V/Q Mismatch. NOTE: Any degree of perfusion decrement (NOT only the ones with completely absent perfusion) should be considered a perfusion defect. It is presumed that a partially occluding embolus could create a perfusion defect with perfusion that was diminished, but not absent. In fact, the pulmonary artery must be narrowed by at least 90% of its normal diameter before perfusion distal to the obstruction is reduced.

Size: Determining the size of a defect can be difficult. A knowledge of segmental anatomy is essential, and use of a schematic chart is encouraged to increase reading reproducibility and interobserver agreement (R). Note that, in general, the actual size of a perfusion defect is UNDER estimated by exam interpreters [13]- most likely due to shine through of radioactivity in contiguous segments [16]. In one study, only 44% of segmental defects were correctly identified as being greater than 75% of a segment [13]. Also- perfusion defects in the medial basal segment of the right lower lobe are often not detectable in any view [13]. Interobserver agreement regarding defect size is poor (about 40%) and even intraobserver agreement is only moderate (about 56%) [15].

* Small defect (small subsegmental): Less than 25% of a segment.
* Moderate defect (moderate subsegmental): > 25%, but < 75% of a segment.
* Large defect (segmental): Greater than 75% of a segment.

Vascular segmental distribution: A segmental defect (independent of size) is a perfusion abnormality which may be triangular or rectangular shaped, periphery based, and specially located within one or several vascular segments. Nonsegmental defects do not conform to segmental vascular anatomy and unlikely to represent PE. Because the term "segmental" is also used in sizing of defects, we prefer to call defect vascular or nonvascular in appearance and distribution.

Defect Descriptors, Identified in Studies Other Than PIOPED:

"Segmental Equivalent" Sizing: The moderate segmental defect is counted as 0.5 of a large segmental defect (two moderate sized segemental defects are therefore equivalent to one large segmental defect). All moderate and large defects can be than added up to result in total segmental equivalent units. (Example: 3 moderate segmental defects and 1 large = 2.5 segmental equivalents)

"Stripe Sign": A thin line (stripe) of activity (perfusion) at the pleural surface of a perfusion defect. The finding is associated with underlying emphysema and is likely related to spared perfusion in the cortex of the lung [6]. Iin the PIOPED study only 7% with a stripe sign actually had a pulmonary embolism corresponding to that segment [6]. Hence, in the absence of other perfusion abnormalities, the finding is considered low probability for PE. In cases in which there is a high clinical concern for pulmonary embolism, SPECT images may help to better define the defect [ R].

"Triple Match": A matching perfusion, ventilation, and CXR abnormality is referred to as a "triple match". [8]. The overall prevalence of PE in all lung zones with triple matches was 26% in the PIOPED population [8]. There were no significant differences between the size of the matching V/Q defects and chest radiographic opacities and the prevalence of PE [8]. The prevalence of PE in small regions (less than 25% of a zone) with triple matches was 27%, compared with 21% in large regions (over 75% of a zone) with triple matches [8]. However, there was a difference in prevalence of PE between lung zones- pulmonary embolism was significantly more common in lower lung zone triple matches (upper - 11%; middle - 12%; lower - 33%) [3,8]. Pulmonary embolism was significantly more common in lower lung zone triple matches [8]. Therefore, a triple-match in the upper or middle lung zones is considered low probability for PE, but a triple-match in the lower lung zone should be interpreted as intermediate probability [8].

Interpretation Criteria

Interpretation is based on fulfillment of specified criteria. Several criteria sets are available, but the most commonly used ones are Biello's and PIOPED. The original PIOPED criteria is the only set that was tested prospectively. Modifications to PIOPED criteria have been suggested on the bases of retrospective PIOPED data review. Interobserver variability in scan interpretations is good when using the PIOPED criteria [18]. The most recent publication of Modified PIOPED Criteria appeared in the form of Society of Nuclear Medicine procedure guideline [ R]:



 

High Probability (80-100% likelihood for PE [12]):

1. Greater than or equal to 2 large mismatched segmental perfusion defects or the arithmetic equivalent in moderate or large and moderate defects. A high probability lung scan confirms a very high likelihood for pulmonary embolism and justifies treatment with anticoagulation (unless contraindicated) [18].

Caveat - It has been suggested that 2.5 mismatched large segmental defects (or the arithmetic equivalent) is a better threshold for calling a scan high probability, as it associated with a 100% probability of PE in the PIOPED population [12]. 

 

Intermediate Probability (20-80% likelihood for PE [12]):

1. One moderate to 2 large mismatched perfusion defects or the arithmetic equivalent in moderate or large and moderate defects.

2. Single matched ventilation-perfusion defect with a clear chest radiograph [12]. 

Caveat - Single ventilation-perfusion matches are borderline for "low probability" and thus should be categorized as "intermediate" in most circumstances by most readers, although individual readers may correctly interpret individual scintigrams with this pattern as "low probability".

3. Difficult to categorize as low or high, or not described as low or high.


 

Low Probability (0-19% likelihood for PE [12]):

1. Perfusion defects matched by ventilation abnormality provided that there are: (a) clear chest radiograph and (b) some areas of normal perfusion in the lungs. Extensive matched V/Q abnormalities are appropriate for low probability, provided that the CXR is clear.

2. Any perfusion defect with a substantially larger chest radiographic abnormality.

3. Any number of small perfusion defects with a normal chest radiograph.

4. Nonsegmental perfusion defects (e.g., cardiomegaly, enlarged aorta, enlarged hila, elevated diaphragm).

5. Multiple matched V/Q abnormalities, even when relatively extensive, are low probability for PE [11]. The prevalence of PE in patients with extensive matched V/Q defects and no CXR abnormality was 14% (low probability).

Normal:

1. No perfusion defects or perfusion exactly outlines the shape of the lungs seen on the chest radiograph (note that hila and aortic impressions may be seen and the chest radiograph and/or ventilation study may be abnormal).


V/Q Scan Interpretation Nuances:

"Gestalt Interpretation" is formed by an experienced reader based on detailed knowledge of the various published lung image interpretation algorithms, as well as clinical data, ancillary findings, and pathophysiological features of PE, which are integrated with the individual case presentation [17]. A gestalt interpretation can be more accurate than any individual criteria set [ R]. The Gestalt interpretation takes into consideration the fact that among patients with no underlying cardiopulmonary disease, fewer mismatched segmental defects are required to give a particular specificity and positive predictive value [15]. For instance, a previously healthy 25 year old female on birth control pills with a normal CXR and a single large segmental ventilation-perfusion mismatch in the lower lung zones on V/Q scanning may be interpreted to have a high probability study by an experienced reader. The Gestalt interpretation has been shown to have good-to-excellent intra- and interobserver variability [17,18].

A single moderate sized V/Q mismatch would qualify for low probability in the original PIOPED schema. However, such a finding was found to harbor PE in 36% of cases in the PIOPED cases [12]. Hence, it has since been considered to indicate intermediate probability.

"Triple Match" in the upper and middle zones is suggested for low probability. If seen in the lower zone, it qualifies for intermediate probability.

"Stripe Sign" is suggested to indicate low probability- 93% of lung zones with the stripe sign had no PE present on angiography in the PIOPED study [11].

A single matched (any size) V/Q defect has a 26% incidence of PE in the PIOPED population, hence, it is suggested to be classified as intermediate probability [12].

Defects associated with pleural effusion: Ipsilateral pleural effusions can be found in up to 35% of patients with acute pulmonary embolism. In the PIOPED study, the majority of patients with PE and pleural effusions had small effusions which caused blunting of the costophrenic angle. When the pleural effusion caused an isolated perfusion defect congruent with the size of the pleural effusion, and when the pleural effusion was limited to the costophrenic angle, the incidence of PE was 25% [12]. Therefore, matching V/Q defects with a small effusion represent intermediate probability for PE. In  patients with no prior cardiopulmonary disease and an effusion that was greater or equal to one-third of the hemithorax, no PE was found, (i.e., low probability) [3, R]. However, some authors outside the PIOPED group argue that any size perfusion defect should be considered intermediate probability in association with any size effusion [ R].

Generalized CXR abnormality: A generalized abnormality on CXR (such as diffuse pulmonary edema or reticulonodular disease) may not cause the perfusion lung scan to be abnormal- in fact, up to 73% of patients with such findings can have normal or near normal perfusion images [15].


Clinical Probability of PE:

The actual incidence of PE is highly dependent upon the pretest (clinical) likelihood for the presence of pulmonary embolism. The utility of the V/Q is optimized when it is interpreted in conjunction with the clinical likelihood for PE. This was well demonstrated in the PIOPED [ R]:

 

Clinical Science Probability

Scan Probability 

 80 - 100%

 20 - 79% 

 0 - 19%

 All Probabilities 

High

 96% 

88%

56%

87% 

Intermediate

66% 

28%

16%

30% 

Low

40% 

16%

4%

14% 

Near normal/ normal

0% 

6%

2%

4% 

Total

68% 

30%

9%

28% 

PIOPED Summary:

The PIOPED study was a Prospective Investigation of Pulmonary Embolism Diagnosis that tested utility of V/Q scanning. All patients enrolled in the study were to have a V/Q scan and a pulmonary arteriogram. Unfortunately, a large number of patients with normal or low probability scans who were enrolled in the study did not undergo arteriograms as many physicians did not want to have their patients exposed to the risks of the procedure in light of the V/Q scan findings. Because of this, there was a selection bias towards patients with intermediate or high probability scans (prevalence of PE based upon arteriogram for patients enrolled in the study was 33%). Such a bias would tend to overestimate the sensitivity, but underestimate specificity of V/Q scanning. Agreement among the readers was excellent for high, very low, and normal scans (92-95%), but agreement for intermediate and low probability scans was not as good [15]. In patients with chronic obstructive pulmonary disease, the sensitivity of a high probability V/Q scan is significantly lower compared to patients with no pre-existing cardiopulmonary disease. In fact, as the complexity of the patients underlying cardiopulmonary disease increases, so does the likelihood that the scan will not be informative (intermediate probability) [15]. Using PIOPED criteria, intermediate probability V/Q scans occurred in 60% of patients with COPD, but in only 13% of patients with normal CXR's [15].

An analysis of the regional distribution of PE on angiography demonstrated that PE occurred more frequently in the right lung, compared with the left, and more frequently in the lower lung zones compared with the middle or upper zones.

False positives

Patients with prior pulmonary embolism can have persistent ventilation-perfusion abnormalities [15]. Pulmonary emboli resolve because of natural thrombolytic processes [15]. A residual perfusion defect can be found in 9% to 30% of patients and incomplete resolution may be more common in patients with underlying cardiopulmonary disease [15]. 

Among young non-smokers approximately 7% may demonstrate subsegmental perfusion defects and 3-4% lobular or segmental defects. As many as 10% of smokers may exhibit some type of perfusion defect. Xenon washout studies have also demonstrated mild delay in regional clearance and heterogeneity of clearance time in young smokers. These changes become progressively more severe and irreversible in long-term smokers.

Multiple nonsegmental, matched ventilation-perfusion defects can be seen following smoking of free based cocaine and snorting heroin. This may be related to the local vasoconstrictive and bronchoconstrictive actions of cocaine [9].

Bronchoalveolar lavage can also produce perfusion defects and lung scanning should be delayed at least 24 hours following the procedure to avoid mis-interpretations [7].


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