What Is A Vq Scan And Pregnancy – A planar ventilation-perfusion (V/Q) scan is often used to investigate pulmonary embolism; however, it has well-known limitations. SPECT overcomes much of this with its ability to produce 3-dimensional image data. V/Q SPECT has greater sensitivity, specificity, and accuracy than planar imaging and has a higher degree of opacification. SPECT enables new ways to display and analyze data, such as parametric V/Q ratio imaging. Compared to CT pulmonary angiography, SPECT has higher sensitivity, lower radiation dose, fewer suboptimal technical studies, and no contrast-related complications. Any nuclear medicine department equipped with modern hybrid scanners can now perform combined V/Q SPECT and CT (using a low-dose protocol) to improve diagnostic accuracy. V/Q SPECT (with or without CT) has applications in other lung conditions and in research.
Since its first description by Wagner et al. in 1964 (1), the planar lung scan is one of the most commonly performed studies in nuclear medicine. However, it is a test that has significant limitations, which have had a detrimental effect on its reputation (2, 3). Because planar imaging is a 2-dimensional technique, it has inherent limitations, particularly related to overlapping anatomical segments. Assigning defects to specific lung segments is often difficult, and variability in segment size and shape between patients makes precise determination of the extent of embolic involvement in individual segments challenging (2). An embolic defect may not be detected if there is a beam from a portion of the lung that is under normal perfusion, causing an underestimation of the degree of perfusion loss (4). Furthermore, the medial basal segment of the right lower lobe is often not visualized on plane scans (2, 5). Added to these factors is the widespread use of probabilistic reporting criteria and the relatively high rate of imprecision, both of which lead to significant dissatisfaction among referring physicians (6, 7). Not surprisingly, contrast-enhanced CT pulmonary angiography (CTPA), with its binary reporting approach (ie, positive or negative), has now become the preferred imaging test for evaluating pulmonary embolism (PE) in many institutions, particularly in the United States. States. (8, 9).
What Is A Vq Scan And Pregnancy
Although SPECT is routinely used in many areas of nuclear medicine because of its ability to image in 3 dimensions, lung imaging remains a 2-dimensional technique in many institutions. Lung SPECT imaging avoids segmental overlap with adjacent lung sections and can more accurately determine the size and location of perfusion defects in individual segments (2).
The Importance Of Quality In Ventilation–perfusion Imaging
Studies comparing planar and SPECT lung scans consistently show the superiority of SPECT over planar imaging. Studies in dogs (10) and pigs (11), as well as studies using Monte Carlo simulations (5), have shown greater sensitivity for detecting PE with SPECT than with planar imaging. In humans, Bajc et al. showed that SPECT is more sensitive than planar imaging (100% vs. 85%) in detecting PE (12). SPECT has less interobserver variation and better delineation of discordant defects than planar imaging in this study. Collart et al. also showed that SPECT is more specific than planar imaging (96% vs. 78%) and has better reproducibility, both intraobserver (94% vs. 91%) and interobserver (88% vs. 79%) (13). In a study of 83 patients, Reinartz et al. showed that, compared with planar imaging, SPECT had higher sensitivity (97% vs. 76%), specificity (91% vs. 85%), and accuracy (94% vs. 81%) (14). In this series, SPECT improved the detection of segmental defects by approximately 13% and subsegmental defects by more than 80%. Based on the collected literature, SPECT has a sensitivity of 80% to 100% and a specificity of 93% to 100% (8). Ventilation-perfusion (V/Q) SPECT imaging has also been consistently shown to have a lower indeterminate rate than planar imaging, typically less than 5% (8, 15-19). Leblanc et al. showed that SPECT has a very high negative predictive value (98.5%) for PE (20). In that study of 108 patients, only 3% of studies were considered indeterminate for PE.
The literature to date is fairly consistent and, taken together, suggests that SPECT has greater sensitivity and specificity, greater reproducibility, and a lower indeterminate rate than planar lung scintigraphy.
As with planar imaging, the usual approach with SPECT is to perform a ventilation study followed by a perfusion study.
Xe most accurately represents regional ventilation; However, this is only used in a limited number of centers due to the need for continuous administration during acquisition and the high cost and short duration.
Pdf] Pulmonary Embolism In Pregnancy
Xe gas has the advantage of a longer half-life, it is a less than ideal choice due to recirculation, low γ-photon energy, and therefore poor spatial resolution (21). Due to this limitation,
Tc-Technegas (Cyclomedica) is more widely used because of its greater availability, low cost, and good image quality (19). Technegas is an ideal SPECT ventilation agent due to its small particle size (30-60 nm), resulting in greater alveolar penetration and less central deposition than nebulizer-produced aqueous radioaerosols such as
Tc-DTPA (22). However, in the United States (where Technegas is not commercially available, a factor that has significantly hindered the transition to SPECT (23)), there is little choice but to use agents such as
The image quality of Tc-DTPA is adequate in many patients, Technegas clearly excels in patients with obstructive pulmonary disease due to its better peripheral penetration (24). The dose usually administered by
Reversible Pulmonary Artery Perfusion Abnormalities In The Postpartum Period As A Precursor To The Development Of Pulmonary Arterial Hypertension
Tc-macroaggregated albumin depends on the ventilator and dose, but is usually on the order of 100–250 MBq if a technetium-based ventilator is used. The European Association of Nuclear Medicine guidelines for V/Q SPECT recommend a dose at the lower end of this range, but ultimately the dose administered must be determined by each institution based on the quality of the image obtained (influenced by factors such as choice of collimator, γ-camera sensitivity and processing parameters) and local radiation protection guidelines (25).
For pregnant patients, the administered dose is usually halved for ventilation and perfusion agents (25), thus requiring longer acquisitions to maintain good quality images. Some centers advocate omitting the ventilation scan; however, radiation savings from this approach are minimal and diagnostic accuracy may be adversely affected (26).
Multidetector γ-cameras are preferred for V/Q SPECT (19). Generally requiring a total imaging time of 20–30 min to complete both ventilation and perfusion, the SPECT scan time is less than that required for traditional 6- or 8-view planar imaging ( 14 , 20 ). Typical acquisition and processing parameters are shown in Table 1. Images are best viewed at a workstation using a dedicated rendering software package that allows automatic or manual image co-registration of ventilation and perfusion data and image review in each orthogonal plane ( 27).
For reporting specialists familiar with planar images, these can be generated from SPECT data using a number of approaches. Although Bailey et al. has described a technique using reprojection (28), many commercial equipment vendors offer a simple approach using angle summation techniques (14). With this approach, images are generated by summing several consecutive projections from SPECT acquisitions. this approach can blur small defects because the data is acquired through the arc; however, the image produced approximates a true flat image (29). These pseudoplanar images provide a familiar and fast view of the lung for rapid evaluation and may be of particular value during the transition phase from planar imaging to SPECT imaging.
Best Practices In Pulmonary Embolism Diagnosis During Pregnancy
More advanced data processing can be performed using SPECT data. First, contrast defects in perfusion SPECT can be further enhanced by subtracting background activity remaining from previous ventilation scans ( 12 , 30 ). Next, by examining the pixel-based V/Q ratio, a quantile image can be generated from the SPECT data. These images can facilitate image reporting and improve demonstration of the location and extent of the defect (Fig. 1) (30). Objective quantification of the V/Q ratio is another processing technique that has been shown to improve accuracy and reduce the number of unspecified studies (31, 32).
(A). Example of a patient with multiple bilateral PE. Ventilation and perfusion images show multiple mismatched perfusion defects. (B) Representative ventilation, perfusion, and V/Q results images The dark area in the V/Q quotient image, indicating a high V/Q ratio, indicates a V/Q mismatch. (Reprinted with permission from (19).)
V/Q studies are typically reported using European Association for Nuclear Medicine guidelines, which recommend that studies be reported positive for PE if there is a V/Q discrepancy of at least 1 segment or 2 subsegments consistent with pulmonary vascular anatomy (17). ). Probabilistic reporting used for flat scanning is not recommended and has not been validated for V/Q SPECT (14, 33).
Multidetector CTPA has evolved to the point where it is often used as the primary imaging investigation in patients with potential PE (34). This is certainly the case in the United States, where it has replaced the V/Q scan as the initial imaging test for PE assessment in many institutions (8, 9). This preference arose for several reasons, including better availability in many centers (especially after hours), rapid acquisition time, ability to diagnose conditions other than PE that may account for the patient’s symptoms, and referrer preference for binary reporting ( 8,