Pulmonary embolism ventilation/perfusion scan: Difference between revisions
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* For a prevalence of PE of 33% (confirmed by angiography), the positive predicitve value (PPV) and negative predictive value (NPV) of the ventilation/perfusion scan for PE are as follows: | |||
** PPV for high probability scan: 88% | |||
** NPV for intermediate probability scan: 84%-88% | |||
** NPV for a normal or near normal scan: 91%-96% | |||
* PIOPED demonstrated that the value of the ventilation/perfusion scan is improved when combined with the clinical pretest probability of PE. For example, among patients with suspected PE and high probability scan, the rates of confirmed PE by angiography were 56%, 88%, and 96% in cases of low, intermediate, and high clinical pre-test proability of PE. | |||
===Comparison with CT Pulmonary angiography=== | ===Comparison with CT Pulmonary angiography=== | ||
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Editor(s)-In-Chief: The APEX Trial Investigators, C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief:
Overview
A ventilation/perfusion scan (otherwise known as V/Q scan or lung scintigraphy) is a study which shows whether an area of the lung is being ventilated with oxygen and perfused with blood. In the setting of a PE, perfusion can be obstructed due to the formation of a clot. The V/Q scan is less commonly used due to the more widespread availability of CT technology, however it may be useful in patients who have an allergy to iodinated contrast. It may also be useful in pregnant patients in an attempt to minimize radiation exposure.
Ventilation/Perfusion Scan
Principle
Technetium (Tc)-99m labeled macro-aggregated albumin particles, when injected, fill small fractions of the pulmonary capillaries. This helps in the assessment of lung perfusion at the tissue level via scintigraphy. When any branch of the pulmonary artery is occluded, the particles will not reach the capillaries rendering the area "cold" on imaging.
Indications
The utilization of V/Q scanning has declined since the advent of more widespread availability of CT technology, however it may be useful in particular subgroups of patients, such as:
- Patients who have a known allergy to iodinated contrast. To read more about contrast allergy, click here.
- In pregnant patients to minimize exposure to radiation.
- For patients who are in a hospital lacking CT technology.
Interpretation
A normal or near normal V/Q scan excludes the diagnosis of PE.[1][2] When the result of the V/Q scan is not normal or near normal, the results of the V/Q scan should be interpreted in combination with the pretest probability of PE. Shown below is a table that summarizes the possible outcomes of a V/Q scan.[2]
| V/Q Scan | Clinical Probability | Interpretation |
| Normal | Any probability | PE excluded |
| Low probability scan | Low | PE excluded |
| High probability scan | High | PE confirmed |
| Any other combination of V/Q scan and clinical probability | Further tests are needed | |
Any defect that is located pleurally, has a triangular/concave shape, and is in the anatomical distribution of a lung segment should be considered segmental in nature. A defect of the right lower lobe involving the medial basal segment may be undetectable in any view.[3]
Supportive Trial Data
- Data regarding the use of the ventilation/perfusion scan for the evaluation of patients with suspected PE is provided by the findings of the PIOPED study. The rates of confirmed PE by angiography for each of the categories of the ventilation/perfusion scan are:
- High probability scan: 88%
- Intermediate probability scan: 33%
- Low probability scan: 16%
- Normal or near normal probability scan: 9%
- Shown below is the sensitivity and specificity of ventilation/perfusion scan for detecting PE:
| Category of ventilation/perfusion scan | Sensitivity | Specificity |
| High probability scan | 41% | 97% |
| High or intermediate probability scan | 82% | 52% |
| High, intermediate, or low probability scan | 98% | 10% |
- For a prevalence of PE of 33% (confirmed by angiography), the positive predicitve value (PPV) and negative predictive value (NPV) of the ventilation/perfusion scan for PE are as follows:
- PPV for high probability scan: 88%
- NPV for intermediate probability scan: 84%-88%
- NPV for a normal or near normal scan: 91%-96%
- PIOPED demonstrated that the value of the ventilation/perfusion scan is improved when combined with the clinical pretest probability of PE. For example, among patients with suspected PE and high probability scan, the rates of confirmed PE by angiography were 56%, 88%, and 96% in cases of low, intermediate, and high clinical pre-test proability of PE.
Comparison with CT Pulmonary angiography
- Spiral CT scanning is the standard modality to non-invasively diagnose a pulmonary embolism.[4]
- Initial studies reported sensitivities for diagnosing emboli to the segmental level (4th order branch) to be as high as 98%, however subsequent studies have found sensitivities to be lower.
- The sensitivity is higher when the clot has a more proximal location.
- Although smaller clots in the subsegmental arteries are not as physiologically relevant as the larger more proximal clots, they may serve as important predictors of future, larger clots.
- A study consisting of 142 patients concluded that the sensitivity and specificity of CT angiography is higher than that of a V/Q scan.[5]
- Obtaining a CT angiography is recommended following an indeterminate V/Q scan. If the pre-test probability is ‘sufficiently high’, and CT angiography is negative, a standard CT angiogram should then be obtained.
- A cost-effective analysis using spiral CT angiography for the diagnosis of PE showed the following results.[6]
- The use of CT angiography in a diagnostic algorithm was the most cost-effective strategy.
- If the sensitivity of CT angiography was < 85%, conventional angiography was associated with a lower mortality, but still remained a more expensive strategy.
- According to the International Commission on Radiological Protection (ICRP) the radiation exposure from a V/Q scan with Tc-99 m macroaggregate of albumi (MAA) is 1.1 mSv.
- The radiation exposure from spiral CT is 2–6 mSv.[7]
- The radiation exposure from plain chest X-ray is approximately 0.05 mSv.
Benefits of V/Q Scan over CTPA
- Less radiation exposure.
- Diagnostic test of choice in an institution lacking a CT facility, or with inexperienced staff.
Benefits of CTPA over V/Q Scan
- Cost effective.
- CT may also identify right heart dysfunction, or provide an alternative diagnoses.
References
- ↑ PIOPED Investigators (1990). "Value of the ventilation/perfusion scan in acute pulmonary embolism. Results of the prospective investigation of pulmonary embolism diagnosis (PIOPED)". JAMA. 263 (20): 2753–9. PMID 2332918.
- ↑ 2.0 2.1 Torbicki A, Perrier A, Konstantinides S, Agnelli G, Galiè N, Pruszczyk P; et al. (2008). "Guidelines on the diagnosis and management of acute pulmonary embolism: the Task Force for the Diagnosis and Management of Acute Pulmonary Embolism of the European Society of Cardiology (ESC)". Eur Heart J. 29 (18): 2276–315. doi:10.1093/eurheartj/ehn310. PMID 18757870.
- ↑ Morrell NW, Roberts CM, Jones BE, Nijran KS, Biggs T, Seed WA (1992). "The anatomy of radioisotope lung scanning". J. Nucl. Med. 33 (5): 676–83. PMID 1569475. Retrieved 2012-01-12. Unknown parameter
|month=ignored (help) - ↑ Schoepf UJ, Goldhaber SZ, Costello P (2004). "Spiral computed tomography for acute pulmonary embolism". Circulation. 109 (18): 2160–7. doi:10.1161/01.CIR.0000128813.04325.08. PMID 15136509. Retrieved 2011-12-05. Unknown parameter
|month=ignored (help) - ↑ Mayo JR, Remy-Jardin M, Müller NL, Remy J, Worsley DF, Hossein-Foucher C, Kwong JS, Brown MJ (1997). "Pulmonary embolism: prospective comparison of spiral CT with ventilation-perfusion scintigraphy". Radiology. 205 (2): 447–52. PMID 9356627. Retrieved 2011-12-06. Unknown parameter
|month=ignored (help) - ↑ van Erkel AR, van Rossum AB, Bloem JL, Kievit J, Pattynama PM (1996). "Spiral CT angiography for suspected pulmonary embolism: a cost-effectiveness analysis". Radiology. 201 (1): 29–36. PMID 8816516. Retrieved 2011-12-05. Unknown parameter
|month=ignored (help) - ↑ "Radiation dose to patients from radiopharmaceuticals (addendum 2 to ICRP publication 53)". Ann ICRP. 28 (3): 1–126. 1998. PMID 10840563. Retrieved 2011-12-06.