Pulmonary embolism ventilation/perfusion scan: Difference between revisions

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'''Editor(s)-In-Chief:''' {{ATI}}, [[C. Michael Gibson, M.S., M.D.]] [mailto:charlesmichaelgibson@gmail.com]; {{AE}}; {{AE}} {{Rim}}
{{CMG}}; '''Associate Editors-in-Chief:''' [[User:Ujjwal Rastogi|Ujjwal Rastogi, MBBS]] [mailto:urastogi@perfuse.org]


==Overview==
==Overview==
[[Ventilation/perfusion scan]] (or V/Q scan or lung [[scintigraphy]]), show whether or not a lung area is being ventilated and perfused with blood (perfusion can be stopped due to obstruction caused by a clot). This type of examination is used less often because of the more widespread availability of CT technology, however, it may be useful in patients who have an allergy to [[iodinated contrast]]or in [[pregnancy]] due to lower radiation exposure than CT.
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 pulmonary embolism (PE), [[perfusion]] can be obstructed due to the formation of a clot. The [[ventilation/perfusion scan]] is less commonly used due to the more widespread availability of computed tomography ([[CT]]) technology, however it may be useful in patients who have an allergy to [[iodinated contrast]]. It may also be useful in [[pregnancy|pregnant]] patients in an attempt to minimize radiation exposure. The diagnostic value of the results of the [[V/Q scan]] is improved when combined with the clinical pretest probability of PE. A high probability scan coupled with a high clinical pretest probability of PE is diagnostic for PE, while a normal scan regardless of the clinical pretest probability excludes PE. For the majority of the cases of suspected PE, however, the [[ventilation/perfusion scan]] does not establish the diagnosis nor exclude PE and further tests are required.<ref name="pmid2332918">{{cite journal| author=PIOPED Investigators| title=Value of the ventilation/perfusion scan in acute pulmonary embolism. Results of the prospective investigation of pulmonary embolism diagnosis (PIOPED). | journal=JAMA | year= 1990 | volume= 263 | issue= 20 | pages= 2753-9 | pmid=2332918 | doi= | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=2332918  }} </ref>


==Ventilation/perfusion scan==
==[[Ventilation/Perfusion Scan]]==
===Principle===
===Principle===
Technetium (Tc)-99m labelled macro-aggregated albumin particle, when injected, block small fractions of pulmonary capillaries and thus helps in assessment of lung perfusion at tissue levels via scintigraphy. If there is an occlusion of any branch of pulmonary artery, the particle will not reach the capillary, rendering the area "cold" on subsequent imaging.
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===
===Indications===
This type of examination is used less often because of the more widespread availability of CT technology, however, it may be useful in patients
The utilization of ventilation/perfusion 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:
# Who have an allergy to iodinated contrast. To read more about contrast allergy, click [[Contrast medium#Allergy Reactions|here]].
# Patients who have a known allergy to iodinated contrast. To read more about contrast allergy, click [[Contrast medium#Allergy Reactions|here]].
# In pregnancy due to lower radiation exposure than CT.
# In pregnant patients to minimize exposure to radiation.
# Hospitals lacking CT facility or inexperienced medical staff.
# For patients who are in a hospital lacking CT technology.


===Interpretation===
===Interpretation===
The following table summarizes the possible outcome of V/Q scan:
A normal or near normal V/Q scan excludes the diagnosis of PE.<ref name="pmid2332918">{{cite journal| author=PIOPED Investigators| title=Value of the ventilation/perfusion scan in acute pulmonary embolism. Results of the prospective investigation of pulmonary embolism diagnosis (PIOPED). | journal=JAMA | year= 1990 | volume= 263 | issue= 20 | pages= 2753-9 | pmid=2332918 | doi= | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=2332918  }} </ref><ref name="pmid18757870">{{cite journal| author=Torbicki A, Perrier A, Konstantinides S, Agnelli G, Galiè N, Pruszczyk P et al.| title=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). | journal=Eur Heart J | year= 2008 | volume= 29 | issue= 18 | pages= 2276-315 | pmid=18757870 | doi=10.1093/eurheartj/ehn310 | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=18757870  }} </ref> 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 interpretation of the different results of the V/Q scan.<ref name="pmid18757870">{{cite journal| author=Torbicki A, Perrier A, Konstantinides S, Agnelli G, Galiè N, Pruszczyk P et al.| title=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). | journal=Eur Heart J | year= 2008 | volume= 29 | issue= 18 | pages= 2276-315 | pmid=18757870 | doi=10.1093/eurheartj/ehn310 | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=18757870  }} </ref>
 
{| style="cellpadding=0; cellspacing= 0; width: 600px;"
|-
| style="padding: 0 5px; font-size: 100%; background: #4682B4; color: #FFFFFF" align=center |'''V/Q Scan''' ||style="padding: 0 5px; font-size: 100%; background: #4682B4; color: #FFFFFF" align=center |'''Clinical Probability''' || style="padding: 0 5px; font-size: 100%; background: #4682B4; color: #FFFFFF" align=center |'''Interpretation'''
|-
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | Normal
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | Any probability
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | PE excluded
|-
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | Low probability scan
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | Low
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | PE excluded
|-
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | High probability scan
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | High
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | PE confirmed
|-
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left colspan="2"|Any other combination of V/Q scan and clinical probability
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left |Further tests are needed
|}
 
===Supportive Trial Data===
* Data regarding the use of the V/Q scan for the evaluation of patients with suspected PE is provided by the findings of the [[Pulmonary embolism landmark trials#Trials assessing the diagnosis|PIOPED]] study. The rates of confirmed PE by [[angiography]] for each of the categories of the V/Q scan are:<ref name="pmid2332918">{{cite journal| author=PIOPED Investigators| title=Value of the ventilation/perfusion scan in acute pulmonary embolism. Results of the prospective investigation of pulmonary embolism diagnosis (PIOPED). | journal=JAMA | year= 1990 | volume= 263 | issue= 20 | pages= 2753-9 | pmid=2332918 | doi= | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=2332918  }} </ref>
** High probability scan: 88%
** Intermediate probability scan: 33%
** Low probability scan: 16%
** Normal or near normal probability scan: 9%


{| border="1"
* Shown below is the sensitivity and specificity of V/Q scan for the detection of PE:<ref name="pmid2332918">{{cite journal| author=PIOPED Investigators| title=Value of the ventilation/perfusion scan in acute pulmonary embolism. Results of the prospective investigation of pulmonary embolism diagnosis (PIOPED). | journal=JAMA | year= 1990 | volume= 263 | issue= 20 | pages= 2753-9 | pmid=2332918 | doi= | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=2332918  }} </ref>
|+
 
! [[Pulmonary embolism ventilation/perfusion scan|V/Q Scan]] !! [[Pulmonary embolism diagnosis#Wells score|Clinical Probability]] !! Diagnosis
{|
|-
|-
| Normal
| style="padding: 0 5px; font-size: 100%; background: #4682B4; color: #FFFFFF" align=center |'''Category of ventilation/perfusion scan'''|| style="padding: 0 5px; font-size: 100%; background: #4682B4; color: #FFFFFF" align=center |'''Sensitivity''' || style="padding: 0 5px; font-size: 100%; background: #4682B4; color: #FFFFFF" align=center |'''Specificity'''
| any probability
| PE excluded<ref name="pmid19634076">{{cite journal |author=Hoeper MM |title=Definition, classification, and epidemiology of pulmonary arterial hypertension |journal=Semin Respir Crit Care Med |volume=30 |issue=4 |pages=369–75 |year=2009 |month=August|pmid=19634076 |doi=10.1055/s-0029-1233306 |url=http://www.thieme-connect.com/DOI/DOI?10.1055/s-0029-1233306|accessdate=2011-12-06}}</ref>
|-
|-
| Low probability scan
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left | High probability scan ||style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left| 41% || style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left| 97%
| Low
| PE excluded
|-
|-
| High probability scan
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left| High or intermediate probability scan || style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left| 82% || style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left| 52%
| High
|-
| PE confirmed
|style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left| High, intermediate, or low probability scan || style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left| 98% || style="font-size: 100; padding: 0 5px; background: #B8B8B8" align=left| 10%
|-
|-
| Variable result/Non diagnostic
| Variable
| Serial [[Deep vein thrombosis ultrasound|lower extremity USG]] or [[Pulmonary embolism other imaging findings#Angiography|Pulmonary angiography]]
|}
|}


===Trial supported Data===
* For a [[prevalence]] of PE of 33% (confirmed by [[angiography]]), the [[positive predictive value]] (PPV) and [[negative predictive value]] (NPV) of the ventilation/perfusion scan for PE are as follows:<ref name="pmid2332918">{{cite journal| author=PIOPED Investigators| title=Value of the ventilation/perfusion scan in acute pulmonary embolism. Results of the prospective investigation of pulmonary embolism diagnosis (PIOPED). | journal=JAMA | year= 1990 | volume= 263 | issue= 20 | pages= 2753-9 | pmid=2332918 | doi= | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=2332918  }} </ref>
The '''[[Pulmonary embolism landmark trials#Trials assessing the diagnosis|PIOPED]]''' data suggested, that the normal perfusion scans are almost never associated with recurrent pulmonary embolism, even if anticoagulation is withheld. The other highlights of this study were:
** [[PPV]] for high probability scan: 88%
:* High-probability scans, however only identified 41% of patients with PE.
** [[NPV]] for intermediate probability scan: 84%-88%
:* In the setting of a high pre-test probability, a high-prob scan revealed PE in 95% of cases.
** [[NPV]] for a normal or near normal scan: 91%-96%
:* Unfortunately, 41% of all scans in PIOPED were interpreted as intermediate, and an additional 16% were interpreted as low-probability.
 
:** Upon angiography, however, 30 and 14% of these patients respectively were found to have PE.
* PIOPED demonstrated that the value of the V/Q scan is improved when combined with the clinical pretest probability of PE.  For example, among patients with suspected PE and high probability V/Q scan, the rates of confirmed PE by [[angiography]] were 56%, 88%, and 96% in cases of low, intermediate, and high clinical pre-test probability of PE respectively.<ref name="pmid2332918">{{cite journal| author=PIOPED Investigators| title=Value of the ventilation/perfusion scan in acute pulmonary embolism. Results of the prospective investigation of pulmonary embolism diagnosis (PIOPED). | journal=JAMA | year= 1990 | volume= 263 | issue= 20 | pages= 2753-9 | pmid=2332918 | doi= | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=2332918  }} </ref>
:** Based on these numbers, there has been a huge movement to abolish the low-probability, and intermediate-probability categories, and have readings either be high-probability, normal, or non-diagnostic.
 
:* It should also be realized that the false-positive rate for high-prob scans was 14%, and that 72% of patients in PIOPED had a clinical – scan combination that required further investigation.
===Comparison with CT Pulmonary Angiography (CTPA)===
* It has been suggested by some authors that in patients with an intermediate pre-test probability of PE, a venous ultrasound would provide the same justification for anticoagulation, as would a confirmed PE. Although, false positive results on venous ultrasound (3 percent patient reported in a study) would result in the use of anticoagulation in patients without PE.<ref name="pmid9148650">{{cite journal |author=Turkstra F, Kuijer PM, van Beek EJ, Brandjes DP, ten Cate JW, Büller HR |title=Diagnostic utility of ultrasonography of leg veins in patients suspected of having pulmonary embolism |journal=Ann. Intern. Med. |volume=126 |issue=10 |pages=775–81 |year=1997 |month=May |pmid=9148650 |doi= |url= |accessdate=2012-01-12}}</ref>
====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


===Comparison with CT Pulmonary angiography===
====Benefits of CTPA over V/Q Scan====
* Spiral CT scanning is now a standard modality to non-invasively diagnose PE.<ref name="pmid15136509">{{cite journal |author=Schoepf UJ, Goldhaber SZ, Costello P |title=Spiral computed tomography for acute pulmonary embolism |journal=Circulation |volume=109 |issue=18 |pages=2160–7 |year=2004 |month=May |pmid=15136509 |doi=10.1161/01.CIR.0000128813.04325.08 |url=http://circ.ahajournals.org/cgi/pmidlookup?view=long&pmid=15136509 |accessdate=2011-12-05}}</ref>
* Cost effectiveness
*:* Initial studies reported sensitivities for diagnosing emboli to the segmental level (4th order branch) as high as 98%, however subsequent studies have found sensitivities to be lower.
* CT may also identify right heart dysfunction, or provide an alternative diagnosis.
*:* The sensitivity is higher with more proximal clot.
*:* Although smaller clot, in the subsegmental arteries, is certainly not as physiologically important as the larger, more proximal clot, they may be important predictors of future, larger clots.
*:* The study consisting of 142 patients concluded that the sensitivity and specificity of CT angiography is higher than that of V/Q scan, as is the inter-observer agreement.<ref name="pmid9356627">{{cite journal |author=Mayo JR, Remy-Jardin M, Müller NL, Remy J, Worsley DF, Hossein-Foucher C, Kwong JS, Brown MJ |title=Pulmonary embolism: prospective comparison of spiral CT with ventilation-perfusion scintigraphy |journal=Radiology |volume=205 |issue=2 |pages=447–52 |year=1997 |month=November |pmid=9356627 |doi= |url=http://radiology.rsnajnls.org/cgi/pmidlookup?view=long&pmid=9356627 |accessdate=2011-12-06}}</ref>
*:*:* They recommend getting a CT angiography as the next test following an indeterminate V/Q scan, however caution that if the pre-test probability is ‘sufficiently high’, a standard angiogram should still be obtained after a negative CT angiography.
* A '''cost-effective''' analysis using spiral CT angiography for the diagnosis of PE showed the following result.<ref name="pmid8816516">{{cite journal |author=van Erkel AR, van Rossum AB, Bloem JL, Kievit J, Pattynama PM |title=Spiral CT angiography for suspected pulmonary embolism: a cost-effectiveness analysis |journal=Radiology |volume=201 |issue=1 |pages=29–36 |year=1996 |month=October |pmid=8816516 |doi= |url=http://radiology.rsnajnls.org/cgi/pmidlookup?view=long&pmid=8816516 |accessdate=2011-12-05}}</ref>
** The use of CT angiography in a diagnostic algorithm was by far and away a more 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 lung scan with 100 MBq of Tc-99 m macroaggregate of albumi (MAA) is 1.1 mSv.
**The radiation exposure from spiral CT is 2–6 mSv.<ref name="pmid10840563">{{cite journal |author= |title=Radiation dose to patients from radiopharmaceuticals (addendum 2 to ICRP publication 53) |journal=Ann ICRP |volume=28 |issue=3 |pages=1–126 |year=1998 |pmid=10840563 |doi= |url=http://linkinghub.elsevier.com/retrieve/pii/S0146645399000068 |accessdate=2011-12-06}}</ref>
**The radiation exposure from plain chest X-ray is approximately 0.05 mSv.


==References==
==References==
{{reflist|2}}
{{Reflist|2}}
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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: ; Associate Editor(s)-in-Chief: Rim Halaby, M.D. [2]

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 pulmonary embolism (PE), perfusion can be obstructed due to the formation of a clot. The ventilation/perfusion scan is less commonly used due to the more widespread availability of computed tomography (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. The diagnostic value of the results of the V/Q scan is improved when combined with the clinical pretest probability of PE. A high probability scan coupled with a high clinical pretest probability of PE is diagnostic for PE, while a normal scan regardless of the clinical pretest probability excludes PE. For the majority of the cases of suspected PE, however, the ventilation/perfusion scan does not establish the diagnosis nor exclude PE and further tests are required.[1]

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 ventilation/perfusion 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:

  1. Patients who have a known allergy to iodinated contrast. To read more about contrast allergy, click here.
  2. In pregnant patients to minimize exposure to radiation.
  3. 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 interpretation of the different results of the 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

Supportive Trial Data

  • Data regarding the use of the V/Q 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 V/Q scan are:[1]
    • 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 V/Q scan for the detection of PE:[1]
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%
  • PIOPED demonstrated that the value of the V/Q scan is improved when combined with the clinical pretest probability of PE. For example, among patients with suspected PE and high probability V/Q scan, the rates of confirmed PE by angiography were 56%, 88%, and 96% in cases of low, intermediate, and high clinical pre-test probability of PE respectively.[1]

Comparison with CT Pulmonary Angiography (CTPA)

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 effectiveness
  • CT may also identify right heart dysfunction, or provide an alternative diagnosis.

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 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. ↑ 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.

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