vq scan of lung

V/Q SCANA type of medical imaging using scintigraphy to

evaluate the circulation of air and blood within a patient's lungs .

The ventilation part of the test looks at the ability of air to reach all parts of the lungs

The perfusion part evaluates how well blood circulates within the lungs.

INDICATIONS Most commonly done to check the presence of a

blood clot or abnormal blood flow inside the lungs.PECOPDPneumoniaPost lobectomy

Pulmonary embolismPathophysiologyRudolph Virchow, 1858

Triad: Hypercoagulability Stasis to flow Vessel injury

Risk FactorsHypercoagulability Malignancy Pregnancy Postpartum status (<4wk) Estrogen/ OCP’s

Venous StasisBed rest > 24 hr Recent cast or external fixatorLong-distance travel or prolong automobile travel

Venous Injury surgery trauma (especially the lower extremities and pelvis)

PresentationDyspneaPleuritic chest painLow-grade feverTachycardia

RADIOGRAPHIC EVALUATION

• CXR• V/Q Scan• Spiral CT with contrast• Angiogram

CXRInitial CXR usually normal. May progress to show atelactasis pleural effusion elevated hemi diaphragm.

CXRChest X-Ray Myth:

“You have to do a chest x-ray so you can find Hampton’s hump or a Westermarck sign.”

Reality:

Most chest x-rays in patients with PE are nonspecific and insensitive

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Chest X-ray Eponyms of PEWestermarck's sign

A dilation of the pulmonary vessels proximal to the embolism along with collapse of distal vessels, sometimes with a sharp cutoff.

Hampton’s Hump

A triangular or rounded pleural-based infiltrate or consolidation with the apex toward the hilum.

Westermark’s Sign

Hampton’s Hump

CXR

Ventilation/Perfusion Scan- “V/Q Scan”

A common modality to image the lung and its use still stems from the PIOPED study.

Relatively noninvasive

In many centers remains the initial test of choice

Preferred test in pregnant patients 50 mrem vs 800mrem (with spiral CT)

Procedure• Ventilation phase a gaseous radionuclide such Krypton ,Xenon or

technetium DTPA in an aerosol form is inhaled by the patient through a mask.

• The perfusion phase involves the I/V inj of radioactive technetium macro aggregated albumin (Tc99m-MAA).

• A gamma camera acquires the images for both phases of the study.

The ventilation and perfusion phases of a V/Q lung scan are performed together along with a chest x-ray for comparison or to look for other causes of lung disease

NORMAL PERFUSION IMAGES

NORMAL VENTILATION IMAGES

PIOPED STUDY

PIOPED STUDY The Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) was a multicenter, collaborative effort designed to determine the sensitivity and specificity of the V/Q scan in patients with suspected acute PE

Defect Descriptors, as Used in PIOPED 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.

PERFUSION DEFECTSMismatched Perfusion defects: -Ventilation is normal in that segment -Indicative of PE.Matched Perfusion defects: -Ventilation defect in that segment. -It is usually seen in parenchymal lung disease.

Normal No perfusion defects or perfusion exactly outlines the

shape of the lungs seen on the chest radiograph.

High probability criteria• 2 or more large (>75% of a segment) mismatched perfusion defects with no corresponding CXR

abnormalities• 1 large and >2 moderate sized (25-75% of a segment)

mismatched• perfusion defects with no corresponding CXR

abnormalities• or 4 or more moderate-sized mismatches with no

corresponding CXR

Intermediate probability

A mixture of matched and unmatched defectsor single moderate-sized mismatch with normal CXRor triple match in lower zoneor matched VQ defect with small effusionor doesn't fit into normal, low or high probability

categories

Low probability Criteria• multiple matched VQ defects, regardless of size,

with normal CXR• triple matched (i.e. matched VQ defect plus CXR

lesion in same area) in upper or mid zone

• or perfusion defects surrounded by normally perfused lung (stripe sign)

• or matched VQ defect with large effusion

• or any perfusion defect with a substantially larger CXR abnormality

RESULTSNormal perfusion scan: means that the patient is very unlikely to have acute

PE.Low probability V/Q scan: means that the patient has less than 20% probability

of having acute PE.

High probability V/Q scan: means that the patient has greater than 80%

probability of having acute PE.

Intermediate probability V/Q scan: means that the patient has between 20 – 80%

probabilities of having acute PE.

Visible renal uptake indicates a right to left shunt faulty radiopharmaceutical preparation Thyroid activity usually indicates the presence of

unbound per technetate in injected dose.

Artefacts may result from clumping of inhaled particles with severe airways disease

ORwhere errors occurred in the preparation or

administration of the particles

CONTRAINDICATIONSNo absolute CI

RELATIVE CIPulmonary HypertensionRight to Left shunts e.g. VSD.

Spiral (Helical) Chest CTAdvantages

RapidAlternative Diagnosis

Disadvantages

Costly Risk to patients with borderline renal functionHard to detect subsegmental pulmonary emboli

CT Angiogram

Pulmonary angiogramGold Standard.Positive angiogram provides 100% certainty that an

obstruction exists in the pulmonary artery.Negative angiogram provides > 90% certainty in the

exclusion of PE.

PULMONARY ANGIOGRAPHY

SummaryPlain chest radiograph – Usually normal and non-

specific signs.Radionuclide ventilation-perfusion lung scan.CT Angiography of the pulmonary arteries – Quickly

becoming method of choice.Pulmonary angiography – Gold standard but invasive.

QUESTIONA 52 yr old male presented with SOB and his V/Q

scan showed multiple matched V/Q defects and his CXR was normal.

Where will you place the pt in terms of probability. Describe the scheme for interpretation of V/Q scan.Describe the different investigation for diagnosis of

PE and its plain CXR findings.

THANK YOU