PEMERIKSAAN SIDIK PARU / LUNG SCINTIGRAPHY

 Ventilation-perfusion scan also referred to as lung scintigraphy or commonly V/Q scan, is a diagnostic test utilizing radioisotopes to evaluate pulmonary ventilation and perfusion. 

While CT pulmonary angiography is currently considered the gold standard and is one of the most commonly used modalities for diagnosing pulmonary embolism, a V/Q scan is useful in assessing the likelihood of pulmonary embolism when intravenous contrast is contraindicated, such as in acute or chronic kidney disease and intravenous contrast allergy.

Ventilation perfusion scan consists of two portions, a ventilation (V) scintigraphy and a perfusion (Q) scintigraphy. An aerosolized tracer is administered to assess lung ventilation by evaluating the distribution of the tracer to the alveoli. The assessment of lung perfusion involves administering an injectable tracer and its distribution to the pulmonary vasculature. 

Ventilation Scintigraphy (V)

This component of the scan assesses air distribution in the lungs. Ventilation radiopharmaceuticals are classified as gases, aerosolized liquid, and aerosolized solid particles.

• 99mTc-diethylenetriaminepentaacetic acid (DTPA) is the most commonly used (57%) radiopharmaceutical in the form of liquid aerosol with a median aerosol diameter of 4.5 micrometers, a half-life of 6 hours, and a photopeak of 140 keV. A dose of 25 to 35 mCi (925–1295 MBq) of Tc-DTPA is administered via a nebulizer using a mouthpiece.
• 99mtc-labeled solid graphite hydrophobic particles is an aerosolized solid with a particle diameter of fewer than 2 micrometers and is currently not approved for use in the United States. by the US Food and Drug Administration. 

Perfusion Scintigraphy (Q)

Perfusion scintigraphy involves a radiopharmaceutical agent 99mTc-macro aggregated albumin (99mTc-MMA) with a particle size of 10 to 100 micrometers which is injected intravenously. The usual administered dose is 40–150 MBq (1 to 4 mCi).[4] The patient lays in the supine position during the injection to allow maximum blood flow to lung apices. The radiopharmaceutical particles then embolize in the capillaries and provide a map of pulmonary blood flow. 

Afterward, lung imaging is performed using either planar imaging with a high-resolution gamma camera, SPECT, or 3D imaging with SPECT/CT.

Imaging

The various imaging techniques for lung imaging are planar imaging, SPECT, and SPECT/CT. Planar imaging uses a gamma camera, and images are obtained in anterior, posterior, lateral, and anterior and posterior oblique views. Planar imaging allows for two-dimensional imaging and is less sensitive compared to SPECT.

SPECT imaging technique has been shown in multiple studies to have better sensitivity, specificity, and accuracy when compared to planar imaging [8]. An analysis of various pooled studies showed that the sensitivity of SPECT ranges from 80% to 100%, and its specificity ranges from 93% to 100%.  SPECT also has a lower indeterminate rate than planar imaging and a negative predictive value of 98%.

SPECT/CT combines SPECT with low-dose CT, allowing for better anatomic information and visualization of an alternative etiology of patient symptoms.

Indications

The commonest indication is for the diagnosis of pulmonary embolism. Other indications include pre-operative evaluation before lung surgery.