Lung perfusion scan

Overview of Treatment

The Lung Perfusion Scan is a nuclear medicine imaging test primarily used to evaluate pulmonary blood perfusion. After injecting a radioactive tracer, a gamma camera tracks its distribution within the pulmonary vasculature, aiding in the diagnosis of pulmonary embolism, assessing the degree of pulmonary vascular obstruction, or evaluating lung function prior to surgery.

This test, when combined with ventilation-perfusion scanning (V/Q Scan), offers higher accuracy. Its core value lies in non-invasively providing dynamic blood flow information of the lungs, especially when pulmonary embolism is suspected but other test results are inconclusive, making it a critical diagnostic tool.

Types and Mechanisms of Treatment

The Lung Perfusion Scan is divided into Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) types. During the procedure, radioactive isotopes such as Technetium-99m labeled (macrocyclic) colloid are used as tracers, injected intravenously and carried by the bloodstream.

The distribution of radioactive particles within the pulmonary vessels is detected by the scanner, generating three-dimensional images. Areas of blockage appear as regions with decreased radioactive concentration, known as "perfusion defects," which are key indicators in diagnosing pulmonary embolism. The entire procedure takes approximately 30 to 90 minutes, with scan frequency adjusted based on lesion location.

Indications

Mainly indicated for patients suspected of pulmonary embolism with normal D-dimer results or when CT pulmonary angiography cannot be performed. It is also used to evaluate pulmonary arterial hypertension, vascular function before and after lung transplantation, or to assess blood flow compensation in chronic obstructive pulmonary disease (COPD).

In specific cases, such as iodine contrast allergy or renal impairment preventing CT scans, pulmonary perfusion scanning becomes the preferred assessment tool. Additionally, for patients with suspected pulmonary embolism at moderate clinical risk (e.g., Wells score), this test can effectively avoid unnecessary invasive procedures.

Usage and Dosage

Patients should remove metal objects before the scan and may undergo a ventilation scan (inhalation of radioactive gas) beforehand. The injected dose of radioactive isotope is approximately 740 to 1850 MBq, adjusted based on body weight and scan requirements.

After injection, rest for 15-30 minutes to allow even distribution of the tracer. During scanning, patients should lie flat and remain still. No anesthesia is required. The tracer is excreted via urine within about 24 hours, and drinking plenty of water is recommended to facilitate metabolism.

Benefits and Advantages

The main advantages include:

• No iodine contrast agent needed, suitable for patients with allergies or renal impairment
• Can distinguish pulmonary embolism from other pulmonary vascular diseases (e.g., pulmonary hypertension)
• Higher detection rate of peripheral lung lesions compared to CT

Compared to pulmonary arteriography, it is less invasive with an accuracy rate of 85-95%. It provides dynamic blood flow information useful in evaluating multiple small emboli or chronic thromboembolic pulmonary hypertension, supplementing other diagnostic methods.

Risks and Side Effects

Main risks include:

• Allergic reactions: about 0.1-0.3% of patients may develop skin rashes or anaphylactic shock due to components of the radioactive tracer
• Radiation exposure: the dose is equivalent to 3-5 chest X-rays per scan, with caution needed for long-term accumulation

Serious risks: Pregnant women may expose the fetus to radiation, so strict contraindications apply. A small number of patients may experience swelling at the injection site or vasculitis, requiring observation for 30 minutes post-procedure.

Precautions and Contraindications

Contraindications include:

• Pregnancy or suspected pregnancy
• Severe blood coagulation disorders that may lead to bleeding at the injection site
• History of severe allergy to radioactive tracers

Before the procedure, inform the doctor if you have: recent other radioactive examinations, renal impairment, thyroid disease, or pregnancy potential. Diabetic patients should temporarily stop iodine-containing medications to avoid interference with imaging clarity.

Interactions with Other Treatments

When combined with anticoagulants, caution is needed regarding bleeding risk, but the medication itself does not affect the test results. If undergoing renal imaging, a 48-hour interval is recommended to prevent radioactive interference. Compared to CT pulmonary angiography, this test has higher accuracy in detecting lesions outside the central pulmonary arteries but is less sensitive to central vascular obstructions, so clinical context should guide the choice of examination.

Treatment Efficacy and Evidence

Multiple studies show that lung perfusion scans have a sensitivity of 97% and specificity of about 85% in ruling out pulmonary embolism. When combined with ventilation scans, diagnostic accuracy exceeds 95%, making it a key step in the staged diagnosis of acute pulmonary embolism.

In patients with chronic pulmonary arterial hypertension, perfusion scans can accurately locate vascular obstructions, with a correlation coefficient of 0.87 compared to right heart catheterization. International thoracic guidelines list it as a second-line diagnostic standard.

Alternatives

Main alternative tests include:

• CT Pulmonary Angiography (CTPA): high spatial resolution but requires iodine contrast
• Pulmonary arteriography: invasive gold standard but with higher complication rates

While CTPA provides more direct visualization of central pulmonary artery obstruction, patients with renal failure should opt for perfusion scans. D-dimer blood tests can be used as screening tools but cannot replace imaging evidence.

 

Frequently Asked Questions

What preparations are needed before undergoing a pulmonary perfusion scan?

Patients should inform the medical team in advance about allergies, pregnancy potential, or renal function issues. On the day of the scan, bring relevant medical reports, fast for 4-6 hours as instructed, and avoid certain medications (such as iodine contrast agents). If you have a pacemaker or metal implants, inform the staff beforehand.

What discomfort might occur after the scan, and how should it be managed?

Some patients may experience pain at the injection site or mild dizziness after the contrast injection, which usually resolves within hours. Severe allergic reactions (such as difficulty breathing or rash) require immediate medical attention and antihistamine treatment. Normal diet can be resumed after the scan, but observe for 24 hours for any abnormal symptoms.

Should daily activities or medications be adjusted after the scan?

Generally, normal activities can be resumed immediately, but strenuous exercise should be avoided for 24 hours to reduce stress on the injection site. Patients on anticoagulants or other medications should follow their doctor's instructions; no special medication adjustments are usually necessary unless directed.

What are the possible next steps if the scan results are abnormal?

Abnormal results may indicate pulmonary embolism or vascular obstruction. Further diagnostics such as CT pulmonary angiography, D-dimer blood tests, or combined ventilation-perfusion scans may be arranged. Treatment options include anticoagulation therapy, surgery, or thrombolysis, depending on the location and severity of the lesions.

What factors can affect the accuracy of the test, and how can reliability be ensured?

Accuracy may be influenced by patient positioning, tracer metabolism rate, or cardiopulmonary function. To improve reliability, patients should remain still during the scan, and results should be interpreted in conjunction with clinical symptoms, other imaging, and laboratory data. Physicians will make comprehensive assessments to avoid misdiagnosis.