Nuclear Medicine: Thyroid Imaging and Ventilation Scans

Nuclear Medicine - Respiratory System

• Radionuclide lung imaging commonly demonstrates pulmonary perfusion and assesses ventilation using inert gases (Xenon) or 99mTc-labelled aerosols.
• Ventilation (V) and perfusion (Q) scans are often called V/Q scans.
• Perfusion scans are usually correlated with ventilation scans.
• If perfusion scans are normal, ventilation scans can be skipped.
• Segmental perfusion defects necessitate a ventilation scan.
• Ventilation imaging is a non-invasive method to evaluate regional lung ventilation (air flow) to determine the type of perfusion defect.

V/Q Scans

• Perfusion scan is typically correlated with the ventilation scan.
• If the perfusion scan is normal, the ventilation scan can often be skipped.
• If segmental perfusion defects are present, a ventilation scan is required.

Ventilation Imaging

• A noninvasive technique used to evaluate lung ventilation.
• Measures air flow to aid in assessing perfusion defects.
• Employing radiolabeled aerosols or radioactive gases.

Pulmonary Perfusion Imaging

• Uses Technetium-99m (99mTc) macroaggregated albumin (MAA).
• MAA localizes via capillary blockade.

Perfusion Imaging

• A non-invasive approach to evaluating pulmonary arterial blood flow.
• Requires a sufficient number of particles for appropriate statistical distribution.
• MAA particle size ranges from 10-100 µm.
• Dosage for adults is 3-5 mCi IV, and for children it's 0.03-0.07 mCi/kg.
• Views include anterior, posterior, lateral, anterior obliques, and posterior obliques.
• MAA has a biological half-life in the lung of 2-4 hours.

Technique (General)

• Tc-99m MAA should be injected during respiration.
• The patient should be supine to minimize the perfusion gradient between apex and base lung.
• The syringe should be gently agitated before injection.
• The injection should be slow for homogeneous pulmonary particle distribution.
• Injected volume is at least 1-2 mL.

Contraindications

• Severe pulmonary hypertension is a relative contraindication.
• Blockade of additional lung capillaries may exacerbate pulmonary hypertension and its cardiac complications.

Ventilation Imaging (Using Radiolabeled Aerosols)

• Technetium-labeled radioactive aerosols are used.
• Unlike radioactive gases, aerosol studies do not allow for dynamic single-breath or washout phase imaging.
• Instead, they show the distribution of aerated lung volume.
• Aerosol particles remain in place long enough for imaging in multiple projections after deposition into the lungs.

Radiolabeled Aerosols—Technique

• The patient inhales a mixture of oxygen and nitrogen containing small amounts of radioactive xenon or technetium.
• The patient is typically positioned supine.
• Flow rates are within the range of 7-10 L/min.
• A mouthpiece with a nose clip is often used to administer the aerosol.
• Half-time clearance time from the lungs for nonsmokers is 45-60 minutes, and 20 minutes for smokers.

Radiolabeled Aerosols—Advantages

• Availability of 99mTc and its optimal imaging energy.
• Limited patient cooperation required.
• Aerosols can be administered in separate rooms from the imaging equipment.

Radiolabeled Aerosols—Disadvantages

• The amount of activity delivered to the patient is relatively small (2-10%) compared with the total available in the aerosol generator.
• Sequential ventilation and perfusion imaging requires adjusted doses to prevent interference from both labeled agents.

Ventilation Imaging (Using Radioactive Gases)

• Radioactive gases (e.g., Xenon-133) can be used.
• This permits sequential imaging of lung ventilation and perfusion.
• Rapid clearance of gases and differences in photon emissions distinguish ventilation from perfusion scans.

Radioactive Gases—Xenon-133 (133Xe)

• Cost-effective and has a half-life of 5.3 days.
• Principal gamma ray energy is 81 keV.
• Low photon energy results in a roughly 50% attenuation by 10 cm of inflated lung tissue.
• Scans are often performed in the posterior position to minimize artifacts due to overlying structures.
• The critical organ with exposure is the trachea.
• Provides a comprehensive assessment of regional ventilation (single breath, wash-in, equilibrium, and washout). This complete characterization makes 133Xe imaging the most sensitive method for detecting and assessing airway disease.

Radioactive Gases—Xenon-133 (133Xe)—Additional

• Images obtained in only one projection and performed before perfusion scans and due to single projection, some regional ventilation abnormalities may go undetected.
• Significant patient cooperation is necessary due to the need for prolonged breathing in a closed spirometer system.

Technique (Ventilation imaging)

• The most complete method involves three phases: single wash-in (or initial breath), equilibrium, and washout.

Technique (Single-breath imaging)

• The patient exhales completely, then inhales 10-20 mCi of 133Xe (370-740 MBq).
• Holds breath for 15 seconds.
• The patient then re-breathes expired xenon diluted with 2L oxygen in a closed system (equilibrium phase).
• A static image is acquired.

Technique (Washout phase)

• Fresh air is breathed, and images are taken (usually for 2-3 minutes), whilst xenon clears from the lungs.
• The duration may be prolonged in those with COPD (Chronic Obstructive Pulmonary Disease).

Ventilation Imaging (Krypton-81m)

• Half-life of 13 seconds and photon emissions between 176 and 192 keV.
• Its high-energy photons allow it to be carried out before or after the perfusion scan.
• Short half-life precludes single-breath and washout images.
• It is expensive and less frequently used.

Normal Lung Scan (Perfusion)

• Posterior projection shows some activity reduction toward the bases due to lung thinning.
• Anterior view typically shows the cardiac silhouette and aortic knob.
• Oblique views provide valuable information but results may be ambiguous, requiring confirmation on standard views.
• Small pleural effusions are best viewed in oblique or lateral projections.

Normal Ventilation Scan

• Homogeneous activity distribution throughout both lungs during initial breath images.
• Maximum inspiratory effort is crucial in ensuring accurate initial breath images to reflect regional rates.
• Activity in lower lung zones clears faster during the washout phase than in upper regions as air exchange is greater.
• Lung zones should be largely clear of xenon within 2-3 minutes with normal half-time being approximately 30-45 seconds.
• The washout phase sensitivity relies on significant rebreathing to ensure equilibrium across different lung segments.

Normal Ventilation Scan (Additional)

• Soluble xenon distributes in fat and blood, and liver deposition may produce false positives, especially towards the end of the washout phase.
• In children, potential abdominal activity can result from swallowed xenon gas..
• Normal aerosol scans mirror perfusion scans but show the visualization of the trachea and bronchi.. Swallowed activity may be seen in the esophagus or stomach.

Clinical Applications

• Suspected pulmonary emboli are primary reasons for ventilation/perfusion scans.
• CT pulmonary angiography (CTPA) can be a leading diagnostic tool initially.
• V/Q scans or perfusion-only imaging are efficient low-dose alternatives for patients with contrast allergies, renal failure, large body size, or other circumstances. Chest radiographs aid in interpretation.

Pulmonary Embolism—Diagnostic Principle

• Pulmonary thromboembolism diagnosis depends on the disassociation between ventilation and perfusion, resulting from the embolus' obstruction of pulmonary arterial blood flow.

Pulmonary Embolism—Diagnostic Principle (99mTc-MAA)

• With 99mTc-MAA imaging, MAA particles cannot enter capillaries distal to the arterial occlusion, hence, perfusion defects in impacted regions.
• Ventilation is typically unaffected.
• The typical characteristic is a wedge-shaped perfusion defect with preserved ventilation (segmental V/Q mismatch).

Radiopharmaceutical Data (Summary)

• Provides data on specific radiopharmaceuticals used, patient preparation, activity administered, collimator type, imaging views, and relevant dose information.