Nuclear Medicine: Thyroid Imaging and Ventilation Scans

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Questions and Answers

What is the reason 131I is not used for routine thyroid imaging?

It poses a high radiation dose and has inferior image quality. (correct)

It cannot identify metastatic thyroid cancer.

It is too expensive for routine use.

It has superior image quality compared to other isotopes.

What characteristic is observed in a technetium-99m pertechnetate scan for a hyperfunctioning nodule?

Cold area with no uptake.

Homogeneous activity throughout the thyroid.

Intense activity focused on the nodule with suppressed surrounding tissue. (correct)

Enlargement of the salivary glands.

How does a nonfunctioning thyroid adenoma appear on a technetium-99m pertechnetate scan?

As a hot spot with high uptake.

As a cold area distinct from the surrounding tissue. (correct)

With symmetry and homogeneous uptake.

As an area with normal thyroid uptake.

Which condition is characterized by an enlarged thyroid with diminished salivary gland visibility on a technetium-99m scan?

Graves disease. Signup and view all the answers

What appearance does subacute thyroiditis have on a technetium-99m pertechnetate scan?

Low activity in the thyroid while salivary gland activity remains normal. Signup and view all the answers

What is a key advantage of using radiolabeled aerosols in ventilation imaging?

Can be delivered in a separate room from the camera. Signup and view all the answers

Which of the following is a disadvantage of using radiolabeled aerosols?

Only a small percentage of activity actually reaches the patient. Signup and view all the answers

What is the principal gamma ray energy of Xenon-133?

81 keV Signup and view all the answers

Why is it important to adjust the relative doses of MAA and DTPA aerosols in sequential imaging?

To prevent one labeled agent from interfering with the other. Signup and view all the answers

What is the half-life of Xenon-133?

5.3 days Signup and view all the answers

What effect does using low-energy photons of Xenon-133 have on imaging?

Causes artifacts from surrounding soft tissues. Signup and view all the answers

What is the primary purpose of performing xenon ventilation scans in the posterior position?

To reduce artifacts from overlying soft tissues. Signup and view all the answers

What type of patient positioning is commonly used when administering radiolabeled aerosols?

Supine position Signup and view all the answers

What is the primary sensitivity advantage of using Xenon-133 (133Xe) imaging for ventilation studies?

It can detect and assess air trapping effectively. Signup and view all the answers

What is a significant limitation of 133Xe ventilation imaging?

Images are obtained in only one projection. Signup and view all the answers

During the washout phase of 133Xe imaging, what is done to monitor xenon clearance?

Serial 15-second images are captured for 2 to 3 minutes. Signup and view all the answers

Which step in the ventilation imaging for 133Xe involves the patient rebreathing a mixture?

Equilibrium phase. Signup and view all the answers

Which feature of Krypton-81m (81mKr) contributes to its infrequent use in clinical practice?

Cost and limited availability. Signup and view all the answers

What characterizes the wash-in phase during the 133Xe imaging process?

The patient exhales completely and inhales xenon. Signup and view all the answers

How does the imaging technique used with 81mKr differ from that of 133Xe?

81mKr studies can occur before or after perfusion imaging. Signup and view all the answers

When assessing a patient with chronic obstructive pulmonary disease (COPD) using 133Xe imaging, what modification might be made during the washout phase?

The duration may be extended to 3 to 5 minutes. Signup and view all the answers

What is typically observed in the anterior view of a normal lung scan?

Identification of the cardiac silhouette and aortic knob Signup and view all the answers

In a normal ventilation scan, how does xenon typically behave during the washout phase?

It clears more quickly from the apices than from the lower portions. Signup and view all the answers

What characteristic indicates the most sensitive detection of airway obstruction?

The washout phase following rebreathing Signup and view all the answers

Which view is best for identifying small pleural effusions?

Lateral or oblique views Signup and view all the answers

What should not be mistaken for trapping of xenon in the right lower lung during the washout study?

Absorption of xenon in the liver Signup and view all the answers

What can be observed in the esophagus and stomach during an aerosol scan?

Swallowed activity from the aerosol Signup and view all the answers

What is the primary clinical application of a ventilation/perfusion lung scan?

Investigation of pulmonary emboli Signup and view all the answers

Which of the following describes the appearance of the lungs during a normal perfusion scan?

Reduction of activity toward the bases due to lung thinning Signup and view all the answers

What is the primary reason for preferring V/Q scans over CTPA in certain patients?

Lower radiation dose and safety in specific populations Signup and view all the answers

What is the main diagnostic principle for identifying pulmonary thromboembolism?

Disassociation between ventilation and perfusion effects Signup and view all the answers

Which imaging method uses technetium-99m pertechnetate for thyroid gland assessment?

Thyroid scintigraphy Signup and view all the answers

What role does TSH play in thyroid function regulation?

Stimulates synthesis and secretion of thyroid hormones Signup and view all the answers

Which of the following accurately describes the behavior of 123I in thyroid imaging?

It is both trapped and organified by the thyroid Signup and view all the answers

What is a typical manifestation of pulmonary embolism in imaging?

Wedge-shaped perfusion defect with preserved ventilation Signup and view all the answers

In which scenario might V/Q scans be specifically preferred?

For young patients and those with normal chest radiographs Signup and view all the answers

What is the primary component that binds circulating thyroid hormones in the bloodstream?

Thyroxine-binding globulin Signup and view all the answers

What is the primary purpose of radionuclide lung imaging?

To demonstrate pulmonary perfusion and assess ventilation Signup and view all the answers

In what situation is a ventilation scan typically not required?

When the perfusion scan is normal Signup and view all the answers

What is the role of Technetium-99m (99mTc) macroaggregated albumin in pulmonary perfusion imaging?

To show regional perfusion defects Signup and view all the answers

In which lung position is pulmonary perfusion considered to be more uniform?

Lying supine Signup and view all the answers

What condition poses a relative contraindication for the administration of 99mTc MAA?

Pulmonary hypertension Signup and view all the answers

How should Technetium-99m MAA particles be prepared before injection?

Gently agitated to prevent settling Signup and view all the answers

What is the typical biologic half-life of MAA in the lungs?

2 to 4 hours Signup and view all the answers

During the perfusion scan, how much volume of MAA should be injected to ensure good distribution?

At least 1 to 2 mL Signup and view all the answers

Study Notes

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.

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Description

This quiz explores various techniques and characteristics involved in thyroid imaging and ventilation scans using nuclear medicine. Questions cover the use of isotopes like 131I and technetium-99m, along with specific conditions affecting the thyroid and lung function. Test your knowledge on the principles and disadvantages of these imaging modalities.