Imaging_of_the_Lungs

Clinical Indications for Lung Imaging

Key Evaluations:

  • Pulmonary embolism (PE): often from deep vein thrombosis (DVT)

  • Chest pain

  • Shortness of breath (SOB)


  • Low blood oxygen saturationAdditional Evaluations:

  • Lung carcinoma

  • Conditions affecting perfusion (e.g. emphysema, asthma)

  • Detection of right-to-left cardiac shunts (particularly in children)

  • Quantitative evaluations of pulmonary perfusion

Patient History Considerations

Critical history questions include:

  • History of cancer

  • Breathing difficulties/pain

  • Coughing up blood

  • Lung disease history

  • Smoking status

  • Symptoms like fever and fainting

  • Use of oral contraceptives

  • Heart problems

  • Recent surgeries and tests

  • Current medications

Patient Preparation

Required:

  • Chest X-ray within 24 hours before imaging to exclude other potential causes (e.g., rib fractures, pneumonia, COPD)

Ventilation Scan Radiopharmaceuticals

[Xe-133] gas:

  • Half-life: 5.3 days

  • Energy: 81 keV gamma

  • Typical dose: 10-20 mCi via inhalation

  • Mechanism: compartmental localization within lung space

[Tc-99m] DTPA aerosol:

  • Half-life: 6 hours

  • Energy: 140 keV gamma

  • Typical dose: 25-40 mCi via inhalation

Imaging Procedures


[Xe-133] Gas Ventilation ImagingSetup:


  • Position: Patient sitting or supine, camera posteriorProtocol includes:

  • Administration during deep breath

  • Capture dynamic images for 30 seconds/frame for 12 frames

Phases of Imaging:

  1. Initial breath (inspiration)

  2. Equilibrium phase (1-2 minutes)

  3. Washout phase (2-3 minutes)

[Tc-99m] DTPA Ventilation Imaging

  • Administer in: Supine or upright position, nebulizer usage

  • Monitor normal breathing for ~5 minutes

  • Images obtained include: Posterior, anterior, lateral, etc.

[Tc-99m] MAA Perfusion Imaging

  • Radiopharmaceutical: [Tc-99m] MAA

  • Half-life: 6 hours, typical dose 2-6 mCi via IV

  • Administration: Supine position to ensure even distribution

  • Imaging positions similar to ventilation scans

Quantitative Imaging:

  • Dose: 1-4 mCi of [Tc-99m] MAA

  • Supine patient for optimal results

Normal Results Interpretation

Ventilation Scans:

  • [Xe-133] gas: Uniform distribution and washout results

  • [Tc-99m] DTPA: Regular aerosol distribution

Perfusion Results:

  • Homogenous uptake observed, with some attenuation by breast/heart

Abnormal Results Interpretation

Ventilation

  • Areas of decreased activity indicate non-ventilated lungs

  • Potential indicators:

  • PE presents normally with ventilation/fusion mismatching

  • COPD shows gas trapping in washout

Perfusion

  • Wedge-shaped decreased activity suggests high probability of PE

  • Matching defects may indicate COPD or lung tumors

Quantitative Lung Imaging

  • Used for evaluating lung carcinoma:

    • Confirm tumor resectability

    • Assess patient’s capacity for pulmonary tissue removal

    • Quantitative Steps:

      • Involve calculating right-to-left ratios using geometric mean values

Review Concepts

  • Types of defects indicative of PE on V/Q scan include: Matching defect, size increase, edge sign, and mismatch defect.

  • Patient setup critical for [Tc-99m] MAA injection, emphasizing supine position.

  • Accurate gas retention can be observed during specific image phases.

Important Notes

  • Avoid back flushing when administering radiopharmaceuticals to prevent clot formation.


  • Pulmonex: A type of scavenger intended for respiratory use, assisting with gas absorption.

  • Drierite: A desiccant which is effective in drying gases and is used in chemical applications.

  • Soda Lime: A substance used in medicine to absorb carbon dioxide during gas exchange by patients in respiratory distress, enhancing ventilation efficiency.