Nuclear Medicine 4203 Scanning & Imaging Respiratory System

Anatomy and Physiology Trachea divides into the right and left mainstem bronchi These divide to form lobar bronchi Right side has upper, middle and lower lobe bronchi Left side has upper and lower bronchi Lobes further divide into segments

Lung Segments

Lung Anatomy Main pulmonary arteries divide into each lung and follow the divisions of the bronchi and bronchioles to the level of the alveoli. Each alveolus is supplied by a terminal pulmonary arteriole, which turns to capillaries. Adults have 250-300 million alveoli

Lung Physiology Gravity and patient position have a significant impact on both ventilation and perfusion. In upright position, intrapleural pressure is significantly more negative at the apices than at the base of the lung. Also in upright position, the apex receives only 1/3 of the blood flow compared to the base.

Radiopharmaceuticals Perfusion: 99mTc macroaggregated albumin (MAA) Localizes by capillary blockage Fewer than 1 in 1000 capillaries are blocked Injection should include 200,000-600,000 particles Normal adult dose is 3-5 mCi of activity and 1-2 ml of volume Syringe should be agitated before injection Should be injected while patient is supine during respiration (some radiologists will prefer upright injection) Care should be taken not to draw back blood into the syringe~this will cause small labeled blood clots~causing focal hot spots on the image.

Radiopharmaceuticals Perfusion: Contraindication to injecting 99mTc MAA Severe pulmonary hypertension Known Right-to Left shunt In both cases, number of particles should be reduced to 100,000-200,000 particles.

Radiopharmaceuticals Radioactive Inert Gas: 133 Xe Half life 5.3 days Gamma ray energy of 81 keV Usual dose of 10-20 mCi Usually done prior to 99mTc MAA perfusion Imaged in posterior view 1-initial breath 2-equilibrium 3-washout Requires patient cooperation Administered using delivery and rebreathing unit.

Radiopharmaceuticals Radiolabeled Aerosols Map the distribution of aerated lung volume 99mTc diethylene triamine pentaacetic acid (DTPA) 30-50 mCi of activity in 2-3 ml volume. Oxygen is supplied to the delivery system. Patient breaths in and out through a mouthpiece and the nose should be pinched off. Advantage is views can be taken in all 8 camera positions, to match perfusion.

Radiopharmaceuticals Technegas and Pertechnegas Delivered in a micro-aerosol generator Still in FDA trials in U.S., but used commonly in other countries. Advantage: pertechnegas can be delivered in only 1-2 breaths and multiple images can be obtained.

Indications Suspected pulmonary embolus Chest pain Shortness of Breath Hypoxia Coughing Chest radiograph should be done 12-24 hrs. prior to VQ scan for comparison. CTA is generally preferred, but a VQ scan will still be warranted if: Pt. has contrast allergy Renal failure Pregnant (this is debatable)

Normal Perfusion Lung Scan Uniform activity seen except a decreased area of cardiac silhouette and aortic knob.

Normal Perfusion

Normal Ventilation 133 Xe Normal half-time washout for Xenon is 30-45 seconds. May be deposited in the liver and result in increase activity in right upper quadrant. Normal 99mTc Aerosol images resemble perfusion images. Normal to see trachea and bronchi Swallowed activity can be seen in the esophagus and stomach.

Perfusion Defects Area of absent or diminished perfusion. Classified as segmental or nonsegmental. Segmental may involve all or part of a bronchopulmonary anatomic segment. These are classically wedge shaped. Nonsegmental do not correspond to anatomic segments and are generally not wedge shaped. These are NOT associated with pulmonary emboli. Can be caused by hilar or mediastinal structures, neoplasms, bullae, pneumonia, edema or other infiltrates.

High Probability Study Pulmonary Embolus

Do they Match? A mismatch refers to a defect seen on perfusion, but is normal on the ventilation. Segmental mismatch is a classic pulmonary emboli.

Analysis of Images Perfusion defect: is it segmental? Yes, then further evaluation is required. Compare to Ventilation scan: It is a mismatch? If yes, Compare to CXR : are there infiltrates, effusions, or masses?

PIOPED II Criteria Prospective Investigation Of Pulmonary Embolism Diagnosis High probability Greater than 80% likelihood of pulmonary emboli Intermediate probability 20-80% likelihood of pulmonary emboli Low probability Less than 20% likelihood of pulmonary emboli Very low probability Less than 10% likelihood of pulmonary emboli Indeterminate Should be used only when technical factors limit the study Normal No perfusion defects

Stripe sign ~ Very unlikely to be pulmonary emboli Fissure sign~ caused by pleural fluid in the fissures, pleural scarring or thickening, or COPD.

Other indications Follow-up on pulmonary embolus To determine if treatment is working Management of patients with COPD Assess lung function in patients with lung cancer Assess lung function pre-operative to lung resection