Nuclear Medicine 4203Scanning & Imaging

Respiratory System

Anatomy and Physiology

Trachea divides into the right and left mainstem bronchi

These divide to form lobar bronchiRight 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

GI Motility online (May 2006) | doi:10.1038/gimo73

Figure 3 Schematic diagram of lung anatomy with cross-sections of bronchi, bronchioles alveolar ducts, and 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 blockageFewer than 1 in 1000 capillaries are blockedInjection should include 200,000-600,000 particlesNormal adult dose is 3-5 mCi of activity and 1-2 ml of volumeSyringe should be agitated before injectionShould 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 daysGamma ray energy of 81 keVUsual dose of 10-20 mCiUsually done prior to 99mTc MAA perfusion

Imaged in posterior view1-initial breath2-equilibrium3-washout

Requires patient cooperationAdministered using delivery and rebreathing unit.

Radiopharmaceuticals

Radiolabeled AerosolsMap the distribution of aerated lung volume99mTc 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 PertechnegasDelivered 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 embolusChest painShortness of BreathHypoxiaCoughing

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 allergyRenal failurePregnant (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 XeNormal 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 StudyPulmonary 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 CriteriaProspective Investigation Of Pulmonary Embolism Diagnosis

High probabilityGreater than 80% likelihood of pulmonary emboli

Intermediate probability20-80% likelihood of pulmonary emboli

Low probabilityLess than 20% likelihood of pulmonary emboli

Very low probabilityLess than 10% likelihood of pulmonary emboli

IndeterminateShould be used only when technical factors limit the study

NormalNo 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 embolusTo 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