Lecture #8: THE PULMONARY SYSTEM

  

Respiration is the transport of oxygen and carbon dioxide to and from the cells of the body. Four processes are involved in respiration:

1.      Ventilation or the flow of air into and out of the lungs. This can be demonstrated via radionuclide Ventilation Lung Scan using either ¹²²Xe, ¹²⁷Xe, ^81mKr or radiolabeled aerosol particles such ^99mTc–DTPA.

2.      Gas exchange or the diffusion of oxygen and carbon dioxide between alveoli and the blood. This can be demonstrated via radionuclide Perfusion Lung Scan using ^99mTc–MAA.

3.      Gas transport or the measurement of oxygen and carbon dioxide to and from the cells through the circulation

4.      Regulation of ventilation

In healthy lungs, ventilation and perfusion (blood flow) are matched for efficient and complete exchange of oxygen and carbon dioxide. Pulmonary disease upsets this balance.

LUNG VENTILATION STUDY

Ventilation imaging demonstrates the flow of air into and out of the lungs. Whereas perfusion imaging is very sensitive for detecting changes in blood flow, it is unable to specify the cause of the change. Ventilation imaging is performed in conjunction with perfusion imaging can increase the specificity of the perfusion study by differentiating perfusion defects that are due to pulmonary embolism from those that are due to obstructive lung disease.

Ventilation imaging may be performed with radioactive inert gases such as ¹³³Xe, ¹²⁷Xe, ^81mKr or radiolabeled aerosol particles such as ^99mTc–DTPA.

1.      Technetium–99m–diethylenetriamine–penta–acetic acid (DTPA) aerosol

a.      Dose                                 :           25 – 35 mCi (via nebulizer)

b.      Oxygen flow rate           :           9 – 15 liters

c.       Aerosol imaging is usually performed before perfusion imaging because it is more difficult to deliver a larger dose of Tc–99m aerosol than it is to deliver a larger dose of Tc–99m–MAA. Because both agents are labeled with Tc99m, it is extremely important that the count rate of the second study is at least four times the count rate of the first study.

d.     Organ receiving the largest dose:

(1)   Bladder wall             :           0.60 rads / 6 mCi

(2)   Lungs                         :           0.30 rads / 6 mCi

(3)   Kidneys                     :           0.12 rads / 6 mCi

(4)   Total Body                :           0.12 rads / 6 mCi

e.      Energy Window             :           20% centered at 140 keV

f.        An advantage of aerosol imaging is that images can be obtained in multiple projections to match those obtained from perfusion imaging.

g.      A disadvantage of aerosol imaging is that aerosol deposition is altered by turbulent flow and central deposition can result in suboptimal study.

2.      Xenon–133 Ventilation

a.      Dose                                 :           5 – 20 mCi (via gas dispenser with return

trap and 3 way valve)

b.      Acquisition protocol

(1)   Be sure a new external filter is in the gas delivery system

(2)   Acquire images in the posterior projection

(3)   Acquire a single breath (ventilation) analog image:

(a)   Instruct the patient to take a deep breath as the Xe–133 gas bolus is injected into the delivery system and then hold the breath as long as possible.

(b)   Acquire a 100k count image

(c)    If the patient starts to breath before 100k counts are acquired, immediately terminate the acquisition

(4)   Equilibrate the concentration of Xe–133 gas within the patient lungs

(a)   Have the patient breathe normally for 3 minutes

(5)   Acquire an equilibrium (airspace) analog image:

(a)   Acquire an approximately 300k count image

(6)   Acquire a series of washout (airway obstruction) analog images:

(a)   Change the system valve so that the patient breathes room air in and exhales Xe–133 into the system trap.

(b)   Beginning immediately, acquire a sequential 30 seconds analog images until the Xe–133 gas is gone as judged from the persistence scope. Acquire a minimum of 4 images.

(7)   Retention of the Xenon after 2–3 minutes of the washout phase indicates obstructive lung disease

c.       Energy window              :           20% window centered at 80 keV

d.     Half – life                         :           5 – 2 days (beta decay)

e.      An advantage of Xe–133 ventilation is that single breath, equilibrium and washout images can be obtained which provide a more complete characterization of ventilation and a more sensitive test for obstructive airway disease. Physiologic information about ventilation can best be obtained from Xe–133 imaging.

f.        The imaging room should be at negative pressure with appropriate exhaust for radioactive gas.

3.      Krypton–81m Aerosol

a.      Dose                                 :           1 – 10 mCi

b.      Krypton is breathed continuously by the patient as it elutes with oxygen from its parent nuclide Rb–81. Multiple projections are possible, but single breath and washout images are not because of the tracer’s short half–life (13 seconds by isomeric transition decay).

c.       The advantage of Kr–18 is that images can be obtained in all views without interference from prior perfusion imaging.

d.     Collimator                       :           Medium energy

e.      Energy window              :           190 keV photopeak

PERFUSION LUNG SCAN

Perfusion imaging demonstrates blood flow to the lungs. Technetium–99m–MAA particles are injected intravenously and are trapped in the vasculature of the lungs. The majority of these particles is 10–90 microns in diameter and thus will block arterioles of this size within the lungs. Areas of the lung where perfusion is decreased or absent will receive little or no tracer. The number of arterioles that are blocked with tracer depends on the number of particles injected. An adult patient should receive 100k – 500k tracer particles, which will occlude less than 1% of the lung arterioles while providing a uniform tracer distribution.

1.      Dose:

a.      6 mCi – if performed in conjunction with a gas ventilation study

b.      2 mCi – if performed before Tc99m DTPA aerosol study

2.      Before intravenous administration of the pulmonary perfusion radiopharmaceutical, the patient should be instructed to cough and to take several deep breaths. The patient should be in the supine position during injection or for case of patient with orthopnea, as close to supine as possible.

3.      Imaging can begin immediately following tracer administration

4.      Acquire static images for the following anatomic views: anterior, posterior, laterals and posterior and anterior obliques.

5.      Because the lung apices are thinner than the lower portions of the lungs, tracer distribution may appear to be somewhat decreased at the apices. The cardiac impression may be seen on the anterior and left lateral views.

6.      Decreased areas of tracer uptake indicate a disruption of blood flow, which can be due to a variety of conditions.

TECHNICAL CONSIDERATIONS

1.      Chest radiograph findings are needed for interpretation of the lung images. Since the symptoms of pulmonary embolism are common to many other conditions, it is necessary to rule out other pathology which may mimic pulmonary embolism.

A routine chest radiograph obtained in both the posterior–anterior and lateral projections is preferred. A portable anterior–posterior chest radiograph acceptable only if patient cannot tolerate chest radiograph examination.

2.      The patient should be placed in the supine position for injection of the radiopharmaceutical. When the patient is supine, the effect of gravity, which hinders blood flow to the lung apices minimized.

3.      Extravasation of the tracer at the injection site, which may cause an insufficient number of particles to be delivered to the lungs, resulting in non – uniformities not related to the pathology of the lungs.

4.      Small blood clots in the syringe, formed when blood is withdrawn into the syringe and allowed to stand for a period of time. When these clots are injected into the patient, they may result in hot spots on the images, again, not related to pathology.

5.      Failure to remove attenuating objects such as jewelry and ECG leads results in cold spot artifacts on images.

6.      Tracer administration with the patient in an upright position, which causes decreased tracer concentration in the lung apices.

7.      Treatment with anticoagulant or thrombolytic therapy should be noted.

8.      Pertinent chest radiograph findings include but are not limited to: (a) consolidation (b) atelectasis (c) effusion (d) masses (e) cardiomegaly (f) decreased pulmonary vasculature.

9.      Result of test for deep venous thrombosis, e.g. compression ultrasonography should be noted.

10.  In patients with acute obstructive lung disease, the use of bronchodilator therapy before lung scintigraphy may decrease ventilating defects and improve the accuracy of the study. Because perfusion defects often change as acute obstruction resolves, patients are best imaged when bronchospasm has resolved.

11.  In patients with congestive heart failure, improve specificity will be obtained if imaging can be delayed until therapy for heart failure has been instituted.

12.  A decubitus or oblique patient position can markedly affect the distribution of ventilation and perfusion as it an result in mismatched pattern.

Sequence of imaging

1.      Ventilation scintigraphy using Xe–133 is usually performed before perfusion scintigraphy using Tc99m. Alternately, a perfusion scintigraphy can be performed first and ventilation scintigraphy omitted if not needed.

2.      The disadvantage of performing perfusion imaging first

a.      With Xe–133 gas or Tc99m aerosol imaging, the perfusion image contributes background activity to the ventilation image.

b.      A decision to perform or not to perform the ventilation study must be made in a timely manner.

3.      The advantage of performing perfusion imaging first

a.      If the perfusion study is normal or matches the chest radiographic findings, the ventilation study can be omitted.

b.      For a single–projection ventilation study, the projection that best shows the defect can be obtained.

4.      Because of the higher energy of the gamma emissions and the short half–life of Kr–81m, images obtained with this gas can be alternated with those obtained with Tc99m MAA.

Interpretation and Reporting

1.      The preferred term in reporting is “Probability of pulmonary embolism is ……”

2.      Based on the modified Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) criteria, the following probability are derived:

a.      High Probability

(1)   ≥ 2 large mismatched segmental perfusion defect

b.     Intermediate Probability

(1)   One moderate to two large mismatched perfusion defects

(2)   Single–matched ventilation–perfusion defect with clear radiograph

(3)   Difficult to categorize as low or high or not described as low or high.

c.       Low Probability

(1)   Non–segmental perfusion defects (e.g. cardiomegaly, enlarged aorta, enlarged hila, elevated diaphragm)

(2)   Any perfusion defect with a substantially larger chest radiograph abnormality

(3)   Perfusion defects matched by ventilation abnormality provided that there are (a) clear chest radiograph (b) some areas of normal perfusion in the lungs.

(4)   Any number of small perfusion defects with normal chest radiograph.

d.     Normal

(1)   No perfusion defects or perfusion exactly outlines the shape of the lungs seen on chest radiographs.