Nuclear Medicine Respiratory System Lecture PDF

Nuclear Medicine Respiratory System Lung imaging Radionuclide lung imaging most commonly involves: The demonstration of pulmonary perfusion The assessment of ventilation using inspired inert gas, usually Xenon, or 99mTc-labelled aerosols. Ventilation (V) and perfusion (Q) scans are often r...

Nuclear Medicine Respiratory System Lung imaging Radionuclide lung imaging most commonly involves: The demonstration of pulmonary perfusion The assessment of ventilation using inspired inert gas, usually Xenon, or 99mTc-labelled aerosols. Ventilation (V) and perfusion (Q) scans are often referred to as V/Q scans. V/Q scans Perfusion scan is usually correlated with ventilation scan. If perfusion scan is normal, ventilation scan can be skipped. If there are segmental perfusion defects, then ventilation scan is needed. Ventilation Imaging Non-invasive method to evaluate regional lung ventilation (air flow) to see what type of perfusion defect is. https://www.britannica.com/topic/lung-ventilation-perfusion-scan ANATOMY AND PHYSIOLOGY Maximal pulmonary blood flow normally occurs in the lung zone at the junction of the lower third and upper two thirds of the lungs. In the upright position, the apex receives only about one third of the blood flow per unit volume as compared with the bases. Normally, ventilation in the lower portion of the lung is about 150% of that in the apex. In the supine position, perfusion is more uniform, Pulmonary Perfusion Imaging RADIOPHARMACEUTICALS Technetium-99m (99mTc) macroaggregated albumin (MAA) It localizes by the mechanism of capillary blockade. Perfusion Imaging Non invasive method for the evaluation of pulmonary arterial blood flow Sufficient number of particles to allow for good statistical distribution. MAA size is 10-100 um Dose is 3-5 mCi, IV. In children 0.03 to 0.07 mCi/kg Views: Ant, post, laterals, ant obliques and post obliques. MAA has a biologic half-life in the lung of 2 to 4 hours, Technique Tc-99m MAA should be injected during respiration The patient supine to minimize the normal perfusion gradient between the apex and lung base. The syringe should be gently agitated before injection (MAA particles tend to settle out in solution). Injection should be made slowly, to assist in homogeneous pulmonary distribution of the particles The injected volume should be at least 1 to 2 mL. Contraindication A relative contraindication is severe pulmonary hypertension because the blockade of additional lung capillaries may acutely increase the condition and its cardiac complications. Ventilation Imaging Using: Radiolabeled Aerosols OR Radioactive Gases Ventilation Imaging Radiolabeled Aerosols Technetium-labeled radioactive aerosols Unlike ventilation studies using radioactive gases, aerosol studies do not allow for dynamic single-breath or washout phase imaging but rather map the distribution of aerated lung volume. Once deposited in the lungs, the aerosol particles remain in place for sufficient time to permit imaging in multiple projections. Radiolabeled Aerosols –Technique The patient inhales a mixture of oxygen and nitrogen containing small amounts of radioactive xenon or technetium The patient is usually in a supine position 30 mCi of 99mTc–DTPA in a volume of 2 mL Flow rates are in the range of 7 to 10 L/min. A mouthpiece with a nose clip is used to administer the aerosol. The half-time clearance time from the lungs is 45 to 60 minutes in nonsmokers and 20 minutes in smokers. Radiolabeled Aerosols – Advantages Availability of 99mTc and its ideal imaging energy. Little patient cooperation is required. The aerosol can be delivered in a room separate from the camera room and can easily be delivered Radiolabeled Aerosols – Disadvantages The small amount of activity actually delivered to the patient (2% to 10%) compared with that available in the aerosol generator. Because both MAA and DTPA aerosols are labeled with 99mTc, sequential imaging of ventilation and perfusion requires the relative doses of each to be adjusted to prevent interference of one 99mTc- labeled agent with the other when imaging is performed. Ventilation Imaging Radioactive Gases Permits sequential imaging of lung ventilation and perfusion in conjunction with 99mTc-MAA because of the rapid clearance of the gases from the lungs and the relative energy differences of the photon emissions. Radioactive Gases – Xenon-133 (133Xe) Inexpensive Has a half-life of 5.3 days and a principal gamma ray energy of 81 keV. The low energy of these photons causes about half of them to be attenuated by 10 cm of inflated lung tissue. Thus overlying soft tissues, such as breasts, can produce substantial artifacts; these are usually avoided by performing xenon ventilation scans in the posterior position. The critical organ for 133Xe is the trachea. Xenon-133 allows for the assessment of all phases of regional ventilation: initial single breath, wash-in, equilibrium, and washout. Radioactive Gases – Xenon-133 (133Xe) Single-breath images represent instant ventilation, wash-in and equilibrium images are proportional to aerated lung volume, and washout phases show regional clearance of activity from the lungs and delineate areas of air trapping. This complete characterization of ventilation renders 133Xe imaging the most sensitive ventilation study for detection and assessment of airways disease.  Ventilation imaging using 133Xe is limited in that images are usually obtained in only one projection and are performed before the perfusion study. The use of a single projection image ensures that some regional ventilation abnormalities will be missed because the lungs are not entirely imaged. requires a considerable amount of patient cooperation because the patient must be able to tolerate breathing on a closed spirometer system for several minutes to reach equilibrium. Technique Although there are several common methods of performing ventilation imaging, the most complete involves three phases: (1) single wash-in or initial breath, (2) equilibrium, and (3) washout. Technique The single-breath phase involves having the patient exhale as deeply as possible and then inhale 10 to 20 mCi (370 to 740 MBq) of 133Xe, holding his or her breath for about 15 seconds while a static image is taken. The equilibrium phase constitutes the rebreathing of the expired xenon diluted by about 2 L of oxygen contained in a closed system. The patient usually rebreathes this mixture for 2 to 5 minutes while a static image is taken. After equilibrium is reached, fresh air is then breathed during the washout phase while serial 15-second images are obtained for 2 to 3 minutes as the xenon clears from the lungs. In patients with chronic obstructive pulmonary disease (COPD), the washout phase may be prolonged to 3 to 5 minutes, if necessary, to assess areas of regional airway trapping. Ventilation Imaging Krypton-81m has a half-life of 13 seconds photon emissions between 176 and 192 keV. Because of its higher-energy photon emissions compared with 99mTc- MAA, 81mKr ventilation studies can be performed either before or after perfusion imaging. The short half-life of 81mKr, precludes single-breath and washout images. 81mKr is expensive, limited in availability, and thus rarely used in clinical practice. NORMAL LUNG SCAN Perfusion Scan In the posterior projection, there is some reduction of activity toward the bases as a result of the thinning of the lungs. In the anterior view, the cardiac silhouette and the aortic knob are commonly identified. Oblique projections are often helpful but may be confusing. In general, defects suspected on the oblique projections should be confirmed on one of the four standard views. Pleural disease Small pleural effusions may best be seen on the lateral or oblique views as posterior sulcus blunting or as a fissure sign, a linear defect caused by fluid in an interlobar fissure https://thoracickey.com/pleural-disease/ Normal Ventilation Scan Homogeneous distribution of activity throughout both lungs; the initial breath image reflects regional ventilatory rate if there is maximum inspiratory effort. During the washout phase, activity clears from the lower portions of the lung at a faster rate than from the apices because the air exchange is greater. Normal Ventilation Scan Usually, the lungs are almost completely clear of xenon within 2 or 3 minutes of beginning the washout phase because the normal half- time for xenon washout is about 30 to 45 seconds. Washout is the most sensitive phase for the detection of trapping caused by airway obstruction so, if xenon gas does not enter an area during equilibrium, washout cannot be evaluated. THEREFOR The sensitivity of the washout phase depends on performing sufficient rebreathing to obtain adequate equilibrium in as much of the lung volume as possible as well as on the length of the washout phase. Normal Ventilation Scan Xenon is soluble in fat and partially soluble in blood, it may be deposited in the liver. This becomes apparent near the end of the xenon washout study and should not be mistaken for trapping of xenon in the right lower lung. In children, activity may be seen in the left upper quadrant of the abdomen because of swallowing of the xenon gas during the study. Normal Ventilation Scan Normal aerosol scans resemble perfusion scans, except that the trachea and the bronchi are visualized. In addition, swallowed activity can sometimes be seen in the esophagus and stomach. CLINICAL APPLICATIONS The most important and frequent indication for a ventilation/perfusion lung scan is suspected pulmonary emboli. CT pulmonary angiography (CTPA) may be the initial test of choice Radionuclide ventilation/perfusion or perfusion only imaging, is an effective lower radiation dose procedure. V/Q scans are often preferred over CTA for patients who have contrast allergies, are in renal failure, or who are too large for the CT-scanner table or gantry, as well as young patients (especially women) and those with clear lungs on chest radiographs. Chest radiographs are important to accurately interpret the radionuclide images Pulmonary emboli- Diagnostic Principle The diagnosis of pulmonary thromboembolism is based on the disassociation between ventilation and perfusion as a result of the obstruction of pulmonary segmental arterial blood flow by the embolus. Pulmonary emboli- Diagnostic Principle With 99mTc-MAA imaging, the MAA particles are unable to enter the capillary bed distal to the arterial occlusion(perfusion defect in the portion of lung supplied by the involved artery). Ventilation is generally unaffected. The most typical manifestation of the pulmonary emboli is as a wedge- shaped perfusion defect with preserved ventilation: the segmental ventilation/perfusion mismatch PER VEN Nuclear Medicine Thyroid Gland Imaging Thyroid Gland Imaging The function of the thyroid gland includes the concentration of iodine, synthesis of thyroid hormones, storage of these hormones as part of the thyroglobulin (Tg) molecule, and their secretion into the circulation as required. Over 99% of circulating thyroid hormones are bound to plasma proteins, primarily thyroxine-binding globulin (TBG).  Thyroid stimulating hormone (TSH) from the pituitary plays the major role in regulating thyroid function Circulating TSH is highly sensitive and represents the most sensitive biochemical indicator of both hypothyroidism and hyperthyroidism Thyroid Scintigraphy Technetium-99m pertechnetate is the most readily available radionuclide employed for thyroid imaging. Pertechnetate ions (TcO−4 ) are trapped by the thyroid in the same manner as iodine but pertechnetate ions are not organified. 123 Iodine is both trapped and organified by the thyroid gland, allowing overall assessment of thyroid function.  Since 123I is cyclotron-produced and has a relatively short half-life of 13.6 hours, it is more expensive and in advance notice is necessary for imaging. Because of its inferior image quality and the high thyroid and total body radiation dose from its β- emission, 131I is not used for routine thyroid imaging other than for metastatic thyroid cancer assessment. Normal 99mTc thyroid scan. Symmetric, homogeneous uptake Hyper functioning nodule in the lower pole of the right lobe Nonfunctioning thyroid adenoma. A technetium-99m pertechnetate scan shows a “cold” area in the superior lateral aspect of the right lobe (arrows). A thyroid carcinoma may present an identical appearance. Appearance of the thyroid on technetium- 99m pertechnetate scans. A, Normal. The thyroid is clearly visible, and the salivary glands are also seen but are somewhat less intense in activity. B, Graves disease. The thyroid is enlarged and has accumulated much of the activity, so that the salivary glands are harder to see. C, Hyperfunctioning “hot” nodule. The nodule is seen as an area of intense activity, and its autonomous hormone production has suppressed the remainder of the thyroid gland, so that the normal thyroid is hard to see. D, Subacute thyroiditis. In this case, the inflammation has caused the thyroid to have difficulty trapping, and the amount of activity in the thyroid is lower than normally expected, whereas the salivary gland activity is normal. References Fred A. Mettler, J., MD, MPH and Milton J. Guiberteau, MD. Essentials of nuclear medicine. Sixth edition