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VQ Scintigraphy
Nuclear Medicine Methods 1 (RADI1126)
Royal Melbourne Institute of Technology
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VQ Scans Lungs Allows visualisation of the physical of the lungs (ventilation) Allows visualisation of the blood flow to the lungs (perfusion) 2 part procedure Part 1: Ventilation Part 2: Perfusion Indications for a VQ scan include: Pulmonary Embolus (to diagnose or exclude) Quantitative VQ assessment for lung resection) Biology: The Alveoli: Techenegas gets stuck in the alveoli At any boundary, the liquid molecules are more strongly attracted to eachother than the gas molecules This unequal attraction produces a state of tension at the liquid surface called surface tension Liquid is drawn closer together Reduces contact with the dissimilar gas molecules Resists any force that increases the surface area of the liquid Water is highly polar and has high surface tension Water is a major component of the liquid film that coats the alveoli Type II alveolar cells produce a surfactant to decrease cohesiveness of the water Surface tension of the alveolar liquid is reduced Less energy is required to overcome those forces to expand the lungs and discourages alveolar collapse Ventilation: Inspiration Exhalation Biomechanics of Inhalation: Initiated diaphragm, supported external intercostal muscles External intercostal muscles lift the rib cage and pulls sternum superiorly Ribs and expand the intrathoracic volume This decreases intrathoracic pressure Air rushes into the lungs Deep (forced) inspirations involve accessory muscles to raise the ribs even further Normally a passive process Lungs are elastic and recoil from the stretch of inhalation Called quiet expiration Forced exhalation is driven abdominal muscles and internal intercostal muscles Increases abdominal and intrathoracic pressure Forces air from the lungs Blood Supply to the Lungs: The lungs are perfused two circulations: Pulmonary Pulmonary arteries supply systemic venous blood to the lungs to be oxygenated Freshly oxygenated blood returns to the heart via the pulmonary veins for systemic circulation Bronchial Provides the lungs with freshly oxygenated blood Supplies all lung tissue except alveoli Most venous blood returns to the heart via pulmonary veins Some venous blood is drained from lungs tiny bronchial veins Ventilation Studies: 99mTc Technegas (Australian) 99mTc DTPA radioaerosol (hard to get patient to breathe in, technologist gets a big dose) 133Xenon gas 99mTc Technegas Atoms of carbon surrounding 99mTc atoms Technegas enters alveoli, it remains trapped along alveoli walls It is NOT absorbed into the blood stream It is NOT exhaled Particles distribute evenly through the lung field Particles are not absorbed or exhaled Allows for PLANAR or SPECT imaging of lungs use SPECT where possible Imaging protocol: Ventilate patient to achieve a count rate of SPECT 20 32 projections per head over 180degrees 128 matrix Planar Projections Anterior, posterior Obliques (RAO, RPO, LAO, LPO) Laterals 150K counts per view 256 matrix Perfusion Scan: Follows ventilation image 99mTc MAA Albumin particles (It is a protein so it is very fragile) Maps pulmonary circulation Localises due to capillary blockage Particles need to be correct size to localise in capillaries Particles too large block arterioles wont get to capillaries Particles too small pass through capillaries 99mTc MAA Albumin Particulate Regulation of size and number is important Particle size (capillary size Particle number can inject maximum 1 million particles Too many particles can kill the patient blocking the capillaries gas exchange will stop causing a PE Restrictions on MAA is volume NOT activity Reconstitution of the MAA cold kit: Swab vial with alcohol swob of 99mTcO4 in 5mL (volume can change with number of particles per vial) Slowly add MAA running 99mTcO4 down the edge of the vial Some vials will have particles Invert GENTLY Do not shake vial Incubate minutes (depending on the manufacturer) Store in the fridge Patient Administration of 99mTc MAA: Check of posterior ventilation (Should be Gently invert MAA vial Withdraw of 99mTc MAA Watch volume, should not exceed max allowed volume (usually 1mL) Max number of particles of 1 million Count rate needs to be 4 times the ventilation count rate Inject patient while they are lying down so MAA is distributed to the whole lung (gravity will push more MAA to lower lungs if vertical) Instruct patient to take deep, tidal breaths same as when patient inhaled techenegas Perfusion: Perfusion administration should achieve a count rate around SPECT 32 projections per head over 180 degree arc 128 matrix Planar Projections Anterior, posterior Obliques (RAO, RPO, LAO, LPO) Laterals 600K counts per view 256 matrix Pulmonary Territories: If the blood flow is cut off to any of these areas there will be a wedge missing from the scan Increase in platelet count and adhesiveness Subsequent decrease in clotting time Increased blood viscosity Dehydration Polycythaemia Vera too many red blood cells Contraceptive Pill increases risk of clotting Pregnancy Active cancer Cancer cells killed chemo release Chemo can damage vessel walls, inducing thrombosis Signs and symptoms of DVT: Localised tenderness along the length of the deep venous system Calf swelling 3cm compared to asymptomatic side Entire leg swelling Oedematous pitting Collateral superficial veins Leg Increased A negative result EXCLUDES the presence of a blood clot A positive result DOES NOT confirm a DVT Formation of a DVT: Deoxygenated blood returns to the heart through the venous system Blood pressure in the leg veins is near zero Venous return relies upon the muscle contraction that occurs during movement When leg muscles contract they squeeze against the wall of the deep veins Blood is forced along the veins Valves present in the veins prevent the blood from flowing backwards Periods of inactivity and immobilisation causes sluggish venous flow or stasis and platelets can then settle, and with factors released the platelets, a fibrin mesh forms around the mass of cells The thrombus occludes the lumen of the vein Proximal to the original thrombus, the entire vein clots Formation of a PE: A fragment of the DVT can detach and become an embolus The embolus passes through the heart into the pulmonary artery The embolus can lodge into the artery, arterioles or capillaries This is then known as a pulmonary embolus Causes severe respiratory distress, pulmonary infarct and often death Saddle embolus is when blocking arteries to both lungs Positive VQ Evidence for Acute PE: Ventilation images will be normal Perfusion images will demonstrate segmental or subsegmental photopaenic defects Looking for cold spots due to branding nature of pulmonary vasculature Perfusion defects will not match any ventilation defects Termed Ventilation images always performed first Matching ventilation and perfusion defects will NOT be acute PE Normal VQ Scan PE Segmental perfusion defects What matching defects might mean...: Consolidation Filling of airspaces with transudate or exudate, cells or protein. Pneumonia is a common cause Atelectasis Collapse of alveoli, caused obstruction of airways, compression of lung from air pr fluid in pleural space, or pressure of tumour outside lung Functioning lung eventually hyperinflate O2 sats normal Increases HR, BP and respiratory rate Pulmonary Infarct Usually from previous, unresolved PE Chronic Obstructive Airways Disease (COAD) KNOW HOW TO EXPLAIN MISMATCH DEFECT AND MATCHING DEFECT Infarcted (dead) tissue will be a matching defect on ventilation and perfusion images PE will be mismatched, ventilation will be normal, perfusion will be defected Asthma Symptoms of COAD COPD: Chronic cough Difficult inhaling and exhaling Diminished capacity of lungs COAD is progressive and irreversible COAD vs PE: COAD makes interpretation of PE very difficult Defects exist on both ventilation and perfusion scans Difficulty assessing whether a defect is matching or mismatched In an ideal world, patients with severe COAD would have CTPA, but its not always possible Quantitative Lung Scans: Evaluate regional pulmonary perfusion prior to lung reduction surgery Quantify regional perfusion Determines the degree of lung resection a patient can tolerate Usually in patients with lung tumours Perform a standard VQ scan Perform regional analysis Create 3 regions of interest on each lung Upper, mid and lower regions Anterior and posterior projections only On both ventilation and perfusion images Assess regional contribution to ventilation and perfusion Perform geometric mean for each region Right to Left Cardiac Shunt: Also known as Cyanotic Heart Disease Congenital Abnormality May be caused to chemical exposure, infection, drug abuse or uncontrolled diabetes during pregnancy Other vascular abnormalities can have the same consequences Blood flows from right ventricle into the lungs and direct into the left ventricle Deoxygenated blood recirculates systemically Results in low blood oxygen levels causing cyanosis Severity depends on the size of the shunt Inject 200MBq IV of 99mTc MAA Perform whole body scan Systemic distribution of activity will confirm shunt 99mTc MAA will lodge in systemic capillaries, especially brain and kidneys Tutorial Activities: 1. Discuss why it is necessary to perform a ventilation study using Technegas before performing the perfusion study using MAA. Because the MAA is used in higher amounts to override the ventilation scans so you can clearly see the blood flow. 2. What routine clinical questions are important to ask a patient prior to a VQ scan? Any history of PE? Any COPD? (Asthma, emphysema (do they smoke?)) Contraceptives? Pregnant?
VQ Scintigraphy
Course: Nuclear Medicine Methods 1 (RADI1126)
University: Royal Melbourne Institute of Technology
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