V/Q lung scintigraphy and A&P

What are the lungs

A pair of spongy, air-filled respiratory organs located either side of the thorax, protected by the rib cage, that exchange gases between inhaled air and the blood.

How many lobes does the right lung have

Three lobes (superior, middle, inferior), separated by horizontal and oblique fissures.

How many lobes does the left lung have

Two lobes (superior and inferior), separated by only an oblique fissure; the left lung is slightly smaller than the right to accommodate the heart.

Why is the left lung smaller and notched

It curves inward at the cardiac notch to accommodate the heart, which sits more to the left side of the body.

Why is the right lung shorter

The diaphragm sits higher below the right lung to accommodate the liver.

Cardiac impression/notch

The indentation on the anterior surface of the (predominantly left) lung caused by the heart; seen as a triangular photopaenic cut-out on anterior static VQ images and also visible on coronal/axial/sagittal SPECT reconstructions.

What is the hilum (root of the lung)

The region where the vessels (pulmonary artery, pulmonary veins), bronchus and nerves enter and exit the lung.

Visceral pleura

The thin membrane on the surface (covering) of the lung itself.

Parietal pleura

The membrane lining the inside of the thoracic cavity.

Pleural cavity

The space between the visceral and parietal pleura, containing fluid that reduces friction and helps the lungs stick to the chest wall during inflation.

Primary function of the lungs

To exchange gases between inhaled air and the blood — absorbing oxygen into the bloodstream and expelling carbon dioxide from it.

Upper respiratory tract components

Nose and nasal cavity, nasopharynx, oropharynx, and larynx.

Role of the nose and nasal cavity

The primary entry point for air; where air is cleaned, warmed/cooled and moistened before travelling further.

Nasopharynx

Lies posterior to the nasal cavity and leads to the oropharynx; connects the nasal passage to the rest of the respiratory system.

Oropharynx

Serves as a shared passageway connecting the nose, mouth and larynx, and contains the muscles that allow swallowing.

Larynx (voice box)

Located between the pharynx and trachea; houses the vocal cords for voice production, and protects the lower respiratory tract from aspirating food into the trachea while breathing.

Sinuses

Air-filled spaces within the skull bones that lighten its weight, produce mucus, and enhance resonance of the voice; include the frontal, maxillary, ethmoid and sphenoid sinuses.

Lower respiratory tract components

Trachea, bronchi, bronchioles, alveolar ducts/sacs, alveoli and the lungs.

Trachea

The windpipe; contains C-shaped (open posteriorly for flexibility) rings of hyaline cartilage and is lined by ciliated pseudostratified columnar epithelium with mucus-secreting goblet cells that clean the airway by moving debris toward the pharynx.

Carina

The point where the trachea bifurcates into the right and left primary bronchi.

Bronchial branching order

Primary bronchi divide into secondary (lobar) bronchi, which divide into tertiary (segmental) bronchi, which divide into bronchioles and then terminal/respiratory bronchioles, ending in alveolar ducts and alveolar sacs.

Alveoli

Microscopic, balloon-like air sacs at the ends of the respiratory tree (numbering in the hundreds of millions) that are the primary sites of gas exchange between air and the bloodstream.

Alveolar wall structure

Very thin, about one cell thick, composed of type I alveolar epithelial cells (for gas exchange) and type II alveolar cells (which secrete surfactant).

How gas exchange occurs

Oxygen diffuses across the thin alveolar and capillary walls into the blood and binds to haemoglobin in red blood cells, while carbon dioxide (a metabolic waste product) diffuses from the blood into the alveoli to be exhaled; driven by diffusion gradients across the large, moist, thin alveolar surface.

Role of alveolar macrophages

Immune cells within the alveoli that engulf and digest debris, bacteria and other foreign particles, keeping the lungs clean and infection-free.

Ventilation (breathing)

The process of moving air in and out of the lungs to facilitate gas exchange, involving coordinated action of the respiratory and muscular systems.

Role of the diaphragm

A dome-shaped muscle at the base of the lungs that is key to breathing; also assists with vomiting, defecation and childbirth by increasing abdominal pressure.

What happens during inhalation (inspiration)

The diaphragm contracts and moves downward, and external intercostal muscles contract raising the ribs, increasing thoracic cavity volume; this decreases intrapulmonary pressure relative to atmospheric pressure, so air flows into the lungs.

What happens during exhalation (expiration)

The diaphragm relaxes and moves upward, and external intercostal muscles relax, decreasing thoracic cavity volume; this increases intrapulmonary pressure above atmospheric pressure, so air flows out.

Passive vs active exhalation

Passive exhalation relies on the natural elastic recoil of the lungs; active exhalation involves contraction of the internal intercostal and abdominal muscles during forceful breathing.

Pathway of pulmonary circulation

Right ventricle → pulmonary artery (deoxygenated blood) → pulmonary capillaries around the alveoli (gas exchange) → pulmonary veins (oxygenated blood) → left atrium.

Systemic circulation pathway (11 steps)

Right atrium → right ventricle → pulmonary artery → lungs (capillaries) → pulmonary vein → left atrium → left ventricle → aorta → upper limb/head and lower limb/abdominal capillaries → superior and inferior vena cava → back to right atrium.

Key rule for heart sides

The right side of the heart receives blood, the left side of the heart leaves (ejects) blood, and arteries always move blood away from the heart.

Branches of the ascending aorta

The left subclavian artery, the left common carotid artery, and the brachiocephalic artery (which bifurcates into the right common carotid and right subclavian arteries) — these supply the upper limbs and head.

Branches of the descending aorta

Supplies the heart, liver, diaphragm, kidneys, pancreas, abdominal vessels and the lower limbs.

Why is understanding circulation direction clinically important in nuclear medicine

It allows the technologist to predict where an IV-injected radiopharmaceutical (e.g. 99mTc-MAA) will travel and lodge, and to understand what happens if an organ encounters aggregated particles such as a blood clot.

What is a V/Q (ventilation/perfusion) lung scan

A nuclear medicine imaging test assessing how well air (ventilation) and blood (perfusion) are distributed throughout the lungs, used mainly to diagnose or exclude pulmonary embolism.

Two parts of a VQ scan

Part 1 — Ventilation scan (assesses airflow using an inhaled radioactive gas/aerosol); Part 2 — Perfusion scan (assesses blood flow using an injected radioactive particulate).

Main indications for VQ scanning

Diagnosing or excluding pulmonary embolism (PE); quantitative VQ for pre-operative assessment before lung resection; evaluating COPD/emphysema, lung infections, and right-to-left cardiac shunting in paediatric patients.

Why order a VQ scan instead of CTPA

Particularly useful when CT pulmonary angiogram or other contrast-based imaging is contraindicated or inconclusive (e.g. contrast allergy, renal impairment, pregnancy).

Normal V/Q study appearance

Both ventilation and perfusion scans show even, uniform distribution of air and blood flow throughout all lung areas, with no mismatched regions.

What does a VQ mismatch suggest

An area that is ventilated (receives air) but not perfused (lacks blood flow), or vice versa — mismatch is suggestive of a pathology such as pulmonary embolism.

Which ventilation agent is used in Australia

99mTc-Technegas is the only ventilation agent currently used in Australia.

What is Technegas

An ultrafine carbon nanoparticle "pseudo-gas" (effectively a micro-aerosol) consisting of carbon-60 (buckminsterfullerene/"bucky ball") atoms surrounding a 99mTc atom.

Why does Technegas work well for ventilation imaging

Particles (~0.1 mm diameter) are hydrophobic, allowing deep penetration into the airways and resistance to central airway deposition; they become trapped along alveolar walls, are not absorbed into the bloodstream and are not exhaled, remaining fixed long enough for imaging.

History of Technegas

Developed in Canberra, Australia in 1984, marketed by Cyclomedica Pty Ltd; in clinical/SPECT use since 1986; also available in Europe; approved as a medical device in the US by the FDA in 2023.

Alternative ventilation agents (historical/overseas)

99mTc-DTPA aerosol (cheap, readily available, especially rural areas); Xenon-133 gas (long half-life of 5.3 days, complex delivery system); Krypton-81m gas (requires a cyclotron-produced 81Rb/81mKr generator, T½ = 13 seconds, 190 keV photon requiring medium-energy collimation, suitable for children but hard to access — used in UK/Europe).

Technegas machine administration steps

1) Prepare carbon crucible with ethanol

2) Add highest concentration of 99mTcO4 (0.1–0.2 mL, ~800 MBq) — about 5–10% (40–80 MBq) is delivered to the patient

3) Simmer crucible 6 minutes at 70°C (can interrupt after 3 min to "double burn" if pertechnetate concentration is low)

4) Prepare and position the patient, rehearsing breathing technique with the patient administration set (PAS)

5) Burn the crucible at 2,500°C (10-minute window to use Technegas after burning)

6) Ventilate the patient.

Why must the crucible not be overfilled

A positive meniscus risks blowing pertechnetate off the crucible into the chamber; burning at 2,500°C with excess material produces "pertechnegas," which causes undesirable uptake in the thyroid gland and stomach — a slightly negative meniscus is the target.

Technegas administration precautions

Use a nasal clip (or patient holds nose with thumb/forefinger) for a tight seal to prevent airborne contamination; patient positioned supine (for even distribution, matching the planned perfusion scan posture); rehearse breathing beforehand to prevent over- or under-ventilation; patient should not be prepared in the camera room to avoid contaminating the camera/room.

Technegas administration technique

Patient takes 2–3 deep breaths of Technegas; count rate is checked (target around 1,000–2,000 counts/second on the P-scope); ventilation images are then acquired (planar or SPECT).

Planar ventilation image acquisition views

Anterior and posterior, obliques (RAO & LPO, LAO & RPO), and laterals (left and right) — 8 projections total.

Planar ventilation imaging parameters

150,000 counts per view OR 180 seconds (whichever comes first), 256 x 256 matrix.

SPECT/CT ventilation acquisition parameters

20 seconds per projection, 32 projections per head over 180° (360° total with dual-head camera), 128 x 128 matrix.

Why is CT not used for attenuation correction in lung VQ imaging

Because the lungs are a mostly air-filled organ, applying attenuation correction can introduce undesirable imaging artefacts; CT is acquired only for anatomical correlation (and to help divide the lungs along lobes/fissures for quantitation).

What radiopharmaceutical is used for perfusion imaging

99mTc-labelled macroaggregated albumin (99mTc-MAA), injected intravenously.

Mechanism of 99mTc-MAA uptake

MAA particles are larger than pulmonary capillaries and become temporarily (and safely) lodged in a tiny fraction of pulmonary pre-capillary arterioles and capillaries, creating a map of pulmonary blood flow/perfusion.

Particle size comparison for MAA

MAA particles range 10–70 microns, compared with pulmonary capillary size of 20–25 microns.

Recommended particle numbers for MAA administration

200,000–700,000 particles per administration recommended; maximum should not exceed 1,000,000 particles; approximately 0.1–0.3% capillary obstruction occurs.

Typical administered activity of 99mTc-MAA

80–220 MBq.

Why does particle number per MBq change through the day

As 99mTc decays, a larger volume (and thus more particles) of the same vial must be drawn up later in the day to achieve the same MBq dose, so particle numbers injected per patient increase as the day goes on — technologists must track this to avoid exceeding the maximum particle count.

Administration requirements for 99mTc-MAA

Aim to inject enough activity to make the perfusion count rate 4–5 times the ventilation count rate (ventilation P-scope count rate is usually around 1,000 counts/second); maximum injected volume should usually not exceed 1 mL; patient should be lying down (supine) and instructed to take deep, tidal breaths matching their ventilation posture, for even distribution.

MAA reconstitution steps

Remove protective disc and swab septum, place vial in a lead shield; add 3.7 GBq in 2–8 mL sterile pyrogen-free sodium pertechnetate (run fluid down the vial side to avoid bubbles); mix by gentle inversion; assay in a dose calibrator and label with radiation warning; reconstitution time is 15 minutes.

MAA quality control

QC performed via TLC using Whatman No. 1 paper in 70% methanol or acetone; radiochemical purity should be ≥90%; finished preparation refrigerated at 2–8°C when not in use and discarded after 6 hours.

Planar perfusion image acquisition views

Same 8 projections as ventilation — anterior and posterior, obliques (RAO & LPO, LAO & RPO), laterals (left and right); always performed after the ventilation scan.

Planar perfusion imaging parameters

500,000 counts per view OR 180 seconds (counts much higher than ventilation's 150,000, since much higher count rates are achieved); 256 x 256 matrix.

SPECT/CT perfusion acquisition parameters

10–15 seconds per projection (slightly faster than ventilation due to higher count rate), 32 projections per head over 180° (360° total), 128 x 128 matrix; CT adds about 2–3 minutes and is for anatomical correlation only, with no breath-hold sequences (patient breathes normally).

Why is ventilation always performed before perfusion

So that matching ventilation and perfusion defects can be properly compared/interpreted — and because a matched defect pattern (both V and Q affected equally) is not characteristic of acute PE, whereas a mismatch (normal ventilation, defective perfusion) is.

How many bronchopulmonary segments are used in VQ static/SPECT reporting

18 segments total (10 in the right lung, 8 in the left lung) are used for lung segmental mapping.

SPECT reconstruction planes used for VQ imaging

Axial/transverse, coronal, and sagittal planes, in addition to the three-dimensional reconstruction.

Where is the cardiac shadow visible

Well described (as a triangular cut-out) on the anterior static projection; also seen in 3D SPECT reconstructions and across axial, coronal and sagittal planes.

Purpose of quantitative lung scanning

To determine regional lung function (e.g. percentage perfusion/ventilation contribution of each lobe), important for pre-operative assessment before lung resection surgery.

How has quantitative lung scanning evolved

Moved from a simple 6-region planar analysis to far more complex and accurate volumetric (3D SPECT/CT) analysis, aided by software that can slice the lungs along lobes and fissures using CT correlation.

Examples of regional analysis software

Vendor-neutral software (e.g. MIM Software, using DICOM data from any camera manufacturer) as well as vendor-specific solutions, used for PET and SPECT molecular imaging quantitation including lung lobe percentage breakdowns.

What is pulmonary embolism

A serious medical condition where one or more arteries in the lungs become suddenly blocked, typically by a blood clot, impeding blood flow and causing potential tissue damage.

What is the most common source of clots causing PE

Deep vein thrombosis (DVT), usually originating in the deep veins of the legs or pelvis.

Pathophysiology sequence of PE

Clot formation (typically in deep leg/pelvic veins) → embolisation (clot travels through the bloodstream to the lungs) → lung impact (blocks pulmonary arteries, impeding blood flow and causing tissue damage) → physiological response (inflammatory response, increased pulmonary vascular resistance, decreased oxygenation) → complications (can lead to right heart failure, hypoxaemia, and in severe cases sudden death).

Common symptoms of PE

Sudden shortness of breath, chest pain worsened by deep breathing or coughing, rapid heartbeat, lightheadedness, or collapse.

How is PE typically diagnosed

Combination of medical history, physical examination, and diagnostic tests such as a D-dimer blood test, CT pulmonary angiogram (CTPA), or ventilation/perfusion (VQ) lung scan.

PE epidemiology in Australia

Affects approximately 8,000 Australians annually and contributes to around 2,000 deaths per year; about one-third of cases are linked to recent surgery or extended hospital stays.

PE mortality statistics

10% die from PE within an hour of first symptoms; 22% die later following misdiagnosis and lack of appropriate therapy; 2% die even when diagnosis was correct; 10% chance of recurrence within 12 months; 30% chance of another PE within 3–5 years.

What is a DVT

A blood clot (thrombosis) that has formed in a deep vein, usually the femoral, popliteal or calf vein.

Virchow's triad (causes of thrombosis)

Endothelial injury (damage to the vessel wall's inner lining), hypercoagulability (increased tendency of blood to clot), and stasis of blood flow (slowed/stopped blood flow) — thrombosis typically results from one or more of these factors.

What is haemostasis

The physiological process that stops bleeding at the site of injury while maintaining normal blood flow elsewhere; it must be finely balanced since inadequate haemostasis causes excessive bleeding, while excessive haemostasis causes thrombosis.

Five steps of haemostasis

1) Vasoconstriction (narrows vessels to limit blood loss)

2) Platelet plug formation (platelets adhere, change shape and stick together)

3) Coagulation cascade (enzymatic reactions convert fibrinogen to fibrin, stabilising the plug)

4) Clot retraction and repair (clot contracts, tissue repair begins)

5) Fibrinolysis (plasminogen converts to plasmin, which digests fibrin and dissolves the clot once healing is complete).

DVT risk factors (causes and increased risk)

Prolonged immobility (bed rest >3 days, long-haul flights, paralysis); major surgery and trauma (especially orthopaedic); medical conditions (cancer, heart failure, inflammatory bowel disease, genetic clotting disorders); hormonal factors (HRT, oral contraceptives, pregnancy); age (risk increases particularly after 60, related to paresis/muscle weakness); obesity; smoking; previous DVT or family history; certain medical treatments (e.g. chemotherapy, erythropoiesis-stimulating agents); venous insufficiency; central venous catheters and pacemakers.

DVT hypercoagulability causes

Recent bleeds after trauma/surgery (increased platelet count/adhesiveness, decreased clotting time)

Increased blood viscosity (dehydration, polycythaemia vera)

Contraceptive pill or HRT

Pregnancy

Active cancer (cancer cells killed by chemo release pro-coagulants; chemo can damage vessel walls inducing thrombosis).

Typical signs and symptoms of DVT

Localised tenderness along the deep venous system; calf swelling >3cm compared to the asymptomatic side; entire leg swelling; oedematous pitting; collateral superficial veins; leg "reddening"/discolouration and warmth; symptoms can vary in severity and some patients have few or none, making clinical evaluation important.

What is D-dimer

A fibrin degradation product; elevated levels suggest an abnormal blood clotting process and are used as a quick, simple blood test to screen for thrombosis.

Interpreting a D-dimer test

A negative result excludes the presence of a blood clot (except in patients over 80 years); a positive result does not confirm a DVT/PE but indicates the need for further investigation (e.g. CTPA or VQ scan) to identify the underlying cause.

Other causes of raised D-dimer besides clotting

Malignancy/cancer, trauma, haemorrhage, severe infections/sepsis, autoimmune disorders, and COVID-19.

What does a positive (PE-indicative) VQ scan show

Ventilation images are normal; perfusion demonstrates one or more segmental or subsegmental photopaenic (wedge-shaped) defects that are not matched on the ventilation study — termed a "mismatch."

Why are PE perfusion defects wedge-shaped

Due to the branching nature of the pulmonary vasculature — a blocked segmental or subsegmental artery cuts off perfusion to the whole wedge-shaped region it supplies.

Why are ventilation images always performed first in VQ scanning

So that any perfusion defect found can be properly compared against a known-normal ventilation pattern to establish a true mismatch (clinically and procedurally the established order).

Do matching ventilation and perfusion defects indicate acute PE

No — matched V/Q defects (both ventilation and perfusion reduced in the same area) are not characteristic of acute PE and instead suggest other pathology such as consolidation, atelectasis, or COPD.

What is consolidation and what does it indicate

Filling of airspaces with transudate, exudate, cells or protein; pneumonia is a common cause; can reduce both ventilation and perfusion in the affected area, producing a matched defect.

What is atelectasis

Collapse of alveoli, caused by airway obstruction, compression of lung by air or fluid in the pleural space, or pressure from a tumour outside the lung; unaffected/functioning lung eventually hyperinflates (O2 sats remain normal) while heart rate, blood pressure and respiratory rate increase.

What is CTPA

Computed Tomography Pulmonary Angiography — a specialised CT technique using IV contrast dye to directly visualise blood vessels and detect blockages (clots) in the pulmonary arteries.

Advantages of CTPA

Great for proximal PE (large central clots); quick and readily available, especially after hours; high sensitivity and specificity, providing rapid precise results; can also reveal other lung abnormalities.

Disadvantages/limitations of CTPA

Not as good for peripheral (distal/small vessel) PE; delivers a large radiation dose to the patient; risk of contrast reactions; nephrotoxicity (not suitable for patients with poor renal function); limited use in certain populations, especially during pregnancy.