Ventilation Technegas

5.3.1 Radiopharmaceutical

^99mTc-labelled ultrafine carbon suspension (Technegas)

5.3.2 Uptake mechanism / biology of the tracer

The regional ventilation distribution can be assessed with a ventilation scintigraphy after inhalation of a radioactive aerosol or gas. Technegas is a submicron sized ^99mTc -labelled carbon radio-aerosol suspension that behaves almost like a gas. The submicron radio-aerosol is inhaled from a dedicated nebulizer system that creates particles of 5-200 nm by burning solid graphite particles and Na[^99mTc]TcO₄ in argon gas at high temperature. If inhaled slowly with 1-3 deep breaths, the particle suspension will deposit uniformly in the lungs by diffusion in the small airways and alveolar regions, where it will stay for days.

5.3.3 Indications

General indications for lung ventilation scintigraphy with Technegas often combined with a perfusion scintigraphy include, but are not limited to:

• Diagnosis and follow-up of PE;
• Evaluate the cause of pulmonary hypertension;
• Quantify regional pulmonary function before surgery/radiation therapy for lung cancer;
• Evaluate lung transplants;
• Evaluate emphysema for lung volume reduction intervention;
• Evaluate congenital heart or lung disease such as cardiac shunts, pulmonary arterial stenosis, and arteriovenous fistulae and their treatment;
• Evaluate chronic pulmonary parenchymal disorders such as cystic fibrosis;
• Confirm the presence of bronchopleural fistula.

5.3.4 Contra-indications

There are only relative contraindications for ventilation scintigraphy.

• When possible the Technegas dose deposited in the lungs should be reduced in pregnant or potentially pregnant patients, and the ventilation scan should be performed only if a preceding perfusion scintigraphy is abnormal indicating PE.
• It is not recommended to interrupt breast feeding [1–3,91].

5.3.5 Clinical performances

Technegas is used as part of ventilation studies either performed as planar or SPECT, and the most frequent indication is diagnosis of PE. A normal perfusion scan excludes PE, and a ventilation scan is not needed. However, if the perfusion scan is abnormal, a ventilation scan is needed for the interpretation. So, in many cases a V/Q scintigraphy is performed. See details under the ^99mTc-MAA perfusion scintigraphy regarding the clinical performance of V/Q scintigraphy.

A ventilation scan without a perfusion scan can be used to evaluate ventilation inhomogeneity, as often seen in obstructive lung disease, and it can be used for evaluation of regional lung function.

5.3.6 Activities to administer

The suggested activities to administer range from 20 MBq to 30 MBq.
In paediatric nuclear medicine, the activities should be modified according to the EANM paediatric dosage card (https://www.eanm.org/publications/dosage-calculator/) and guidelines for lung scintigraphy in children [94]. The minimum recommended activity to administer to the nebulizer is 100 MBq.

5.3.7 Dosimetry

The effective dose per administered activity is 15 µSv/MBq [3].
The effective dose for an inhalation of 30 MBq Technegas is: 0.45 mSv.

Caveat

“Effective Dose” is a protection quantity that provides a dose value related to the probability of health detriment to an adult reference person due to stochastic effects from exposure to low doses of ionizing radiation. It should not be used to quantify the radiation risk for a single individual associated with a particular nuclear medicine examination. It is used to characterize a certain examination in comparison to alternatives, but  it should be emphasized that if the actual risk to a certain patient population is to be assessed, it is mandatory to apply risk factors (per mSv) that are appropriate for the gender, the age distribution and the disease state of that population."

5.3.8 Interpretation criteria/major pitfalls

For interpretation of combined ventilation and perfusion scintigraphy, the reader is referred to the chapter on ^99mTc-MAA perfusion scintigraphy.
Ventilation defects often occur in moderate to severe COPD, asthma, cystic fibrosis, lung fibrosis, atelectasis, lung tumour, pneumonia, pleural effusion, and pulmonary infarction.
Major pitfalls include uneven Technegas lung distribution with hot spots due to sedimentation in the larger airways, which can occur if the patient is severely obstructive, or if the inhalation procedure is started to late. The inhalation should be done within 3 min (maximally 10 min) after generation of the particles, because these grow by aggregation. Such hot spots could travel up the airways by mucociliary and cough clearance.

5.3.9 Patient preparation

A recent standard chest radiograph or a CT scan can help in the interpretation. However, this is not needed if a CT scan is performed as part of a SPECT/CT procedure.

5.3.10 Methods

The detailed recommendations are available in the EANM Pulmonary Embolism Guidelines. Further information can be found in the published literature [93].