European Nuclear Medicine Guide
99mTc-labelled ultrafine carbon suspension (Technegas)
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.
General indications for lung ventilation scintigraphy with Technegas often combined with a perfusion scintigraphy include, but are not limited to:
There are only relative contraindications for ventilation scintigraphy.
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.
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.
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."
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.
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.
The detailed recommendations are available in the EANM Pulmonary Embolism Guidelines. Further information can be found in the published literature [93].