Nuclear Medicine
CLINICAL DECISION SUPPORT
The regional ventilation distribution can be assessed with a ventilation scintigraphy after inhalation of a radioactive aerosol or gas. [99mTc]Tc-DTPA is the most frequently used radio-aerosol for ventilation studies. The radio-aerosol is inhaled from a nebulizer that preferably creates particles of 1-3 μm (mass median aerodynamic diameter). If inhaled slowly with tidal ventilation, the particles will deposit uniformly in the lungs by sedimentation in the small airways and alveolar regions.
Specific indication for the use of [99mTc]Tc-DTPA radio-aerosol is to evaluate alveolo-capillary permeability in conditions such as interstitial lung disease, exposure to gases and particles, pulmonary infections, etc.
General indications for lung ventilation scintigraphy with [99mTc]Tc-DTPA often combined with a perfusion scintigraphy include but are not limited to:
When possible the 99mTc-DTPA dose used should be decreased in pregnant or potentially pregnant patients and in children (and guidelines for lung scintigraphy in children [94].
It is not recommended to interrupt breast feeding [91].
Most [99mTc]Tc-DTPA radio-aerosol studies are 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]Tc-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 can be used for regional lung function evaluation.
[99mTc]Tc-DTPA radio-aerosol clearance can be used to evaluate alveolo-capillary permeability. [99mTc]Tc-DTPA disappears over the alveolo-capillary membrane through the tight-junctions with less than 2% clearance per min in healthy non-smokers, but two to five times faster in current smokers and in some other conditions. Such rapid clearance is reflected by the early depiction of radioactivity in the kidneys. Greatly increased permeability is found in active interstitial lung disease (e.g. active pulmonary fibrosis, sarcoidosis, pneumonitis, respiratory distress syndrome), after gas exposure (e.g. tobacco smoking, fire smoke, ozone), pulmonary infections (e.g. pneumocystis carinii pneumonia), some other diseases (e.g. graft rejection, inflammatory bowel disease) and physiologic conditions (e.g. exercise). Slightly decreased permeability can be seen in a few conditions such as primary ciliary dyskinesia, after radioiodine treatment of lung metastases, diabetes with vascular complications, and in the elderly.
The suggested activities to administer to the nebulizer range from 400 MBq to 900 MBq so that the activity to reach the lungs is 30-50 MBq [95].
The effective dose per inhaled activity is 4.9 µSv/MBq [3].
The effective dose for an inhalation of 30 MBq is: 0.15 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 that an uneven radio-aerosol lung distribution with hot spots or large airways deposition can occur if the patient is obstructive, inhaling to fast, or if the nebulized aerosol particles are too large (>3 µm). This can result in a suboptimal study, if the study is part of a V/Q scan for the workup of PE. Correction for clearance of [99mTc]Tc-DTPA during the time span of ventilation SPECT has been applied by some investigators. The main limitation of the use of the [99mTc]Tc-DTPA alveolo-capillary permeability test is its non-specificity and limited value in smokers who have increased permeability.
A recent standard chest radiograph or a CT scan can help in the interpretation. However, this is not additionally 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.