Key words
lung ultrasound - bio effects - pulmonary hemorrhage - safety - chest
Introduction
This best practice recommendation gives an overview of current statements and novel
findings regarding the safety aspects on the interaction of ultrasound on lung tissue.
Based on these data, a best practice recommendation is given to minimise potential
risks during routine lung ultrasound applications.
The use of sonography on lung tissue is a valuable Point of Care diagnostic covering
almost all medical disciplines [1]
[2]
[3]
[4] which is currently summarised by an international consensus [5]. However, due to total reflection at the air-tissue interface, such as occurs during
pleural sonography, potential bio-effects of this interaction should be considered.
Consideration of these bio-effects should always be balanced with the clinical benefits
of using a non-ionising imaging modality such as sonography.
Current literature status concerning lung ultrasound safety
Current literature status concerning lung ultrasound safety
Ultrasound, when used under diagnostic exposure conditions, can cause pulmonary capillary
haemorrhage (PCH) in peripheral lung which has been investigated extensively on several
animal models [6]. Lung ultrasound (LUS) induced PCH has not been investigated in humans on a pathological
level such as in animals. In contrast, observational safety studies in children [7] and during transoesophageal sonography [8] showed no complications including no symptomatic pulmonary haemorrhage.
In large animal models, it was clearly shown that PCH occurred over an acoustic output
range that is typical of that emitted during clinical sonographic B-mode (brightness
mode) imaging [9]. In addition, it has been shown that sonographic modes with longer pulses such as
those used in Doppler induce PCH at lower output levels (low Mechanical Index (MI))
[10]. In addition, shear wave elastography (SWE) and acoustic radiation force impulse
(ARFI) sonography techniques emit push pulses with higher intensity and longer durations,
that induce reliable PCH on direct pleural exposure as shown in pre-clinical studies
[11]
[12].
Even though ultrasound induced PCH seems to be a threshold phenomenon, reduction of
scan duration, independent of scanning mode, significantly decreases the likelihood
of PCH induction and its extent [13]
[14]. In obese patients, PCH is much less likely to occur during lung sonography due
to the high attenuation of the soft tissue of the chest wall [15].
Earlier statements of the British Medical Ultrasound Society (BMUS) and American Institute
of Ultrasound in Medicine (AIUM) point to the likelihood of PCH induction at MI values
greater than 0.3 [16]
[17].
Sonography induced PCH was shown to be limited to a peripheral depth of 1–2 mm and
is related to the size of the transducer. PCH is asymptomatic, does not cause alveolar
rupture and does not require interventions [18]
[19]. Diagnostic concerns arise, however, due to the fact that PCH can generate LUS signs
such as the vertical hyperechoic artefacts (B Lines) and White Lung Syndrome (WLS)
and may lead to an incorrect presentation of LUS artefacts and therefore diagnosis
[11]
[13].
To the best of our knowledge, there is no literature on LUS safety in diseased lung
nor regarding effects of contrast enhanced sonography on lung. Due to lower MI values,
typically used during contrast enhanced ultrasound (CEUS), it may not affect the lung.
But studies are required to prove the safety of CEUS on lung tissue.
Therapeutic ultrasound applications are emerging, where focal ablation in proximity
to lung is performed [20]. Pre-clinical animal studies showed PCH induction in lung tissue during shock wave
treatment of liver [21] and heart tissue at peak negative pressures (PNP) above 1.5 MPa. Such values are
similar to diagnostic ultrasound thresholds [22], however due to the use of higher intensities and lower frequencies than in diagnostic
sonography, PCH may arise on a larger surface. Even though the lung is not directly
targeted, pre-focal and post-focal intensities may expose the lung surface above the
PCH threshold. Therefore, treatment planning should consider a sufficient safety margin
between focal position and lung during application of therapeutic ultrasound in proximity
to lung.
Best Practice Recommendations
Best Practice Recommendations
Scan Settings and Preparation
A LUS specific Pre-Set should be used or scanner settings in line with the guidelines
should be set up prior to any LUS examination [23]
[24]. LUS specific Pre-Sets are nowadays available on modern scanners but cover a wide
acoustic output range (0.4–1.4 MI). Therefore, the initial output should be adjusted
(MI ≤ 0.4) independent of PreSet configuration before any lung examination.
Safety Indices during applications
Independent of mode, sonography of the lung with an MI value of less than or equal
to 0.4 can be performed safely without limits on exposure time. Use overall gain and
TGC (time gain compensation) for optimal imaging adjustments. For specific diagnostic
imaging requirements, the output can be increased up to an MI value of 0.7.
In clinical cases where adiposity may limit the field of view, or acoustic obstacles
exist in the sonication path, a maximal MI value of 1.0 should not be exceeded in
order to minimise the probability of cavitation. In such cases, justified by diagnostic
needs, the operator should be aware of the likelihood of PCH induction falsifying
diagnostic findings.
The use of the ALARA (As Low As Reasonable Achievable) principle is strongly recommended
whenever LUS is performed. When exceeding the initial MI value and depending on the
examination requirements, exposure times should be kept as short as possible (1–2
breath cycles).
The use of Doppler during LUS should be applied with an MI ≤ 0.5 and with exposure
times as short as possible.
SWE and ARFI sonography techniques should be performed only if the region of interest
(ROI-where the shear wave is generated) is located in consolidated, peripheral lung
tissue, avoiding direct pleural exposure.
Lung sonography in the neonate should always be performed with the lowest MI value
possible and not exceeding 0.4. The use of Doppler as well as SWE and ARFI should
not be applied on neonatal subjects until further studies have shown that it is safe
to use for this vulnerable patient class.
A summary of output setting recommendations is shown in [Table 1].
Table 1
Summary of recommended initial Output Index (MI) for Lung Ultrasound depending on
mode and application.
|
Mode/MI
|
B Mode
|
Pulsed Doppler
|
Elastography (SWE, ARFI)
|
|
General Imaging Initial setting (start)
maximum (if needed)
|
≤ 0.4
≤ 0.7
|
always ≤ 0.5
|
peripheral consolidations only, not recommended for pleural examination
|
|
Neonatal
|
≤ 0.4
|
not recommended
|
not recommended
|
Education and Future directions
Specific teaching and education for lung sonography should include principles of safety
indices and their recommended limits for lung sonography.
The safety profile of SWE and ARFI when applied to lung tissues requires more scientific
evaluation to prove its diagnostic safety record before further recommendation.
No lung specific safety index has been introduced to date. However, most of the research
literature shows good correlation of PCH thresholds with MI, even though current research
would suggest that cavitation is not the cause of PCH [25]. A specific safety index for lung is justified which should include TI (Thermal
Index), MI and pulse duration but requires evaluation in future studies [18]
[26].
