Do current Philips ultrasound systems exceed the recommended safety limits during routine prenatal ultrasounds?

Abstract

Background

Biological safety of ultrasound is a complex and nuanced subject that is poorly understood by ultrasound users. Little is known about the acoustic output and thermal index levels during the routine use of modern ultrasound machines in prenatal scanning.

Methodology

This study was a retrospective review of thermal index (TI) values encountered during 300 prenatal ultrasound examinations (100 in each trimester) performed on any one of 13 Philips Epiq 7 or Epiq 5 systems, representing approximately 106.5 h of real‐time scanning. The TI levels were compared to three international guidelines on the biological safety of ultrasound.

Results

The routine use of current Philips systems was associated with low TI levels. Of the 300 examinations reviewed, virtually all were compliant with the BMUS and Nelson safety guideline. Whether the examination was compliant with the WFUMB guideline is open to interpretation. The highest level of TI encountered was 1.1. In no instance did the TI level incur into the ‘not recommended’ range or into a range where specific user action was required to reduce the TI within 1 min. The most frequent action associated with TI > 0.7 was the use of M‐mode to document the fetal heart rate. In the four instances where TI peaked at 1.1, 3 were associated with the use of M‐mode and one with B‐mode. Spectral Doppler was not implicated in high TI levels. These results are surprising and open up a range of opportunities for future study.

Background

Ultrasound is the only imaging test routinely employed in pregnancy. In the developed world, low‐risk women receive on average 2–3 prenatal ultrasounds and as many as 4–5 ultrasounds.1 It is not uncommon for high‐risk women to receive ultrasounds on a fortnightly, weekly or more frequent basis. Although ultrasound is generally considered safe, ongoing concerns about biological safety have resulted in the development of specific safety guidelines by professional ultrasound societies2, 3 and expert groups.4 An important component of the safe practice of prenatal ultrasound involves the ultrasound user (sonographer, sonologist) monitoring the power output display standard (ODS) parameters on the real‐time display and taking appropriate action to minimise the acoustic exposure to the fetus. The relevant ODS indices in prenatal ultrasound are the Thermal Index in Soft Tissue (TIs) which is used in pregnancies <10 weeks gestational age (8 weeks conception age) and the Thermal Index in Bone at focus (TIb) in pregnancies ≥10 weeks of gestational age.2, 3, 4 The mechanical index is of lesser relevance in obstetrics because cavitational effects do not occur in the gravid uterus due to the paucity of free gas.

A number of studies across the North America,5, 6 Europe,7, 8 the Middle East,9, 10 Australasia11 and the developing world12 have demonstrated that the users of ultrasound demonstrate inadequate knowledge of ultrasound safety. The generalised belief in the safety of ultrasound has resulted in user complacency, despite abundant reference literature, industry guidelines and additional safety training.6, 11 In practice, most ultrasound users do not monitor the Thermal Indices (TI) and do not know how the TI can be reduced by adjusting the system controls and limiting the power output.8, 11, 13 Even though substantial reductions in acoustic output are possible without compromising the clinical content of ultrasound images,14, 15, 16 uninformed or complacent users cannot be expected to take the appropriate action to comply with guidelines intended to ensure the biological safety of the fetus.

While the onus on the safe use of ultrasound ultimately rests on the end user, ultrasound manufacturers have been encouraged to develop systems that incorporate safety features such as limiting the power output of the system at start up.3 However, little is known about the power output characteristics of modern ultrasound machines during routine prenatal examinations. In the event that the routine use of ultrasound exceeds the recommended safety boundaries in terms of Thermal Indices (TI), ultrasound users need to be aware and take appropriate action. Conversely, if ultrasound systems are already set up in such a way that they do not exceed the recommended TI, ultrasound users and their clients can be reassured about the relative safety of our current practices.

The aim of this study was to review 300 prenatal ultrasound examinations (100 for each trimester of pregnancy) to determine whether ultrasound systems exceed the recommended TI levels set by three international guidelines.

Methodology

This study was a retrospective sequential review of 100 first trimester (50 early pregnancy and 50 nuchal translucency), 100 second trimester (fetal anatomy) and 100 third trimester (surveillance) ultrasound examinations performed at Waikato Hospital and Hamilton Radiology (Hamilton, New Zealand) in July–August 2018. Patients were identified using a sequential search of the picture archiving and communication systems. All examinations were performed on any one of 13 Philips Epiq 7 or Epiq 5 ultrasound systems (Philips Ultrasound; Bothel, WA, USA) by qualified sonographers with a minimum of postgraduate degree or by trainees (sonography trainees, radiology registrars) under the direct supervision of a qualified sonographer. All examinations had been allocated normal booking times of 20 min for early pregnancy (EP), 30 min for nuchal translucency (NT), 30 min for 3rd trimester examinations and 45 min for fetal anatomy scans.

The following data from each scan were recorded: transducers used, presets, whether the correct TI was displayed (TIs or TIb), the maximum and minimum TI encountered throughout the examination, whether M‐mode, colour Doppler and spectral Doppler were used to assess the embryo/fetus and specific system settings and user actions associated with an elevated TI. The duration of the ultrasound examination was approximated as the time elapsed between the last and first ultrasound image. The results were tabulated and compared to three widely accepted comprehensive industry guidelines on the biological safety of ultrasound in prenatal ultrasound (Table 1):

1. The British Medical Ultrasound Society’s Guidelines for the safe use of diagnostic ultrasound equipment (BMUS guideline),2

2. World Federation of Societies for Ultrasound in Medicine and Biology’s Clinical safety statement for diagnostic ultrasound (WFUMB guideline)3 and

3. Ultrasound biosafety consideration for the practicing sonographer and sonologist by Nelson, Abramowicz and Church (Nelson guideline).4

Table 1.

Synopsis of safety guidelines for the thermal index in prenatal ultrasound applications.

BMUS2
0.7 < TI ≤ 1.0	60 min
1.0 < TI ≤ 1.5	30 min
1.5 < TI ≤ 2.0	15 min
2.0 < TI ≤ 2.5	4 min
2.5 < TI ≤ 3.0	1 min
WFUMB3
TI ≤ 0.7	Recommended level
TI > 0.7	Only used if necessary
TI ≥ 3.0	Not recommended
Nelson et al.4
TI < 0.5	Recommended level for extended periods of scanning
TI 0.5–1.0	<30 min
TI > 2.5	<1 min

In order to evaluate our data with reference to the WFUMB guideline, we needed to define the term ‘necessary’ or ‘not necessary’ in the context of the user being permitted to use a ‘TI > 0.7 if necessary’. As imaging step was defined as ‘necessary’ if the required clinical information could not be obtained in another way, such as using colour Doppler to examine a fetal anatomical structure (fetal heart, 3‐vessel cord) or using high‐definition zoom to obtain greater image detail. The recording of the fetal heart rate (FHR) using M‐mode was defined as ‘not necessary’ because FHR can be assessed visually, estimated from a video‐clip or measured from the Doppler waveform of the umbilical artery. None of the fetuses in our series required specific echocardiographic M‐mode investigation for tachyarrhythmia or cardiac anomaly.

Patients were excluded if the embryo/fetus was demised at the time of the ultrasound. Early first trimester gestations with uncertain viability were included regardless of final outcome.

Ethics approval

The study was approved by the Waikato District Health Board Clinical Audit Service (study reference number: 3927P).

Results

Three hundred ultrasound examinations were reviewed (100 in each trimester of pregnancy), representing approximately 106.5 h of real‐time scanning.

In the first trimester gestations, the EP scan (n = 50) duration ranged from 2 min 22 s to 20 min 23 s (mean = 10 min 4 s, median = 9 min 58 s). The predominant transducer used was C5‐1 in 54% of cases, C10‐3V (transvaginal) in 40% and C9‐2 in 6%. The correct TI (TIs) was displayed in 48(96%) cases. In addition to B‐mode, ultrasound users also utilised M‐mode in 42(84%) cases, colour Doppler in 2 (4%) of cases, but spectral Doppler was never used to assess the embryo. The two instances where the sonographer utilised colour Doppler involved early gestations of uncertain viability one of which proved to be live and the other demised on follow‐up examinations. The maximum TI across all examinations is shown in Table 2. The maximum TI for EP scans ranged from 0.2 to 1.1 (mean = 0.5, median = 0.5). The examination satisfied the criteria for the recommended level of TI in 100% of cases by the BMUS guideline, 96% of cases by the WFUMB guideline and 100% of cases by the Nelson guideline (Table 3).

Table 2.

Overview of thermal index values encountered in our cohort.

Trimester	Type of Examination	Maximum TI	Mean TI	Median TI
First	EP scan	1.1	0.5	0.5
	NT scan	1.0	0.7	0.7
Second	Anatomy scan	0.9	0.6	0.6
Third	Third trimester scan	1.1	0.8	0.8

Table 3.

The percentage of ultrasound scans in each TI category for each guideline.

Guideline		First Trimester EP Scan, %	First Trimester NT Scan, %	Second Trimester Scan, %	Third Trimester Scan, %
BMUS2
Guideline categories
0.7 < TIS ≤ 1.0	60 min	98	100	100	97
1.0 < TIS ≤ 1.5	30 min	2	0	0	3
1.5 < TIS ≤ 2.0	15 min	0	0	0	0
2.0 < TIS ≤ 2.5	4 min	0	0	0	0
2.5 < TIS ≤ 3.0	1 min	0	0	0	0
Scans compliant with guideline		100	100	100	100
WFUMB3
Guideline categories
TI ≤ 0.7	Recommended level	90	54	91	24
TI > 0.7	Only used if necessary	6	16	9	2
TI ≥ 3.0	Not recommended	0	0	0	0
Scans compliant with guideline		96	70	100	26
Scans in the ‘not recommended range’		0	0	0	0
Nelson et al.4
Guideline categories
TI < 0.5	Recommended for extended periods of scanning	48	9	16	3
TI 0.5–1.0	<30 min	50	91	84	94
TI > 2.5	<1 min	0	0	0	0
Scans compliant with guideline		98	100	100	97
Guideline does not address scenario		2	0	0	3

The NT scan (n = 50) duration ranged from 4 min 27 s to 40 min 23 s (mean = 18 min 7 s, median = 15 min 52 s). The predominant transducer used was C5‐1 in 72% of cases, C9‐2 in 26% and C10‐3V in 2%. The correct TI (TIb) was initially displayed in 86% of cases, however, if the entire examination is taken into account, then the correct TI was consistently displayed in only 68% cases due to users frequently switching between presets which inadvertently triggered the display of the incorrect TI. In addition to B‐mode, ultrasound uses also utilised M‐mode in 47(94%) cases and colour Doppler in 45(90%) cases to assess the fetus. Spectral Doppler was never used during NT scans. The maximum TI ranged from 0.2 to 1.0 (mean = 0.7, median = 0.7). The examination satisfied the criteria for the recommended level of TI in 100% of cases by the BMUS guideline, 70% of cases by WFUMB guideline and 100% of cases by the Nelson guideline.

In the second trimester gestations (n = 100), the scan duration ranged from 4 min 6 s to 62 min 51 s (mean = 31 min 19 s, median = 30 min 10 s). The longest examination of 62 min 51 s involved a twin pregnancy. The predominant transducer used was 5C1 in 81% of cases and 9C2 in 19%. The correct TI (TIb) was displayed 98% of the time. In addition to B‐mode, ultrasound uses also utilised M‐mode in 3 (3%) of cases, colour Doppler in 97 (97%) cases and spectral Doppler in 1 (1%) case due to early on‐set fetal growth restriction. The maximum TI ranged from 0.1 to 0.9 (mean = 0.6, median = 0.6). The examination satisfied the criteria for the recommended level of TI in 100% of cases by the BMUS guideline, 100% of cases by the WFUMB guideline and 94% of cases by the Nelson guideline.

In the third trimester gestations (n = 100), the scan duration ranged from 3 min 31 s to 24 min 14 s (mean = 10 min 50 s, median = 10 min 18 s). The predominant transducer used was C5‐1 in 99% of cases and C9‐2 in 1%. The correct TI (TIb) was displayed 100% of the time. In addition to B‐mode, ultrasound users also utilised M‐mode in 97% of cases, colour Doppler in 78% of and spectral Doppler in 50%. The maximum TI ranged from 0.3 to 1.1 (mean = 0.8, median = 0.8). The examination satisfied the criteria for the recommended level of TI in 100% of cases the BMUS guideline, 26% of cases by WFUMB guideline and 97% of cases by the Nelson guideline.

Overall, the TI across all 300 examinations was relatively low. There was no single instance of a TI value in the ‘not recommended’ category or in a category requiring the user to take action within 1 min. The highest TI encountered was only 1.1 and this occurred three times during third trimester scans due to the use of M‐mode and once in a EP scan due to the use of B‐mode with high‐definition zoom. A summary of user actions which were associated with TI > 0.7 and TI > 1.0 is provided in Table 4. The three most common actions leading to increased TI involved the use of: (i) the use of M‐mode; (ii) B‐mode with high‐definition zoom; and (iii) colour Doppler with small region of interest (ROI). Despite the prolific use of spectral Doppler in the third trimester, spectral Doppler was almost never implicated in elevated TI and was not associated with the four instances of highest TI recorded.

Table 4.

User actions that were associated with TI > 0.7 and TI > 1.0.

User Actions Associated with TI > 0.7
Transducer	User Action	Number of Cases
C5‐1	M‐mode of FHR	82
C9‐2	B‐mode, high‐definition zoom	10
C9‐2	M‐mode of FHR	7
C9‐2	Colour Doppler, high‐definition zoom, small ROI	4
C5‐1	Spectral Doppler of umbilical artery	2
C10‐3v	B‐mode, high‐definition zoom	2
C10‐3v	Colour Doppler, small ROI	1
C9‐2	Colour Doppler, small ROI	1
C5‐1	B‐mode, high‐definition zoom	1

User Actions Associated with TI > 1.0
Transducer	User Action	Total Number of Cases
C5‐1	M‐mode of FHR	3
C10‐3v	B‐mode, high‐definition zoom	1

All examinations complied with the BMUS guideline. Virtually all examinations complied with the Nelson guideline with the exception of four cases where a TI of 1.1 was briefly encountered. The Nelson guideline does not specifically address a scenario with TI ranges of 1.0–2.5. We discovered a high rate of non‐compliance with the WFUMB guideline, particularly during NT and third trimester scans. In all cases, the problem arose when the user activated M‐mode for the purpose of recording the FHR and the thermal index rose >0.7. This level of TI is reserved only for cases where such imaging is ‘necessary’. Interestingly, all second trimester scans achieved 100% compliance with the WFUMB guideline because M‐mode was almost never used in the second trimester for reasons of a different imaging protocol. This further supports our opinion that M‐mode of FHR is not necessary, because it was not consistently utilised across the different trimesters.

A surprising and unexpected finding of this study is that the use of M‐mode to record fetal cardiac motion was associated with higher TI than the use of spectral Doppler. We confirmed this experimentally with a sample of test images. Figure 1 demonstrates test images obtained in different imaging modes under the same default system settings with M‐mode showing the highest TI value.

Figure 1.

Test Images in B‐mode (top left), M‐mode (top right), Colour Doppler (Bottom Left) and Spectral Doppler (Bottom Right) Under the Default Obstetric Preset Demonstrate That the Highest TI Occurs During of M‐mode.

Discussion

The three main guidelines on TI levels during prenatal ultrasound vary significantly in detail and scope, however a common agreement can be found on several points:

1. Thermal index of ≤0.7 can be used for prenatal examinations lasting up to 30 min

2. Thermal index of >2.5 should be restricted to <1 min of scanning

3. Thermal index of >3.0 should never be used

We found the BMUS guideline was the most lenient and WFUMB guideline most conservative (Table 5).

Table 5.

Advantages and disadvantages of each guideline.

Guideline	Stringency	Advantages	Disadvantages
BMUS2	Most lenient	Clearly defined boundaries for all TI levels	Complex Difficult to memorise At higher TI levels, the user is required to keep track of time which is practically impossible
WFUMB3	Most stringent	Simple Easy to memorise	Open to interpretation at TI > 0.7 due to the phrase ‘if necessary’
Nelson et al.4	Lenient	Simple Easy to memorise	Non‐specific for a wide range of TIs (1.0–2.5)

The WFUMB guideline is open to interpretation and debate because it contains the phrase ‘only used if necessary’. When the use of M‐mode as defined as ‘not necessary’ for the recording of normal FHR, as many as 74% of third trimester scans were non‐compliant with the WFUMB guideline. However, if M‐mode had been defined as ‘necessary’, then the non‐compliance rate would have diminished to 0%. Whether measurement of FHR is necessary is debatable. Another consideration with regards to FHR recording centres around the user’s compliance with industry guidelines. A recently released consensus template for the reporting of third trimester obstetric examinations, jointly endorsed by the Australasian Society for Ultrasound in Medicine, the Royal Australian and New Zealand College of Obstetricians and Gynaecologists and the Royal Australian and New Zealand College of Radiologists contains the FHR as one of the required fields. It could be argued that from the clinical perspective, precise measurement of FHR in normal fetuses may not be necessary, but from a compliance perspective, it is necessary.17

The use of different guidelines in different regions and the ability of the user to choose a guideline to follow is problematic because the same user action resulting in the same TI may be viewed as perfectly safe in one locality and questionable in another. This is particularly the case when the TI exceeds threshold values of 0.7 and 1.0. The lack of consensus on what constitutes a safe level of TI may also be a source of confusion for the users of ultrasound. It is important to point out that just because one guideline is more stringent in term of TI boundaries does not mean it is any safer. More stringent guidelines, however, are more closely aligned with the spirit of the As Low As Reasonably Achievable (ALARA) principle.

The results of our study indicate that the routine use of current Philips Epiq 5 and 7 ultrasound systems on default settings safe and compliant with 2 of the 3 international guidelines. There was no single instance where the routine use ultrasound systems was associated with TI in excess of 1.1. However, caution is advised on several points. First, the Philips systems display either TIs or TIb, but not both. This means that the change of user preset in the middle of the examination may inadvertently trigger the display of the incorrect TI. If the incorrect TI is used, the user cannot judge the biological safety of the instrument. Second, the use of M‐mode was found to trigger increased TI compared to B‐mode, colour and even spectral Doppler. Furthermore, given that spectral Doppler was associated with lower TI values than M‐mode, consideration should also be given to the possibility of using spectral Doppler for the purpose of FHR recordings. The use of spectral Doppler particularly in the first trimester is contentious,18 but it may be feasible to study ultrasound systems experimentally using a combination of user settings to gain a better understanding of the TI level during M‐mode compared with spectral Doppler. Even though increased TI has previously been reported in association with the use of colour and spectral Doppler in pregnancy,19 we did not observe elevations of thermal indices in these modes of imaging.

Our study has several potential limitations. The first limitation is that the data set for our analysis was based on Philips ultrasound systems (Philips Epiq 5 and 7). This choice was made for practical reasons as these systems are the most widely available machines in our region servicing a population of half a million people and are some of the most commonly used systems in Australia and New Zealand. At present more than 1000 Epiq systems are deployed across Australia and New Zealand (e‐mail from Kathryn Smith, Philips Healthcare, April 2019). As these systems are often used by several users, thousands of sonographers and sonologists are currently using these systems. There is a wide range of other manufacturers of ultrasound systems the authors encourage other research groups to replicate this study on their ultrasound equipment.

A second limitation is that the data was gathered from recorded images, not from real‐time live scanning. While the image record provides a comprehensive representation of the examination, it is possible that the user may have taken unknown steps during the examination that may have transiently increased the TI higher than recorded on images. We consider this unlikely for several reasons. First, several thousand still images were scrutinised representing approximately 106.5 h of real‐time examinations. Secondly, images of the imaging modes with the highest TI such as M‐mode, spectral and colour Doppler are usually well documented due to their intrinsic clinical value. It is unlikely that the user would have interrogated the fetus using these modes without recording images.

The third limitation involves the lack of ability to determine the power output level used during the examination. Philips ultrasound systems do not provide an on‐screen indicator of the power output. This information is only available on the second page of the touch‐panel display and is not captured on images. Based on existing literature and clinical and teaching experience, the authors strongly suspect that our users never adjusted the power output control, however, this could not be empirically verified.

Another limitation involves the approximation of the timing of the examination. We estimated the duration of the ultrasound based on the acquired image record; however, it is feasible that the sonographer scanned for a longer period before or after obtaining the first and last image or conversely the sonographer paused the examination without live scanning. It is very difficult to accurately estimate acoustic exposure time of ultrasounds even if the examinations had been viewed or recorded in real‐time because of the frequent freezing–unfreezing of images for the purpose of measurements, annotations and recording. These actions result in a highly fragmented timeline.

Finally, all examinations in this series were done by a qualified sonographer or by trainees under the direct supervision of a sonographer. There were no clinician‐operators. It is conceivable that sonographers and clinician‐operators may exhibit different system utilisation practices which may affect the TI.

Conclusion

The routine use of current Philips systems was associated with low TI levels. Of the 300 examinations reviewed, virtually all were compliant with the BMUS and Nelson safety guideline. Whether the examination was compliant with the WFUMB guideline is open to interpretation. In no instance did the TI level incur into the ‘not recommended’ range or into a range where specific user action was required to reduce the TI within 1 min. The most frequent action associated with TI > 0.7 was the use of M‐mode to document FHR.