A comparison of one shot and continuous modes for
Shear Wave elastography on the Toshiba Aplio 500
Sandra O’Hara1,2, MMS DMU AMS AFASA
1
2
SKG Radiology, West Perth, Perth, Western Australia
Department of Medical Radiation Sciences, Curtin University, Perth, Western Australia, Australia
Introduction
When using the shear wave elastography
mode, the Toshiba Aplio 500 system supplies
two methods to obtain readings of meters per
second (m/s), and kilopascals (kPa) to assess
the stiffness of tissues in the human body;
the one-shot and continuous methods. The
one-shot method activates the main pulse
over one frame to measure the resultant speed
of the shear wave and elasticity in the tissue
being examined. A single image is produced
to register the stiffness measurements. The
continuous mode provides a live acquisition of
the elastographic and propagation maps, and
multiple images are acquired over time. It is also
possible to cine-loop back through the frames
to register multiple stiffness measurements
from the one acquisition.
The main pulse used in shear wave technology
is a sound beam that is modified to a high
intensity in order to produce the shear
wave propagation.The one-shot method is
considered to be the most accurate method
clinically as it provides a truer representation
of data at a particular point in time, and at a
particular location. The one-shot method also
provides a higher amplitude push pulse, and a
greater number of tracking pulses. Following
the application of the one-shot method, a
mandatory crystal cool time of 5 seconds
is usually observed before a further tissue
interrogation can be made.
In continuous mode, multiple frames can
be sampled and stored in a cine loop with
3 levels of frame rates available. The frame
rate level of 1 equates to 0.4 frames per
second (fps); with levels 2 and 3 providing
0.7 and 1 fps respectively, with small delays
during image acquisition to allow for crystal
cooling. The compounding of images in the
continuous mode can result in greater errors in
measurements as higher levels of noise may be
introduced into the output. Higher frame rates
produce weaker amplitudes of the main pulse
and a reduced number of tracking pulses.
The ideal clinical situation is to use the
continuous mode to find the most appropriate
anatomical window for shear wave
interrogation and then activate the one-shot
method to obtain data. Some patients however,
may be noncompliant to breathing instructions
and technically it may be difficult to achieve
results using a one-shot interrogation. This
investigation therefore compares the use and
measurements of the one-shot and continuous
modes on the adult liver using the Aplio 500
system.
Sandra O’Hara1,2, MMS DMU AMS AFASA
1
SKG Radiology, West Perth, Perth, Western Australia 2 Department of Medical Radiation Sciences, Curtin
University, Perth, Western Australia, Australia
Sandra O’Hara is the Tutor Sonographer at SKG Radiology in Perth, overseeing one of the largest
Ultrasound training programs in Australia. With over 25 years of experience as a Sonographer,
Sandra obtained the General DMU in 1993. She is experienced in general, obstetric, vascular and
musculoskeletal ultrasound, and is committed to maintaining high standards of practice in the
field of Sonography. Sandra completed a Master of Medical Sonography in 2015 with the research
for this qualification incorporating 3 peer reviewed publications. More recently Sandra has been
awarded WA Sonographer of the year and has become an Associate Fellow of the ASA.
www.medical.toshiba.com.au
Method
Data was collected over a 3-month period at
SKG Radiology in Perth, Western Australia. All
data was collected using the Toshiba Aplio 500
series (Japan) (Version 6 & 6.5) paired with the
6C1 curvilinear transducer.
The mean age of the 30 participants was 40
years (range 24 – 57), with 18 of the participants
having no history of liver disease and 12 with a
confirmed diagnosis of Hepatitis C.
All participants had their livers interrogated
using both continuous and one-shot modes.
The continuous mode measurements were
registered at the lowest frame rate setting of 1,
or 0.4 fps.
Continuous mode measurements were
registered with gentle respiration, and
one-shot mode with gentle cessation of
respiration during expiration. The same imaging
methodology was utilised for both one-shot
and continuous modes.
As demonstrated in figure 1, an elastogram
box size of 2.5cm x 3cm with a 10mm circular
ROI was used to obtain the mean speed and
elasticity of the shear wave. A maximum of
two ROIs were placed within each speed or
pressure map and concordant propagation
map simultaneously, at similar depths within
the liver. All ROI’s were obtained in anatomical
segments 7 or 8 of the liver. All of the mean
measurements were registered with an output
standard deviation of less than 20%. The mean
output from the region of interest (ROI) was
registered for each participant a minimum of 13
times. Outlying measurements were excluded
so as to obtain a maximally homogenous
dataset of 10 measurements.
In Transient elastography (TE) based techniques,
measurements of tissue elasticity registered in
kPa are obtained, and 10 measurements are
used to assess for homogeneity. This is achieved
by assessing the interquartile measurement
for the dataset. The interquartile range is
divided by the median for the data set and
multiplied by 100 to calculate a percentage. A
percentage of less than 30% is considered to be
a homogenous dataset.
Currently in TE and shear wave elastography of
the liver the median of the 10 registered means
is the reported measurement. Reflecting the
standard for the reporting of liver fibrosis, the
median value of the 10 measurements was
used to compare the one shot and continuous
modes. To assess for homogeneity of the shear
wave dataset of 10 means registered in both
one shot and continuous modes, the registered
interquartile range was divided by the median
for all participants as described previously. All
datasets achieved a median/interquartile range
of less than 30% for the registered kPa values
with the same calculation for the registered
speed or m/s values equating to approximately
half of the kPa value. This calculation is used
to mirror results obtained from the use of
Transient elastography (TE) in the assessment of
stiffness of liver tissue.
Figure 1: Speed and propagation maps displaying
2.5cm x 3cm elastogram and two 10mm ROI’s
registering the mean speed and SD for the ROI.
www.medical.toshiba.com.au
Statistical analysis
Data analysis was performed using SPSS version
26.0 (SPSS V26.0, Chicago, USA). Descriptive
data were presented as mean ± standard
deviation (SD).
The variables were input to assess normality
using a Kolmogorov-Smirnov Test. The data did
not differ significantly (p>0.05) from normality
with exclusion of the one case outlying from
the normal distribution (29 participants). The
authentic outlier at higher median speed was
included in the analysis with no effect on the
analysis observed.
The one-shot method was used as the
reference measurement, and the continuous
method was assessed for difference from the
one-shot method. We defined any observed
difference between the two measurements
as measurement bias. Bias was assessed
in two ways. (1) The mean of the array of
30 differences, one difference from each
participant, was compared to an ideal value
of zero using a 1-sample t-test. The null
hypothesis, H0: mean bias = 0, was tested
Figure 2.
www.medical.toshiba.com.au
against a two-sided alternative, at the 5%
level of statistical significance, (p<0.05). (2)
Additionally, the bias values were plotted
against the averages of the pairs of measured
lengths using Bland-Altman plots (Bland &
Altman, 1986) This provided a visual indication
of how any measurement bias varies, if at all,
with a range of input shear wave speed and
elasticity values in respect of both magnitude
and trend.
a mean difference of 0.016 m/s (SE 0.16);
t(30)=-0.99,p =0.33. The Bland Altman plot
seen at figure 2 shows a large number of
values near the zero bias line, and a very slight
positive bias of 0.016m/s. The registered oneshot and continuous mode median speed
outputs obtained for each of the 30 cases is
also presented at figure 3 in the format of a
scatterplot, which also displays the line of equal
speed.
Results
The mean difference for the elasticity of
tissue for continuous mode and one-shot
mode was not statistically different from
zero, with a mean difference of 0.143 kPa (SE
0.15); t(30)=-0.95,p =0.35. The Bland Altman
plot seen at figure 4 shows a large number
of values near the zero bias line and a very
slight positive bias of 0.143kPa. The registered
one-shot and continuous mode median kPa
outputs obtained for each of the 30 cases is
also presented at figure 5 in the format of a
scatterplot, which also displays the line of equal
elasticity.
One-shot and continuous mode measurements
were obtained in all 30 participants. For the
one-shot mode and continuous modes
respectively, the mean of the median shear
wave speed values was 1.46 m/s (SD 0.40) and
1.48 m/s (SD 0.41.) respectively with mean
values of 6.87 kPa (SD 5.58) and 7.013 kPa (SD
5.8) for the respective elasticity measurements.
The mean difference for the speed of tissue
for continuous mode and one shot mode
was not statistically different from zero with
Figure 3.
Figure 4.
Figure 5.
Discussion
The one-shot shear wave acquisition mode is
recognised as ideal clinically, as a single frame
is acquired with increased penetration and
high resolution. The continuous mode acquires
multiple frames with decreased penetration
and increased noise, which may compound
errors arising from image noise. The continuous
mode is however recognised as advantageous
in assisting the operator to identify the best
anatomical window to obtain reliable shear
wave propagation.
appropriate method with which to obtain
clinically relevant results on this system.
Comparative results demonstrate that the Aplio
500 system delivers highly correlated results
between one-shot and continuous modes.
We therefore contend that during the usertraining phase, the continuous mode utilised
at its lowest frame rate, may be considered an
acceptable method for the novice operator to
obtain reliable shear wave measurements. As
operator skill increases however, the one-shot
mode should become the standard method
used clinically. Additionally, we have shown
that in a non-compliant patient the continuous
mode, at its lowest frame rate, is likely an
The elastogram maps are presented to show
both speed of arrival time of the shear wave
and elasticity (pressure) of tissues. Loss of
colour filling in the speed and pressure map
is indicative of regions where shear wave
propagation has not been successful.
www.medical.toshiba.com.au
The Toshiba Aplio 500 platinum ultrasound
system supplies a shear wave elastography
application that is unique from other vendors.
The operator can assess the reliability of the
shear wave propagation through multiple
maps that provide a pre-emptive indication of
the reliability of placement of the ROI before an
analysis.
The propagation map is unique to Toshiba. This
map shows the user areas where increased
reliability is more likely. Regions registering a
parallel arrangement of propagation lines have
increased reliability for resultant shear wave
speed. Regions where there is a loss of parallel
configuration show decreased reliability and
thus should be avoided. The propagation map
is also a visual aid to determine tissue stiffness;
the closer the lines are the softer the tissue, the
wider apart they are the stiffer the tissue.
The Toshiba system also supplies the standard
deviation of the registered mean speed or
elasticity measurement, which is useful in
the context of several areas of tissue being
examined. It is recommended by the vendor
that the registered SD of the mean should
be kept to less than 20% of the mean, or an
artefactual increase in the mean speed may
be registered in the results. All registered
measurements in this research achieved a
standard deviation of less than 20%. These
measures of reliability checking assist the
operator in achieving a placement of the
ROI in regions where a reliable shear wave
propagation output is expected.
Figure 6 is an example of the speed map
indicative of shear wave speed utilised in dual
mode; with the propagation map showing the
ROIs placed in the region of greatest reliability,
and as is demonstrated a low SD of the mean
has been achieved. Figure 7 is an example of
ROI placement in a technically challenging
patient.
This research shows that it is possible with
this system to achieve an extremely close
replication of results comparing the one-shot
Figure 6: Shear wave propagation and ROI placement.
mode with continuous mode; as long as the
frame rate is kept at the minimum level for the
continuous mode method. The results show a
statistically insignificant difference between the
median speed and elasticity measurements for
one shot and continuous modes.
As can be seen in figures 2 and 4, one value was
registered above the 95% confidence interval
and some values approached this interval, thus
we need to consider if these differences will be
medically significant. An overestimation of shear
wave values may alter the patient treatment
regime and the tools provided by the Toshiba
Aplio 500 system to quantify the reliability of the
shear wave propagation should be considered
when deciding which mode is most appropriate
for the examination.
Figure 7: Shear wave propagation and ROI placement demonstrating a technically
challenging case with one reliable ROI placement achieved.
Conclusion
The Toshiba Aplio Shear Wave application
supplies the user with multiple indicators of
the reliability of the shear wave propagation.
These tools should be used in all cases. The
operator should consider using a combination
of continuous and one-shot modes, especially
when working with a non-compliant patient.
The reliability of the propagation map will
be synchronous with the registered standard
deviation of the mean, and therefore these
maps and SD assessments should always be
utilized. Regions of higher reliability will register
a lower SD and a reading registered with an
SD greater than 20% of the mean should be
considered unreliable.
The one-shot mode is recognised as the ideal
method to obtain reliable data of shear wave
speed and elasticity of tissues. Continuous
mode, even though shown in our study to give
very similar results to one-shot mode in most
patients, should only be used if one-shot mode
is unsuccessful; and always at the lowest frame
rate available.
In conclusion the mode that achieves the
best propagation maps and lower standard
deviations should be the preferred mode for
the shear wave examination.
Reference
Bland, J. M., & Altman, D. G. (1986). “Statistical methods for assessing agreement between two methods of clinical measurement.”, The Lancet, February 8.
www.medical.toshiba.com.au