Receive Beam-Steering and Clutter Reduction for Imaging the Speed-of-Sound Inside the Carotid Artery
Round 1
Reviewer 1 Report
Speed of sound can be a very useful metric in tissue classification, but there are complex technical issues in its measurement using conventional ultrasound. This paper examines some of these issues and proposes a methodology for the production of SoS images, particularly for the imaging of the carotid artery.
This is a nice paper, well-written, clear and it presents some interesting and useful results. It is well-referenced and contains much useful and relevant background information. The approach of using a phantom for the evaluation of the ultrasound methodology is sensible and the details of the phantom construction are clear. The presentation of the largely qualitative results from the phantom experiments is appropriate and the differences between phantom experiments and clinical application are examined appropriately. This paper presents a useful study on which further development can be made towards clinical application.
The writing style is good and it is all well-presented (but care has to be taken in the final version to try to keep the figure captions on the same pages as the figures). The references are comprehensive.
Author Response
We thank the reviewer for the encouraging comments.
The manuscript is now reformated and all figure and corresponding figure captions are at the same page.
Reviewer 2 Report
The authors have tried to assess atherosclerotic plaque composition in the carotid artery using receive beam steering instead of transmit beam steering and pairwise data subtraction to enable phase tracking of blood echoes. The manuscript discusses about a critical problem which needs to be addressed. The manuscript is well written and can be accepted for publication subject to following minor revisions.
The authors need to describe more about the current state of the art for atherosclerotic plaque imaging and the necessity for an improvement on it.
The authors need to specify the exact time gained by their receive beam steering compared to transmit beam steering per acquisition, this quantification will help the readers in understanding the significance of the technique.
In figures 2 and 3, the authors need to add a schematic representation of the cross-section of the imaging area and label the images(Blue and red part of the image). This will help the readers understand the images better.
In figures 2 and 3 the authors need to increase the font size to make it more readable.
The authors need to explain more on the artifacts noticed in Fig 3(b).
The authors have performed only in-vitro phantom studies. However, in-vivo imaging conditions are very different. The authors need to comment on how they tried to mimic the in-vivo conditions and how their model is expected to perform in-vivo.
Author Response
The authors need to describe more about the current state of the art for
atherosclerotic plaque imaging and the necessity for an improvement on
it.
We have slightly rephrased the introduction to make better highlight the state-of-the-art ultrasound imaging techniques used for plaque imaging and their limitations. Since this study intends to improve the applicability of US we concentrated in the introduction only on ultrasound techniques. The part reads now: Pathomorphologic studies in relation to US echogenicity suggest that low risk “stable” plaques mainly consist of fibrous or calcified tissues, which appear as echogenic, while high risk plaques appear as heterogeneous or translucent [36,37]. Nevertheless, the accuracy of the B-mode US is still not satisfactory, and thus it is nowadays combined with US elastography to improve the diagnostic accuracy. These imaging techniques provide however only limited morphological information for assessing plaque rupture risks.
In figures 2 and 3, the authors need to add a schematic representation
of the cross-section of the imaging area and label the images(Blue and
red part of the image). This will help the readers understand the images
better.
The exact location of the cross-section of the phantom was added in Figure 1. The images shown in figure 2a,b and 3a visualize phase shift maps and the blue and red part of the image shows time differences in us as shown with the color bar.
In figures 2 and 3 the authors need to increase the font size to make it more readable.
has been done
The authors need to explain more on the artifacts noticed in Fig 3(b).
it is mentioned in the manuscript that the artifacts are caused by phase noise (see ln 273-276)
The authors have performed only in-vitro phantom studies. However, in-vivo imaging conditions are very different. The authors need to comment on how they tried to mimic the in-vivo conditions and how their model is expected to perform in-vivo.
This point has been extensively discussed (see ln 320-350). A short paragraph was added discussing the possible spatial resolution of the technique presented.
Reviewer 3 Report
In this paper, the authors adapt their speed-of-sound algorithm toward utilisation in the carotid artery. First, the authors attack the problem of speckle decorrelation due to blood motion by speeding up their acquisition time. This is done by using source-receiver reciprocity and replacing transmit focusing with receive focusing (thus less transmit-acquisitions are required). Second, the blood echos are enhanced by suppressing tissue clutter through subtraction of subsequent frames of data (as is sometimes done in Doppler imaging). This highlights blood signal, and reduces stationary tissue signal (as in Doppler imaging). This paper is a first step toward the application of SoS imaging when blood motion is present, is well-written and appropriate for publication in Journal of Imaging.
The following questions would be helpful to address in the manuscript, to aid the reader:
- In the preliminary in-vivo carotid measurements, what was the time between Tx-angles? This gives the reader an idea of the level of blood decorrelation expected within the 5 shots for a flow as high as the carotid artery (L148-149).
- In figure 2(c), there are artifacts (yellow) indicating ~1500 m/s SOS below the vessel that are not seen in Figure 2(d). Do the authors have an idea for why these are seen when using transmit focusing, whereas they are not when using the combined transmit and receive focusing?
- What are the realistic spatial resolution limitations of SoS imaging for this application? In the introduction, the aims are to measure the SoS for lipid pools (often < 1 mm), and fibrous caps (~100 micrometers). However, these structures are quite small compared to the artery lumen itself, which the phantom mimics. What are are the realistic limitations for SOS imaging of atherosclerotic plaques?
Author Response
- In the preliminary in-vivo carotid measurements, what was the time between Tx-angles? This gives the reader an idea of the level of blood decorrelation expected within the 5 shots for a flow as high as the carotid artery (L148-149).
the exact time for an aquisition is now given in table 1
- In figure 2(c), there are artifacts (yellow) indicating ~1500 m/s SOS below the vessel that are not seen in Figure 2(d). Do the authors have an idea for why these are seen when using transmit focusing, whereas they are not when using the combined transmit and receive focusing?
these artifacts are very similar in both cases but due to the small differences in the SoS and the offset of -8 m/s let the artifacts appear in a slightly different color. This fact is now mentioned in the manuscript (ln 305-312)
- What are the realistic spatial resolution limitations of SoS imaging for this application? In the introduction, the aims are to measure the SoS for lipid pools (often < 1 mm), and fibrous caps (~100 micrometers). However, these structures are quite small compared to the artery lumen itself, which the phantom mimics. What are are the realistic limitations for SOS imaging of atherosclerotic plaques?
A realistic estimate about the spatial resolution has been added (see ln 344-350)
