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Power Doppler Imaging to Evaluate Flow-limiting Stenoses

Department of Radiology, Ochsner Clinic and Alton Ochsner Medical Foundation, 1516 Jefferson Highway, New Orleans, LA 70121

Editor:

I read with considerable interest the article by Dr Claudon and colleagues in the January 2001 edition of Radiology (1). Dr Claudon and colleagues tested the technical settings of power Doppler ultrasonography (US) to evaluate stenoses by using a phantom. The authors concluded that power Doppler US cannot be used to measure stenoses accurately. I, too, have considerable interest in power Doppler imaging as a method to evaluate flow-limiting stenoses (2). I agree that power Doppler imaging is not the preferred method to measure a stenosis. In our recent article (2), we suggested that power Doppler imaging be used as a screening test, but not as a definitive test. Interestingly, we found that we were more accurate in identifying flow-limiting stenosis at the 60% level, or greater if we used parameters of power Doppler imaging to identify stenosis greater than 40%. Our empirical findings support the present article findings that power Doppler imaging, if used alone, may lead to underestimation of stenosis.

However, the authors suggested that "optimal settings (which should be used) were a gain of 30%, pulse repetition frequency below 1,500 Hz, and a low filter level." We used these parameters to scan some healthy individuals and found that we were unable to detect any flow in the common carotid artery. The parameters that we used to obtain adequate power Doppler information were a gain of approximately 70%, pulse repetition frequency of at least 1,500 Hz, and filter setting of medium to high (Figure). It is important to appreciate that there evidently is a considerable difference in optimal parameters in vitro and in vivo.

View larger version (129K): [in this window] [in a new window] [Download PPT slide]   Figure 1a. Sagittal US images of the same normal common carotid artery obtained with use of (a) parameters suggested by Claudon et al and (b) parameters suggested by Bluth et al.

View larger version (124K): [in this window] [in a new window] [Download PPT slide]   Figure 1b. Sagittal US images of the same normal common carotid artery obtained with use of (a) parameters suggested by Claudon et al and (b) parameters suggested by Bluth et al.

REFERENCES

1. Claudon M, Winninger D, Briançon S, Pesque P. Power Doppler US: evaluation of the morphology of stenoses with a flow phantom. Radiology 2001; 218:109-117.
2. Bluth EI, Sunshine JH, Lyons JB, et al. Power Doppler imaging: initial evaluation as a screening examination for carotid artery stenosis. Radiology 2000; 215:791-800.

Dr Claudon and colleagues respond:

Department of Radiology, University of Nancy, Hôpital de Brabois-Enfants, 54511 Vandoeuvre les Nancy, France, e-mail: michel.claudon@wanadoo.fr

We thank Dr Bluth for his letter regarding our experimental phantom study (1) findings on the role of power Doppler imaging in the evaluation of stenoses, which concludes that this mode cannot be used to measure stenoses accurately. We are aware of Dr Bluth’s interest in this field and of his recent article (2) on the results of a multicenter study on the evaluation of power Doppler imaging as a potential screening method for carotid artery stenosis. From empirical findings, Dr Bluth recognized some degree of underestimation of stenoses. This confirms the main result of our phantom study, which showed that the degree of stenosis is more or less underestimated in all cases. This underestimation of the degree of stenoses could partly explain a sensitivity of only 60% that Dr Bluth’s group achieved in identifying flow-limiting stenosis at power Doppler imaging.

The objective of phantom studies is to reveal advantages and limitations of a given technique. We fully agree with Dr Bluth that there may be a substantial difference between optimal parameters in vitro and in vivo. In the discussion section of our article, we clearly indicated that when a high-frequency transducer was used, "with the experimental conditions of the present study, which only mimic and do not duplicate in vivo conditions, optimal settings were a gain of 30%, pulse repetition below 1,500 Hz, and a low filter level" (1). Therefore, the combination of settings, found in our experimental study as optimal for the depiction of the morphology of stenoses, cannot directly be used to image in vivo vessels as Dr Bluth and colleagues did. For example, we mentioned that in our protocol, the block of US gel pad containing the stenosis was submerged in water. This condition is totally different from the in vivo situation because of the marked difference in attenuation between water and tissues. Consequently, the gain level needs to be increased to compensate for the attenuation from overlying tissues in vivo, as logically Dr Bluth did when he scanned a carotid artery (Figure, part b).

To support his contentions, Dr Bluth provided two images to demonstrate that he and his colleagues were unable to detect any flow in the common carotid artery when they used the settings from our experimental study. In addition, these two images give us the opportunity to further discuss the problems related to the wider use of the power mode in the evaluation of vessels and stenoses:

1. Dr Bluth and colleagues used a lower transmission frequency than we used in our experimental study (4–7 MHz versus 5–10 MHz). This may result in a different sensitivity to flow on the basis of fundamental Doppler principles.
   
2. The insonation angle that Dr Bluth and colleagues used decreased from approximately 70°–80° (Figure, part a) to 65°–70° (Figure, part b), as measured from their images. In our experimental study, the angle was found to be a nonsignificant parameter with a 10–5 MHz probe, but it was significant (P < .01) with a 4–2 MHz probe. Because Dr Bluth and colleagues used an intermediate emission frequency probe (7–4 MHz), the insonation angle might play a role and slightly decrease the signal level in Figure, part a, as compared with that in Figure, part b.
   
3. On the image (Figure, part b) obtained with the technical settings proposed by Dr Bluth and colleagues, the apparent diameter of the vessel, as measured from the power Doppler imaging data, progressively decreased from the left to right side of the same vessel, with a reduction of approximately 15%, as measured from this printed image. This could have been related to different factors, including the insonation angle, which increases from left to right, and the cardiac cycle phase, which can change the flow velocity distribution within the common carotid artery. Whatever the reason, such a variation of approximately 15% in the measurement of the common carotid artery diameter would influence the calculation of the degree of any associated stenosis.
   
4. Finally, this is an image of a normal common carotid artery. There is no evidence that with use of the parameters proposed by Dr Bluth and colleagues, an accurate depiction of both feeding and stenotic vessels could be provided for a large range of stenoses. We have shown that, under some conditions, a marked reduction in signal in feeding vessels compared with the signal in the stenotic segment mainly results from the wall filter response used in the system. This would lead to the underestimation of the degree of stenosis.
   

The objective of our experimental study was not to propose optimal settings, since they are subject to adaptation to patients, US units, and transducers. We only wanted to make radiologists and referring physicians aware of the risk of underestimating the degree of stenoses, even if this risk is lower with high-frequency transducers, which are currently used for carotid artery evaluation.

In their multicenter study, Dr Bluth and colleagues mentioned that "adjustments were made to the wall filter and pulse repetition frequency, depending on the patient’s respiration and vessel pulsatility, to achieve maximum depiction of the vessels." On the basis of our phantom study results and response to their comments and images, we believe that a more precise definition of all power Doppler imaging technical settings, as initially recommended by Bude and Rubin (3), is useful in making the evaluation of carotid artery stenoses more accurate and reliable.

REFERENCES

1. Claudon M, Winninger D, Briançon S, Pesque P. Power Doppler US: evaluation of the morphology of stenoses with a flow phantom. Radiology 2001; 218:109-117.
2. Bluth EI, Sunshine JH, Lyons JB, et al. Power Doppler imaging: initial evaluation as a screening examination for carotid artery stenosis. Radiology 2000; 215:791-800.
3. Bude RO, Rubin JM. Power Doppler sonography. Radiology 1996; 200:21-23.

This article has been cited by other articles:

				T. Wessels, J. U. Harrer, S. Stetter, M. Mull, and C. Klotzsch Three-Dimensional Assessment of Extracranial Doppler Sonography in Carotid Artery Stenosis Compared With Digital Subtraction Angiography Stroke, August 1, 2004; 35(8): 1847 - 1851. [Abstract] [Full Text] [PDF]

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