Without applying the proposed spectral broadening correction method, UD could significantly overestimate (see ) all three parameters (PSV, MV EDV) by 26.6 cm/s (or 29.0%), 13.4 cm/s (or 28.2%) and 7.2 cm/s (or 25.2%), respectively. The overestimation of PSV by the pre-correction UD data could be further confirmed by comparing to the MRA measurements as shown in . More specifically, as compared to the MRA measurements, the pre-correction UD data would have significantly overestimated the peak systolic velocity values by 29.2 cm/s. This observation is consistent with those of other studies, where overestimations by UD have been as high as 47% (Wendt et al., 1992
; Hoskins, 1996
). The potential for this inaccuracy is illustrated well in one of the waveforms investigated. At cross-section 3 in subject D (Figs. and ), UD overestimated the velocities by as much as 60% compared to both the PC-VIPR MRA and the velocities predicted by “animal-specific” CFD simulations.
In , we found stronger spatial velocity gradients near the cross-section D-3 as compared to the other two measurement sites (D-1 and D-2). Although we observed that the complexity of the flow streamlines (i.e.
the presence of helical flow and recirculation zones) in and around the bifurcation aneurysm increases significantly compared to the upstream parent artery and further downstream away from the bifurcation aneurysm (), no turbulence was identified. Thus, we stipulate that the presence of strong velocity gradients such as this causes significant spectral broadening (Campbell et al., 1989
) at the cross-section D-3, though the actual cause of this discrepancy could be multi-factorial and is unknown. It is likely that the simple correction for Vcorr
is inadequate for these extreme conditions, leading to ultrasound Doppler derived velocities that significantly overestimated (60%) the true velocity for Section 3 in Subject D (Hoskins, 2002
It is also interesting to note that there were only weak correlations for the PI (r=0.35 and 0.25 for the pre- and post-correction UD data, respectively) and the RI (r=0.23 and 0.14 for the pre- and post-correction UD data, respectively) determined from the UD and PC-VIPR MRA measured velocity waveforms. Since the PI and RI have been used for various clinical applications to assess the resistance of blood flow (Petersen et al., 1997
; Pinggera et al., 2008
), further investigations are warranted to delineate conditions for which such parameters lead to erroneous results.
Doppler ultrasound is the method of choice for evaluation of blood flow in many clinical applications e.g.
carotid artery evaluation. Its advantages over other techniques include its portability, real-time imaging capability, high temporal resolution and, instant information on peak velocities under various physiologic conditions. Improvements in ultrasound flow imaging are ongoing (Hoskins, 2002
; Ivancevich et al., 2008
; Haworth et al., 2008
). Any assessment of the potential clinical utility of these and other new developments in velocity assessments using ultrasound requires that they be evaluated and compared in vivo
against state of the art PC-MRA techniques (Gu et al., 2005
; Wigstrom et al., 1996
; Markl et al., 2003
; Johnson et al., 2008
) or other gold standards. To our knowledge, this is the first study to compare in vivo
ultrasound Doppler measurements with results from a 4D accelerated PC-MRA technique, in this case the PC-VIPR approach.
As proposed by Steinman and colleagues, the incorporation of ultrasound and PC-MRA data that combine “subject specific” geometry information and velocity waveforms into computer flow models for simulation of hemodynamics (Steinman et al., 2003
) allows calculation of expected/idealized ranges of flow velocities. We recognize that neither CFD-calculated (Roache, 1997
) or 4D PC-MRA measured flow is not sufficient to serve as the gold standard to validate the UD measurements. However, they do, in our opinion, serve as tools for performing valid comparisons of measured flow parameters. This is important, especially for in vivo
flow determinations where validation of the accuracy of techniques is incomplete. Our work explored the use of this approach.
We also demonstrated that corrections for spectral broadening are needed in order to obtain reliable ultrasound Doppler measurements (see Tables and ), especially for peak systolic velocity measurements. While the proposed simple correction scheme could be easily used in a clinical setting since all parameters used in this study are readily available to clinicians, broad application of the ultrasound Doppler approach may be more limited since our measurements included only superficial arteries studied using a high-frequency linear array transducer. For this preliminary study, we also limited our effort to investigate the correlations between MRA and ultrasound velocity parameters and waveforms to only a small number of animals. In the future we will extend our effort to obtain volumetric flow information (e.g. flow rate) using 3-D ultrasound flow imaging to compare these to measurements made by the PC-VIPR MRA technique. Further studies are needed to validate Doppler ultrasound use with other transducers and other scanning conditions.
Besides the small number of animals (14 waveforms in 4 subjects), another limitation of this study is that the locations of flow measurements were manually aligned using anatomical features. The accuracy of such alignment was not verified in this study. In future work, fiducial markers both visible under Ultrasound and MR will be implanted for more precise and accurate alignment.