Harmonic power mode Doppler using microbubble contrast agents: an improved method for small vessel flow imaging

Abstract

By virtue of' their ahility to enhance the weak echoes received from flow in small blood vessels. ultrasound contrasl agents render flow in a lower level of the circulation detectahle to colour Doppler imaging. Contrast enhanced colour Doppler is, however. prone to the artifacts of "blooming". a consequence of amplitude thresholding of the colour display and multiple aliasing, due the low pulse repetition frequencies needed to detect flow in sniall vessels.We combine contrast imaging with power Doppler, which is a more appropriate colour mode for contrast enhanced Doppler imaging. The power mode itself, though, imposes additional limitations which can destroy its ahility to denionstrate small vessel flow in a clinical setting. in which solid tissue moves.Power Doppler is niore susceptihle to the "flasli" artifact hecause of the high power associated with echoes from nioving solid structures. an artit'act exaccrhated hy the widespread use 0 1 frame averaging in power niode. This imposes a lundaniental limitation on the ahility of a conIrast agent to dcrnonslratc llow in small vessels wilhin the ahdomcn, lor example. In this study. we eniploy harniofi Doppler as a method to overcome these limitations. We dcnionstrale that the comhination of harnionic Doppler and the enliancenient (11 the Doppler signal by the contrast agent produces an in vivo gain o f more than 30dB in ( h e signal-io-clutter ratio. Harinonic power Doppler iniaging dernonslratcs llow in Ihe cortex of the kidney lo within 2nini ol the capsule. even in the presence of respiratory morion. Histological correlation t i l the Doppler images wilh sections of the cortical vasculature suggest that llow is detected in intcrlohular arterioles o l mean dianieter 44pni. We conclude Ihat his new nielhod holds promise lor the clinical detection o f sniall vessel flow i n moving solid tissue. INTIIOI )UCTI( )NSus pc nsic i 11s o 1 s I ~i hi I i red ni i c r( o h ti h h Ics 11 se d as u I lr;isou lid contrast agcnls have hcen shown lo he capahle 01 producing enhancements o l the hackscaltcrcd echo I'roin arterial hlood of hetween I O and 25 dB [ I ] tollowing injcclion i n very sniall quantities into a pcriphcral vein. The cl~lect o n spectral and colw~r Doppler is dramatic. with signals lroni Ilow in vessels too small or too deep. and, therefore, hilhcrto undctcctahle. heing rendered visihlc on colour Ilopplcr iniaging 121. Two nia.ior prohlenis, however. conlronl small vcsscl Ilow iniaging using colour Doppler and echo-enhancing agents.First, the ellect o f the arrival o f Ihc contrasl ;igenl i n a colour region 01 interest is t t i produce "hltroniing" 01. tlic colour iniage, whcrehy signals Ironi m j o r vascular largets spread our to occupy the entire region. Thus. allliough Ilow from sinallcr vessels may he dcleclahlc. their colour images niight he swamped hy anitactual tlow signals. The origin o l this artilact is the amplitude tliresholding that govcriis niosl colour displays in conventional ( o r 'velocity') mode imaging, Increasing the hackscallcrctl sig...

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