Congratulations to Amir Manbachi

My bad for being a couple of months late is posting this: Congratulations to Amir Manbachi on the successful defense of his Master's thesis, "Characterization of common carotid artery geometry and its impact on velocity profile shape". Amir is now doing a PhD on the topic of ultrasound-guided therapy, and is also patiently waiting for me to get my act together so we can submit a paper on his Master's work!

Paper published in J Biomech Eng

In 2008 we demonstrated a significant correlation between certain geometric variables and the amount of disturbed flow at the carotid bifurcation. Now working with Qi Zhang and Mort Friedman -- yes, that Mort Friedman, the father of the geometric risk hypothesis -- we extended and improved these correlations in "Use of factor analysis to characterize arterial geometry and predict hemodynamic risk: application to the human carotid bifurcation." To me the strength of factor analysis is not just its ability to rationally combine variables to create factors that improve correlations, but also the ability to identify why certain variables (in the present case, bifurcation angle) are not correlated with disturbed flow. Less directly, the debates we had about why certain geometric factors are better correlated with disturbed flow helped inspire us to identify better geometric variables (to be presented at SPIE Medical Imaging 2011), which can only lead to better factors, etc. And in case you didn't notice the subtle use of bold text, yes there is an issue of terminology that I'm still trying to get used to!

Paper published in Ultrasound Med Biol

In 2005 we showed how Doppler ultrasound spectra could be synthesized in real-time from CFD data, provided that simplistic assumptions were made about the sample volume (SV) power distribution and intrinsic spectral broadening (ISB). In the first paper arising from Luis Aguilar's doctoral thesis work, "On the Synthesis of Sample Volumes for Real-Time Spectral Doppler Ultrasound Simulation", we show how acoustic monopoles can be used to overcome the SV assumption. For Luis' next trick, he will show how this approach can also be used to overcome the ISB assumption, and probably much, much more...

Too many papers, too little time...

Doing a literature search this morning, I stumbled across "The reliability of high resolution MRI in the measurement of early stage carotid wall thickening" by Boussel et al. What struck me immediately was their Figure 2 (left), which nicely shows that MRI tends to overestimate normal wall thickness compared to ultrasound intima-media thickness (IMT) measurements, something the authors said was "potentially due to the inclusion of the adventitia by MRI".

As it turns out, I had downloaded a copy into my Papers database on January 5, 2008. Why is this relevant? Because, in Febrauary 2008 we submitted a paper entitled "On the overestimation of early wall thickening at the carotid bulb by black blood MRI...". Looking at their figure now (and probably then), I noticed how similar it looked to our predictions. So now, almost two years later, I plotted their data (hand digitized using a great shareware program, getdata) over the data from Figure 8 of our paper. As shown to the right, the trends are indeed remarkably consistent.

There are some discrepancies, of course. Boussel et al.'s data were acquired with 0.6-mm pixels, but would seem to fall between our predictions of 0.3 - 0.5 mm pixels. This may be because their images were segmented manually, whereas our predictions were based on automated edge-detection. (Thanks to Bill Kerwin from the University of Seattle for later explaining to me why the latter probably overestimates the problem relative to the former.)

It would have been nice to have included this comparison in our paper, for it bolsters our conclusion that the spatial resolution of MRI, rather than the adventitia, is the culprit. Oh well, better later than never! But, and per the title of this post, there are just too many papers out there to be able to read and remember them all. My Papers database has 3446 PDF files as of today, most of them probably relevant to my research, but the vast majority probably inhaled and forgotten like a fast food dinner, rather than savoured and remembered like a gourmet meal. A weak metaphor perhaps, but maybe science could do with a "slow food" movement...

Shit my dad says about radiology

For those of you disconnected from mainstream media, Shit My Dad Says -- I refuse to sanitize the title -- documents the pearls of wisdom passed down from a father to his son over the years. Being Twitter-free and Facebook-phobic, I only heard about this a few weeks ago from my daughter, who is a devoted follower, and then a few days ago I received the recently-published book, unsolicited, from my father as a birthday present. (Hmm, I wonder what this says about me as a father and as a son, Dr. Freud?)

What does this have to do with Biomedical Simulation? Well, a few minutes on Google and then PubMed reveals that the father, introduced in the book as having worked "in nuclear medicine at the University of California-San Diego", is indeed a former academic radiologist at UCSD, specializing in nuclear and adolescent(!) medicine, with quite a few papers in journals that I read. (OK, it's a stretch, but allow me this tenuous brush with greatness.)

All this to say: if you don't believe there's wit and wisdom behind the media hype, check out Samuel E. Halpern's editorial "Of Models and Men" from 1977. They don't write 'em like that anymore!

All hail Lewis Fry Richardson (1881-1953)

First time I came across his name was indirectly, as an undergrad, learning about something called "Richardson extrapolation", a method for inferring the convergence of a numerical analysis. Then, much (much) later, as I was rethinking turbulence in blood, I came across Richardson as one of the pioneers of modern turbulent flow theory, and as the author of this unsurpassedly clever and concise description of turbulence:

Big whorls have little whorls
that feed on their velocity,
And little whorls have lesser whorls
and so on to viscosity.

Shortly thereafter -- OK, so I didn't pay close attention to my reading of Gleick's "Chaos" many years back -- I learned about Richardson's anticipation of fractals via his musings on the measurement of coastlines, something that arose out of his interest in divining the mathematical rules underlying human conflict; and of his anticipation of chaos theory through his pioneering work in weather prediction. By virtue of what some might call his mania for the latter, he arguably founded modern numerical analysis and finite difference methods.

If that weren't enough, thanks to my reading of Abigail Swillens' fine PhD thesis, I learned that Richardson filed the first patent for underwater echo-ranging (apparently inspired by the sinking of the Titanic), a precursor to sonar and, by extension, medical ultrasound.

In short, virtually everything I do in my research can be traced back, with far fewer than six degrees of separation, to Lewis Fry Richardson. For more about Richardson's life and works, there's the at-your-fingertips Wikipedia of course, but also a nice 1998 review in the Annual Reviews of Fluid Mechanics.

Paper published in J Biomech Eng

Not long ago we reported differences in the characteristic shape of older vs. young adult carotid flow rate waveforms, which got us wondering whether and how such age-related differences might affect predictions of disturbed flow. In "Carotid bifurcation hemodynamics in older adults: effect of measured versus assumed flow waveform", we went a step further by testing the impact of not only waveform shape, but also mean flow rates, using the actual subject-specific measured flow condition as a gold standard. As it turned out, errors in mean flow had a greater impact on nominal metrics of disturbed flow, particularly oscillatory shear index (OSI); however, these effects were on the order of those due other assumptions we typically make regarding, say, blood rheology or inlet velocity profiles. Take-home message: don't sweat the waveform shape, but if you can measure the mean flow rates, go for it!