Corrected BPD or Head Circumference?

Background:

There was an article about Corrected-BPD measurements being published in the ASA journal, so I thought I would enliven the debate with a bit of a steaming rant...  I had been working in a tertiary hospital and all these really bad scans were being referred to me (and the Prof) for rescans, and so I was pretty jack of poor technique at this point in time.  Beware, if you caught me for a DMU Prac Exam round this time!  

I expected to be torn to shreds on this article, but no-one was game to answer back.  If this had been posted on the OBGyn.net list (if I knew about it then...) I would have had my goolies in a vice sooner than you could say axial-corrected BPD!

Good quotations though!

 

 

 

 

We measure shadows, and we search amongst ghostly errors of measurement for landmarks that are scarcely more substantial.            Edwin Hubble, Astronomer.

 Donít lie if you donít have to.            Leo Szilard, Nuclear Physicist.

 

I can remember the arguments/discussions going on in our ultrasound room from 1977 on, about how to perform the bi-parietal diameter (BPD) measurement. [I was "Student Radiographer" at the time.  It was Trevor Beckwith, Gerald Hayward, Tom Dominikovich and Peter Motteram having these discussions.]  The purists in those days used to be adamant that a good BPD could only be performed on the A-mode oscilloscope.  The terminology was "leading edge to leading edge", as we didnít want to introduce error due to poor axial resolution.  Then there were discussions about which linear echoes on the B-mode image actually constituted the falx and which were the septum cavum pellucidum (the WHAT?), or was that the 3rd ventricle? Someone countered that the landmarks donít actually matter, as the BPD should be the widest section of the skull wherever that level may be found.  Then there were those measurements done with a map-reader on Polaroid prints.  Then we had a character generator (primitive computer) on which we could perform calibrated measurements.  Calibrated is the word... not!  Aspect ratio, video distortion, light-pen accuracy, etc and so on: these were part of the everyday potential errors.

A lot of the controversy about this measurement was derived from its significance in pregnancy dating, and the potential for assessing growth disturbances later on, plus being a trigger for the detailed search of fetal malformations, particularly intracranial stuff.  Now we have other methods for assessing much of this.  We look directly at much more of the fetus, so much so that the 18 week scan has been compared in its completeness to the detail of an autopsy.  We have various Doppler and AFI and serial AC measurements for growth disturbances.  Is the magic wearing off the BPD?  I hope so. Well, whatever criteria you use, measurement of the BPD was claimed as being the most accurate measurement of gestational age known to sonographer-kind with an error of one week to 10 days.  In fact, more and more institutions are instituting (hence their name) a first trimester scanning policy utilizing the Crown-Rump length, not only because of the increased accuracy of 5 days to 1 week (and they say thereís no such thing as progress) but also because of the current measurement du jour , the nuchal translucency.  This hopefully will make BPD discussions such as this one as obsolete as mediaeval debate about the number of angels that could dance on the medulla oblongata of a pin.

Lets us look at some of the causes of error in BPD measurement, and try to determine whether the lack of a corrected BPD measurement is a significant cause of fetal death, decreased caesarean rates (God forbid!), missed golf games, matricide, dogs and cats living together, other abominations, etc.  What are the causes of error in ultrasound?  This is a highly personal list and should be used in evidence against me at any time. 

Numero uno is measurement botch-ups.  Errors of scan plane selection, faults of measurement obliquity, sins of caliper placement, minuscule images where the BPD is one tenth of the screen size (minify the image and you magnify the potential for error), excess gain ballooning the white bony wchoes, etc... these are a few of my favourite reasons for wrong-headedness.  (Top pun, that!)   But is this really as bad as I make out?  Is it not inevitable that variations of up to 2mm (1 weeks or more) are not uncommon even in fastidious scanning in the third trimester?  Have inter- and intra-observer error studies been performed in every scanning lab?  Are variations on one patient averaged out or is the second measurement affected by the first?   How accurate are our calipers anyway?  Nicolaides only uses 1mm measurement increments in his nuchal translucency charts to overcome our unconfirmed trust in our machines' accuracy. 

What about the effects of the type of chart you are using?  The ASUM chart is pretty much accepted in Australia, but some early criticisms suggested a preponderance of European descent, middle class, Carlton residents.  What about genuine biological variability?  What about variation due to fetal sex?  Fetal position?  Does the chart you are using reflect your patient population, based on racial or other characteristics?   Is your chart based on outer to inner or and outer to outer measurements? - yes, they exist.  Who actually did the measurements in the study which you use for your charts - where they experienced sonographers, or first-year research fellows?  Was it a multi-center study where quality control could be an issue?  Even experienced sonographers make mistakes, but inexperienced sonographers make a lot more.  Were the charts longitudinal or cross sectional studies?   What was the number of data points used?  Were at-risk or abnormal fetuses excluded?  How was the gestational age assessed?   Was the GA averaged up to the nearest whole week (yes, it happens)?  Was the raw data used to derive the charts?   Was the data collected prospectively or retrospectively?  Was the regression curve correctly derived?   Are you using the 5th and 95th centiles, or two standard deviations?

Head circumference (HC) has been recently shown to more accurate and reliable than the BPD.  This HC was better if based on direct measurement rather than calculated from the BPD and OFD (the derived HC).  What does this say?  It might be inferred that the OFD is probably not a reliable measurement.  Most noticeably, when the same image as the BPD is used to measure the OFD, refractive shadowing and speed of sound artifacts result in obvious subjectivity in the placement of the calipers at the occipital end.  Using a second image for OFD has been suggested, a rotated image where the occiput is well shown, but this raises other points.  Were the techniques used to generate the corrected BPD and HC charts based on this method of measurement?  If not, can they reasonably be expected to apply?  Also it is hard enough getting people to look properly at the fetal intracranial structures, let alone do two images just to get one measurement (the corrected BPD.)   The corrected BPD is actually very close in accuracy to the derived HC because mathematically they are the same, the difference only being a multiplying constant.  Therefore using both corrected BPD and derived HC is a redundant exercise.  Also, the derived HC should be done with an outer to outer BPD, so donít forget that potential error.

Measurements corrected for head shape certainly reduce error, but mainly in the later parts of the second trimester and in the 3rd trimester.  For accurate dating up to the 18week scan, studies have shown that the BPD is as accurate for practical purposes as the corrected BPD or HC, but with a slightly wider standard deviation (+/- 8 days as opposed to +/- 7 days.)   Abnormal head shape will certainly affect the BPD measurement, but is it significant enough to warrant the use of the corrected (however well) BPD at 18 weeks?  Hadlock does not think so.  He advocates this technique only when the cephalic index (but how reliable is that?) is more than 1 standard deviation beyond the normal range, but is aware that it is no better than HC.  And what about head shape where the distortion is superior, like a cone-head type baby. (Have you seen my baby snaps?)  No-one ever corrects for that! [except Terry duBose, except him!]

I am not aware of any studies performed to assess if the difference in these measurements has any clinical significance.  Most studies have shown that obstetrical ultrasound in general is an expensive waste of time, and possibly harmful, given the false reassurances of normality in the woefully performed scans documented in the RADIUS trial.  The pedantic conclusions of our BPD discussion here are in fact so much obscure hermeticism.  

Conclusion:  Corrected BPD measurement is in my opinion not worth doing because the majority of sonographers couldnít do a decent BPD to begin with if their jobs depended on it (luckily they donít, but they should). Individual errors of technique are probably far more significant than those of slight head shape variation (within the normal range of CI), plus there are the multitude of potential errors in the charts you are using.  Corrected BPD is the same measurement as derived HC anyway!  The directly measured HC may indeed be the best measurement after the 2nd trimester, but at 18 weeks it probably does not make enough difference to alter pregnancy management.

Phillip L Ramm

References:

F. Hadlock, Ultrasound determination of menstrual age, in Callen P., Ultrasonography in Obstetrics and Gynecology 3rd Ed. 1994, Saunders.

 Royston P, Wright EM; How to construct Ďnormal rangesí for fetal variables. Ultrasound Obstet Gynecol 1998;11:30-38

 Altman DG, Chitty L; New charts for ultrasound dating in pregnancy. Ultrasound Obstet Gynecol 1997;10:192-197

  Altman DG, Chitty L Charts of fetal size: 1.Methodology.  Br J Obstet Gynaecol 1994;101:29-34