LETTER TO THE EDITOR
Reply
Anna S. John, MD
Royal Brompton Hospital, Cardiovascular MR Unit, Sydney Street, London, SW3 6NP, United Kingdom
Thorsten Dill, MD
a.john{at}rbh.nthames.nhs.uk
We agree with Dr. Friedrich and his colleagues that the technique we used in our study (fast low-angle shot [FLASH]) is not as new as the steady-state free-precession (SSFP)-based techniques. However, it would also be incorrect to assume that using SSFP techniques eliminates all potential errors with aortic valve area measurements. Both FLASH and SSFP techniques have their own sources of artefacts. While it is true, as Dr. Friedrich and his colleagues pointed out, that FLASH techniques are susceptible to signal loss as a result of spin-dephasing effects, if the same parameters are used for all subjects (including slice thickness, flow compensation gradients, echo time, repetition time flip angle), one can be reasonably confident in making area measurements as pointed out in our study. The SSFP techniques have less sensitivity to the dephasing effect, but they have different sources of artefacts, including sensitivity to areas of magnetic field disturbance (calcifications, tissue interfaces, and so forth) and sometimes severe artefacts in areas of highly complex flow. As a result of these differences, many use a combination of FLASH and SSFP techniques for magnetic resonance (MR) valve assessment in practice.
During study data acquisition (1), both FLASH and SSFP based sequences were tested. We found that the amount of artefact in the SSFP cines was often higher than in the FLASH cines, especially in aortic valve cross sections (2). We therefore decided to use FLASH rather than SSFP cines for planimetry of the aortic valve in the aforementioned study.
Perhaps the overall message to readers who may be considering using MR for valve area measurements should be the following: The appearance and size of signal voids on MR images are highly dependent on a number of parameters of the pulse sequence chosen. Therefore, a FLASH or SSFP sequence at one institution or scanner cannot be immediately assumed to be the same as another institution or scanner. It is prudent in all cases to attempt to replicate the parameters reported in the literature, but then to perform a small validation study before adopting a new technique with confidence.
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References
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- John AS, Dill T, Brandt RR, et al. Magnetic resonance to assess the aortic valve area in aortic stenosis: how does it compare to current diagnostic standards? J Am Coll Cardiol. 2003;42:519–526[Abstract/Free Full Text]
- John AS, Kilner PJ, Mohiaddin RH, Lorenz CH, Pennell DJ. Comparison of TrueFISP and FLASH in valve disease. (abstr)J Cardiovasc Magn Res. 2001;3:84–85
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