Comparing MRI pulse sequences for a specific clinical task

As our utilization of MRI in cervical spine trauma increases, we read with interest the recent article by Killeen et al. [1] entitled ‘‘Inversion recovery versus T2weighted sagittal MR imaging in cervical spine cord injury.’’ Examination of the tissue contrast in the images contained in this article reveals that the images labeled ‘‘FLAIR’’ do not have the typical properties demonstrated by the FLAIR pulse sequence [2, 3]. On the basis of the images and the text, we initially concluded that one of three possibilities existed: (1) the inversion times were not set for FLAIR imaging; (2) no inversion pulse was applied; (3) a publication error occurred which was undetected at the galley proof stage, leaving images mislabeled ‘‘FLAIR’’ (despite the fact that the authors cite only FLAIR articles in the references). A brief, personal communication with the senior author of the article in question revealed that the images referred to as FLAIR were in fact short tau inversion recovery (STIR) images, in accord with our first suspicion that the inversion time was improperly set for FLAIR imaging. Our main purpose in writing you, however, is not to single out an error made by our colleagues. Instead, we would like to address the essential elements needed for writing useful articles which compare different magnetic resonance imaging (MRI) pulse sequences, particularly since MRI will likely be the modality which sees the most increase in utilization among emergency radiologists. In addition, we feel that a discussion of the physics behind the tissue contrast and signal-to-noise properties for inversion recovery sequences in general would also be of use to the readership, particularly as it applies to the characteristics of the images in the Killeen article. Comparing MRI pulse sequences for specific clinical tasks is a worthy and important step in improving the utilization of MRI. Such endeavors have a long history and many articles can be found in the current literature [4, 5]. These articles share a fundamental principle: reporting the precise parameters associated with every sequence tested. Since the interpretation of tissue contrast requires an understanding of the interplay between the pulse sequence (with its parameters) and the underlying tissue relaxation times (T1, T2, and T2*), this interpretation is rendered impossible without a complete set of imaging parameters. Furthermore, the tendency to use manufacturer-supplied acronyms for a pulse sequence hinders an interested reader from reproducing results or even using the same technique(s) on different manufacturer platforms. As the definition of the acronym suggests (‘‘fluidattenuated inversion recovery’’), FLAIR images are intended to demonstrate attenuated signal from fluid. The standard implication for neuroimaging is moderate to heavy T2-weighting accompanied by the presence of black cerebrospinal fluid (CSF) which appears bright on T2-weighted images. None of the images in the article demonstrates FLAIR tissue contrast. For example, the CSF is bright in Figs. 1 and 2. An interested reader will consider possible reasons for this ‘‘unFLAIR’’-like tissue contrast by examining the specific sequence parameters used in the acquisition. However, the precise parameters associated with every sequence tested are not reported, including TR, inversion time (TI), TE (or effective TE), specifics of the actual imaging module (e.g., whether fast spin echo or conventional spin echo was used), image matrices, field of view, number of slices, slice thicknesses, and overall scan time. (Scan times are particularly important in judging a sequence and, if Emergency Radiology (2002) 9: 178–180 DOI 10.1007/s10140-002-0215-x