In Vitro Comparison of Water Displacement Method and 3 Tesla MRI for MR-Volumetry of the Olfactory Bulb

Rationale and Objectives

Magnetic resonance imaging olfactory bulb (OB) volumetry (OBV) is already used as a complementary prognostic tool to assess olfactory disorders. However, a reference standard in imaging for OBV has not been established. The aim of this in vitro study was to compare volumetric results of different magnetic resonance sequences for OBV at 3 T to genuine OB volumes measured by water displacement.

Materials and Methods

The volumes of 15 human cadaveric OBs were measured using the water displacement method in this institutional review board–approved prospective study. The magnetic resonance imaging protocol at 3 T included constructive interference in steady state (CISS), T2-weighted (T2w) three-dimensional (3D) sampling perfection with application-optimized contrasts using different flip-angle evolutions (SPACE), T2w two-dimensional (2D) turbo spin-echo (TSE), and T1-weighted (T1w) 3D fast low-angle shot (FLASH) sequences. Two blinded observers independently performed two OB volumetric assessments per bulbus and sequence. Intraobserver and interobserver reliabilities were assessed by intraclass correlation coefficients. Bland-Altman plots were analyzed to evaluate systematic biases and concordance correlation coefficients to assess reproducibility.

Results

For both observers, intraclass correlation coefficient analysis yielded almost perfect results for intraobserver reliability (CISS, 0.94–0.98; T2w 3D SPACE, 0.93–0.98; T2w 2D TSE, 0.98–0.98; T1w 3D FLASH, 0.95–0.99). Interobserver reliability showed almost perfect agreement for all sequences (CISS, 0.98; T2w 3D SPACE, 0.89; T2w 2D TSE, 0.93; T1w 3D FLASH, 0.97). The CISS sequence yielded the highest mean concordance correlation coefficient (0.95) and the highest combination of precision (0.97) and accuracy (0.98) values. In comparison with the water displacement method, Bland-Altman analyses revealed the lowest systematic bias (−0.5%) for the CISS sequence, followed by T1w 3D FLASH (−1.3%), T2w 3D SPACE (–7.5%), and T2w 2D TSE (−10.9%) sequences.

Conclusions

Compared to the water displacement method, the CISS sequence is suited best to validly and reliably measure OB volumes because of its highest values for accuracy and precision and lowest systematic bias.

Olfactory bulb (OB) volumetry (OBV) has become a growing topic in the field of understanding olfactory dysfunction, for which studies have indicated a prevalence of about 20% in the Western world . OB volume decreases have been identified in all of the five most frequent etiologies of smelling disorders (ie, sinonasal disease , postinfectious , posttraumatic , neurodegenerative diseases , and idiopathic olfactory disorders ). However, volume increases have also been shown (eg, after successful olfactory rehabilitation in sinonasal diseases ), which exemplifies the reversibility of OB volume changes. These dynamic changes are most probably due to the fact that the human OB retained the ability of neuroneogenesis and therefore exhibits high structural plasticity, in which its volume is correlated to afferent neural activity transmitted by the olfactory receptor neurons . OBV has already been used as a complementary prognostic tool for radiologic diagnosis to predict outcomes in olfactory disorders .

In 1997, Yousem et al described magnetic resonance (MR) imaging (MRI) as a feasible method for OBV on the basis of T1-weighted (T1w) sequences at 1.5 T. Further development of MRI techniques and the use of MRI at higher field strength (ie, 3 T) could facilitate the quantification of volumetric results of this small paleocortical structure because the increased signal-to-noise ratio can be invested in better spatial resolution . Therefore, future studies dealing with the subject of high-resolution MR OBV should be based on standardized imaging, among other factors, for compatibility reasons. However, until now, a systematic radiologic comparison of various standardized sequences for OB volumetric purposes, which logically justifies a reference standard in imaging for upcoming studies, has not been finalized.

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Materials and methods

Subject Recruitment, Sampling, and Experimental Design

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Imaging Procedures

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Imaging Data Analysis

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Statistical Analysis

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Results

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Table 1

CCCs, Interobserver Reliability, and Reproducibility ( n = 15)

Sequence CCC (ρ c ) 95% CI Pearson’s ρ (Precision) Bias Correction Factor c b (Accuracy) CISS First evaluation 0.92 0.80–0.97 0.95 0.97 Second evaluation 0.90 0.73–0.96 0.91 0.99 Average 0.95 0.88–0.98 0.97 0.98 T2w 3D SPACE First evaluation 0.70 0.41–0.86 0.84 0.83 Second evaluation 0.83 0.60–0.93 0.90 0.92 Average 0.80 0.57–0.91 0.91 0.88 T2w 2D TSE First evaluation 0.85 0.63–0.94 0.86 0.99 Second evaluation 0.83 0.58–0.94 0.84 0.99 Average 0.85 0.63–0.95 0.86 0.99 T1w 3D FLASH First evaluation 0.93 0.82–0.98 0.94 0.99 Second evaluation 0.89 0.70–0.96 0.89 0.99 Average 0.94 0.84–0.98 0.94 0.99

CCC, concordance correlation coefficient; CI, confidence interval; CISS, constructive interference in steady state; FLASH, fast low-angle shot; SPACE, sampling perfection with application-optimized contrasts using different flip-angle evolutions; 3D, three-dimensional; T1w, T1-weighted; TSE, turbo spin-echo; T2w, T2-weighted; 2D, two-dimensional.

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Discussion

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Conclusions

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Acknowledgments

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In Vitro Comparison of Water Displacement Method and 3 Tesla MRI for MR-Volumetry of the Olfactory Bulb

Rationale and Objectives

Materials and Methods

Results

Conclusions

Materials and methods

Subject Recruitment, Sampling, and Experimental Design

Imaging Procedures

Imaging Data Analysis

Statistical Analysis

Results

Discussion

Conclusions

Acknowledgments

References

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