Visual Grading Characteristics (VGC) Analysis of Diagnostic Image Quality for High Resolution 3 Tesla MRI Volumetry of the Olfactory Bulb

Rationale and Objectives

The aim of this study was to evaluate the suitability of 3-T magnetic resonance imaging (MRI) for olfactory bulb volumetry, comparing image quality obtained using different sequences on the basis of physical characteristics in combination with observer performance.

Materials and Methods

Twenty-two healthy volunteers (11 men, 11 women; mean age, 25 years) underwent 3-T MRI of the frontal skull base in this prospective study. Imaging was performed using a constructive interference in steady state (CISS) three-dimensional Fourier transformation sequence, a three-dimensional T2-weighted (3D-T2w) sequence, and a two-dimensional T2-weighted (2D-T2w) sequence. The relative performance of sequences was assessed using visual grading characteristic analysis. Interobserver agreement was assessed using κ statistics. For evaluation of physical image quality characteristics, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were calculated and compared using Wilcoxon’s test. SNR and CNR measurements were correlated with visual grading results.

Results

Visual grading characteristic analysis showed significantly better image quality ratings for the CISS sequence compared to the 3D-T2w and 2D-T2w sequence, and the 2D-T2w sequence performed significantly better compared to the 3D-T2w sequence ( P < .001). Interobserver agreement was substantial (κ = 0.66–0.73). Wilcoxon’s test revealed significantly higher SNR and CNR values for the 2D-T2w sequence compared to the 3D-T2w and CISS sequences, and SNR and CNR values for the 3D-T2w sequence were significantly higher compared to those for the CISS sequence ( P < .001 for each). Significant correlation between SNR and CNR and visual grading was found for the 3D-T2w sequence (SNR: ρ = 0.510, P = .015; CNR: ρ = 0.546, P = .009).

Conclusions

High-resolution 3-T MRI resulted in excellent values for SNR and CNR, suggesting the appropriateness of the sequences for olfactory bulb MRI volumetry. Visual grading characteristic analysis revealed the CISS sequence to be the most suitable for olfactory bulb volumetry.

The topic of olfactory bulb (OB) volumetry is becoming increasingly important in understanding olfactory dysfunction and has already been used as a complementary prognostic tool for radiologic diagnosis, to predict outcomes in olfactory disorders , or as a marker in neurodegenerative disorders such as Alzheimer’s disease . The OB is the first cerebral olfactory structure, which processes afferent information from the olfactory receptor neurons . Furthermore, the human OB retains its ability to renew cell population, because in the OB, progenitor cells differentiate into neurons until well into adulthood . Therefore, the OB exhibits high structural plasticity, whereby OB volume is correlated with afferent neural activity . Dynamic volumetric changes have been demonstrated for different etiologies of smelling disorders. A decrease in volume has been verified for trauma , infection , idiopathic olfactory disorders , schizophrenia , Alzheimer’s disease , depression , and sinonasal disease . One recent study also showed an OB volume increase after successful olfactory rehabilitation in sinonasal diseases . Additionally, a positive correlation of OB volume with age-related olfactory function has been reported .

Magnetic resonance imaging (MRI) of the olfactory system became possible in the late 1980s . Further development of MRI techniques, including improvements in spatial resolution, enabled the assessment of OB volume . Although MRI is a feasible method for OB volumetry , further studies at 1.5 T were inconsistent in the application of the sequence types (eg, T1-weighted [T1w] vs T2-weighted [T2w]) and differed in numerous scanning parameters (eg, slice thickness) or the use of technical equipment (eg, surface vs head coils) . Data from a systematic comparison of sequences for detection of human olfactory nerve anatomy at 1.5 T are limited. As a result, a reference standard in imaging for OB MRI volumetry has not been established. The limited resolution gained at 1.5 T complicates the comparative quantification of volumetric results of this small paleocortical structure. One major advantage of high-field MRI (3 T) is an increased signal-to-noise ratio (SNR). This higher SNR can be invested in better spatial resolution . Until now, a systematic radiologic comparison of sequences for OB volumetry at a field strength of 3 T has not been performed.

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Materials and methods

Participant Recruitment and Sampling

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Imaging Procedures

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Data Analysis

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Table 1

Ordinal Scale Step Categorization for Visual Grading Analysis

Step Image Quality Limitations for Volumetry 1 Excellent No limitations 2 Good Minimal limitations 3 Sufficient Moderate limitations, no substantial loss of information 4 Restricted Relevant limitations, clear loss of information 5 Poor Image not usable, loss of information, image must be repeated

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SNR¯¯¯¯¯¯¯=1n∑ni=1SICSFiσi, SNR

¯

=

1

n

∑

i

=

1

n

SI

i

CSF

σ

i

,

where SI CSF is the SI of CSF in the OB cistern, and σ is the SI standard deviation of the pure image noise.

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CNR¯¯¯¯¯¯¯=1n∑ni=1∣∣SICSFi−SIOBi∣∣σi, CNR

¯

=

1

n

∑

i

=

1

n

|

SI

i

CSF

−

SI

i

OB

|

σ

i

,

where SI CSF is the SI of CSF in the OB cistern, SI OB is the SI of the OB, and σ is the SI standard deviation of the pure image noise.

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Statistical Analysis

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Results

Participant Recruitment and Sampling

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VGC Analysis

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Physical Measurements of Image Quality

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Discussion

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Conclusions

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