Diagnosis of Parkinsonism Using Nigrosome 1 Imaging at 3T: Comparison of Interobserver Agreement between GRE Magnitude Images (MEDIC) and Susceptibility Map-weighted Images (SMWI)
Eung Yeop Kim1, Yoon Ho Nam2, Young Noh3, Young Hee Sung3, Byeong Ho Goh1, Joon Hyung Ann1, and Jongho Lee4

1Radiology, Gachon University Gil Medical Center, Incheon, Korea, Republic of, 2Radiology, Seoul St. Mary Hospital, Seoul, Korea, Republic of, 3Neurology, Gachon University Gil Medical Center, Incheon, Korea, Republic of, 4Electrical and Computer Engineering, Seoul National University, Seoul, Korea, Republic of

Synopsis

Susceptibility map-weighted imaging (SMWI) improves both CNR and SNR in comparison with conventional SWI. In this work, we compared SMWI with MEDIC (a multi-echo GRE imaging that combines magnitude images via sum of squares) for nigrosome 1 imaging at 3T in terms of interobserver agreement and diagnostic performance in 74 subjects (44 with Parkinson’s disease). Two experienced and two less-experienced reviewers visually assessed both imaging sets separately. Compared with MEDIC, SMWI showed higher kappa values and larger AUCs, regardless of level of experience. As a result, nigrosome 1 imaging using SMWI significantly improved diagnostic performance as well as interobserver agreement.

Purpose

Susceptibility map-weighted imaging (SMWI) is a recently proposed magnetic susceptibility imaging method that utilizes a quantitative susceptibility map (instead of a phase image) as the mask for susceptibility weighting. This new approach improves both CNR and SNR in comparison with conventional SWI.1 We hypothesized that nigrosome imaging using SMWI has higher interobserver agreement as well as higher diagnostic performance than that of MEDIC. The aim of this study was to compare SMWI and MEDIC for nigrosome 1 imaging.

Methods

A total 74 subjects (median age, 71 [IQR, 65-75]; 30 males; PD [n=44], DIP [n=13], healthy subjects [n=10], NPH [n=2], essential tremor [n=1], MSA-C [n=1], MSA-P [n=1], PSP [n=1], and vascular parkinsonism [n=1]) underwent 3T MRI using a 32-channel head coil. Oblique axial 3D MEDIC imaging was obtained vertical to the longitudinal axis of the midbrain. The scan parameters for MEDIC were as follows: TR, 88 ms; minimum TE, 11.1 ms; maximum TE, 66.9 ms (the number of echoes, 6; echo spacing, 11.1 ms); FA, 10°; ETL, 6; thickness, 1 mm; number of slices, 28; matrix, 384×384; FOV, 192×192; reduction factor of parallel imaging, 2. From the multi-echo k-space data of MEDIC, QSM was caculated using iLSQR,2 and then SMWI was generated by using the QSM images as a weighting mask (threshold value, 1 ppm; number of multiplications, 4).1 Details of the SMWI processing for the nigrosome 1 imaging are described in a separate abstract. Four independent raters (two experienced [a neuroradiologist and a neurologist; R1 and R2] and two less experienced [third- and fourth year radiology residents] reviewers; R3 and R4) classified the bilateral nigrosome 1 as ‘normal’ (iso- or hyperintensity in the central portion of the presumed nigrosome 1 area), ‘possibly abnormal’ (hypointensity in less than 50% of the presumed nigrosome 1 area and ‘definitely abnormal’ (hypointensity in equal to or more than 50% of the presumed nigrosome 1.5,6 Each side was rated separately separately on both MEDIC and SMWI. For a simplified statistical analysis, the subjects were re-classified as abnormal if any abnormality was determined on either side of the nigrosome 1 area; a subject was classified as normal when both nigrosome 1 regions were determined as normal. As both PD and MSA-P showed abnormality in nigrosome 1, they were considered as an abnormal group. The other subjects were categorized as a normal group. The generalized kappa test was used to test if there is any improvement of interobserver agreement between MEDIC and SMWI. ROC curve analyses were conducted to test if there is any significant difference between the reviewers.

Results

Compared to MEDIC, SMWI showed more conspicuous borders of the substantia nigra and nigrosome 1 regions (Fig. 1), and helped to determine if there was abnormality in the presumed nigrosome 1 regions (Fig. 2, 3). The MEDIC imaging had the generalized kappa of 0.989 (CI, 0.896-1.082), whereas SMWI showed a higher kappa value of 1.339 (CI, 1.232-1.446) without an overlap of CIs, suggesting significant improvement of interobserver agreement. In the MEDIC imaging, the AUCs of R3 and R4 were 0.741 and 0.692, respectively, whereas those of R1 and R2 were 0.870 and 0.826, respectively. The ROC curve analyses showed significant difference between the experienced and less experienced reviewers (P = 0.0138 between R1 and R3, P = 0.0031 between R1 and R4, P = 0.0211 between R2 and R4). In SMWI, the AUCs of R3 and R4 increased to 0.894 and 0.894, respectively. Both R1 and R2 also showed higher AUCs (0.933 and 0.933, respectively). There was no significant difference among the all reviewers in SMWI (P > 0.05).

Discussion

Here we demonstrated that the SMWI improved the diagnostic utility of the nigrosome 1 imaging at 3T by increasing interobserver agreement between the experienced and less-experienced reviewers and by enhancing diagnostic performance. In previous nigrosome 1 imaging studies at 3T, various imaging techniques, including PRESTO,3 SWAN,4 and MEDIC5,6 have been applied. All these methods are proprietary and had relatively limited SNR and CNR. Thus, it is desirable to standardize an imaging technique for visualization of nigrosome 1. Although MEDIC had higher SNR, it exhibited relatively low susceptibility weighting, limiting its diagnostic sensitivity for the nigrosome 1 imaging. On the other hand, SMWI provided high quality images with good contrast and SNR. SMWI could also enable us to measure susceptibility values in the substantia nigra by using the QSM map, thereby quantifying the level of degeneration.

Acknowledgements

No acknowledgement found.

References

1. Gho SM, Liu C, Li W, et al. Susceptibility map-weighted imaging (SMWI) for neuroimaging. MRM 2014;72:337-346. 2. Li W, Wang N, Yu F, et al. A method for estimating and removing streaking artifacts in quantitative susceptibility mapping. Neuroimage 2014;108:111-122. 3. Schwarz ST, Afzal M, Morgan PS, et al. The 'swallow tail' appearance of the healthy nigrosome - a new accurate test of Parkinson's disease: a case-control and retrospective cross-sectional MRI study at 3T. PloS One 2014;9:e93814. 4. Cosottini M, Frosini D, Pesaresi I, et al. Comparison of 3T and 7T susceptibility-weighted angiography of the substantia nigra in diagnosing Parkinson disease. AJNR Am J Neuroradiol 2015;36:461-466. 5. Noh Y, Sung YH, Lee J, et al. Nigrosome 1 Detection at 3T MRI for the Diagnosis of Early-Stage Idiopathic Parkinson Disease: Assessment of Diagnostic Accuracy and Agreement on Imaging Asymmetry and Clinical Laterality. AJNR Am J Neuroradiol 2015 Aug 20. [Epub ahead of print] 6. Sung YH, Noh Y, Lee J, et al. Drug-induced Parkinsonism versus Idiopathic Parkinson’s Disease: Utility of Nigrosome 1 Imaging at 3T. Radiology 2015 in press.

Figures

Fig. 1. A 53-year-old female patient with drug-induced parkinsonism (DIP). The normal-appearing nigrosome 1 is visualized as hyperintense areas (red arrows) on both sides. The border of substantia nigra is more conspicuous on SMWI (yellow arrows). Dopamine transporter PET shows no abnormality.

Fig. 2. An 80-year-old female patient with Parkinson’s disease. The abnormality (hypointensity) in nigrosome 1 is more easily appreciated on SMWI than on MEDIC. The border of substantia nigra is more conspicuous on SMWI (yellow arrows). Dopamine transporter PET shows bilateral abnormality.

Fig. 3. A 45-year-old male patient with PD. The abnormality (hypointensity) in nigrosome 1 is more easily appreciated on SMWI than on MEDIC. The border of substantia nigra is more conspicuous on SMWI (yellow arrows). Dopamine transporter PET shows bilateral abnormality.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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