INTRODUCTION

Parkinson’s disease is a neurodegenerative disease with movement disorders caused by degeneration of the substantia nigra in the midbrain. Neuromelanin imaging is an imaging sequence that demonstrates the neuromelanin of the substantia nigra as high signal intensity; normal substantia nigra cannot be demonstrated at 1.5 T. Therefore, a method is required for quantitative evaluation of the substantia nigra at 1.5 T.
Three-dimensional (3D) phase-sensitive inversion recovery (PSIR) can yield T₁-weighted images with high tissue contrast and T₁ values simultaneously. We hypothesized that the T₁ value and area of the substantia nigra could be measured using PSIR at 1.5 T and that these properties could be used to evaluate the degeneration of the substantia nigra. This study aimed to investigate whether the substantia nigra can be quantitatively evaluated by acquiring a T₁ map using 3D PSIR at 1.5 T.

MATERIALS AND METHODS

In this study, 1.5-T Ingenia (Philips Healthcare, the Netherlands) and a ds Head 15-ch coil were used. The study protocol was approved by the Institutional Review Board of the National Hospital Organization Saisyun Medical Center. Phantoms were made by diluting an intravenous solution of gadopentetate meglumine (0.5 mmol/mL, Fuji Pharmaceutical Co., Ltd., Japan) with distilled water to nine different concentrations, adding agar at 1.0 w% of the total mass. An in vivo study was performed on nine healthy subjects (age: 20–51 years; nine men; average age: 31.8 years). All examinations were performed after obtaining written informed consent from the subjects.
PSIR involves 3D turbo field echo acquisition at two different inversion times (TIs) after applying a nonselective inversion recovery pulse. The magnitude and reference images are reconstructed by signals obtained at the first (TI₁) and second (TI₂) TIs, respectively. Furthermore, the PSIR image is reconstructed using the phase information obtained from the reference image and coil sensitivity, which maintains the signal polarity. Figure 1 shows the representative magnitude, reference, PSIR, and T₁ map images of the substantia nigra. The T₁ value can be calculated using the signal intensities of the reference and PSIR images.
In the phantom study, phantoms were imaged using PSIR and inversion recovery spin-echo (IR-SE) sequence, and the T₁ value of each phantom was measured to verify the accuracy of the PSIR T₁ measurement at 1.5 T. TIs of IR-SE were set at 50, 100, 200, 400, 600, 800, 1000, 2000, 3000, and 4900 ms to calculate the T₁ value. PSIR was imaged with a combination of five TI₁ and TI₂ conditions—TI₁, 300, 400, 500, 600, and 700 ms and TI₂, 2300, 2400, 2500, 2600, and 2700 ms.
T₁ values of the phantom were measured for each imaging setting of TI₁ and TI₂. Additionally, the Bland–Altman plot was used to calculate T₁ in each TI settings. In the in vivo study, the midbrain of nine healthy volunteers was imaged by PSIR at four imaging conditions: TI₁, 300, 400, 500, and 600 ms (TI₂ was also used from 2300 ms to 2600 ms in 100 ms increments.). T1 values were calculated pixel-by-pixel with all combinations of TI₁ and TI₂.
The regions of interest (ROIs) of the substantia nigra were set on the slice that widely demonstrated the substantia nigra. The ROI boundary threshold was determined based on the T1 value of the cerebral peduncle (Fig. 2). The T₁ value and area of the substantia nigra were measured using T₁ maps. The T₁ value and area of the substantia nigra were compared to those reported in previous studies. In addition, the T₁ value of the substantia nigra in each TI setting was compared to that reported in previous studies. The area of the substantia nigra in each TI setting was also compared to that reported in previous studies.

RESULTS

In the phantom study, the measurement error in the T₁ value caused by PSIR relative to the T₁ value, which was measured in the IR-SE sequence, was a maximum of 16% (Fig. 3). In the in vivo study, the T₁ value of the substantia nigra measured by PSIR was 828.4 ± 18.0 ms, and the difference from the T₁ value reported in previous studies was of approximately 1.1%. Furthermore, the area of the substantia nigra measured by PSIR was 123.9 ± 22.5 mm². There were no significant differences in the T₁ value or area of the substantia nigra among TI₁s (Fig. 4).

DISCUSSION

We demonstrated that PSIR could measure the T₁ value of the substantia nigra with high accuracy at 1.5 T. Setting the ROI in the substantia nigra based on the T₁ value of the cerebral peduncle enables accurate evaluation of the T₁ and area of the substantia nigra. In the future, the measurement of the T1 value and area of the substantia nigra by PSIR may enable quantitative evaluation of the degeneration of the substantia nigra at 1.5 T.
In conclusion, PSIR enables quantitative evaluation of the substantia nigra by measuring its area and T₁ value in 1.5-T magnetic resonance imaging.

Acknowledgements

No acknowledgement found.