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Quantitative magnetization transfer imaging in relapsing multiple sclerosis.

Subjects and Methods Nine patients (6 women, 3 men, aged 25 to 50 years) with clinically definite MS and 8 healthy controls (4 women, 4 men, aged 18 to 45 years) were studied. All patients suffered from a relapsing-remitting course and had an Expanded Disability Status Scale (EDSS) score ranging from 1.0 to 5.0. Conventional MR imaging and quantitative MT imaging were performed on a 1.5T unit (Gyroscan ACS-NT, Philips) using the standard head coil. The standard protocol included dual-echo spin echo imaging (TR=2060 ms, TE=20/80 ms) and precontrast and postcontrast T1-weighted spin echo imaging (TR=510 ms, TE=14 ms). The postcontrast scan was performed 5 minutes after intravenous application of 0.1 mmol/kg Gd-DTPA in MS patients only. Quantitative MT analysis was performed with a FastPACE sequence (TR=580 ms, α1=45°, α2=90°, phase cycling=90°/0°). Parameter maps showing MTR, k and T1sat were calculated pixel by pixel using the modulus and phase images of the PACE acquisition as described in reference 2. T1free maps were calculated according to Eq.[2]. Conventional imaging was used to identify the following tissue categories: Normal appearing white matter (NAWM), diffuse white matter changes and focal non-acute and acute lesions. Focal non-acute lesions were subdivided according to their hypointensity on T1-weighted images as isointense, mildly (intensity > gray matter) and markedly hypointense (intensity < gray matter). Acute lesions were subdivided into densely enhancing lesions and ring enhancing lesions. Areas of marked hyperintensity around enhancing lesions which disappeared on follow-up scans were considered to reflect edema. Results In volunteers we analyzed 64 white matter regions considered as normal (NWM). In MS patients we analyzed a total of 296 areas. Quantitative results are summarized in Table 1. While the MTR was significantly reduced in NAWM and dirty WM when compared to NWM, T1free was increased in dirty WM only. Focal non-active lesions showed a gradual decline of MTR and k values with greater T1-hypointensity. Inversely, T1free was noted to increase with T1-hypointensity. Different and more complex patterns could be observed within active lesions. While the MTR was lowest in ring enhancing lesions and highest in edema, T1free was lowest in densely enhancing lesions and highest in edema. Overall, the transfer rate k correlated very well with the MTR when excluding edema and dirty WM. However, the transfer rate was more sensitive for tissue changes.