title: Quantitative analysis of the diffusion-weighted steady-state free precession signal in vertebral bone marrow lesions.
abstract: OBJECTIVE
  : Diffusion-weighted steady-state free precession (DW-SSFP) sequences have shown great potential for the differential diagnosis of benign osteoporotic and malignant neoplastic vertebral compression fractures, which appear hypo- to isointense or hyperintense in DW-SSFP magnetic resonance imaging, respectively. In contrast to other diffusion weighting sequences, the DW-SSFP signal depends not only on the apparent diffusion coefficient (ADC), but also on the tissue relaxation times and sequence parameters. The purpose of the present study was to provide a detailed analysis of the DW-SSFP signal in benign and malignant vertebral lesions (VLs) and in vertebral bone marrow (VBM) to understand the observed signal alterations and their dependence on tissue and sequence parameters.
  
  
  METHODS
  : Magnetic resonance imaging was performed in 40 patients with benign (n = 20) or malignant (n = 20) VLs to determine the fat fraction and tissue parameters (ADC, T1, T2, T2*) for both the water and fat signal. With these values, the DW-SSFP signal was simulated and compared with the measured signals for different diffusion gradients by determining the signal intensity ratio between the SSFP signals of the lesions and of normal-appearing VBM for both malignant and benign VLs.
  
  
  RESULTS
  : The simulated DW-SSFP contrast agreed well with the measured contrast and provided a very good differentiation between benign osteoporotic and malignant VLs. ADCs were significantly different in both lesion types (malignant 1.36 vs. osteoporotic 1.77 × 10 mm/s); however, the observed contrast differences were caused predominantly by an opposed-phase readout in combination with significantly different T2* values (malignant 22 vs. osteoporotic 14 ms) and fat fractions (malignant 3.9% vs. osteoporotic 12%) in the lesions as well as significantly different fat fractions in normal-appearing VBM (malignant 42% vs. osteoporotic 52%) of both patient groups.
  
  
  CONCLUSIONS
  : Although the ADCs of the evaluated malignant and benign VLs showed highly significant differences, the influence of diffusion on the DW-SSFP signal contrast is relatively low compared with other tissue parameters due to the very complex signal mechanism of the SSFP sequence. Thus, the observed DW-SSFP signal contrast of different VLs (hypo-/isointense vs. hyperintense signal) is rather fat- and T2*-weighted than diffusion-weighted. The intermediate diffusion weighting of the applied SSFP sequence, however, helps to shift the different contrasts into a signal range that is easily visually accessible.
authors: Josef Lissner Laboratory for Biomedical Imaging, Department of Clinical Radiology-Grosshadern, LMU Ludwig Maximilian University of Munich, Germany.
authors: Andreas
authors: A
authors: Biffar
authors: Andrea
authors: A
authors: Baur-Melnyk
authors: Gerwin P
authors: GP
authors: Schmidt
authors: Maximilian F
authors: MF
authors: Reiser
authors: Olaf
authors: O
authors: Dietrich