Original ContributionsLow field T1ρ imaging of myositis
Introduction
Differences in the longitudinal (T1) and transverse (T2) relaxation times and proton densities of tissues determine contrast in magnetic resonance imaging (MRI). Relaxation of water protons in tissues is complicated by tissue heterogeneity and biologic variability. For example, muscle tissue exhibits multicomponent relaxation,1, 2, 3, 4, 5 which has been thought to arise from chemically different compartments containing, i.e., fat, protein, and water1 or different environments of tissue water.2, 4, 5 Consequently, T1 and T2 may not alone provide unique contrast between three or more heterogeneous tissue types in clinical MRI.
The spin-lattice relaxation time in the rotating frame, T1ρ, is an alternative and additional contrast parameter, which can be measured with the spin-locking (SL) method.6, 7, 8 T1ρ has been suggested to share the tissue-characterizing qualities of both T29 and low Bo field T1 relaxation.7 Moreover, because T1ρ is highly dependent on the locking field B1, it provides a useful tool to manipulate contrast between tissues with different B1 dispersion behavior.
The purpose of this study was to appraise the value of T1ρ in relation to T1 and T2 in characterizing normal and diseased muscle tissue. In the active stage of neuromuscular diseases, the involved muscles may be edematous, whereas in the chronic stage the muscle fibers are atrophied and replaced by fat and connective tissue. Involved muscles are, therefore, likely to consist of chemically different compartments with independent relaxation rates and different 1/T1ρ dispersion behavior. We determined the 1/T1ρ, 1/T1, and 1/T2 of muscle in 13 myositis patients and in 13 normal volunteers and evaluated 1/T1ρ, 1/T1, and 1/T2 contrast between muscle and fat, and between normal and diseased muscle.
Section snippets
Materials and methods
We examined 13 patients with idiopathic inflammatory muscle disease (aged 48–72 years, eight female, five male) at the Turku University Central Hospital. There were six patients with inclusion body myositis (IBM), three with dermatomyositis, two with polymyositis, and two with collagenosis-associated myositis. The duration of the disease varied between half a year and more than 5 years. One of the patients with collagenosis-associated myositis had a primary scleroderma and the other had
Results
1/T1, 1/T2, and 1/T1ρ of tibialis anterior and gastrocnemius and the relaxation rate-based contrast between muscle and fat are presented in Table 1. 1/T2 (p < 0.0001) and 1/T1ρ (p < 0.0001) of TA and GC were clearly lower in the patients than in the volunteers, whereas there was no difference between the corresponding 1/T1 values (p > 0.06). We could not observe any difference in the 1/T1, 1/T2, and 1/T1ρ values between the different myositis groups. In the normal volunteers, 1/T2 and 1/T1ρ was
Discussion
Earlier investigations have already shown the feasibility of MRI in evaluating neuromuscular diseases.8, 10, 11, 12, 13, 14, 15, 16, 17 Several neuromuscular diseases lengthened both T110, 14 and T215, 16 of the affected muscles. Similarly, in the active stage of the disease, dermatomyositis increased the signal intensity of the involved muscles in the T2-weighted images.13, 16, 17 In a study of polymyositis, some of the patients had focal areas of hyperintensity in the T2- but not in the T1
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