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The reversibility and linearity of the T2 -temperature dependence of adipose tissue allows for the monitoring of the temperature in the subcutaneous adipose tissue layers.

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MRI can be used for monitoring temperature during a thermocoagulation treatment of tumors. The aim of this study was to demonstrate the suitability of a 3D steady-state free precession sequence (3D Fast Imaging with Steady-State Precession, 3D TrueFISP) for MR temperature measurement at 0.23 T, and to compare it to the spin-echo (SE) and spoiled 3D gradient-echo (3D GRE) sequences. The optimal flip angle for the TrueFISP sequence was calculated for the best temperature sensitivity in the image signal from liver tissue, and verified from the images acquired during the thermocoagulation of excised pig liver. The treatment of hepatic metastases by local heat delivery has drawn considerable interest over the last few years. The efficiency of thermoablation is improved if real-time monitoring of heat distribution is available during the treatment (1-3). At the moment, MRI is the only medical imaging modality that can provide noninvasive temperature information from tissue.Temperature monitoring with MRI is possible using the temperature sensitivity of the proton resonance frequency, i.e., chemical shift (4), diffusion (5), or longitudinal relaxation time T 1 and equilibrium magnetization M 0 (6 -8). The applicability of each method depends on the field strength of the MR scanner used, and on the target anatomy (9).Low-field open magnets are well suited for performing tumor ablations. They offer maximum access to the patient; dedicated tools for interventional MRI, such as needle tracking systems and in-room controls; less-intense needle artifacts; and lower-cost procedures. A disadvantage is that temperature-measuring methods utilizing the temperature sensitivity of the proton resonance frequency are difficult to use, as the change in chemical shift is directly proportional to field strength. For example, the sensitivity is 6.5 times less sensitive at 0.23 T than at 1.5 T.Motion artifacts hamper diffusion-based temperature measurement methods (10), making them impractical for use in the liver. With low-field open scanners, the easiest method for obtaining temperature data from the liver is to use the temperature dependence of T 1 and M 0 . Their temperature sensitivity is still high at lower magnetic field strengths, and they are easily measurable with standard sequences and hardware.The accuracy of temperature measurement with T 1 and M 0 depends strongly on the type of MR sequence used and on the choice of acquisition parameters, such as the repetition time (TR) and flip angle (␣). Temperature resolution may be affected by a factor of 5, depending on the type of sequence used for the measurement (11). The parameters need to be carefully chosen to optimize the temperature measurement while maintaining a reasonable temporal and spatial resolution.Spin-echo (SE) sequences, gradient-recalled acquisitions in the steady state (GRASS), and spoiled gradientrecalled acquisitions (SPGR) have been employed for monitoring temperature by using the temperature dependency of T 1 (7,8,11,12). A steady-state sequence (3D Fast Imagi...

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T1 rho dispersion, or the frequency dependence of T1 relaxation in the rotating frame, was used for in vivo muscle tissue characterization in 13 patients with primary skeletal muscle disease and in eight normal subjects for comparison. T1 rho dispersion measurements represent a new approach to magnetic resonance tissue characterization, possibly reflecting the macromolecular constituents of tissue. A definite, statistically significant, difference was found between the relative T1 rho dispersion values of normal and diseased muscle tissue. T1 rho dispersion measurements and images may increase the accuracy of identification of diseased muscles. Early identification of affected muscles is important for accurate diagnosis by muscle biopsy.

The potential of T1 rho dispersion, spin lock (SL), and magnetization transfer (MT) techniques to differentiate benign and malignant head and neck tumors was evaluated. Twenty-four patients with pathologically verified head and neck tumors were studied with a .1-T MR imager. T1 rho dispersion effect was defined as 1 -(intensity with lower locking field amplitude/intensity with higher locking field amplitude). T1 rho dispersion effects were higher for malignant than benign tumors (P = .001). With T1 rho dispersion effect .14 as the threshold, sensitivity for detecting a malignant tumor was 91%, specificity was 77%, and accuracy was 83%. A strong correlation between T1 rho dispersion effects and SL effects and between T1 rho dispersion effects and MT effects in the head and neck tumors was found (r = .87, P < .001 and r = .90, P < .001, respectively). High T1 rho dispersion effects are not specific indicators of malignancy, because chronic infections, some benign tumors, and malignancies may overlap. Low T1 rho dispersion effect values are characteristic of a benign tumor.