Marta Z. Lewis

Imaging spin probe distribution in the tumor of a living mouse with 250 MHz EPR : Correlation with BOLD MRI

Electron paramagnetic resonance imaging (EPRI) promises to provide new insights into the physiology of tissues in health and disease. Understanding the in vivo imaging capability of this new modality requires comparison with other physiologically responsive techniques. Here, an initial comparison between 2D EPR spatial imaging of a narrow single line injectable paramagnetic trityl spin probe and 2D slice‐selected carbogen subtraction BOLD MRI is presented. The images were obtained from the same FSa fibrosarcoma grown in the leg of a C3H mouse. This tumor was unusual in comparison with others imaged with subtraction BOLD MRI because of its peripheral distribution of intensity. The spatial distribution of the EPR spin probe showed the same peripheral distribution. The pixel resolutions of these images are comparable. These images provide an early in vivo comparison of EPRI with a well‐established imaging modality. The comparison validates the in vivo distribution of spin probe as imaged with EPRI, and provides a proof of principle for the comparison of BOLD and EPRI. Magn Reson Med 47:634–638, 2002.

Magnetic resonance measurement of response to hyperoxia differentiates tumors from normal tissue and may be sensitive to oxygen consumption.

Karczmar GS, Kupertnan VY, River JN, Lewis MZ, Lipton MJ. MR measurement of response to hyperoxia differentiates tumors from normal tissue and may be sensitive to oxygen consumption. Invest Radiol 1994;29:S161–S163.

Magnetic resonance imaging of rodent tumors using radiofrequency gradient echoes

This paper evaluates the use of radiofrequency (RF) magnetic field gradient echoes to provide contrast in magnetic resonance (MR) images of model tumors. Decay of RF gradient echoes as a function of evolution time was measured and sensitivity of the decay to changes in blood pressure was evaluated. Previous investigators have demonstrated that static field (B0) gradient echoes provide MR image contrast which is sensitive to the rate of self-diffusion of tissue water and may also be sensitive to the rate of tissue perfusion. Gradient echoes produced by RF magnetic field gradients provide a useful alternative to the conventional B0 methods. Unlike B0 gradient echoes RF gradient echoes are relatively insensitive to local magnetic susceptibility gradients and to magnetic field gradients produced by eddy currents. Differences between the two methods may be particularly significant for studies of tumors where large concentrations of deoxyhemoglobin and other paramagnetic substances may cause significant susceptibility gradients. Mammary adenocarcinomas subcutaneously implanted in the flanks of female Fisher rats were studied. Magnetic resonance experiments were performed at 2 T. A surface coil was used to provide an RF gradient and to excite and detect signals from the tumors. The decay of echo amplitude as a function of evolution time was measured and the decay at short and long evolution times was analyzed independently to calculate two apparent diffusion coefficients (ADCs). The preparation was extremely stable and the standard error for 10 consecutive measurements of gradient echo amplitude made over 30-60 min with an RF gradient strength of 50 kHz/cm, gradient duration of 1 ms (i.e., 50 cycles/cm), and echo evolution time (td) of 1 s was generally +/- 0.8%. The ADC calculated from the decay at short evolution times was approximately 3 x 10(-5) cm2/s. The ADC calculated from the decay at longer evolution times was approximately 0.5 x 10(-5) cm2/s. Both ADCs decreased immediately following sacrifice and administration of Hydralazine. The experiments demonstrate that measurements of RF gradient echo amplitudes in tumors can be made in vivo with a high degree of reproducibility and suggest that RF gradient echo amplitudes are sensitive to acute physiological changes in tumors. This method may be useful for characterization of tumors and prediction and monitoring of effects of therapeutic agents.

Prospects for Assessment of the Effects of Electrical Injury by Magnetic Resonancea

Magnetic resonance (MR) methods allow noninvasive studies of tissue anatomy, blood flow, and metabolism, with high spatial resolution. Therefore, MR methods may be useful for noninvasive assessment of the effects of electrical shock on living tissue and on the response of tissue to therapy. Serial studies of a single experimental animal can be performed before and after an injury and throughout the entire course of therapy. Such studies would allow the correlation of physiological and metabolic changes during therapy with the ultimate therapeutic outcome. MR methods that are developed using model systems may eventually be used to guide treatment of patients. A wide variety of MR methods can be applied to the study of effects of electrical injury. Examples of applications of these methods to study tissue pathology are described below and possible applications to the problem of electrical injury are discussed.