Dynamic MR mammography: three-dimensional real-time visualization of contrast enhancement in virtual reality.

RATIONALE AND OBJECTIVES: In view of the increasing use of breast magnetic resonance (MR) imaging to supplement x-ray mammography. the authors developed a method for fast and efficient analysis of dynamic MR images of the female breast. MATERIALS AND METHODS: The MR image data sets were acquired with a saturation-recovery turbo fast low-angle shot sequence to detect the kinetics of the contrast agent concentration in the whole breast at a high temporal and spatial resolution. A morphologic three-dimensional fast low-angle shot data set was also acquired. The dynamic image data sets were analyzed with tracer kinetic modeling to describe the physiologic processes underlying the contrast enhancement in mathematical terms and enable the estimation of functional tissue-specific parameters, which reflect the status of microcirculation. To display morphologic and functional tissue information simultaneously, the authors developed a multidimensional real-time visualization system (with three-dimensional texture mapping), which enables a practical and intuitive human computer interface in virtual reality. RESULTS: The spatially differentiated representation of the computed functional tissue parameters superimposed on the anatomic information offers several possibilities: (a) more discernible contrast enhancement, (b) inspection of the data volume in three-dimensional space by means of rotation and transparency variation, (c) location of lesions in space and thus faster and more natural recognition of topologic coherencies, and (d) fast and efficient overview in compressed form. CONCLUSION: A feasibility study demonstrated that multidimensional visualization of contrast enhancement in virtual reality is a practicable idea. Detection and location of multiple breast lesions may be an important application.

Estimation of heat transfer and temperature rise in partial-body regions during MR procedures: an analytical approach with respect to safety considerations.

In order to assess thermal response to RF exposure during MR procedures at the tissue level, simple analytical solutions to the non-stationary Pennes' bio-heat equation were obtained using the Green's function approach. Two thermal models appropriate for partial-body exposure were analyzed: In the first model, the temperature field at the periphery of an idealized volume RF resonator was modeled. The analytical solution reveals that tissue response to RF heating is characterized by an equilibration time and length. Both parameters are inversely related to tissue perfusion and vary for the soft-tissues considered between 0.27-25 min and 1.5-12 mm, respectively. None of the tissues investigated increase in temperature more than 0.5 degrees C for each W/kg of power dissipated. Secondly, a homogeneous tissue solution was derived that predicts the temperature-time course to an MR examination with time-varying specific absorption rates (SAR). Since SAR limits indicated in current MR safety standards relate to running SAR averages computed over an appropriate period of time, an expression was formulated that gives an upper limit for the temperature rise averaged over the same period of time, as a function of both the upper limit of running SAR averages and the duration of the MR examination. The analysis revealed that the partial-body SAR limits indicated in the IEC standard may not guarantee under all circumstances compliance with the basic restrictions concerning temperature rise.