Synopsis

Respiratory motion is an important problem in Magnetic Resonance Imaging (MRI), contributing to image blurring during data acquisition and coil detuning. Using the concept of an ideal (perfectly matched and tuned at all available ports) RF transmit coil and the VHP-Female v4.0 dynamic CAD model, we estimate the detuning of a full-body RF coil detuning during the respiratory cycle. Our results show that the computed resonant capacitance values change by at most 0.5%.

Purpose

Method

An approximate method for modeling continuous respiratory motion in a CAD human virtual model subject to electromagnetic finite-element analysis has been realized. Its concept relies on using affine transformations (three-dimensional translations, rotations and scalings) to create polynomials of deformation for every structure involved in respiration, which are implemented in commercial FEM software packages in the form of a parametric sweep. This method does not require multiple copies of the CAD model or multiple project files. It enables use of arbitrary sampling times and an automatic reposition of on-body and in-body devices, such as medical implants. The method was applied to the Visible Human Project® (VHP)-Female phantom, a platform-independent, full-body triangular surface based electromagnetic computational model. Such an approach is not exact, but it may have sufficient accuracy when the parametric sweep is carefully designed. It will allow us to employ any temporal resolution, which is impossible with discrete models. To construct an anatomically relevant breathing cycle, we followed the anatomical data collected from Refs. [1-9] as close as possible.

The user can initialize a discrete generic global variable,, define object geometry parameters as certain unique functions of , and then move/rotate/deform every object of a multi-object structure independently within the framework of the same project file, greatly facilitating large numeric studies that require these complex motions. In the present case, the 42 structures involved in respiration include 24 ribs, the heart, liver, lungs, stomach, skin shell, fat shell, 6 abdominal muscles, 2 erector spinae muscles, and 4 pectoralis muscles. No intersections between any structures are experienced as a result of these affine transformations. Figure 1 provides a depiction of minimum and maximum ribcage movement in the axial plane using this method. While air and total body volumes corresponding to these motions are automatically updated there is essentially no change in the mass of the virtual human model.

We next construct and simulate models mimicking maximum exhalation (), zero pressure gradient (), and maximum inhalation () using the perfectly matched RF coil model from Ref. [14]. This method uses a resonant-model of a coil optimally driven at all possible ports. The coil is loaded with a dynamic virtual human VHP-Female v. 4.0 Refs. [11, 12, 13].

Results

Table 1 shows averaged simulation results for the virtual human VHP-Female 4.0 in a high-pass fully body birdcage coil shown in Fig. 2 at the shoulder landmark. Our numerical results obtained with FEM software ANSYS Electronics Desktop reveal that detuning due to respiration is generally very small. The maximum deviation occurs in between maximum exhalation and maximum inhalation. A series of figures depicting Specific Absorption Rate (SAR) plots for various breathing stages is given in Table 2. Each tissue is augmented with accurate material properties.

Discussion and Conclusion

Acknowledgements

References

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