Prediction of heating patterns of a microwave interstitial antenna array at various insertion depths

From the RSNA refresher courses: MR imaging in hyperthermia

There is growing clinical evidence that the combination of radiation therapy and hyperthermia, when delivered at moderate temperatures (40 degrees-45 degrees C) for sustained times (30-90 minutes), is of benefit with regard to palliative relief of cancer, tumor response, local control, and survival. Adequate measurement of the temperature distribution achieved with the hyperthermia is a key element in successful therapy. Thermal dosimetry, even invasive dosimetry, is a complex topic when applied to the heterogeneous tissue of a tumor and associated organ systems. Imaging in hyperthermia therapy is performed primarily for estimation and control of temperature. Magnetic resonance (MR) imaging has unique parameter dependences that make it possible to monitor hyperthermia therapy by detection of proton resonant frequency changes or diffusion coefficient changes. In addition, MR imaging can be used to assess vascular parameters that not only allow selection of suitable patients for therapy but may also allow demonstration of response to therapy. Finally, as the use of thermally sensitive liposomes for delivery of chemotherapeutic agents is developed, MR imaging may allow determination of local drug dose.

Effect of phase modulation on the temperature distribution of a microwave hyperthermia antenna array in vivo

Perfused, canine skeletal muscle and the brain tumour of a cancer patient were heated with an array of four parallel, interstitial antennas placed on the corners of a 2-cm square and driven at 915 MHz. The temperature distributions along the axial and diagonal catheters were measured with equal-phase driving of the antennas and with several time-varying schemes of driving phase differences among the antennas. When equal-phase driving was replaced by a rotating scheme of 90 degrees driving phase differences, the tissue area in the junction plane heated above a normalized index temperature of 0.6 increased by a factor of about 1.25. With a rotating phase of 135 degrees, the same area increased by a factor of about 1.6. The axial temperature distribution was not affected significantly by driving phase.

A theoretical model for input impedance of interstitial microwave antennas with choke

PURPOSE

Two important characteristics for interstitial microwave antennas used in clinical hyperthermia are: (1) a good impedance match to minimize reflected power; and (2) a good power deposition pattern which is independent of insertion depth. A major problem of the miniature coaxial dipole antennas used for interstitial hyperthermia is the fact that the impedance and power deposition patterns of these antennas change with insertion depth. One possible solution is the addition of a coaxial choke. A theoretical model for calculating the input impedance of interstitial microwave antennas having a coaxial choke is presented, which may serve as the first step in the design of such antennas.

METHODS AND MATERIALS

A theoretical model for calculating the input impedance of coaxial microwave antennas with and without a choke is presented using insulated antenna theory. The theoretical model was used to calculate the input impedance of several prototype antennas having various choke and feedline dimensions, and comparison was made with experimentally measured impedance measurements in tissue-equivalent phantom.

RESULTS

The choke section of the antenna is not ideal if conventional plastic insulation is used as the choke dielectric, because the desired radiating length of the antenna is significantly shorter than the quarter-wavelength in the choke dielectric. Impedance calculations based on the theoretical model correlate reasonably well with experimentally measured impedance. Based on these calculations, the effect of parameters such as choke layer thickness and choke dielectric constant are discussed for a 915 MHz antenna with choke.

CONCLUSION

The theoretical model can serve as a design aid for optimizing choked microwave antenna designs, as well as predicting the impedance match of a given antenna design at a given insertion depth. The model allows the effect of some variables not accessible experimentally such as termination impedance to be studied, which may also be useful in the understanding of these antennas. Calculations are easily performed on a desktop computer.

Interstitial microwave hyperthermia and brachytherapy for malignancies of the vulva and vagina. I: Design and testing of a modified intracavitary obturator

A vaginal obturator was fabricated to be used in combination with implanted catheters to provide microwave hyperthermia and brachytherapy to the vulva and vaginal wall. This site is difficult to heat or irradiate solely with interstitial techniques. The obturator was modified to provide grooves for the mounting of interstitial catheters into the outer wall and was matched with a template for circumferential implants. Power deposition tests were done using arrays of three microwave antenna designs: dipole (hA = hB = 3.9 cm), helical (3.9 cm coil, shorted), and modified dipole (1.0 cm helix on dipole tip) to test the performance of the obturator. The obturator and four non-obturator catheters were positioned in muscle-equivalent phantom. Two obturator catheters along with two free-standing catheters formed the obturator array. Four freestanding catheters formed the non-obturator array. Power deposition or specific absorption rate (SAR) measurements were made along the central axis, bisect, and diagonal transect of each array. SAR results showed that antennas in the obturator wall radiated as dipole theory predicts, although with less power density when compared to antennas in the same catheters spaced 1.8 cm from the obturator. This could be compensated for by increasing the power to the antennas in the obturator by 42%. Adjacent pairs of antennas were placed 90 degrees out of phase for 0.25 sec and rotated around the array. Phase rotation demonstrated that the central array SAR peaks could be lowered from 100% to 50% SAR, with dipole antennas thus resulting in lowered peak temperatures and the ability to heat larger volumes by improving the distribution of power. With helical antennas, there was 50% SAR at the array center when operated coherently without phase rotation. Three patients were treated with the obturator and a custom-made template using dipole antennas, and temperatures were measured in five obturator catheters. Therapeutic heating was measured in the catheters on the obturator between antennas in contact with the vaginal mucosa.