Faculty Bibliography

BACKGROUND: Ultrasound induced hyperthermia is a useful adjuvant to radiation therapy in the treatment of prostate cancer. A uniform thermal dose (43 degrees C for 30 minutes) is required within the targeted cancerous volume for effective therapy. This requires specific ultrasound phased array design and appropriate thermometry method. Inhomogeneous, acoustical, three-dimensional (3D) prostate models and economical computational methods provide necessary tools to predict the appropriate shape of hyperthermia phased arrays for better focusing. This research utilizes the k-space computational method and a 3D human prostate model to design an intracavitary ultrasound probe for hyperthermia treatment of prostate cancer. Evaluation of the probe includes ex vivo and in vivo controlled hyperthermia experiments using the noninvasive magnetic resonance imaging (MRI) thermometry. METHODS: A 3D acoustical prostate model was created using photographic data from the Visible Human Project. The k-space computational method was used on this coarse grid and inhomogeneous tissue model to simulate the steady state pressure wavefield of the designed phased array using the linear acoustic wave equation. To ensure the uniformity and spread of the pressure in the length of the array, and the focusing capability in the width of the array, the equally-sized elements of the 4 x 20 elements phased array were 1 x 14 mm. A probe was constructed according to the design in simulation using lead zerconate titanate (PZT-8) ceramic and a Delrin plastic housing. Noninvasive MRI thermometry and a switching feedback controller were used to accomplish ex vivo and in vivo hyperthermia evaluations of the probe. RESULTS: Both exposimetry and k-space simulation results demonstrated acceptable agreement within 9%. With a desired temperature plateau of 43.0 degrees C, ex vivo and in vivo controlled hyperthermia experiments showed that the MRI temperature at the steady state was 42.9 +/- 0.38 degrees C and 43.1 +/- 0.80 degrees C, respectively, for 20 minutes of heating. CONCLUSION: Unlike conventional computational methods, the k-space method provides a powerful tool to predict pressure wavefield in large scale, 3D, inhomogeneous and coarse grid tissue models. Noninvasive MRI thermometry supports the efficacy of this probe and the feedback controller in an in vivo hyperthermia treatment of canine prostate

Several methods have been proposed for overcoming the effects of radiofrequency (RF) magnetic field inhomogeneity in high-field MRI. Some of these methods rely at least in part on the ability to independently control magnitude and phase of different drives in either one multielement RF coil or in different RF coils in a transmit array. The adjustment of these drive magnitudes and phases alone to create uniform RF magnetic (B(1)) fields has been called RF shimming, and has certain limits at every frequency as dictated by possible solutions to Maxwell's equations. Here we use numerical calculations to explore the limits of RF shimming in the human head. We found that a 16-element array can effectively shim a single slice at frequencies up to 600 MHz and the whole brain at up to 300 MHz, while an 80-element array can shim the whole brain at up to 600 MHz

We present a method to calculate the electric (E)-fields within and surrounding a human body in a gradient coil, including E-fields induced by the changing magnetic fields and 'conservative' E-fields originating with the scalar electrical potential in the coil windings. In agreement with previous numerical calculations, it is shown that magnetically-induced E-fields within the human body show no real concentration near the surface of the body, where nerve stimulation most often occurs. Both the magnetically-induced and conservative E-fields are shown to be considerably stronger just outside the human body than inside it, and under some circumstances the conservative E-fields just outside the body can be much larger than the magnetically-induced E-fields there. The order of gradient winding and the presence of conductive RF shield can greatly affect the conservative E-field distribution in these cases. Though the E-fields against the outer surface of the body are not commonly considered, understanding gradient E-fields may be important for reasons other than peripheral nerve stimulation (PNS), such as potential interaction with electrical equipment

Previous researchers have successfully demonstrated the application of temperature feedback control for thermal treatment of disease using MR thermometry. Using the temperature-dependent proton resonance frequency (PRF) shift, ultrasound heating for hyperthermia to a target organ (such as the prostate) can be tightly controlled. However, using fixed gain controllers, the response of the target to ultrasound heating varies with type, size, location, shape, stage of growth, and proximity to other vulnerable organs. To adjust for clinical variables, feedback self-tuning regulator (STR) and model reference adaptive control (MRAC) methods have been designed and implemented using real-time, online MR thermometry by adjusting the output power to an ultrasound array to quickly reach the hyperthermia target temperatures. The use of fast adaptive controllers in this application is advantageous because adaptive controllers do not require a priori knowledge of the initial tissue properties and blood perfusion and can quickly reach the steady-state target temperature in the presence of dynamic tissue properties (e.g., thermal conductivity, blood perfusion). This research was conducted to rapidly achieve and manage therapeutic temperatures from an ultrasound array using novel MRI-guided adaptive closed-loop controllers both in ex vivo and in vivo experiments. The ex vivo phantom experiments with bovine muscle (n = 5) show that within 6 +/- 0.2 minutes, the tissue temperature increased by 8 +/- 1.37 degrees C. Using rabbits' (n = 5) thigh muscle, the in vivo experiments demonstrated the target temperature reached 44.5 degrees C +/- 1.2 degrees C in 8.0 +/- 0.5 minutes. The preliminary in vivo experiment with canine prostate hyperthermia achieved 43 +/- 2 degrees C in 6.5 +/- 0.5 minutes. These results demonstrate that the adaptive controllers with MR thermometry are able to effectively track the target temperature with dynamic tissue properties.

AIM: The aim of the study was to determine and compare the areas of brain activated in response to colorectal distention (CRD) using functional magnetic resonance imaging (fMRI) and c-fos protein expression. METHODS: For fMRI study (3.0 T magnet), anaesthetized rats underwent phasic CRD, synchronized with fMRI acquisition. Stimulation consisted of eight cycles of balloon deflation (90 s) and inflation (30 s), at 40, 60 or 80 mmHg of pressure. For c-fos study two sets of experiments were performed on anaesthetized rats: comparing (A) brain activation in rats with the inserted colorectal balloon (n = 5), to the rats without the balloon (n = 5); and (B) rats with inserted balloon (n = 10), to the rats with inserted and distended balloon (n = 10). The pressure of 80 mmHg was applied for 2 h of 30 s inflation and 90 s deflation, alternating cycles. RESULTS: Functional MRI revealed significant activation in the amygdala, hypothalamus, thalamus, cerebellum and hippocampus. Significant increase in c-fos expression was observed in amygdala and thalamus in the first set of experiments, and hypothalamus and parabrachial nuclei in the second. CONCLUSION: The two methods are not interchangeable but appeared to be complementary: fMRI was more sensitive, whereas c-fos had much greater resolution.

A numerical model of a female body is developed to study the effects of different body types with different coil drive methods on radio-frequency magnetic (B 1) field distribution, specific energy absorption rate (SAR), and intrinsic signal-to-noise ratio (ISNR) for a body-size birdcage coil at 64 and 128 MHz. The coil is loaded with either a larger, more muscular male body model (subject 1) or a newly developed female body model (subject 2), and driven with two-port (quadrature), four-port, or many (ideal) sources. Loading the coil with subject 1 results in significantly less homogeneous B 1 field, higher SAR, and lower ISNR than those for subject 2 at both frequencies. This dependence of MR performance and safety measures on body type indicates a need for a variety of numerical models representative of a diverse population for future calculations. The different drive methods result in similar B 1 field patterns, SAR, and ISNR in all cases.

All participants provided informed consent to participate in this study, which was approved by the institutional review board of Milton S. Hershey Medical Center. The purpose of the study was to determine the feasibility of cartilage T2 mapping in the evaluation of response of femoral and tibial cartilage to running exercise. Quantitative magnetic resonance (MR) T2 maps of weight-bearing femoral and tibial articular cartilage were obtained in seven young healthy men before and immediately after 30 minutes of running by using a 3.0-T MR imager. There was no statistically significant change in T2 profiles of tibial cartilage. There was a statistically significant decrease in T2 of the superficial 40% of weight-bearing femoral cartilage after exercise. These in vivo observations agree well with published ex vivo results and support the hypothesis that cartilage compression results in greater anisotropy of superficial collagen fibers.

RF behavior in the human head becomes complex at ultrahigh magnetic fields. A bright center and a weak periphery are observed in images obtained with volume coils, while surface coils provide strong signal in the periphery. Intensity patterns reported with volume coils are often loosely referred to as 'dielectric resonances,' while modeling studies ascribe them to superposition of traveling waves greatly dampened in lossy brain tissues, raising questions regarding the usage of this term. Here we address this question experimentally, taking full advantage of a transceiver coil array that was used in volume transmit mode, multiple receiver mode, or single transmit surface coil mode. We demonstrate with an appropriately conductive sphere phantom that destructive interferences are responsible for a weak B(1) in the periphery, without a significant standing wave pattern. The relative spatial phase of receive and transmit B(1) proved remarkably similar for the different coil elements, although with opposite rotational direction. Additional simulation data closely matched our phantom results. In the human brain the phase patterns were more complex but still exhibited similarities between coil elements. Our results suggest that measuring spatial B(1) phase could help, within an MR session, to perform RF shimming in order to obtain more homogeneous B(1) in user-defined areas of the brain

Image inhomogeneity related to high radiofrequencies is one of the major challenges for high field imaging. This inhomogeneity can be thought of as having 2 radiofrequency-field related contributors: the transmit field distribution and the reception field distribution. Adjusting magnitude and phase of currents in elements of a transmit array can significantly improve flip angle homogeneity at high field. Effective application of some well-known parallel imaging and other receive array post-processing methods removes receptivity patterns from the intensity distribution in the final image, though noise then becomes a function of position in the final image. Here simulations are used to show that, assuming high signal-to-noise ratio, very homogeneous images in the human head can be acquired with the combination of transmit arrays and some receive array reconstruction methods at frequencies as high as 600 MHz

There is evidence in the literature indicating a significant static field inhomogeneity in the human breast. A nonhomogenous field results in line broadening and frequency shifts in MRS and can cause intensity loss and spatial errors in MRI. Thus, there is a clear rationale for determining the regional variations in the static field homogeneity in the breast and providing strategies to correct them. Herein, the nature and extent of the static magnetic field at 3 T were measured in central planes of the human breast using both phase maps and multivoxel MRS techniques. In addition, the effect of first- and high-order shimming and of spatial saturation pulses on the static field inhomogeneity was evaluated. Both the theoretical and the measured field were found to be primarily linear in nature, with a reduction of 300 Hz from the nipple to the chest wall. First-order shimming reduced this inhomogeneity by 65%. Interestingly, the combination of spatial saturation pulses and first-order shimming was more effective than high-order shim alone. Since many clinical scanners do not have either higher-order shim or automated higher shimming algorithms that work in the presence of fat, the suggested combination provides an effective means to correct inhomogeneities in the breast