GLOW Cluster Projects
The Medical Physics computer cluster has contributed significantly to several research projects that would not have been possible without a large computational resource. It has been valuable for running Monte Carlo based radiation transport simulations for a variety of applications including brachytherapy source characterization and dosimetry, fast neutron beam characterization and dosimetry, proton dosimetry, and radiation detector characterization and design. It has also proven valuable for design-type work where literally hundreds of potential designs can be run and evaluated in a short period of time. Some of the projects using the cluster include:
Improved Well-type Ionization Chamber Design
Well-type ionization chambers are used for measuring the strength of radioactive seeds implanted into cancer patients. The cluster is being used to run Monte Carlo based radiation transport simulations to model existing well chambers to characterize their response and analyze known deficiencies. The cluster was used to design an improved well chamber and a prototype was constructed to compare modeled and as-built performance. A 3-dimensional model of one well chamber is shown in the figure to the right.
The ultrasound research group makes extensive use of simulations to model echo signal wave-forms and construct simulated images for media containing randomly distributed scatterers. The most computationally intensive aspect of the simulation is solving for the transducer field, where a 2-D integral over the surface of each rectangular element in an array transducer's active aperture is required. The work uses a frequency domain model for the transducer field, along with superposition to derive a time domain echo signal. To get adequate frequency resolution for long duration echo signals that cover a 80% bandwidth pulse, it may be necessary to compute beam profiles for as many as 1,000 frequency components. In forming an image the superposition calculation must be done for as many as 150 separate ultrasound beams, each beam selected by proper phasing of the signals among transducer elements.
As an example, a small phantom with dimension of 4cm x 4cm x 1cm will contain on the order of 155,000 scatterers. The simulation program calculates the field for 960 frequency points. The received signal for each of the 120 beam lines that make up a "frame" of RF data is computed by summing echoes from all scatterers. Thus, it takes about 3 hours of computer time for a single frame. Much longer computation times are necessary to simulate certain beam-steered acquisitions because many of the symmetries used for non-steered beams are not applicable.
Ge Detector Characterization
The cluster is being used to characterize and correct for defects in a high purity germanium spectrometer used to analyze radioactive seeds. These radioactive seeds are implanted in patients to treat cancer. The characterization of the detector requires 8192 separate mono-energetic simulations that consume approximately 5 hours of computer time each. The detector characterization also requires numerically solving a linear algebra equation with a 8192x8192 matrix that is also solved on the cluster (using Octave).