Michael A. Miller

Development of anatomically realistic PET and PET/CT phantoms with rapid prototyping technology

The utility of PET and PET/CT in research and diagnosis of cancer, cardiac and neurological disorders has been widely demonstrated. Phantoms with well defined geometries that accurately model radiolabeled tracer concentrations and photon attenuation coefficients are suited for characterization of imaging systems, but not as well suited for evaluating methods sensitive to detailed anatomical structure, such as algorithms for monitoring tumor response. An ideal phantom would have the shape and activity distribution of a realistic tumor and would be useful in evaluation of automated image analysis systems. Such a phantom, imaged at sites involved in clinical trials, would be valuable for evaluating consistency and accuracy. We have developed a method of creating such phantoms by incorporating radioactive tracer as dye for a cellulose powder based rapid prototyping system. This allows us to create phantoms with spatial resolution limited only by the stereolithography printer system (slice thickness is 0.18 mm, printing resolution is 600 dpi). We have evaluated the method by printing several small phantoms with 18F and measuring activity in a gamma counter. The relative standard deviation of the activity of multiple identical phantoms was 2%. Activity in unlabeled parts was less than 2% of adjacent labeled regions. We have created and printed realistic phantoms based on the SPL human brain atlas [1,2], the Paxinos & Watson rat brain atlas [3] and from PET/CT images of human lung nodules, showing that this is a practical method for making complex radioactive phantoms that model real anatomy. We are proceeding with further development to allow us to produce phantoms with multiple activity concentrations, tunable photon attenuation coefficients and long lived isotopes.

Absolute temperature MR imaging with thulium 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetramethyl-1,4,7,10-tetraacetic acid (TmDOTMA−)

MR thermometry based on the water 1H signal provides high temporal and spatial resolution, but it has low temperature sensitivity (∼0.01 ppm/°C) and requires monitoring of another weaker signal for absolute temperature measurements. The use of the paramagnetic lanthanide complex, thulium 1,4,7,10‐ tetraazacyclo‐dodecane‐1,4,7,10‐tetramethyl‐1,4,7,10‐tetraac‐ etate (TmDOTMA−), which is ∼60 times more sensitive to temperature than the water 1H signal, is advanced to image absolute temperatures in vivo using water signal as a reference. The temperature imaging technique was developed using gradient echo and asymmetric spin echo imaging sequences on 9.4 Tesla (T) horizontal and vertical MR scanners. A comparison of regional temperatures measured with TmDOTMA− and fiber‐optic probes showed that the accuracy of imaging temperature is <0.3°C. The temperature imaging technique was found to be insensitive to inhomogeneities in the main magnetic field. The feasibility of imaging temperature of intact rats at ∼1.4 mmol/kg dose with ∼1‐mm spatial resolution in only 3 min is demonstrated. TmDOTMA− should prove useful for imaging absolute temperatures in deep‐seated organs in numerous biomedical applications. Magn Reson Med, 2009.

A non-iterative method for emission tomographic image reconstruction with resolution recovery

We propose a non-iterative method called Local Regression FBP (LR-FBP) for emission tomographic image reconstruction with resolution recovery. The method processes the sinogram volume data by running a two-dimensional local regression (LR) algorithm over the image projections - this has the effect of improving data signal-to-noise-ratios (SNR) as it fits the image projections thus reducing noise. Resolution recovery is then performed on the fit sinogram and the Filtered Back Projection (FBP) reconstruction using a ramp filter is then applied. The simulation studies presented show that LR-FBP gives lower bias and improved resolution compared to conventional FBP. Imaging studies using the IndyPET-III Small Animal PET scanner and the Siemens/CTI Biograph-16 PET/CT scanner show qualitative improvements in image resolution and contrast, as well as possible quantitative improvements in contrast recovery.