During the last eight years our group has developed radial acquisitions with angular undersampling
factors of several hundred that accelerate MRI in selected applications. As with all previous
acceleration techniques, SNR typically falls as least as fast as the inverse square root of the
undersampling factor. This limits the SNR available to support the small voxels that these methods
can image over short time intervals in applications like time-resolved contrast-enhanced MR
angiography (CE-MRA). Instead of processing each time interval independently, we have developed
constrained reconstruction methods that exploit the significant correlation between temporal
sampling points. A broad class of methods, termed HighlY Constrained Back PRojection (HYPR),
generalizes this concept to other modalities and sampling dimensions.
We present Computed Tomography (CT) acquisition and reconstruction schemes for low-dose neuro-angiography based
on the method of HighlY constrained back PRojection (HYPR). Simulated and experimental low X-ray radiation dose
scans were prepared using the techniques of interleaved view angle under-sampling and tube current reduction.
Dynamic CT Angiograms (CTAs) were produced for both standard and low dose images sets. The spatial correlation
coefficient, r, between the two reconstruction approaches was determined for each time frame and the SNR and CNR
values in arterial ROIs were calculated. The undersampled HYPR reconstructions produced r values of > 0.95 at undersampling
and dose reduction factors of 10 and SNR and CNR were more than doubled using HYPR techniques at a tube
current of 25 mA. HYPR approaches to contrast enhanced neuro-imaging provide not only volumetric brain
hemodynamics but also the ability to produce high quality maps of standard perfusion parameters. The synergy of
volumetric hemodynamics and assessment of tissue function provides the medical imaging community with high quality
diagnostic information at a fraction of the radiation dose in a single contrast-enhanced scan.
In this study we develop a novel ECG-gated method of HYPR (HighlY constrained backPRojection) CT reconstruction for low-dose myocardial perfusion imaging and present its first application in a porcine model. HYPR is a method of reconstructing time-resolved images from view-undersampled projection data. Scanning and reconstruction techniques were explored using x-ray projections from a 50 sec contrast-enhanced axial scan of a 47 kg swine on a 64-slice MDCT system. Scans were generated with view undersampling factors from 2 to 10. A HYPR reconstruction algorithm was developed in which a fully-sampled composite image is generated from views collected from multiple cardiac cycles within a diastolic window. A time frame image for a heartbeat was produced by modifying the composite with projections from the cycle of interest. Heart rate variations were handled by automatically selecting cardiac window size and number of cycles per composite within defined limits. Cardiac window size averaged 35% of the R-R interval for 2x undersampling and increased to 64% R-R using 10x undersampling. The selected window size and cycles per composite was sensitive to synchrony between heart rate, gantry rate, and the view undersampling pattern. Temporal dynamics and perfusion metrics measured in conventional short-scan (FBP) images were well-reproduced in the undersampled HYPR time series. Mean transit times determined from HYPR myocardial time-density curves agreed to within 8% with the FBP results. The results indicate potential for an order of magnitude reduction in dose requirement per image in cardiac perfusion CT via undersampled scanning and ECG-gated HYPR reconstruction.
X-ray cone-beam computed tomography (CBCT) is of importance in image-guided intervention (IGI) and image-guided radiation therapy (IGRT). In this paper, we present a cone-beam CT data acquisition system using a GE INNOVA 4100 (GE Healthcare Technologies, Waukesha, Wisconsin) clinical system. This new cone-beam data acquisition mode was developed for research purposes without interfering with any clinical function of the system. It provides us a basic imaging pipeline for more advanced cone-beam data acquisition methods. It also provides us a platform to study and overcome the limiting factors such as cone-beam artifacts and limiting low contrast resolution in current C-arm based cone-beam CT systems. A geometrical calibration method was developed to experimentally determine parameters of the scanning geometry to correct the image reconstruction for geometric non-idealities. Extensive phantom studies and some small animal studies have been conducted to evaluate the performance of our cone-beam CT data acquisition system.
In order to provide high sensitivity rapid imaging at 3.3 mm (90 GHz) for the Green Bank Telescope - the world's largest steerable aperture - a camera is being built by the University of Pennsylvania, NASA/GSFC, and NRAO. The heart of this camera is an 8x8 close-packed, Nyquist-sampled detector array. We have designed and are fabricating a functional superconducting bolometer array system using a monolithic planar architecture. Read out by SQUID multiplexers, the superconducting transition edge sensors will provide fast, linear, sensitive response for high performance imaging. This will provide the first ever superconducting bolometer array on a facility instrument.
We are constructing an 8 by 8 bolometer camera, as a 90 GHz facility instrument for the 100 m Green Bank Telescope (GBT). The bolometers use transition-edge-superconducting (TES) sensors read
out with a time-based SQUID multiplexing system. The receiver will be one of the first astronomical instruments to use such detectors. Our TES bolometers require cooling below 290 mK. To obtain these temperatures we use He3 and He4 sorption refrigerators that cycle from a two-stage pulse-tube cryocooler. The He3 stage has an operating temperature of 252 mK and a hold time of 77 hours with a 10 microwatt load. A combination of the large collecting area of the GBT and the low noise of the detectors will enable us to map 15 arcsecond by 15 arcsecond areas of sky to 200 microJansky in one hour.