Karl K. Vigen

Undersampled projection reconstruction applied to MR angiography

Undersampled projection reconstruction (PR) is investigated as an alternative method for MRA (MR angiography). In conventional 3D Fourier transform (FT) MRA, resolution in the phase‐encoding direction is proportional to acquisition time. Since the PR resolution in all directions is determined by the readout resolution, independent of the number of projections (Np), high resolution can be generated rapidly. However, artifacts increase for reduced Np. In X‐ray CT, undersampling artifacts from bright objects like bone can dominate other tissue. In MRA, where bright, contrast‐filled vessels dominate, artifacts are often acceptable and the greater resolution per unit time provided by undersampled PR can be realized. The resolution increase is limited by SNR reduction associated with reduced voxel size. The hybrid 3D sequence acquires fractional echo projections in the kx–ky plane and phase encodings in kz. PR resolution and artifact characteristics are demonstrated in a phantom and in contrast‐enhanced volunteer studies. Magn Reson Med 43:91–101, 2000.

T1 independent, T2* corrected chemical shift based fat–water separation with multi‐peak fat spectral modeling is an accurate and precise measure of hepatic steatosis

To determine the precision and accuracy of hepatic fat‐fraction measured with a chemical shift‐based MRI fat‐water separation method, using single‐voxel MR spectroscopy (MRS) as a reference standard.

Undersampled projection-reconstruction imaging for time-resolved contrast-enhanced imaging

In time‐resolved contrast‐enhanced 3D MR angiography, spatial resolution is traded for high temporal resolution. A hybrid method is presented that attempts to reduce this tradeoff in two of the spatial dimensions. It combines an undersampled projection acquisition in two dimensions with variable rate k‐space sampling in the third. Spatial resolution in the projection plane is determined by readout resolution and is limited primarily by signal‐to‐noise ratio. Oversampling the center of k‐space combined with temporal k‐space interpolation provides time frames with minimal venous contamination. Results demonstrating improved resolution in phantoms and volunteers are presented using angular undersampling factors up to eight with acceptable projection reconstruction artifacts. Magn Reson Med 43:170–176, 2000.

Time‐resolved, undersampled projection reconstruction imaging for high‐resolution CE‐MRA of the distal runoff vessels

Imaging of the blood vessels below the knee using contrast‐enhanced (CE) MRI is challenging due to the need to coordinate image acquisition and arrival of the contrast in the targeted vessels. Time‐resolved acquisitions have been successful in consistently capturing images of the arterial phase of the bolus of contrast agent in the distal extremities. Although time‐resolved exams are robust in this respect, higher spatial resolution for the depiction of tight stenoses and the small vessels in the lower leg is desirable. A modification to a high‐spatial‐resolution T1‐weighted pulse sequence (projection reconstruction‐time resolved imaging of contrast kinetics (PR‐TRICKS)) that improves the through‐plane spatial resolution by a factor of 2 and maintains a high frame rate is presented. The undersampled PR‐TRICKS pulse sequence has been modified to double the spatial resolution in the slice direction by acquiring high‐spatial‐frequency slice data only after first pass of the bolus of contrast agent. The acquisition reported in the present work (PR‐hyperTRICKS) has been used to image healthy volunteers and patients with known vascular disease. The temporal resolution was found to be beneficial in capturing arterial phase images in the presence of asymmetric filling of vessels. Magn Reson Med 48:516–522, 2002.

Tissue mimicking materials for a multi‐imaging modality prostate phantom

Materials that simultaneously mimic soft tissue in vivo for magnetic resonance imaging (MRI), ultrasound (US), and computed tomography (CT) for use in a prostate phantom have been developed. Prostate and muscle mimicking materials contain water, agarose, lipid particles, protein, Cu++, EDTA, glass beads, and thimerosal (preservative). Fat was mimicked with safflower oil suffusing a random mesh (network) of polyurethane. Phantom material properties were measured at 22 degrees C. (22 degrees C is a typical room temperature at which phantoms are used.) The values of material properties should match, as well as possible, the values for tissues at body temperature, 37 degrees C. For MRI, the primary properties of interest are T1 and T2 relaxations times, for US they are the attenuation coefficient, propagation speed, and backscatter, and for CT, the x-ray attenuation. Considering the large number of parameters to be mimicked, rather good agreement was found with actual tissue values obtained from the literature. Using published values for prostate parenchyma, T1 and T2 at 37 degrees C and 40 MHz are estimated to be about 1,100 and 98 ms, respectively. The CT number for in vivo prostate is estimated to be 45 HU (Hounsfield units). The prostate mimicking material has a T1 of 937 ms and a T2 of 88 ms at 22 degrees C and 40 MHz; the propagation speed and attenuation coefficient slope are 1,540 m/s and 0.36 dB/cm/MHz, respectively, and the CT number of tissue mimicking prostate is 43 HU. Tissue mimicking (TM) muscle differs from TM prostate in the amount of dry weight agarose, Cu++, EDTA, and the quality and quantity of glass beads. The 18 microm glass beads used in TM muscle increase US backscatter and US attenuation; the presence of the beads also has some effect on T1 but no effect on T2. The composition of tissue-mimicking materials developed is such that different versions can be placed in direct contact with one another in a phantom with no long term change in US, MRI, or CT properties. Thus, anthropomorphic phantoms can be constructed.

Optimized high-resolution contrast-enhanced hepatobiliary imaging at 3 tesla: a cross-over comparison of gadobenate dimeglumine and gadoxetic acid.

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Phase-contrast with interleaved undersampled projections.

mapping has important applications in MRI, including functional imaging, tracking of super‐paramagnetic particles, and measurement of tissue iron levels. However,

Effectiveness of MR angiography for the primary diagnosis of acute pulmonary embolism: clinical outcomes at 3 months and 1 year.

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Fetal Origin of the Posterior Cerebral Artery Produces Left-Right Asymmetry on Perfusion Imaging

measurements can be confounded by several effects, particularly the presence of fat and macroscopic