Accelerated Imaging Techniques for Chemical Shift Magnetic Resonance Imaging

Chemical shift imaging is a method for the separation two or more chemical species. The cost of chemical shift encoding is increased acquisition time as multiple acquisitions are acquired at different echo times. Image acceleration techniques, typically parallel imaging, are often used to improve the spatial coverage and resolution. This thesis describes a new technique for estimating the signal to noise ratio for parallel imaging reconstructions and proposes new image reconstructions for accelerated chemical shift imaging using compressed sensing and/or parallel imaging for two applications: water-fat separation and metabolic imaging of hyperpolarized [1-13C] pyruvate. Spatially varying noise in parallel imaging reconstructions makes measurements of the signal to noise ratio, a commonly used metric for image for image quality, difficult. Existing approaches have limitations such as they are not applicable to all reconstructions, require significant computation time, or rely on repeated image acquisitions. A SNR estimation technique is proposed that does not exhibit these limitations. Water-fat imaging of highly undersampled datasets from the liver, calf, knee, and abdominal cavity are demonstrated using a customized IDEAL-SPGR pulse sequence and an integrated compressed sensing, parallel imaging, water-fat reconstruction. This method is shown to offer comparable image quality relative to fully sampled reference images for a range of acceleration factors. At high acceleration factors, this technique is shown to offer improved image quality when compared to the current standard of parallel imaging. Accelerated chemical shift imaging was demonstrated for metabolic of hyperpolarized [1-13C] pyruvate. Pyruvate, lactate, alanine, and bicarbonate images were reconstructed from undersampled datasets. Phantoms were used to validate this technique while retrospectively and prospectively accelerated 3D in vivo datasets were used to demonstrate. Alternatively, acceleration was also achieved through the use of a high performance magnetic field gradient set. This thesis addresses the inherently slow acquisition times of chemical shift imaging by examining the role compressed sensing and parallel imaging can be play in chemical shift imaging. An approach to SNR assessment for parallel imaging reconstruction is proposed and approaches to accelerated chemical shift imaging are described for applications in water-fat imaging and metabolic imaging of hyperpolarized [1-13C] pyruvate

3D bite modeling and feeding mechanics of the largest living amphibian, the Chinese Giant Salamander Andrias davidianus (Amphibia:Urodela)

Biting is an integral feature of the feeding mechanism for aquatic and terrestrial salamanders to capture, fix or immobilize elusive or struggling prey. However, little information is available on how it works and the functional implications of this biting system in amphibians although such approaches might be essential to understand feeding systems performed by early tetrapods. Herein, the skull biomechanics of the Chinese giant salamander, Andrias davidianus is investigated using 3D finite element analysis. The results reveal that the prey contact position is crucial for the structural performance of the skull, which is probably related to the lack of a bony bridge between the posterior end of the maxilla and the anterior quadrato-squamosal region. Giant salamanders perform asymmetrical strikes. These strikes are unusual and specialized behavior but might indeed be beneficial in such sit-and-wait or ambush-predators to capture laterally approaching prey. However, once captured by an asymmetrical strike, large, elusive and struggling prey have to be brought to the anterior jaw region to be subdued by a strong bite. Given their basal position within extant salamanders and theirPeer ReviewedPostprint (published version

Quantification of fetal organ volume and fat deposition following <i>in utero</i> exposure to maternal Western Diet using MRI

<div><p>Purpose</p><p>To examine the feasibility of using MRI to identify differences in liver size and fat deposition in fetal guinea pigs exposed to an <i>in utero</i> environment influenced by maternal consumption of a Western diet.</p><p>Materials and methods</p><p>Female guinea pigs fed either an energy-dense Western Diet (WD), comprised of increased saturated fats and simple sugars, or a Control Diet (CD) from weaning through pregnancy, underwent MR scanning near term (~ 60 days; term ~ 69 days). Maternal weights were collected at mating and at MR scanning. T₁-weighted, T₂-weighted, and IDEAL water-fat images were acquired at 3 Tesla. The images were used to segment maternal adipose tissue, fetal liver, fetal brain, fetal adipose tissue, and total fetal volumes and to measure maternal and fetal hepatic fat fractions.</p><p>Results</p><p>Weights of WD sows were lower prior to pregnancy (<i>P</i> = .04), however their weight gain over pregnancy did not differ from the CD group (<i>P</i> = .98). The WD sows had less total adipose tissue (TAT) at MR scanning (<i>P</i> = .04), while hepatic fat content was significantly elevated (<i>P</i> = .04). When controlling for litter size, WD fetuses had larger livers (<i>P</i> = .02), smaller brains (<i>P</i> = .01), and increased total adipose tissue volume (<i>P</i> = .01) when normalized by fetal volume. The WD fetuses also had increased hepatic fat fractions compared to CD fetal livers (<i>P</i> < .001).</p><p>Conclusion</p><p>Maternal Western Diet consumption prior to and during pregnancy induces differences in maternal liver fat content, fetal liver volume and liver fat storage, as well as changes in fetal adipose tissue deposition that can be measured <i>in utero</i> using MRI.</p></div

Correlation of fetal adipose tissue volumes with hepatic fat fraction.

<p>(A) Total adipose tissue (TAT) to fetal volume ratio as a function of fetal hepatic proton density fat fraction (PDFF). (B) Intra-abdominal adipose tissue (IAAT) to fetal volume ratio as a function of fetal hepatic PDFF. Linear fit lines for all data are shown with 95% confidence intervals.</p

Weight and fat deposition measurements for Control Diet and Western Diet sows.

<p>(A) Weight measurements taken before pregnancy, on the MR scan date, and weight change during pregnancy (CD: <i>n</i> = 4; WD: <i>n</i> = 4). (B) Total adipose tissue volumes (CD: <i>n</i> = 4; WD: <i>n</i> = 4). (C) Hepatic proton density fat fraction (CD: <i>n</i> = 4; WD: <i>n</i> = 4). Data are presented as box and whisker plots as described in the methods section for CD (black) and WD (grey) sows. *<i>P</i> < .05.</p

Coronal images of a pregnant guinea pig on a lifetime Western Diet.

<p>(A) T₁-weighted image with fetuses contoured in yellow and fetal livers denoted by white arrows. (B) T₂-weighted images with fetal brains contoured in yellow. (C) IDEAL fat-only image. (D) IDEAL fat fraction map. Images have been cropped to highlight the fetuses.</p

Fetal fat deposition for Control Diet and Western Diet fetuses.

<p>(A) Total adipose tissue (TAT) to fetal volume ratio and intra-abdominal adipose tissue (IAAT) volume to fetal volume ratio. (B) IAAT to TAT volume ratio. (C) Fetal hepatic proton density fat fraction. Data are presented as box and whisker plots as described in the methods section for CD (black) and WD (grey) fetuses. <i>n</i> = 11 and 9 for CD and WD, respectively. *<i>P</i> < .05.</p