Selective fourier transform localization

Brooker, H. R.; Mareci, Thomas H.; Mao, Jintong

doi:10.1002/mrm.1910050503

Cited by 78 publications

(56 citation statements)

References 24 publications

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“…Because the liver is located in an oblique fashion in the abdomen, the oblique localization was achieved by the simultaneous use of vertical and horizontal gradients. A Gaussian-weighted phase-encoding scheme was used to improve the signal-to-noise ratios without sacrificing the efficiency of localization (19). A total of 505 phase-encoding steps over 32 k-space values was acquired per cycle of Gaussian-weighted sampling.…”

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confidence: 99%

Role of Hepatic Glycogen Breakdown in Defective Counterregulation of Hypoglycemia in Intensively Treated Type 1 Diabetes

et al. 2006

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Impairment of hypoglycemic counterregulation in intensively treated type 1 diabetes has been attributed to deficits in counterregulatory hormone secretion. However, because the liver plays a critical part in recovery of plasma glucose, abnormalities in hepatic glycogen metabolism per se could also play an important role. We quantified the contribution of net hepatic glycogenolysis during insulininduced hypoglycemia in 10 nondiabetic subjects and 7 type 1 diabetic subjects (HbA 1c 6.5 ؎ 0.2%) using 13 C nuclear magnetic resonance spectroscopy, during 2 h of either hyperinsulinemic euglycemia (plasma glucose 92 ؎ 4 mg/dl) or hypoglycemia (plasma glucose 58 ؎ 3 mg/dl). In nondiabetic subjects, hypoglycemia was associated with a brisk counterregulatory hormone response (plasma epinephrine 246 ؎ 38 vs. 2,785 ؎ 601 pmol/l during hypoglycemia, plasma norepinephrine 1.9 ؎ 0.2 vs. 2.5 ؎ 0.3 nmol/l, and glucagon 38 ؎ 7 vs. 92 ؎ 17 pg/ml, respectively, P < 0.001 in all), and a relative increase in endogenous glucose production (EGP 0.83 ؎ 0.14 mg ⅐ kg ؊1 ⅐ min ؊1 during euglycemia yet ϳ50% higher with hypoglycemia [1.30 ؎ 0.20 mg ⅐ kg ؊1 ⅐ min ؊1 ], P < 0.001). Net hepatic glycogen content declined progressively during hypoglycemia to 22 ؎ 3% below baseline (P < 0.024). By the final 30 min of hypoglycemia, hepatic glycogen fell from 301 ؎ 14 to 234 ؎ 10 mmol/l (P < 0.001) and accounted for ϳ100% of EGP. In marked contrast, after an overnight fast, hepatic glycogen concentration in type 1 diabetic subjects (215 ؎ 23 mmol/l) was significantly lower than in nondiabetic subjects (316 ؎ 19 mmol/l, P < 0.001). Furthermore, the counterregulatory response to hypoglycemia was significantly reduced with small increments in plasma epinephrine and norepinephrine (126 ؎ 22 vs. 448 ؎ 16 pmol/l in hypoglycemia and 0.9 ؎ 0.3 vs. 1.6 ؎ 0.3 nmol/l, respectively, P < 0.05 for both) and no increase in plasma glucagon. EGP decreased during hypoglycemia with no recovery (1.3 ؎ 0.5 vs.

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confidence: 99%

Role of Hepatic Glycogen Breakdown in Defective Counterregulation of Hypoglycemia in Intensively Treated Type 1 Diabetes

et al. 2006

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“…Starting at k 0 rad ϭ 0, rad (k i rad ) ϭ 1/⌬k rad is given by Eq. [7] and (k i rad ) ϭ 1/⌬k is given by the following formula:…”

Section: D Density-weightingmentioning

confidence: 99%

“…Starting at k 0 rad ϭ 0, rad (k i ) is calculated analogous to the 2D case (Eq. [7]), and ⍀ (k i rad ) is determined by the following formula:…”

Section: D Density-weightingmentioning

confidence: 99%

“…The SRF characterizes the spatial origin of the signals contributing to a distinct voxel (6). If averaging is necessary, acquisition-weighting by variable accumulation numbers for different k-space positions can strongly suppress the sidelobes of the SRF, resulting in a significant reduction of the contamination (7,8). This kind of acquisition-weighting, which is based on variable accumulation of the different PE steps, is in the following referred to as "accumulation-weighting" or "AW."…”

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confidence: 99%

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Efficient k‐space sampling by density‐weighted phase‐encoding

Greiser

Kienlin

2003

Magnetic Resonance in Med

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Acquisition-weighting improves the localization of MRI experiments. An approach to acquisition-weighting in a purely phaseencoded experiment is presented that is based on a variation of the sampling density in k-space. In contrast to conventional imaging or to accumulation-weighting, where k-space is sampled with uniform increments, density-weighting varies the distance between neighboring sampling points ⌬k to approximate a given radial weighting function. A fast, noniterative algorithm has been developed to calculate the sampling matrix in one, two, and three dimensions from a radial weighting function w(k), the desired number of scans NA tot and the nominal spatial resolution ⌬x nom . Density-weighted phase-encoding combines the improved shape of the spatial response function and the high SNR of acquisition-weighting with an extended field of view. The artifact energy that results from aliasing due to a small field of view is substantially reduced. The properties of density-weighting are compared to uniform and to accumulation-weighted phase-encoding in simulations and experiments. Density-weighted 31 In metabolic MRI in vivo, low concentrations commonly lead to low signal-to-noise ratios (SNRs). Although fast imaging methods with readout encoding offer comparably high sensitivity (1-4), pure phase-encoding (PE) in all spatially resolved dimensions is still the gold standard in sensitivity-limited MR experiments (5). The low achievable spatial resolution results in severe signal contamination from neighboring regions due to the sidelobes of the spatial response function (SRF). The SRF characterizes the spatial origin of the signals contributing to a distinct voxel (6). If averaging is necessary, acquisition-weighting by variable accumulation numbers for different k-space positions can strongly suppress the sidelobes of the SRF, resulting in a significant reduction of the contamination (7,8). This kind of acquisition-weighting, which is based on variable accumulation of the different PE steps, is in the following referred to as "accumulation-weighting" or "AW." The term "conventional" is used in this context for a uniformly sampled (UW) experiment with equal accumulation numbers. Compared to conventional uniform (UW) weighting, AW improves the SRF without any loss of SNR at identical resolution and experiment duration (6 -8). However, due to the higher accumulation numbers in central k-space and the need for an increased number of different PE steps to preserve spatial resolution, ⌬x nom , AW results in a smaller achievable field of view (FOV) compared to a UW experiment with identical total number of accumulations and spatial resolution. An improved form of acquisition-weighting is presented, "densityweighting" (DW).DW is based on a variation of the density of sampling points in k-space. Compared to AW, DW preserves the favorable shape of the SRF, but extends the accessible FOV. In DW the overall number of different sampling positions is maximized to the total number of scans. Furthermore, the sampling poin...

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“…However, the reduction in ringing typically results in increased noise variance (2,3), an undesired effect for in vivo CSI. Mareci et al (4) pointed out that k-space data can be windowed without a trade off in noise variance, as long as the k-space sampling density is modified during acquisition to be proportional to the applied window function.…”

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confidence: 99%