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Zhiyue J Wang

Publications and source records attributed to Zhiyue J Wang.

10 recordsLinked to original sources
PubMed2006

Correcting the effects of background microcirculation in the measurement of arterial input functions using dynamic susceptibility contrast MRI of the brain.

In dynamic susceptibility contrast MRI, the shape of the arterial input function (AIF) is commonly obtained in the near vicinity of the middle cerebral artery (MCA). However, the tissue regions where the AIF is sampled also have significant perfusion, which contributes to T(2)* changes. We investigate whether correction of this effect will introduce significant changes in the measurement of the AIF and, subsequently, the assessment of the mean transit time (MTT). Clinical dynamic susceptibility data from 13 patients with brain tumors were analyzed. Patients received either single or double doses of Magnevist followed by a saline flush through a power injector. In the correction procedure, DeltaR(2)* was sampled in a region of gray matter approximately 1-2 cm away from the MCA and then subtracted from the DeltaR(2)* sampled in the immediate vicinity of the MCA. We demonstrate that in the brain, this correction of DeltaR(2)* due to tissue perfusion leads to a narrower width of the AIF curve obtained with DeltaR(2)* (mean+/-S.D.=7.3+/-2.0 and 6.4+/-1.7 s, before and after correction, respectively, P<.001 using a two-tailed paired t-test). Furthermore, the peak of the AIF also moved to a slightly earlier time relative to the time of arrival (mean+/-S.D.=4.7+/-0.9 and 4.3+/-0.8 s, before and after correction, with P<.001). With the use of the corrected AIF, the measured MTT had increased values in areas of both gray and white matter.

Algorithms↗
PubMed2006

Achieving plane-wise uniform B1 amplitude in a 3D volume for high-field MRI: a computer simulation study.

PURPOSE: To demonstrate the possibility of achieving plane-wise uniform B(1) field amplitude for human head imaging with an array coil configuration by using computer simulations. MATERIALS AND METHODS: We considered the use of an excitation array coil that employed composite excitation elements. Each composite excitation element consisted of three small current loops centered close to each other with axes along the x, y, and z directions, respectively. The excitation elements were distributed to surround a model human head. The vector B(1) field from each current loop was calculated using the FDTD numerical method at 170 MHz. Analytical target RF field patterns with plane-wise uniform B(1) field amplitude were derived and approximately constructed from the fields of individual current loops through a least-squares procedure. RESULTS: The RF field patterns generated by the computer simulations closely followed the target field patterns. Highly uniform B(1) field amplitude was obtained within parallel sagittal planes or parallel axial-to-coronal oblique planes in the brain with the expected plane-to-plane variations. CONCLUSION: In principle, patterns of B(1) amplitude distribution with a high degree of plane-wise homogeneity can be achieved simultaneously in multiple parallel planes in a 3D volume.

Brain↗
PubMed2005

1/T2 and magnetic susceptibility measurements in a gerbil cardiac iron overload model.

PURPOSE: To measure the transverse relaxation rate (1/T2) and magnetic susceptibility of the heart in conditions of iron overload by using magnetic resonance (MR) imaging and to correlate these with the tissue iron concentration in a gerbil model. MATERIALS AND METHODS: With prior approval by the institutional animal care and use committee, iron overload was induced with one to 15 weekly subcutaneous injections of iron dextran. Nine gerbils had one to five injections, 10 had six to 10, and eight had 13-15. T2 of the whole heart was measured ex vivo (n=27), and the magnetic susceptibility of the tissue was estimated through measurement of the tissue lysate (n=25). The iron level was measured (in milligrams of iron per gram of wet tissue) with chemical analysis after MR imaging. While 1/T2 and magnetic susceptibility are not equivalent measures of the chemically determined tissue iron level, correlations were expected and were identified by using linear regression models. RESULTS: Iron concentration range was 0.28-1.95 mg/g wet tissue. Iron concentration was strongly correlated with 1/T2 (r=0.92, P <.001, and the root of the mean squares error of the linear prediction, epsilonRMS, was 0.17 mg Fe/g wet tissue with a repetition time of 700 msec). Iron concentration also was strongly correlated with magnetic susceptibility (r=0.90, P <.001, epsilonRMS=0.19 mg Fe/g wet tissue). Multiple regression analysis with combined 1/T2 (with repetition time of 700 msec) and magnetic susceptibility data led to a slight increase in r and decrease in epsilon(RMS) (r=0.93, P <.001, epsilonRMS=0.16 mg Fe/g wet tissue). CONCLUSION: The results of this animal model study demonstrate that 1/T2 and magnetic susceptibility values can be used for estimation of the iron level in the heart.

Animals↗
PubMed2005

Late proton MR spectroscopy in children after traumatic brain injury: correlation with cognitive outcomes.

BACKGROUND AND PURPOSE: Proton MR spectroscopy has demonstrated reduced levels of N-acetylaspartate (NAA) in normal-appearing occipital and frontal regions of patients with acute nonpenetrating traumatic brain injury (TBI). We studied the relationship of frontoparietal NAA, choline (Cho), and creatine (Cr) to test the hypothesis that reduction in NAA is predictive of cognitive outcome. METHODS: Proton spectra were collected by using conventional 2D chemical shift imaging in five healthy children and seven children (6 weeks to 3 years) with severe (n=4), moderate (n=2), or mild (n=1) TBI. Spectra in the anterior and posterior regions of the left and right frontoparietal areas were averaged for analysis by using LCModel, with a phantom-established basis function, for quantification of NAA, Cho, and Cr concentrations. Intellectual function, expressive language, and arithmetic capability were measured within 4 months of imaging. RESULTS: NAA/Cho concentration was lower in TBI patients than in control subjects, but no group differences were present for Cho or Cr. Hemispheric levels for NAA, Cho, and Cr were higher on the left than on the right, but we found no effect of region and no interactions. Cognition was lower in the TBI group than the control group and correlated with NAA levels. Left frontal Cho was also correlated with arithmetic scores, whereas Cr was not significantly correlated. CONCLUSION: NAA levels remain low after TBI and are related to cognitive function. Neurometabolite values are greater in the left frontoparietal region than in the right, and the left frontal Cho level is related to arithmetic ability.

Adolescent↗
PubMed2004

MRI measurement of hepatic magnetic susceptibility-phantom validation and normal subject studies.

A magnetic resonance (MR) imaging method with the potential for assessing hepatic iron overload from measurements of hepatic magnetic susceptibility in vivo is described. Using the blood in the portal and hepatic veins as an internal reference, this technique uses the orientation dependence of signal phase to measure the susceptibility of the liver parenchyma. Computer simulations were done to investigate the requirements on spatial resolution and contrast ratio between the vessels and the background liver tissue for data acquisition. Validation studies were conducted using tube-embedded gel phantoms doped with iron-dextran from 0 to 10 mg Fe/mL to mimic healthy and iron-overloaded livers. The phantom measurements were conducted without motion and flow, under respiration-like oscillatory motion, and with flow. Studies on six normal human subjects demonstrated excellent reproducibility and precision. All images were collected at 1.5 T using a 3D T(1)-weighted turbo field echo sequence for inflow MR angiographies with full flow compensation and capable of cardiac synchronization, navigator gating, and motion correction.

Algorithms↗
PubMed2003

MR diffusion imaging and MR spectroscopy of maple syrup urine disease during acute metabolic decompensation.

Maple syrup urine disease (MSUD) is an inborn error of amino acid metabolism, which affects the brain tissue resulting in impairment or death if untreated. Imaging studies have shown reversible brain edema during acute metabolic decompensation. The purpose of this paper is to describe the diffusion-weighted imaging (DWI) and spectroscopy findings during metabolic decompensation and to assess the value of these findings in the prediction of patient outcome. Six patients with the diagnosis of MSUD underwent conventional MR imaging with DWI during acute presentation with metabolic decompensation. Spectroscopy with long TE was performed in four of the six patients. Follow-up examinations were performed after clinical and metabolic recovery. DWI demonstrated marked restriction of proton diffusion compatible with cytotoxic or intramyelinic sheath edema in the brainstem, basal ganglia, thalami, cerebellar and periventricular white matter and the cerebral cortex. This was accompanied by the presence of an abnormal branched-chain amino acids (BCAA) and branched-chain alpha-keto acids (BCKA) peak at 0.9 ppm as well as elevated lactate on proton spectroscopy in all four patients. The changes in all six patients were reversed with treatment without evidence of volume loss or persistent tissue damage. The presence of cytotoxic or intramyelinic edema as evidenced by restricted water diffusion on DWI, with the presence of lactate on spectroscopy, could imply imminent cell death. However, in the context of metabolic decompensation in MSUD, it appears that changes in cell osmolarity and metabolism can reverse completely after metabolic correction.

3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide)↗
PubMed2003

In vivo measurement of brain metabolites using two-dimensional double-quantum MR spectroscopy--exploration of GABA levels in a ketogenic diet.

A localized proton 2D double-quantum (DQ) spin-echo spectroscopy technique was implemented on 1.5 T clinical MRI scanners for the detection of gamma-aminobutyrate (GABA) in the brain. The 2D approach facilitates separation of peaks overlapping with GABA in 1D DQ-filtered (DQF) spectra. This technique was applied to four normal adult volunteers and four children with intractable epilepsy. The coefficient of variation of the level of GABA and overlapping macromolecules at F2 = 3.0 ppm and F1 = 4.8 ppm was 0.08 in normal subjects. Three patients received 2D MRS scans before and after initiation of the ketogenic diet (KD): one patient showed a trend of decreasing GABA throughout the study, and two patients showed low initial GABA levels that increased over time. In addition to major metabolites and GABA, low-level metabolites (valine, leucine, and glutathione) were also identified in the 2D spectra.

Adult↗
PubMed2003

Magnetic susceptibility quantitation with MRI by solving boundary value problems.

Magnetic susceptibility measurement is of considerable research interest in MRI and MRS. A rigorous method was previously developed to quantify the susceptibility of an arbitrarily shaped uniform object in an inhomogeneous external field. However, it requires using the field distribution information on a spherical surface or shell in the surrounding homogeneous medium enclosing the object. In this work, a new approach was developed through solving the boundary value problems of the Laplace equation, which has an advantage that the boundary providing the necessary field distribution information can have an arbitrary shape. This method has been validated on rectangular boundaries with both numerical simulation as well as experimental data. It has also been realized that MRI provides an experimental means of solving some boundary value problems of partial differential equations, if proper boundary condition can be set up.

Computer Simulation↗
PubMed2002

Evaluation of iron overload by single voxel MRS measurement of liver T2.

PURPOSE: To overcome the difficulty of poor signal-to-noise ratio of magnetic resonance imaging (MRI) in evaluating heavy iron overload by using a single voxel magnetic resonance spectroscopy (MRS) technique. MATERIALS AND METHODS: A single voxel STEAM pulse sequence with a minimum TE of 1.5 msec and a sampling volume of 36.6 cm(3) was developed and applied to 1/T2 measurement of the liver in 14 patients with thalassemia whose liver iron concentration was determined through biopsy. RESULTS: The iron level ranged from 0.23 to 37.15 mg Fe/g dry tissue with a median value of 18.06. In all cases, strong MR signals were obtained. 1/T2 was strongly correlated with the liver iron concentration (r = 0.95, P < 0.00005). CONCLUSION: The single voxel MRS measurement of T2 in liver iron overload overcomes the difficulty of lack of detectable signals in conventional MRI when the iron level is high. There is an excellent correlation between the iron level and 1/T2.

Adolescent↗